Vapor barrier during renovation

[farbrorBlaa]

Hello!

I have taken down a wall to create a room from the hall and living room. I will be putting gypsum on the ceiling, and the question is whether I should install a vapor barrier or not...?

The house is from '55 and was renovated and extended in '87. In the new part, there's plastic, but not in the old part. A carpenter who is going to help me thinks I shouldn't use plastic because I might get "point load" of moisture in the attic since not all the rooms will have plastic. Currently, I have issues with mold growth on the roof soffit in the attic and am therefore in the process of installing an FTX system to increase ventilation and create a slight negative pressure in the living area.

What would you do, plastic or not??

// Pär

 

[jon_h]

I would have refrained, with the same argument as your snickare.

 

[Linnex]

What does he mean by point load?
Will it increase in other places if it decreases where you apply plastic?

 

[farbrorBlaa]

Yes, he means that the plastic should keep moisture away so that the ventilation can handle it. If you plast a room and not the others, the moisture will follow the plastic until it reaches an "unplast" room. If there are then cracks or holes through which moisture can penetrate, there will be an increased moisture load right there, i.e., a larger amount of steam can penetrate at certain points and condense against the raw boards. If I understood him correctly...?

// Pär

 

[jon_h]

The problem you describe really only exists if there is poor ventilation and the humid air can move into the structure, known as convection. Without a vapor barrier, this will happen in many places, but with a vapor barrier in only certain places, there is a higher load on the places that do not have a barrier. However, the actual diffusion of moisture (the movement of moisture through the structure itself) does not increase in the places that do not have plastic.

 

[Milkshaken]

Don't use plastic

 

[snickarboden]

I can only agree on not using plastic and personally think that people have an overconfidence in vapor barriers. It doesn't solve any problems as I see it in an older house, but often causes quite big problems a little further down the line instead.

/Kent

 

[farbrorBlaa]

The problem you describe really only exists if you have poor ventilation and the humid air is able to move out into the construction, so-called convection. Without a vapor barrier, this will happen in many places, but with a vapor barrier in only certain places, it becomes a higher load on the places that do not have a barrier. The actual diffusion of moisture (the moisture moving through the construction itself) does not increase in the places that do not have plastic, however.

Hi Jon_h
Would you then put plastic or not? The ventilation should hopefully be good after the installation of ftx.

//Pär

 

[jon_h]

As I wrote in the first post, I had refrained.

 

[farbrorBlaa]

As I wrote in the first post, I would have refrained.

Oops! I should have seen that .. sorry

//pär