vapour barrier or not?

[limeranger]

I'm getting ready to insulate the garage I'm building and am getting conflicting stories about wether to vapour barrier it or not since it will only be heated when I'm out there, probably most of the weekend and occasionally through the week in the evening. A friend of mine built a camp (also only heated occasionally) 4 years ago and has already replaced the floor due to condensation rotting it out, I realize mine is concrete but I don't want to create a mold problem either. The garage is detached from the house and uses a forced air oil furnace for heat. I did notice last year while only the ceiling was insulated and the walls were not that the walls furthest from the furnace would become soaking wet after running the furnace and be covered in ice afterwards. I was told not vapour barriering would allow this to breath, but it seems odd not to. Any advice?

 

[MrWhy19]

This is a complicated issue and depends on the climate and if moisture is generated inside, or if the outiside has more moisture than inside. As I understand it, moisture will tend to migrate from the wet to the dry side of a wall. That tells me a cold climate will have dryer outside air than usually exist inside a building. A shop may be different because there are no appliances, showers, or cooking going on. If the inside air is warm and the outside is cold, then moisture may condense where it hits an exterior sheath or siding. If the wall is well insulated then it should not condense on the inside. However if there is moisture moving from the inside through the wall then the mosture will condense where ever it contacts a cold surface. I would guess your climate is more cold/dry than warm/humid. Therefore I would want to stop any inside moisture from getting into the wall. i.e. vapor barrier on the inside. When the shop is warmed up, the warm air will hold more moisture and that moisture will want to migrate through the wall and condense when it hits a cold outside surface. Probably the inside of the exterior sheathing. Not good. But, if you run an air conditioner in the summer the opposite could happen. Warm outside moisture moving through the wall to get to the dry cooler inside. In that case you might want the barrier on the outside as we do here in Texas. Moisture that does migrate to the inside is extracted by the AC unit.

Some principles:
1. Warm air will hold more moisture than cold. Usually the colder the air, the dryer it is.
2. When warm moist air contacts a cold surface, like a window or a cold wall, condensation will occur. This might be on the inside surface of the exterior sheathing, or if the wall is not insulated, condensation would occur on the inside part of the wall. Depends on where the warm air first contacts a cold surface.
3. Moisture tends to migrate through a wall from the warm moist side to the cold side.
4. Any oil burning unvented heater will put a lot of moisture in the air. Rusted tools.

 

[BooUrns!]

While Mr.Why19 is dead-on with his description of vapour pressure, I don't see a reason to try and install a moisture barrier if the garage is detached and infrequently heated. Perhaps check the furnace venting and air quality (CO2 content) when the furnace is in use.

 

[limeranger]

I live in southern New Brunswick Canada The air is typically considered cold and dry. The furnace is out of a mobile home and vents through a selkirk chimney through the roof. There is currently no duct work to move the air around the shop just a single big opening on the top. It has no a/c. The walls nearest the furnace seem to stay dry just ones furthest away develop the moisture. These walls at the time had no siding just tyvek and sheathing. Hopefully when i insulate it this will help.

 

[belvedere]

I would install a barrier. Otherwise, the moisture will migrate into your insulation, sit there, and mold. JMHO

 

[limeranger]

That was the what I was thinking, but the argument against i'd heard was that not having it would allow it to breath and dry out.

 

[79pacecar]

I insulated & sheetrocked my shop about 18 years ago & installed a vapor barrier, I heat it the same way your talking about and over the years I have had to pull a sheet or 2 off to run a wire or something & have never seen any problems.. Based on that I would do it the same way again.. So my advice is to use a vapor barrier.

 

[tcianci]

When you see a problem in an uncompleted wall assembly, all bets are off as to what is really going on. But the comments here have been correct. Your Tyvek , unsided wall is quite capable of letting moisture in, I would toss the Tyvek and put a layer of 15lb felt and then the siding. Also the thing no one has touched on is that fiberglass insulation basically does nothing as far as insulating when it is wet. Why do you think the insulation is supplied from the manufacturer with a vapor barrier, they are not interested in your building, they want to ensure the performance of their product. Use felt on the outside and a vapor barrier in the inside and your wood and your insulation will be protected.