New warm flat roof - construction problem

[NeilP27]

Hi

I'm currently having a single storey kitchen extension built. The architect has specified an EPDM covered warm flat roof. Our builder has built up differently to the design.

The drawings show from bottom up, joists, firrings, plywood deck, vapour barrier, insulation board, membrane. There are valley gutters at the edge, made up of thinner insulation board on top of ply.

The builder has built: joists, firrings, insulation board, plywood deck. Gutters made of ply, with no insulation.

i.e. the difference is - no vapour barrier, and ply on top instead of underneath the insulation boards. I suspect he has done this because he has substituted cheaper boards which you can't apply the membrane onto (unlike the ones he was supposed to use).

I have discussed with the builder who says he intends to insulate under the gutters with insulation board between the joists. He says he has followed his his normal construction and this is fine.

Several things worrying me about this:
1. Potential for condensation due to lack of vapour barrier
2. Potential for insulation boards to compress around joists over time due to no ply deck underneath to spread the load
3. Insulation between joists under gutters likely to worsen any condensation problem / set up potential for damp timbers
4. The (recticel eurothane) boards he has used are according to the manufacturer intended for pitched roofs between rafters / walls, not flat roofs

Am I right to be concerned about these issues / is the builder spinning me a yarn?

Any help / advice appreciated - thanks in advance!

 

[^woody^]

I can't see why he has specified a vapour barrier it is doing nothing - where does the vapour go when the roof covering is impermeable?

There would normally be ply support layer on the furrings to hold the insulation.

Insulation boards can be used on all roof types, and are not specific to flat or pitched.

Roofs can be insulated between joists, but it depends on the overall design. Typically there should be some continuous insulation either over the top or under the bottom of the joists. This prevents what is known as a thermal bridge, which shows as dark lines in the ceiling paint where the cold joists are. Get building control to confirm that his roof is acceptable. Call them now that the roof can be seen, as they won't otherwise check it.

 

[tony1851]

If it's a warm roof, surely the vapour barrier should be directly under the insulation?

 

[NeilP27]

Thanks both - that's helpful.

The way he is proposing to insulate between the joists under the gutters will create a thermal bridge through the joists where there is no insulation barrier between surface of roof and bottom of joist.

Re the vapour barrier, yes the design says that this goes directly under the insulation. My understanding is that it is to stop any moist air from the room underneath (kitchen) finding its way into the roof structure where it could condense and cause problems.