insulating a room, part 2

[js.b1]

Ah, bats do the following...

"any water penetrating the outer leaf will drain down the surface of the batts. The direction of the grain of the mineral wool ensures that the insulation will stop water crossing the cavity to the inner leaf."

Chopped mineral wool is thus most likely treated because the fibre orientation can not be guaranteed. That said there have been reports of it becoming saturated... too much cement in the mortar & non-porous bricks, gutter fault, sarking felt rotted and allowing water into non- sealed cavity, or simply wind driven rain which is problematic for mineral wool. Poly bead might be more tolerant, but in those situations internal insulation and an empty cavity seems better albeit hugely more expensive and disruptive.

[stuart noble]

I've grown geraniums in Rockwool bat offcuts :-)

AFAIK the only waterproof insulation is the closed cell foam, such as Celotex. I don't know about polybeads. Certainly the individual beads would be but en masse they may well hold water by the J Cloth method.

[js.b1]

Anything with an open cell structure can hold water.

The idea of water repellence on chopped-wool is to prevent wicking/ capillary action, I believe they spray polybead with same.

What surprises me is the wool cavity bats rely on strand orientation - most likely up/down? In which case what is the betting that some are miss-installed, never mind gaps left which are a more serious known problem re water tracking.

I'd prefer PIR bat in a cavity with air gap to the outer leaf, closed cell, water can still run down the rear of the outer leaf.