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  #1  
Old 12/08/08, 10:37 PM
 
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does dry soil insulate?

So I've been reading a lot about PSP structures and I just finished watching six hours of video about them.

So this is an underground structure that is really cheap if you have timber on your land.

Basically, you make a wood box out of posts, cover it in black plastic and then bury it. Of course there are lots more details.

But my question of the moment is pretty specific.

I drew this little picture in my paint program so I can ask this question.



So the two black lines represent the black plastic.

The teal colored stuff would be soil that is wet.

The tan colored stuff would be soil that is dry.

Does dry soil insulate?

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  #2  
Old 12/08/08, 10:51 PM
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What is the R-value of soil?

The resistance of soil to heat flow (R-value) varies a great deal, depending on the type of soil and the moisture content. In general, soil is not a good insulator. For a fine-grained soil with 20 percent moisture content, the R-value is about 1 per foot, roughly the same as concrete.

Because of this low R-value, it is important to insulate foundations, including slabs-on-grade, crawl space walls and full basements. Insulating the first few feet below grade is the most critical area, but we recommend full-depth insulation.
http://www.engext.ksu.edu/ees/henerg.../basement.html
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  #3  
Old 12/09/08, 12:10 AM
 
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Everything insulates. Dry soil insulates a tad better than wet soil, yes.

Neither one insulates worth a hoot tho.

Underground houses depend on earth being about 55 degrees year around _below_ the frost line. You need your blue & tan stuff to be deeper than your frost line, and the house will keep itself about 55 degrees all year 'round naturally. Then you need to insulate only a little to allow for the 15 degree temp difference you want to maintain in the house, rather than the 60-100 degree temp difference many house walls are subjected to.

The trade off is you need to deal with the moisture issues also created, aside from leaking soil (you hope the plastic controls), condensation from that 55 degree earth in a humid summer is a very real issue that needs dealing with. This is not something to ignore, and it is condensation, so the plastic layer is not going to do anything for it. It is humidity coming out of the hot air touching your cool walls & floor & ceiling.

--->Paul

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  #4  
Old 12/09/08, 08:52 AM
 
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Hi,
I would not depend on dry soil for insulation. The people who know a lot about underground homes like Rob Roy and John Hait are very definite about the need for insulation.

Dry soil can be used for heat storage, as in the Passive Annual Heat Storage designs -- if you have not seen these, have a look on this page:
http://www.builditsolar.com/Projects...es/plansps.htm
Search down the page for "Passive Annual Heat Storage"

Gary

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  #5  
Old 12/09/08, 10:33 AM
 
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When I did a bunch of research on this type of project. I was going to be using a pond liner for the water barrier along with a mastic layer. Pond liners arent that expensive and will last almost forever if not subjected to UV light.

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  #6  
Old 12/09/08, 11:58 AM
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Soil is a great isolator - but not a good insulator. Go down far enough, and the soil temp will basically be constant all year long.

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Old 12/09/08, 04:22 PM
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Dry soil has an insulating value of 0.1R from the articles I've read. Adding a 4" layer of closed cell foam insulation below the plastic barrier will GREATLY increase the insulation value and add more water protection from downward coming water. Shingle the layers of foam. Check out information about PAHS (Passive Annual Heat Storage). Here is a link to get started with:

http://www.earthshelters.com/

That said, I strongly recommend against using timber in contact with soil, even dry soil. Rob Roy's houses had a lot of problems in the long run. You are holding up a lot of weight. Look into ferro-cement (FC), arches, barrel vaults, and such which are a good way of using a little concrete to build a very strong long lasting structure. See our tiny cottage which we built this way. We used brick, stone, granite and concrete both RC and FC. Our walls are dry stacked concrete block. The roof is a 1.5" ferro-cement barrel vault. Eventually it will be earth sheltered.

http://sugarmtnfarm.com/blog/labels/Tiny%20Cottage.html

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Old 12/09/08, 09:15 PM
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I'm pretty sure sod has a good thermal mass and R value. Correct me if I'm wrong, but the difference is the air spaces in the dirt caused by multiple factors like roots and grass, so it's different than just throwing dirt over a structure. Settlers built sod houses in Nebraska and they required minimal heating. I'll have to read Rob Roy's book on underground houses--read about him in The Natural House. He also does cordwood houses, I think.
Vikings did something similar to your design with the wood frame contacting the eight inch thick sod except above ground. When wood touches soil it does rot it. Obviously you wouldn't want something that primitive, but the buildings are really cool looking.
This is sort of unrelated now that I look at the question again.

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Old 12/10/08, 08:07 AM
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Remember that the settlers contented themselves with lower home temperatures than many people do today. I laugh when I see people talking about turning their thermostat down in the winter to 72°F or 65°F. In our old farm house it took a lot of work to get it up to 55°F. On the other hand, our new stone cottage floats at that and is very easy to raise to a comfortable 60°F.

In other words, check out the local ground temperature (real life data) and consider how warm you like your home to be.

The best natural insulator is snow.

As to the Vikings and Rob Roy vs timber in contact with the soil, realize that the wood did rot and the houses did not last without a lot of maintenance. Rob Roy had a lot of trouble with his houses. Read the later books and discussions before you invest time, effort and risk in the wood underground houses. It's your head. I thought they were cool too, at first.

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  #10  
Old 12/10/08, 09:13 PM
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On a similar note, if anybody wants to answer, I am interested in the old sod houses on the prairie. I've come up with a couple of designs where the RBA (roof bearing assembly) is set over top of the sod walls and then the roof is connected. Since everybody says dry dirt rots wood, would a couple layers of paint suffice against rot? I know when building raised beds for gardening out of wood that painting greatly extends its life, and those come into contact with perfect rotting conditions all the time.
Our house is at 68 degrees right now. But consider if it's something like -20, which it still gets to in ND, 55 degrees is pretty warm. ND is just an example of prairie weather. I actually live in MD.

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  #11  
Old 05/01/09, 08:31 PM
 
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So .... dry soil has an r-value of about 0.1 ....

I would think that a dry, sandy soil would have a higher r-value than a dry, clay soil ...

How much do those insulation boards cost?

rambler, your point about condensation is an excellent one. I suspect that adding some of the annualized thermal inertia could help with some of that. So that the temp behind the walls might be something more like 65 - thus reducing (though, not eliminating) the condensation issue.

So if the total thickness of my roof is 36 inches, I have an overall r-value of 3.6. Maybe less because more than half of that can be wet soil. But maybe more, because that soil could contain a lot of air gaps.

Add to that, that the dry part is acting as a sort of thermal mass.

But here's another weird thing, following the link that highlands gave: "It takes six months to conduct heat 20 feet (6 m) through the earth." --- so if I have three feet, do I have 1 month of buffer? So if I heat the insides to 70 ... and let's say I've also heated a foot of dirt, does it take a month to bleed out?

I'm looking around trying to think of what I might be able to use that is already lying around ....

Well, intensive googling tells me that snow has an r-value of 1 (I'm assuming that that is for 1 inch). It also mentions that "Wood chips and other loose-fill wood products" have an r-value of 1 ....

The trouble with wood is that is can catch on fire, or get moldy .... But it will be at least a foot of dry dirt away from me. So maybe I don't care.

I guess my idea is this: I have my wood roof, and a layer of poly over that. Then a layer of 12 inches of soil. Then a layer of poly. Then a layer of 4 inches of dry sawdust. Then another layer of poly. Then 20 inches of soil.

BTW: more stuff about PAHS: http://www.norishouse.com/PAHS/UmbrellaHouse.html

So the PAHS folks have about double the house footprint in an umbrella with ... R-8? And they end up not having to heat their home at all. So I wonder if I do something closer to 20% smaller and R-5 if I might have to fire up a wood stove on cold days and pay one tenth for the umbrella. ??

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  #12  
Old 05/01/09, 08:43 PM
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Paul, use those rules about "six months to conduct and such" really loosely. It depends greatly on the location, seasonal issues, snow cover, soil wetness, etc. Do what you can to keep the soil dry, increase its insulation value, etc. Insulate your mass. Have mass within your insulating envelope. Get solar gain on the insulated mass. Protect yourself from wind. Lots of little details. Lots of looseness in the formulas. Don't let hard numbers get too distracting. Reality is dirty.

I too would avoid the wood chips. Just because they'll break down in time and change your system greatly, may attract insects, etc. If you want to increase the insulating value then consider sand. Drains well. Or maybe surplus styrofoam packing peanuts mixed into sand. Just don't use the cellulose kind. Be aware they can float up through the soil. Use a fabric on top, flat stones, etc.

Our house requires very little heat. It stays above freezing without any extra heat. Comfortable with a fraction of a cord of wood per year. Big windows. Lots of mass inside the insulating envelope. It will be even better when I insulate the roof.

Cheers

-Walter
Sugar Mountain Farm
in the mountains of Vermont
http://SugarMtnFarm.com/blog/
http://HollyGraphicArt.com/
http://NoNAIS.org

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Old 05/02/09, 07:52 AM
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Your drawing reminds me a bit of the spud cellars we used out west for bulk storage of taters when I was a kid. The old timers would dig a shallow pit, maybe four feet deep, about 30 ft wide and maybe 60 to 80 feet long. They used poles to frame up the shed in the pit. walls were just the soil along the edge of the pit, pole rafters to the peak with a interior wall about 8 feet in from each side wall to help support the roof. The pole rafters were set about every 2 ft down each side of the cellar, and were then covered with woven wire, wheat straw was then applied about 6 inches thick and the whole thing covered in just a few inches of soil. On one end they framed out a door way about 10 or 12 ft wide to back the trucks or wagons down inside. Those cellars were quite toasty and warm in temps well below zero in the winters. Taters cannot stand to be frost bit and the cellars kept them well above freezing with no additional heat other than the basic ground temps.

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Old 05/03/09, 01:29 AM
 
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So here it is. From http://en.allexperts.com/q/Architect...insulation.htm

Quote:

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)

So if I have 12 inches of dry earth and 24 inches of wet earth, that would be 3.96 + 1.2: about R-5.

And some of my walls might be 10 feet thick - so that would be an r-value of ... 39.6 when dry?

Hmmmm .... I suppose if I put down a couple inches of newspaper, that might add some r-value too.
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Old 05/03/09, 01:57 AM
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Originally Posted by Paul Wheaton View Post
So here it is. From http://en.allexperts.com/q/Architect...insulation.htm




So if I have 12 inches of dry earth and 24 inches of wet earth, that would be 3.96 + 1.2: about R-5.

And some of my walls might be 10 feet thick - so that would be an r-value of ... 39.6 when dry?

Hmmmm .... I suppose if I put down a couple inches of newspaper, that might add some r-value too.
If you have 12 inches of dry earth, not even counting the wet earth which is much heavier, you better have something pretty durn stout under it or you will have a hole filled with dirt. dry loam earth weighs in at roughly 75 lbs per cubic foot. Thats the same as square footage at a foot thick.
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Old 05/06/09, 12:59 PM
 
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Yup.

Do these pictures look like something that will be stout enough?

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Old 05/06/09, 01:37 PM
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Quote:
Originally Posted by Paul Wheaton View Post
Yup.

Do these pictures look like something that will be stout enough?
They may hold for a while, on very short spans, depending on the logs used. Three cubic feet of dry earth figures out to be in the 250 pound range, if the top two feet are wet, it will go well over 300 pounds, thats a ton and a half across a 10 foot span at a foot wide, or over 15 tons on a ten by ten area. I would not want to be in the cellar on a rainy day though, the added weight just might be too much. I have seen what happens when earth covered buildings collapse. Its not pretty. I would feel much better with the following, which I have seen used successfully for many years. Those logs, covered with woven wire, then a layer of straw about 6 to 8 inches thick, covered with 3 to 4 inches of earth. much less weight, much better insulation and a whole lot safer. IF you pile the straw higher in the center then put a sheet of plastic over the straw to shed water off the sides and away from the structure, then enough soil to grow some sod, you should have a pretty fair system.
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Old 05/06/09, 04:28 PM
 
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Well, the fact that Sepp has a hole bunch of these up in the Alps that have been doing just fine for years makes me think they are generally quite safe. And that's three feet and three inches of wet soil plus snow load and animals. So if I convert a foot or two of that to dry soil, it would seem that I'm already ahead of the game.

At the same time, I think you make a really good point.

Loose straw would be a much better insulator than straw bales. And with only a few inches of soil, it won't get smashed much. Hmmm .... I wonder .... how about this order:

12 inches of soil
layer of newspaper (quarter inch thick)
layer of plastic
layer of newspaper (quarter inch thick)
loose straw/sawdust/pine straw/etc. (8" loose -> 4" packed)
layer of newspaper (quarter inch thick)
8 inches of dry soil
layer of newspaper (quarter inch thick)
layer of plastic
layer of newspaper thick enough to end up with something fairly smooth
logs / wood roof

So I've reduced the wet soil by a factor of 3, then added 8 inches of dry soil and a couple inches of newspaper and some sawdust/straw ....

Granted, the sawdust/straw will compress, but I think that with only 12 inches of soil above it, it won't compress any more than what you get in a regular bale of straw. So, in theory, you might be one step closer to having an annualized thermal mass.

Hmmmm .... hang on .... are you saying that one cubic foot of dry soil weighs about 80 pounds? Wouldn't a cubic foot be about what would fit in a five gallon bucket? A gallon of water weighs 8 pounds; five gallons is 40 pounds. Dry soil has lots of air pockets in it. So I would guess it would be less than 40 pounds. Of course, I've been wrong about lots of things before ....

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