Insulating my loft??

4 Sep 2013
Reaction score
United Kingdom
I want to insulate my loft , I have increased the height of the joists from 80mm to 140mm and thrown away all of old mineral wall as it was damp and not in great condition. I have fixed all damp issues now.

So also want to board over joists for light storage using 22mm chip board. I therefore have 140mm depth for insulation. I know if you use mineral wool only the level required is 270mm. Obviously I cannot do this but want to retain the same insulation value that 270mm of mineral wool would give me.

My suggestion is to to use 100mm of mineral wool as a base and then 50mm insulation board (celotex) on top of this pushing down the mineral by 10ml. I know you shouldn't squish mineral wool too much but I hoped if it was only 10ml this would be ok.

So would 100mm mineral wool with 50mm celotex board on too be as efficient as 270mm mineral wool?
Sponsored Links
I should think its reasonably good but I'd guess it is slightly less insulating, you could do the U-value calcs and work it out.

It may be better to celotex with quilt over.
Ok, so celotex down first and them mineral wool on top.

Do you know what is best to seal joints between joists and celotex?

Also I plan to have led down lights so need to look at how to insulate around these. I'm planning to put 22 mm chip board on top too so this should help.
Sponsored Links
I'd just cut the boards with care and if needed shove the dust/trimmings down any gaps, you could foil tape here if you wished but I would not bother, just foil board to board.

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local