PIR insulation between joists

bpj · 22 September 2021 at 11:56AM

Hello all,

Our loft insulation is currently inadequate (100 mm of slightly sorry looking glass wool), and I'd like to do my bit to preserve gas in these trying times.

I plan to add another 200 mm or so glass wool in the eaves, taking care not to restrict airflow. I would rather not raise the height of the boards in the centre of the space, so I'm thinking of using 100 mm PIR boards instead for this area.

I appreciate this is a more expensive option, but is there anything else I'm missing? In particular, I know it's important to leave a gap for air flow when adding PIR to rafters. I assume this isn't necessary for the joists, given they're less likely to get damp and don't have any air flow in the first place.

Thanks in advance for any thoughts.

FaceHead · 23 September 2021 at 3:33PM

I'm in a very similar situation, so am bumping your thread to encourage participation for my own ends.

I also have 100mm of glass wool that has put quite a shift in already. Agree that +200mm is ideal, but with little head height, I'm loathed to raise the floor. I (1.8m) can just about stand up right on the ridge line of the roof - raising the floor 200mm would make it a hunching around job.

PIR does seem to be prohibitively expensive, however, as:
200mm Glass Wool @ £3.44 /m2 Isover Spacesaver Loft Roll Insulation Glass Wool 200mm - 4.5m2 Pack | Insulation Superstore®

100mm PIR @ £17.99/m2
EcoTherm Eco-Versal General Purpose Insulation - 2.4m x 1.2m x 100mm | Insulation Superstore®

For my loft which is about 5m x 7m (35msq), that's then £120 vs. £630.

In fact, I'd like to remove my 100mm glass wool and put down 150mm PIR (roughly equivalent to 250mm glass wool) @ £25.97 /m2, so now a lot of mess and a slightly unpalatable cost of £907
EcoTherm Eco-Versal General Purpose Insulation - 2.4m x 1.2m x 150mm | Insulation Superstore®

Agree that it seems to be purely cost that prevents people from doing this.