How much can I compress loft insulation?

[Simplez]

Current roof joists are 8.5cm, and loft legs come in 17.5cm and 30cm. Loft insulation is 27 or 30cm

If I go with the shorter loft legs that will give me 26cm so will need to compress the insulation about 4cm or 13% or 1cm if I go for 27cm insulation.

Or I go for the 30cm legs and leave a 8.5cm space under the boards and lose that in height.

Really unsure whats best to do.

Also as to loft insulation how does Knauf roll insulation compare to Rockwool? As its about 1/3 the cost.

 

[endecotp]

You can easily compress regular loft roll by that much, but note that its insulation effectiveness will decrease in the same proportion.

 

[Simplez]

You can easily compress regular loft roll by that much, but note that its insulation effectiveness will decrease in the same proportion.

But should I have an air gap above the insulation and below the loftboards to stop condensation forming?