The old timers around here swore by sheep manure. (Highly compacted hay into tiny pellets). They always buried waterline with a layer of sheep manure and it is said to never freeze. (I live in Maine where the frost line is considered 4 feet deep).
Gilbert Fritz wrote:I'm looking for an underground insulation option for a hot frame that is cheap, nontoxic, and won't break down quickly. Any ideas?
If you are building an underground hot frame would not the soil around it be the insulation?
is this insulation to retain heat or keep it out?
what about water infiltration?
how are you going to make it water tight so any insulation will work?
Insulation is always good but it is easier to give ideas that will work if we know what your design/ plan for the structure is.
To insulate you are trapping air, so just about any material that will trap and hold still air should be a good choice.
Blue foam board exposed to light will break down, it will be a matter of getting it out of direct light to keep it stable.
Some items I've seen reused as insulation, that work pretty well;
used soft drink and juice bottles, whole with lid or cut in half.
Straw bales, once sealed with plastering they are great.
waxed cardboard tubes, saw a guy using toilet paper rolls that he soaked in wax, he was stacking them before putting up sheet rock on the interior, said he was experimenting, so I don't know how well they will last.
Wine bottles, used just like the soft drink bottles.
Dried, fiber rich dung.
cob with extra straw worked in. All these have been used for many years with some success.
For a hot frame (cold frame?) if you only have the light infiltration exposed to the sun, some black paint or black painted water jugs would go a long way at retaining heat and letting it go at night.
The earth will want to equalize the temperature inside the hot house to the average soil temperature around it. If you isolate (thermal and water) a large block of soil, as with the PAHS and wofati designs, you can raise the soil temperature in that block to match your target temperature for the hot house (it is an inexact science, so your mileage may vary). If you have a high water table, you will need to isolate underneath as well as on the sides and top.
There has been a lot of discussion about underground insulation on other threads. I have advocated the use of perlite as a cost effective (hopefully) alternative to foams that will not crack with the inevitable settling. It is used in conventional construction as under-slab insulation. Pumice and foamed glass aggregate can also be used. Mineralized wood chips may be another alternative.
It will lose some insulative value while it is wet, but after it dries out, it will return to normal. Perlite, pumice and foamed glass are ostensibly closed cell, however, on the exterior surface of the aggregate, there are exposed cells that will latch onto water, while the interior of the aggregate will remain dry. You will want to sandwich any insulation with a waterproof membrane and engineer effective drainage into your project. Long-term studies have shown that even flotation foam can get waterlogged after prolonged exposure to water.
If you are dealing with a high water table, don't go underground. Try super-insulation and/or earth berms, above grade. You can insulate earth berms to get a similar effect to underground, especially if they are well compacted.