Plasticization during extension

[henrik762]

Hello!

I'm building a 10 square meter extension intended to be a new entrance. A new exterior wall is being constructed, and the old exterior wall will be converted into an interior wall.

One thing that occurred to me the other day is that I don't know how far down to plastic the old exterior wall. I wrote and asked earlier but didn't get an answer, so I'm hoping for better luck now.

The idea was to plastic the entire exterior wall and roof, and then only go down partway on the old exterior wall, but that won't be completely sealed since I can't attach the plastic in the roof between the new and old sections.
So the question is whether an extension should be plasticized as if it were a new house, i.e., floor to ceiling everywhere. Or is it enough to plastic the exterior walls and roof?

 

[Aser]

Hello!

I'm building an extension of 10 square meters intended to become a new entrance. A new exterior wall is being constructed and the old exterior wall is being turned into an interior wall.

One thing that struck me the other day was that I don't know how far down to place the plastic on the old exterior wall. I wrote and asked earlier but got no response, so hoping for better luck now.

The idea was to put plastic over the entire exterior wall and roof, then just go down a bit on the old exterior wall. But then it won't be completely sealed since I can't attach the plastic that runs in the ceiling between the new and the old section.

So the question is whether an extension should be plastic-sealed as if it were a new house, i.e., from floor to ceiling everywhere. Or is it enough to seal the exterior walls and roof with plastic?

Did you come to any good conclusion? I've been wondering myself how it should be done. Either try to seal between the studs as tightly as possible or put a vapor barrier on one side and plastic on the other.

 

[Oldboy]

If it is well-insulated, it should be covered with plastic (vapor barrier or retarder of some sort). It is only needed against cold exterior surfaces, i.e., exterior walls and roof, (and should be on the warm side). Otherwise, no vapor barrier/retarder is needed. However, it is always critical to ensure that it is airtight everywhere so no drafts can occur anywhere. It is always challenging to make it completely windproof when extending onto an existing house. Windproof and vapor-proof are two different things.

Whether the existing house has a vapor barrier or not does not matter.