Question regarding passive roof construction

Eoghan Barra

Evening all,

Following the advice of others who posted in this forum, we are having the space between the rafters in our new dormer pumped with cellulose. The inside of the rafters are sheeted with osb, taped at the joints as an airtight barrier. We're also putting in a service cavity, with 50mm of rockwool flexi between 2x2 (horizontal) latts @ 400mm centres, which will then be slabbed.

My question, which is really for anyone who went with this type of roof build-up, is this: how did you keep the rockwool from falling out before slabbing?

Our roof pitch is 35°, which possibly doesn't help, in the sense that the steeper the pitch the more likely that the press-fit insulation will stay in place by itself (?).

Any suggestions welcome.

harry21

Hey, I've never done this but maybe a spray adhesive applied to the OSB and then place rockwool.

Is there an ption to replace roakwoll with rigid insulation?

sas

We nailed 2*2 to the rafters increasing the depth for cellulose and then didn't bother with insulation in service cavity.

With your build up the point of rockwool flexi is that it's supposed to hold itself in place.

Eoghan Barra

Harry 21, already have rockwool, and wouldn't want to use rigid insulation anyway.

The adhesive spray is a useful suggestion though, thanks for that. What brand/type of adhesive spray would you have in mind?

brophis

I have a similar buildup in my timber frame. 50mm service cavity with 2*1.5 laths at 400 centres. The 400mm wide rockwool flexi was rigid enough to stay between the battens even on the ceiling. Some of the slabs of insulation were a bit floppy and usually a dry wall screw or two was enough to hold it in place before the slab went up.

A ball of twine does re job nicely either but more work in it

Eoghan Barra

Thanks very much for your interesting reply Brophis.

But I must admit I'm a bit baffled; the rockwool flexi batt I tried kept just flopping out each time I tried to install it, yet you even managed to get it to stay put in a ceiling? There must be something to it that I haven't copped.

And how does the twine thing work?

brophis

Eoghan Barra wrote: »

Thanks very much for your interesting reply Brophis.

But I must admit I'm a bit baffled; the rockwool flexi batt I tried kept just flopping out each time I tried to install it, yet you even managed to get it to stay put in a ceiling? There must be something to it that I haven't copped.

And how does the twine thing work?

How rigid is the stuff you have? You could pretty much stand the stuff i had without it falling over. I've found it can vary a from pack to pack, I had to hand pick some of the more rigid ones for ceiling and some of the less rigid ones went in the walls.

The distance between the battens was less than 400mm whereas the flexi is 400mm so it fits in snugly. I found packing/curling in at one side worked best rather than let it curl out at the middle as it was more likely to fall.

As for the twine, my battens are fixed at 90 degrees to the studding so its easy run twine in behind them and then out in front of the flexi then back in behind the next batten in a zig zag pattern.

Eoghan Barra

Thanks again for your reply Brophis.

I'll try some of the other batts, maybe from different packs, and see if I have any more luck with them.