Thermal resistance (R value) of earth 3

[seventhstream]

Hello.. would anyone know why I could get typical R values of earth fill? I am looking into the feasability of replacing the thermal insulation of a couple feet of earth with some sort of insulation.... this would be for the roof of a buried concrete structure. We could then try to reduce the amount of earth fill on top of the structure. thanks in advance!

 

[LeoSC]

The paper in the following link has some information which may be useful, although it is specific for replacing soil in rock trenches

http://irc.nrc-cnrc.gc.ca/fulltext/nrcc43093/

 

[seventhstream]

thank you very much. I've seen this as well, seems this subject is complicated. I appreciate your time.

 

[Focht3]

The thermal coefficients are influenced by many factors, one of the most important of which is soil moisture content. This subject is, indeed, complicated - because soils are inherently heterogeneous and nonlinear.



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[DRC1]

Contact Geotherm in Canada. They seem to the leaders in this area.

http://www.neptune.on.ca/~geotherm

 

[CarlB]

In Alaska I've heard many times the "rule of thumb" that 1 inch of insulation is equivalent to 1 foot of soil. I believe this is quite conservative; having run the Modified Berggren frost penetration model many times indicates 1 inch is equivalent to 2 feet or more of soil.

 

[seventhstream]

thanks gentlemen.... I especially appreciate the rule of thumb. The science of all of this complicated, but obviously the simple stuff is that resistance of dirt is a function of its moisture content. I am looking to replace a couple feet of fill above a concrete reservoir with an insulation, but I keep getting stimied by the variable resistance for earth.... oh well....I know we insulate foundations. I wonder how those R values get determined? Seems to me you need to at least estimate the resistance of the dirt that you are building in. thanks again for your comments, I appreciate your know how and generosity

 

[Focht3]

As I recall, the "R" values represent the insulating value of a given thickness of granite; i.e. an R value of 19 is equivalent to 19 inches of solid granite.

Rather than focusing on the "dirt", why not determine how much insulation you need and then figure out how much soil you should replace? And remember that the insulation is a lot lighter than soil, so you can create a significant reduction in dead load by using Styrofoam or similar lightweight hydrophobic closed cell material instead of soil. The reduction in the roof load alone might be enough to justify the increased construction cost.



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[seventhstream]

thank you Focht3....in the construction world, they use R value as the reciprocal of thermal conductivity...but thanks for your comments. Apparently it is not something that is general enough for use...you barked right up my tree with your comments; we are trying to estimate the amount of resistance required to protect the concrete. I'd like to take a stab at estimating what sort of R value I can get from earth fill (dirt)and supplement it with rigid insulation. I was also looking into what is called "green roofs" - they insulate below the dirt, but I've not found anything that exposes their heat loss calculation. oh well....

 

[HMROSS]

I don't visit this site often but here is my contribution.

Thermal Mass is different than heat transfer resistance. R-Value is heat transfer resistance only. Typical R-value for earth is 0.25 per inch; however, the thermal character of the soil mass is quite different than that of material designed and used primarily for heat transfer resistance (R-value) like polystyrene or fiberglass. When changing from soil to a less massive "insulating" material, you should understand the design performance with regards to heat capacity of the building and/or soil mass (K-value). A massive soil wall or roof can store heat energy to even out the temperature swings of day--lightweight insulation does not perform this way. The thermal value you get from 18-inches of soil far surpases the 4.5 R-value (0.25 per inch). Other factors such as water and vegitation can signifcantly change the performance of an earth sheltered home or building.

 

[seventhstream]

thank you HMROSS... that was an excellent description of what I am trying to hunt down....(little did I know as I started this thread!) Is there a way to equate the two? To replace the thermal mass of 2' of earth with a thermal insulator of certain R value? I am looking to insulate the top of water filled vault....without putting lots of dirt on it... any comments are welcome....Thanks again!

 

[HMROSS]

jhosborn,

Most of your questions are answered in a book by John Hait, "Passive Annual Heat Storage (PAHS). Think heat flow when comparing the two. You can not just replace one with the other and get the same response (heat/storage/flow). Hope this helps.