Will Replacing 100mm Fibreglass Loft Insulation with 100ml PIR Board Suffice?

[uptoyou]

Hi all,

Lived in house 15 years and loft insulation is still from when we moved in (house is 1970s); very patchy and the loft floor has been partially boarded with whatever wood the previous owner had laying around.

Joists are 50 x 100 and insulation is fibre glass (see picture?), absent in places, about x 4 high in other places - all over the place!

I have a very small ceiling height in the loft and need it for storage.

Q. Can I replace the existing insulation with 100mm PIR board, which will take it to the top of the joists, and board straight on top without need for any other insulation on top?

*I if I had to go higher and maybe put loft legs down, the additional height would make access towards the area where the joist meets the rafter almost inaccessible and most of the loft space unusable so I'm hoping just 100mm PIR will at least improve the insulation slightly.

Thanks everyone!

 

[cdbe]

See if you can stretch to 100mm mineral between the joists and 100mm PIR on top. You can board over the PIR with practically any thin sheet material, eg 6mm plywood.

In the eaves you could attach raised shelves to the trusses and fit 300mm mineral wool under.

 

[uptoyou]

See if you can stretch to 100mm mineral between the joists and 100mm PIR on top. You can board over the PIR with practically any thin sheet material, eg 6mm plywood.

In the eaves you could attach raised shelves to the trusses and fit 300mm mineral wool under.

Thanks cdbe
Issue I have is that the roof at its highest point from the top of the joist is only 1.16m, so 100mm PIR effectively takes me to 1m at the highest point and barely any room to get in and around.
I could possibly go 25mm PIR on top if you think the insulation benefit of PIR that thin is worth the cost?

 

[bsr]

Having a second layer of insulation over the top makes a big difference as it improves air tightness and reduces cold bridging (the heat coming through the rafters).

 

[uptoyou]

Having a second layer of insulation over the top makes a big difference as it improves air tightness and reduces cold bridging (the heat coming through the rafters).

Thanks bsr - would 25mm PIR on top of 100mm mineral be worthwhile when considering cost v additional insulation value?

 

[cdbe]

Better than nothing - I would PIR between instead of mineral wool if you can only put a thin layer of PIR on top.

 

[uptoyou]

Better than nothing - I would PIR between instead of mineral wool if you can only put a thin layer of PIR on top.

So 100ml PIR in the gaps and then lengths of 25mm PIR on top?
Did I read somewhere I need to leave a 25mm gap between the floor and the bottom of the PIR, meaning it would sit 25mm proud of the joists?