Do you need to have a vapour barrier on the interior of a warm flat roof?

[plockhart]

Hi,

 

I am building a timber framed garden room, which has a warm flat roof. The construction of the roof is:

• 18mm osb lower deck
• vapour barrier
• 100mm PIR
• 11mm upper deck
• EPDM

 

I have been working on the interior now, and I am starting to work on the vapour barrier there. Intend to foil tape over the joins of the PIR board in the walls to form a barrier. Then I intend to roll a vapour barrier across the insulated concrete floor, and join it up to the foil on the walls. What do I do as it goes up to the ceiling? It will meet the lower deck of the ceiling, which acts as a break in the vapour barrier.

 

The wall construction from inside to out is:

• plasterboard
• 38mm service void
• 25mm PIR
• 95mm studs (with PIR inbetween)
• breathable membrane
• 38mm battens
• weatherboard

 

The ceiling finish will just a 12.5mm sheet of acoustic plasterboard attached to the underside of the joists. Attached is a work in progress photo. Are you meant to add a vapour barrier on the underside of the lower deck as well (and thus I would have to go around all the joists)? Or add a vapour barrier to the underside of the joists before attaching the plasterboard (sealing around holes made for electric cables). Or do you do nothing, as it is just an 18mm osb sheet between the vapour barriers and it doesn't matter?

 

Thanks in advance!

 

 

[Iceverge]

Airtighess is the real issue rather than  vapour diffusion.  Air leaks take the airborne moisture where it shouldn't go orders of magnitude more than diffusion. 

 

You need a continuous airtight layer throughout the building. At the moment that is in your plastic sheet above the 18mm OSB.

 

This will be tricky to join to the inside walls at this stage. 

 

I would probably just make the roof airtight layer the SB. Tape it to the joists, and the walls then. Including another membrane wouldn't be ideal in my view.

 

 

 

[plockhart]

Ok thanks! Yes it's funny when I was doing all the plans I overlooked this until last night and had a `huh` moment. 

 

Just so I understand right, do you propose using insulation tape along all the edges where the joists meet the lower deck, and then taping where the joists meet the wall?

 

Or are you talking about bringing the roof vapour barrier down along the exterior of the building to meet with the insulation at the eaves? If it is that then alas I already have my fascias on with nails.

[Iceverge]

Ideally you'd be able to join the roof polythene to the foil facer of the PIR. 

 

Not practical now of course. 

 

I would use the OSB (sorry for the earlier typo)  as the roof airtight layer.

 

Tape it to the joists and tape the perimeter to the  boards.