Floor structure vapor barrier

[chrille112]

We are in the process of reinsulating the floor joists in our early 1900s house with glass wool.

According to Rockwool, you should have a vapor barrier between the joists and the floorboards:
http://www.rockwool.se/råd+och+anvi...ering+vid+nybyggnad/projektering+av+krypgrund

But according to Paroc, you shouldn't:
http://www.stenull.paroc.se/isolera_hemma/?grupp=3&item=2

Does anyone know what the situation actually is?

 

[chrille112]

Someone, please... had hoped to close the floor tomorrow!

 

[Mats]

well, I'm just as interested

 

[chrille112]

I have written in this thread as well, where it's getting a bit clearer at least... but I still don't know which option it will be tomorrow

http://www.byggahus.se/forum/isolering/108779-byta-ut-golvisoleringen-sagspan-eller-inte-2.html

(sorry for the double posting, necessity knows no law....)

 

[Krawk]

The reason there's no simple answer is that the floor structure is situated between warm, semi-humid air and cold, usually very humid air. Therefore, the direction the moisture wants to travel is uncertain and can vary throughout the year. If it is constantly dry in the foundation, you want the vapor barrier on the warm side, just as in an exterior wall, to prevent moisture from traveling down through the insulation and condensing. However, if it is humid in the foundation, this moisture will travel upwards and "get stuck" under the plastic instead of being able to pass up through the floor.

 

[chrille112]

We have had some issues with moisture in the foundation (RH around 75-80%), so with your reasoning, would it be best to skip the plastic then?

Admittedly, we will install a dehumidifier so we can stay around 60%, but it is still relatively humid...

 

[Krawk]

With a dehumidifier controlling the climate in the foundation, it should (?) make sense to have plastic on the warm side to prevent moisture migration downwards. The value of the floor structure being able to breathe is also not as significant anymore.

 

[chrille112]

Why is it no longer meaningful? Sealing in the moisture with plastic doesn't feel quite right... and the idea that warm air would go down into the floor structure seems strange to me!

But it seems like everything points to using plastic, so I guess that's what it will have to be, right..?

 

[Krawk]

It would decrease in significance because the dehumidifier (hopefully) takes care of the moisture instead.

Warm air also does not move down through the joists. However, moisture does, as diffusion occurs regardless of air movement; nature always strives for equilibrium.

 

[chrille112]

Now it's almost time to lay the floor, and since both the flooring manufacturer and forum members point towards plastic, I'm thinking of going with that.

Thanks for all the help!

 

[Mats]

Thinking about doing the same in a room where I have some issues with floor drafts.
I have a plinth foundation so it should be self-drying under there.