DIY Doctor UK DIY Forums

We have moved into a house that, in one part has a pitched ceiling (plasterboard, cavity and slates). the cavity has some loose-fitting cellotex - about 25mm I think. The room gets much colder than the rest of the house and I want to add 100mm cellotex. The is a 100mm deep purlin running down the length of both side of the pitch. I thought to attach 50mm battens and fill with cellotex then repeat and plasterboard. Trouble is, I can't find the existing battens and my detector is (I think) upset by the existing cellotex foil. Could I use platerboard fixings to hold the new battens? Too much weight?

Plasterboard fixings should be fine but it depends of course on how well the existing plasterboard is fixed. The Redidrive fixings we show in our fixing to plasterboard project are very good indeed and will hold up kitchen units so a few lbs of cellotex shouldn't be a problem.

OK, thanks - is there any milage in using the bubble-foil insulation rather than cellotex? I understand that I could hold it up using battens and then put plasterboard onto that? It sounds much easier to do than cellotex?

Check out the SAP energy rating topic for advice. Multifoil insulation is easy to put up and I estimate your current roof has a uvalue of 0.72. adding 30mm Triso Super 10 or similar multifoil would make it 0.32. Adding 100mm Celotex would make it 0.22. If you really want to warm it up then celotex is your best method however as you have noted not the easiest. Another approach is to use a thermal plasterboard like Gyproc Thermaline Super which is a phenolic foam bonded to 9.5mm plasterboard. This is 50mm thick overall. This would give you 0.32 for your roof and you could simply fix right through this onto the existing plasterboard provided you use a decent fixing as mentioned in previous post.

Ah, nearly there i think, thanks! Would a dream combination for ease of installation be a phenolic-backed plasterboard with mechanical fixing (like Kingspan K18 I think) at 50mm+12.5mm and underneath that a layer of the multi-foil all fixed by the fixings you have recommended - or, if I hack the existing ceiling via 125mm drywall fixings (if I can find them)? Thanks again.

Hmm, second thoughts, having just read the BRE report on mulit-foils, i think I'll give using that a miss and concentrate on using phenolic board of the greatest thickness I can find fixings for (I have only found one place - on eBay - where are drywall screws longer than 100mm (presuming I need these screws to go into the fixings you origninally mentioned?). THanks

I have a pitched roof on the offshoot to my house (built 1879) which has two skylights. To allow more light from the skylights into the area below I want to remove the existing horizontal ceiling and affix plasterboard sheets to the rafters. the roof is constructed of concrete tiles on plastic roofing felt on top of wooden boarding (using floor boards) secured to the rafters. I have read on websites that you need to have an airflow between the roofing and the insulation within the depth of the rafters to avoid condensation forming which will affect the integrity of the rafters. However I'm not sure that there will be any airflow within the construction I have described. Advice on how I should insulate this roof would be appreciated. Please note also that the area of roof I want to insulate in this way is not the full length of the offshoot, i.e. there will still be an area of pitched roof above horizontal ceiling - so do I need to insulate the vertical partition between the two areas?