Vapor barrier or vapor retarder, that's the question?

[Liten hemmabyggare]

We have built a completely new roof on the 1930s villa and will insulate with 300 mm mineral wool.
Once it's finished, it will be an attic space with a cathedral ceiling and exposed purlins/braces.

The question is whether we should use a vapor barrier or vapor retarder? Everything in the attic is removed, making it easy to access everywhere and tape the seams, etc., to make it airtight. The side walls will also be re-insulated. Question 2: Should the same plastic be used on the walls?

 

[klaskarlsson]

We have built an entirely new roof on the 1930s villa and will insulate with 300 mm rock wool. When it's finished, it will be an attic with cathedral ceilings and exposed tie beams/knee walls.

The question is whether we should use a vapor retarder or vapor barrier? Everything in the attic is gutted, so it's easy to access everywhere and tape seams, etc., to make it tight. The side walls will also be insulated anew. Question 2: Should the same plastic be used on the walls?

There are a few different schools of thought.
But if you're sure you can get everything completely airtight (including any penetrations), then you can use a barrier.

Personally, I would have used a vapor retarder in such an old house, as I'm skeptical that you can make it airtight (and that it will hold over time), and it feels better to allow moisture to escape in both directions. Did you have any mold problems in the roof/walls before? If not, a retarder might be better in that case.

On the other hand, you have added 300 mm of rock wool insulation, which points more towards a barrier since that's significantly more than older houses usually have...
Hard to say.

 

[Viktor.J]

Why vapor barrier instead of vapor brake?

For me, it's an important question to answer. If you still want to go with a vapor barrier, you probably need to explain how the walls, floors, and roof are constructed, what the ventilation looks like, etc.

 

[Daniel 109]

Why steam brakes before the barrier is a better question, I think.

 

[Rickard.]

Why is steam brake in front of barrier is a better question, I think.

I agree, there's probably no one who recommends/"sells" (300mm) stone wool and steam brake without then it's always steam barrier that's applicable. Steam brake in the roof is also a bit controversial just as it is and even more so if you don't follow a product line straight through.

 

[Viktor.J]

From what I understand, there is quite a consensus that an old house (in wood) which is built to be diffusion-open should be maintained that way.

If I understand correctly, it is an attic that has been converted, but is it insulation of the floor structure up to the attic or insulation of the roof?

 

[Rickard.]

From what I understand, there is quite a large consensus that an old house (in wood) that is built to be vapor open should be maintained as such.

If I understand correctly, it is a furnished attic, but is it insulation of the joist against the attic floor or insulation of the roof?

That "consensus" has probably already been skipped when they built a new roof (whatever that exactly means) with 300mm of rock wool.

 

[Viktor.J]

That "consensus" has probably already been skipped when building a new roof (whatever that exactly means) with 300mm stone wool.

True.

 

[Nissens]

From what I understand, there is quite a large consensus that an old house (in wood) built to be vapor-permeable should be kept that way.

If I understand correctly, it's a finished attic, but is it insulation of the floor structure up toward the attic or insulation of the roof?

This thread says something completely different...

 

[Liten hemmabyggare]

From what I understand, there is quite a large consensus that an old house (made of wood) built to be diffusion-open should be kept that way.

If I understand correctly, it's an attic that has been converted into a living space, but is it insulation of the floor structure up to the attic or insulation of the roof?

Thanks for the answer! A clarification is that it is roof insulation which is now new with added rafters, windproof membrane, air gap, new roof boards, and sheet metal. The only thing left is the old load-bearing construction with purlins, collar ties, and braces. All previous walls are demolished, so it will be easy to lay the foil (vapor barrier or vapor brake) then furring strips on it and gypsum.

 

[Rickard.]

https://www.rockwool.com/se/produkter-och-konstruktioner/takisolering/snedtak/paralleltak/