Should I insulate a sloping roof?

[John G. Franklin]

Hi all,

I have a small bedroom in my house which has a ceiling which is part sloping, where the house roof is and part flat where the loft is.

The loft part is insulated but the sloping part has no insulation between the plasterboard and the loft space/rooft. Above the plasterboard are the joists, approximately a 160mm air gap and then roof planks with no gaps, on which there are tiles. The length of the sloping part is about 180 cm. I think the roof is sealed at the end, I can't tell very easily. There is a gap at the end for between the outside wall and the plaster board.

The sloping part of the rooft is perhaps 50-60% of the area of the room's ceiling and so I believe a lot of heat is lost through that part during the winter.

However to insulate without ripping out the plasterboard means sliding something down between the joists, without blocking off the vertical gap at the end.

I was thinking of somehow sliding 100mm rockwool insulation down between the joists, though not sure how, since it will need some mechanism to push it all the way without bunching up. However, this will leave only 60mm of air gap and I'm worried that because the roof is quite sealed, that ventilation of the area will change and cause a damp problem.

The other alternative I considered was sliding down 50mm to 100mm of polystyrene sheeting, but this is more expensive than the rockwool and so I wonder whether I should just pay for the heating instead.

Any suggestions?

Thanks in advance.

[Andrew]

I had a similar problem - although my roof was flat. I wanted to "push" insulation 2-3m between the joists from one end. I tried and failed with polystyrene (the joist space was too irregular to get a good fit). I ended up using that expensive "pink panther" wrapped insulation, and a length of 20mm diameter plastic waste, (with an elbow and a 20cm offcut at one end, making it L-shaped). Fold about 50cm of the insulation over the L and push into the space. Once the insulation is almost all the way in, hold on to the insulation and you can push the folded bit out so that it lies flat.

Provided I didn't catch a splinter on the joists, I was surprised how well this worked (particularly having first tried with unwrapped insulation).

If you are insulating a slope, stapling the insulation to the joists at the top should be sufficient to keep it in place.

However, this doesn't solve the second problem of ventilation. I've recently posted a thread ("Insulating Flat Roof") to get some comments on a solution placing some lightweight ducting on top of the insulation.

regards, Andrew

[Christian McArdle]

60mm should be enough gap. I believe 50mm is recommended if you want a ventilated roof.

I'd use 100mm of Celotex. This is a rigid foam insulant, so won't bunch up. If there had been no sarking (I assume this is what you mean by roof boards with no gaps), then I'd have used 150mm, as there would be sufficient ventilation between the tiles not to need a gap.

Modern roof construction often uses breatable membranes instead of impervious sarking, so doesn't need ventilation air gaps.

One other issue is due to lack of a vapour barrier beneath the insulant. You may find that the bottom of the insulant (or the body of it, if rockwool) will get soaked, ruining the plasterboard underneath. At least this would give you an opportunity to replace the old ceiling! (with foil backed plasterboard).

Christian.