Install vapor barrier in old roof

[herge]

I have 2 interior ceilings that need to be redone (stretch fabric from -46) and these completely lack a vapor barrier up to the attic.

While I'm fixing the new ceiling, I thought I might try to install a vapor barrier up to the attic, but what's the best way to do it? I assume it should go down the walls to the floor, but since I'm not planning to mess with the walls right now, can I do part of the job? Or is it not worth it?

 

[Stefan1972]

What kind of insulation is it?

 

[herge]

Planer shavings and regular insulation on top of that. Would probably want to replace the shavings in the future.

 

[Finndjävel]

Do you have any problems with moisture in the attic? Otherwise, it's unnecessary to try to fix something that works.

 

[heimlaga]

I would suggest some kind of cardboard or tar paper as a vapor barrier. Fold it down a little behind the cornice and then it's good. At some point, you can re-insulate with eco-fiber if you want without getting moisture problems.

Plastic and modern moisture barrier thinking become a bit too tight for an old-fashioned diffusion-open construction. Plastic is associated with mineral wool and fiberglass. The plastic is only there because these modern insulation materials lack the ability to dissipate moisture and dry out.

 

[herge]

Ok I hear what you're saying, but the argument "too tight" is hard for me to buy, rather "overkill"? Nothing benefits from moisture?

 

[Finndjävel]

The problem is that the joint where the plastic ends may be exposed to extra moisture, which can lead to problems.

 

[Fred vom Jupiter]

If you seal with plastic everywhere you can, there will still be some places you miss, like above walls and closets. When the air and steam that previously escaped evenly across the entire ceiling now only escapes through a few small spots, the moisture impact becomes concentrated there.

Where I've read recommends a vapor brake instead of a vapor barrier.

I have a similar ceiling with stretched fabric, sparse paneling, tar paper, and wood chip insulation. The tar paper likely acts as both moisture protection and prevents the wood chips from falling between the boards. I also plan to replace the wood chips with cellulose insulation at some point, but there are always other projects to tackle.

 

[Stefan1972]

Ok I hear what you're saying but the argument "too tight" is hard for me to buy, rather "overkill"? Nothing benefits from moisture?

No, but wood shavings/sawdust can handle normal moisture migration without any issues because it dries by itself. If there's more moisture migrating than that, you might need to solve that problem first. I would probably recommend some form of cellulose insulation on the shavings instead of the usual one you mention, and then let the shavings stay where they are because as insulation, they work better than their reputation suggests.