Cold Roof: Vented or Unvented ?

[Cypermethrin]

I have done a lot of searching on this topic and its clear that (at least to me) its unclear as to what the best plan is with a cold roof.

I have an existing 4 x 4.5m flat roof build in the mid 1970's and is enclosed on three sides by other parts of the house - so only one side could be vented through the eaves and to add further complexity it has a 6 tile pitched parapet too. the original roof was unvented with 10cm of rockwool packed imbetween the joists up to the deck above - no vapour barrier - no venting.

The roof sits on 9x2" rafters and sits on firings strips to allow water runoff to the "open" side.

So far we have had new OSB put on top of the existing decking boards and EDPM installed. All good so far.

My original plan was going to be:
EPDM
OSB
5 cm void
20 cm rockwool
Vapour barrier
Plasterboard with VB
ensure eaves on "open" side had appropriate venting holes

However - reading some of the posts on here it would seem that packing the entire joist depth with rockwool ensuring there are no air voids and using a vapour barrier would essentially prevent any potential build-up of condensation and negate the need for any venting.

I know +woody+ has kindly explained on numerous posts previously that this "composite roof" containing a full insulation layer from upper deck board to vapour barrier removes the potential for condensation to even occur - but all the other literature I look at (e.g., Bauder and others) all advocate venting above the insulation/eaves etc

Thanks for your counsel in advance

Tim

 

[garyo]

I'm in exactly the same boat on a 1.5M by 3M flattie, enclosed on 3 sides and only able to vent on a 1.5M side, so look forward to your replies Tim.

I would use foil backed PIR and then expanding foam and foil tape the bejesus out of it. I wouldn't trust that rockwool wouldn't sag/relax over a couple of years.

 

[Cypermethrin]

Hi Garyo - I would use foil-backed PIR but the joists are so uneven that getting any kind of uniform seal anywhere along the length would (to my skill level) be impossible - hence falling back on the rockwool.

Perhaps I am overthinking all of this .....

 

[chappers]

It can be said that full filling the void can eliminate condensation and in the past BC used to allow this as an alternative to a warm roof, however that won't be the case with rockwool fill.
Best bet would be to vent it. But it seems you have already gone too far to do that effectively.
Ventilating at the open eaves end will now be better than nothing but the correct method would have been to cross batten to create cross air flow between the rafter voids and then fitted a few mushroom vents on the upside of the pitch with ventilation at the eaves.
Your best bet now is probably to proceed as you suggest with a VCL below the joists immediately above the PB