Overheating

[stubiff]

Apologies if this is the wrong forum.

 

Am looking at a timber frame (probably) build in the future.

Although one requirement will be retaining heat, another one, possibly as important, will be not overheating in the summer.

 

From what I've read then decrement delay can help with this so maybe looking at something like wood fibre insulation or similar.

Has anyone experience of this, or designing for it.

 

Alternatively, would you recommend other methods, e.g. MVHR with summer bypass, MVHR with cooling, UFH with cooling, AirCon, solar shades, anything else.

 

Thanks.

 

 

[ProDave]

Yes.  Our house has 100mm thick external wood fibre insulation board, and then Frametherm 35 filing the timber frame.  The combination has worked out very well indeed, the house has a very slow thermal time constant meaning it takes a long time to warm up and a long time to cool down.

 

If it gets hot in summer, the time constant is such it does not heat up much in the day, and a night time purge with windows open to cool it down keeps it all in check.

[stubiff]

Thanks for the reply.

Do you have any other cooling/ventilation (apart from opening the windows!).

Assume going for wood fibre instead of the Frametherm would make the delay even longer.

 

[ProDave]

7 minutes ago, stubiff said:

Thanks for the reply.

Do you have any other cooling/ventilation (apart from opening the windows!).

Assume going for wood fibre instead of the Frametherm would make the delay even longer.

 

I have mvhr.  I do have an ASHP that will do cooling and might one day incorporate a couple of fan coil units to give proper cooling on the few occasions it would be useful.

 

It was originally going to be wood fibre EWI with wood fibre beads filling the frame, until i worked out Frametherm 35 gave just about the same insulation and decrement delay for half the price and was a DIY job to fit whereas the wood fibre beads would need to be blown in.

[joe90]

The good news about insulation is it works both ways, keeps heat in and keeps it out . Problems only occur when excess glass is used in the build. (Or people go opening windows in hot weather and let’s the heat in ?‍♂️).

[Big Jimbo]

1 hour ago, joe90 said:

The good news about insulation is it works both ways, keeps heat in and keeps it out . Problems only occur when excess glass is used in the build. (Or people go opening windows in hot weather and let’s the heat in ?‍♂️).

Try telling my Mrs and kids that......"Oh it's so hot, lets open all the bi-folds, windows etc

[stubiff]

4 hours ago, joe90 said:

The good news about insulation is it works both ways, keeps heat in and keeps it out . Problems only occur when excess glass is used in the build. (Or people go opening windows in hot weather and let’s the heat in ?‍♂️).

Some insulation is not as good at keeping heat out though. 
And the difference is, heat coming in affects the house/household, heat going out is just lost energy. 

[Roundtuit]

We've got a timber frame with a cavity and brick outer skin, so decrement delay isn't really an issue, but we do have a lot of south-facing glass downstairs (4m bifolds and 3 sets of French doors), and as I suspected, need to manage solar gain. 

This time of year it can easily get to 22-24C inside when it's in single figures outside.  In summer, temperature can creep up during the day and mvhr purge doesn't really shift it tbh.

The original plan was for an external timber pergola walkway to provide shade, but I held off making any decisions until we'd lived in the house for a while. Now I think I'm going to give internal blinds a try for the French doors, and a sail shade for the bifolds.  Next stop will be an aircon unit run off the ashp (powered by the pv panels I anticipate).

 

[tonyshouse]

Shading needs to be external otherwise heat is generated inside the thermal envelope - practice with dust sheets, gazebos, anything outside 

 

a physics model of the house would have informed decisions, west facing glass can be a major problem 

[gc100]

9 hours ago, Roundtuit said:

We've got a timber frame with a cavity and brick outer skin, so decrement delay isn't really an issue, but we do have a lot of south-facing glass downstairs (4m bifolds and 3 sets of French doors), and as I suspected, need to manage solar gain. 

This time of year it can easily get to 22-24C inside when it's in single figures outside.  In summer, temperature can creep up during the day and mvhr purge doesn't really shift it tbh.

The original plan was for an external timber pergola walkway to provide shade, but I held off making any decisions until we'd lived in the house for a while. Now I think I'm going to give internal blinds a try for the French doors, and a sail shade for the bifolds.  Next stop will be an aircon unit run off the ashp (powered by the pv panels I anticipate).

 

this much like mine ans was planning pretty much the same. I have just fitted some automatic blinds in some rooms so might see how well they perform at keeping the heat out 

[joe90]

9 hours ago, Roundtuit said:

We've got a timber frame with a cavity and brick outer skin, so decrement delay isn't really an issue, but we do have a lot of south-facing glass downstairs (4m bifolds and 3 sets of French doors), and as I suspected, need to manage solar gain. 

This time of year it can easily get to 22-24C inside when it's in single figures outside.  In summer, temperature can creep up during the day and mvhr purge doesn't really shift it tbh.

The original plan was for an external timber pergola walkway to provide shade, but I held off making any decisions until we'd lived in the house for a while. Now I think I'm going to give internal blinds a try for the French doors, and a sail shade for the bifolds.  Next stop will be an aircon unit run off the ashp (powered by the pv panels I anticipate).

 

Yes mine is very similar , everyone said my conservatory would overheat (south facing) but in the shoulder months I open the bifolds into it and it heats the house, in mid summer they stay shut to keep excess heat out. I did buy a roll of greenhouse shading to hang in it to stop plants (and me) from burning ?.

[SteamyTea]

5 minutes ago, joe90 said:

Yes mine is very similar

We don't get excessively high air temperatures and we normally have a breeze as well.

Different up in the home counties and in more built up areas.

 

I seem to remember looking at the temperature difference between Penzance and Newcastle, only about 1°C colder up on the East Coast, on average.  But the temperature range was was much bigger than Penzance.  And it is not as windy.

Sun is higher in the sky down here as well, this can, depending on house orientation and design, reduce passive gain.

Edited March 19, 2021 by SteamyTea

[joe90]

Just now, SteamyTea said:

Different up in the home counties and in more built up areas.

I remember @Jeremy Harris doing all the passive house calculations but his heating load was way smaller than calculated and his cooling load way higher, it was down to local climate, he lived in a south facing valley which is not factored into any calculations.

[SteamyTea]

21 minutes ago, joe90 said:

he lived in a south facing valley

With virtually no air movement at all.

[Bitpipe]

We have a MBC passive standard build which employs a twin wall (and roof) design pumped with cellulose based insulation. Has a very long decrement delay and is very effective in reducing overheating.

 

Obviously solar gain is the main culprit so we also specced external blinds on east  for wall and roof windows where we have no natural shade (and street facing so privacy). Gable ends are north and south so not too much glazing there. West has large sliders but facing a mature garden so evening sun and more shade. Windows are set in 100mm from external wall which helps also.

 

MVHR will not significantly heat or cool your house - by design the air flow is low volume - perfect for ventilation - but not meaningful to move hot or cold air.

 

 

[SteamyTea]

Some Charts showing variation in UK mean, min and max temperatures over the years.

 

And a bonus one, hours sunshine

 

[Bitpipe]

One thing to add, a few here with the MBC passive slab with UFH and ASHP run the slab cool in summer and it acts as a heat sink.

 

I wish I'd put in ducting for a split air con as that would take the edge of any summer overheating and effectively run free on the PV.

[Dan F]

This is a good read. Deals with everything in a pragmatic priority order.  https://elrondburrell.com/blog/passivhaus-overheating-design/

[gc100]

21 hours ago, Bitpipe said:

One thing to add, a few here with the MBC passive slab with UFH and ASHP run the slab cool in summer and it acts as a heat sink.

 

I wish I'd put in ducting for a split air con as that would take the edge of any summer overheating and effectively run free on the PV.

We’ve put Ac in for this very reason 

[JamesP]

43 minutes ago, gc100 said:

We’ve put Ac in for this very reason 

I am planning to put in some through the wall ducting before I close off a  first floor bathroom ceiling to enable fitting later, a multi split AC to provide cooling for the upstairs bedrooms during the warmer days. The UVC is usually back to full temperature by 10am via the PV diverter so the PV can offset the AC during the later daylight hours.

Would you let me know what system you fitted? 

Thank you.

Edited March 20, 2021 by JamesP

[stubiff]

@Dan F thanks for the link, and for all the other replies. 
Before I’d read the link then my summary was going to be, ideally:

Have some decrement delay. 
Have some but not a massive amount of solar gain windows and externally shade them. 
Active cooling of some sort.

Reduce/eliminate internal heat generation, e.g. HW tank. 

i.e. There is no easy/one solution VC and it sounds like it is quite difficult or costly without active cooling. 

 

Speaking to my other half again, then cooling or at least more comfortable summer temps is the highest priority. 
Will probably start a new thread to talk about cooling. 
 

Thanks. 

[stubiff]

Anyone else with experience of wood fibre, cellulose or equivalent, for decrement delay.

Thanks.

 

[jack]

41 minutes ago, stubiff said:

Anyone else with experience of wood fibre, cellulose or equivalent, for decrement delay.

 

Another one here with thick, cellulose-filled walls. It works very well, although at the end of a long hot spell, all that heat is retained in the walls, and it can take another couple of days of cooler weather for the internal temperature to fall much. 

 

Personally, I wish we'd made provision for active cooling, at least for the bedrooms. Downstairs we cool the slab with our ASHP, and that works amazingly well.

[Dan F]

On 20/03/2021 at 13:14, stubiff said:

Speaking to my other half again, then cooling or at least more comfortable summer temps is the highest priority. 
Will probably start a new thread to talk about cooling. 

 

The amount of cooling you need may be less than you'd imagine, although this depends on shading/overhangs/glazing etc.   A decrement delay also helps here.

 

These are the numbers for out build (300m2. A number of large south-facing windows, but all will overhangs)

- Heating load:  3.3kW

- Cooling load:  335W   (1.3kW without blinds)

 

Given this, we're using UFH cooling on ground floor, and MVHR cooling on the first floor.   MVHR cooling output is very low, but we're confident it will be enough to ensure first floor is cool enough, especially given we have external blinds.   Hopefully we'll be in this summer and can give real world feedback on this setup, but only just plastering still..

 

- Cooling power of UFH:                                          3.6kW

- Cooling power of ComfoPost (first floor only): 1.1kW

 

 

 

[Olf]

On 18/03/2021 at 21:12, stubiff said:

Some insulation is not as good at keeping heat out though

Example? And what happens when you put it wrong way round?

But seriously, insulation does not 'recognise' direction of heat transfer, acts identically either way.