warm roof - risk of condensation

MizMix · 19-07-2016 10:25pm #1

Hi All,

Our arch had specced a cold roof system (a kingspan KSxxxx panel from memory). Our preferred builder (only guy local - ish who we could find who builds well in exceed of airtighness regulations etc) went against the spec (its a mono pitch, flat roof- informed me of noise issues etc). He has proposed a warm roof system- the architect mentioned a major risk of condensation and isn't happy with it. The builder has provided a detailed drawing showing sealed membranes, vapour control layer. My feeling is with the the right detailing there isn't a risk and perhaps it's just a system our architect isn't experienced with so he's pushing against the idea for that reason alone? Or is there any disadvantages of a warm roof system to be aware of?

sydthebeat · 19-07-2016 10:40pm

sounds like the builder knows more than the architect...

a mono pitched cold roof construction is difficult to ventilate correctly, so the best thing to do is to negate the requirement to ventilate completely by using a warm roof construction.

once there is enough insulation over the rafter plane to bring the dew point outside of the structural timbers then its the best option.
There is more risk of condensation issues with a cold roof construction, especially using phenolics around structural timber.

BryanF · 20-07-2016 5:55am

Has builder shown all insulation above / outside of structure? What dept of pir?

MizMix · 20-07-2016 8:56am

Yes our gut is telling us the builder knows more particularly when it comes to best practice, exceeding regs etc.

I don't know what dept the pir (I'll check)- he showed us a sketch with different levels: airtight membrane- rafter with mineral wool- rigid insulation - battens over rafters - counter battens - single skin box profile sheet. All to achieve U value of .1

sydthebeat · 20-07-2016 9:55am

MizMix wrote: »

Yes our gut is telling us the builder knows more particularly when it comes to best practice, exceeding regs etc.

I don't know what dept the pir (I'll check)- he showed us a sketch with different levels: airtight membrane- rafter with mineral wool- rigid insulation - battens over rafters - counter battens - single skin box profile sheet. All to achieve U value of .1

what thickness of rigid insulation, and where EXACTLY is this proposed?

any chance of a pic of the sketch?

BryanF · 21-07-2016 12:02am

sydthebeat wrote: »

what thickness of rigid insulation, and where EXACTLY is this proposed?

any chance of a pic of the sketch?

'
Exactly'

MizMix · 25-07-2016 9:21am

I'm waiting to find out what the thickness is- it's the one detail we don't have to hand. Here's a rough sketch he provided to explain the concept of the warm roof to us and our architect.

Our architect wants to stick with a kingspan panel which was originally giving .15 u value which the builder felt wasn't good enough (plus he said there would be considerable issues with sound on a monopitch). The arch said they can improve it to .11 u value with mineral wool between the rafters but the builder feels a warm roof is superior. I'm just waiting to find out the exact details of what he's proposing.

MizMix · 25-07-2016 10:43am

Full details: 200mm of .034 mineral wool between rafters (rafters will be 175 + 25 lath to underneath). Then 120mm of .31 EPS on top of rafters with joints taped to stop cool airflow. Insulations are vapour permeable and u value should be .1

Arch is questioning the U value and feels there's risk of condensation plus is questioning stability of the structure with rafters at 600cc.

Any thoughts? Our builder came highly recommended to us and is really up to speed on airtightness etc but obviously we are not experts and we want to make sure everything is sound (particularly as the architect is questioning everything)

Mellor · 25-07-2016 10:58am

Questions for Builder;
1. How is he getting 200mm between a 175mm rafter?
2. Why the ventilation above the insulation?
(2. is prob related to 1.)

Questions for the architect;
1. Thicknesses/materials of insulations?
2. How is he proposing to ventilate?

MizMix · 25-07-2016 11:20am

I'll ask those questions- the arch didn't have anything specced for the roof aside from kinspan panel but now he's proposing 120mm of mineral wool.

Re ventilation- we will have a MHRV but not sure that's what you mean for the roof

Calahonda52 · 25-07-2016 11:32am

MizMix wrote: »

Full details: 200mm of .034 mineral wool between rafters (rafters will be 175 + 25 lath to underneath). Then 120mm of .31 EPS on top of rafters with joints taped to stop cool airflow. Insulations are vapour permeable and u value should be .1

Arch is questioning the U value and feels there's risk of condensation plus is questioning stability of the structure with rafters at 600cc.

Any thoughts? Our builder came highly recommended to us and is really up to speed on airtightness etc but obviously we are not experts and we want to make sure everything is sound (particularly as the architect is questioning everything)

Am happy to be all wrong here, its only Monday:D.

This description is not consistent with the sketch.

The lathe is at right angles to the rafter so the m/w is going to be compressed against the membrane
You say in the sketch that there is an airtight membrane on inside.
What exactly is this: plastic sheeting?
then you have m/w
then taped eps which is perhaps impermeable, even if not foil backed and is certainly less impermeable than the m/w.
Then there is the vapour open felt.

The problem with some (builders and architects and engineers) with these, more complex material layers is that some, I repeat some, think you can look at a suggested build up layer from supplier X and then substitute cheaper materials, with vastly difference performance criteria, for some layers in the build up.

This mix and match approach is okay when looking at a Chinese menu: not much difference between 1,3,14 and house wine or 2,6, 11 and house wine but is not correct here.

My understanding that the vapour resistance profile in roof and wall design is that it should reduce from inside to outside, the thinking being that it is easier for it to escape and the vapour holding capacity of the air is reduced as the temperature drops.

ps I have some notes on this which I will dig out once the 3 grandkids go to bed...soon

Mellor · 25-07-2016 11:32am

MizMix wrote: »

Re ventilation- we will have a MHRV but not sure that's what you mean for the roof

The builders sketch posted above shows a ventilated eaves and a vented cavity above the insulation.

MizMix · 25-07-2016 1:45pm

OK looks like we need to organise a meeting to go through the details again. It's over our heads (the technical side of it)- we chose this builder as he is positioned very much as an expert in achieving the highest standard of efficiency, air tightness etc and we are paying a premium for him. However it's so hard to judge when you are a novice (like I am) in understanding the details.

Calahonda52 · 25-07-2016 1:57pm

Thanks.
In terms of context, its thai a new build and if so can we have some context of the scale of the roof, especially the unsupported span and the slope.

Airtightness in a flat roof has, IMO, more to do with ensuring the structural integrity of the construction rather than energy efficiency per se because if it is done incorrectly and posture gets into the roof space, the timbers may rot.
I am not trying to scaremonger here, just get you to realise that, in this specific case, its not about achieving 0.xx on the blower door test.

There are other issues to consider, including, but not restricted to:
moisture content of the roof timber
moisture content increase due to construction.moisture content just before too timbers are sealed up.

when you talk to the team, get the exact specification of each of the layers, by exact I mean exact.
Feel free to PM me any links you get and I will have a peep
back later.

MizMix · 25-07-2016 2:25pm

Yes it's a new build. It's approx 200m2. It's a narrow U shape- lots of exterior walls. The house width is 3.9m (i.e. exterior wall to exterior wall) so it's extremely narrow which I'm presuming structurally makes things easier (i.e. no large span of unsupported roof).

Thanks for your help- I'll get the exact details

CaraK · 15-09-2019 7:37pm

Did you get a solution to this yet?

warm roof - risk of condensation

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