Inverted Warm Roof

[OrangeJuice09]

Hi,
I want to keep the roof height to a minimum and thus planned to install a cold roof rather than a warm roof. Because this requires venting, which can be problematic, the inspector suggested an 'inverted warm deck' / hybrid.

The cold roof plan was to put in 100mm of insulation between rafters level to the bottom of the joists which allowed 50mm of clear space above for air flow. Under the insulation, fit 30mm insulated plasterboard and vent both sides.

The inspector suggested, to avoid venting, consider a Inverted warm deck: to fit 150mm of insulation between the rafters pushed up so it touches the OSB (leaving no gap between the insulation and OSB). Then to add a vapour barrier (green polythene sheet) tacked to the underside of the joists. Then add standard plasterboard.

I was wondering if this approach is well known and proven as I haven’t come across it before.

Thank

 

[^woody^]

Yes it's common

 

[OrangeJuice09]

Can I make a few small holes for light fittings in the vapour barrier ?

 

[^woody^]

Only if you are sure no air will get through. Some sealed fittings may not be suitable or may not seal adequately at the ceiling.

Also the localised heating can promote condensation if there are air voids present in the roof, so extra care is needed.

 

[Notch7]

Hi,
I want to keep the roof height to a minimum and thus planned to install a cold roof rather than a warm roof. Because this requires venting, which can be problematic, the inspector suggested an 'inverted warm deck' / hybrid.

The cold roof plan was to put in 100mm of insulation between rafters level to the bottom of the joists which allowed 50mm of clear space above for air flow. Under the insulation, fit 30mm insulated plasterboard and vent both sides.

The inspector suggested, to avoid venting, consider a Inverted warm deck: to fit 150mm of insulation between the rafters pushed up so it touches the OSB (leaving no gap between the insulation and OSB). Then to add a vapour barrier (green polythene sheet) tacked to the underside of the joists. Then add standard plasterboard.

I was wondering if this approach is well known and proven as I haven’t come across it before.

Thank

it is common

slightly questionable whether it works

none of the insulation manufacturers have tested a hybrid model nor do they recommend it

I would say provided you ensure robust detailing, you should be ok

but check out Steve roofer channel on youtube

 

[^woody^]

slightly questionable whether it works

Do you know of any that have not worked?

How do SIP roof panels work (or not work) are they questionable too?

 

[stuart45]

Don't know why the.inspector called it an inverted roof. That's when the insulation goes on top of the roof covering membrane.

 

[OrangeJuice09]

Don't know why the.inspector called it an inverted roof. That's when the insulation goes on top of the roof covering membrane.

Thanks for your help.
What should be the roof called if not an 'inverted warm deck'?

 

[Notch7]

Do you know of any that have not worked?

How do SIP roof panels work (or not work) are they questionable too?

they can fail because insulation used in between makes it difficult to ensure robust VCL detail

None of the insulation manufacturers offer any hybrid solutions for warm roofs.

Personally I cant see m any architects including hybrid roofs designs on a building regs application -they wouldnt want the risk of being sued

Im not saying they will necessarily fail and I think its a sensible solution, but its not infallible



A SIP panel roof isnt really the same thing is it -the SIP is manufactured in a controlled environment and there are no gaps

 

[^woody^]

they can fail because insulation used in between makes it difficult to ensure robust VCL detail

None of the insulation manufacturers offer any hybrid solutions for warm roofs.

Personally I cant see m any architects including hybrid roofs designs on a building regs application -they wouldnt want the risk of being sued

Im not saying they will necessarily fail and I think its a sensible solution, but its not infallible



A SIP panel roof isnt really the same thing is it -the SIP is manufactured in a controlled environment and there are no gaps

OK. Well a SIP is exactly the same principle, ie a roof with insulation within and no voids. So the principle is proven, and proven to work, so it's wrong and misleading to describe it as questionable as to whether it works.

As to site or factory construction, again it's misleading and wrong to imply that factory manufacturer guarantees success and site may not. Anything not built correctly can fail, and this type of roof is no more likely to fail than any other part of a building.

 

[Deluks]

to fit 150mm of insulation between the rafters pushed up so it touches the OSB (leaving no gap between the insulation and OSB). Then to add a vapour barrier (green polythene sheet) tacked to the underside of the joists. Then add standard plasterboard.

This seems too easy.
Surely this solution would allow cold bridging of the joists from plasterboard to the the outer deck?

 

[OrangeJuice09]

This seems too easy.
Surely this solution would allow cold bridging of the joists from plasterboard to the the outer deck?

I'm assuming timber is relatively a low thermal conductor, especially a joist which is 150mm ? Not sure how to verify this though - https://www.australply.com.au/technical/thermal-properties

 

[Mike13]

Timber joists make a huge difference to the final U value of an insulated wall. If you use the American R values for timber and kingspan, in the ratio of 2" and 16" for example, work out the average R value for an 18" slice, invert and you have the U value for your roof, it will be significantly less than Kingspan alone.

 

[catlad]

I certainly wouldn't use normal plasterboard when foil backed plasterboard will give you more of a robust vapour barrier.

 

[OrangeJuice09]

A bit of on old thread, but I'm about to build the roof, does anyone know of anyone who as tried this approach ?
I know @^woody^ says its common, but are there any statistics?