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vapor barrier under concrete floor?

12 years ago

Hi all,

We are getting ready to pour the concrete floor in our basement, and then we will do the garage floor. We have decided to do 2 inch rigid insulation under the cement in the basement floor and are wondering if we should also do a vapor barrier?

In terms of the garage floor, we are laying down the 2 inch rigid insulation around the perimeter of the garage floor, with just the gravel in the centre of the garage. The insulation is rather costly and the garage will not be heated (hence only putting it in the perimeter). However, we also wondered if we need a vapor barrier under the garage floor?

I am getting conflicting info on the necessity of a vapor barrier so I would appreciate hearing what others who live in a heat dominated climate have done under their garage and basement concrete floors. BTW, I live in southern Ontario so it is not that cold (sort of like a Niagara Falls NY).

Thanks,

Carol

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Comments (5)

worthy
12 years ago
Two-inches of XPS is classified as vapor semi-impermeable, so no further barriers are needed. A vapour barrier under the slab is optional in a typical garage. But if you're storing collectible vehicles, it would be a good idea, as it slows vapour inflow into the garage.

(I build in the GTA.)
ontariomom
Original Author
12 years ago
Thanks Worthy for your help. I didn't know the XPS was classified as semi-impermeable. We are not storing collectable cars in the garage, just regular cars, bikes and other storage. We have very sandy soil. I'm guessing poly plastic under the slab would keep the stored goods dryer. What did you put under your garage floor?

Carol
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Sorry. The addition is 16x16 and 4 years old. Its poured concrete, and is under the kitchen, so lots of water lines. The rest of the house varies from 30 to 100 years old. I have 6mil poly on the floor, which is very uneven as its ledge. Only 2-3 feet of working room, and hard on the knees. I want to put the barrier on the walls and lap it it over the poly on the floor to make a better seal. The insulation is a benefit as it will help keep the space a little warmer. Currently, I have fiberglass insulation pushed up in the joists, but I am considering taking that down. Also, some of the wall space has 2" XPS foam boards attached with contact cement. I also still have condensation down there, and some mold, but I plan to get a dehumidifier next summer. The only remaining question is foam or glass bats in around the box of the house.
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worthy
12 years ago
I didn't know the XPS was classified as semi-impermeable.

See here, first link, p.7.

Actually, I've never used any vapour barrier under garage floors.

For these and a few other questions you've posted, there would have best been recommendations in your building specs from a local P.Eng.* who reviewed the plan. However, I know many/most designers here don't go that extra step. Considering even the cost of a modest home, I think the $500-$600 cost would have been worth it.

*mrs. worthy's OBY/GYN is also a P. Eng., but we've never solicited construction information from him!
ontariomom
Original Author
12 years ago
Hi Worthy,

I will check out the link you posted -- thanks! The architect did consult with a structural engineer several times (and we paid more than $600.00 for this service). Not sure if the engineer was involved in doing specs for the garage floor, but I doubt it. I agree having a P.Eng review plans is important. I start second guessing when the contractors offer different opinions than what the architect has in his specs.

Carol
worthy
12 years ago
when the contractors offer different opinions than what the architect has in his specs.

They try this on me all the time too, sometimes in as a belittling way as possible. ("Have you ever built a house before?") More often than not, it involves them saving money and effort rather than improving the durability and longevity of the building.

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