Loft Insulation advice needed

[willardx]

Hello there,
I'm a bit confused! I've only got 20mm of fragmented rock wool insulation between my roof joists at the moment and thought I should save the planet (and a few quid) and get it sorted. I just had a surveyor bloke for an insulation firm round and he quoted about £220. Now the price is fine but what he said would be done was all my loft boards and junk removed and 250mm of fluffy stuff laid on top of the existing. This means I basically lose my storage space.
My joists are about 300-350mm apart and are only 100mm deep.
Ive looked at different products and the 2 options seem to be Screwfix Airtec double insulation - which is foil type stuff or B&Q Kingspan Thermaboard (in various depths). These could possibly be cut an put under the loft boards. Does anyone have any other solutions? Failing that does anyone know if either of these is equivalent to the required 250mm of fluffy stuff that is required or what thickness of Thermaboard would you need to get same insulation properties?
What do you pro's recommend?
Ta
Will

 

[JohnD]

If your joists are only 100mm deep and you like walking about up there, you might consider counterbattening it with more timbers laid at right-angles to the existing. this spreads the load when you walk about. You can put them at intervals that suits whatever flooring you use. If screwed together, and boards screwed on top, it will be quite a lot more rigid.

It is a bit slow and labour intensive so you could do it yourself or find a local carpenter.

If you were to do it yourself then you might as well lay the extra insulation yourself at the same time, and use rolls of it, or get your carpenter to do it if he is willing. Insulation contractors are quite cheap because they put very little effort into laying the stuff.

This would give you 200mm of insulation plus the boarding. The insulation effect of this will only very marginally less than the 250mm (because you get diminishing returns from each extra layer) and I would be (am) quite content with it. Someone may know a link to a website that calculates insulation values so you can see the difference.

Going from 20mm to 100mm will make a big difference

Going from 100mm to 200mm will make a small difference

Going from 200mm to 250mm will make a tiny difference

 

[Buzzark]

A layer of the thin mylar insulation would work fine under your floorboards. You can leave the existing insulation in place too.
I think it's equivalent to about 250mm of other insulation by itself at around 10mm think.

100mm ceiling joists I think are the norm but like the previous poster mentioned you won't want a lot of weight up there.

 

[willardx]

cheers chaps