Insulation question (vaulted ceiling)

SodiumCooled

Hi, we will soon be having insulation installed in a new build. We have a vaulted ceiling in one location that will require a mix of wool insulation between rafters and an insulated slab. I will be using a fairly large and reputable insulation company to do the work but while I am not an expert I am slightly at odds with one aspect.

The rafters are 7x2 so approx 178mm and they want to maximise insulation between rafters and want to use 150mm of Metac. I asked about maintaining a 50 mm air gap and installing vent cards etc but I was told there is no need with Metac as its breathable? Is there any truth in this as I thought an air gap is important. As above these are not some cowboy outfit and would insulate a large amount of the builds in the region (new and upgrading).

Any advise would be appreciated. Thanks.

writetojd

Apologies double post. BS5250 allows you to omit the vented cavity if you have a breathable roof covering, breather felt and natural slate or concrete tile. Manufactured slate requires battens and counter battens.

SodiumCooled

Thanks, we have breathable felt and roof is battened with 2x2 but not counter battened. Slates are fibre cement slates. Maybe they are assuming it's counter battened as it wasn't discussed.

SodiumCooled

I spoke with the insulation company again last week and they said that in their opinion there is no need for cross battening to install 150mm metac in a 178mm rafter - breathable felt is sufficient as the metac is fully breathable and they install in this way all the time. He also said metac will say the same. Confused now as these are supposed to be the experts.

writetojd

Its not about the insulation or the felt being 'vapour open' (breathable is a very loose term) its the roof covering that is the issue. Manufactured slates are uniformly made and closely spaced therefore don't allow the same airflow as concrete tile or natural slate in order to offset condensation risk. 50mm needs to be maintained below the felt. 150mm insulation into 178mm rafters doesn't encourage enough airflow to deal with the condensation risk associated with this construction. Id be worried and ask them to point to where that says its ok. BS5250 (code of practice for condensation risk) says otherwise.

SodiumCooled

There are also soffit vents if that makes any additional difference.

I reiterated my point a few times on the 50mm gap requirement but they were adamant its not needed in my situation and they install like this all the time with no issue. I will speak with my engineer on it and get his take.