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Old 08-22-2009, 06:32 PM   #1
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Vapor Barrier Question

I am insulating an outbuilding I have. I know that traditionally the vapor barrier goes on the "warm in the winter" side and that is how I am handling all the exterior walls.

However, I have 2 twists on this I need help with:

1. The outbuilding will be divided into 2 halves on the main level and separated by a wall. One side will be a garage and the other a workshop. Both will be heated, but to different temperatures. The garage will be kept at just above freezing (35-40 F or 2-4 C) while the shop will generally be kept around 55 F/12 C when I am not using it and around 65-70 F/18-21F when I am in there or need the extra heat. So which side does the vapor barrier go on?

2. There is also a loft that I want to turn into a "guest house". It is above both the garage and the shop. Most of the time it will be keep it just above freezing so the pipes don't freeze, however when someone is staying there it will be whatever temperature is comfortable for them. Which side does the vapor barrier go on in this case?

These ones have me puzzled. Any help would be great.



Last edited by awhitehouse; 08-22-2009 at 06:46 PM.
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Old 08-22-2009, 09:27 PM   #2
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The idea of a vapor barrier is to keep moisture from penetrating into the insulation, then freezing within the insulation. So traditionally, the barrier goes on the warm side (the house side), so that in winter there will be limited amounts of moisture penetrating into the insulation from the moist air in the house. Similarly for the floor of an attic, the vapor barrier is against the floor, to keep moisture from the house from penetrating into the insulation and freezing.

That said, there is a lot of disagreement about how important vapor barriers are. I just had my 1959 house insulated, it had no insulation in the walls at all, and of course no vapor barrier. The contractor blew in cellulose insulation, and put up no vapor barrier at all. This is standard practice, since installation of a vapor barrier in the case of retrofit is expensive, and according to the cellulose insulation institute, completely unnecessary.

So in your case, the garage presumably will not have any running water in it, so the humidity level should be about the same as outside. Probably no need for a vapor barrier at all. The guest house, if it has running water (i.e. kitchen), should (by tradition) have a vapor barrier on the inside wall. If there is no running water, probably no need for a vapor barrier at all.


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Old 08-22-2009, 09:53 PM   #3
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I would call your local Building Department to find the requirements for your area. If you need to heat the garage, it sounds as if you may have a cold environment. Vapor barriers and insulation will be more strict where you live. Special requirements for garage/house fire separation, bedroom egress (escape), light and ventilation may be required for your minimum safety. With it inspected you will have peace of mind that what you build will be safe for your family and friends.
Be safe, G
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Old 08-28-2009, 06:52 PM   #4
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I actually did call the building department here. The person (who sounded like they were 16) on the other end basically said "good question" and then after a lot of "hmmms" and "ummmms" gave me the answer about going with vapor barrier in the shop side of the wall and on the top of the insulation for the loft.

But the answer didn't give me a lot of confidence in what to do. So I have posted her and a couple other boards. Unfortunately I have gotten answers saying both shop side and garage side and both are convincing arguements (garage side says melting snow and ice and rain water is more moist than the shop so put vapor barrier on that side).

I am leaning towards just using rigid foam insulation between the shop and the garage. Then the issue of the vapor barrier goes away. It is a small enough wall that the extra costs won't break me. However, for insulating the loft, I am still at a loss of what to do.
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