Insulating a dormer house
Discussion
I live in a 4 bedroom dormer style house and the upstairs can be bloody freezing at times. There appears to be a real lack of insulation up there so I am planning when the weather gets better to start ripping out some of the walls / ceilings (parts 4 and 5 in the image below) and insulate behind them. There seems to be plenty of insulation in the loft but the walls have nothing behind them.
(My house is slightly different to this pic in that the roof starts at the top of the ground floor so the entire upstairs is in the roof).
Some questions:
1) Not sure whether to overboard the ceilings or just rip them down. Either way, I assume I'd just put up some celotex / kingspan insulation, cover with plasterboard, then get them skimmed? How thick would it need to be for it to be effective?
2) With regards to ripping out the walls (shown in point 4 on the pic) and insulating behind them, I suppose celotex / kingspan would be the best thing again and just fitting between the joists/studs? Or should I also put a load of rockwool in the floors behind those walls?
3) As the bedrooms are all split by stud walls upstairs, is it worth putting insulation in those walls or just stick to the outside walls?
4) Anyone know when the best deals on insulation are? I remember seeing deals in the sheds for about £3 a roll of the rockwool type insulation, they now seem to be about £30. Obviously I'm going to need a lot if I'm going to do this so want to keep it as cheap as possible.
Sorry for all the questions, always lived in well insulated new builds before, and never had to think about this kind of thing...
(My house is slightly different to this pic in that the roof starts at the top of the ground floor so the entire upstairs is in the roof).
Some questions:
1) Not sure whether to overboard the ceilings or just rip them down. Either way, I assume I'd just put up some celotex / kingspan insulation, cover with plasterboard, then get them skimmed? How thick would it need to be for it to be effective?
2) With regards to ripping out the walls (shown in point 4 on the pic) and insulating behind them, I suppose celotex / kingspan would be the best thing again and just fitting between the joists/studs? Or should I also put a load of rockwool in the floors behind those walls?
3) As the bedrooms are all split by stud walls upstairs, is it worth putting insulation in those walls or just stick to the outside walls?
4) Anyone know when the best deals on insulation are? I remember seeing deals in the sheds for about £3 a roll of the rockwool type insulation, they now seem to be about £30. Obviously I'm going to need a lot if I'm going to do this so want to keep it as cheap as possible.
Sorry for all the questions, always lived in well insulated new builds before, and never had to think about this kind of thing...
Edited by JohnStitch on Monday 27th January 14:28
On your drawing, number 5. If the studs are the roof joists, and its built as a cold space roof, you need to keep the airflow in the roof to stop it getting damp. If the insulation blocks the airflow, it will cause more issues than you'll solve by having a warmer bedroom. I wouldnt go deeper on the insulation than the depth of the joists. Once its insulated properly the loft space will become a lot colder, so airflow is paramount.
I had a similar style house, although I removed part 4 as angle wasn't as acute. Pretty easy if messy cutting the boards, get as tight as you can, seal any gaps with expanding foam, if the joists/studs aren't Straight it certainly helps. Get a proper gun around £20, and screw on canisters.
I know a bloke that does most of the jobs on my house (carpenter by trade, but can pretty much apply himself to most things). My plan is to take 2 weeks off work, employ him for those 2 weeks on a day rate, then blitz the place, which will give me the opportunity to learn stuff from him too.There's loads of other jobs to do too, but this one is my priority. As the quality of the plastering on those walls is of questionable quality then it gives me a good excuse to rip them out and start again.
If this job reduces my heating bills I'll be happy!
If this job reduces my heating bills I'll be happy!
DoubleSix said:
I was with you up until this bit:
thx
Its fine, we all started somewhere. Hopefully from top to bottom, the gap between the wood is equal, 400mm centres. If they're not straight, say 400mm going to 390mm, it makes them more of a pain to fit, as the boards need to shaped, they need to be tight to be effective. I ended up filling the gaps with foam, then taping over with foil tape, seemed to work OK.bigdom said:
......if the joists/studs aren't Straight it certainly helps. Get a proper gun around £20, and screw on canisters.
Sorry, I know advising noobs is tiresome!thx
What I've done for a new flat-roofed dormer (the old smaller dormers from the 1970's that we demolished were completely and totally uninsulated) is between the 8" deep flat roof timbers I infilled/fitted 100-120mm kingspan, a fews nails to hold, then foamed in place. It needs to be between all the timbers - so where there's a stud wall that needs to come out to ensure the whole roof is treated the same. No gaps or voids!!
Get a proper foam gun - about £20 or so from screwfix etc - waaaay more controllable than the DIY type that come with their own nozzle and 6" tube, and you can stop halfway through a canister without wasting any. Ebay for the foam - out-of-date stuff comes up quite a lot, and is perfectly fine.
This 120mm infilling between 200mm timbers left more than the minimum 50mm air gap you need for a cold roof between the underside of the roof boards and the insulation, allowing the timbers to breathe. Ideally you also need a vented soffit one end, and a vented ridge at t'other, so there is enough ventilation to get any condensation out of the roof.
Then I put 50mm kingspan right across the underside of the timbers and infill insulation, and then screwed plasterboard underneath that, before plasterer skimmed the lot.
As the insulation board is foil-backed, you can foil-tape all the joints before the plasterboard goes over it, and make a perfect airtight box... until you put the plasterboard screws in. I think you're supposed to put a full plastic membrane in, but I didn't. Worth checking up to date building regs.
You can get pre-insulated plasterboard which is perfect for this job, but even as seconds it isn't cheap.
Same goes for the pitched areas of roof allowing for a continuous 50mm airgap between insulation and roofing felt, and the dormer walls/cheeks - insulate by infilling and then underdraw with board/insulation.
Get between your dormer floor joists at each end too - there'll be a gale blowing under the floor from the main wall cavities if it's anything like ours was.
You need long plasterboard screws (I used 125mm or something ridiculous in some tricky areas, as we used 75mm insulation under the rafters (your sloped/pitched area 5), with plasterboard over that, and got through either 6,000 or 9,000 screws in total - whichever it was, my cordless drill arm looked like Popeye's by the end of it).
Metal screws are a thermal bridge so I believe you can now get clever screws/systems made from basalt fibres to hold the plasterboard and insulation in place. We've had a thermal imaging camera survey since, and you can actually see each little round thermal bridge made by the screws.
Huge difference. Massive. But I reckon you'd get close by just underdrawing everything floor to ceiling with 50mm insulated plasterboard, foaming every nook and cranny you can find, and being careful with all the detailing to prevent any draughts.
Get a proper foam gun - about £20 or so from screwfix etc - waaaay more controllable than the DIY type that come with their own nozzle and 6" tube, and you can stop halfway through a canister without wasting any. Ebay for the foam - out-of-date stuff comes up quite a lot, and is perfectly fine.
This 120mm infilling between 200mm timbers left more than the minimum 50mm air gap you need for a cold roof between the underside of the roof boards and the insulation, allowing the timbers to breathe. Ideally you also need a vented soffit one end, and a vented ridge at t'other, so there is enough ventilation to get any condensation out of the roof.
Then I put 50mm kingspan right across the underside of the timbers and infill insulation, and then screwed plasterboard underneath that, before plasterer skimmed the lot.
As the insulation board is foil-backed, you can foil-tape all the joints before the plasterboard goes over it, and make a perfect airtight box... until you put the plasterboard screws in. I think you're supposed to put a full plastic membrane in, but I didn't. Worth checking up to date building regs.
You can get pre-insulated plasterboard which is perfect for this job, but even as seconds it isn't cheap.
Same goes for the pitched areas of roof allowing for a continuous 50mm airgap between insulation and roofing felt, and the dormer walls/cheeks - insulate by infilling and then underdraw with board/insulation.
Get between your dormer floor joists at each end too - there'll be a gale blowing under the floor from the main wall cavities if it's anything like ours was.
You need long plasterboard screws (I used 125mm or something ridiculous in some tricky areas, as we used 75mm insulation under the rafters (your sloped/pitched area 5), with plasterboard over that, and got through either 6,000 or 9,000 screws in total - whichever it was, my cordless drill arm looked like Popeye's by the end of it).
Metal screws are a thermal bridge so I believe you can now get clever screws/systems made from basalt fibres to hold the plasterboard and insulation in place. We've had a thermal imaging camera survey since, and you can actually see each little round thermal bridge made by the screws.
Huge difference. Massive. But I reckon you'd get close by just underdrawing everything floor to ceiling with 50mm insulated plasterboard, foaming every nook and cranny you can find, and being careful with all the detailing to prevent any draughts.
Edited by andy43 on Monday 27th January 19:59
DoubleSix said:
My rafters are only 100mm deep, how do you get requisite 130mm and maintain the space between insulation and the felt?
Depends on the felt.If breathable then a min air gap of 25mm so you could use 70 between the rafters and 60 below the rafter.
Either tape the joints (aluminum tape) or foil backed board.
Personally I'd batten the underside of the rafter to 125mm and use 100 between them.
Cheers, that makes sense. So having increased the depth of the rafter, how do ensure the kingspan stuff doesn't get pushed back into your void. Is it common to batten the sides of the rafter to prevent this?
OP sorry for hijacking your thread, hopefully im asking q's that will benefit you!
OP sorry for hijacking your thread, hopefully im asking q's that will benefit you!
We have a similar problem but everything's decorated and there's no way I'm going to strip it all out. It's tile hung with plastic membrane and an inch or two of rockwool, nothing in the triangular space behind (4). I don't suppose I can inject something in there?
I also wondered about the space under the floorboards through which the wind whistles thanks to vents in the soffits. Our floors (3) are cold, is there anything I can do to limit the airflow without causing problems?
I also wondered about the space under the floorboards through which the wind whistles thanks to vents in the soffits. Our floors (3) are cold, is there anything I can do to limit the airflow without causing problems?
DoubleSix said:
My rafters are only 100mm deep, how do you get requisite 130mm and maintain the space between insulation and the felt?
I have the same design. Builder put in 4" polystyrene, which leaves 3" gap to the felt. Seems to work OK, it isn't cold upstairs, but having a large open hall does allow heat to rise.Gassing Station | Homes, Gardens and DIY | Top of Page | What's New | My Stuff