Considering the choice of vapor barrier.
I have received two quotes.
One with wood fiber and a variable vapor barrier.
The other with paper insulation and a vapor barrier with an SD value of 18.
The company that manufactures the variable vapor barrier is based in Norway. The air there is at least as humid as here. On the other hand, their vapor barrier is more expensive, which in itself is an incentive to sell it.
The company with the paper insulation is based in inland Sweden. The air there is dry, meaning it's a simple climate for a house.
I will cover the frame with facade boards, a 60 mm thick wood fiber board with a density of about a quarter. Ventilated, with a 45 mm air gap between the facade board and the outer panel. A seller from the facade board supplier recommended osb as a vapor barrier. With an SD value of 2.5 and a requirement for a factor of five more breathable on the outside than the inside, it probably works in my area and certainly in drier places. That "probably" makes me go for a vapor barrier. (It's slightly complicated by the fact that I have to put plywood on both the inside and outside of the frame at a few places, totaling about 20 m2, to prevent the house from collapsing in strong winds.) The same company manufactures a vapor barrier with an SD of 5, a sturdy fabric that weighs 130 grams/m2 and costs 2.9 euros/m2, barely one euro more per m2 than the vapor barrier with SD18.
Initially, it's undeniably clever to have a vapor barrier that opens if it's humid, but for it to be beneficial, it must be more humid inside the wall than outside. If it is drier inside the wall than outside, the wall will absorb moisture. But with prescribed ventilation and a negative pressure of 10-20 pascals inside the house, the indoor air will be drier than the outside air and likely also the air in the wall.
In humid weather, it's often summer. During the day, it can be cooler indoors than outdoors. Often doors/windows are open during summer, disrupting the negative pressure. Then it is a big advantage if moisture inside the wall passes through the vapor barrier instead of condensing on it. On the other hand, the wood fiber insulation should buffer so much moisture that no condensation can occur. Additionally, convection is very small in a wood fiber insulated wall.
A vapor barrier with an SD value of 18 I think is too tight. I want a reassuring margin to a factor of five, but don’t see the benefit of SD18. Those working at the company selling this vapor barrier started nervously rambling, "you need to ask a designer about that" when I asked any questions. Not the smartest guys in the world, evidently. Moreover, a company located inland where conditions differ from mine with wonderfully humid, raw, and cold air blowing in from the sea.
Yes, these are roughly my thoughts on the matter.
Why should I choose a variable vapor barrier?
Why should I choose a vapor barrier with SD18?
Grateful for opinions.
I have received two quotes.
One with wood fiber and a variable vapor barrier.
The other with paper insulation and a vapor barrier with an SD value of 18.
The company that manufactures the variable vapor barrier is based in Norway. The air there is at least as humid as here. On the other hand, their vapor barrier is more expensive, which in itself is an incentive to sell it.
The company with the paper insulation is based in inland Sweden. The air there is dry, meaning it's a simple climate for a house.
I will cover the frame with facade boards, a 60 mm thick wood fiber board with a density of about a quarter. Ventilated, with a 45 mm air gap between the facade board and the outer panel. A seller from the facade board supplier recommended osb as a vapor barrier. With an SD value of 2.5 and a requirement for a factor of five more breathable on the outside than the inside, it probably works in my area and certainly in drier places. That "probably" makes me go for a vapor barrier. (It's slightly complicated by the fact that I have to put plywood on both the inside and outside of the frame at a few places, totaling about 20 m2, to prevent the house from collapsing in strong winds.) The same company manufactures a vapor barrier with an SD of 5, a sturdy fabric that weighs 130 grams/m2 and costs 2.9 euros/m2, barely one euro more per m2 than the vapor barrier with SD18.
Initially, it's undeniably clever to have a vapor barrier that opens if it's humid, but for it to be beneficial, it must be more humid inside the wall than outside. If it is drier inside the wall than outside, the wall will absorb moisture. But with prescribed ventilation and a negative pressure of 10-20 pascals inside the house, the indoor air will be drier than the outside air and likely also the air in the wall.
In humid weather, it's often summer. During the day, it can be cooler indoors than outdoors. Often doors/windows are open during summer, disrupting the negative pressure. Then it is a big advantage if moisture inside the wall passes through the vapor barrier instead of condensing on it. On the other hand, the wood fiber insulation should buffer so much moisture that no condensation can occur. Additionally, convection is very small in a wood fiber insulated wall.
A vapor barrier with an SD value of 18 I think is too tight. I want a reassuring margin to a factor of five, but don’t see the benefit of SD18. Those working at the company selling this vapor barrier started nervously rambling, "you need to ask a designer about that" when I asked any questions. Not the smartest guys in the world, evidently. Moreover, a company located inland where conditions differ from mine with wonderfully humid, raw, and cold air blowing in from the sea.
Yes, these are roughly my thoughts on the matter.
Why should I choose a variable vapor barrier?
Why should I choose a vapor barrier with SD18?
Grateful for opinions.
Last edited:
Know-It-All
· Västra Götaland
· 12 287 posts
You've basically answered the question yourself. The variable steam brake has an advantage, and in this context, is it a marginal price difference?
W witten said:
What are the advantages of PE plastic compared to some form of vapor barrier?
The inside should be a little more than 5 times theoretically as the vapor permeability varies with humidity and moisture content of the materials involved. But at significantly more than 5, you might as well use PE plastic as Witten suggests.useless said:
Is it really an advantage? Why? Are there studies/measurements done in the lab?
In dry weather, it closes to an SD well over 20. Then you miss the formidable opportunity for the wall to dry inward. On the other hand, it's an advantage to have a tighter vapor barrier when it's cold, which usually coincides with dry air.
The price for example: 6 SEK/m2 instead of your suggestion of 20 - 30 SEK/m2.F Fritt harmoniskt hus said:
I agree that the feature is unnecessary and actually not needed even then since walls dry outwards if there is an air gap. Moisture in walls is such an absent problem in correctly built ventilated walls that these expensive products completely lack application.
The only time it is justified to use the variable steam brake is when cooling down the house in the summer to below 20°C for an extended period. Then you get reverse moisture migration in the structure.
13,000 kr or 3,300 kr, a difference of 10K in a deal worth many millions. A lower price is no incentive in this case.KnockOnWood said:
Is the tape for the open vapor barrier systems also more expensive than for plastic?
This should be the benefit of variable vapor barriers. That the wall, due to a humid summer, contains a lot of moisture. The sun heats the facade on the outside so much that the airgap cannot dry out the wall, causing moisture bound in the insulation to evaporate and in the worst case, condense on the inside. Admittedly an extreme case if you have a wall insulated with cellulose fiber and facade board on the outside.Matti_75 said:
Or if, due to a prolonged heatwave, you cool the house on the inside, then there will definitely be condensation on a membrane that is not vapor open, especially if the wall is covered with a wind barrier and insulated with mineral wool.
It depends on how thick the wall is. Somewhere there is a condensation point. If it is inside the wall, it will get moist. That's why the frame is clad in a board with the fitting name "sarking board," for example, Steico Universal if you have thick walls.W witten said:
What do you mean by "Somewhere there is a condensation point. If it is inside the wall, it becomes humid."?
Not at all. What causes something to dry is that water evaporates, which can only happen to a certain limit based on the air's humidity. If heat drives out moisture in the air gap, a prerequisite for further drying is that it is sufficiently ventilated.
At a certain temperature, the vapor in the air will turn into small water droplets. Known as dew during the summer and as condensation on cold surfaces.D Daniel 109 said:
Which air are you talking about? If it's the indoor air, you address it by having a moisture barrier. The dew point for indoor air is in the wall regardless of how it looks if the outdoor temperature is below the dew point.