Dan Feist

Experiences Mitigating Moderate Overheating in Design

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Posted (edited)

Hi,

 

We had had PHPP calculations done for our build which shows 3-4% over-heating as is.  While this seems low PHPP considers the building as a whole and so the south-facing rooms with a lot of glazing are therefore potentially likely to be >25C on significantly more than a week or two a year.

 

Given this, and given the warming climate, we are interested in reducing overheating to closer to 0% if possible.  There are multiple ways to do this though and I'm struggling to understand the best route:

 

1) Active cooling (via MHVR or slab)  [shouldn't be required with other approaches, and adds additional running costs]

2) Increase overhangs  [overhangs are already a decent size and I'm concerned too much overhang would mean too much shade late spring]

3) Increase ventilation during hot days by keeping windows open [unsure if this would be enough, and wouldn't be able to keep downstairs patio doors open at night]

4) Add exterior blinds [these could be used on hottest days and also for i) additional shading in August when overhang is less effective ii) privacy]

 

Adding exterior blinds opens up a fifth option though.

5) Add automated exterior blinds + decrease existing overhangs.  [This should, in theory, solve any overheating concerns + also maybe maximize solar gains also?]

 

Anyone with any experience of these options and choosing one over the other either in design or later? Or any suggestions?

 

For reference, 1st-floor overhangs are currently 75cm, while ground floor (2.1m high doors) overhang is on average 1.4m.

 

Thanks,

 

 

Edited by Dan Feist
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Our experience has been that overhangs don't have any useful effect during the times of the year when we get a lot of solar gain, which is Spring and Autumn.  The low sun angle makes it penetrate more deeply into the house.

 

Ventilation works whenever the outside air temperature is cooler than inside, but has the opposite effect to that intended as soon as the outside air temperature warms up.  We don't like the house to be warmer than about 23°C, and we've already had several days this year where the outside air temperature has exceeded that, so daytime ventilation just makes things worse on those days.  Night time ventilation often works well, though, as the temperature overnight usually dips.

 

Exterior blinds is the one  thing I wish we'd been able to fit, as I know from experience of staying in a place in Portugal that had them that they work very well.  Retrofitting external blinds can be difficult; for us it would now look a bit odd, but if designed in they can be very unobtrusive.

 

I would be inclined to retain the overhangs, as they only have any effect in Summer, and it's extremely unlikely that you will need any solar gain when the sun is relatively high in the sky.   You can easily do a sketch of the elevation and superimpose the angle of the sun at any time of the year to get an idea as to how effective any overhang may be.

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+1 for fitting the (automated) exterior blinds and retaining overhangs.  I wish we had blinds but it is very difficult for us to try and retrofit them now.

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We have a small area of South facing glazing and it doesn't currently have an overhang. I will be adding a brise soleil which will reduce solar gain in May, June and July. The worst area for us is the large glazed gable facing WNW which has solar gain in May, June and July late in the day and to which we had external solar film applied. This has worked well. Your best option is built in automated external blinds or SageGlass.

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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.

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Excellent feedback, thanks all.   We are in the south-east and currently planning to use MBC 300mm PH system.  Our windows are all south (large)/north (smaller), nothing the east and only a couple of the west.

 

So in summary:

- Ask for 23C to be modelled as a maximum in PHPP ideally.

- Don't rely on additional ventilation via windows.

- Definitely use external blinds on south elevation at least and ideally automate them.

- Could tweak overhangs, but
   i) don't make them bigger as there may be too much shading in spring/autumn, 

   ii) be careful with reducing them too much as it won't increase winter solar gain, and they will provide additional summer shading (especially important if blinds aren't automated) 

 

Did anyone get PHPP overheating numbers during the design phase? @jack  @JSHarris @lizzie you all suggest overheating is an issue for you post-build, just wondering what your PHPP values for overheating were pre-build (if you had them done)?

 

@jack Can MHRV/Slab active cooling be retrofitted or does it need to be designed in from the start?  I assume we'd need an MHVR system that supports the addition of cooling, and a heat pump that supports reverse mode?  By "duct cooler", are you referring to active cooling being added to MHVR?

 

thanks!

 

 

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3 minutes ago, Dan Feist said:

 

@jack Can MHRV/Slab active cooling be retrofitted or does it need to be designed in from the start?  I assume we'd need an MHVR system that supports the addition of cooling, and a heat pump that supports reverse mode?  By "duct cooler", are you referring to active cooling being added to MHVR?

 

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.

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Both @jack's build and ours are MBC passive system builds.  PHPP showed a 3% overheating risk for us, which I (wrongly) assumed to be OK.  As @jack mentioned, though, our house is in the bottom of a deep sheltered valley, facing South and is deep in a cutting in the hillside, so we get higher local temperatures than the weather/climate data predicts.

 

Our slab cooling was just a matter of switching the ASHP into cooling mode, rather than heating mode.  Almost all ASHPs will reverse and cool pretty much as well as they heat.  Our MVHR cooling uses a Genvex Premium 1L MVHR unit, that has a built in air-to-air heat pump, that can either cool or heat the ventilation air.  In practice we never use it for heating, as heating is really a non-issue for our house, but it's handy for providing a small amount of comfort cooling.  The effect of the cooling from the MVHR is modest, as the ventilation rate is so low that it never shift a lot of heat, but it's useful, all the same.  If doing this again I'd not have bothered with the active MVHR, but would have fitted a cheaper split aircon system.

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4 minutes ago, jack said:

Yes, by "duct cooler" I mean this type of thing.

 

While it is called a "Duct Mounted Cold Water Cooler", and has the requisite condensate drain, am I right to assume it would work perfectly well as a duct heater in winter to take the chill off incoming air, which will be ever so slightly cooler than room air during the winter?

 

image.thumb.png.1fca86bdf4fbab4d5241bf6b5a556399.png

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3 minutes ago, Dreadnaught said:

... am I right to assume it would work perfectly well as a duct heater in winter to take the chill off incoming air, which will be ever so slightly cooler than room air during the winter?

 

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.

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This is an interesting read https://passipedia.org/basics/summer#literature

 

Although if you compare fig. 4 with fig. 7, they seem to make out that tilted window ventilation makes all the difference and is enough to keep temps down, of course, that doesn't match with your experiences which I'm inclined to trust more.  They also show the effectiveness of different types of blind.

 

@jack Yes, by "duct cooler" I mean this type of thing. It's also run off the ASHP in cooling mode.

 

I was assumed retrofitting something like "Zehnder ComfoCool" if we found we needed some active cooling. A duct cooler would probably be harder to retrofit I assume. Like the idea of cooling UFH, will need to look into that and look out not only for HP's that support it but also suitable setup and control system I assume..

 

BTW, any suggestions on good external blinds that are easy to automate without have a proprietary control system?

 

Thanks,

 

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5 minutes ago, Dan Feist said:

This is an interesting read https://passipedia.org/basics/summer#literature

 

Although if you compare fig. 4 with fig. 7, they seem to make out that tilted window ventilation makes all the difference and is enough to keep temps down, of course, that doesn't match with your experiences which I'm inclined to trust more.  They also show the effectiveness of different types of blind.

 

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.

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23 minutes ago, Dan Feist said:

I was assumed retrofitting something like "Zehnder ComfoCool" if we found we needed some active cooling. A duct cooler would probably be harder to retrofit I assume.

 

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.

 

23 minutes ago, Dan Feist said:

BTW, any suggestions on good external blinds that are easy to automate without have a proprietary control system?

 

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.

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Posted (edited)

I would add 2 items.

 

1 - Some form of automated or touch of a button outlet for nighttime purge ventilation at the top.

 

2 - This interesting detail of a briese soleil that does not look like a chip cutter, from the House Of the Year if it was Designed By An Architect programme. Old Shed New House. They have a belt of trees quite close, which will help with the autumn and spring.

 

201906-briese-soleil-old-shed-new-house.jpg.4cd4b70e11521fa8ae526f0ad5bc378b.jpg

 

Ferdinand

Edited by Ferdinand

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My calcs were messed up...nightmare...too late by the time I found out, yes I did question but was assured all was ok.  I would have done external blinds as part of the build had I known.

 

The film we fitted is helping no doubt about it but it irks that I had to do this instead of getting it right first time.

 

Having been idly chatting about fitting aircon in some areas if not all..........not progressed further than idle chat at the mo.

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Posted (edited)

@lizzie Wondering how you found out they were messed up, or you just assume they must have been because of results?  Did you get them done professionally?  

 

Edited by Dan Feist

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Just now, Dan Feist said:

@lizzie Wondering how you found out they were messed up, or you just assume they must have been because of results?  Did you get them done professionally?  

 

Yes professionally done.....I spotted things that seemed a bit odd and questioned but was reassured, all cobblers.  I should have probed further.

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2 hours ago, Dan Feist said:

This is an interesting read https://passipedia.org/basics/summer#literature

 

Although if you compare fig. 4 with fig. 7, they seem to make out that tilted window ventilation makes all the difference and is enough to keep temps down, of course, that doesn't match with your experiences which I'm inclined to trust more.  They also show the effectiveness of different types of blind.

I would agree with figs. 4 & 7 but only if the window ventilation can be carried out every day. It is possible to purge the warm air over night but if you are away and not able to do so the heat will build up inside the house and it will take along time for the fabric of the house to cool down in the same way as it takes time to warm up a cold house using air heating only.

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I think a key concept here is robustness .. to coping with 95% or 99% of whatever may happen during the lifetime of the house.

 

Tod ensign to a more conservative temperature in PHPP and to (say) 1-2% not 3-4% of time exceeding that threshold seems to be a god approach.

 

And given that solving the problems afterwards can cost into 4 or 5 figures, it seems worthwhile to provision for a variety of remedies .. since circs may move in several directions.

 

Ferdinand

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2 hours ago, PeterStarck said:

I would agree with figs. 4 & 7 but only if the window ventilation can be carried out every day. 

 

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.

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I know it got a brief mention earlier in the thread, but don't discount using SageGlass. Yes, it's expensive, but so would automated external blinds and active cooling be, and I'd suggest SageGlass is a far more elegant solution. 

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Posted (edited)

I have family who build electric external roller-shutter style security blinds into an extension in 2001.

 

1 - The security aspect was overegged for them as they are in a row of semis and garages so getting to the back is difficult anyway. For an isolated or private house - yes.

2 - They do use them for solar blocking sometimes.

3 - They do really need to be designed in first wrt soffits etc, then they can be properly invisible.

4 - You do ideally want a "touch and release" option not just "touch and hold" switches. By the time you have stood next to 4 blinds for 30s each, it is tedious.

5 - Ideally also an "all close" or "all open" switch.

6 - "Easy to use" is very important; or you won't use them.

 

Ferdinand 

Edited by Ferdinand
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@Ferdinand good tips thanks

 

@NSS can the sageglass be retrofitted into the Internorm frames?

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