Do I need a vapor barrier for my tool shed?

Old 10-15-04, 12:22 PM
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Do I need a vapor barrier for my tool shed?

Hey, guys. I'm building a 10'x12' tool shed on concrete posts. I've got the concrete posts in the ground and have framed the joists. I'm gonna use 3/4" exterior grade plywood for the sub floor. I was mentioning this to a friend of mine and he stated that I should definitely use a vapor barrier for the floor.

Now I did some research and it appears that the only time vapor barriers are used for floors are when the floor is a full concrete slab. I couldn't even find an example of using it when the subfloor is supported by concrete posts. I imagine I'd stretch it on top of the joists, staple them down then put the plywood subfloor down.

Now I'm not putting heat in my shed so I never really have to worry about heat flowing from inside to out. Anyway, the question becomes, do I need one? I've been working off of the home depot book and they (once again) only mention it in the full concrete floor section of the tool shed build, although they don't mention what type of floor you need it for (if any of that makes sense). Anyway, thanks for any help you can give.
Old 10-16-04, 10:13 AM
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As long as you have air flow under the shed, a vapor barrier should not be needed. However if you totally enclose the perimeter of your shed to where no air can freely flow, a vapor barrier would be recommended. You could also install vents to permit air circulation if you enclose the bottom.
One other thing to think about if you enclose the bottom of your shed. Rats, mice and other assorted critters love dark moist areas to make their homes.
Good luck with your project and post back if you have more questions.
Old 10-16-04, 03:10 PM
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I would use poly (4mil.) as a vapor barrier on top of the joists since it sounds like you are in cold wet environment. The nice thing is for the couple bucks it costs you termites have a hard time fiquring there is wood under it since it doesnt smell like wood. I used plastic on the studs on my house 22 years ago and after pulling some siding because if termite penetration I found they hadn't done any damage, there was some dry rot on the bottom of the panels (T111) and even where the termites came out where the studs were they didn't penetrate the plastic and the dry rot didn't penetrate the plastic (which it normally would have). A true vapor barrier would be laid on the ground to prevent ground moisture from coming up.

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