You don't say how 'long' (top to bottom) the sloping ceiling is. But unless it's very small in length, the likelihood of getting anywhere near an airtight finish with the fit of PIR, foil-backed or not, or fitting an effective separate vapour barrier by sliding it/them down a gap between the felt and plasterboard is as close to zero as makes no difference.
Best 50mm insulation material
- Thread starter ZACCCC
- Start date
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If you've now decided to go over the existing ceiling with insulation, I don't get why you don't take down the existing and do the job properly. You're going to have to create a finished job in the end, no matter what.
The ceiling slopes up from the eaves for slightly less than 2 metres and then levels out. So I'll be cutting the boards to something less than 2 metres and then sliding them in from the loft.
The reason I've decided to put up another ceiling under the existing one using insulation backed PB rather than taking the existing ceiling down is partly to save all the mess of taking down the full ceiling and partly because I think I can get a reasonably tight fit by measuring carefully before cutting the boards and also because the double ceiling with insulation backed PB should block even more heat loss.
The reason I've decided to put up another ceiling under the existing one using insulation backed PB rather than taking the existing ceiling down is partly to save all the mess of taking down the full ceiling and partly because I think I can get a reasonably tight fit by measuring carefully before cutting the boards and also because the double ceiling with insulation backed PB should block even more heat loss.
It is not possible to seal insulation against timber rafters - foil or no foil! But it's irrelevant anyway. In your first post you said you were leaving a 50mm gap above the insulation for air flow. You do not have to consider condensation if you have airflow above the insulation.
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Jeds, yes there will be a 50mm gap between the insulation boards and the roofing felt.
(The other lads seem to be saying that there should also be a vapour control layer underneath the boards above the plasterboard.)
(The other lads seem to be saying that there should also be a vapour control layer underneath the boards above the plasterboard.)
Well that's the problem with a forum like this. You've got to be able to distinguish between advice and accurate advice. I'll grant you that isn't always easy but to point you in the right direction; I'm right and they're wrong.
What is there to stop water vapour condensing on the underside of the roofing felt, should that roofing felt be non-breathable?
Same as any old style sloping roof. The way they were always ventilated was a 50mm (2") vented gap above the insulation. As long as it's vented any vapour will be diluted and ultimately carried away.
And how is an old raked ceiling vented? Where does it get its air-flow?
OP, I think you will find your method very flawed when you try it out. There's no guarantee the rafters will be parallel, or wont have nail heads sticking out or other obstacles. I also doubt you'll get the "flex" you'd need to slot the boards in, unless you cut them into small pieces.
You really need to try and stop any cold air getting to the back of your plasterboard, so gaps must be avoided.
If you don't have a vapour barrier, you will likely get damp cold spots in the winter wherever theres gaps in the insulation.
It really would be so much better to take the ceiling down and fit your first layer properly, staple your vapour barrier in place and then add insulated plasterboard.
You really need to try and stop any cold air getting to the back of your plasterboard, so gaps must be avoided.
If you don't have a vapour barrier, you will likely get damp cold spots in the winter wherever theres gaps in the insulation.
It really would be so much better to take the ceiling down and fit your first layer properly, staple your vapour barrier in place and then add insulated plasterboard.
The top is open to the loft. The bottom terminates at the vented eaves.
I don't disagree with this. As I said first off, it would be much better to take the ceiling down, insulate between, and then 40mm continuous insulation beneath and re-board. But it is a common solution to slide insulation down from the loft and, as long as the gap is vented, it is better than nothing.
Thanks again for all the advice I've had on here.
I bought two sheets of Xtratherm and I intended to put them between the joists/rafters by sliding them down into place from the loft. A couple of the lads on here (freddymercurystwin, SNM, regsmyth ...) advised me (quite correctly as it turned out) that this would be very difficult to do and just about impossible without leaving gaps everywhere. Well, I did try but soon realized it couldn't be done and therefore decided to take down the whole ceiling (or at least the sloping part to the eaves - horrible dirty job!) I've now completed this and am next going to put the Xtratherm boards in, getting as tight a fit as I can.
After I've put the boards in I'm going to put up insulation backed plasterboard, preferably a type that has a built in vapour control layer. Does anybody have any recommendations on what type would be best? Thanks.
I bought two sheets of Xtratherm and I intended to put them between the joists/rafters by sliding them down into place from the loft. A couple of the lads on here (freddymercurystwin, SNM, regsmyth ...) advised me (quite correctly as it turned out) that this would be very difficult to do and just about impossible without leaving gaps everywhere. Well, I did try but soon realized it couldn't be done and therefore decided to take down the whole ceiling (or at least the sloping part to the eaves - horrible dirty job!) I've now completed this and am next going to put the Xtratherm boards in, getting as tight a fit as I can.
After I've put the boards in I'm going to put up insulation backed plasterboard, preferably a type that has a built in vapour control layer. Does anybody have any recommendations on what type would be best? Thanks.
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