Airtight Warm Roof Questions

supermilk1

Hi,
A couple of questions for any Airtightness Guru's out there.
Scenario is I'm carrying out a full refurb on an old property in the next couple of months. One of the changes is the rear roof pitch which will change to a flat roof with roof membrane laid across 150mm insulation which will be fixed to a 18mm WBP board.
The front elevation of the roof wont change and will be sprayed in between the rafters with insulation. I intend to spray foam the perimeter on the rear flat pitch also to make the edges air tight.
My main query is , Is WBP airtight?? If I seal the joints on the outside with an airtight tape before laying the 150mm insulation on top will this act as my air tight envelope in this area?
If its is not, i see the 150mm insulation on the roof as unnecessary and something a lot less like 100mm or even 80mm would be fine as it would only be stop condensation under the membrane.
Note also that there is a suspended ceiling underneath the flat roof where I would run the airtight membrane also to act as a vapour barrier. Also, air tightness will be practiced throughout the rest of the building.
Hope this makes sense! TIA

Calahonda52

Are you happy the foam is a/t?
will the 150mm be alu clad both sides?
If it is and it is properly taped and sealed at the joints and where it meets the perimeter, it becomes the a/t layer and the wpb is superfluous from an a/t perspective.
what is the final roof material?
.
have you checked the flat roof area as a % of total, it may be uninsurable as a property?

supermilk1

Hi,
Yes, according to the contractor he says the foam will be air tight to the front and perimeter.
So to try and help you understand the layout , there will be a 1st and 2nd floor flat roof section. Apologies when I refer to my flat roof it will have an 11 degree fall hence the suspended ceiling below. The final roof covering is a Kaliko roof membrane.
On the higher flat roof section it will have Alu trim on each side to form a drip. On the lower section it will have a parapet to one side and the rear. The other side will have an alu trim drip.
Is this what you are referring to with alu clad each side?
So would you just tape the joints of the 150mm insulation? And make sure the foamed perimeter is sufficiently airtight?
TIA

Calahonda52

TKS FOR REPLY.
No, what is the makeup of the 150mm insulation

supermilk1

Apologies, it will be either 150mm Kingspan or Quinntherm roof board with a foil each side.
Thanks for your help, just found what I am looking for on the Quinntherm website. I'll have to give them the twist! Thanks for your input

Calahonda52

One thing you might reflect on is excessive solar gain from the thin tin roof in the context of decrement delay.

Will also be an issue with the foamed rafters if a vaulted ceiling
.
If it were my gaff, and the home insurer was happy with my % of flat roof, I would create a ventilated space between the thin tin roof and the queen-span, same idea as a rain-shield design on a wall

MicktheMan

supermilk1 wrote: »

Yes, according to the contractor he says the foam will be air tight to the front and perimeter.

TIA

hmmm .... will/can he guarantee this? I would never rely on spray foam to be my airtight barrier.