Re-insulating flat roof - advice sought!

[elfinblues]

I have read lots of posts on DIYnot forum to try and understand our problem with a cold ground floor extension and many have been really useful, though there are often different opinions, which gets a bit confusing for someone who is completely new to the subject of flat roof insulation! Basically, our one-story, ground floor extension, which was built by the previous owners of our house, is so cold in autumn, spring and especially winter as to be virtually unusable, unless you really like having frostbitten fingers! In the course of doing up the shower room, study bedroom and kitchen which it houses, we want to strip it right back and retro-improve the insulation as best we can. I am happy to get on and do jobs like insulating and plasterboarding myself – not a problem – but I want to make sure that I do the insulating correctly, as I know that getting it wrong can lead to problems with condensation, damp, etc. So, if anyone here is able to help, I could really do with telling our own situation and asking for your advice on whether I’ve got the right way forward planned.

If I understand correctly, a warm flat roof is almost always preferable to a cold flat roof, and that in new builds cold roofs are pretty much unknown these days. However, our building already exists, and it has a badly/partially insulated cold flat roof, with soffit ventilation strips running down both sides of the roof to ensure a through-flow of ventilation. It loses so much heat, though, so my plan is to take down the existing plasterboard and remove whatever roll-out insulation there is above it, and replace it with Celotex (of undecided width – what would be best?) jammed up between the joists, taking care to leave a gap between the top of the insulation boards and the underside of the roof deck (though I’m not sure what depth of gap) but no gaps around the insulation boards, and then to re-plasterboard, ensuring that the plasterboards are tight up against the insulation, with foil-backed plasterboard (with the foil acting as the vapour barrier – right?). In addition, we also plan to take out the existing downlighters and replace with pendant lights. Does all this sound about right?

I’m sure that a better course of action would be to get the whole 5m x 5m roof redone so that it can be changed to a warm-roof setup but, to be honest, we have such a lot to do on our house that we need to compromise between the course of action we take and the cost. So, do you think what we propose would work, and do a good job at keeping the extension a lot warmer, or is getting someone in to fit a warm roof really the only sensible way to go? I am especially concerned to get this right because the extension houses a shower room and a kitchen – both major sources of water vapour.

Thanks in advance for any help or advice anyone can give me.

 

[GrogSym]

Someone will be along to answer your question more thoroughly, but I think you are on the right tracks. If these are particularly damp rooms then I would advise putting an additional damp proof membrane between the plasterboard and insulation to ensure no water ingress.
I would also ensure thathte airflow above the insulation is indeed a flow (ie a through draft and not just air vents doing nothing). A 50mm gap is sufficient between insulation and roof, best to use some kind of spacers to stop you pusing the celotex up too far.
Wear a good quality mask when you are cutting the insulation, it's really rough on the lungs. Depth to go for - the depth of your rafter, minus 50mm ie as much as you can while still leaving a gap!
I am currently doing my flat roof extension (albeit the walls and not the roof!), so this is just my DIY knowledge from my own research. Maybe the pro's will tell you different...
Now I must go and look at that image that I signed up to see

 

[catlad]

Take a look on the celotex web site and it will show you how to do it.

 

[elfinblues]

Thank you GrogSym. Is it still worth putting in an additional damp proofs membrane, though, when you ultimately have to screw up through it to fix your plasterboards to the joists?

And thank you Catlad. I had a look at the Celotex website and found the "Between and Under Joist Applications" sheet. It tells me to use Celotex boards both between AND underneath the joists - the product it tells me to use underneath the joists seems to be plasterboard backed with an insulation board with built in vapour barrier. So, I guess this is to insulate the joists themselves as well as the gaps between the joists. Could anyone tell me, is doubling up in this way genuinely necessary? I obviously want to insulate between the boards, but it hadn't occurred to me that I might want to insulate the joists themselves.

Any advice anyone can give would be massively appreciated. Thank you.

 

[Ric2013]

Personally I would insulate the joists if possible, even if you can only squeeze a thin layer of celotex in.

The reason for this is that your celotex will have a conductivity of about 0.02 W/mK (this means that for every degree celsius difference between the warm and cold side of the board, 0.02W of heat will escape per square metre, if the board were 1m thick (so if the board were 0.1m thick you lose 0.2W per degree per square metre per degree)).

By contrast, your softwood have a conductivity of 0.13W/mK, ie it conducts 6.5 times as much heat as the Celotex for a given thickness. Effectively this means that a large part of your ceiling area will be relatively uninsulated.

The surface temperature of your plasterboard will therefore be low in places due to the cold bridging of the joists and you will be less comfortable than otherwise. The human body's sense of comfort is not just related to 'comfort temperatures': radiant heat plays a large part.

Basically, if your walls and ceiling are cold, you will have to overheat the air in the room to achieve comfort. So insulating the joists may not save much gas, but you will feel more comfortable.

 

[tony1851]

Applying some insulation below the joists does help reduce heat loss through the joists, though they only make up around 12% of the area of the ceiling and the reduction is fairly small.

Without under-joist insulation, over time you will also get 'pattern-staining'. This is due to the ceiling immediately below the joists being a little cooler than the rest.

 

[hardmetalking]

You can insulate on top of the joists if you replace the roof.