Expert: Richard Taylor, AIA - 5/30/2005

Question
I am asking this question in order to help my daughter with a science project. She will be building simple models (probably just cardboard boxes) to test heat loss in an ordinary home versus that in an earth sheltered home. In order to have any semblance of worthwhile data, it seems that the above-gound home should have some sort of insulation that approximates, proportionately,  that of a real house. [The earth sheltered house should also be proportionately accurate, maybe using 1 inch of dirt for every 3 inches of building height.]The question is, what material would you use to achieve this? Any other tips come to mind? Many thanks for any input you can offer. - Amanda

Answer
Hi Amanda - thanks for your question!

The insulating value of building material is measured in "R-value", which is the resistance to the flow of heat energy.  The higher the R-value, the greater the resistance and the better the insulation.

Dry materials are better insulators since water is a good conductor - so dry insulation is better than wet insulation.  That's especially important with earth-insulated buildings.  Density is also important - you may want to use sand, which might more accurately mimic the density of dirt at your scale.

You'll have to scale these down to the sizes you're working with, but here are some very rough approximate r-values of materials per inch of thickness:

Dry earth: 0.33 per inch
Wet earth: 0.05 per inch
Wood: 1.25 per inch (assume the same for cardboard - not the corrugated kind but solid cardboard)
Fiberglass insulation: 3.5 per inch
Styrofoam: 5.0 per inch (use packing material or cut up a Styrofoam cup)

So in order to compare a house wall with 3 inches of fiberglass insulation and 1/2 inch of plywood siding (total r-value of 11.125) to the same r-value of earth-sheltering, you'd need about 36 inches of dry earth.  Once the earth wet, you'd need 222 inches (18 feet) to get the same effect.

However, earth-sheltering isn't that simple.  Keep in mind that the earth has a consistent temperature of about 55 deg below about 8 feet, so you're actually gaining heat from the earth for much of the year.  And earth absorbs and retains heat from the sun, which can have an effect on the house.  True heat-loss heat-gain calculations are quite complex.

Most of the heat loss in a house happens through "loose" construction; that is, the "cracks" around windows, doors, etc.  Another area of large loss is the roof.  These are the areas we concentrate on in design.

Good luck with your project - hope this helps!

Richard Taylor, AIA
Richard Taylor Architects, LLC
www.rtastudio.com
www.betterhomedesign.blogspot.com