Warm pitched roof

[deuce22]

Hi.

 

Can somebody with experience please let me know, the most efficient way of completing a warm roof. It is the first time that I will actually be doing a roof this way.

The roof is temporarily covered and so I will be adding on top of this. My understanding is: insulation is fitted first, then OSB, breather membrane and counter battened. I've found these for fixing insulation https://ukflatroofing.com/insulation-fixings/warm-deck-insulation-fixing-for-plywood-deck-box-100-ejot-150mm-timber-deck-td-150-95-100 and I'm not sure on whether to use screws or twist fixings.

Any advice would be appreciated.

Thanks.

[Temp]

That is one way but check the membrane manufacture allows it in contact with OSB. I think Kingspan do one.

 

Otherwise fit counter battens and then the membrane allowing it to drape between them. This is to allow water to run down under the tile battens rather than potentially pool above them.

[Temp]

PS The membrane should be a vapour permeable type. I've used VP400 in the past and it's strong stuff.

[deuce22]

Thanks Temp.

 

Ive got a roofer helping me, but he hasn't fitted a warm roof before either. I was planning to fit eve's and ridge ventilation, but was told not to by the BCO, this is just a standard thing that I've always done. I also asked if I need to fit a vapour barrier under the rafters or any where within the roof build up and he said no.

 

I was planning to get the membrane for fitting to sheet material, but I think I'll do what you suggested as I do prefer it draping between each timber. The roofer thinks I'm fitting OSB unnecessarily and should just counter batten directly onto the insulation.

[Temp]

Yeah, no ventilated void needed a warm roof construction.

 

Note for someone reading this in 5 years time... It's possible to have a "warm loft" with a "cold roof construction" and that would have a ventilated void.

[Temp]

What thickness of insulation are going for?

[deuce22]

Ok, no ventilation needed. What about a vapour barrier underneath the rafters?

 

I'm using 150mm PIR and completely filling in between 8x2 rafters with mineral wool.

[andyscotland]

If you are insulating on top of and between rafters then I think that's a hybrid roof rather than a warm roof.

 

There are some concerns about hybrid roofs, due to the risk of condensation within the structure. This will be more problematic with a non-breathable insulation (e.g. PIR) on the exterior layer. I'd think you definitely need a fully sealed vapour barrier below the rafters - really you should have a condensation risk analysis for the full buildup to confirm it's going to be ok.

 

I'm a bit surprised your BCO hasn't flagged/objected to this TBH as I know a lot of the insulation manufacturers quite strongly advise against hybrid roof construction.

[deuce22]

Thanks for pointing that out Andy.

 

I've had 3 BCOs come to the site and not one of them has commented on it. This build up was advised by the company who provided my SAP calculations.

I'm now thinking that I should just fit vented soffits and ridge as well as the vapour barrier below the rafters.

[andyscotland]

Do you have a drawing/section of your plan just to be certain we're not talking at cross purposes?

 

Is there any insulation/vapour barrier at the ceiling level below the loft?

 

What u-value are you trying to hit?

 

If it's as I picture it then adding ridge/soffit ventilation (inside/below the wool & PIR) would significantly undermine the insulation as the cold air will just go round it, it would potentially make the PIR largely pointless.

 

You could explore using a breathable insulation instead of PIR (wood fibre boards or similar).

 

Or you could speak to the manufacturer of your PIR and see if they'll do a calc/condensation analysis for your buildup.

[deuce22]

I haven't got a cross section.

 

I'm not trying to hit a specific U value. I asked the SAP company to get it as low as possible with the materials I was using and the depth of the rafters. I increased to 8x2s rather than 7x2s that were specified, due to their advice.

 

These calculations were completed about 12 months ago and not one person has made any comments on this. I've never used this method before, I normally fill, in between and below rafters.

 

I'll speak with a manufacturer today and see what they say.

 

Thanks.

[deuce22]

I've just spoken with, both Kingspan and Celotex and they've both stated that it won't be an issue with condensation as its only 150mm.

[andyscotland]

Fair enough, that's good news. Always worth checking these things!

[deuce22]

Definitely worth checking.

 

I lived in a property years ago and on very cold days, ice would form in the the roof space and you'd here it cracking as the day warmed. That is the last thing I would want on a new build.

 

Thanks for pointing it out.

[andyscotland]

I definitely wouldn't want that, doesn't sound good at all! 😯

[Temp]

On 09/10/2022 at 10:23, deuce22 said:

Ok, no ventilation needed. What about a vapour barrier underneath the rafters?

 

I'm using 150mm PIR and completely filling in between 8x2 rafters with mineral wool.

 

Putting insulation between the rafters as well, effectively moves some of the rafter towards the cold side so a condensation risk analysis should be  done. 

 

On 11/10/2022 at 10:14, deuce22 said:

I've just spoken with, both Kingspan and Celotex and they've both stated that it won't be an issue with condensation as its only 150mm.

 

Not sure I understand that. Are they saying 150mm PIR and OSB is sufficiently vapour permeable? Or did they do a proper condensation risk analysis for you based on the proposed build up?

 

[deuce22]

I am coming to the end of fitting the insulation on top and I'm not too confident in this setup. I was thinking of adding the mineral wool in between and then more PIR underneath the rafters. Would this have any benefit to stopping condensation?

Who would do a condensation risk analysis?

[andyscotland]

I think PIR below would make it worse : it will bring the "dew point" (the point where it's cold enough for moisture in the air to condense) further in towards the house. So increases the risk that is somewhere inside/between the rafters, rather than above them. In theory it would also (if fully taped) act as a vapour barrier so reducing moisture levels but I wouldn't rely on the foil faces of PIR if there's any doubt, they are easily scratched/punctured.

 

I think Kingspan/Celotex will do you a full u-value calculation including a condensation risk for free if you ask. I've had Ecotherm do them for me before. Though I heard some of the manufacturers were starting to restrict that service to trades/bigger projects - not sure if that's correct.

[deuce22]

andyscotland

I'll give Celotex a call and see what they say. I've spoken with Kingspan and they said that there was no need to vent the roof with the current spec.

Thanks.

[BartW]

On 05/11/2022 at 07:00, deuce22 said:

andyscotland

I'll give Celotex a call and see what they say. I've spoken with Kingspan and they said that there was no need to vent the roof with the current spec.

Thanks.

Depending oj the final roof coverings, ventilation may be required. 
 

good example is Cedral slate. 
 

we too did a 150mm warm PIR roof, fyi. Vented top and bottom.