jack

Actually, I suppose it depends on your house design, but we really like the look of the external venetian blinds on our house. When we specced them we assumed we'd open them all the way every day, but at the front, we just tilt them about halfway open in the mornings. I think they look better down than out of sight! They also prevent people seeing in during the day, but allow you to see out between the slats.

The "windows open" graph represents even more onerous behaviour than that: it shows the theoretical case where the windows (presumably all of them!) are cracked open at all times when the temperature outside is lower than that inside, and closed at all other times. I don't think there are many who would be able to consistently implement this in real life, so this is clearly an unrealistic - or at least extreme best-case - scenario.

I think it depends what you start with. Even if you start with the correct MVHR, you still need to ensure space to retrofit the ComfoCool unit - probably not a deal-breaker with a bit of planning. The ComfoCool looks pretty expensive - around £4000 from a quick look. You could install a big split system air conditioner for that sort of money. With planning, the duct cooler needn't be difficult to retrofit. You need to plan where it will go, and ensure there's space for you to install it, but other than that it's just a flow and return line from the ASHP circuit, plus a route to a drain for the condensate. Actually, planning the hydraulic circuit and control is probably the tricky bit, especially if you're combining it with underfloor cooling. Ours came with the windows. I think Luxaflex might make some, as does Hunter Douglas. They do need to be carefully detailed to work with your windows, apertures and exterior cladding/finishes.

I'm in the south east. Even during very warm periods, the temperature outside is generally lower than inside by mid-evening (roughly 9pm, from memory). By midnight it's generally in the teens, dropping to mid teens or even lower during the early hours of the morning. It's then mid-morning before the heat builds up again. If your home circumstances allow you to open windows as soon as the temperature drops enough, and keep them open until the temperature rises the next day, you nearly always have a full 12 hours of purging going on, and often quite a bit more. So yes, I think what they propose seems likely to work, but in their graph, the bedrooms do spend many weeks in the 22-24°C range. While obviously this is a lot better than the high 20s alternative, it's still pretty warm. I don't know what temperature our east-facing bedroom gets to in the mornings, but the radiant heat off the internal blockout blinds first thing in the morning is really unpleasant. It may be that the room doesn't actually get very hot, but my perception is that it's very uncomfortable. Subjectively, I also absolutely hate being too hot, especially when I'm trying to sleep. I place a higher value than most on my bedroom not being too hot.

Yes, that's right. I believe the sole difference is the condensate drain. If you're only heating, you don't need one, but if you're cooling or doing both, you do.

If you have UFH installed in the slab by MBC (which is what most of us have done), then it can be cooled using an ASHP. Nearly all of them support reverse mode, although it may not be well-documented. In our case, we had to change settings via the service mode, but instructions for doing this easily found on the internet. Yes, by "duct cooler" I mean this type of thing. It's also run off the ASHP in cooling mode. All MVHR systems support cooling, in the sense that you can always cool the air that they supply before it's distributed to the outlets. Condensation management may be more important here than in the UFH heating application - your entire floor isn't likely to reach the sorts of temperatures that would encourage condensation, but it's certainly possible in a duct. You don't need the temperature to be freezing cold though. For me, the aim would just be to reduce the temperature a few degrees below the current room temp, to reduce how high the peak gets during the day. I don't know what temperature that would be, but I imagine that something in the high teens would be fine.

No. I've repeatedly had things written on a plan - even pointed it out to the tradesperson before they started - and come back to find they've done it their own way. Worst was the scaffolder. Gave him a simple diagram with measurements relative to the (already laid) slab. There're three bits with quite a big overhang so the scaffolding needed to step out at those points. It's noted (and highlighted) on the drawing, and I explained it to him before he started. Went to work, came back to find he'd just followed the line of the foundation the whole way around. Got him back, explained the problem, gave him another copy of the drawing, left for work, came back to find he'd only adjusted one of the overhang areas despite me pointing all of them out on the plan. He eventually got it right. And then wanted to charge me extra for adjustments.

For comparison with your situation, we have Passivhaus-level airtightness and insulation, with virtually the same construction method as @JSHarris (300mm cellulose in the walls, 400mm in the roof, concrete slab on 300mm EPS). My experience is that once heat starts building up, it takes a long time to dissipate. A single hot day doesn't make much difference, but a string of them can result in the house becoming very uncomfortable, and staying uncomfortable for days more until it cools down. Re: PHPP, you can aim for a lower percentage of overheating, but you can also reduce the temperature that you define as overheating. I think the default is 25°C, which is actually very warm in a well-insulated house. Like Jeremy, we find more than about 23°C (even less upstairs) to be unpleasant. It might be enlightening to see what impact setting an overheating temp of, say 23°C makes to the PHPP overheating estimate. Re: overhangs, what you have sounds good. Calculating the optimum overhangs is actually quite complicated, because their impact changes throughout the seasons, as does the desired contribution from solar gain. Obviously you want max solar gain in winter, decreasing into spring and out of autumn, and as little as possible in summer. However, local conditions will have an impact on this. For example, Jeremy's house is in a very warm microclimate, with a large double-height window to the south. In contrast, we have a reasonable amount of shading from trees, and not a huge amount of solar gain to the south. He has a much shorter heating period than us, but uses active cooling more in hot weather. He'd probably want deeper overhangs than us, all things being equal. All that said, external shading is much better than overhangs in our experience. We have external venetian blinds on some large windows to the west and they kill nearly all of the solar gain. In contrast, we have floor to ceiling bedroom windows to the east with no external blinds and a 1+metre overhang, and the sun at this time of the year has that room roasting by 9am, even with blockout blinds (heat just radiates off the blinds into the room). We did wire up to retrofit external blinds on these windows and I'll be installing those when I get the time. If you have the opportunity to include external blinds (or other external shading), I'd consider it, especially to the east and west. As Jeremy says, ventilation only works when the outside air is cooler than inside. The good news is that most of the time in the UK, the air temperature falls significantly overnight. You might therefore think about how you can maximise night purging options. We have a large remote-control roof window, which does help. If I were doing this again, I'd include retractable insect screens on bedroom windows to allow good airflow overnight while excluding flying insects. Depending on your layout, you could consider having a downstairs security door with insect screen that can be locked overnight while letting air in. I wouldn't discount active cooling, especially if you're planning PV. Personally, I wish we'd made provision for a duct cooler to bring down the temperature of the air coming into the rooms (bedrooms in particular) on the hottest days. Air-conditioning is another option to consider making provision for now (ducts for wiring and fluids, if not the units themselves). We do use active cooling of the slab downstairs and it's just extraordinary how pleasant an environment it creates on a hot day (concrete floors help!) People walk in and are stunned - it's like walking into a cave or an old stone church on a hot day. I personally wouldn't be without underfloor cooling if I were doing this again. If I were to choose one thing from the above, I'd say external blinds (or other external shading) are the most effective. That and maximal overnight purging and/or active cooling of some sort will get you a long way towards being comfortable through most of the worst periods of summer.

I did some fiberglassing recently and this was exactly my experience. The first lot ended up heating in the bucket to the point where it went off extremely quickly (I only used 2% catalyst and it was only about 10 degrees). In fact, it got so hot that I actually burned my arm on the frame of the roller when I took it out of the bucket when it was partly cured. In the end, I concluded that you needed to quickly get it out of the bucket and onto a coolish surface, or use small amounts each time. I also concluded that the ambient temp was more or less useless for calculating the amount of catalyst. The real temps that mattered were the starting temp of the resin and the temp of the surface to which it was being applied. Once I figured that out it mostly (mostly!) went okay.

Of course it is - it was just a poor attempt at humour from me!

You sure? This is bloody awful. What do these people think when they leave a job like this?

Do you know what ICF manufacturers say about this? I seem to recall it being important not to pour concrete on top of cured concrete in ICF, but that may have been in the context of basements and waterproofing.

Yup. Can you imagine jetwashing down into a deep recess within the blocks? And where's the water/retarder going to drain to? I know there are gaps, but do you really want concrete retarder thrown about all over the insides of your blocks right before you pour?

I use a flow temp as high as 16°C and that seems to work fine. That article is BS. At the temps we run, I barely get a mist of condensation on the small amount of exposed metal pipework around the UFH manifold. It doesn't even generate enough to drip onto the floor. I doubt the floor temp falls much below around 20°C, so definitely no condensation there. I don't even bother with a thermostat - just turn cooling mode on when the weather gets hot enough for long enough, and program it to run when the sun's up so it's mostly powered by PV.

Indeed. Isn't there are an issue with joins between fresh and cured concrete in ICF (as in you need to make provision for joins between pours done more than a day or so apart apart)?

Literally word for word what I'd say about our experience with balconies. They add positively to the look of the house at the back, but were very expensive, introduced the only cold bridges in our entire design, and will never be used.

Only you can put a value on it, for sure. Personally, I'd find the money by sacrificing other things, because it makes that much of a difference to my life. As one example, I've had chronic sinusitis problems in winter for decades. In the freezing cold hovel I rented for a year while we built, I had three or four nasty attacks without any triggering illness like a cold. Since moving into the new house 3.5 years ago, I've had two minor attacks (one triggered by the flu and therefore pretty much unavoidable). Anyway, I'm not going to argue any further. I just wanted to provide the perspective of someone who's lived in a lot of places without MVHR who would never choose to do so again.

During hot periods, I think the approach of keeping the house closed up when it's hotter outside than in is useful irrespective of whether you have MVHR or not. It's just that when you have MVHR, it's more effective. I'm not that worried about closing things up until the weather reaches about 22-23 deg C, as that's the maximum temp downstairs when we have a series of warm days (we have underfloor cooling downstairs). If it's really hot, then the sliding door stays closed (bonus: it's south facing, so somewhat less solar gain when closed) and people use the side door to get to the garden. Not as nice as having everything open, but then I'd rather keep the inside of the house as comfortable as possible during hot weather.

I don't know where the idea of an MVHR preventing the opening doors and windows comes from, but it's repeated all the time and it's a complete fallacy. We open ours all the time when the weather is right. Interestingly, other than winter, we get a lot of use out of the MVHR in summer. Without it, the house would get a lot hotter during a string of hot days. Remember that the effect of MVHR is not to keep the inside warm - it's to maintain any temperature difference that exists between inside and outside. So in hot periods in summer, we have all the windows open at night and early in the morning to purge as much heat as possible. As soon as the temperature outside reaches the temperature inside, we shut everything and let the MVHR work to keep the heat outside where it belongs.

Hi, and welcome to the forum. To provide yet another opinion, I've lived in over 20 houses in my life, from the south of England, to Sydney, to New York. Climates ranging from very hot to freezing cold, wet and dry, in houses large and small, well-insulated and not. My current home is the first one with MVHR. Having lived with it for a while, if I were to ever build another house, it would be a non-negotiable item right near the very top of the list after decent insulation and airtightness. It's just that life-changing. Absolutely true - but if you haven't lived with it over the course of the seasons, you don't know what you're missing out on. There's something weirdly "right" about the air quality, especially in winter. First-time visitors are always surprised that the air feels warm but really fresh.

Sorry, are you saying £1300 per SQM including the plot, using a main contractor? I know land in Scotland is relatively cheap, but that seems like an extremely aggressive price You say it's £150k above what you were hoping for, but it makes a massive difference whether you were hoping to build for £500k or £150k.

That's my guess. I'll take a pic next time I'm up there.

I recently noticed a very thin patchy area of black dust on the wall adjacent our MVHR outlet. I suspect it's the result of moisture (particularly from showers) condensing onto the adjacent surfaces in cold weather, and providing a habitat for mould. I plan the clean it off with a weak bleach solution and see how long it takes to come back. Edited to add: you could try Bordeaux mixture. I can't, because my area is painted

My suspicion is that it doesn't need to have a lot of effect if it's being used in conjunction with floor cooling downstairs. We have a lot of PV. The aim would be that during hot weather, the cooling is on permanently from as soon as the sun is up when you know it's going to be a hot day. It should cost little to nothing to run, because when the sun's out on a cloudless day, we're generating a lot of surplus energy most of the time. My experience living in hot climates is that it's easier to keep the environment cool if it's prevented from getting too hot in the first place. Even being able to reduce the peak temperature by 2-3 degrees across the day would be very helpful - big difference trying to sleep at 23 deg C versus 26 deg C! Interesting idea. The main issue is where to put it!

Why not do (or at least allow for) both? We have slab-cooling and it's remarkably effective at keeping the house cool. It definitely doesn't cool upstairs sufficiently, and I'd love the option of air cooling as well. I may try and retrofit at some stage - it would've been far easier if I'd planned it from the start. Another possibility is fan-assisted radiators that cool as well as heat. They seem extremely expensive, but just for bedrooms they might be worth the cost.