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.

Rob Horton wrote:
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?

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

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

Not much. Its winpdproofing that ciounts for meore.

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?


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


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?


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

Thanks.

In article ,
Rob Horton writes:
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?

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).

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

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

I could improve things by using chipboard with some sort of foam
insulation bonded to the underside.

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

Will an air gap be needed between
this and the existing fiberglass insulation?

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.

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?

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.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]

Rob Horton wrote:
Just looking for a definitive opinion on the following situation(s).

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

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.

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

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

Indeed.

this and the existing fiberglass insulation?

Probably not.

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?

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.

--
Cheers,

John.

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