Insulation and earth sheltered construction

I just finished building a pit greenhouse frame , the front however is exposed and has a door and a stone wall covering it. I have some 4" poly iso insulation, and placed that in the front to add to the minimal insulation value of the stone wall. My question is do I place insulation on the other 3 walls, the back is aprox 4' deep, and it tapers on the sides to the front where it is aprox 40" buried? I know I want the benefit of the Earth tempering, should I only place the insulation along the walls, just below the windows, to down aprox 1' or further into the soil? Or go to the bottom of the interior wall?

I would think you want to insulate anywhere above the frost line.

I'm in North Central Massachusetts, so the frost line is almost 4' deep, so that would mean covering the whole inner wall. Unless there is another opinion that I hadn't considered yet.

Hi Ted,

I think I am tracking (from just a verbal description) with about 80% of what is being asked (drawing or blue print would really help.)

This is one of those..."it depends"...kind'a things...in my view and experience.

Frost line is often a "misnomer" as "cold penetration is greatly affected by moisture content, soil type, and ground water depth and the local "geothermal" characteristics of the local geology...I live in Vermont and see spots in the dead of winter with -20 highs for days and there are spots where water still is perking to the surface...That's a "warm spot."

Target temperature for the greenhouse's low is part of this equation as well.

Usually...modern insulation types and applications go on the "outside" of whatever foundation matrix has been selected. This way the "high R" is outside the "thermal mass" (aka flywheel) of the wall itself. For Wofati, Earthship, Earth Lodges, Kiva, "Bikooh Garden/Walipini, Bermed and related fossorial architectural forms (modern or vernacular) there are "umbrella systems" of all fashion from just earth to pumice stone, and aerated clay rock, and/or lava stone. Mineral wools too, are often configured into these many systems when in the contemporary application.

This sounds like a "retro fit" after the fact and/or "building is up" and now wanting more insulation than what the berm is offering?

Without an "insulated shroud" I am not sure if going below ground at all will give you much gain for effort? Hard to tell..

All your options and thoughts on the subject sound viable for a retrofit...

Regards,

j

Study break! I made this concept drawing in 2011, and now I'm returning to the idea of multiple inhabited layers for protection from your climate.

I have worked with Paul to produce this wofati umbrella conceptual drawing.

I have worked with Paul to produce this wofati conductive wall section.

Frost depth is directly related to being exposed to freezing,or rather lack of heat for extended periods of time and time required to thaw/ freeze; think thermal mass.

where the inside of the greenhouse shouldn't be exposed so your frost line should turn horizontal/vertical to the walls of the green house to a point.

there fore the insulating property of the earth around the area should provide.
the reason you put it underground to start with.

Hi All,

I am only posting again because of Davin has brought such a fantastic cross sectional schematic depiction to the conversation (excellent!)

Davin illustrates (with minor exception at most) of a "modern day" replication of several traditional vernacular "fossorial buildings" of both First Nation cultures here in the Americas, as well as, other forms found globally. The only variations is in materials and slight differences in detailing. The lack of "air voids" and some material variances and region climate/soil variations are what make some of these (Nordic culture) last perhaps only a generation at best (maybe two) before "reconstruction" is required, or some in the more arid (or stone based) forms that may last well over a thousand years...(e.g. Hopi, Zuni, Mandan, Anasazi, Asian, Middle Eastern, etc...) If the rubber matting (Umbrella) of these structures do not fail (even a little bit) and wood destroying organisms do not take refuge, these "modern interpretations" of historic fossorial structures should last as long as those two crucial elements (drainage/water shedding umbrella, and wood compromise) do not take effect.

In the traditional forms (of the longest lifespan and durability) the crucial design elements and material choices are what become paramount to focus on. First would be the "umbrella." This water shedding feature in vernacular forms is typically a layered system of pure clays, cobb mixes then a "shingling" of flat stone in such a fashion as to naturally direct water away from the living space below. Under this stone, is often more clay and/or cobb mixtures to further protect and insulate the space. Then, in the more enduring designs, an "air space" is formed by either stone (best choice) or a rot/pest resistance species of wood, or even a "cobb ribbing." Then the "timber frame" and/or log superstructure that holds the mass above. Of course this is all built in reverse, and each culture (usually dependant on climate and material resource) have there subtle differences...

The only real "improvement" I have observed that could be added to the Wofati design that Davin has shared is to include an air space between the wood and the "backfill materials"...Perhaps include a more substantial "draining matrix" as found in the Kidan designs of Asia and other cultures (see Permies article on Raised Earth Foundations) and then backfilling with stone and/or sand gravels for at least 300 mm on top of the structure and berming this out toward the foundation base where the thickness could go as far as a meter or more before a cobb and then pure clay soil back filling...then the regular native soils as they are...

Some other "augmentation elements" that could be added from the waste stream that would further insulation, protect and "form" the critical "umbrella" would be adding a layer of felt carpet padding and polyester/rayon based carpet materials, cardboard, and/or newspaper. This could be further treated with borates, and natural oils like flax to inhibit mold, and further shed water...

Great drawings Davin...

As for "forest depths" there is a great deal of "misinformation" as well as repeated lack of complete understanding this very natural phenomenon...which isn't full understood completely yet today by soil scientist and others that study such events. Frost, and related frost heave can not take place if there is no or very little moisture. The entire "Wofati" concept and that of other vernacular fossorial structures greatly changes the "frost behavior" of the macro region within a building site. Just changing a backfill from "native soils" to stone, gravel/sand and or clay/cobb will greatly change its behavior and effect on the architecture...Simply scraping away topsoil down to a mineral soil, anding a cobb the a plank of thick wood will create a "thermal heat trap umbrella" that will change alleged "frost depth." Further bedrock and geothermal variants that are very different even withing a few hundred meters can have great effects on this "frost line." Some area's here in Vermont for example will perk water to the surface year round or have bed rock that never keeps snow on it and the surrounding soils never freeze completely or to any great depth, as just a few examples...

Regards,

j

Hello everyone.

I did some ramblings on some potential solutions over here:
https://permies.com/t/48609/natural-building/Storing-heat-beneath-feet-umbrella

The specific case discussed in this thread (if using clay) can work but there are some things to consider :
1. a good clay "membrane" is good only if it's sufficiently moist and has enough thickness "t" (but how much exactly ?)
2. to keep the clay layer protected from extreme outside cold/heat and water, the normal soil on top has to have a certain thickness "d" (at least 60cm but how much actually ?)
3. the insulation must work in somewhat damp conditions (no plastic membrane to keep things dry)

If not using clay, i am unsure on how long the artificial membrane can last.
The best PEHD, >1mm thick or Firestone EPDM, >3mm thick, have a rating of cca. 50 years.
Even the geotextiles (some waterproof and some not) used underneath the highways don't have a longer lifetime.
The thinner and different materials are much less.

And the issue is that they will fail mechanically well before any chemical bonds will break.
I know popular wisdom would indicate that plastic in the soil, protected from it's archenemy - the sun, will last indefinitly but even the manufacturer does list a limited lifetime.

The clay protected system is major work compared to the "plastic" one but, if properly executed, should last a lot longer.

PS
Don't ever let deep rooted plants grow on top of the clay layer, especially those in the thistle family.
They are prolific hard, deep clay drill machines.