best loft insulation for joists 330mm apart?

[Masterpop]

Hi,
I'm looking to insulate my loft with 100mm depth (glass mineral wool, wickes loft roll insulation base layer). However, my joists are 330mm apart whereas the roll is suitable for 400mm and 600mm apart. Can I still use it? ie. can i put the 400mm width into 330mm, or should I look for a wider piece and cut to size?

also, has anyone used the wickes loft roll insulation? if yes, how does it perform?


many thanks in advance.

on top of the joists, i intend to use slabs of insulation with chipboard on top.

[Perry525]

If you use polystyrene it comes in 100mm/4inch thick sheets that are 8 by 4 feet, these will give you a better result and they can be cut with a knife or saw making it a tight friction fit.

The problem with your proposal, is that for an insulation to work, it needs to be a tight fit between the joists, and you cannot do this with fibre glass.
Fibre glass is transparent to water vapour and both water vapour and air can and does move through it, taking the heat with it.
If you have a situation where water vapour is rising through the house, passing through the plasterboard ceiling and into the fibre glass the water vapour can freeze inside the fibre glass.
When the ice melts it makes the fibre glass wet, the firbre glass then acts as an express route out for your expensive heat.
Whereas, a closed cell product like polystyrene or other foam boards is water proof and wind proof resulting in a far better result.

[stoneyboy]

Masterpop,
Go to your local builders merchant - they will probably stock 1.2m wide rolls which you can then cut to fit your joist spacing.
end

[ajrobb]

[quote="Perry525"]The problem with your proposal, is that for an insulation to work, it needs to be a tight fit between the joists, and you cannot do this with fibre glass.
Fibre glass is transparent to water vapour and both water vapour and air can and does move through it, taking the heat with it.
If you have a situation where water vapour is rising through the house, passing through the plasterboard ceiling and into the fibre glass the water vapour can freeze inside the fibre glass.[/quote]

Fibre glass or mineral wool are commonly used as loft insulation without a significant vapour barrier. I think this requires a well ventilated loft so the moisture coming from the house can be diluted with dry air within the insulation BEFORE it can condense. If you propose to block in your wool with closed cell insulation (or boards) on the cold side, I think you MUST have a solid vapour barrier on the warm side.

I agree, use a closed cell insulation between the joists to give you a vapour barrier at the warm side.