Plastning or dukning

[charl_6]

I'll keep it brief.
House built in '67, 95mm insulation. Crawl space, cold attic.

I've torn down the ceiling and all the old insulation is gone. An exterior wall is being rebuilt (long side) due to damage.

The question is about. Plastic or a “smart membrane,” meaning a diffusion-open membrane that allows moisture to pass through depending on the temperature. The latest update from the spray insulation company was 40mm loose fill (glass) with this smart membrane from Isola in the attic. The idea was to intentionally not insulate the attic too much so that minimal heat spills up (sounds wise). The exterior wall being rebuilt is planned to have plastic. Thus, all other walls have windproof paper since '67. An FTX system will also be installed in the future. The heating system is water-based floor heating throughout the house. So, not all exterior walls will get new plastic. What's being rebuilt is an exterior wall long side and the ceiling.

What would you have done? Plastic or membrane? Different schools here and I can't make sense when I read up.

/charl

 

[Jonatan79]

Skip the plastic, except perhaps in the bathroom ceiling which should be tight.

 

[charl_6]

Skip the plastic except maybe in the bathroom ceiling which should be sealed.

Hi!
Thanks for the response. You mean to skip plastic entirely on interior ceilings and exterior walls. But why plastic in the bathroom? Can't I just as well paint the ceiling with a moisture barrier?

 

[Jonatan79]

Yes, use steam brakes otherwise. You don't want moisture in the attic, so the plastic is good for joints, like where the wall meets the ceiling.

 

[vectrex]

Do you really mean 40mm insulation in total in the ceiling? That's very little. I assume you mean 40cm, which seems more reasonable?

 

[charl_6]

Do you really mean 40mm insulation in total in the ceiling? That's very little. I assume you mean 40cm which seems more reasonable?

haha I mean of course 400mm, that is 40cm. Sorry for that.

 

[charl_6]

Yes, use steam brake otherwise. You don't want moisture up in the attic, so that's why plastic is good for joints like where the wall meets the ceiling.

are you only thinking about wet areas now? I don't understand why I should cover the joints where the wall meets the ceiling with plastic and the rest with a sheet of new modern quality.

 

[useless]

With so much insulation, it will be cold in the attic, and it's important that no moisture seeps up from the dwelling. Use a tight vapor barrier.

Moreover, the smart vapor retarder is also made of plastic...

 

[charl_6]

With so much insulation, the attic will be cold, and it is important that no moisture seeps up from the dwelling. Use a tight vapor barrier.
.

Moreover, the smart vapor brake is also made of plastic...

So the risk of moisture/mold is thus smaller with plastic. That feels safer. Also, cheaper with plastic.

 

[vectrex]

So the risk of moisture/mold is thus less with plastic. That feels more reassuring. Also cheaper with plastic

That's right. The reason people usually say not to use plastic in old houses is because it's often difficult to get it airtight everywhere, and then you risk increased moisture load where there is no plastic. In practice, it doesn't matter whether you use diffusion-open fabric or diffusion-tight plastic. It's not diffusion that causes moisture problems in old attics. It's convection.

So make sure to get it as airtight as you can and use whatever product you want for it. Fabric, plastic, or paper. But make sure it is airtight and puncture it as little as possible.

 

[charl_6]

That's right. The reason it's often said not to use plastic in old houses is because it's usually difficult to seal everything thoroughly, which increases the risk of moisture where plastic is absent. In practice, it doesn't matter whether you use diffusion-open fabric or diffusion-tight plastic. Moisture problems in old attics are not caused by diffusion. It's convection.

So make sure to seal as tightly as you can and use whatever product you like for that. Fabric, plastic, or paper. But make sure it is tight and puncture it as little as possible.

Thank you for your clarity. Actually leaning towards plastic.

 

[useless]

And make sure to have functioning ventilation.

 

[charl_6]

And make sure to have proper ventilation.

Yep. FTX will be installed.
What do you think about ventilation of a cold attic? I just read on Ljungby fuktkontroll that too much ventilation of the attic can be harmful, especially if you're increasing insulation. At my place, there are ventilation openings at each eave, a small gap 28 of them, plus a vent grille about 200x200mm at each gable end of the house. I was thinking of installing spacer boards at each eave but will thus have difficulty controlling ventilation if I do so. What's your take on this?