Glass Mineral wool for Roof

demontfort

Hello,

I had 50% of the slates on my roof replaced last year, the work included the addition of a breathable membrane. I now want to add insulation between the rafters to get building control sign off. I was planning to insulate using Knauf glass mineral wool between the rafters with a further layer of wool on top to meet the 300mm requirements. Does this sound reasonable? I want to be sure wool is ok  or should I use insulation boards? Do I need to leave a gap between the wool and membrane?

Thanks in advance

DM

FreeBear

Unless the loft space is being used as a habitable space, insulation needs to go between the ceiling joists. The loft should have plenty of ventilation to keep damp at bay.

demontfort

Thanks, the part of the loft I am insulating isn't currently being used although the rest of the loft is used as bedroom and was previously professionally insulated and boarded over. I would however like to start using the extra part as a habitable space hance looking to insulate the rafters and then board over the insulation.

grumbler

In this case, IIRC, it's 100mm or rockwool between rafters with 50mm gap beteween it and the membrane, plus 100mm of celotex(-like) under.

demontfort

Thanks both, you've helped me figure it out. Wool between the rafters with a 5cm ventilation gap to the membrane then board over the rafters with Celotex. That should keep Building Control happy.

ComicGeek

demontfort said:

Thanks both, you've helped me figure it out. Wool between the rafters with a 5cm ventilation gap to the membrane then board over the rafters with Celotex. That should keep Building Control happy.

You would need to size the insulation thickness to achieve the U-value required for building control sign off - that value will depend on what building regs edition it's being assessed against.

You might have to install Celotex between the rafters and below to achieve the value, or have a thicker insulation below the rafters if you're using mineral wool between rafters.

You don't want to guess and get it wrong.

FreeBear

ComicGeek said:

demontfort said:

Thanks both, you've helped me figure it out. Wool between the rafters with a 5cm ventilation gap to the membrane then board over the rafters with Celotex. That should keep Building Control happy.

You would need to size the insulation thickness to achieve the U-value required for building control sign off - that value will depend on what building regs edition it's being assessed against.

You might have to install Celotex between the rafters and below to achieve the value, or have a thicker insulation below the rafters if you're using mineral wool between rafters.

You don't want to guess and get it wrong.

I'd go Celotex between the rafters and underneath. Much easier to maintain that air gap consistently, and the boards are nearly twice as good thermally as fibreglass. So if 300mm of fibreglass is recommended, you only need 150mm of Celotex for the same u-value.

ComicGeek

FreeBear said:

ComicGeek said:

demontfort said:

Thanks both, you've helped me figure it out. Wool between the rafters with a 5cm ventilation gap to the membrane then board over the rafters with Celotex. That should keep Building Control happy.

You would need to size the insulation thickness to achieve the U-value required for building control sign off - that value will depend on what building regs edition it's being assessed against.

You might have to install Celotex between the rafters and below to achieve the value, or have a thicker insulation below the rafters if you're using mineral wool between rafters.

You don't want to guess and get it wrong.

I'd go Celotex between the rafters and underneath. Much easier to maintain that air gap consistently, and the boards are nearly twice as good thermally as fibreglass. So if 300mm of fibreglass is recommended, you only need 150mm of Celotex for the same u-value.

You would use a mineral wool batt between rafters though, not fibreglass. If you use a mineral wool batt with thermal conductivity of 0.034, then that would need 230mm to match 150mm of Celotex, or 270mm of fibreglass.

Using an insulated plasterboard with thermal conductivity of 0.018 or 0.022 would also change the thicknesses.

That's why the OP needs a proper U-value calculation to work it out, rather than guessing.

Establish the required U-value for the roof. Use the Celotex on-line calculator (if using Celotex products) to work out the thicknesses needed, and keep a copy of the calculation sheet to prove to Building Control.