Warm roof - How to insulate an stud upstand for rooflights?

[ex_directory]

Hi, we have just built a 'hybrid' roof, 40mm insulation below deck and 80mm insulation above deck on a flat roof with waterproof singe ply membrane on top.

In the roof are two rooflights with 400mm upstands from joist level. These are made of stud work, cladded on the outside with OSB leaving a 75mm void.

What is the best way to insulate this void to avoid any long term problems with condensation etc. i.e. fully fill the void with insulation?

 

[freddiemercurystwin]

These things are best thought about before you build the uptands really, fill the voids and add some to the outside too. http://www.sarnafilroofassured.co.u...shing to Site Manufactured Rooflight Kerb.pdf

 

[ex_directory]

The cross section drawing is exactly what we have.

What I was hoping to learn is maybe what would be the best insulation to put in that upstand void and if I may be prone to condensation problems at all given if it is filled up there will be no void for air and hence it is neither a warm or cold roof section.

 

[freddiemercurystwin]

Use a rigid insulation within the stud like Kingspan or Celotex. If it has insulation on the outside and you are bonding your sarnafil to that then the timber upstand itself will be warm. Use a foil back plasterboard or a foil faced insulation with foil tape on any joints for good measure.

 

[ex_directory]

OK, will get the void completely filled.

On the main deck there will be vcl, insulation then single ply membrane.

On the cladded upstand there will not be any insulation, just a VCL and then single ply membrane. So I guess again it is neither cold nor warm but hopefully not a condensation risk.

 

[freddiemercurystwin]

I don't follow, on the upstand there should be single ply, insulation, timber stud with voids infilled with insul, vcl, plasterbaord.

 

[ex_directory]

I probably did not explain well.

From outside to inside on the upstand...

UK weather
single ply
vcl
20mm cladded deck (OSB)
70mm timber stud with voids infilled with insulation
foil backed plasterboard
air inside house

 

[freddiemercurystwin]

Eh!!!!!!!

The whole idea of a VCL is to stop any moisture entering the insulation from the inside., not from the outside, that's what the membrane does. Putting it next to the membrane is completely pointless and incorrect. If you're gonna use foil backed plasterboard you don't need any addition VCL, the foil acts as the VCL. Don't know why you're adverse to layer of rigid insulation on the outside frankly though?

 

[ex_directory]

Your right... I'll try again...

UK weather
single ply
20mm cladded deck (OSB)
70mm timber stud with voids infilled with insulation
foil backed plasterboard
air inside house

The reason to avoid the insulation on the outside of the OSB is cost, that is cost of rooflight getting bigger by xmm all the way around as opposed to insulation cost!

On the main deck adding VCL between deck and insulation is just belt and braces. Bearing in mind cut outs for recessed lighting etc.