Vapor Barrier Under a New Concrete Basement Floor

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Posted by: from Ottawa
1/8/2013 at 10:05:17 PM

I'm in the process of removing the existing basement floor in my house along with the rock and dirt underneath, down to the undisturbed soil. Then put in the proper amount of crushed stone but have a question on the best product and ways to do the vapor barrier.

What is the best approach? Oh, I'm not going below the current footings or walls so no need for recommendations on underpinning.



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Erroll from MVP Construction in Surrey
Date/Time1/8/2013 at 11:39:57 PM

Not sure how much room you have to work with. For best results and optimum insulation it's best to use a good compactable sand. Compact it. Then lay down ridgit insulation. Then your vapour barrier of 10 mil poly. Make sure to tape all the seams, or around any plumbing pipes. Make sure you have a minimum overlap of a foot on seams. Use tuck tape.

Now you can either rebar with a 10m bar or use a wire mesh. Try and seat the rebar so it is at about an inch to inch and a half below the finished slab.

Make sure your plumbing and anything else you want right in the slab is correct before pouring.

Good Luck

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Date/Time1/9/2013 at 12:44:06 AM

6 Mil Poly Vapor Barrier will work fine, tape the joints and caulk the edges to the concrete wall at the perimeter. If you have soil gas issues, such as Radon gas, this would be a good time to put a gas mitigation system in.

Check with your local building authority if you live in an area with gas issues. It is not that expensive or difficult but is very important if needed.

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Date/Time1/9/2013 at 2:09:17 AM

Hello Edward,

Do or did you have any moisture problems on your existing concrete slab before? If yes, then it is very important to break the capillary action by filling in the proper amount of crushed stone ! Don't use sand for this !!!

Thereafter (on top of your crushed stone) you could use a layer of sand to cover the sharp edges of the stone. On top of this layer sand you should place a layer of a Poly Vapor Barrier to prevent the rigid foam from soaking water. Most rigid foams are not "clost celled" and will wick water if exposed to it. In this case your insulation would be going to zero and the foam would act like a sponge !!!

It is not a bad idea to install another double layer of the Poly Vapor Barrier on top of the rigid foam a) to prevent the foam from going wet during the concrete install and b) this layer will act as a cleavage membrane so the concrete slab can "move" a bit. Around the perimeter you should install a 10 mm thick foam strip a bit higher than the thickness of your concrete slab. This is very important for the necessary movement of your slab because it will always a bit expand and also a bit shrink.

Now you can install your concrete slab with a thickness of at least 4 inches! Keep in mind you'll need expansion joints about every 10 to 12 feet. Don't forget to rebar with 10mm bars (don't use mesh because it is not appropriate) and install the bars in a raster of 4 to 6 inches by 4 to 6 inches. The "bar-grid" must be located about 2/3 deep from the finished surface. That means you have to put at least 1 inch to 1 1/2 inch thick "spacers" (don't use wood as spacers !!!) Underneath your grid to keep it from the ground (the rigid foam) when you pour the concrete in. It is not good at all if the "bar-grid" is laying on the ground while pouring the concrete in and thereafter lift the grid up a bit like some people do it, because it would be not properly embedded at the right place inside the concrete slab !

If you don't need insulation underneath your slab, it is also possible to pour the concrete right on top of your first "sand-bed". This layer of sand will also act like a cleavage membrane. And instead of installing a Poly Vapor Barrier you could use an additive to mix the concrete which would make the concrete water tight.

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Date/Time1/9/2013 at 2:10:25 AM

The extension joints, rebar and movement joints around the perimeter are necessary in an case !!!

3D-Tile-Design - Betram Tasch

Maple Ridge, BC, (Greater Vancouver)

+1 (778) 886-9503

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Date/Time1/9/2013 at 9:48:31 PM

Hi Ed,

I do believe that you have gotten sum bad advice. In forty years of custom contracting I have never even heard let alone done any of those things that have been presented to you.

The basement floor is simple 5" 3/4 clear stone 31/2"of concrete. If there is a water problem put 4" drains in the stone to a sump hole. If the moisture is coming through the walls then the problem must be addressed from the out side.

Concrete is concrete, it is always cool and it always cracks. The best way to warm a floor is to have two or three cold air returns at the basement floor level, leaving your furnace fan running slow will draw the cool in and allow the floor to warm.

If you use sand it will trap the ground water below the floor and the build up pressure will cause big problems. The clear stones allow the ground water to flow easily to the proper drainage.

You can insulate the floor but it will still be cold unless the rooms are properly heated. Even the best insulated house in the world will be cold if not heated properly.

There are many factors to consider.

Hope this is helpful.

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Gus from Triworx Design in Vaughan
Date/Time1/10/2013 at 12:37:04 AM

As per code; Use 3/4" crushed stone, 2' rigid insulation (min) then pour your concrete. Temperature

bars has to be placed mid point the thickness of your slab. If your slab is 4" thick, your temp bars has to be 2" below the slab.

Around the perimeter where the wall and slab meets, a half inch Expansion joint has to be made.



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Date/Time1/13/2013 at 4:21:35 AM

Hi Eric,

With all due respect, but if you build a concrete slab in the way you described (nevertheless if this is by code or not) it is no wonder why your slabs brake. As you said: There are many factors to consider and one of them is the proper reinforcement of a concrete slab

@ the Gus from Triworx Design

A reinforcement which is placed in the middle of a concrete is in most cases pretty useless. Depends what the concrete slab is designed for, it has to be placed either below or above the middle of the slab and in some cases at both positions, below and above the middle of the slab.

3D-Tile-Design - Betram Tasch

Maple Ridge, BC, (Greater Vancouver)

+1 (778) 886-9503

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Edward in Ottawa
Date/Time1/29/2013 at 11:39:53 AM

Thanks for your replies it was very helpful.

I have one quick question on the finishing of the cement floor. I will be laying down an engineered laminate flooring and have been advised that since I'm putting this down over the cement floor it would save some money if I did not polish finish the cement but just hand trowel it so its smooth and level. I will have 1 1/2" of insulation under the 4" slab.

Should I just have them to do a hand trowel finish or pay the extra and have it polished with the machine?



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