I am putting 300mm celulose into the walls of a timber new build, Will it sag over time and cause a void under the windows and at the top of the walls or does it stay put? Any thoughts

I'm intrigued that your walls are 12" thick - this is more than twice the thickness of low energy housing in Canada. Sounds like a lot of extra material will be required for little benefit. If you need high insulation levels, use a better product than the cellulose and you'll save in many ways.

Paul in Montreal.

Warmcell. Fitted by them via the wet blown system.
I want breathable walls and plan to use feather edge boards on the outside with a breather membrane. Then the 300mm warmcell with unfired bricks and clay plaster on the inside. Hopefully a low carbon foot print and quite unexpensive to build?!?

Here's a reply from Laren Corie on the Yahoo LittleHouses group to my question about whether you let cellulose settle then top it up or something:

" That is not the way it is done. The natural settling density
of cellulose is between 1.3 and 1.6 pound per cubic foot. So, by
"Dense Packing" it into walls and cathedral ceilings, at between 3
and 3.75 pound per cubic foot, you can assure that any vibrations
will expand it rather than settle it. It is like an over-stuffed pillow
that will push out its stuffing if ever opened. Over time, it expands
and equalizes, into every tiny crevice in the wall. It expand instead
of settling.

" This, "Dense Pack" technique uses, on average, 2 to 2.5 times
as much material, so it is not without cost, but there are other very
important benefits to "Dense Pack" One is that it seals against air
leakage, close to as well as spray foam insulations, but at a far lower
cost. The other is that it can absorb large amount of moisture without
any significant effect. This also serves to save other materials in the
wall from water related damage. In tests at the University of Colorado,
cellulose was shown to be far more effective than fiberglass of the same
Rvalue (about 25% IIRC). Also while bat insulations, like fiberglass,
rockwool, etc, loose as much as 50% of their 72°F rated insulation
value, as temperatures drop to -20°F, the Rvalue of cellulose actually
increase (~20% IIRC). So, as temperatures drop, and you need
insulation the most, fiberglass performs worse, and cellulose performs
significantly better than its rated RValue.

" In the real world, moisture in walls is a major issue, and the fact
the dense pack cellulose can absorb over a cup of water, per cubic
foot, without even a significant change in Rvalue, make a real
difference, that does not get included in any of the Rvalue
comparison number."

Unfortunately, I left his group early in the year when it got overrun with political comments. Pity as there was good information there.

It would, perhaps, have been more accurate to say "Ed Davies quoted" than "Ed Davies wrote".

"Have I been totally unobservant? I'm sure this is the first time I've ever seen a statement about R value varying with ambient temperature. Can anyone explain (in simple terms) the mechanism behind this?"

It seems to be to do with convection kicking in at a certain temperature gradient.

http://www.homeenergy.org/archive/hem.dis.anl.gov/eehem/92/920510.html

No, I don't know why cellulose improves. I think the text from Laren Corie I quoted is actually discussing dense packed cellulose rather than loose fill. Perhaps it's to do with water vapour condensing out and being absorbed, somehow. There's perhaps some risk of getting into the static/dynamic discussion here but any ideas, anybody?

NO !!!

I would always advocate using a vapour barrier and it seem that you are too. No way would I risk there being a problem in a wall with condensation.

When breathing walls were first introduced they too had a vapour barrier. At the time it seemed to me to make a nonsense of the breathing bit.

There are also celulose batts available, which might be better if you're worried about the loose fill sagging. E.g. Vital, http://www.benkoski.co.uk/images06/pdfs/VITAL_Spec.pdf. Same performance as Warmcel.

quick check on the website says only 600 width, no 400. Does say special dimensions to order so maybe UK importer has arranged that as it is standard rafter/joist spacing in UK?

According to the Vital website for another language, they do make it in other sizes as standard.