David Moodie wrote:

"Dean Richard Benson" wrote in
message newsan.2004.01.21.18.28.06.252332@spamlessharker hill.co.uk...

On Wed, 21 Jan 2004 16:29:12 +0000, Christian McArdle wrote:


Yes. The law of diminishing returns says that 350mm is the best
environmentally speaking. Any more and the environmental cost of
manufacturing and transporting the insulant is higher than the energy

saved.

100mm is definitely on the low side. 200mm would be better. The

difference

between 200mm and 350mm isn't that great.

Some useful info in there. Leads me to even more questions....

- with currently having a 100mm joist height, should i either increase the
joist height (cross joists) or just cross-lay the insulation over the top
of the 100mm.

- If you insulation up to 350mm, how are you meant to ever reach places in
your loft without falling through the ceiling ;P



The problem is that with a 100mm joist height you aren't meant to store
anything, other than insulation in your lost. Its one of those cases where
the builders use the absolute minimum they can get away in order to save a
few quid and you end up with some valuable storage space that you
effectively can't use!



Yes, and that is where cross laying more joists , maybe herringbone
bracing and a few hangers up to teh ridge/radfters makes sense. Total
insulation depth comes up to maybe 8 inches with only where the joists
cross as 'cold bridges' and boardiong out the loft stops draughts getong
in any less than perefectly insulated places.

I have just complete boarding out my loft with 150mm insulation
(rockwool) and its made a lot of difference. Its icy up there when the
wind blows (silly building regs vents are enough to take every ounce of
moist air out in about 1.2 seconds). Once you get to this sort of
insulation depth, the real danger is tiny cracks that allow air movement
alongside celotex and through the rockwool.

If yu have as I have, hollow rockwool filled walls, you then discover
that things like e.g. power sockets, or the odd crack round a window
frame, lets in a tiny icy draught...from air moving THROUGH the
insulation and getting into the rooms.

Going to IMMlike levels of insulation only works if you can stop up each
and every one of these micro draughts: This takes patience, and
decorators caulk and frame sealer inside, and attention to detail up in
the loft as well.

I sometimes wonde if much more limited ventialtion, and an electrical
dehumidifier in the loft might not be a more energy efficient way to
keep a cold roof dry, and allow the loft airspace itself to become an
insulator...



cheers

David