Flat roof insulation vent or not?

[HJC1972]

Hi, I am about to finish insulating the roof on my garage. It is approx. 6.5mtrs X 2.5mtrs wide with the joists running across the width. It currently has a cold deck and I have 120mm celotex to put between the rafters. The joist are notched into the bricks with the top of the joists sitting level with the top of the bricks. The ply roof substrate also sitting onto the bricks and extending over the sides.

I am unsure if I should put the celotex tight up under the ply thus leaving a cavity beneath this and the plaster board that I will eventually dry line it with. Alternatively, should I fit the celotex sitting just above the plaster board with the cavity above. If I do this do I need to knock out a brick at each end between each andd every rafter or could I perhaps knock out a brick every second rafter and dril some vent holes, say, approx. three or four 30mm in the joists to allow the air to move down the length?

Alternatively again, if I was to fit the insulation tight to the underside and use a vapour barrier would this suffice?

Your help appreciated.

 

[HJC1972]

Bump.


Anyone got any thoughts?

 

[noseall]

The idea of a 'warm deck' is that the insulation covers the joists as well as the void between.

What this means is it prevents the risk of cold bridge down the joist.

I would advise fitting the celotex tight to the underside of the deck then use foil tape to seal any gaps.

Depending on the depth of the joists, it may then be wise to either fully fill the space or fix a 25mm insulation underdraw prior to fixing the plasterboards.

The idea is that you still have the risk of cold travelling down the joists....

 

[HJC1972]

Thanks for the reply, Noseall. I'm still a bit confused though.

Just reading up a little it seems the biggest problem I am likely to have is condensation build up on the cold side of the celotex boards if I leave an unvented cavity above them. The joists are 8x2 with an approximate gap between each one of about 18" with about 13 or 14 joists in all. What I had in mind was to take the facia board off one side and one end of the garage and then to knock out half a brick between each joist and then put in, say, 4 louvered vents into the facia boards. Any ideas if this would be anough, or too much venting?

Also, does a vapour barrier on the underside of the joists, or insulation (foil backed) plaster board negate the need to vent the cavity above the insulation?

Cheers.

 

[noseall]

If you are cute with the cutting and fitting of the insulation board, i.e. cut them a snug as possible, push them tight against the deck then seal any remaining gaps and use foil tape also, then there is no reason to suggest that condensation will occur.

Fitting the insulation robustly will save the headache of all that venting.

As i said, regardless of all that insulation between the joists, the joists themselves can transmit cold downwards and cause condensation issues. This is why a warm deck is so much easier.

 

[freddiemercurystwin]

There will still be a risk of condensation occurring as the joists will allow cold bridging to occur. Unless you go to the effort of insulating above the joists then your roof will be hybrid of cold and warm and the risk of condensation will always be there. this is why when doing a garage conversion BC usually expect either ventilation above or insulation above.

 

[HJC1972]

Ok, thanks for the answers. Another thought then: I have no problem with venting the cavity above, if I put the unsulation lower down, but I can only do it at one side of the garage. Would it be feasable to vent the garage at the ends and then drill vent holes through the rafters, all 13 of them, with, say, 4 0r 5 30mm holes, to allow the air to move sidways trhrough the rafters as opposed to running along the length of the rafters?

Cheers for your thoughts, it's much appreciated.