TerryE

We've had our second planning application in the system since mid December. Our first application was in May and the main reason for the delay between these was negotiating with Highways over the new vehicle access and parking to our existing farmhouse. (The existing access and parking falls within the new-build plot.) This delay was due to a combination of us being new to this and not understanding the rules to this bureaucratic game, and the highways engineer being just totally disorganised. Very frustrating and annoying for us, but what can you do? We have been checking the LPA website most days. The acceptance Notice of Decision was posted on the Thurs 12th (also the expiry date for our application). Hurrah! On the following Monday, I rang our planner to ask a few follow up Qs, and got an answer phone message that the 13th was his last day and he'd left the LPA. Arggggghhhh!! – I felt like I had just walked out of the front door to a building only to have the ceiling collapse behind me; not because of any loss (even though we like the guy), but more the idea of our having to change the planner that we'd been negotiating with at such a critical stage in the process. Having compared our NoD to others that have been recently posted on the LPA website, it seems broadly typical. A lot of the conditions are defensive measures on the part of the LPA to give them mechanisms to enforce the application should a builder try it on. We had six pre-commencement decisions for details to be agreed: three relating to the stone and slate of the outer skin and the access onto the highway; one on surveyed levels for the existing and proposed ground levels, floor levels, eaves and ridge heights in relation to adjacent buildings; one on the planting plan and one on an archaeological survey. The planting plan one was silly but easy to fix: we'd already provided one in our application, but it wasn't in the format that they liked, so we essentially had to represent the planting scheme as a marked-up site plan with lots of text boxes and arrows, rather than a figure and a couple of pages of text. IMO, the archaeological survey requirement is both silly and capricious. Apparently a survey ½ mile away below the village uncovered some Roman/Iron Age finds in 2010. So we now have to dig a survey strip across our 14.5m wide plot under the supervision of a qualified archaeologist; get him to write this up and file it before we can start our foundations. There have been 5 full infill development with permission granted in the village since 2010; none of the other four had a survey condition, despite 3 being closer to the previous site and ours being the only one that had previously contained (Victorian / Edwardian) farm outbuildings; the others were green field / garden. We also dozed the site down to the virgin clay 25 years ago to lay a tarmac drive so I don't think that it will yield anything more than the odd Victorian bottle shard. However, the cost and time impact of appealing this condition will probably be more than the cost of gritting our teeth and doing it – as I said to Jan, at least we don't have Newts!! We have six conditions before specified phases of work; for example they want to agree the paint (which must be white or off white) that we will use for exterior paintwork. I would class these as mostly stupid / annoying, and the LPA having a dose of Big Brother complex. We have another 3 conditions before occupation / use. So overall we can live with all of these, but even if we were to ignore their petty ones (e.g. the colour of paintwork, there's no way that the LPA would realistically issue an enforcement notice (the village falls into a category the LPA classify as "modestly sustainable"), so why bother attempting this level of minutiae control? However, overall I think that the LPA provided a good and effective service. Two of our neighbours strongly objected to having a house built beyond the bottom of their gardens, but the planner agreed with us that what we were proposing was the best compromise given that it provided a reasonably sized infill dwelling that would fit well within the village. We'll just have to be careful about one of these neighbours during the build itself; I suspect that he'll be complaining to the enforcement officer about anything that he possibly can – which is a pity because I'd prefer to stay on good terms with neighbours and do a bit of give and take where practical. So upwards and onwards. The main issue for us here is now to reach the necessary agreements without this taking another 3 months off our timelines.

We've sort of covered this topic buried in various earlier threads, but since I need to use this info for my heating calcs, I thought useful to cover this in a short summary post. Characterising the components of heat transfer across a solid / air surface really does come down to basic physics and we just need to crank the numbers into the two main factors at play in this: Radiation. Any surface is radiating heat but is also simultaneously absorbing similar radiation from everything in line of site. If everything is at the same temperature, then this all cancels out and no net flow of energy occurs. However even with a small imbalance in temperatures, because of the amount of radiation being transferred, results in a net energy flow. The physics depends on the Stefan–Boltzmann law, and when you crank the numbers for a surface at roughly 20°C, this works out at ~5.7 W/m²K. OK, this has to be factored by something called the emissivity which can be as low as 0.03 for a mirrored aluminium surface, but for normal painted surfaces like house walls, it's nearer to 0.9. Also remember that the whilst the area can easily be calculated for very smooth surfaces, any texturing (like clothes or carpet pile) can dramatically increase this. Nonetheless, a good general rule of thumb is to assume 5 W/m²K. Conduction. This is atoms of air bumping into the walls and transferring energy that way. Air, being a gas, is light on atoms compared to the solid wall, and so is a poor conductor, but it is also free to move and so the air region in contact with the wall is continually replaced due to any air movement. Once the temperature difference between the wall and the air is more than a couple of degrees, then the heating of the air itself generates convection and this make the heat transfer even more efficient. However in internal spaces, where there are no major drafts or temperature induced convection differences, this conduction makes a relatively small, say 30%, contribution, and radiation is the dominant component. So a good overall figure for bare surfaces is ~7W/m²K and this is what I use in my active slab calculations. This means that when doing U-value calculations, I can treat any material/air interface as having an effective thermal resistance of its reciprocal, that is roughly 0.15 m²K/W within the R-value calculations. Note that the references often assume some level of internal air movement, and so quote a lower thermal transmittance value of 0.12 m²K/W. Also if the surface includes a reflective / foil layer then the emissivity can drop significantly (though not to the 0.03 figure that I quoted earlier unless a high-spec multi-layered material is used); a typical foil-backed plasterboard might achieve an effective emissivity of around 0.3 which is why this is often quoted as having a transmittance value of 0.4 m²K/W. The bible which gives all of these magic figures is the BRE Conventions for U-value calculations document and the data given therein broadly corresponds to the above. Incidentally the average human has a surface area of roughly 2m², so radiates 5 x 2 x (33 - 20) (=130) watts if naked in a room at 20°C. Clearly the more clothes that you wear, the less your effective surface temperature, and the less your radiant heat losses; so with a light covering and a jumper on the torso, this might drop to 100W or so. This echoes a point made in one of Jeremy's earlier posts: how cold you feel in a house relates to your overall heat loss and in still air maybe 60-80% of this total is due to radiant losses rather than conductive/convective ones. So the temperature of the wall surfaces is just as important as the air temperature in determining this comfort level. Being in a room with walls and air at 20°C can feel just as comfortable as being in a room with walls at 17°C and air at 24°C.

I am writing these posts for two main reasons. The first is for my benefit, in that I find that if I have got to the point where I can explain my thinking to others, then I've got to grips with the problem myself. The second is that I might just help others going down this same path, by documenting my thought processes. It's three months since I wrote the Part I of these three Thermal Design posts, and I concluded this by saying that I intended to adopt Jeremy Harris's UFH For A Low Temperature Slab concept. Since then I have done quite a lot of modelling to understand how the house has a system will react to changes in input and output (as discussed in my last post). Basically, the house fabric has a huge thermal inertia, so I can use average weather conditions and ignore diurnal temperature changes, and even the odd day or so of unseasonal weather. The only wild-card in the heating equation is the issue of solar gain. Part 2 was a bit of a mixed bag which summarised these issues, and to be honest in the process of getting to grips with some of these, I ended up pretty much rewriting chunks of this and cutting out a lot. It's take me some time to get my head around some of the secondary issues, to get to the point where I could do this post. I realised that once I approached this issue with the right mindset, then achieving this control is going to be quite straightforward. Perhaps the best place to start this is by a showing a graph of the net heat balance for my planned new house: This is just a summary of the heat balance output from my spreadsheet of overall heat balance by calendar month. It shows three curves (note that this these curves now reflect poorer wall U values, a shift from 0.12 to 0.16): Gross Heat Balance excluding solar gains. This is similar to the one that Jeremy calculated for his house and roughly similar to one that you might calculate for you own house if it is a near-PassivHaus specification. Note that daily household electricity usage ultimately ends up as waste heat within the fabric of the house and acts as a heating source, so this is included in the overall heat equation -- which is why this curve goes positive (up to 7kWhr/day in July/Aug) as well as negative (down to -12 Whr/day in Jan/Feb). Ditto but less slab losses. I explain why I break these loses out below. Ditto but adding in expected solar gains. These gains are just the season estimates based on the PVGIS data as I describe in my last post. My house is aligned on an S/E axis with no large south facing windows, yet even for my window configuration solar gain is quite an issue to be addressed. Using an Active Slab So the essence of Jeremy's and my approach is to have an actively controlled slab. By this I mean that I can to define a target temperature set point for the slab itself, as measured by a sensor in the slab, and there is an automated mechanism for adding heat and dumping heat so that it's temperature can be automatically maintained 24x7 within a defined dead-band about that set point. The slab temperature is coupled to the room temperature: if the temperature difference between these is ΔT and the area of the slab is A, then the heat transfer between the slab and the area is linearly dependent on these, say h.A.ΔT, where h is a constant. A good ballpark for h is 7 W/m²K. This will could vary from house to house because it is dependent on floor coverings and treatments, but it is unlikely to be outside the 5-10 W/m²K range. If I crank in the numbers for my heat balance curves and slab area, a ΔT of less than 1°C will create this range of heat transfer even in the depths of winter. For example, if I set the slab temperature at 22°C in January, (ignoring solar gain for now), then maintaining the slab at this temperature will pump enough heat into the house to keep the room temperature at roughly 21°C. Likewise if I maintain the slab at 20° in the summer by dumping heat, then this will dump enough heat out of the house to keep the room temperature at roughly 21°C. I am using the heat flow between the slab and the house's internal air to keep the overall house temperature near a desired temperature. The whole house is treated here as a single zone for temperature control purposes. I am not trying to separate out individual rooms or even floors. Yes we will lose heat directly into the ground through the slab, but this doesn't directly factor into the temperature at which I need to set my slab. This type of slab control is two sided: I need to be able to dump heat as well as add it. However, the average rates at which I need to add or dump heat are pretty low -- never more than ± 0.5kW. Because the slab itself has a significant thermal capacity (~ 2kWhr per degree), I have a lot of freedom in how this heat adjustment is timed, if I am willing to tolerate a degree or so temperature variation during the day. I am planning to use a monoblock inverter ASHP for heating broadly the same as Jeremy discusses in his topic that I referenced above. One variation to Jeremy's solution that I am still assessing is to add an external buried ground loop as an extra controlled zone on my UFH. As the ground is varies in the range 8-12°C (depending on season) at 2m depth, dumping 400W or so from a slab at ~20°C doesn't take a large loop size or flow rate. (I will cover these details in a separate thread.) Dealing with Solar Gain This is a difficult issue to get to grips with. As tried to discuss in an eBuild topic, Rfc -- U F Controlled Slab + Control For A zero energy home (ZEH) on this, I can get a pretty good estimate of the average season gain for my window configuration and this is not trivial. Some (less than half) of the sunshine will end up on a ground floor and directly heat the slab. Having the slab circulation pump on (e.g. even 20 mins every hour if not otherwise heating) will effectively redistribute this heat across the entire slab, cooling the sun-facing rooms and heating the sun-shaded rooms. Once in the slab, these heat deltas will simple get factored into the overall slab control. In terms of overall temperature control, it is the major proportion -- that where the sunshine lands on other than the slab -- and the heat from which then gets transferred into the room air is more problematical. However, in my case I have a simple remedy which will work for most of the year -- which is to leave my MVHR set in summer bypass mode. With this, I can define a trip threshold for exhaust air temperature (say 23°C) above which, the MVHR automatically starts to bypass the heat exchanger dumping hot air and drawing in cold air. This is an automated heat dump which in my case can effectively dump up to 1.5kW in winter though a lot less in summer. The TBD issue is whether I also factor predicted solar gains into my slab temperature offsets, and this is one that I can't really model effectively, and (to be honest) not one where I think that I need to. I am not directly controlling the heat input into the slab, only its temperature set-point. If the air temperature does rise above the slab set-point then heat will flow into the slab from the air. My system won't demand extra heat input (and heat that I pay for). I want to avoid the scenario where my system is actively heating the slab at the same time as the the MVHR is in active bypass mode, but accepting a 3°C offset should effectively eliminate this situation. High summer is going to be the period that I need to consider more. This is when I will almost certainly need to dump quite a lot of heat, but the MVHR heat dump is going to be the least effective and this is where I might need to factor solar gain into the slab setpoint, but this is a case of refinement on my real house. I am lucky in that my house plan and orientation means that solar gain isn't going to be a major problem for me.

I was really unhappy with my discussion of this on my last blog entry, so I want to get a better quantitative understanding of how this would impact my house design, so I decided to write a simple 1-D explicit form finite mesh simulation which could be used to explore various wall and roof profiles. I initially intended to do this as a spreadsheet so that others could have a play with their own designs but without needing to get into code and programs, but stability issues means that the mesh size was just too small for this to be usable, so I ended up coding this up as a program. (Save this wall-thermals_c.txt as a C file.) The source is fully documented inline so I won't repeat this here. It's simple to compile and run on any system with a C compiler, e.g. Linux / Mac / Windows with MinGW. If it is useful and there is a consensus for an alternative language then I'll port it. If anyone is interested, then I can post some sample graphic analysis, and also reflect any comments later. But some general observations: The thermal lag through my walls is measured in days not hours. The wall acts as a huge filter to remove even daily fluctuations, so don't worry about sizing heating for worst night-time conditions for ~0.14 U-value or less walls. You need to think of time averaging any external temperatures over days not hours. You really don't want to think about saving heating, when you go away on holiday. I looked at how long it took to heat the walls up on a typical February day from cold limiting the heating to 6kW. The walls just sucked that heat in and it took over a week to get the to the point where they were still warming towards a steady-state thermal profile but at double the base heating load! These are a couple of profiles for a daily 0 - 10°C external cycle. Note that I will have an external stone skin which does nothing in terms U-values, but does act as huge thermal damper of heat variations. There are significant heat flows in the walls, but the second plot show the "one sigma" for these as a function of depth in the wall. There is bugger all ripple getting through to the internal walls. (Windows have no thermal lag so if you have large areas of window in your house then this won't be the case.) I still want to do a bit more playing to look at the effects of solar gain, etc., but anyway if you can read C (the code should be understandable to anyone who knows Fortran / Basic, etc.) or the code documentation, then have a browse and tell my what you think.

The latest turn in our planning journey has occurred. As part of pre-planning we send an early draft of our submission tried to consult with LPA Highways department and got this reply: We will have two drives and access onto the road from our plot which is being divided onto two: the existing drive which will serve the new house at the bottom of our garden, and a new drive will be added in the reduced plot left with the old house. We initially planned to have turning provision in both hard standings, but on the LPA case officer's recommendation we removed the turning area from the new house (as we like everyone else on the street reverse out, and trimmed the turning area for the new house a little) -- he was very keen to retain as much of the existing stone walls and planting as possible. He also dismissed the idea of needing a speed survey during his site visit, since it was clear to him that that traffic wouldn't get anywhere near 30mph. So despite the DaS clearly stating that the new drive has a turning head and that we will instate the splays required by the LPA that on the LPAs advice the additional turning head had been removed from the existing drive to maximise green planting we had the following response from the Highways officer: So they hadn't bothered to read the DaS; had invented a mythical pre-application advice, and were playing hard-ball (see my highlight). I now had to find a course between the conflicting advice of the case officer and the Highways people who seemed to want us to demolish our boundary stone walls and turn the garden into a car park. After a quick chat with the case office, he then confirmed by email: But at least, he informally confirmed that we'd cleared all other pending issues, but that we should still withdraw our application until we could resubmit complete with traffic survey And so after discussing options for survey companies with a couple of eBuilders, I selected a few more local ones through Google (I found the best search term was Highways speed survey TA22). I talked to / swapped emails with 4 of them. Their general advice was consistent: a radar gun based survey would be impractical because the traffic down the road is so low that it would take hours or days to get the vehicle numbers needed to make the survey meaningful; a 7-day loop on the road survey was both cheaper and more appropriate in these circumstances. Two only offered gun surveys and the others were in the £250 + VAT range. In the end I chose PCC Traffic Information Consultancy because they were based reasonably near to us and their engineer was very helpful and knowledgeable. He also asked us to check a couple of issues with the local Highways. So I then phoned the Highways and during the conversation, the Local Highways Engineer casually dropped into the conversation that there wasn't really any hurry because they don't accept surveys taken during school and other holiday periods, so another two months hold-up! We had our next wobble with Highways, because the just wouldn't agree about our lines of sight at our proposed new access. The Highways officer said that it was only X metres to critical north side, but we measured Y on the ground. We eventually tracked this discrepancy down to the fact that our TA had used the OS 1:1200 map for his road layout, but which ever OS bod had digitised the road in the 1970s or 1980s from the aerial photo had taken a shadow-line from some trees as the line of the road, and hence the road (as shown on the map) bent by an extra metre just past our house. Not much, but enough to shift the minimum stopping distance that we would require by over 5 metres. In the end, the survey came back supporting our claim that our proposed drive was within the Highways guidelines (luckily thanks to the 5m adjustment), and we can now prepare a second application for submission. End of panic, and a lot of hassle and delays.

I discussed my overall static thermal design of my house in Part I. In this second post, I wanted to discuss the dynamic characteristics of its design – that is how the house will respond over time as external temperatures vary, but I've decided to break this into two parts leaving how I propose to control the internal temperature to a later post. Again, this content is a self-developed analysis, because I have yet to find any decent design guides to build upon – the Internet either seems to be full of qualitative hand-waving at one extreme or the reader requires degree-level maths/engineering to understand the content at the other. In my view any intelligent reader with at least O level Maths (or GCSE A-C in new money) should be able to understand the general principles here and make informed quantitative decisions relating their application in design. So in this post, I will try to apply this to my own design. I will accept any constructive feedback and will update this accordingly. I look at rates of heat loss and heat gains in steady state when doing the static design – that is I assume that nothing varies of time. Such heat losses are typically a summation of factors which relate to an intrinsic property of the house fabric known as the Thermal Resistance; this relates to the extent to which a given material resists a heat flow when a temperature difference exists across it, though conventionally its inverse, thermal conductance is usually quoted in the case of sheet materials and is usually referred to as the U value, measured in W/m²K (where K refers to degrees Kevin; the same as per °C when measuring temperature deltas). By way of an example, let's say that I want to maintain my house at a target temperature of 20°C. If the external temperature is 10°C then the U value calculations that I showed in Part I give a gross heat loss of 0.65 kW for the house fabric including ventilation, with this increasing by 68 W for every extra °C in delta temperature. However, I need to offset this loss by the intrinsic heat inputs to house which includes most of the electricity usage, roughly 0.7 kW (as I discuss below). On top of this we have 3 warm human bodies of the occupants (0.3 kW say), and any solar gain. However, let's assume that it is grey overcast weather so I can ignore this last factor, and so this all gives a rough gross heat input of 1 kW into the house, or if I take off the heat losses a net heat gain of roughly 0.35 kW. Given this heat excess, the house will slowly warm up and will only stop warming up when the heat losses and heat input are in balance, that is when the temperature has risen by (350/68)°C at roughly 25°C internal temperature. Maybe in reality, we'd have started opening windows to cool down, but my point is that we might need such manual intervention to prevent this scenario occurring even in spring or autumn. This might be difficult for the reader to accept, but the problem with relying on our experience is that we've all been brought up in poorly insulated housing stock which has a net heat deficit for at least 3 seasons a year. If I look at the trend for typical U values (in W/m²K) for house walls over built over recent decades: the 60s – 1.8; by the 80s – 1.0; the 90s – 0.5, 2010 to 0.35. The target figure for my house is 0.12 and it will also have MVHR, so its overall heat losses are over 15 times smaller than the house that I grew up in. So we are used to heating systems (usually automatic central heating) to top up the heat in the house when the temperature falls below a threshold, but the heat losses are so small with this type of energy-efficient design that the intrinsic heating can frequently exceed the heat losses. With a near PassivHaus type design, any house will be in a state of heat excess at least half the time. If we want a controlled comfortable living environment, then knowing how we will dump excess heat is just as important as generating heat. Understanding our heat inputs Because the whole house design is nearer equilibrium, I need to have a good understanding of what our likely heat inputs are. These broadly fall into three categories: Electricity. I have been tracking our electricity use in our current house for over 7 years. We have Economy 7, and our daily usage is currently 10kWhr (day rate) and 6 kWhr (night rate). We don't use electricity for space heating (but we do top up our DHW using E7) and all of our lighting is energy efficient. Nonetheless, this usage still rises by perhaps 2+2 kWhr in the winter quarter. We have the usual array of electrical equipment as well as using an overnight timer-based cold-fill dishwasher and washing machine. We are now retired, so there isn't a noticeable weekday / weekend usage pattern. However, our live-in son is the main consumer of electricity with his games PC, Xbox, etc. running a lot of the time. (The only noticeable dip in usage is when he goes on holiday!) Apart from the small amount of DHW that goes down the plug hole, this electrical energy all ultimately ends up warming the air or the fabric of the house. So this average heat input in our case varies from roughly 650W in the summer to 850W in the winter. Body heat. This varies according to body mass and activity levels but is as low as 70W when sleeping, maybe 100W when sedentary and up to double this if moderately active. So a reasonable estimate for 3 lazy house occupants is 300W. Small but worth including in the totals. Solar gain. The energy in direct sunlight is roughly 1kW/m² directly facing the sun. If you want to estimate the overall solar gain, then you can use the PVGIS calculator. It's main purpose is to predict PV array outputs, but it also gives you in one of its output columns Hd, the average daily sum of global irradiation per square meter, by calendar month. You can plug in your location and the area (of glass) and orientation of the windows on each wall to get an average solar irradiation. Your need to allow for any shading and the fact that windows have roughly 70% transmittance at the sun's colour temperature. Here is the predicted plot for my windows. I feel that it is quite difficult for us to vary these without investment or material lifestyle changes. Of course we will look at the energy efficiency of new appliances, PCs, etc. for the new house, but given that we've eliminated most of the "low hanging fruit" over the last 5 years, I think that we will have a comparable run rate in the new house. If you are doing this same exercise, then I recommend that you do your own, albeit similar analysis, as your usage and patterns might well be very different. The approach that I discussed for solar gains is useful to determine ballpark values to feed into your overall heating calculations. Yes, it is intermittent and unpredictable the UK and the actuals could vary significantly from this on a daily basis. However when I compare this graph with ones that Damon published for his actual intra-day energy collected (Earth Notes: Grid-tie PV Power) and look at the spread against his predicted (which I did by analysing his CSVs), the actual daily figures nearly always vary between 0 to 2x this predicted value with a 1σ of roughly 0.4x so this still a reasonable approximation for heat calculation purposes. The Role of Thermal Inertia What this analysis doesn't tell me is how quickly the house will react to this net heat excess and how I can control this. To do this I need to consider the impacts of a second material property usually called the thermal inertia or mass (but more strictly is the Thermal Capacity) and which relates to its ability to store heat energy. It is normally quoted in J/KgK for bulk materials, but since I prefer to calculate everything based on lengths, areas and volumes, I find it easier just to multiple this figure by the density of the material to give a per unit volume equivalent, that is in J/m³K. You can look these coefficients for standard materials up on the Internet (e.g. by Googling "thermal capacity for concrete", etc. I also find the Engineering Toolbox site a useful resource for this). These properties can very dramatically for different materials: for example EPS has a high thermal resistance but low volumetric capacity; in contrast, concrete has a poor resistance but reasonably high volumetric capacity. Thermal resistance is linked to flow of heat, that is power or the rate of energy usage – measured in watts (W), whereas the capacity is linked to total energy stored – measured in joules (J) so any equations linking the two involved multiplying in a time factor. In my example, if I assume that the excess heat is transferred to the internal air in the first instance and from this to the slab (as discussed in this post), the slab will exchange heat with the air at roughly 0.7 kW per °C temperature difference between the slab and the room air. So once the overall air temperature is roughly half a degree warmer than the slab, the air and the slab will rise in temperature together, lock-stepped at this offset. The slab is the major component in the thermal capacity of the house, and concrete has a thermal capacity of 1.9 mJ/m³K, so a 0.35 kW excess will raise the slab 1°C in 7 × 1,900,000 / 350 secs or in roughly 10 hours, so it will take days rather than hours to reach this 25°C endpoint. The ratio of the thermal capacity to thermal conductivity for a given body, say one m² of external frame and wall has the units (J/K) / (W/K) which is in seconds when you do the cancelling out. This is known as the thermal time constant of the body (often referred to by the Greek t, tau or τ). This has a well defined meaning (as link explains) for homogeneous materials, but you can think of this much as the half life is used in radio-active materials: in this case its a measure of how quickly a change is temperature at one side starts to be reflected at the far side. The τ value of my house walls is about 16 days. What this means in practice is that I don't need to worry about the daily variation in external temperatures or even the odd really cold or hot day when calculating expected heat losses or gains. The fabric of the house smooths out all of this sort of variation long before it is felt internally. (I discuss this further in my later post Modelling Thermal Lag).

Janet and I want an energy efficient house, but what does that mean in practice? The whole concept is still largely rejected by the UK building industry. In our initial research, we either found books like the House Builders Bible which are good but superficial introductions on the concepts but without serious detail or at the other extreme academic papers on micro details. There is precious little in between, and to be honest we have found far more gems of knowledge in this site. All my experience and intuition concerns living in a traditionally built house. An energy-efficient house is just a different beast entirely, so I discarded my intuition and put my trust in the physics, maths and engineering. Likewise, we only considered the views and recommendations of those who have actually lived in this type of house. One of the first things that I did was to build up my own version of Jeremy Harris's Heat Loss Calculator.xls (which he first developed on this GBF topic). I plugged in the numbers for our own house design, but in reality there are only a few parameters derived directly from the house geometry that drive this calculation (my numbers are in brackets): The internal footprint of the slab (71m²) The internal surface area of the external walls, less windows (179m²) The internal surface area of the roof, less roof windows (93m²) The area of windows (23m²) The total volume of the internal living spaces (419 m³). Each of these is multiplied by a factor derived from the design to give a heat loss per °C: a U value in the case of the first four. The last is more complex in that I had to build up a composite heat-loss based on the rate of air exchange, its unit mass and specific heat, and importantly the recovery efficiency of the MVHR. For a typical winter external temperature of 4°c and an internal 21°c, this gives a delta of 17°c for fabric heat losses. The slab delta is somewhat different in that the ground temperature under slab is far more constant – say 10°c at the centre of the slab and maybe 6°c at the edges in the winter raising to 15°c in the summer. Plugging in our current design values (from our frame supplier, MBC) gives the following heat loses for a typical January day: Slab: 97 W (9%) – 71 m² x 0.105 W/m²/K x 13° Walls: 364 W (32%) – 179 m² x 0.120 W/m²/K x 17° Roof: 171 W (15%) – 93 m² x 0.105 W/m²/K x 17° Windows: 313 W (28%) – 23 m² x 0.800 W/m²/K x 17° Air change: 180 W (16%) – 419 m³ x 0.025 W/m³/K x 17° That's 1.1 kW in total, or as I sometimes say to friends, the whole house could be heated by a single 1-bar fire. Clearly this heat loss varies according to season, so if I plug in an overall temperature profile for my location, I then get the following daily heat losses in kWhr : The house is reasonably balanced as a system: no single component dominates the heat losses. However, this wouldn't be the case if we dropped the heat recovery element of the MVHR, for example, as this would increase the air change heat losses by roughly 5x, becoming the majority of total heat loss. This is why the inclusion of MVHR is such an important component of energy-efficient design. If we look at the risks and sensitivities in this sort of calculation, then they broadly fall into two categories: failures in airtightness and thermal bridging at boundaries. (Googling these highlighted terms will give background explanations of what these are). All internal surfaces in a properly implemented energy-efficient house are within a few degrees of the internal temperature, which also means that there are no internal condensation surfaces. A serious consequence of thermal bridging is that surface temperatures can drop significantly at the bridges below the internal dew-point, causing surface condensation. Failures can occur with sloppy design or poor attention to detail during construction, so I believe that it is a lot more important to find a frame manufacturer / assembler who gives us confidence that these issues will be effectively addressed (as failures here could cost kilowatts of heat loss or mouldy surfaces) rather than whether the walls have a nominal U value of 0.12 or 0.14 (which varies the total heat loss by roughly 60W at most). However, this heat calculation isn't the whole story: We typically have three occupants in the house and being alive we radiate heat – roughly 300-400W between the three of us. The 23m² of windows can let in up to 1 kW/m² incident energy in direct sunlight. Our average daily electricity usage in our current house is 16 kWhr / day. We will cut this a bit in the new house, but this is a combination of lighting, DHW and running electrical equipment – fridge, washing machine and my live-in son's Gaming PC + Xbox. Apart from some of the DHW used (which literally goes down the plug hole), all of this energy eventually ends up as waste heat within the living environment, and therefore adds to the general heat budget – that's roughly 1kW plus the solar gain. This 24 kWhr/day or more means that we will often have a heat excess within the house. On the other hand clearly these temperatures are profile averages and there will be periods where the temperatures will be lower, and below zero for extended periods. However, even a doubling of the temperature deltas only give a running heat loss of approximately 2 kW. My overall conclusions are: Our overall heat budget will be in near equilibrium for large parts of the year. At most the sustain heat input requirement will be of the order of 2kW peak. We need to manage heat excess efficiently and automatically up to say 2kW. I want to expand on this last "automatically" point: in our current farmhouse with its 2ft thick walls, the house environment is sufficiently stable for a large part of the summer and autumn that we turn off all heating and leave windows ajar all day, relying on natural ventilation. We feel that warmer weather should result in periods where we can do the same in our new energy-efficient house. However, we don't want to be forced into the situation where we have to dash around the house a few times a day opening and closing curtains, blinds and windows just to keep the house at a stable temperature: in general, the house should look after itself. This imbalance (or a lot more on sunny days with the solar gain through windows) is a material issue and to put this in perspective consider: The mass of the liveable airspace in the house is some 500Kg with a heat capacity of just over 0.5 mJ/K or 0.14 kWh/K. The specific heat of the slab is less than that of air (0.75 kJ/kg K), but the mass of over 7m³ of slab concrete is significantly more (16.4 tonne) giving a heat capacity of 3.42 kWh/K and the plaster board, etc. within the walls adds perhaps another 10% to this capacity. There are various equations for the heat transfer between the slab and the air above it but a good ballpark is 10 W/m²/K – that is roughly 0.7 kW/K for the entire slab. The slab and the other fabric which sit within the thermal envelope of the house has over 20 times the heat capacity of the air inside the house. So if we were to heat the input air from the MVHR to 10°C above room temperature, this will transfer about 0.7 kWh heat into airspace of the house in one hour at 0.5 ACH – the same as running the slab at 1°C above room temperature. In extremes we can easily lift the slab temperature say 5°c to increase the heating slab effect five-fold, but at another 2x the air heating route will start to be problematic with noticeable effects on air quality and background noise if we increase the ACH to do so.. Our initial intention was to use an integrated MVHR + ASHP(Genvax), but there was always a concern that this would be inadequate to cope with extreme cold spells, so we planned to use a supplementary closed wood burner. However the problem with any stove is the minimum output (typically 2-3 kW) which is simply far too much for the living room to cope with. So we abandoned that idea: there's no point in installing a stove that you will never use in practice. This analysis plus Jeremy's reasoned argument also convinced us that an active UFH slab was the way to go, for the following reasons: It's a relatively low cost option. It addresses a saleability risk: "I want to fit a gas boiler" With a suitable ASHP it enables active cooling as well as active heating. In circulation mode it is extremely effective at distributing the heat throughout the entire slab from any hot spots caused by direct sunshine through the windows. With this reasoning the advantages of a combined ASHP+MVHR just seemed to dissolve, and we've now decided to abandon the Genvax in favour of a standard passive MVHR system. We have still to chose the ASHP, but we are looking at a low power (say ~5 kW) monoblock inverter-based design alone the lines of Jeremy's active slab approach, but more on this in later posts as I finalise details. We also need to think about the vertical temperature gradients in the house (our hallway / landing rises through all three floors into the loft space). Hence this is only part I of the thermal design.

We've live in a 300 year-old farmhouse with lots of beams, wobbly walls and character -- and the odd draught. It's a large family house and, after 30 years living it, we feel that it is now time for a change. We aren't interested in a Grand Design; we want a modest design that is a good balance of function, of being practical and cost-effectiveness. Given this, our main drivers in selecting our design were: Comfort and space. We want a smaller, cosy house with minimal running costs, low maintenance, and energy-efficient; with ample room for ourselves and a bedsit-style bedroom for a son who lives with us; with enough space to accommodate our two other children and their partners when they visit. We estimate we need an internal floorspace of roughly 200m² to achieve this. Constraints on external footprint. Our plot is 14.5m wide, and our house is aligned across the plot because of the overall placement considerations. Minimum clearances to the left and right give us an external length of 11.5m. The LPA also requires that the overall style of the house fits in with the street scene, so must be faced with local stone and have real slate roof. It can have a maximum depth of 6.5m (1m less than we initially planned), though the planner also suggest we could have a rear gable, but again plot constraints (minimum rear separations) limit this to 2m. There are also constraints on the ridge height. I know that we all bitch about LPAs, but in our case our planner is a nice guy and has been very helpful. Yes he has laid down restrictions on how we can use the site, but to be perfectly honest on reflection we agree on nearly all that he's suggested, and the rest was so marginal that it was easier just to go along with him. Our build has to be pleasing to the eye for ourselves and our neighbours; it has to fit in well. For example, we didn't like the idea of restricting the depth and adding the gable, but we found that this works really well when we planned it out and the house is more interesting to look at than a boring Monopoly house shape. We have really pushed the plot to it's sensible limits in terms of the house footprint and envelope, and now we have fill that envelop to meet our needs. External wall thickness. We have to balance a desire for high thermal performance and the impact of a deep wall cross section on the internal living space. When you are working outside-in you realise just how much of the slab footprint is taken up by the external walls: at a U-value of 0.15, the walls already account for nearly 20%. Usable living space has a value to us, so it's just not cost-effective to thicken the walls to drop the U-value further. We also have to go for a timber-framed build as we can't achieve this scale of U-value and have a cut stone skin any other way. The stone skin is a nominal 10cm deep but can be 25% deeper and stone needs backing or a controlled stand-off; we are using SureCav to mitigate the width impact of the stone skin, but a safe overall budget for this is still 17.5cm. A high-spec frame can achieve 0.15 with an overall wall depth of 40cm (though we may still have to go up to 45cm when we finalise the profile). Use of loft space. We aren't going to achieve our target living space on two floors, so we will incorporate the loft space within the liveable environment. However, ridge-height considerations mean that we will have to limit our roof pitch to 45° and we will have ceiling heights less than we would have preferred (2.4m on ground floor and 2.3 on first). Even so, the loft pitch profile will still be tight, so we can't class this loft as full living space; however, it will still prove very useful floor space for two main functions: (a) this give us a good space for a MVHR / equipment / storage room; plus (b) an occasional-use / guest bedroom. General layout. Our thinking is to keep the overall layout simple. The window-less gable constraint gives the house an internally feeling very similar to that of a double fronted terraced layout. We will have entrance hall and utility block in the centre of the house with the living room and kitchen/dinning area straddling this on the ground floor; bedrooms straddling it on the first. The bulk of the services are contained within this utility block. We want a fairly clean modern styling internally, and our main extravagance is that the hall space will be fairly large with the centre void carrying up into the roof space and floating staircases to the 1st and loft floors. Apart from the visual impact and sense of space, this also has a very pragmatic benefit of making it a lot easier to get bulky furniture and equipment onto the upper floors. Aligning all of the toilets, bathrooms, utility services on all three floors into a single block significantly simplifies pipe runs, etc. The only "outriders" are the hot and cold feed to the kitchen. Another design decision was to make the internal fore-and-aft walls which divide the central hall/utility block from the rest of the living space aligned on all floors and load bearing. This eliminates the need for internal structural steel, and (since the maximum floor spans are now ~3.5m) reduces the depth of the inter-floor voids and giving more volume to living space. Thermal design. This is very important to us. We are aiming for an overall zero-carbon design, albeit it with some careful payback constraints. This bullet merits it own post which I will do next. So the final layout will look something like this. Note that we've since decided to abandon the fireplace in the living room (though we'll leave the flue capped and in place to help buyer-proof the build), and the 1st floor void will be carried into the loft.

As I discuss in my first post, we have a large garden that is side-on to our road, and it is large enough for us to divide off a strip at the far end from our farmhouse to act as the plot for our new build, whilst leaving an acceptable plot for the farmhouse. However it is just large enough to do this, and neither plot is going to be generous. We therefore need to balance where we position the new boundary so as to give the new house a sufficient plot, whilst leaving the existing house with a plot that won't impact its saleability. The site plan shows a plot of some 26 x 14.5m with the new house 11.5m long sitting in the middle of this. This is where the garage currently is and between our neighbour's extension (which is largely obscured by a plum tree in her garden) and the plum tree in ours. We have little freedom in the build orientation or placement given the overall floor space (and which I discuss below). The plot is within the centre of the village and parallel to the road, so this means that we have quite a few neighbours who could be impacted by the new development – ten in all, though only five have gardens that border the new development (if we include the farmhouse itself). So we need to be sensitive to the views of our neighbours views and the planners in positioning the new build. I've orientated the site plan so the adjacent road and access onto it is at the bottom. Our farmhouse is to the right; this was first built in circa 1680 and added to in later centuries. A friend owns the cottage to the left (visible in the photo); this and most of the adjacent houses on the road date to the mid 1800s or earlier and are built in the locally quarried stone (excepting one 1990s in-fill). On the opposite side of the plot from the road is an estate build in the 1970s on what was originally the farm's land. Three of these estate houses back onto the plot, so these neighbours will have the new-build at the bottom of their garden. The main axis of the house is aligned with the adjacent cottage extension, with the trees in its garden breaking the visual line between the cottage and the new build. The small mature plum will be retained within the remaining farmhouse garden to right, near the boundary and centred on the gable, will also soften the visual impact of the house from the farmhouse. To the front we have an old stone wall and hedge and along the road boundary with some damsons in the hedge. There is a 10ft high laurel hedge to the rear. The planners want the build to be sympathetic to the local style, that is faced with local stone, a slate roof and having a general cottage style in terms of doors, windows, etc. We agree that this will help the house to sit well in the street context. We can't have windows in either gable in order to avoid overlooking issues, and so the house has a very “front and back” feel to it. Also the fore and aft positioning of the house is pretty much dictated by the aesthetics of the axis alignment and the need to balance distance from the rear neighbours and having adequate off-road parking to the front. Overall, we feel that our proposed position for the new-build sits well within the plot. We've had our first round of planning application, which we unfortunately had to withdraw after Highway Agency comments (to do a traffic survey during school term-time and to address some details with the new vehicle access to the farmhouse plot – which I've already discussed in a separate eBuild topic). However, the overall the LPA feedback in regard to the new-build itself and it's positioning within the plot was very positive.

Thirty years ago my company wanted to relocate my group to Milton Keynes. I was working in the West End, and Jan and I were living in Croydon at the time. We had just started our family, so the opportunity for a paid relocation out of a terraced house in suburbia into a larger family home in the country was just too good to miss. We ended up buying a somewhat run-down farmhouse in a village between Milton Keynes and Northampton. Jan said: “Think of the potential!”; to which I replied “Think of the work; this is going to be a 10-year project!”. Well, it took us nearer to 20 years to finish the place. We were always cash limited, so we did nearly all the work ourselves, but it proved a beautiful home to raise our family: we had lots of room and a large garden. Today, two of our kids have 'flown the coop' and set up their own homes with their partners; one briefly flew, and then returned. A large rambling 300-year old farmhouse might look beautiful, but it is now far too big for us; the house and garden are high-maintenance, and it is expensive to heat in the winter. This burden is only going to get worse as we get older, so it is now time for us to downsize. But where and to what? We have friends locally, and we like area. We want a smaller house, an energy-efficient low-maintenance one, but also with enough space to include a bed-sit for our son, and to be able to put our other two kids up when they regularly visit for their mum's excellent cooking and free booze. However, when we started looking at local properties, the choice wasn't to our taste or they were just too expensive for what you get. Our garden has enough space at the opposite end from the house for an infill development – it's now probably the only such plot remaining in the village as all of the other plots, the two timber yards, the bus business, the garage, the DIY place and even the old Methodist Chapel have all been bought up and developed. We have been approached a few times over the years by builders offering to buy the end of the garden, so we thought: why not do this ourselves? We could build a new house for our own use and then split the plot. So at the beginning of this year we started exploring costs and options, and the more that we looked at this, the more compelling the case became. We discovered this site during our searches. We have found it – and especially Jeremy's The House at Mill Orchard blog – extremely informative, so we felt that we should also follow his example and write up our experience for the benefit of others who are considering the same path. I will cover how our requirements and site have constrained our design choices in my next post.