"Set Square" wrote in message ...
With regard to the insulation, how tall are the joists? If it's an old
house, they're probably no more than 100mm - if that.

It's a 1950's ex council house - I believe the joists are 100mm, and
the insulator guys were going to lay the insulation over the joists
(at right angles).

There's no point
whatsoever putting in 350mm of insulation if you are going to compress it
into 100mm. 350mm of insulation is only effective if it traps a 350mm layer
of air! Admittedly the boarding will add some insulation, but you really
need to pack the joists up as high as possible with something solid to
support the boards.

I absolutely agree with the physics of this, however from my point of
view I can get the whole insulation done in the next few weeks, and at
some point later in the year I can board over only a relatively small
portion of the loft, (around the hatch and tanks etc). So, 100mm of
compressed insulation is not as effective as 350mm uncompressed, but
taking that as given, and going back to my original question I still
wonder about the sensibility of screwing the boards to the joists
*through* the insulation, (which I am guessing will be compressed to
about 20-30mm at that point). I then end up with 20-30mm compressed
insulation *and* a board above a joist which has got to be better than
just the board alone????