Yet another loft insulation boarding question

[Rob Horton]

Just looking for a definitive opinion on the following situation(s).

My loft has only 100mm of fiberglass insulation between the joists. The top of this insulation is, obviously, not flat. Some times the insulation is little bit above the joists and sometimes below.

I need to use the loft for storage and am considering laying chipboard across the top of the existing joists. Only part of the loft will be covered. The existing insulation will, essentially, be in contact with the chipboard. There will be no designed in air gap between the insulation and chipboard. Is this ok?

As I understand it, the chipboard will also add a little bit of extra insulation itself.

I could improve things by using chipboard with some sort of foam insulation bonded to the underside. Will an air gap be needed between this and the existing fiberglass insulation?

Would it be better to use something like celotex beneath ordinary chipboard and would the foil covered side need to be face down in contact with the existing insulation?

Thanks.

[The Natural Philosopher]

That's actually advantageous in a 'cold roof' with ;lots of ventilation. Reduces airflow through it.

Not much. Its winpdproofing that ciounts for meore.

Nope. And a good idea. Or cross batten the joists, add more rockwool, and chip over that..

Foil irrelevant in this case. Celotex is the best insulant per unit thickness there is.

[Andrew Gabriel]

The chipboard will become the cold barrier against the insulation, so that's the point where any moisture which comes through the ceiling will condense. If you have a vapour barrier on the warm side of the insulation (foil-backed plasterboard), then you won't get any moisture coming through, so it won't matter. At the other end of the scale, if you have a cracked lath and plaster ceiling over a kitchen or bathroom, it almost certainly will matter. If it's a plasterboard ceiling over a room which isn't high humidity, then the moisture penetration rate at the ceiling will probably be close the the moisture penetration rate of the chipboard without damaging it. So it all depends.

Other options are to raise the floor with air flow under it, or to make a slatted floor so it's not airtight (but you also need to make sure you don't make it airtight with whatever you pile onto it).

Probably quite insignificant compared with even a small ammount of insulation.

You could, particularly if you want to get your loft insulation nearer to current new build standards.

Probably not. The key thing is if the temperature at the boundary can drop below the dew point of the air there, but even if it can in theory, it's going to struggle to lose the latent heat energy resulting from condensation and stay that cold, in the middle of thermal insulation. Will depend on thickness of the foam.

The foil should face down (although all the stuff I've used has foil on both sides).

Other thing -- be aware of what weight loading your loft floor will take. It's probably a lot less than you might imagine, if you're thinking of piling it up like you could a room.

[John Rumm]

or better - several definitive opinions - all different! ;-)

Some, but not worth writing home about in the presence of real insulation.

Probably not.

This would be the simplest solution. Foil faced PIR foam has quite good crush resistance in these situations, so simply lay it over the floor and screw your chipboard sheets to the floor joists through the insulation.

If you go for "seconds" or similar then you could probably do most of a loft for a few hundred quids worth of insulation.