Loft conversion insulation

[irlontop]

Hi There,

I'm a bit lost and hoping for some advice, about to convert our attic to an office essentially,

I am being advised numerous options for insulation,

Ceiling insulated with Super foil multi layer insulation, 52mm warm board & plastered.

Walls insulated with 150mm Earth wool insulation, plaster boarded & plastered.



Does anyone have experience with the multi layer foul insulation?

Would I be better off going with another option?

 

[freddiemercurystwin]

Multi layer foils are a misnomer, you need an air cavity above and below negating any potential space-saving advantages.

 

[irlontop]

Thanks for reply,

so by the time you ensure the airgaps are there there isn't any space saving and therefor some form of PIR board is a better option?

But it is easier to install and quicker? as not cutting the insulation board to fit between each rafter.

 

[freddiemercurystwin]

Correct though typically you'd need to do PIR between the rafters and a continuous layer of either PIR or multi foil beneath.

 

[irlontop]

Thank you,

my rafters are only 100mm in depth, so not much room to put PIR board in there as I need a 50mm airgap also,

What was i was hoping to do was the multifoil then battens (for airgap, then 57.5mm warm board onto the battens, which is about 50mm of insulation,

I guess I could stuff half of the rafters with some earthwhool?

 

[geraldthehamster]

I think Building Regs currently specify 175mm if using board insulation (Celotex, etc). Though presumably you're not aiming to do a Building Regs loft conversion. technically you only need an air gap for older style impervious roofing felt, and it's not essential if it's a modern breathable membrane. You could fit some between the rafters, and the rest over. 50mm between and 100 over, or vice versa, would make it pretty well insulated. No point doing a half arsed job

 

[JohnD]

It's easier and cheaper to use mineral wool, and it is better for blocking draughts and fitting closely, but it only has about half the insulating power as the same thickness of foam slab. So it will not usually give enough insulation.

 

[^woody^]

You won't beat PIR board for performance for any given thickness of insulation layer. Cut it carefully, seal the gaps and you won't go wrong.

Air foil is expensive per m2, requires more care (more to go wrong) and still has an element of snake oil about it. It may still require greater depth of the insulation layer and might need supplementary quilt/board insulation.

Quilt of any kind, whilst cheaper per m2 needs greater thickness to perform as well as PIR, and is impractical for any layer across the underside of rafters or inner face of eaves walls.