How to Insulate a Slab on a Grade
"Concrete foundations, including slabs on grade, wick heat away from a building's floors and increase the costs of heating the building," according to the U.S. Department of Energy. In many regions, insulation applied to the edges of the slab will reduce heating costs by as much as 20 percent and will pay for the cost of the insulation in five to 10 years. Slab on grade insulation is installed shortly after the slab is poured and before the foundation trenches are back filled. Does this Spark an idea?
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Things You'll Need
- Internet access
- Web browser
- Rigid insulation
- EPDM membrane
- Finish material of choice
Instructions
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1
Check your local building codes to be sure rigid perimeter slab insulation is approved for use in your area. Some states in the southern U.S. prohibit the use of this insulation underground because of the increased chances of termite infestation.
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2
Find your area's actual heating degree days by checking with your local weather station. You can find your weather station by entering your zip code at the National Oceanic and Atmospheric Administration (NOAA), National Climatic Data Center, website. If your heating degree days are fewer than 2,499, stop. You don't need to insulate your slab.
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3
Compare your area's actual heating degree days to the chart published by the U.S. Department of Energy on its website. (See the link in the Resources section of this article.)
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4
Select the appropriate R value of insulation needed for your area from the chart referenced in Step 3.
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5
Place the insulation directly against the slab's exterior edges extending it down to the bottom of the frost line. In some jurisdictions, a termite gap may be required between the sill plate and the top of the insulation, so check your local building codes for proper placement in your area. This step can be done once the concrete forms are removed from the trenches after the slab has been poured.
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6
Cover the insulation with an ethylene propylene diene Monomer (EPDM) membrane to minimize wicking of moisture from the concrete and to discourage termites. This material is often used for pond liners.
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7
Apply the desired finish to the top 6 inches of insulation that will be exposed once the soil has been back filled against the foundation.
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8
Back fill the soil against the insulation.
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Tips & Warnings
Some finish material choices for Step 7 include stucco, pressure treated wood, brick and aluminum flashing.
Consider using rigid insulation that has a termiticide (usually boric acid).
Inspect for termites regularly.
Install metal termite flashing below the sill plate.
Some jurisdictions may require insulation beneath the floor portion of the slab extending inward a certain number of feet from the edges. Check local codes.
Remove all wood from foundation trenches before back filling.
Be sure the bottoms of the turned-down edges of the slab are well drained. Install slab drainage pipe and gravel if necessary to keep moisture and water from pooling along the edges of the slab.
