Here comes something for all the experts:
Vapor barrier under the floor, on the joist with an open foundation, yes or no and why/why not?
I’m building a vacation home with an open foundation. Inside the insulation in the walls and ceiling, I have installed Bison Vapor Barrier.
The floor is constructed from:
4.5 fiber cement board
220 MU/joist
Vapor barrier
28 pine floor, screwed.
When we reached the point of installing the vapor barrier and floor, I hesitated. Is it appropriate to have a vapor barrier under the floor? Should I skip it entirely?
In the bathroom, for example, we must skip it as we will glue and screw the chipboard.
Is there anyone with smart insights or experiences? Pros/cons?
It seems like the risk of moisture getting into the joist from the indoor air is small, right? But what about potential ground moisture moving up through the floor? We neither want it to enter nor be trapped in the joist, which is why a vapor barrier might be appropriate.
Help a confused person!
Vapor barrier under the floor, on the joist with an open foundation, yes or no and why/why not?
I’m building a vacation home with an open foundation. Inside the insulation in the walls and ceiling, I have installed Bison Vapor Barrier.
The floor is constructed from:
4.5 fiber cement board
220 MU/joist
Vapor barrier
28 pine floor, screwed.
When we reached the point of installing the vapor barrier and floor, I hesitated. Is it appropriate to have a vapor barrier under the floor? Should I skip it entirely?
In the bathroom, for example, we must skip it as we will glue and screw the chipboard.
Is there anyone with smart insights or experiences? Pros/cons?
It seems like the risk of moisture getting into the joist from the indoor air is small, right? But what about potential ground moisture moving up through the floor? We neither want it to enter nor be trapped in the joist, which is why a vapor barrier might be appropriate.
Help a confused person!
I don't think there's any risk in placing a vapor barrier against the floor as you describe (on the warm side). However, I'm not sure it's necessary. Generally speaking, I think you can say that the higher up in the building, the more important the vapor barrier is, because there's a higher air pressure difference higher up (so least important against the floor).
However, it's good to ensure that the building is as airtight as possible. A vapor barrier certainly contributes to that. But if you want to ensure warm floors, it should be better to seal further out in the construction (but with some diffusion-open material).
However, it's good to ensure that the building is as airtight as possible. A vapor barrier certainly contributes to that. But if you want to ensure warm floors, it should be better to seal further out in the construction (but with some diffusion-open material).
Some form of vapor barrier should be used against an open crawl space foundation; the conditions are similar to those for an exterior wall. However, it is not the easiest to incorporate this into the floor structure without perforating it with numerous screw holes. The function of the vapor barrier, compared to regular wind barriers, is to prevent diffusion through the structure, driven by differences in vapor pressure across the structure. What they have in common is that they prevent convection through the structure, driven by differences in air pressure.
The ground is not waterlogged but rather quite well-drained, although it is rich in clay in some areas. It is generally silty till. Under the house, I have placed age-resistant plastic and covered it with 10-20 cm of 8-16. The plinth height above ground is about 40 cm. The fiber cement boards are generally considered to be diffusion-open as I understand it, so if I omit the vapor barrier (which I must do in the bathroom), I have no real diffusion barrier in the floor structure.
I'm not really afraid that it will be a major problem regardless of how I do it, but on the other hand, I'm not an expert on the subject, so I'm very grateful for any thoughts and ideas.
I'm not really afraid that it will be a major problem regardless of how I do it, but on the other hand, I'm not an expert on the subject, so I'm very grateful for any thoughts and ideas.
Now, I don't really think they had 220MU 1200 years ago
In the bathroom, you have the waterproofing, so you already have a vapor barrier there. If you equate the bathroom floor with an external bathroom wall, there is a vapor retarder in the external wall, this is to prevent material from being trapped between two vapor-tight layers. So you can continue with the vapor retarder in the bathroom floor and substitute particle board with plywood. However, it's very important that the underside of the joists is completely airtight.
In the bathroom, you have the waterproofing, so you already have a vapor barrier there. If you equate the bathroom floor with an external bathroom wall, there is a vapor retarder in the external wall, this is to prevent material from being trapped between two vapor-tight layers. So you can continue with the vapor retarder in the bathroom floor and substitute particle board with plywood. However, it's very important that the underside of the joists is completely airtight.
The underside is probably airtight, all joints are taped, and sealed around the connection to the plinths. But it is not vapor-tight, I suppose.B bossespecial said:
Is plywood in the bathroom floor recommended? Shouldn't it also be screwed and glued as I planned with moisture-resistant chipboard? It would remain difficult with a vapor barrier there, but in the rest of the house, it sounds like a good idea to continue with the vapor barrier.
The outer layer should not be vapor-tight, only windproof. Plywood is more moisture-resistant than particleboard, even if you choose a more moisture-resistant particleboard.