Flat roof insulation query

Broseley

Hi we have a flat roofed house with the roof made of asphalt (covered with GRP). I want to get insulation for the roof. It would be very expensive to insulate over the roof as it would involve having a new roof covering over it. It would also be expensive to insulate the ceilings inside the house. However there is a space about 2 feet deep between the bedroom ceilings and the asphalt. This is ventilated with air bricks on all sides. I am wondering if it would be feasible to put insulation in there - either the foam or beads - pumped in like with cavity wall insulation. Or would it result in a lack of ventilation and cause condensation? If I consult an insulation company (via the Green Homes Grant) they are bound to say yes as they'd make money, so I would like an unbiased opinion please.

FreeBear

The air bricks are there to ventilate the void. Insulating above them will be a pointless exercise as the heat will just go out through the air bricks. Insulating below the level of the air bricks, and using a permeable insulation (rockwool or fibreglass) would avoid build up of damp. Would certainly NOT recommend and foam insulation pumped in to the void (horrible, nasty stuff), and any loose material (polystyrene beads or rockwool/fibreglass) will fall out if the ceiling ever needs replacing or repairing. Pumping insulation in would also be tricky as you have no real way of knowing if there is an even coverage - All too easy to have a mountain at the edges and nothing in the middle.

If possible, it would be far more effective to take the ceilings down and fit Celotex/Kingspan type boards between the ceiling joists. Put a vapour control membrane up, and perhaps another layer of Celotex to get the required thickness needed for building regs compliance. Fix plasterboard (screwed, not nailed up), and then skim. You then end up with a nice warm ceiling without any cracks, and you have the chance to re-jig lighting to suit your taste.

Jeepers_Creepers

"It would also be expensive to insulate the ceilings inside the house."

Price this easiest of options up before you dismiss it. All it'll require is for insulated plasterboard to be screwed to the existing ceiling and skimmed over. Instant transformation.

Any thickness you add will make a huge difference, even 1" stuff. 2" would be pretty optimal - this will give a high level of insulation, and going any thicker than this will have an increasingly diminishing level of advantage. I wouldn't bother.

If you instead add insulation above the ceiling, then you need to make sure it's fitted snuggly and tightly so that (a) it's effective and (b) there are no voids where moisture from the room below could condense out.

Obviously you'll lose an inch or two of headroom, so you need to be happy with that. But you'll know you'll have gained the full advantage from what you fit - no 'wondering' :-)

Broseley

Thanks, Freebear and Jeepers_Creepers we had dismissed the idea of insulating the ceiling from below or taking the ceilings down as this art deco house has all the ceiling/wall joins as a curved continuation which we don't want to lose. Also wouldn't it give issues with the wiring in the ceiling - if we ever needed to access it? We have insulated between the floor joists in the bedrooms when we have had the opportunity to do so (new carpets). I just thought pumping something into the air gap between the ceiling and roof might be an easy option.

Jeepers_Creepers

That sounds like a lovely ceiling :-) Fair do's you don't wish to touch that.

You have a 2' void betwixt this ceiling and the exterior roof above? What is the access like - is it crawlable? I think, in your case, I'd play safe and just add conventional loft insulation - the rolls of itchy stuff (or a non-itchy alternative).

First snugly fill in the gaps between the ceiling joists to whatever depth these are so it all ends up flush with the joist tops, and then lay at least a further 4" (6" or 8" if you can) across it all at right angles to the joists, again snugly touching each strip. 

It all sounds very well ventilated up there, so I'd have few concerns. Obviously make sure you don't cover or restrict any actual vents or required air gaps. 

Broseley

Jeepers_Creepers said:

That sounds like a lovely ceiling :-) Fair do's you don't wish to touch that.

You have a 2' void betwixt this ceiling and the exterior roof above? What is the access like - is it crawlable? I think, in your case, I'd play safe and just add conventional loft insulation - the rolls of itchy stuff (or a non-itchy alternative).

First snugly fill in the gaps between the ceiling joists to whatever depth these are so it all ends up flush with the joist tops, and then lay at least a further 4" (6" or 8" if you can) across it all at right angles to the joists, again snugly touching each strip. 

It all sounds very well ventilated up there, so I'd have few concerns. Obviously make sure you don't cover or restrict any actual vents or required air gaps. 

Thanks, no the gap is not accessible, the only reason we know it's there is the existence of air bricks all around the roof and a discrepancy between the level of the roof and the ceilings. So any insulation would have to be pumped in through the air bricks.

FreeBear

Broseley said:  the gap is not accessible, the only reason we know it's there is the existence of air bricks all around the roof and a discrepancy between the level of the roof and the ceilings. So any insulation would have to be pumped in through the air bricks.

You might be able to get fibreglass/rockwool blown in using the same kit they use to do cavity wall insulation. My concern would be getting an even coverage, especially into the corners - You might need to cut out a few bricks at strategic locations. Would also need scaffolding for safe working at height.

Broseley

I'm thinking about using the green homes grant for this, but there is only one company that does insulation listed, and they are 150 miles away!