TerryE

I've been tracking with and engaging with self-builders on this forum and the previous eBuild forum for about 8 years. During that time Jan and I split our old plot, built a new energy-efficient low-maintenance home to retire in, and have now lived in it for over 4 years. What I find very interesting is tails of self-builders here and how they fall into a spectrum: At the blue end of the spectrum, you have a lot of builders whose builds seem to go to plan and budget; everything comes good with the final house living up to expectation. OK, many have the odd bump along the way, but they address the issue(s) head-on and overcome or mitigate them. I think that we fell into this category. At the other red end, you have some where their build is a tail of woe; of one disaster after another; of being "unfairly let down" by suppliers and tradesmen; of time and costs spiralling out of control. And you've got all shared in between, but even so spread seems to be more of a double hump than a normal distribution-style bell curve. So I think that a more interesting Q is not "what was the worst mistake" but more: why do some people succeed and some fail so badly? And the corollary: what do I need to do so that I and my build is in the falls in the first category. I would suggest that there are some key success factors: Research, research, research. Understand your issues and risk factors and establish viable solutions well before you start to implement them. If you don't have the skills to do this in any area, then you have to be prepared to hire the expert skills that you need at a realistic commercial price. If someone offers you a deal that sounds too good to be true then 9 times out of 10 that is because it isn't true. Realistic cost and time budgets. If you start out with an infeasible cost plan, then magic doesn't happen; things don't just turn out for the good; you will inevitably make mistakes that just compound and derail the project. You need to scope your build project honestly and include sensible time and cost contingencies. Clearly defined scopes and responsibilities. Every aspect of the build must be "owned" by someone, and both you and they must agree on that. Pay particular attention to three-way interfaces, because these are a hot-spot of failure. Expect that mistakes will happen. We all make mistakes, even the best of us. So you must be continuously on the watch for them. It doesn't matter who is responsible. You and your build will ultimately suffer if they aren't identified and mitigated in a timely manner. This means you (or your PM) must be prepared to do continuous quality assurance on the build. Check and validate each stage before starting on the next(*). When mistakes are found, then agree an action plan to address them and execute it as soon as practical. Respect your tradesmen and subs, and be flexible with your mitigation. (Trust but verify). This is a corollary to the previous point. Most decent tradesmen include some contingency in pricing a job. If the final cost is within that contingency then they will be flexible. Once they've used up the contingency, then they'll start to cut corners. So even if a mistake is down to someone else, then be flexible and try to identify a mitigation that minimises everyone's costs, because next time it might be your mistake. I could add more but I hope that you get my drift. (*) There was a notorious case in the history of the forum where one self builder seemed to have a "magic happens" personal philosophy. The foundation sub screwed up the pour of the slab and one edge slumped by up to 40mm along one wall. No one picked this up and things really started to come apart when the TF arrived and they tried to erect it. Pretty much everything that could go wrong did go wrong. By contrast, when my slab team did ours, I thought that they did a great job, but I still closely monitored sub-base layer compaction, then personally checked and OKed dimensions and diagonals before and after pour; ditto levels across the slab with my Dumpy. We did have a slump of about 3mm in one area of the living room floor, but I decided that this wouldn't compromise frame erection. (I later let the slater know before he started putting down the finished floor.) Was this my job? Not really, but I would have ultimately borne the consequences if the slab had been off-spec, so I still did it. (We had a different cock-up with our rebar design which could have been as bad for us, but I picked it up in time and agreed on a mitigation with the supplier before to pour.)

Our Larson strut cassettes were assembled in factory on assembly jigs. Too much of a PITA making these up on site, IMO

I have a photo of the old fence line, but if you look at the LR regs they are only to ~ +/- 0.5m accuracy and the boundary is what the boundary is on the ground, or at least as it was before (IIRC) 1995 otherwise adverse possession doesn't apply. The only way to have a more accurate boundary is to go through the boundary dispute process, and have it "proved" by a chartered land surveyor. I am talking about a wedge of land about 3½m long going from 0 to ½m at the widest. The dispute process can cost £1,000s and £10,000s if you end up in court over it. All for 1m2 of land — life is just too short: it really isn't worth getting into a dispute over this. As far as LR is concerned, there is a major flaw in the process of initial registration, as in our case because we bought the plot with a farmhouse on it in 1984 before registration was required in Northants. All of the houses around our plot were sold to new owners after 1994 when registration became mandatory, and the boundaries were registered during these sales, but there was no requirement to get your neighbours agreement that the registered boundary is the agreed and actual boundary during the actual registration process. This has led to countless disputes for others. Luckily most of our boundary, the line isn't too far out. However on the North side of our old farmhouse, we had a 4ft wide path leading alongside our property giving access to our front door and that was part of our plot when we bought it. (The path was described as part of our property in the original 1912 bill of sale and deeds.) However, when the neighbouring cottage was sold in 1996, they incorrectly recorded the boundary at the farm wall, effectively pinching the path. It then got sold another couple of times and no one noticed the mistake, until I tried to register my own plot in 2015. The neighbour played hard to get in agreeing to do a TP4 to transfer the path back to me, and I ended up having to give him a £1K sweetener for his "time and inconvenience".

Having f***ing annoying neighbours. They objected to building compliance that I had built my house too close to their fence. So I got a visit from the LA compliance officer who measured up and decided that my house was 0.6m closer to their corner boundary than on the plan. Simple, he said: just put in a minor material amendment to record to new position. I talked to my builder about this, and he said: do not submit an MMA under any circumstances, because you are admitting that the house is in the wrong position; what happens if they turn it down? Anyway something seemed off, so I double checked the distance of the front of the house to the road: it was maybe 10cm further from the site boundary at the access, certainly nowhere near 60cm. Then the penny dropped: I have owned the plot for 35years, and I used to have a 1.2m deep laurel hedge running along the back of it. This neighbour property used to back onto my property and my side neighbour's at a T junction, but now there was a 0.5m step in the boundary. I double checked with the guy who used to own the side property 10 years before, but still lives in the village: yup they'd replaced the fence and swung one end around behind the cover of the laurel, so instead of being 0.6m away from the laurel root it was hard against it. Good old adverse possession. I stuck to my guns and told the LPA people that I was submitting an MMA because the property was the right distance from the road, and since the neighbours house hadn't moved any closer to our road, the distance between our house and theirs was according to plan; the problem was that they'd possessed a bit of my land and moved their fence line, which was a civil issue between me and them, and nothing to do with planning. I suggested that they revisit the site and remeasure with me. In the end they backed down. The total area of this pinched wedge was just over 1m2. Annoying as it was, it just wasn't getting into a boundary dispute over it. Morale: Use fence posts attached to concrete spurs to mark your boundary. This makes it a lot harder for neighbours to play silly buggers with fence lines.

In terms of cost and simplicity using a distribution manifold and standard flexible ducting as @joe90 shows above is far more straightforward and easier to balance. Ours is pretty much the same as his and @ProDave have shown. We doubled up on 3 vents in our house: the LR inlet; and the inlet and outlet in our kitchen / dining room, because the required flow rate was higher that we could achieve with a single pipe when balancing the system: you can only throttle back an individual terminal manifold.

The sort of Σ( area × U-value × ΔT) calc that I did in the blog entry below for Jan daily ΔT average and maybe by month should be within 10% and the following one. It is certainly enough to explore marginal infrastructure costs and heating run-rate costs. This is of course assuming that as-build performs broadly as-designed, that you haven't introduced any bridging or insulation flaws. PHPP introduces 2-D adjustments such as Psi-values, but TBH, if these are at all material then you've done something wrong in your wall / ceiling / slab mate-up.

@SteamyTea, One of the strengths of the StackOverflow sites is that if I am learning about a new topic, then I will do a search by descending votes on the associated tag and then scan through the questions and drill down on any that look interesting. The nearest that we have is the "Sort by" selector which allows you to sort topic by #comments and #views descending which does tend to hoist the most valuable topics to the first page or two. On another forum that I used to be a moderator on, the mods introduced a "Moderators Picks" pinned topic which was a list of interesting / best topics for that sub-forum. The trouble with doing this is that it is high maintenance and gets misleading if not regularly maintained. Note sure that we can square this circle.

No apologies asked for or needed. I am just trying to say that there is a lot of gold and gems there – in terms of useful knowledge and experience – amongst threads and blog posts. Well worth mining! 😊

@AccidentalSkydiver, If you don't want to go the supplied TF route, then I feel that following @ProDave's approach of buying in (mostly) pre-cut 8×2 or engineered equiv, then making up cassettes onsite for onsite erection might prove the best choice for you. External panelvent board-out and 100mm external insulation; breather membrane, of coarse just remember to mark off the verticals lines on the outer -- this makes fitting wall ties a lot easier as they do need to be fixed the joists and not just the panelvent 🤣. If you go for pumped cellulose, Then you can internal board out with OSB3 as you go along to give racking strength, then do the pumping before 1st fit. Of course, you will probably need an SE to sign-off on the entire structural design for B Control approval. One other thing if you go this route: go for a warmslab foundation if you can -- with integral UFL loops in the slab. Look on the blogs and YouTube. Search for the company that we used: MBC. I am not saying that you should use them (though they did do an excellent job for us) -- some members here (e.g. @Stones) went this route DIY, but these videos and blogs give you an idea of what I mean. Having the entire base to FFL with a +/- 2mm accuracy gives you a clean datum for cassette make-up and frame erection. 2×6 vs 2×8 is mainly a structural design issue for your SE, I guess.

Rishard, I am not trying to be difficult here, but you have to remember this is a pro-bono community. The regular posters who answer most questions tend to get pretty fed up answering some subtle variant of the same Q for the 20th time. I always search a forum for the answer to a Q before asking. 90% of the time good Q and A resolves my issue, and when I do ask it is on some specific point that is more likely to get an answer.

Rishard, These sorts of Q aren't needed if you can use the search facilities, on forum and Google. 30s to ask this sort Q and how much time do expend other members to spend answering?

@Marvin, I just cranked the maths on the figures that you provided, not on my figures. For my own house the numbers are a lot more compelling. I have a 3 storey house with ~ 250m2 floor-space with UFH on the ground-floor slab only. The inflow on the MHVR inlets is around 19°C, but we keep our internals at around 22½ °C. We don't have any heating on the top two floors (except a couple of small oil filled rads which my HA system turns on for a few hours every night during the coldest three months). If the 0.5 ACH was coming in at 0-5°C overnight instead of 19°C, then we would have needed some form of decent CH system for the top two floors. You just can't look at one component and try to optimise it in isolation (not unless your name is Boris 🤣). You have to treat the house as a whole. You also can't just use averages, because if you want to avoid freezing your bollocks off, then your system still has to cope with that Dec-Feb window when the night temperatures can hover not too far above 0°C. We did our MVHR for ~£2K, labour DIY. We avoid maybe £4-5K of CH costs and needing to install gas, etc. Ignoring the avoidance of replacement CH solutions, the payback period was under 3 years. As well as living in a fresh damp-free environment. IMO, this one was an absolute no-brainer. You do need reasonable access for maintenance / cleaning, say once a year or so.

IMO, they are easier to balance and a lot easier to run: no pipe Ts or joints; just a 1-1 from the central manifold to the individual vent,

Pretty much identical to mine except that I put the manifolds in a cupboard in the loft so the pipe go down into the floor rather than up into a wall

If you want an installer to take design authority and prepare the building control documentation and certification, then this is one valid route but I feel you should expect to pay ~2-3K for this service plus typical manpower rates for installation. On the other hand if you want to DIY, then I suggest that you need to take design authority and do your own calcs. IMO, BPC provide a useful design validation, but it's essentially a "free" service funded on their supplier margins. Having 4m high ceilings is the sort of thing that they might miss when working from plans. Again, this is a common theme discussed in the past. The BReg ventilation reqs are rather high in terms of volume exchange. A lot of us cranked the flow-rates up high to do the certification report, and then dialled the rate back (say to 30%@ normal; 50% @ boost) for living in.

We used these guys: BPC. I got them from others on this site who recommended them. I downloaded all the manuals, installation guides, etc, and dis my own design but passed it past them for validation. They suggested a few tweaks on vent placement, but OKed the rest.. Others just shipped them their house plans and asked them for the layout and equipment list. They then shipped us everything on a palette. Based on a couple of spot-checks, they were pretty competitive (this was ~5 years ago). As I said above we did the install ourselves as part of 1st fix and commissioned the system pretty soon after the the internals were plastered out. All pretty straightforward, really.

Conor, You've got your commission order out of sequence. MHVR should have been just be about the first thing to commission after 2nd fit as soon as you have a temporary electricity supply up. Pretty much everyone here on the forum (at least those that have sized and commissioned their MVHR systems correctly) agree that you live in a house that always smells fresh and that never have problems with damp or condensation. The tickle vents and four holes will be a PITA to seal properly once your MVHR is commissioned. The MVHR system was one of the things that Jan and I did ourselves. The sizing calcs are straightforward as is a star manifold installation, though to do need to properly calibrate and commission the system. If done properly, the B Insp will accept a self-certified commissioning report. It performs really well.

SHair ~ 1 kJ/kg/K, Densair ~ 1.2 kg/m3. So 108 × 24 × 14 × 1.2 × 1 /3600 = 12 kWh Yes you are correct in that it is not ½kwh / day. It is roughly 25× that. Your inference is wrong. In a near passive class house, air losses are by far the largest heat loss component if you don't install MVHR.

Much as with @Temp and @Russell griffiths we have a warm-roof, where the roof profile was sarked, partly for structural reasons (to give racking stiffness to the loft storey) and partly just to provide a physical barrier between the roofing and the house interior. This was then "felted" with roofing membrane and vertically battened. The slater later added the horizontal counter battens in line with his slating plan. We had a continuous horizontal breather at the eaves and under the ridge tiles to facilitate airflow between the felt and the slate to keep the under-slate area ventilated and dry.

I understand where you are coming from and you clearly have to guard against potential poor workmanship, but IMO that's a matter quality control and not a cause to accept that you've lost control of your subs before you begin.

Ours is -- by the MVHR -- because the top floor in the roof is part of our living space. One a more serious point, having a ventilated cold roof is one approach, but not the only one from a design and construction PoV -- Yes, you've got to get the thermal design, RH gradients vapour barriers etc correctly placed, but this is all entirely doable and we've talked all of the issues to death on previous threads on the forum. The main advantage of a cold roof is that poor builders are less likely to screw it up.

Have a read through my blog @Jeremy Harris and others. You have Slab losses, wall losses, fenestration losses, roof losses, air exchange losses, and thermal bridging losses. You need to design a house with a sensible balance to optimise so that you get a delta £s invested on any one that gives the best drop in heat losses and therefore delta £s in running costs. As with my and Jeremy's calcs, you don't need anything too sophisticated like the PHPP model which as so many terms and adjustments that it is intractable to make trade offs. With TF, as Dave says 8×2 + an extra layer of slab insulation can get a U-value of 0.12; slab and roof same ballpark; decent triple glazing under 1.0. Once you are in this domain 40+% of your heat losses are from air exchange if you have a pretty airtight house and MVHR; up to 85% if you use poor quality conventional build techniques and no MVHR, so you need to make your house airtight by design. The MVHR means that it always smells fresh because you are continually replenishing with (reheated) external air. If doesn't matter how good the nominal design is if your builder doesn't bother putting in the roof or wall insulation where you can't see -- as many have discovered with a FLIR camera once they've moved in. Ditto thermal bridges on steel I-beam crossing the warm / cold divide. The single biggest cost component in our house was the natural local stone skin -- which was a planning requirement, as well as looking nice. As to the near-passive performance, adding this was maybe 15% of the cost of the skin, but then again I use a 3kW inline heater to heat my water-based UFH system and our internal temp is ~ 22½°C everywhere, all year round. Putting the water-base UFH loops in the slab cost ~$2K and no other CH installation, rads on walls etc needed. (The 22½°C is because a pair of OAPs like it that way.)

An estate agent gave me a piece of advice that proved invaluable: if you think that you might ever convert your loft into living space or warm storage, then by far the cheapest time to do it is when you build the house; you can always defer second-fit until when you need the space. So we went for warm-loft and now my adult son who lives with us uses the loft floor as his bed-sit.

That's for the reasons that I discussed. The Q just isn't meaningful as phrased. Very east to ask, but you are expecting a lot of time and effort from other members, and any answers taken out of context will be useless. Your design / construction and heating approach can impact your ASHP bills by a range of 5-8x. It's a bit like asking: what is the average MPG for a vehicle; well it all depends on the type of vehicle: bike, motor bike, eBike, Tesla, conventional compact ICE, Diesel, gas guzzler, ... IMO, you really need to do your own research instead of expecting members to give you lots of detail on meaning of life Qs.

@SBMS, this is really a "how long is a piece of string" type of survey. I don't think that it's going to help you very much at all. Why? The heat demand is nothing to do with the heating technology and everything to do with the thermal design and actual performance of your build. So you need to start there, with the trade-offs between cost, design and construction risk and performance. We have a very airtight, passive-class build with decent MVHR; just UFH in the floor slab, and no other heating for the two upper floors, apart from a couple of small oil filled electrical rads that my HA system turns on for a few hours each night in the worst 3 winter months. I don't even have an ASHP because I can't make a payback case based on my (electric resistance heating for my water UFH loop) energy bills. Next if you use electric heating for your DHW (SunAmps, cold-fill DW and WM), then you can get away will running your ASHP at something like 30°C output temp and you should get a CoP of 4-5. If you use conventional rads then you will need an output temp nearer 60°C and then you'd be luck to get a CoP of 2½. Just because Dave used 1 mW electricity doesn't mean you will achieve something similar 2× better or 4× worse. My suggestion about not bothering about ASHP to heat your water has been covered on other topics. If you heat your water using E7 rates and use some highly insulated PCM-based system such as SumAmp and a HEP20 manifold system, than parasitic the heat losses are so minimal that you really don't spend that much heating your water using electric resistance elements -- certainly not enough to merit investment in more complex DHW solutions.