Another question about unventilated cold roofs

[Martin Phillips]

I am confused by the different things I read. I have very shallow joists and a low ceiling height. I need to replace ceiling and for Building Control reasons "now" need to get close to the U-Value of 0.18. I can do this, without dropping the ceiling much further by using Actis Triso but they say I need a 50mm void between the deck and the Actis for ventilation (not sure who exactly is insisting on this - Actis or BC). This seems only possible if I put 2 roof vents between each pair of joists. My flat roof will look like a hedgehog and seems eminently likely to increase the chance of leaks!

If I completely tape all edges of the Actis Triso and seal any voids under it, perhaps even adding a further vapour barrier, can I forget about the ventilation? By the way, the original single plasterboard ceiling had no insulation or vapour barrier!

Please help!!!

Martin

 

[chappers]

You can do what you like, but BC won't like it.
I know it's not very helpful to you as you are trying to save head height, but it is only 25mm, could you not batten out the ceiling at 90 degrees to your joists to create cross ventilation, then you won't need anywhere near as many vents, a couple at each end would probably do.

Make up would be roof deck, joists, counter-battens, actis, battens,, plasterboard, total would be circa 95mm from bottom of joists

 

[Martin Phillips]

Thanks, this makes perfect sense! The roof is only 11.4 sq m. Extra Aqua Flat roof ventilators will vent 36 sq m. which will more than do it if I follow your suggestion......do you agree?