Is there any reason not to use cellulose fluff in cavities? The website FAQ for 'Thermofloc' says it can't be used in cavities, but I've just taken my edge-tiles off to expose the top of the cavity and there was some gappage in the glasswool fluff that is in there. I have a bag of thermofloc to hand to top it up and can't see why it shouldn't work. Once above the skeiling level it's not actually doing any insulating - it's just filling in the space to stop air circulation. It might get damp if the brickwork lets through in driving rain, but then so would the glassfibre, and what's in there is totally dry.

Is it to do with slumping over time because it's relatively heavy?

I recall that for filling timber cassettes they wet it to make it 'stick' in place (even when dried out again - papier-mache style).

The bag also says it is not suitable for blowing, which also seems odd. This stuff is routinely blown into timber cassettes isn't it?

I do like the way it comes as a compact and quite solid block which you have to break up in a box with a paint-stirrer.

I couldn't find a supplier of glassfibre 'bits' fill at all. I could find EPS balls but I'm not a huge fan of those as they tend to escape. And there was plenty of mica, but that's quite poor insulation.

Posted By: wookeyIs there any reason not to use cellulose fluff in cavities?

No doubt the main issue would be a high risk of deterioration, or worse, due to damp. It's startling to see how much water can run down the inner face of the external leaf in heavy rain.