Jeremy Harris

It's a good question to ask. Generally, some screeds have a very slightly better thermal conductivity than concrete, but a similar heat capacity. There are some screeds that have a higher heat capacity, but they are the ones that are very, very slow to harden, and generally a bit of a pain when it comes to trying to bond things like tiles or anything else to. Unfortunately there is a fair bit of mythology surrounding a lot of building-related stuff, often not deliberate, just a consequence of the natural conservatism of the building trade in general, and a reluctance to change older ways of working. If you want to store a lot of heat in something like a slab, and release it slowly over a long period of time (eee Terry's useful thread here: ) then it's hard to beat plain concrete as a multi-function material. Not only does it have the strength and stiffness needed to provide a sound foundation system, but it also has a reasonable heat capacity for a solid material (about 880 J/kg°C , compared with about 4181 J/kg°C for water, so about 20% of the heat capacity per unit mass of water which is pretty good for a building material) and a reasonable thermal conductivity, about the same as water for the sort of medium density concrete used in floor slabs (water's about 0.58 W/m.K, medium density concrete varies from around 0.4 to 0.7 W/m.K). It's worth noting that as a general rule of thumb, you can ignore heat stored in any part of the structure deeper than around 100mm, as the time taken for heat from that depth to flow to the surface and contribute to the heat input to the building is usually so long as to be insignificant in terms of thermal modelling the internal thermal time constant. The latter is important for comfort, and it's worth looking at the above ground structure to determine how that will respond. A good example of something often overlooked is the decrement delay of insulation. There's a good explanation of that in this link: http://www.greenspec.co.uk/building-design/decrement-delay/ that's worth a read. I can vouch for the fact that having a relatively high decrement delay insulation system, in our low overall mass new house, has a very marked impact on temperature stability, particularly helping to keep it nice and cool in hot weather.

One related area to this is the U boat bunkers at Brest. I remember sailing around from L'Aberach to Brest to visit them in the early 80's, and the tale was that the French had given up trying to demolish them because the 40 year old concrete was now a fair bit stronger than it was when they were built. Given that we struggled to damage them using Barnes Wallis's Tallboys, when the concrete had only been there for a couple of years or so, I can imagine just what a job it would be to demolish the thing. As an aside, the division at Boscombe Down I occasionally flew with years ago had two Tallboys either side of the entrance door - they are pretty impressive bombs when stood on end!

We have a pretty standard "passive slab", which is also the foundation for the house. The under-insulation build up varies a bit with the type of soil, but in our case we excavated to a depth of 150mm over the whole area of the house, plus about 300mm all around. This was filled with type 3 stone (18mm to 35mm, no fines) compacted in layers until it was 150mm deep (so the same as the ground level. On top of this was laid a 25 to 50mm thick layer of coarse blinding grit, very carefully levelled. The EPS insulation was laid directly on to this, with L shaped sections all around the edges and the interior filled with over-lapping 100mm thick EPS sheets. The DPM was laid under the last 100mm thick insulation sheet. The steel reinforcement fabric was then laid in place, including a reinforced ring beam around the edge and for two internal load bearing walls. The UFH pipes were fitted, then the concrete was poured and power floated smooth. The whole process of laying the foundation to finished and level slab, with UFH pipes in, took 4 days. So, the build up, from the bottom, is 150mm compacted type 3, plus grit blinding, 200mm of EPS, DPM, 100mm of EPS then 100mm of reinforced concrete on top, with the 12mm floor finishes (travertine and bamboo) on top of that. It's documented in our build blog, starting at about this entry, with photos: http://www.mayfly.eu/2013/10/part-sixteen-fun-and-games-in-the-mud/

As someone's already mentioned, "thermal mass" isn't a measurable parameter, has no units associated with it, and no known way of calculating what it actually is for any material. Concrete and screed are very similar in terms of heat capacity (defined as the amount of heat energy required per unit mass, or volume, per unit change in temperature) and thermal conductivity (defined as the rate at which heat energy flows through a given thickness of material for a particular temperature differential). There won't be a significant, or even probably detectable, performance difference between laying UFH pipes in the slab and laying them in a screed on top of a slab, in practice, particularly if the overall heating requirement is low, so the UFH temperature is also low. If the heating requirement is significant, say more than about 40 to 50 W/m2, then there may be a very slight advantage in terms of initial heat up time to laying the UFH in a screed, the question is really whether it's worth all the extra cost and time involved. My own view is that, for a well-insulated house, with a decent level of airtightness, and MVHR to significantly reduce the ventilation losses, then the heating requirement is so low as to make it not worth considering all the additional cost and time involved in laying UFH in a screed on top of a slab, especially as it increases the overall floor height build up and the screed adds nothing structurally. We have a 100mm thick slab, with the UFH pipes cable tied to the top of the steel reinforcement fabric on 200mm centres, so the centre of the UFH pipes is around 42mm below the surface of the floor. Our slab was power-floated dead flat, so the floor finishes (12mm thick travertine stone and 12mm thick bonded down bamboo) are laid directly on to the slab. This was a pretty cheap and quick way to get a heated finished floor.

At a guess it won't be "fully"* cured for at least 10 to 12 hours, but it'll be around 80% or more cured within a couple of hours. After 24 hours it will be around 90% cured, about as cured as it's going to get, anyway, in the short term. *concrete doesn't really fully cure for decades, as chemical changes continue to harden it through life, which is one reason some well-mixed old concrete is so damned hard to break up.

I've started sticking downloads like this at the top of our website, to make them easier to find. The heat loss spreadsheet is here: http://www.mayfly.eu/wp-content/uploads/2017/01/Fabric-and-ventilation-heat-loss-calculator-Master.xls . There are a couple of other tools that may be useful there, too, at the top of this page: http://www.mayfly.eu/

It depends on the insulation material; we have 300mm of EPS, but if that were PIR it would reduce to about 200mm and still have a similar U value. I don't know what your heating requirements are, in terms of heat loss, but if, say, you needed to run the UFH at around 24 deg C in order to heat the house OK, with a typical ground temperature of around 8 deg C (reasonable for most of the UK, except particularly cold regions) then the heat loss into the ground with a U value of 0.15 W/m.K is going to be around 2.7W/m2 and the heat input to the room (with a room temperature of 21 deg C) is going to be about 27 W/m2, so about 10% of the heat input will be wasted. That's the maximum allowable normally under building regs, as it happens. For comparison, our house has a lot lower heating requirement; the UFH floor temp is usually around 22 to 22.5 deg C, and our under slab insulation has a U value of just under 0.1 W/m2.K. For a 21 deg C room temperature and 22.5 deg C floor temperature, our UFH heat loss into the ground is about 1.45 W/m2 and our heat output into the room is about 14 W/m2, so a bit over 10%, even with 300mm of insulation. One of the paradoxes of improving the thermal efficiency of a house is that you pretty much always increase the UFH percentage heat loss, because of the ratio between heat delivered to the room versus heat delivered into the ground under the house. If you want to calculate the heat delivered into the room from a UFH system, then the formula is P (in watts/m2) = 8.92 x (floor surface temp - room temp) ^1.1

The answer is as others have said, but as a very general rule, anything that is going to become an integral part of the structure, including some landscaping if it is a planning requirement, can be zero rated, or the VAT on materials you buy can be reclaimed. If it's a service that's provided, that isn't contributing to an intrinsic part of the structure, then it can't be zero rated. The list of things you have to pay VAT on is quite long, but common examples are scaffolding, skips, toilet hire, surveys and legal fees.

As above, more insulation, ideally. UFH will increase the floor slab temperature, so it increases the level of insulation needed in order to keep the heat losses down into the ground at a sensible level.

Late last year I had a similar phone call, trying to sell me a new PV inverter that was claimed to do the same thing. I wish I'd tagged them along for fun, but at the time I was just annoyed at being cold-called (as we're registered with the TPS and the number is ex-directory). I just kept asking them where they'd got my phone number from, and in the end was told they had been "given it" by the FIT register. I have no idea whether someone's hacked the FIT register or whether they really are giving away, or selling, personal data, but either way our number should have been flagged as being TPS registered. Not worth the effort of chasing it up, as the number they called was the old house, and with luck we'll be moving soon. Somehow I don't think I'm going to give the new number to the FIT people, though.

I didn't bother getting that far, but it is a load of tosh, anyway. If you reduce the flow temperature, as you have to with UFH, it makes no difference to the amount of energy needed to heat the room at all, as that's determined by the heat loss. Arguably it always increases the losses slightly, too, as the very best UFH will always be slightly less efficient that a heating source that isn't in contact with a thermally conductive outside surface. With a decent level of insulation underneath UFH this loss can be reduced to a small enough percentage to not really worry about, but it's always there, just the same. It's a bit like the snake oil "voltage optimiser" claims a while back, although I think they've now had to rein in their advertising claims. They were claiming that by regulating the house supply to a steady low voltage (pretty sure they regulate to just above lowest allowable, around 220V, IIRC) then the energy used would be lower. In reality all lowering the voltage does for many appliances is result in less power being available, so they stay on longer in order to deliver the fixed amount of energy required. In the case of something like a kettle, the energy use is actually increased with one of these things fitted, as the lower voltage means a longer time to come to the boil and that means greater heat losses from the case. I'm never quite sure why companies like this seem to think they can just arbitrarily break well-proven laws of physics.

The Itho is like the Qooker, too, in that it uses a pressurised boiler that sits at around 105 to 110 deg C. The pressure is one of the ways they not only get the higher temperature at the tap, but the high level of aeration that reduces the scalding risk. I agree that if you're used to just putting the kettle on, then there's not much reason to go for one, but we've had instant, near-boiling water on tap for around 7 or 8 years now, and have rather got used to it, so for us, being able to get rid of the big "hot pot" that sits on a work surface and needs filling up every couple of days was a big motivation towards fitting an all-in-one kitchen tap that dispenses boiling water. It also makes very nice, scum-free, tea, perhaps rather too well as I was drinking far too much of the stuff a year or two ago, and can say that it doesn't do a lot for your general well being, when drunk in excess.

There are no cavity trays, as such, but TRADA have some good guidance on how to deal with the door and window heads where you have cladding. I pretty much followed the TRADA detail, except with flexible non-lead flashing rather than specially formed aluminium flashing. The idea is to allow the cavity above the door or window to freely drain, whilst allowing ventilation. With render board I don't think there is the same need to ensure water drainage, as it should be pretty impermeable, which is probably why Knauf has that sealed detail.

Multisolve is just naptha, which is heavy petroleum spirit, also referred to (especially here in the UK) as petroleum ether, and is a slightly less effective solvent than toluene. Some may know it better as lighter fuel, for Zippo lighters and the like. Toluene is often referred to as the "universal solvent" because it's a pretty good description of the stuff. You can buy 5 litres of petroleum ether for around £38, whereas 5 litres of Multisolve will cost you around £90 or so.

Do you know what type of Sikaflex it was? If it had a solvent-like smell then it was probably an isocyanate-based, moisture curing polyurethane, the sort of sealant that's used a lot on marine stuff and as a car body panel adhesive/sealant. If it was this, then toluene may well help to un-bond it from the glass, but it won't dissolve it, it will just soften it so it loses adhesion, and may well take a bit of time to do even that. In the past, I've found that using a bare Stanley knife blade will often remove 99% of the cured stuff from glass, and the rest can be removed by either a solvent or by polishing the glass with something like jewellers rouge. I have to say that it probably won't be easy, as the PU Sikaflex really does stick like sh** to a blanket! If it had either no distinctive smell, or a sort of smell a bit like cabbage (best I can describe it!) then the chances are it was a modified silicone polymer, like CT1, and something like acetone or methyl ethyl ketone (MEK) may well soften it. Generally, a bit of effort with a moderate solvent will get this off glass, as it doesn't bond as well as the PU stuff as a rule. Uncured polyurethane sealants can be pretty easily cleaned off with isopropyl alcohol, or even something like methylated spirit, quite well, but neither will touch the stuff once it's cured. I'm not sure what will clean off MSP sealants when uncured, as I've not used them very much. IPA works reasonably well at cleaning off uncured neutral cure silicone, but probably the best general purpose clean-up solvent for silicones generally is limonene - like in the citrus clean up cloths.

The key here is that if you want to use plastic fittings or pipe then you have to provide the evidence to BC that it will withstand the temperature OK. If you use copper, in compliance with the approved document (Part G) then you don't have to prove anything, you can just follow the guidance and be sure you're OK. In our case I used plastic waste pipes from the PRVs, and wrote a short justification based on the specification of the pipe used and the maximum possible temperature of the discharge (75 deg C from a Sunamp, but less than 15 litres, so it's debatable whether I needed to do this). My other PRV is a 1.5 bar one on a system with a capacity of around 70 litres and an absolute maximum temperature of 40 deg C (the highest the ASHP can run at). In both cases the temperature was within the safe working range of all the fittings, and so was compliant with the regs, even though it's not a standard detail in Part G. Worth remembering that the approved documents aren't the actual regulations, they are just some ways that the regulations may be complied with, but often aren't the only way of doing things within the regs themselves.

This thread now has close to 600 posts. I'm just waiting to see what happens when it gets to post 666...........................

As above, our boiling water tap is really quite hard to operate unintentionally, because it is locked by a button that has to be pressed down and the tap itself has a fairly strong spring keeping it closed that you have to overcome. Like the Qooker, boiling water from our Itho comes out aerated and I think you'd have to make a conscious effort to try and scald yourself with it, it's not something that would be easy to do accidentally, I think. All I can say is that we bought a "hot pot" hot water dispenser around 8 or 9 years ago, that holds about 3 or 4 litres at near boiling water, and has a small pump to dispense hot water on demand, either by a mug operated switch behind the spout, or by a push button on top. We've got used to having near-boiling water on tap, not just for quick cups of tea, but also for cooking. You can cook some vegetables, and other short cooking time stuff, just by putting them in a pan and covering them with boiling water, then leaving them for a few minutes, which saves using the hob. Once you've got used to always having boiling water on tap it's quite hard to imagine living without it, but if you've never had it you won't miss it.

We fitted a heat-detecting alarm in the kitchen, not a smoke-detecting alarm, linked to an upstairs smoke-detecting alarm, positioned close to the bedroom doors. The reason for this is that heat-detecting alarms don't give false alarms when cooking, whereas smoke-detecting alarms can. This was on advice from our building inspector, after I'd bought and had installed two smoke alarms and he checked them. Luckily they all use the same base, so it was easy to just unplug the kitchen smoke alarm and replace it with a heat alarm. It does mean I have a new, unused, smoke alarm as a spare.

It was a very odd pub, to be honest, and the landlady really didn't like anyone but her local regulars coming in. The bar was a sort of serving hatch in the wall, that she kept closed and you had to knock to get her to open it and serve you. Nick may know of it, if it's still there, as it's 20 odd miles up the valley from him, I think. It was the Ancient Briton, on the A4067, just North of Glyntawe (the caving club is up on the hill at Penwyllt, owns a row of ten cottages up there). Back then it was a "no women" pub, with a sign outside saying such. That area at that time was mainly English-speaking, and it was pretty clear in that small pub when conversations switched language, as soon as the regulars realised that we weren't locals. In the end we were stopped from going there, as the landlady added a "no cavers" notice outside, under the one that said "no women", for no reason that anyone, even the locals, ever found out. We switched to drinking in Abercraf, at the Copper Beech, where the landlord made us very welcome, even letting us use his function room for meetings.

Be good for a laugh, it made me chuckle when Nick posted it! I spent the time driving from Cornwall up to South Wales Caving Club most weekends for a few years learning Welsh from cassette tapes, purely so I could eavesdrop on conversations in one of the local pubs, where the regulars switched to speaking Welsh every time we walked in. It was good for a laugh, as after about 6 months I felt confident enough to engage one of them at the bar, in conversation, in Welsh. The expression on his, and his mates, faces was a picture - you could see the cogs whirring around as they desperately tried to remember how rude they'd been about the English whilst we were in there...................

I agree about plasterboard back boxes, they always seem to feel less than solid. I used deep steel boxes, spaced out with ply backing boards to get them at the right depth inside the plasterboard cut out. There's nothing I can see about the distance from corners, and I can't recall seeing anything about this is any version of part M I've read. I would hazard a guess that we may well have outlets around 350mm, maybe less, from a corner in one or two places, and it wasn't picked up, despite the fact that Part M compliance was about the only thing our completion inspection thoroughly checked.

It does seem odd. When we were looking around at suppliers, all offered us the chance to see either completed houses (with the owners consent) or see houses under construction, and most had previous customers as references. Similarly, I've been asked by the company who built our house if I'd mind chatting to potential customers, and in a few cases, giving them a look round. I've agreed to this, on the basis that I can be completely honest with anyone that asks. Having seen one potential supplier's standard of build on a small development (which was dire) I'd say that it's really important to have a look at the previous work, and get references, of anyone that you're thinking of spending so much money with, and I'd be a bit suspicious if a supplier was unwilling to let you do this.

I reckon you need to go and buy a lottery ticket, right now! I mean, what are the odds that the stud would end up smack bang in the centre of that inlet hole?

It would be appalling if fitted to something like our old 1988-built bungalow, that's for sure, as that has a concrete floor with no insulation at all underneath, so a large percentage of the heat would just be conducted through the concrete into the ground below and get wasted. However, if there was a new concrete floor laid onto a decent layer of insulation (say, at least 100 to 150mm of PIR, or more if using EPS) then I guess it would work just fine. The question then would be, why bother? If laying a new floor anyway it'd be easier to just include the UFH pipes within it, where they are out of harms way. The thermal response time really isn't going to be significantly improved by having the pipes on top, as most of the heat is still going to flow into the floor slab when the thermostat calls for heat, anyway. It might make a few minutes difference on a response time of around half an hour to an hour, but I doubt it'd be noticeable in practice. It looks as if it could be expensive, too. I doubt that special machine is cheap.