Hybrid Warm/Cold Roof

[Richard Dabell]

Hi Guys,

I'm in the process of doing a loft conversion (full width dormer), due to height restrictions, I'm not able to do a warm roof which would be my preffered choice, and due to build complexity around the ventilation aspect and also lack of space for downlights, I dont want to do a cold roof.
I have drawn examples of 4 roofing details which if possible I'd like your comments on. I personally cant see fault in my hybrid design other than a vague argument that there would be a cold bridge above the joists.
So could someone from technical advise why either of the hybrid's I show not be done?

Image album is here - https://imgur.com/a/ZrAMt

Cold Roof - https://i.imgur.com/baEc8de.png
Warm Roof - https://i.imgur.com/MUg1IVC.png
Hybrid 1 - https://i.imgur.com/P7xctwD.png
Hybrid 2 - https://i.imgur.com/fn8kcz6.png

There obviously isnt a lot of difference between Hybrid 1 and 2, but just wanted to show both options.

Any advice would be much appreciated. I have tried speaking with Celotex tech but they wont offer any advice unless the design is either warm or cold.
I've not yet spoken with my building control inspector about it, I just wanted to see what others are doing.

Many thanks,
Richard

 

[Notch7]

I see that celotex dont think a hybrid roof is a good idea

http://blog.celotex.co.uk/technical/flat-roof-refurbishment-the-perils-of-the-hybrid-construction/

They would argue that it does not show robust detailing and therefore they cant put their name against it.

I suppose the question is whether you are confident you can make a complete vapour barrier and prevent any warm air reaching the cold side. Its tricky because it is very difficult to seal the inbetween insulation. I suppose you could tape underneath every joist so that the foil tape joined up all the celotex. You also do the same thing over the rafters before putting your osb boarding.

One problem you have on site is building it so that it is completed and the flta rood deck laid without any rain getting into the structure. Any that gets in is going to be trapped. It will dry out eventually but could be a very long time.

If you can make sure you are very thorough in your construction I would think it should be ok.

I see in the blog, they mention that building regulations dont say a hybrid cant be used.


You would need to foil tape all the joints or include a vapour barrier to prevent any moisture getting though any gaps and into the cold side.

If you use inceptor micro downlighters or similar, they will go in a 50mm void depth (they are 55mm deep, so with 15mm plaster will fit into a 50mm void in joist).

I once supplied al the joinery for an orangery, the customer used their own builders to fit the roof. I supplied the construction drawing and design. It was to be a warm roof, however the chippie decided it was too tricky to lay the firrings onto the rafters and the insulation after, so he fitted all the celetox flat and then did all the firrings before the top deck. So he ended up with a void directly below the grp that was unvented. Ive never heard any feedback to say there has been any issues, I wonder if its been slowly rotting!

 

[tomfe]

Get a dewpoint calculation done or find a calculator online.
Normally with a hybrid roof you need more insulation above the deck than below.
Are you sure you need 200mm joists?