Mixing vapor brake and vapor barrier

[RocketScienceSmurf]

I am renovating a cabin built in the 50s and when I contacted a manufacturer of grooved insulation boards for underfloor heating, I was recommended to use a vapor barrier under the floorboards. I have previously been in contact with, among others, Bison, who suggested a vapor retarder for this type of residence, which I feel is better than a vapor barrier when it comes to walls and ceilings. However, one thought that struck me was potentially using a barrier on the walls and ceiling in the area that will become a sleeping alcove, and then using a vapor retarder on the walls and ceiling in the rest of the cabin.

What are the advantages and disadvantages of mixing barriers and retarders depending on the room? My reasoning includes that, due to various reasons, I will find it difficult to effectively use barriers everywhere, which is why I like the idea of letting the cabin "breathe" a little more. But in the space where you spend the night, I think the vapor pressure is somewhat higher, and since I don't plan to have any windows or other openings in the walls of the sleeping alcove, I can easily create a space that is completely sealed in terms of the walls and ceiling.

 

[Jonatan79]

I am renovating a cabin built in the 1950s and when contacting a manufacturer of grooved insulation boards for underfloor heating, I was recommended to use a vapor barrier under the floorboards. I have previously been in contact with companies like Bison who, for this type of dwelling, suggested a vapor brake which I feel is better than a vapor barrier when it comes to walls and ceilings, but a thought that struck me was to possibly use a barrier also on walls and ceilings in the area that will become a sleeping alcove and then use a vapor brake on walls and ceilings in the rest of the cabin.

What would be the advantages and disadvantages of mixing barrier and brake depending on the room? My reasoning includes the fact that for various reasons it will be difficult for me to properly install barriers everywhere, which is why I like the idea of allowing the cabin to "breathe" a little more, but in the area where you spend the night, I think that the vapor pressure is somewhat higher and because I don't plan to have any windows or other openings in the walls of the sleeping alcove, I can easily achieve a space that is completely sealed against walls and ceiling.

A barrier in the ceiling in the bathroom is enough.

 

[Avemo]

It's far from an exact science. I believe the biggest problem with vapor barriers in old houses is that there is a risk that small leaks remain undetected and cause significant damage before they are noticed. Without the vapor barrier, the moisture could partly dry out inwards, and it would be noticed earlier. That risk doesn't exist in the floor. Another potential problem is if there is a leak where air frequently flows from the room into the wall. Then condensation can accumulate that is not so easy to dry up. If everything is perfect, a vapor barrier is better. If it's not perfect, a vapor retarder is somewhat more forgiving. I don't think it's a big deal to mix vapor barriers and vapor retarders. They have their pros and cons on the walls where they are placed.

 

[RocketScienceSmurf]

It is far from an exact science. I believe that the biggest problem with vapor barriers in old houses is that there is a risk that small leaks remain undetected and cause significant damage before they are noticed. Without the vapor barrier, the moisture could partly dry out inward and it would be noticed earlier. That risk does not exist in the floor. Another potential problem is if there is a leak where air often flows out from the room into the wall. Then condensation can accumulate, which is not so easy to dry. If everything is perfect, the vapor barrier is better. If it is not perfect, a vapor brake is somewhat more forgiving. I don't think it's a big deal to mix vapor barriers and vapor brakes. They each have their pros and cons on the walls where they are placed.

Thank you for the answer. That was roughly my understanding as you describe, that if you manage to make the installation more or less perfect, a barrier is best since it does not allow any moisture movement at all. But I have never attempted this before, and I know I will have some penetrations at certain places, and with that in mind, I think it is a good idea in my case to have a vapor brake on certain areas to reduce the risk of concentrating moisture problems at places where I would have failed to make a vapor barrier completely airtight.

 

[Matti_75]

The first question is whether the cabin should be heated constantly?
There is no significant difference between vapor barrier/brake when it comes to holes and leaks. The moisture will find its way out where it is open.

 

[Oldboy]

Also depends on how thick and well-insulated the walls are. The thicker the walls, the longer it takes for any moisture that finds its way in to dry out.

 

[RocketScienceSmurf]

The plan is for the cabin to be heated year-round. There will be underfloor heating in the living room, kitchen, and WC. Mechanical exhaust ventilation with a fan for extraction will be placed in the outer wall in the WC section and fresh air intake on the opposite side in the living room.

The current insulation is 50-70mm in the walls, and I have started work on adding extra insulation with west coast boards on the outside.

 

[Oldboy]

Permanently heated, then it's possible to have a regular vapor barrier. Personally, I believe that a vapor brake is preferable.