Overheating

[Bitpipe]

Just now, Olf said:

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.

 

Decrement delay. https://www.greenspec.co.uk/building-design/decrement-delay/

 

We have blown cellulose fibre (warmcell) in a fairly deep cavity which has a high decrement delay in a timber frame build. An equivalent u value of insulation with a  a shorter delay would not prevent internal overheating as effectively on a hot sunny day.

[SteamyTea]

27 minutes ago, Olf said:

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

Careful, some materials do.

[Olf]

15 hours ago, Bitpipe said:

Decrement delay

Fair point. But imho  of little use if disconnected from reality.

PIR boards manufacturers are silent on that aspect, so not easy to find reliable sources, but there is a nice picture on the website of a supplier of wood fibre boards:

for a roof structure of U=0.13 W/m²K (minimum required by Building Regs) PIR based solution gives 8.1h phase shift, woodfibre 16 hours.

So let's say there is a very sunny day (radiation heating roof surface is dominant factor of total heat trying to enter the building) so already at 10am the roof is warmer than inside. For PIR roof that heat will get in 8 hours later, so 6pm - if the air temperature will be pleasant enough by then then this can be counteracted with natural ventilation, but on some days this will not be the case. Woodfibre is a clear winner, as the same sun generated heat from 10am will only arrive to the inside at 2am. However to get the same U value, one needs 50% thicker insulation layer and much stronger roof structure - so the question is how many days will benefit from this longer phase shift achieved at extra cost?

Another case: U values better than 0.13. For both materials phase shift will extend, but PIR will actually benefit more, as will move the peak heat ingress towards the night hours. I'm not sure if there is any limit for what delay wood fibre structure could achieve, but I wouldn't want it to get close to 24h - meaning to receive heat from the day before...

 

 

 

[jack]

19 minutes ago, Olf said:

So let's say there is a very sunny day (radiation heating roof surface is dominant factor of total heat trying to enter the building) so already at 10am the roof is warmer than inside. For PIR roof that heat will get in 8 hours later, so 6pm - if the air temperature will be pleasant enough by then then this can be counteracted with natural ventilation, but on some days this will not be the case. Woodfibre is a clear winner, as the same sun generated heat from 10am will only arrive to the inside at 2am. However to get the same U value, one needs 50% thicker insulation layer and much stronger roof structure - so the question is how many days will benefit from this longer phase shift achieved at extra cost?

Another case: U values better than 0.13. For both materials phase shift will extend, but PIR will actually benefit more, as will move the peak heat ingress towards the night hours. I'm not sure if there is any limit for what delay wood fibre structure could achieve, but I wouldn't want it to get close to 24h - meaning to receive heat from the day before...

 

Be careful interpreting the phase shift information as being all there is to this. The delay is just the time it takes for a temperature change "signal" to propagate through the structure, but there's a significant damping factor that reduces the extremes of hot and cold the bigger (both geometrically and in a thermal capacity sense) the structure. 

 

Think about the ground as an extreme example. There's a reason that the temperature beyond a certain depth at a given location tends to be stable, irrespective of the temperature swings at the soil surface.  

[Adsibob]

Very interesting read, thank you. How would one incorporate decrement delay insulation into the design of a loft conversion? Architect has just designed a standard loft conversion with not much thought to keeping it cool other than to say "install aircon". I rather not if I can avoid it, and prefer to use some wood fibre in the roof insulation (we have a very high roof, so easily space for some Pavatex wood fibre boards or equivalent product like Steico.

I've ordered the windows for the loft (which are all west facing) to have solar control glass, so will mitigate some solar gain there, but would people recommend building up the walls with wood fibre as well. @ProDave you mentioned your walls were 100mm of wood fibre, plus Frametherm 35. Would that work in a loft conversion?

[Olf]

21 hours ago, jack said:

there's a significant damping factor that reduces the extremes of hot and cold

Absolutely! In both cases regardless  at which moment in time, at most U x surface area x temperature difference of heat energy will come inside - in most of the cases manageable to compensate by natural (as in open window/door) ventilation.

I'm not against wood fibre based insulation, just see its heat capacity as bonus rather as a starting point. So it is down to personal circumstances to weight all the factors - and for me extended decrement delay (especially considering its practical effect as elaborated earlier) would be one of the last on pros/cons list.

 

I see glazing being THE enemy to fight in overheating war, and here marketing is even worse - digging through 'double glazed, A rated' to get to the detail is a nightmare

 

[Bitpipe]

9 minutes ago, Olf said:

I see glazing being THE enemy to fight in overheating war, and here marketing is even worse - digging through 'double glazed, A rated' to get to the detail is a nightmare

 

Agree - the best decision we made wrt overheating was to spec external motorised blinds for our east and south facing windows (both wall and roof). We noticed how strong the morning sun was when in the caravan on site and mitigating that coming into the house makes a huge difference. 

 

Evening west sun is less of an issue and we just have internal voile to reduce that glare.

 

I'd say next biggest culprit is warm air so having MVHR (for ventilation, not injecting cooling) and being able to keep cool evening air in the house the next day when it's warmer outside helps. We have an atrium with velux so can effect a stack ventilation at night to purge the house and then try best to keep the cooler air in.

[Adsibob]

1 hour ago, Bitpipe said:

 

... best decision we made wrt overheating was to spec external motorised blinds for our east and south facing windows (both wall and roof)...

@Bitpipe I'm thinking of installing an external motorised blind for a large rooflight - which company did you go with?

[Bitpipe]

1 hour ago, Adsibob said:

@Bitpipe I'm thinking of installing an external motorised blind for a large rooflight - which company did you go with?

 

I installed Velux Integra windows and used their motorised blind as it can take power from the window and can run from the same wireless contoller.

 

For the windows, they are Roma blinds but were factory fitted by Gaulhoffer and just wired into the mains and rocker switches on the wall - they also have wireless options etc.

[Adsibob]

On 23/03/2021 at 21:03, Adsibob said:

How would one incorporate decrement delay insulation into the design of a loft conversion? Architect has just designed a standard loft conversion with not much thought to keeping it cool other than to say "install aircon". I rather not if I can avoid it, and prefer to use some wood fibre in the roof insulation (we have a very high roof, so easily space for some Pavatex wood fibre boards or equivalent product like Steico.

I've ordered the windows for the loft (which are all west facing) to have solar control glass, so will mitigate some solar gain there, but would people recommend building up the walls with wood fibre as well. @ProDave you mentioned your walls were 100mm of wood fibre, plus Frametherm 35. Would that work in a loft conversion?

Just reviving this thread in case anybody had an answer to this? I have a fair bit of space in my roof, so trying to work out what is the best combination of build ups to keep it as cool as possible in the summer. Appreciate the comments made by others about importance of mitigating solar gain through glazing, and I am taking the steps I can in respect of that, but need to finalise the build up of my roof and this is quite important as i plan on working out of a home office there most summers.

[SteamyTea]

13 minutes ago, Adsibob said:

I have a fair bit of space in my roof, so trying to work out what is the best combination of build ups to keep it as cool as possible in the summer

Fit PV, that will take out 20% of the energy.

If you have a decent amount of room, active ventilation between tiles and room may help, pipe the warmed air to a heat pump to warm your DHW.

You could make up a small roof section and do some tests in your garden. I did this over at the other place.

http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=5643&page=1

 

http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=7304&page=1

10 years ago.

 

Edited April 5, 2021 by SteamyTea

[Adsibob]

11 minutes ago, SteamyTea said:

Fit PV, that will take out 20% of the energy.

If you have a decent amount of room, active ventilation between tiles and room may help, pipe the warmed air to a heat pump to warm your DHW.

You could make up a small roof section and do some tests in your garden. I did this over at the other place.

http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=5643&page=1

 

http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=7304&page=1

10 years ago.

 

We aren't installing a heat pump or PV. In fact I enquired about installing PV on the flat roof and the companies I spoke to said they only install it on pitched roofs.

[SteamyTea]

6 minutes ago, Adsibob said:

We aren't installing a heat pump or PV. In fact I enquired about installing PV on the flat roof and the companies I spoke to said they only install it on pitched roofs.

Fit mirrors, that will work a treat.

Intact you could easily cast tinfoil into clear resin. That would make it waterproof as well.

No reason you cannot fit PV to s flat roof. Do it yourself.

[Adsibob]

9 minutes ago, SteamyTea said:

Fit mirrors, that will work a treat.

Intact you could easily cast tinfoil into clear resin. That would make it waterproof as well.

No reason you cannot fit PV to s flat roof. Do it yourself.

Maybe the companies I spoke to didn't have the expertise, but at the time I was told that wind issues prevented them from going there. I guess I could just brace the panels with something strong and rigid. We were originally going to get some through the Ikea/Solarcentury scheme, but then the government pulled the subsidies and Ikea pulled out of the joint venture with Solarcentury, which was a shame. Stupid government.

Edited April 5, 2021 by Adsibob

[Dan F]

On 05/04/2021 at 16:42, Adsibob said:

Maybe the companies I spoke to didn't have the expertise, but at the time I was told that wind issues prevented them from going there. I guess I could just brace the panels with something strong and rigid. We were originally going to get some through the Ikea/Solarcentury scheme, but then the government pulled the subsidies and Ikea pulled out of the joint venture with Solarcentury, which was a shame. Stupid government.

 

http://www.windandsun.co.uk/products/PV-Mounting-Structures/Flat-Roof-Mounts#.YH9kaJNKjUY

[Thorfun]

On 22/03/2021 at 09:50, Dan F said:

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

Hi @Dan F. are you finished and in yet? just wondering if you had any sort of update on the ComfoPost with the recent hot weather we're having? we're having a Zehnder MVHR unit and wondering if the ComfoPost does enough or if I need to think about fan coil or AC solution.

 

Cheers.

[Dan F]

25 minutes ago, Thorfun said:

Hi @Dan F. are you finished and in yet? just wondering if you had any sort of update on the ComfoPost with the recent hot weather we're having? we're having a Zehnder MVHR unit and wondering if the ComfoPost does enough or if I need to think about fan coil or AC solution.

 

Cheers.

 

The Comfopost is in but we aren't yet.

 

Do you have any cooling load calculations? That's the best way to understand if Comfopost is enough or if you need to plan for fancoil.  Also, what is your external blind/overhang situation like?

 

[Thorfun]

Just now, Dan F said:

 

The Comfopost is in but we aren't yet.

 

Do you have any cooling load calculations? That's the best way to understand if Comfopost is enough or if you need to plan for fancoil.  Also, what is your external blind/overhang situation like?

 

the vast majority of our windows/sliders have external blinds and where they don't we will have a brise soleil so we should be able to control the heat getting in nicely when combined with the proposed Loxone HA system.

 

we had a TM59 Thermal Analysis Report conducted and they concluded:

 

"Most assessed areas would not overheat according to the TM59 methodology, some rooms may experience short periods of elevated temperatures, however, these have a low enough occurrence to be not considered as overheating."

 

So I'm not sure that active cooling would be necessary. I presume we could 'bolt on' the comfopost post installation if required but that's a question I can pose to Enhabit who have designed our MVHR system. I was just thinking I could run a couple of extra pipes from the ASHP for possible fan coil units at a later date during the build. even if they're never used they're there just in case.

 

hope you get in to enjoy your new house soon! ? 

[Adsibob]

3 hours ago, Thorfun said:

the vast majority of our windows/sliders have external blinds

@Thorfun where did you get your external blinds from?

[Thorfun]

1 hour ago, Adsibob said:

@Thorfun where did you get your external blinds from?

not got them yet as we're not out of the ground yet, but I will be getting Warema blinds from Corner Star (https://cornerstaraluminium.com). Depending on where you are there might be a Warema seller closer to you but I've been impressed with Jon @ Corner Star so far in my dealings with him. but the proof is in the pudding as they say and I won't have them installed for quite a few months so I can update on my dealings with them more after that.

[Dan F]

6 hours ago, Thorfun said:

the vast majority of our windows/sliders have external blinds and where they don't we will have a brise soleil so we should be able to control the heat getting in nicely when combined with the proposed Loxone HA system.

 

we had a TM59 Thermal Analysis Report conducted and they concluded:

 

"Most assessed areas would not overheat according to the TM59 methodology, some rooms may experience short periods of elevated temperatures, however, these have a low enough occurrence to be not considered as overheating."

 

So I'm not sure that active cooling would be necessary. I presume we could 'bolt on' the comfopost post installation if required but that's a question I can pose to Enhabit who have designed our MVHR system. I was just thinking I could run a couple of extra pipes from the ASHP for possible fan coil units at a later date during the build. even if they're never used they're there just in case.

 

hope you get in to enjoy your new house soon! ? 

 

They didn't give you a cooling load though? With a cooling load number you can compare with cooling power of comfopost (given airflow rate).  We got this from PHPP cals.

 

From what you describe though,  assuming high levels of insulation and airtightness too, it sounds like you are same as us roughly then, and Comfopost would probably be enough for some supplementary heating/cooling. 

 

If this is the case, MVHR approach is going to be easier and less obtrusive as pipes already there and no need to plumb fan coil units in.  You could still put pipes in incase, but we were very confident, given calcs, that even with global warning fan coil units wouldn't be needed.

 

We have done this only for first floor. Its worth doing now to ensure Comfopost fits somewhere and that first floor MVHR pipes are insulated.  Enhabit did our cooling load calculations, MVHR design and have supplied Comfopost for us.

 

Give me a shout if you need any more specific info.

Edited June 6, 2021 by Dan F

[Thorfun]

13 minutes ago, Dan F said:

 

They didn't give you a cooling load though? With a cooling load number you can compare with cooling power of comfopost (given airflow rate).  We got this from PHPP cals.

 

From what you describe though,  assuming high levels of insulation and airtightness too, it sounds like you are same as us roughly then, and Comfopost would probably be enough for some supplementary heating/cooling. 

 

If this is the case, MVHR approach is going to be easier and less obtrusive as pipes already there and no need to plumb fan coil units in.  You could still put pipes in incase, but we were very confident, given calcs, that even with global warning fan coil units wouldn't be needed.

 

We have done this only for first floor. Its worth doing now to ensure Comfopost fits somewhere and that first floor MVHR pipes are insulated.  Enhabit did our cooling load calculations, MVHR design and have supplied Comfopost for us.

 

Give me a shout if you need any more specific info.

cheers. sadly we didn't get PHPP calcs and our architects used a firm they use to do the thermal modelling before I found Enhabit otherwise I would've got them to do it. but it's done now. I'll drop Enhabit a quick mail to discuss the potential of putting the comfopost in now or leaving space for it later and will make a decision from what they come back with. 

 

Enhabit are doing our Solar PV and ASHP/DHW as well. been very happy with them so far.