insulating ceiling - advice needed

seneca

Hi folks,
not sure this is the right forum and please bear with me as I try to explain (it's tricky)
My hallway is always difficult to warm in the winter. It's a wide space and has the staircase in it. the sloped ceiling above the stairs has no rooms above it, just the sloping outside roof. So, any heat in the hallway tends to rise to the ceiling and the large are of sloping ceiling above the stairs. There it has only wallpaper, plasterboard, roof baton and then roof tile. There is no insulation, so the heat goes straight out.

I would like to insulate this sloped ceiling area above the stairs, but without ripping off the plasterboard etc to get to the roof beams. Is this possible? I reckon I have around 8cm to play with, maybe a little more, without it being obvious I have put a false ceiling there (the whole section is framed by a coving type border.)

Don't know much about the materials needed, but I imagined:
- batton against the ceiling (through plasterboard into roof beams - once located!)
- finding some kind of thin insulation sheet (what's the thinnest, most efficient kind) between the batons.
- plasterboard onto batons (maybe the kind with foil on it)
- wallpaper over plasterboard, so it matches the rest of the hall.

Is this plan plausible? is there a better way of doing it? what materials will i need? I'm a reasonable competent diy-er, but would like to make as little mess as possible (too much traffic in hallway and stairs.)
any advice would be greatly appreciated.

evilgoose

hello - I'm not an expert - but best check the building regs - there are ones (I think) regarding the head clearance for ceilings on stairwells. Also check that once you've put your false ceiling up that you've got the space for getting your furniture up and down from there (my apologies if I'm teaching to suck eggs!)

You can get thermal plasterboard which will help. There is also an insulation product from Kingspan - it foil coated polyurethane (i believe) comes in several thicknesses and is an effective insulator.

beaker141

Have a look at a product called "Thinsulex", it is a multi layer foil insulation blanket that gives insulation equivalent to quite a lot of kingspan.

Also consider a product called Kingspan K17 or K18, that has the insulation already attached. Do check the strength of the roof though before adding all this weight to it !!

seneca

thanks for the response.

The roof is quite high, hence by adding 8-10cm there would still be plenty of headroom.

I have been told that a better way to do it is to take down the existing plasterboard, put insulation (some kind of blanket?) between the rafters and then place a foil backed plasterboard back.

If I were to do this I think I would rather use some kind of solid slab insulation. That would seem easier to get up into the space than a floppy blanket. Not sure what I should get though. I really only have access to wickes and jewsons, where I'm situated.

Tucker

I recently did something similar to a bay window, which had no insulation, just ply board then render, beyond the plasterboard.

I used kingspan insulating boards in 50mm width (you can get 75 and 100 etc), which are more expensive, but far better to work with, as you saw them to a size to fit between the joists or rafters and push them in snuggly. However, they do make a bit of mess when you saw them, so do it outside if you want to reduce the amount of tidying up....

I then used the foil backed plaster board.

seneca

I've removed a small section of the "plasterboard" and found that it's 1cm of plasterboard (i think) and then that old form of wooden strips (don't know what they are called) with lots of mortar/plaster between them.
Anyway, there is a 12cm space between the board and the underneath of the roof tiles. I must say it is a bit worrying that if a roof tile were to slip/fall out, the rain would be falling directly onto the back of my ceiling.
The family are going away for a week, so I can go the whole hog and rip the ceiling off and insulate between the rafters. Or seeing as the ceiling is made of those wooden strips, should they be left alone. For all I know they might be important. I don't want to do something that may cause future damage. What's the best thing to do?

does there need to be a gap for ventilation? If so, is the gap between the insulation and roof tile, or between insulation and plasterboard, or both? Whats the best soild insulation and plasterboard to go for (I've got a wickes and Jewson near by.)
sorry for so many questions, but I know it is important roofs are ventilated.

beaker141

The old wooden slats are how they used to plaster in the old days (Plaster and Lath). You can rip them off but be ready for an almighty mess.

I think 50mm airgap should be left between the top of your insulation and the roof tiles, if your rafters are 12cm deep then I would get 70mm Kingspan boards and fit them in between the rafters. If you cut them snugly they will stay in place with friction. I'm sure Jewson will stock Kingspan/Celotex or equivalent, not sure about Wickes.

Its then a personal choice of plasterboard or insulated plasterboard.

designermatt_2

Best way of doing this really is to remove the plasterboard.. probably plaster and lathe. If you are gonna be plastering otherwise its the best bet. Don't use thinsulex it relies on having a low emissivity air gap each side of the insulant which over all thickens your construction and doesnt give the results claimed by many multi foils (although it has to be said Thinsulex dont make these claims.

When insulating like this you should ensure teh roof itself has ventilation. Chances are that it will be ventilated by having old loose fitting felt or sarking boards. To prevent interstitial condensation place Celotex between the rafters leaving at least a 50mm gap ABOVE the insulant. Then undercloak this with a further layer of Celotex. Tape all the joints of the Celotex with a silver foil tape and you have a very effective vapour barrier. Silicon seal the edges of the boards where they meet the walls and then plaster board over.. . If your rafters are say 150 in depth you could put 90mm between and 45mm underneath and this would give you a 'U'-Value of 0.2 which is the same as the requirements for a modern roof. If your rafters are only 100mm in depth place 50 between and about 70 underneath to get the same effect. You should inform Building Control as you will be upgrading a thermal element and should therefore make an application. You wil find many authorities wouldnt worry about this however. If you are determined not to remove the ceiling, (really cant emphasise enough how much of a better idea it would be to do so) you could just batten out and fix the insulation both between and under the battens and then board over.. You are relying on hitting joists you cant see though and will probably just be screwing to lathes which may later come lose from the extra loads (plasterboard and insulation).

seneca

I’ve made a small hole through the plaster and lathe to have a look at what’s underneath. There is no sarking board or felt (a bit worrying, but other than removing the roof, there's nothing I can do about it.) The rafter is approx 12cm, so I should be able to use 70mm insulation with the 50mm gap above. This should leave the bottom of the insulation flush with the bottom of the rafter. I can now use a board which has plasterboard (12.5mm) on one side and insulation on the other (25mm.) I plan to fix this to rafters with screws. The plasterboard has a vapour barrier and there is foil on both sides of the 70mm insulation between the rafters. The only gap is above the insulation – 50mm. Does this sound ok? Is it ok to have the 2 bits of insulation, between and below the rafter, touching? I don’t need a gap between those do I? I won’t have a problem with all those vapour barriers will I (three in total). I got these exact specs from an online insulation dealer and kingspans own data sheet, so presumably they are ok.

I have been looking online and came across Kingspan tp10 (70mm) for between the rafters and kingspan k18 kooltherm dry-lining board (37.5mm.) I can’t find anywhere near me in Birmingham that sells these so I’ll get them online.

Does this sound ok?