Insulate the upper floor of an Älvsbyhus

[Fatbird]

Hi, I have an idea to use 50mm foam plastic against an air gap instead of using 3mm oil-hardened board. Älvsbyhus uses a plastic instead of raw boards, so I'm a bit unsure.

The construction would be from outside to inside something like
Roof plastic - air gap 45mm - foam plastic 50mm - then crossed approximately 250mm glass wool insulation - vapor barrier - gypsum board.

Thoughts?

 

[justusandersson]

I would probably advise against it, as the foam plastic partially counteracts the purpose of the air gap.

 

[Daniel 109]

You need to have a material that allows moisture to pass through, I'm unsure if the cellplast is sufficiently permeable. Additionally, it's a bit unwise to have cellplast there due to fire protection reasons.

 

[fsn]

I would probably advise against it, as the foam board somewhat counters the purpose of the air gap.

Can you elaborate on that?

 

[justusandersson]

The cellular plastic is not diffusion-tight but diffusion-slow. In this way, it can help enclose organic material (parts of the rule construction) in a relatively vapor-tight layer. The air gap is needed because there are two diffusion seals surrounding the entire construction. Additionally, I share Daniel's point of view regarding the fire risk.

 

[Fatbird]

I don't know if I was unclear or if I'm misinterpreting the answer, I can understand the part about fire, so I should still have an air gap of about 45mm then according to älvsbyhus you should put a 3mm board, I was thinking instead of the board to put styrofoam panels that are 50mm. The board that's supposed to be there according to the plan, is there such a big difference in how much air it lets through? I mean that board is oil-treated so it should almost be tighter.

 

[justusandersson]

It's not about air tightness but about water vapor tightness. There's a big difference.

 

[Kallebo]

Such discussions appear frequently. I think you could use frigolit if you are not worried about the fire risk. Sure, it is probably somewhat more vapor-tight, but the difference compared to the vapor barrier on the inside is still very large.

 

[fsn]

The cell plastic is not diffusion-tight but rather diffusion-resistant. It can thus help to enclose organic material (parts of the frame structure) in a relatively vapor-tight layer. The air gap is needed because there are two diffusion seals surrounding the entire construction. Also, I share Daniel's viewpoint regarding the fire risk.

Now, as mentioned, there was never any question of not having an air gap.

I also don't understand the purpose of EPS, is it for a little better insulation or what is the purpose?

Personally, I would use this instead https://www.byggmax.se/skivmaterial/byggskivor/plywood-och-osb/trossbottenskiva-p07040

I also agree about the fire risk with cell plastic. Whole roofs are sold in cell plastic, completely unacceptable.

 

[Fatbird]

The purpose is that since I still have to use a board, in order to reduce the workload and cost, I'll use cellplast instead of masonite.

 

[justusandersson]

I believe that fire risk should be taken seriously. Once you've seen what a fire in cellplast can cause, you don't want to experience it again. Especially the combination with plastic as a sealing layer on the roof makes it advisable to avoid cellplast. A spark from a fire in the yard that gets under the roof tiles, burns through the plastic, and lands on the styrofoam is not an entirely unthinkable scenario.