Plastic/foil or not?

[Hunneberg]

Hello!

I am planning to extend an existing old building that is used as a guest cottage. It is occasionally heated but mostly remains cold. It is old and has reed/clay and timber in the walls. I have removed the exterior panel and installed insulation between the timbers on the side where the sauna will be built. The sauna will be used about once a week, and the space it occupies may occasionally be heated when guests are present. The walls of the sauna building will consist of exterior panel, air gap, wind barrier, insulation, air gap, and interior panel. My question is whether I should have sauna foil between the insulation and the inner air gap against the existing building, ceiling, or elsewhere?

Best regards, Stefan

 

[Nissegandhi]

Hello!

I am planning to extend an existing old building that is used as a guest house and heated occasionally but mostly remains cold.
It is old and has reed/clay and timber in the walls. I have removed the exterior panel and added insulation between the timber on the side where the sauna will be built. The sauna will be used about once a week, and the space it is in may be heated occasionally when guests are present. The walls of the sauna building will be exterior panel, air gap, windbreaker, insulation, air gap, inner panel. And the question is whether I should have sauna foil between the insulation and the inner air gap towards the existing building, roof, or other?

Best regards, Stefan

Having plastic foil in houses that are cold in winter is a bit tricky. In spring, it can be -10 degrees at night, and it gets below freezing indoors, then the sun comes up, and maybe it becomes 15 plus degrees on the sunny wall, there is a risk of condensation on the outside of the plastic, that is, inside the wall. I would avoid the plastic.

 

[Hunneberg]

Having plastic wrap in houses that are cold in the winter is a bit tricky. In spring, it can be -10 degrees at night and it gets below zero indoors, then the sun comes out and it might be 15 degrees on the sunny side, there is a risk of condensation on the outside of the plastic, meaning inside the wall. I would avoid the plastic.

Thank you for your response! Yes, there seem to be many opinions on both sides, but what you wrote makes sense! Definitely leaning towards going without plastic.

 

[Weimik]

Having plastic film in houses that are cold in the winter is a bit tricky. In spring, it can be -10 degrees at night and it gets below freezing indoors, then the sun comes out and it might be 15 degrees on the sunny side, and there's a risk of condensation on the outside of the plastic, i.e., inside the wall. I would avoid the plastic.

But wasn't the question about what will now become an interior wall? And then there won't be much sun there.

That said, I'm not advocating for plastic, but it depends on other circumstances.

 

[Hunneberg]

But the question was about what is now going to be an interior wall? And then there won't be much sun there.

That doesn't mean I'm advocating plastic, but it depends on other circumstances.

Yes, it was about the interior wall, behind the inner panel's air gap. I thought that's where he meant warm and cold air meet and condense?

 

[Weimik]

Yes, it concerned an inner wall, behind the inner panel's air gap. I thought that's where he meant warm and cold air meet and condense?

There might be some room for misunderstanding here.
What I mean by inner wall is that which is not an outer wall. If I understood you correctly, you're doing an extension where one sauna wall will be built against the guest cottage. It thus becomes an inner wall from what was previously an outer wall. But I might have misunderstood what you wrote.

If you have plastic, it should be on the warm side, which is normally innermost in the wall, as close to the inner panel as possible. But when you close the house for the winter, it's not guaranteed that it will always be warmer inside than outside. That's what @Nissegandhi meant (I believe).

If it indeed becomes an inner wall between the sauna and the guest cottage, it becomes another question, that's what I meant. That inner wall could at least theoretically be considered for having plastic, depending on how the building is constructed. On the inside towards the sauna, that is.

 

[Hunneberg]

There's probably some risk of misunderstanding here.
What I mean by interior wall is what is not an exterior wall. If I understood you correctly, you are making an extension where one of the sauna walls will be built against the guesthouse. So it becomes an interior wall from what was previously an exterior wall. But I may have misunderstood what you wrote.

If you have plastic, it should be on the warm side, which is normally the innermost part of the wall, as close to the inner panel as possible. But when you occasionally shut down heating in the house, it's not certain that it's always warmer inside than outside. That's what @Nissegandhi meant (I think).

If it becomes an interior wall between the sauna and the guesthouse, then it's a different matter, that's what I meant. Theoretically, you might consider having plastic in that interior wall, depending on how the building is constructed. Closest to the sauna, that is.

You didn't misunderstand; it will become an interior wall from a previous exterior wall. However, I removed the outer panel, so it's really just a framed wall that I've now also added insulation to. And the question involves both that and the newly built walls. The former exterior wall becomes an interior wall between 2 rooms that have roughly the same temperature (heated simultaneously), but there's some risk if the sun hits one room and the other is in the shade, with a closed inner door between. Then I'm considering how it can properly dry out when using the sauna and it's cold outside, but it should work thanks to the residual heat slowly decreasing and drying out as I understand it.