Is this really a warm roof? (pics)

[glock339]

Hi wondering if anyone could give their opinion on the diagram bellow?

I've built a standard style flat GRP warm roof with the insulation above the deck & joists etc in the past. I now need to build another flat roof but this time I'm EXTREMELY limited for height so I was thinking of resorting to a cold roof to save on a much needed couple hundred mm. While Googling diagrams of different insulation methods I came across the one bellow on a well know UK DIY website, which would be ideal for my situation. It stresses the fact that no air gaps can be present etc & I'd be prepared to put in the extra time needed for this... but it goes against everything I've seen/read everywhere else so I'm wondering if it's such a good idea or should I stick to the usual style ventilated cold roof?

I'd prefer to go with the method bellow but it seems a bit to good/easy to be true!

Cheers.

 

[Leofric]

I wouldn't take any notice of these diagrams ,they don't even mention a vapour barrier. You would be better looking on the website of the manufacturer of the insulation you intend using. Their technical details will show the build up for different types of roofs with U values.

 

[^woody^]

I'd prefer to go with the method bellow but it seems a bit to good/easy to be true!

It's true, and it's good.

But yes, it does need a vapour check at ceiling level and a good standard of installation to avoid no air voids.

 

[garyo]

They're often referred to as a hybrid roof, so if you search the archives you'll find a few useful threads discussing the merits and drawbacks. Whether your building inspector will entertain them seems to be key, given that most of the insulation manufacturers don't specify it as a method of installation.

 

[glock339]

Thanks for the replies, I've only just realised I had some.

You would be better looking on the website of the manufacturer of the insulation you intend using

I've spent eons looking on manufacturer websites but so far only ever found the usual warm roof above deck or ventilated cold roofs, which is why I thought I'd ask on here to see if anyone had experience of the one in the diagrams.

It's true, and it's good.
But yes, it does need a vapour check at ceiling level and a good standard of installation to avoid no air voids.

I thought you must still need a barrier, I'd be prepared to spend the extra time on making sure the insulation was fitted extremely well so may be worth thinking about this method. Cheers.

They're often referred to as a hybrid roof

Ah I'll try searching for Hybrid roof thanks!



Just to throw a complete curve ball in dose anyone have experience/opinions on using closed cell spray foam on this kind of set up? I see the manufacturers claim you can spray it onto the underside of the roof & it expands to fit snug between the timbers with no air cavities plus doesn't require a vapour barrier with it being closed cell. It sounds a bit to easy & I know spray foam insulation can be bit of a hotly debated topic so thought I'd ask if anyone knows anything about it as I don't!

Thanks again.

 

[catlad]

I don't see a problem with spray in theory

 

[Leofric]

I've spent eons looking on manufacturer websites but so far only ever found the usual warm roof above deck or ventilated cold roofs

I wonder if there is a reason for that

 

[Notch7]

Insulation manufacturers wont do construction details for hybrid roofs because its difficult to prevent thermal bridging and detailing the vcl is awkward.

This blog provides some useful info:

https://www.ealconsult.com/warm-roofs-go-wrong/

If you want to go with fully insulated in between, use celetex, cut 12mm small and use spray foam both sides -use gun grade and adjust the gun down enough so foam goes deep in the the gap without blowing out.

Then foil tape all round.
I would use foil backed plasterboard as well.

 

[Leofric]

Insulation manufacturers wont do construction details for hybrid roofs because its difficult to prevent thermal bridging and detailing the vcl is awkward.

This blog provides some useful info:

https://www.ealconsult.com/warm-roofs-go-wrong/

Good points - so insulation manufacturers do have a reason for not including 'hybrid' flat roofs in their recommended construction details.

 

[Kaymo]

Hope I am not hijacking this thread, but as my dilemma is very similar, I thought it better than posting an additional thread. Coincidentally, I just found the exact same diagram as the OP while searching around examples of flat roofs, and I also found one or two similar pics on a couple of roofers sites. I'm in the process of pulling down the ceiling plasterboard for my flat roof extension, which currently just has some glass fibre insulation roughly pushed up against the underside of the roof board, probably there since the extension was built around 1990, and no evidence of condensation.

I had originally wanted to convert to a warm roof, but not possible because the insulation would come up above the level of the windows on the original house that the extension is built on to. I can't do what my understanding of a cold roof is either, because there is no possibility of through ventilation as the roof joists.

It seems like I have two options, one is to do like below, but with some holes drilled in the outside wooden facia between each joist end, to ventilate at least from one end; or to completely fill the cavity like in the OP diagram. I find the theory that if there are no voids then there is no air to carry moisture, but can't any moisture just travel through the wood in the way the interstitial condensation manifests?

From: https://www.diydoctor.org.uk/projects/building_a_flat_roof.htm

 

[glock339]

Thanks to everyone for the further replies.

I don't see a problem with spray in theory

I'm going to try & do a bit of research to see if it's an option as this would do nicely IF it is.

Insulation manufacturers wont do construction details for hybrid roofs because it's difficult to prevent thermal bridging and detailing the vcl is awkward.
This blog provides some useful info:https://www.ealconsult.com/warm-roofs-go-wrong/

If you want to go with fully insulated in between, use celetex, cut 12mm small and use spray foam both sides -use gun grade and adjust the gun down enough so foam goes deep in the the gap without blowing out.

Then foil tape all round. I would use foil backed plasterboard as well.

Cheers that was an interesting read & I'd not thought of foil backed plasterboard

Hope I am not hijacking this thread

Nah fill your boots mate.

 

[Kaymo]

Just thought I'd share this link containing links to German research - https://foursevenfive.com/blog/unvented-flat-roofs-a-technical-discussion/

Summary is that, in the opinion of the writers, ventilation of a flat roof can be a higher risk than it being unventilated. The suggestion is that typical flat roof ventilation is rarely sufficient to drive off moisture introduced by convection, unintended air leaks, construction moisture etc. They instead propose an intelligent vapour barrier at ceiling level, which keeps internal vapour out of the roof space, but allows it to travel in the other direction, for example during summer when solar heating of the roof drives moisture towards the inside.

 

[glock339]

Just thought I'd share this link containing links to German research

Interesting read, I was just about to start looking at what's best to use as a vapour barrier but I think that answers my question.