Rafter Insulation

kieranhr

To be fair, it was your fellow moderator Syd that opened the gate to the next field. We just happened to wander out there after him.

antiskeptic

sydthebeat wrote: »

but as a synopsis, if you want to remove the ventilation gap in a cold roof construction, you need to install a vapour barrier perfectly on the warm side of the insulation with no gaps or chance of loss of performance over teh life of the build. The actual standard that allows this states that doing so is practically impossible anyway.

saying that you can push your metac right up against a breathable felt is simply not correct.

Edit

Forget I asked - I clicked on the link Syd put up. Sorry

sydthebeat

antiskeptic wrote: »

I'm not doubting what Syd says (especially not the impossibility of constructing a perfect vapour barrier). But could someone explain this to me?

If there's a breathable felt under the slate and on top of the Metac, what's the use of the air gap. Does not the vapour go through the Metac and through the breathable membrane?.

No, because at some point close to the top of the metac the "vapour" turns into "droplets" of condensation.... this point is called the dew point.

breathable felt allows vapour through it.. but it certainly doesnt let water droplets through it.

the air continuous gaps "dries out" the formed water on teh metac / roof timbers... like a washing line allows clothes to dry out.

antiskeptic

sydthebeat wrote: »

No, because at some point close to the top of the metac the "vapour" turns into "droplets" of condensation.... this point is called the dew point.

breathable felt allows vapour through it.. but it certainly doesnt let water droplets through it.

the air continuous gaps "dries out" the formed water on teh metac / roof timbers... like a washing line allows clothes to dry out.

Gotcha!

BoardsMember

Hope it's ok to jump on this thread, if not tell me and I'll set up another one.

I saw a renovated house recently use spray foam insulation. I'd be keen to look at something similar ourselves. My concern is about breathability and avoiding doing the wrong thing. Our roof is old, but secure. There is no membrane under the tiles, there was a layer of lime of something plastered on donkeys years ago, this is disintegrating and mostly gone. A couple of yeas back I put in floor insulation in the attic and it has make some improvement, I used rockwool, two layers perpendicular to each other.

The wind whistles through it so I thought with foam insulation I could get the whole attic area fully sealed. The wind gets in under the tiles but also between the top of the wall and the roof. What I dont know is can one put in this foam type stuff and seal off the attic completely. I have spoken to a few suppliers and some say they staple a breathable membrane onto the rafters near/against the tile, and spray onto that, and also seal at the top of the walls. I'm just wondering if I am letting myself in for long term risks of rotting etc if I do this?