Insulate under basement floor slab, soil gases?

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Posted by: from Edmonton
5/24/2016 at 10:44:56 AM

Building a new home and trying to get an opinion regarding placing 1 1/2" of rigid insulation under the floor slab. The floor may or may not be heated but in any event would like to add some protection against moisture finding its way through the slab as the site is fairly wet. Presently here is an interior drainage system and 4" of sand. There isn't room to put gravel on top without losing some basement headroom; however this is also a possibility.

Also concerned about the potential issue of radon/soil gases. Although radon is not thought to be an issue in our area, the interior weeping tile system could also perhaps be used to capture and vent soil gases in the event that testing reveals elevated levels. Of course there is also the option of putting a DRIcore type panel over the concrete so long as the slab is not heated.

Looking for thoughts (pros/cons) regarding these panel systems.

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Date/Time5/24/2016 at 5:18:48 PM

Ridged foam board will not stop water or gases but it will help warm slab / Gravel is a must for proper drainage and Dricore will take any cold foot worries away (I prefer making own sub floor Delta Wrap then Plywood 4/8 Sheets 5/8 in think water resisting plywood stagger joint screw into slap silicone holes tapcon screws)



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Date/Time5/24/2016 at 5:42:23 PM


You need the gravel bed under the concrete slab. There is no question about that. The Dricore panels will provide a place for water to run below if you ever get any in the basement. A cracked foundation with a slow leak or multiple other possibilities that can cause a leak.

I always install them in a basement contract unless an area is being tiled.

Hope this helps.


James Fram

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Date/Time6/17/2016 at 10:47:53 PM

Hi Richard,

You are very wise to be addressing your concerns of the "fairly wet site" and potential "soil/radon gases" before pouring your slab. The costs of repairing long term damage to your slab and foundation and living space from these conditions is exponentially more than the costs right now, not to mention possible health risks. Concrete is porous and can remain damp or saturated for long periods of time, depending on the moisture conditions around it, wet or damp basements, finished or unfinished, smell terrible and are unhealthy living spaces. I do not recommend rigid insulation in this instance. I have removed saturated rigid insulation many times (even the top end closed cell extruded type 4) and the material weighed what seemed to be 2 to 3 times it's dry weight and sometimes would break under it's own weight. Wet insulation has little or no R-value, and the shiplap joints every 2 feet will not protect you from the gases, even if they don't spread apart during the pour.

Spray foam insulation installed below the slab and right up the foundation walls, before you pour the slab, will provide the driest and most consistent environment possible. I have attached a video link that shows installation and radon testing of spray foam insulation below a slab. This is the best basement protection available.


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