Fibreglass insulation and boarding a loft

[Chris Hogg]

Modern fibreglass loft insulation stands proud of the rafters by a couple of inches or so. This means that if boarding out the loft for storage etc. the f/g insulation inevitably gets compressed, and I assume becomes less effective. Is there a way round this, or doesn't it matter that much? Is there any insulation board that is more efficient than f/g roll, thus providing the same degree of insulation but with less thickness, that could be laid between the joists under the boarding (e.g. Kingspan or similar)?

[Dave Liquorice]

Only if you have stuff that is thicker than the raters are high... You can get it in 100mm thickness. In my mind I'd rather have something like fibre glass(*) against the ceiling than a rigid insulating board. The thinking being that there could be a clear gap between the ceiling and the board allowing a wind to whistle through taking away heat. Cutting an fitting board to be snug would be slower than something soft. Over the top of that lay celotex type panels and board over the top of the that for walking/storage.

(*) Hate the damn stuff it's terrible to work with unless you completley seal your skin away from it. I'm going to look hard at:

a view to replacing all the grotty FG here with Thermafleece and boarding out some of the loft spaces. But as it is wool it "breaths" quite a bit and I'm not sure if it's suitable to be boarded over. They are also quite close so may be able to get a good price if I collect...

[John Rumm]

I am assuming you mean joists here and not rafters (i.e. the timbers that define the slope of the roof)...

Compressing the fibre glass will reduce its effectiveness a little, however its insulating properties are not just down to the entrained air as many would have you believe, so reducing the height of 6" insulation to 4" is not the same loss in effectiveness as only having 4" in the first place.

There are other more effective materials. The rigid PIR foam based boards (Celotex, Ecotherm, Kingspan etc) perform significantly better for a given thickness. However they are harder to fit between the joists (would need to be individually cut in), and cost a fair bit more. You could however lay a thin board (say 30mm) right over the joists and existing insulation and screw the loft boards down to the joists through the insulation. The insulation is rigid enough to resist crushing (the same construction technique is frequently used on "warm deck" flat roofs)

[Chris J Dixon]

When only a portion of the loft is boarded, and that is covered with filled plastic storage boxes, cardboard boxes, rolls of carpet offcuts and so forth, how much less insulation does this provide than additional fibreglass?

Chris

[mail]

Dear Chris Your post and the replies imply that you are content with 4" of compressed insulation. Part L (which is pretty c*** anyway) requires

300mm. This is best done 6" in the rafters and 6" at 90 degrees. Be very careful of so-called natural products. Biodegradation risks are considerable apart from the counter argument that to harvest wool off a sheep is natural! (above the LD50 for mammals, the natural insecticides are just as lethal as the un-natural but I don't want to go there on this post!). So were I you I would make a platform for the stuff I wanted to store and limit it considerably as 4" CJs are not designed for much other than self-load and plaster. If you are determined only to have 4" and are prepared to put in the time I would use he Kingspan type PU foam and spend the time fitting it with a foarm gun for the gaps. The other suggestioni of combining what you have there and sheets cross wise over the top would be much much better chris G

[tony sayer]

So how does it work then?...

[John Rumm]

IIUC the material itself is a reasonably decent insulator. (you can get a rigid insulation that is made by compressing and heating layers of fibreglass "wool" such that it becomes a board not unlike celotex etc)

[NOSPAMnet]

Is any insulation effect you might lose by it being compressed not made up for by the flooring you are putting on top of it plus all the other stuff that will be put up there... boxes etc etc etc.?

[Dave Liquorice]

Yes glass does not conduct heat well. If you ever do any glass work with heat involved just beacuse you can safely handle it a one point don't assume you can an inch away. I have had the burns...

[dennis]

Glass is a poor insulator when it comes to keeping heat in. Just feel how cold a single glazed window is in winter and watch the ice form. Nearly all the insulating properties of glass and mineral wool does come from the trapped air the same as it does from double glazing. You need to have a minimal amount of actual fiber conducting heat through the insulation for it to work well, that is not compressed much so the fibers make as little contact with each other as possible while still being dense enough to stop any air flow.

[meow2222]

Options:

1. squash insulation, then add more on top of the boards, or dont. Puts force on the PB though
2. Fill to the joist tops, board, put more insulation on top or not
3. Add timber to top of joists to raise their height. More insulation then fits, plus nothing need lie on top of the boards. Also more wood means less flexing of the joists. However a fair bit more work.

When loaying insulation on top of boarding, bear in mind that when you roll it back to store stuff, the stored goods in boxes also insulate.

NT

[Chris Hogg]

Many thanks for all the replies and suggestions.