Q: Well I've finally picked up the shovel and started building a modest 18 x 14 one-room affair. I'm having a little trouble trying to decide what to do as far as insulation. I've read what appear to be conflicting ideas about insulating an earth-sheltered home like this. Many folks insist upon insulating the whole thing (floor, all wall area, roof) with something like foam board insulation, insisting that if you don't you'll lose all your heat to the surrounding soil.

However, I was reading an older earth-sheltered book ("Earth Sheltered Housing Design: Guidelines, Examples and References," by the Underground Space Center University of Minnesota 1979) which offered different advice. Basically the authors use some computer models to predict the energy flow and usage of several earth-sheltered and underground designs. In their discussion of walls and floors, the authors argue that insulating the entire wall/floor structure defeats a main advantage of earth-sheltered housing: the great thermal mass of the surrounding soil which moderates both cold winter temps and warm summer temps. Specifically, they compared two wall insulation schemes: one where insulation was run down the entire length of the 10' concrete wall (on the outside, next to the soil); the other where insulation was run only on the 5' of the wall closest to the surface. Their computer model concluded that the partial insulation method was best in overall energy efficiency. While the full insulation method saved 5% in winter heat losses, that came at the sacrifice of 10% improvement in summer cooling for the partial insulation method. Apparently they didn't test a wall with no insulation whatsoever. As for floors, their model predicted that 1" foam board insulation under the flooring would reduce *winter* heat loss through the floor only 5% (ie a 1% decrease in the total building heat loss). Adding that same 1" in the roof, however, resulted in a 20% decrease in ceiling heat loss (ie a 11% decrease in the total building heat loss). Thus, it made more sense to insulate the roof rather than the floor. This conclusion was reinforced by their model for summer use, where an insulated floor would significantly reduce the effective mass of the floor as an aid in cooling.

As for condensation and human comfort factors, the authors recommended a vapor barrier under the floor (e.g. plastic sheeting?) and the use of carpet/area rugs to reduce the "cold floor on feet" situation. 'Depends on the season again, however. When its 95 degrees outside, I like the idea of a cold floor on bare feet ;)

For now I was planning on leaving the floor uninsulated. I will be finishing the floor later this fall, and in the mean time it will just be dirt. If I decide I need insulation there later, I can do a little digging and add it. And of course I'll be insulating the heck out of the ceiling/roof when I put that on. But I need to decide what to do with the walls, since I will be building them and backfilling here in the next few weeks. I was thinking about following the findings of the MN study and installing foam bard on the 4' or so of wall closest to the surface (and to all walls not bermed with earth of course, which I'll be making out of earthbags), but leaving wall surfaces deeper than 4' uninsulated. As for type of insulation, I was looking at a Celotex product (tuff-r I think), which is extruded polystyrene with a radiant-barrier foil on one side, maybe in 1" thick size. I would sure appreciate any thoughts you might have on the insulation issue.

A: (Kelly) I have generally been of the school of thinking that favors total insulation, because it gives you more control, but I have also encountered advocates of leaving the floor uninsulated, for the reasons that you quote. I have not seen the data from the 1979 book, but I find it quite interesting. It seems to me that it depends on the climate where you are building which approach would be better. If you look at the year-round energy load for maintaining comfortable interior temperature, then it might be more economical to insulate the floor mass in colder climates where winter heating is the big drain, and perhaps leave it uninsulated where summer cooling is the greater issue. At over 8,000 feet in the Colorado mountains, I recommend insulating the floor, and that has worked out well for me.

The use of a vapor barrier under the floor makes sense also, to keep moisture from coming up, and to mitigate against the possibility of radon gas in the house. In fact, if the soil tends toward holding moisture, say from a high water table, that could also make a difference in how much heat might be transferred by direct contact with the ground inside.

I am not familiar with the specific insulation that you mention, but anything that is designed for direct burial with sufficient R-value for the application should work. I use crushed volcanic rock for this purpose in my house because it is natural.

C: (Laren Corie) A rough rule of thumb for stabile ground temperature (which usually starts at about 10 ft down) is 90° minus the latitude. 55° is usually found around 35° latitude, at sea level. Above sea level you need to subtract another 2°/1000 ft elevation. That is only one of many variables. The ground above that level only 'averages' that temperature, and goes through ever greater swings, the closer to the surface it is. Annual average air temps will also match that same number, so it is easiest to determine your approximate stabile ground temperature by starting with finding out the average annual air temperature. Of course the air temperature covers a huge range, but even though it may be isolated from the ground by such factors as snow cover, and plant cover, it all averages out, pretty consistently.

When you want to maintain a space at or near the stabile ground temperature, it is often far more effective to insulate the ground around the structure from the atmosphere above, than to just insulate the structure from it. The ground mass, coupled to the deep stabile ground below, will function as a huge heat sink. Earth over the roof can never function in that manner. It is usually better to build a vented, very well insulated light weight roof, with low cost insulation, then spend your "saved" funds on extruded polystyrene for the sub-grade insulation of the earth mass which is intimately coupled to the stabile ground temperature. Calculate how much earth your structure will displace per foot of depth, then you can know how much fill you will produce per foot of digging. Set the sheets of sub-grade insulation, along with a layer of 6 mil poly vapor barrier, as you back-fill. This system also works well for a structure that is to be heated.

Q: I'm building a 1000sqft rock house, in addition to the exterior rock walls I will have a rock wall going through the center of the house to act a roof support and the thermal mass. I want to insulate the inside of all the exterior walls and was planning on building an interior partition wall with steel studs and using cellulose or fiberglass batting. One wall is earth bermed 3 foot deep. Will I have any problems with moisture in any of these walls that might determine which type of insulation I use? I live in New Mexico and its usually very dry here.

A: (Paul Shippee) Very good question. Glad you are thinking ahead like this.You will certainly have moisture problems in the insulation placed against the exterior rock walls, because these rock walls will present a pretty (or not so pretty) cold surface to the insulation and moisture migration from the interiors will find its dew point readily somewhere inside there. The rock is a good heat conductor so will be nearly the same temperature as the outside air.

On the solar side... If you have a lot of south windows, then you will need all your exterior and interior rock walls for thermal mass to modulate the room temperature swings during winter days and nights. This is a comfort consideration as well as a performance issue. I think you would like to have the beautiful rock visible on the exterior, right? But the very best would be to insulate these rock walls on the outside... admittedly, a not-so-aesthetic compromise. One solution some folks have used is to build two rock walls, one for interior thermal mass, the other for exterior aesthetics, but at greater $$$ expense. There will still be the potential moisture problem, but you can vent the cavity, and then utilize all that great interior rock thermal mass to store heat and minimize temperature swings. Think it over!

Q: I am renovating a garage in Los Angeles into what I hope will be a cute rustic hide-away. Since there is minimal space between the roof and ceiling (about 5 inches) I was wondering if it would be possible to plant grass or plants, or put straw bales on the roof as a form of insulation? Any thoughts or suggestions?

A: Putting earth on your garage roof to grow plants would need lots of structural support to hold up the extra weight. That may not be practical for your structure. Straw bales placed on the outside will get wet and be of little insulation value when wet. If you have access to a 5 inch space between roof and ceiling, then it will be practical to blow or place fiberglass or cellulose  (or some other kind of insulation), or place batt insulation, in there. In Los Angeles you won't need much insulation for winter. R19 (5-6 inches) in the ceiling is fine. In summer, the insulation will keep the heat out, and you can put aluminum color roofing material (or white) to reflect some of the sun off the roof surface.

Q: I am building a home in Venice, CA. I am about 1/2 mile from the beach and groundwater was found at 13' below grade on my property, and at 15' and 17' below grade on other properties in the area. The foundation of my home is a basement with an 18" concrete mat and 12" concrete walls. Beneath the slab will be 4" sand with 10 mil vapor barrier in center, and 4" gravel beneath the sand. Walls will be waterproofed with Miradri 860 membrane and Mirafi G100N drain board with adjacent 12" gravel to subdrain. Bottom of slab is at 10.67' below grade and will have Miradri 860 beneath it, also. The basement slab and 1st floor (4-1/2" concrete) will be heated with a hydronic radiant heating system. The contractor installing the hydronic system assures me that there will not be heat loss to the surrounding earth. Do you agree? Do I need to provide additional insulation for the basement slab and walls? How would this interact with the waterproofing?

A: I am thinking that if ground water table rises (say with global warming effects) then your house with the thick walls and 10 ft below grade slab may float upward? You mention 12 " gravel around the walls, which is good, and a "subdrain"? Where does this drain to? You may need a sump pump or two below the basement slab if water table rises even 2 ft. ??

About the heat loss from the heating system... you call it a hydronic radiant heating system, by which I assume you mean tubing in the floors? So, indeed your basement slab and the edges of your 1st floor slab will lose a lot of heat through the concrete walls and floor as concrete is a conductor of heat, not an insulator. Now, when you add the ground water ingredient... this will increase the heat loss to the moisture in the soils surrounding all areas... soil moisture wicks heat away fast and will suck heat through the concrete.

So, you will need, in addition to the stout waterproofing you already are planning, some insulation for the exterior of slab and walls. You could insulate the interior of the walls instead of exterior, but not the floor slab. It's not only water but moisture which migrates through the soil that will rob your heat under the slab and carry it away. Extruded styrofoam 2 " thick will be good, but check with waterproofing and foam mfg about chemical compatibility between them. You can always place a layer of 4 mil polyethylene between them to be sure.

On the other hand if underground temperatures in Venice are around 60-65F and you keep a cool basement you may only need 1 inch foam to conserve heat over the long term. The point here is to encourage the heat to go into the house, not into the soil moisture.

Q: I see one of the great advantages to these homes is that they are easier to heat. Do the same processes work for cooling? I live in WARM Central California....during the summer we have temps well over 100 for 4 months...Is there any way to get away from the A/C..?

A: (Kelly) Mmost well-designed passive solar homes also perform well during the hot season. This is because they are designed to keep the sunlight out of the house during this time, and they are generally quite well insulated, so the heat doesn't have a chance to penetrate that much. Another factor is what is called thermal mass; a well designed passive solar home also has a lot of thermal mass built into the interior in the form of tile floors, rock or adobe walls, a brick fireplace or such, and this tends to keeps temperatures on an even keel all year round.

The very best thermal performance can be expected from homes that are earth-sheltered, so that the heat of the day cannot easily reach the home, and all of the thermal mass of the earth is buffering the temperature fluctuations. Yes it is really is possible to live comfortably without air conditioning.

Q: I am looking for information on the “R” value of earth. The location is Boise, ID. The soil has a high clay content. Could you direct me to a site that I may be able to gather information on soil / earth R-values?

A: Earth does not have an “R” value as such. This is because as it conducts heat slowly it also stores some of that heat, thus changing the effective R-value. So it is a dynamic situation with the R-value changing as it transmits and conducts heat. This is as opposed to a steady-state R-value such as those reported for lightweight insulator materials like fiberglass and styrofoam.

It is possible to estimate an approximate R-value for earth, but to do that one needs to know the thickness of earth, the orientation, the temperatures on each side— then one can compute an approximate R-value for a specific and limited heat transfer situation. So basically it's steady-state (for light weight materials)  vs. dynamic (for massive materials)...

Q: Winter temps need not be more than 65 degree F. The fact earth sucks heat in winter is not really a concern because I like it cool.

A: (Kelly) The natural static temperature in some areas is about 65 degrees F., so this would be perfect for you; you wouldn't even need to insulate the floor and bermed portion of the house to achieve what you like.

Q: I am building an earth sheltered home in the Austin, TX area. It is built into a hillside, with the south and west sides exposed. Those sides have 4' overhangs, and all glass is high performance low e glass. The walls are 10" concrete, and we will have 3' of earth on the roof (including the overhangs). The floors are 4" of concrete, exposed. The builder has, in the past, faced the south and west walls with 2" of foam insulation and stucco. The main consideration is heat gain in the summer, when the temperature varies from 95 in the day to only 90 at night. My question is whether or not we can eliminate the foam insulation on the walls without overheating in the summer? My guess is it is really an issue of the rate of heat transfer through the 10" walls. I am hoping the rate of heat transfer through the walls will be slow enough to allow the thermal flywheel effect to benefit the overall performance of the house. If I can eliminate the foam I can drop the finishing costs of the exposed walls substantially, but I am worried about potential overheating by the end of the summer season before cooler weather arrives in November. (I have a blog of construction at marant1946.blogspot.com if you wish to include it in this post)

A: (Kelly) The safest approach is to insulate those walls, which will give you more control over the situation. While it is true that there will be some thermal flywheel effect that could be beneficial in the winter, the rest of the time you will be dealing with unwanted heat gain. It might be that the house will have so much mass that it can handle such heat gain, but you might not know this until it all gets "up to speed", which can take as much as a year...I suppose that you could start out by testing the situation with no insulation for the first year or two, and then add it later if necessary.