How diffusion-tight is a gypsum board. Are there better/more suitable options?

[justusandersson]

There is always a risk that water will seep in. After more than fifty years as a professional in the construction industry, I can say that I have seen many damages caused by unnecessary use of plastic but practically none due to the opposite.

 

[Daniel 109]

Never replaced a window? The sealant usually changes color because there is no sealing inside. To give an example.

I have an acquaintance in the industry who replaced a roof that wasn't two years old due to fatal errors in the moisture barrier.

 

[justusandersson]

I do not advocate for skipping vapor barriers in normal wall constructions where the majority of the wall's insulation consists of mineral wool. In this thread, I have only spoken about walls where the outer and dominant material is something else.

The thread starter's basement wall should have a wind-tight material behind the gypsum board because the leca stone is not plastered on the inside and therefore cannot be considered wind-tight.

 

[Olf Oggler]

I am not advocating for avoiding vapor barriers in normal wall constructions where the main part of the wall insulation consists of mineral wool. In this thread, I have only talked about walls where the outer and dominant material is something else.

The basement wall in question should have windproof material behind the drywall since the leca is not plastered on the inside and therefore cannot be considered windproof.

But now it's mostly soil on the other side of the leca wall. That must be considered windproof, right? At the top, the wall is of course exposed to wind and weather. But it feels pointless to have plastic only on the upper part. Or?

 

[justusandersson]

You should not have plastic. There are many types of wind barriers that are not diffusion-tight.

 

[Olf Oggler]

Alright. OK sorry. I misunderstood. Of course, you can have some wind shielding that is not diffusion-tight.

Isn't the gypsum board enough of a wind shield? But, of course, it seals tightly against the concrete ceiling. But down by the sill, there will probably be drafts. What type of membrane should one use if you want it to be windproof but still diffusion-open?

 

[justusandersson]

There are many wind barriers. Ask your building merchant. For example, Paroc XMW 095 wind barrier.
The joints between the drywall panels are not tight.

 

[Olf Oggler]

There are many wind barriers. Ask your building supplier. For example, Paroc XMW 095 wind barrier.
The joints between plasterboards are not airtight.

Won't it be too airtight with such wind barriers?

Spackled plasterboard joints with strips should be quite airtight, right?

 

[justusandersson]

You must distinguish between steam-tight and airtight, two completely different things. Exterior gypsum is usually sealed with tape but it doesn't look as nice on the inside. Nothing dramatic will happen if you don't seal it. It's up to you.

 

[Olf Oggler]

With some form of "windbreak" (airtight membrane that allows vapor to pass through) on the inner wall that goes all the way down to the concrete slab and seals against the concrete, this will prevent convection in the lower part of the wall. I imagine that the lower part benefits from a little air exchange there. That's just how I thought.

But I understand that you can't have both air exchange in the lower part of the wall and airtightness.

The only thing that makes me consider still having some form of membrane in the walls is that in the long run, you want a house that is as airtight as possible. I'm trying to maintain a certain negative pressure in the house with FTX, etc.

 

[Mikael_L]

But now to the question. How well does a regular gypsum board breathe? Are there other wallboards that breathe better?

To answer the question directly here, it depends, as mentioned, on what you mean by "breathe."

A gypsum board is relatively airtight, meaning that air doesn’t pass through due to different air pressures on each side. The joints are often quite leaky before any treatment, like taping and filling, is done. Most building materials, except for many types of insulation, are often reasonably airtight.

However, a gypsum board is significantly vapor-permeable compared to most board materials. That is, how much moisture in vapor form can pass through it. If you want it to be less vapor-permeable, a coat of paint on the board does a lot for that. For a truly vapor-tight solution, use a layer of vapor-tight plastic behind the board.

 

[Olf Oggler]

Does anyone have a recommendation for a windproof membrane that is diffusion-open? Preferably something that is commonly available and can be purchased without having to order, etc.

 

[Mikael_L]

https://www.byggmax.se/isolering/byggplast-och-vindskydd/vindskydd-bison-p6303018
https://www.bauhaus.se/vindpapp-windy-base-2-75x12-5m.html
https://www.xlbygg.se/mjallby/sorti...ydd/vindskydd-t-vind-274cm-25m-standard-32716
https://www.byggmax.se/isolering/byggplast-och-vindskydd/vindpapp-ac-350-p17040

Some examples.
Personally, I used T-vind, but Bison and Icopals are probably almost identical.