"Dave" == Dave Liquorice writes:

On 18 Jun 2004 15:42:44 +0100, John Richer wrote:
(3) Make the joists deeper by adding 2x2s or 3x2s on top to get
more insulation, then reboard. This is probably too hard, and
I'm hoping 4 inches of insulation is enough.

100mm is pretty thin by modern standards I think they recomend
200mm these days but that would be open on the exposed sides the
boarding will have some insulating effect as well.

It's more faff to lay extra timbers but on the whole probably
better.

You want to lay them across the existing timbers not along. Fill
to the top of the existing ceiling joist with glass fibre or
WHY. Lay the new timbers then fill to the top of them. This means
that the only direct through timber path from the ceiling to
flooring is the small areas where the timbers cross rather than
the full area of each joist. Laying the new joists cross ways
also helps to spread the load across the whole ceiling as
well. Ceilings are not normally designed to take much weight...

....but I think I am OK for load bearing in this case: it's a 3-storey
Victorian house and the joists in the ceiling of the 2nd floor look
closely spaced and large. The boards are also original Victorian in the
main so I think it was designed to be a storage space.

So your interesting idea of cross laying new timbers and insulation is
intriguing - also means it wouldn't be too hard to access the ceiling
for maintenance. But maybe 100mm celotex is enough.

If you use celotex or similar you need to arrange ventilation
between it and the ceiling to avoid possible damp problems. Glass
fibre is breathable.

Thanks for raising this - but is it really an issue in this location - a
ceiling void? I'll look at the celotex literature... If I really need a
50mm air gap there then I am only going to squeeze in 50mm of
insulation...