Soil on foundation wall (C) Carson Dunlop AssociatesInsulating R-Value of Soil or Dirt

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Insulating or R Value of soil: this article describes the insulating value of soil or dirt such as the insulating value of soil against a building wall or foundation wall.

We include soil R-values and we discuss the effect of moisture and soil density on R-values or heat loss rates.

Our page top sketch, courtesy of Carson Dunlop Associates, illustrates the effects of soil density and moisture as a source of pressure on a foundation wall.

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What is the R-value for earth, dirt, soil, backfill, or earth berms? How about the R-value of gravel & sand?

Photograph of our contractor's radiant heat folly, a really bad radiant heat slab installation that had to be abandonedReader question: Sir: Does InspectApedia have an R-value for earth when used as a berm on an exterior concrete house wall? Thank you R.J.

Reply: Earth or soil has an R-value of about R 0.25 to R-1.0 per inch at 20% moisture content and other assumptions discussed here

Sand or gravel aggregate has an R-valueof about 0.08 to 0.11 per inch at 20% moisture content or less. Some sources quote a much higher R-value for sand and gravel aggregate but you need to look at the thickness or amount of sand or gravel when you see such quots. For example some cite sand and gravel aggregate as providing R 1.11 FOR 8 INCHES of thickness. That's equivalent to an R-value of R 0.1388 per inch, but without considering moisture level.

But really, the insulating value of earth depends .... as we elaborate below. A complete table of the R-values of soil and other mateirals is found

At left we illustrate the preparation of a radiant floor slab in contact with soil. A contractor SNAFU left exposed soil (visible in our photograph) that conducted heat away from the floor - discussed separately

As we note below, the R-value of the wet soil (sketch center) will be much lower than dry soil outside of the same volume of dry soil (sketch left). Freezing at the upper level of such wet soil also will affect its heat transfer rate as well as risking foundation damage as we show here.

A short answer to the R-Value of Dirt - about R 0.125 to R 0.25 per inch.

Some sources we researched assert that "one inch of 'insulation' is equal to about two feet or more of soil. If we take 'insulation' to be a bit more specific, say the most commonly-used material, fiberglass, that's about R3 /inch for fiberglass, or if we believed the soil R-value rule of thumb about dirt, that's about 24/ 3 = about R 0.8 for arbitrary "dirt" insulation value.

If 24" of soil = R3 the R-value of 1" of soil = (3 / 24 ) or R 0.125

- Thanks to reader Timothy Carlson, 8 June 2015 for correcting this calculation.

R 0.125 per inch for soil sounds pretty reasonable if we assume about 20% moisture content, and if we consider for comparison or a "sanity check" that the R-value of uninsulated concrete is about R 0.8/inch.[1] Other engineering sources cite the R-value of earth as about R 0.25 per inch or double our calculation. Without normalizing for soil properties and moisture content, these numbers are very arm-waving rules of thumb.

But these soil R-values may be rather unreliable given the discussion below about the effects on heat transfer of soil properties and soil moisture. Heck even snow does better, at about R1 per inch. In addition to avoiding the confusion that comes from an unreliable R-value for earth (take R 0.25 if you like), discussions of earth berm housing and underground housing usually consider the effects of thermal mass on building comfort, not just R-values.

R-values measure resistance to heat flow or transfer between materials. But thermal mass considers the storage effects of the mass of soil (or concrete block or ?) or other materials that comprise and surround a building.

Thermal mass stores heat and returns it during cooler periods, evening out swings in building temperature. So let's keep in mind that while the R-value of two feet of soil outside of a building wall, say, may be between R 0.135 and R 0.5, that 24" of dirt has much greater thermal mass than the same quantity (in equivalent R-value) of an insulating material such as fiberglass or solid foam insulation.

What all of this means is that it is a mistake to try to equate thermal mass and insulating values, and it makes no sense to forget about heat flow rates in or out of a structure if you are paying to heat or cool a building.

Details about the Insulating Properties of Dirt, Soil, Backfill, or Earth Berms

The R-value of earth depends on the type of soil and its water content. Even more significant can be the movement of groundwater through the surrounding soil, as moving water will significantly increase the rate of heat transfer from warm to cool areas.

At least important to anyone asking this question will be the assumptions about

The soil temperature Ts at some depth where it is stable (such as below the frost line in a freezing climate, perhaps as deep as 20 feet. A Journal of Light Construction online forum discussion of soil insulating properties includes the observation that

" [earth provides a ] huge amount of thermal mass, and that's what you'll be working with or fighting against. The soil temperature at about 20' is equal to the year round ambient temperature, so that will tell you what you'll be working with/against.

If you want the room warmer or cooler than that, it's easier to install insulation and create a thermal mass inside that insulated envelope, if the ambient temperature is close to what you want, well, you don't need heat."[2]

For a more scholarly discussion of the insulating properties of soil you should consult a heat transfer engineer or a soils engineer. But here are my views of some important parameters to consider when assigning an insulating value to soil:

Material I've reviewed about earth sheltered homes and schemes that use electric radiant heated floors over un insulated soil (where electricity is dirt cheap), but I'd prefer to evaluate that "design" with comments by heat transfer experts since it seems to me that any system that pumps heat into un insulated ground in a cold climate is spending a significant portion of their heating dollar to return heat to Mother Earth rather than to Mommy upstairs.

The claim that "heat you pump into the ground under or around a home doesn't really go anywhere" is in violation of the basic laws of thermodynamics and is simply not so. Heat flows from warmer to cooler materials.

Sure we can expect there to be a temperature gradient in cool soil beneath or against a heated building, but heat flows from warmer to cooler materials, it doesn't magically stop dead at some arbitrary distance. Just where energy costs are very low and are expected to stay low might it sound plausible to use un insulated earth for heat storage under or around a building.

References for the insulating properties or R-Values of soil or dirt or earth

Reader Question: more on how to figure out the R-value of soil or dirt

Hello, just noticed that your insulation value for dirt is inaccurate. if you are saying that 24inches of earth insulates the same as 1 inch of fiberglass, or R3, than that means 8 inches of dirt has R1, or that an inch of dirt is R 0.125. Am i wrong? Cheers, - G.R. 2/29/2013


In this article, just above, we include a longer discussion of this question about the insulating properties of soil or dirt.

In fact there is no single right soil R-value answer without considering soil moisture levels and soil density, particle composition, but our research did find some interesting scholarly articles that gave a range of values. Above we give quite a few source citations on this topic.

In sum, if you like a dirt R-value of R=0.25 per inch of soil (which is within the range for soil R-values we discuss), then 24-inches of dirt at that R-value and moisture assumptions would be about (0.25 x 24 = 6) or R-6.

Reader Question: 2006 IECC: effectiveness of foundation perimeter insulation and insulation recommendations for radiant-heated floor slab designs

I would like to know what the persons that wrote and researched this article thinks about what Montana has on research. On their web page MONTANA SLAB EDGE INSULATION ANALYSIS FOR 2006 IECC ADOPTION [PDF]. There seem to be so many theories on this.

One thing we have found that if the soil conditions are quite damp, there definitely needs to have some type of insulation under the slab.

Another theory I have read is that the heat as it goes down, which it will, some is that it radiates horizontally, which makes insulating the edge quite well. - Wendell Schubloom

Reply: thorough under-slab and perimeter insulation and proper tubing depth are critical for radiant heat floor slab designs

Wendell, there is not actually any contradiction between the Montana (DOE) research you cite above and radiant heat floor slab insulation requirements. The study you cite does not focus on radiant slab heating designs but or a more narrow question about the benefits of foundation/floor slab perimeter insulation.

The DOE photo (below left) shows a typical Montana construction practice that gives a thermal break between a concrete floor slab (not yet poured) and the exterior foundation wall.

I've read quite a lot of supporting research on slab and slab perimeter insulation for radiant heat flooring, and I have some direct experience with installing radiant heat and more with inspecting radiant heat flooring problems.

Typical Montana interior slab insulation design - U.S. DOEQuoting from the conclusions of the Montana DOE-sponsored study you cite, [2] [photo at left showing interior foundation insulation before the slab is poured, U.S. DOE, op cit.]

This study shows that insulating slab edges with R-10 insulation to 4-ft depth along the slab edge saves about 3% annual energy and reduces annual fuel cost by between 1 and 2%. The energy savings vary slightly depending on the insulation configuration and building type.

Although the current installation practice in Montana does not extend the interior footing insulation to the top of the slab, based on empirical data, this study concludes that irrespective of the insulation installation configuration, Montana buildings will save energy by insulating the slab edge with R-10 insulation to a depth of 4 ft. The payback period could vary from 4 years for small retail commercial buildings to 12 years in small office buildings.

This study, using eQUEST, Version 3.0 simulation modeling, compared full versus partial slab perimeter insulation schemes and found that there was useful energy cost savings even with partial insulation.

The study data includes comparison with fully-insulated slabs too, but most important for our discussion, it does not address radiant-in-floor-slab heating designs that, without full insulation, can find an easier heat flow into the ground than into the building - not what we want to see nor pay for in heating bills. Quoting:

The local practice of insulating the slab footing on the interior allows heat loss along the slab perimeter and thus does not achieve the full savings that could be achieved with full edge insulation configurations, but the savings are still significant.

The risk in misinterpreting the Montana study conclusions above would be to apply them generally to radiant heat floor designs and that to improperly infer that complete under-radiant-heat-floor-slab insulation is not needed in cold climates.

That study makes a general conclusion for all Montana buildings and by no means does the conclusion adequately address radiant in-slab heating system designs. The fallacious concept held by the contractor in our horror story was that "once you heat up the earth below your building it will start "giving back" heat to the building and you'll be just fine. His theory was nonsense, as both expert advice and actual field experience proved.

The earth in a cold climate like Montana or Minnesota, is for practical and design purposes, an infinite heat sink. A radiant floor slab heating system will, if improperly designed, keep pumping heat into the ground as long as the heat is turned on. Forever. We saw this in astronomical heating bills and a cold building interior in the Minnesota home discussed above. Heat always flows, and continues to flow from a warmer material into a cooler material.

Heated the soil beneath a building where insulation was incomplete, inadequate, or omitted, will never reach some magic perimeter after which it stops sending heat into the surrounding soil any more than an ice cube placed into the sea will stop melting because it's "cooled down" the water around itself.

As the principal author of the original material at RADIANT HEAT MISTAKES I relied largely on the concrete industry and the radiant flooring industry's radiant floor slab design specifications and advice [1] as they, above all, have a huge vested interest in their installations being successful.

There is no doubt that in virtually every radiant-heat-floor-slab design we need continuous insulation under the slab and at slab perimeter, though the appropriate insulation amount might vary depending on the local climate.

The folks who seem to disagree have been people like the bully contractor who himself admitted he had never read instructions, attended a class, nor asked for expert advice. As is often the case with small contractors in remote areas and without expertise, he was "winging it". Don't try mentioning "thermodynamics" or "heat flow theory" to a bully.

Just how bad an un insulated, under-insulated, or incompletely insulated floor slab will perform with radiant in-slab floor heating depends on some additional variables: climate, soil moisture (read thermal conductivity as you suggest), and critically, the depth of tubing in the slab. In ALL cases we want the insulation in place.

But in the horrible installation we describe in these articles, the contractor not only provided incomplete and no perimeter slab insulation, he also buried the tubing so deep in the concrete that heat moved much more down into the cold earth than upwards into the occupied space.

There was so much heat loss that we could not get the room temperature up even in cold but not bitter cold weather, and even though the same contractor had done a great job insulating the upper portions of the structure's roof and walls. (He was a framer/carpenter, and should not have attempted radiant slab installation nor tile work.) That's why we had to abandon the whole radiant floor installation.

If the floor slab had been very well insulated, the installation still would not have performed well because of the excessive tubing depth in the slab ( over 12" down in some sections ).

I appreciate the Montana reference and have added it to this article below at references [2].


Continue reading at INSULATION AIR & HEAT LEAKS or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see INSULATION R-VALUES & PROPERTIES - R-values & U Values of various materials

Suggested citation for this web page

SOIL R-VALUES at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


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