No ventilation in cold flat roof = condensation!

[jamie765]

Had a builder do the shell/structure of my loft conversion earlier this year and I'm DIYing the rest.

Whilst putting insulation in I've discovered he hasn't left ventilation at either end of the cold dormer roof. It's felt on top, with 150mm (145mm) ceiling joists, plus firrings. Am using 120mm Celotex in between joists, with 25mm Celotex underneath. Taking the firrings and 120mm Celotex into account, the ventilation gap (between top of insulation and bottom of roof OSB) spans from 63mm at one end to 33mm the other end - I know, a little less than the recommended 50mm ventilation gap, but that particular detail is the least of my worries at the moment! Because...

I've already discovered severe condensation when I removed one of the roof insulation boards and found the dormer roof OSB wet to the touch! The top of the ridge beam is also wet to the touch.

I know it will be (relatively) easy enough to vent at the far end of the dormer roof - using soffit or fascia vents. However I know I need the air to circulate i.e. vented at both the 'far' dormer soffit end, as well as the ridge end. Am toying with the idea of mushroom vents on top of the dormer roof, but there are 13 openings and obviously I don't want 13 mushroom vents on my dormer roof! (would need one for each opening as there's currently no way for the air to circulate between them)

I'm also thinking I could notch and drill holes in each and every joist to the maximum regulations permitted, that would create some sort of airflow in between joists, and possibly result in needing far less mushroom vents? (could I get away with just 3 - one in the middle and one at each end?) I've attached a diagram showing joist holes and notches I've calculated. I'd have to batten 44mm x 44mm timber to the bottoms of the joists and bring the 120mm Celotex down to that level to allow for 30mm holes to still have the required 50mm height clearance from the hole to the top of the joist.

Also concerned about the roof leaking if I get a roofer to put mushroom vents into a felt roof, whether it's 1, 3 or 14...

Any other solutions to provide ventilation?

 

[Alastairreid]

Full fill the void hard against the deck, fill any spaces with expanding foam, tape all joints and use a vapour barrier.

Was there a reason not to make a warm roof?

 

[^woody^]

Before doing as Alastair says above, make sure the deck and timbers are dry, so as not to trap moisture in the structure.

You will probably need celotex across the whole underside too.

 

[jamie765]

Thanks for replies.

I wish it was a warm roof but didn't know much about cold and warm roofs when the builder did it, and now it's too late.

If I full fill the void and push insulation up, wouldn't that create even more of a problem? Aren't cold roofs supposed to have that ventilation gap? If pushing the insulation up to the ceiling is an option, why aren't all cold roofs made that way - would save on needing to ventilate the void?

Seeing videos like below has struck fear in me, especially when my house was only built in 2017 (and is therefore much better insulated/warmer than most houses):

 

[Alastairreid]

It would then be a hybrid roof, as the insulation is packed hard against the underside of the deck and great care taken in filling all the voids, tape, vapour barrier and underdraw the vapour cannot reach the deck to condense.
Speak to your Bco.

 

[jamie765]

If the 120mm insulation is pushed up tight under the roof decking, would I then need to add more insulation to fill the void that would appear underneath this 120mm insulation? As they are 150mm joists, plus firrings.

Would the insulation need to be flush in line with the bottom of the joists, like it looks right now? As if the whole joist void is filled with insulation, getting it level at the bottom (where plasterboard would start) may prove difficult because of different levels due to the firrings...

And would I still need to fit 25mm Celotex under the joists too?

 

[Alastairreid]

celotex under the joists is to stop cold bridging which show up as shadowing on the finished ceiling.

Take up any differences in the insulation using expanding foam to get the insulation flush with the bottom of the joists.

 

[John D v2.0]

Take up any differences in the insulation using expanding foam to get the insulation flush with the bottom of the joists.

You should give a health warning using expanding foam overhead in those quantities! Horrible stuff! I'd be more inclined to use mineral/glass wool as you'd spend a fortune in tins of foam and most of it would end up on your head and impossible to remove.

 

[Alastairreid]

you would put the foam on the board before putting it up, better not using mineral wool as it has air pockets.

 

[John D v2.0]

you would put the foam on the board before putting it up, better not using mineral wool as it has air pockets.

I'm only a diyer but it sounds like you're suggesting making a 120mm board into a 150mm one using expanding foam, then fitting it, which sounds extremely expensive and difficult and still very messy.
Also mineral wool is supposed to trap air pockets, that's how it prevents heat transfer as air is a good insulator. Celotex works in exactly the same way.

 

[Alastairreid]

No! 120mm followed by 25mm closed cell insulation board! foamed into position to make a total depth of 150mm.

Mineral wool allows the passage of moisture! closed cell insulation boards does not.

 

[jamie765]

Thanks, out of curiousity - is this a common solution to this issue? I only ask as I've been pulling my hair out researching this cold roof ventilation issue for the last 2 days and haven't come across this suggestion, only people saying how the void needs to be vented both ends. Hoping BC will agree with this option as it seems like a much better option that what I suggest in my first post!

With this solution, I assume I would keep the soffit vents closed off on the inside? (as mentioned in my original post, builder actually blocked it in from the inside)

Including firrings, the height from top of roof deck to bottom of joists goes from 183mm to 153mm...i.e. a void of between 63mm and 33mm if I pushed the existing 120mm Celotex up to the deck. So I guess I would then need to purchase some 30mm, 40mm, 50mm and 60mm Celotex and fit under the 120mm boards accordingly, to get it as flush as possible with the bottom of the joists?

 

[Alastairreid]

Your aim is to get rid of the cold underside of the decking and prevent moisture reaching it! the 120mm board does that alone, fill any spaces between the boards and joists with expanding foam then pack it out as level as you can...30mm underdraw with taped joints followed by vapour barrier then gyproc. As i said earlier put it past your Bco for clarification.
Ideally a warm roof, and as you have issues installing adequate cold roof ventilation your Bco should be open to compromise.

You could also consider spray foam insulation, it can be bought in kits.

 

[jamie765]

Gotcha...so the principle aim is NOT just to get rid of the void in between the joists?

Sorry if I'm being thick but want to make sure I get this right. Just to clarify, I push up the 120mm to underside of deck, then ONLY use foam in the void (which will range from 63mm at one end to 33mm the other end) to get it as level as possible with underside of joists? I don't need to put further 30/40/50/60mm Celotex under the 120mm, I can solely use foam? (wouldn't foam actually be more difficult to get level?)

And then put more Celotex under the joists (I already purchased 25mm a while back, not 30mm...)

 

[Alastairreid]

You have 150mm joists, so you can put 150mm of insulation in! Use the foam to fill any spaces around the edges.
Dont bother messing about trying to get the insulation level with the underside of the joists....you can then install the 25mm boards on the underside of the joists followed by the vapour barrier and gyproc.