Insulation under concrete basement floor slab

Originally Posted By: kgrabowski
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I have a client who is building a new house and putting in geothermal heating. She has asked me if its a good idea to add insulation under the concrete floor slab. They are going to use a 10 inch gravel base underneath the slab. She wanted to know what the pros and cons were when adding insulation underneath the floor.

Any ideas?


Originally Posted By: bkelly2
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Kerri do a little research here.

"I used to be disgusted, Now I try to Be amused"-Elvis Costello

Originally Posted By: clawrenson
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Some R2000 builders incorporate it in their design. It does help provide additional thermal efficiency - even below the slab below grade for basements.

Here is another source -

Ontario Home Inspections Inc.

Originally Posted By: mcyr
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Hi. Insulation under the slab would provide you with an R-factor of 5 per inch and would be recommended if radiant heat was involved. In your case the geothermal design might not help depending on what is be accomplished. If you have a basement, insulation would prevent the cool soil in keeping your basement cool in the summer, but keep it warmer in the winter.

The explanation below hopefully helps.

Geothermal heating is a method of heating and cooling a building. It takes advantage of the natural stable warmth stored in the earth. Normally the earth is around 55?F (12.8?C). This is accomplished by one of a number of methods. A heat pump uses the extracted water or transfer fluid as a heat source in winter and a heat sink in summer. Some systems use part of the heat to heat the building's hot water.

Types of geothermal systems

Long, shallow loops buried in the ground
Pipes circulating fluid through or drawing water from a nearby body of water
Deep wells - usually two or more wells over 200 feet deep
Either of these systems can be either open-loop or closed loop. In a closed system, water and antifreeze (or other transfer fluid) is circulated through the heat exchanger and back out through the loops continuously. In an open system, water is drawn from depth in a deep well system or from a body of water in a shallow system. Once this water passes through the heat pump, it is released back to its source, generally as far from the intake as possible. Some closed loop systems bleed off a portion of their working fluid to keep the source temperature stable.

Geothermal heating is one of the most efficient ways to heat a building but it has high initial costs for drilling the wells deep enough into the earth to take advantage of the earth's temperature. It also has costs involving electrical usage to run pumps and other electrical accessories.


Originally Posted By: roconnor
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Geothermal heating systems under slabs are not common in my neck of the woods, but be careful placing anything directly below a slab that does not allow some moisture to escape (some insulation can act somewhat as a vapor barrier depending on the type).

The top of the slab can cure (lose moisture) much quicker and may cause what is called "slab curling", which also tends to crack the slab. It is problematic if the right mix isn't used and especially if the slab is not "wet cured" for at least 7 days in decent weather.

Some type of granular soil layer between the slab and any moisture barrier is usually recommended by ACI as long as there will not be a moisture-sensitive floor covering placed on the slab.

See the "Technical Links - Structure" for more info.

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Originally Posted By: mcyr
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I would recommend that since you are in Winnipeg, MB, that a foundation exist like Maine and a vapor barrier be installed under the slab that is at least 10 mils thick. The barrier would be lapped at least 6" and taped using the manufacturers product.
You could get by using the standard 6 mil poly, but that is susceptible to physical damage while the slab is poured.

I would strongly recommend to research the geothermal requirements on this issue.

Hope this helps and welcome you to any more questions you may have.