DIY Doctor UK DIY Forums

I have the issue in that I have a section of sloping cieling to the front and back of the house following the slope of the simple dual pitch roof which runs laterally across. So at the moment am looking to upgrade the insulation in that area...despite being c 1990 I believe there was very little if any insulation provided in this gap when built...but it looks like the last owner tried to fit some PIR board into the small space ...but as stoneyboy predicted the fit is very poor and in this draughty area I suspect they are doing virtually no good at all...
I am contemplating stuffing loft roll down the gaps, after removal of the PIR boards, which I think is doable...what will be problematic is keeping a gap for ventilation...the ovelal depth of the sloping cielings is about 1.5 metres and I haven't found a bespoke product to suit the circumstances....at the moment I think my options are:
1) do nothing, not very satisfactory
2) Stuff rockwool down the gaps, make up for loss of ventilation by putting some air bricks in the two gable walls (its detached) or getting those devices which allow ventilation in the laps of the sarking felt
3) Use profile box section plastic roofing sheets to put under the felt prior to filling with insulation, either between every rafter (expensive) or say every 3 or 4,
4) Sod it, just fill the gaps with earth wool and then monitor to see if condensation occurs on the roof space at the most likley times (winter)...i.e. inadequate ventilation.
Access to the gaps is even more of a problem as there are large purlins at about the point.
Anyway any advice very welcome and thanks for your time in reading this...
Regards
Graham

Moderator note: Split from old post.

Hi ghh,
However you try to insulate the gap from within your loft you are likely to end up with gaps which may allow cold bridging on the ceiling below.
You will have most success by stripping off the tiles battens and felt and then fitting PIR insulation sealed in place with expanding foam. PIR slabs have much better insulation properties than Rockwool. You can either use slabs 20mm thinner than you rafter height to maintain ventilation or fill gap completely and fit air bricks in the gable ends.
An alternative is to insulate inside the rooms with the sloping ceilings using PIR backed plasterboard.
There is no easy solution.
Regards S