ventilation gap

paul127

Just wondering if there is still need for a 50mm air gap between breathable fealt and fiberglass insulation ? if there is how come spray foam insulation dose not need the air gap ?
Thanks in advance Paul

Poor Uncle Tom

Regulations state a 50mm air gap is required. Some believe the spray foam is breathable and therefore the 50mm doesn't apply. IMO, if you are using the spray foam you need to counter batten on the outside to make up the 50mm required.

beyondpassive

The vent gap became a 'deemed to satisfy' requirement to prevent condensation as traditional sarking felts had a large temperature difference across them. If you have a breathable construction that has no sharp changes in temperature then you have no dew point and no condensation, equally if you have a vapor control layer on the inside, that will prevent moisture entering the roof build up. I presume you are talking about insulating on the slope. Fibreglass is in no way appropriate for this.

paul127

The make up is breathable fealt 150mm rafter filled with 150mm metec vapour control layer and insulated slad on inside just wondering is there need to counter batten ?

beyondpassive

In Lieu of the breathable felt, you could use T+G softboard, 22mm is often used. It performs quite well, preventing wind heat loss from outside and summer overheating.

If the mineral wool will bulge the felt to the extent that there is a risk it touches the undersideof the slates then you need to double batten. How will the insulated slab work with cables and wiring? Won't The vapor check get punctured by the electricians?

I'd recommend putting a high spec of insulation and airtightness in the roof, if you have rooms in the roof. These types of rooms suffer from large temperature swings and bad air quality. This is where the softboard is usefull I'd batten out the rafters to 200-250mm line with OSB instead of vapour check and create a services zone filled with mineral wool. Cables and downlights can then fit in this zone.