Increasing Loft Insulation

21 Nov 2011
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United Kingdom

I have recently moved into a 1970's house and there 100m loft insulation between the joists.

I want to increase the insulation but also board the loft so can use it for storage. In my previous house chipboard planks were screwed to the joists on top of the insulation, but the insulation did not go higher than than the joists.

These days I believe 270mm of insulation is recommended, I am not keen on placing insulation over\across the joists as would need to increase the height of the floor significantly,

Could I increase the insulation to match the height of the joists and then lay preinsulated floorboards such as below on top of the joists?

Has anyone done similar or got any other suggestions? The loft is big as moved to a four bed house and I want to retain the balance between storeage area, insulation and cost of course.

Thanks in advance.
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How deep are your joists? That board will be better than nothing but I couldn't see what the insulation material is and no clues as to the U value, at £30 per square metre it is a fairly pricey fix (plus, of course, you'll be losing headroom).

If you're not going to store ocean going liners up there you could increase the depth of your joists (using ripped 1" plywood nogged at suitable intervals), fill the gap with loose fibreglass (not as cheap as it used to be but still only about £5/sq metre of 150mm) then slap standard chipboard on top. More work but quite a cost saving if you have a 20 sq m loft
I believe the B&Q product is 100mm expanded polystyrene glued to chipboard, in small panels which are presumably easy to carry into the loft. They work out very expensive.

A comparable method which I think better, is to lay slabs of Celotex or Kingspan over the loft joists, then flooring in chipboard or ply over that, screwing it down to the loft joists with long screws. Short joints must be over a joist or nog, unless you think you can trust glue.

You might be able to carry 2400x600mm sheets into the loft, I can, just. It is easier if there is someone below pushing them up to the hatch.

Celotex and similar have about twice the insulating power as the same thickness of mineral wool quilt.

Take a vacuum cleaner up with you as the materials are very dusty if you have to cut them.

There used to be a Knauf installation guide on YouTube for this method (or it might have been one of the other makers). Their "Supadeck" product gave a U-value of 0.16 W/m2K when laid over 100mm mineral wool between the joists, which meets modern standards.
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