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Flooring and Carpeting/Heat insulating carpet underlayment


Hi, have an above-ground concrete slab founding my unheated three season front porch in Southern Wisconsin.

I want to make said space my bedroom.

The slab is a massive heat sink.

Mostly for the purpose of insulating against the slab, I am going to carpet it.

I have no moisture accumulation issues that I know of.

My question is, what is the underlayment I may put under the carpet that will have the highest R value?

Thank you.

My memory from reading years ago is that about 10% of the building's total heat loss is through the floor.  That said, it is hard to convince your freezing feet of this as concrete is indeed a great heat sink.

Being retired, I no longer have access to the hard data we developed.  R value of floor coverings is a function of the density, thickness and conductivity of the material itself.  Below are some measurement ranges I do have.   

A rebonded polyurethane pad that is 7/16" thick, with a density of 6 pounds per cubic foot has an R value of about 1.7, while a denser pad of the same thickness registers around 1.4, the higher density making it more heat-conductive.  The problem here is that density is reduced by by increasing the amount of air bubbles (cells) in the material, which makes the pad softer and less resilient.  The 6 pound density is the minimum mentioned in many carpet manufacturers' warranties,with 7/16" being the maximum thickness.  These specs were established to help prevent the carpet from wrinkling and, according to information from the pad industry, to improve the wear of the carpets. I don't have any data for 1/2" pads, but logic says the R values to be a bit higher. Pad manufacturers may have such data readily available.  

Thicker and heavier weight carpets offer higher R values, ranging from about 1 for a FHA minimum weight cut pile (25 oz.)(often called "builder grade") up to about 3.5 for a 90 ounce nylon cut pile.  

Combinations of pad and carpets range from about 2.5 to 5.5.

While these numbers may seem low, but compared to wood and laminate, both below 1.0, they reduce heat flow much better than the latter.  Moreover, a carpeted floor does not "suck" the warmth from a bare foot like a solid floor.  

I realize this information may seem thin, but there is little reliable data available.  The Carpet Cushion Council has some data at, that is similar to the above.  Note that most of that data relates to pads for commercial buildings.  

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Carey Mitchell


Any question regarding carpet, including specifications, maintenance, installation, regulation.


Retired after 44 years in carpet manufacture, technical and research. Served 33 years as Director of Technical Services for the world's largest carpet manufacturer.

The Carpet and Rug Institute, Technical Chair - 12 years; American Society for Testing and Materials (40 years); American Association of Textile Chemists and Colorists (39 years); Society of Cleaning and Restoration (Board of Directors); US National Textile Research Consortium; Cleaning Industry Research Institute (Board of Directors and founder); Canadian General Standards Board Technical Advisory Committee; The Fiber Society; Alzheimer's Association of NW GA (Board of Directors); Trout Unlimited; Federation of Fly Fishermen

AATCC Review (Textile Chemist and Colorist Journal); Journal of the American Industrial Hygiene Association; International Journal of Flooring Sciences Cleaning Digest; Cleanfax Magazine; Floor Covering Weekly; Floor Covering News; Proceedings of the Technical Conference of the Polyurethane Association; Proceedings of the Symposium on the Science of Cleaning;

BS in Chemistry and Business Administration, University of North Georgia

Awards and Honors
Carpet and Rug Institute, Smrekar Award, 1998; Carpet Industry Leadership Award, 1994; Cleanfax Magazine Person of the Year, 1996; Cleaning Digest, person of the year, 1997; Institute of Inspections,Cleaning and Restoration Certifications award,1997; ISCT Lipscomb Award, 2004

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