metac 220 fullfill

getonin

Hi

Can you use metac 220 in 225mm deep rafters under a slate roof up against a black thick felt used in the build in 1993 if you use vario moisture control underneath and finish it with insulated slab? ie no air gap!

If so how has it worked out over the last few years?

CAn you use Knauf omnifit 34 instead?

Thanks

Penn

I doubt it. You're not providing sufficient airflow to the rafters, which means moisture will build up and potentially rot the timbers. Why not use 175mm rigid insulation between the rafters (allowing for a 50mm ventilation gap behind which should be sufficient) and then insulated board under (attic side) to achieve the required u-value and reduce cold-bridging?

Biker1

180mm Metac 34 or 180 Knauf Omnifit 34 have the same thermal conductivity and will leave sufficient air gap between insulation and felt, plus easier to fit than rigid insulation. Along with a 62.5mm insulated board to underside of rafters you would achieve a u-value of 0.14w/m2k.

getonin

Thanks for the comments.

I understand that a 50mm gap should be left for airflow generally.

However Metac says specifically on it's data sheet that it is suitable for fullfill rafter insulation with the use of certain membranes.

So my question really is, does anyone have any experience in using metac and completely filling the space and was this done in a roof with the 1993 era black felt?

Biker1

https://www.boards.ie/discussion/comment/118222495#Comment_118222495

No one can give a definitive answer to the question. It can take years for the rafters to show signs of rot due to lack of ventilation. I very much doubt if Metac or any other type of insulation was approved to be in contact with that type of felt.

Penn

https://www.boards.ie/discussion/comment/118222495#Comment_118222495

The type of felt that's allowable to negate the air gap would have to be substantially breathable. The thick black felt for the age of your house construction is unlikely to be that breathable.

Personally, I wouldn't advise it, even sight-unseen. There's little point when you're going to have to dryline the underside of the rafters anyway to minimise cold-bridging through the rafters. Just leave the 50mm air gap behind the insulation, and dryline underside of the rafter to achieve the required U-value.