Is this flat roof design correct?

[Jthomas]

Our architect has specified the following for a flat roof:
zinc sheeting on membrane
on 18mm ply
on 100mm Kingspan
(shouldn't there be vapour control layer here?)
on 18mm ply
on firrings
on 150x50 joists
underdraw joists with 50 mm insulation backed plasterboard and skim

I have a couple of concerns:
Why is there no VPC, should there not be a vapour layer between the lower ply and kingpin?

Why is there insulation below the joists, won't this compromise the 'warm' roof space and lead to the potential for condensation?

many thanks for any help

Jard

 

[tony1851]

Yes, there should be a vcl under the 100mm insulation, otherwise water vapour will rise between the joints and condense on the (cold) plywood immediately below the zinc.

The insulation below the joists would not necessarily be a problem, as long as it is thinner than the main insulation on top of the deck

 

[chappers]

neither kingspan nor celotex make provision for insulation over and under rafters. They both say not to do it. Any reason why you cant fit 126 celotex over the top as that would pass regs.

 

[tony1851]

That's correct, but in the BRE publication Thermal Insulation: Avoiding Risks it states that you can have some insulation below the joists/rafters, as long as the greater part of the thermal resistance is on the cold side of the deck.
Maybe the arch. put a little below the joists to prevent pattern-staining? though agree it could be done cheaper by putting it
all above the deck.

Aside from this, one thing which can be an issue with a warm flat roof is thermal stresses on the finish. The zinc will get very hot in the summer due to being directly above the insulation, and very cold in the winter, due to being insulated from heat below.
Will zinc be able to accomodate the resuting repeated expansion and contraction without tearing? Don't know myself but just a thought?

 

[chappers]

That is a good point , recently worked on some steel framed buildings and the big steel purlins were battened and the 12mm celotex underdrawn to prevent pattern staining.
Zinc does get very hot, you will burn yourself in summer and does arch up slightly in the middle of each tray on a hot day, but the clips it is fitted down with allow the zinc to expand along it's length.

 

[Jthomas]

Thanks for the comments - I think I'll go with a Visqueen barrier and extra thickness in the Kingspan layer rather than insulated ceiling board.

I'm struggling with this warm roof construction though. Surely it's impossible to prevent 100% vapour from getting passed the vapour barrier (eg breaks in the barrier etc) in which case what happens to vapour that does get through, wont this ultimately cause problems?

thanks

Jard

 

[tony1851]

You'll never keep 100% of the water vapour down. But with a warm roof, the vcl should be on top of the deck, immediately under the insulation, and then turned up at the edges and sealed to the roof finish. This way you can get a fairly seamless finish. With a cold roof where the vcl will be lower down (eg across the underside of the joists), it is easier to puncture it, eg by lighting cables and fixings.