R-value for under-the-slab insulation in Atlanta, Georgia?
fedepfel
| Posted in Energy Efficiency and Durability on
Hello,
Building a house in Atlanta, GA, with exposed concrete floors (no radiant heating). What R-value should we use for under our slab on grade in this area of the country?
Thanks!
Federico
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Replies
Federico,
There is no code requirement for under-slab insulation in Climate Zone 3, but it's always a good idea. The problem is termites. Consult with your local building department and local contractors to find out local practice.
Ideally, you would at least install vertical rigid foam at the slab perimeter -- say, 2 inches thick by 2 feet wide -- but voracious termites often make this approach impossible.
Thanks,
yes, we found termite resistant insulation http://www.achfoam.com/Termite-Resistant-EPS.aspx
we are planning to use this product. In order to make it cost effective, we want to avoid over insulating under the slab. We consulted several local contractors and 9 out of 10 are not familiar with this practice. We are thiking that a 5 centimeter layer of termite resistant EPS with an R value of 10 and PSI 25 will be enough.
Any thoughts?
thanks again,
f
Atlanta seems best to deal with staying cool and so slab insulation may not be the way to go. Perimeter yes.
You can use Cellofoam PermaBG+. It is an insecticide impregnated XPS, and the manufacturer is in Conyers, GA (just outside of Atlanta for GBA readers). I had them cut my order one inch thick by 4 inches for the slab edge and 3 feet wide for the perimeter. If interested, call Dan at 864-940-5716. Give him a couple of days notice, and then you can pick it up. Shipping is prohibitively expensive since this is a "hazardous" product. The total cost for my basement was only a few hundred dollars.
As Martin said, code does not require foam under the slab in our climate. But I thought it was worthwhile.
Federico,
5 cm. is a little less than 2 inches. Most EPS insulation has an R-value of R-4 per inch, so 2 inches of EPS has an R-value of about R-8. If you use XPS, which has an R-value of R-5 per inch, you'll end up with R-10.
If you want to calculate the energy savings associated with any insulation measure, including rigid foam under a slab, you can use an energy modeling program. If you aren't familiar with energy modeling programs, you can hire an energy consultant to do the calculations for you.
Energy is very cheap in the U.S. right now, so a payback calculation on horizontal underslab foam in Georgia may show a very long payback. These calculations are uncertain, however, because no one knows the future price of energy.
Many people invest in above-code insulation for reasons other than economics. There will be some comfort benefit to increased insulation, especially during the winter. (In Atlanta, homeowners spend more for heating than air conditioning.) And it will never be easier to install insulation than during construction; after all, it's impossible to retrofit foam under a slab after the house is built.
Steven,
what did you use to secure the insulation to the edge of the slab?
Atlanta ground temps 62... the slab is at room temp of say 75 degrees for 9 plus months? Start to heat and R toward the slab without foam has got to be pretty good I would think from most of the year being exposed to 75 degrees. I still say just the perimeter with the whole slab being a mistake for more than one reason. Heating degree days for the cold months in Atlanta are equal to where I live for April and in April our heat needs are minimal.
With enough household activity a well insulated home in Atlanta should heat itself if not an uninhabited Mcmansion and therefore have more of an issue with the need to be cooled.
I would just do perimeter insulation.