Editing Earth shelter (section)

===Insulation===
One or more insulation board or foam layers are added outside the waterproofing. If the insulation chosen is porous, a top layer of waterproofing is added. Unlike the conventional building, earth shelters require the insulation on the exterior of the building rather than inside the wall. One reason for this is that it protects the waterproof membrane against freeze damage, another is that the earth shelter is able to better retain its desired temperature. There are two types of insulation used in earth shelter construction. The first is close-celled extruded polystyrene sheets. Two to three inches glued to the outside of the waterproofing is generally sufficient. The second type of insulation is a spray on foam (e.g. [[polyurethane]] solid foam insulation). This works very well where the structure's shape is unconventional, rounded or difficult to get to. Foam insulation requires an additional protective top coat such as foil or fleece filter to help it resist water penetration.

In some low budget earth shelters, insulation may not be applied to the walls. These methods rely on the earth's U factor or thermal heat storage capacity below the frost layer. These designs are the exception however and risk frost heave damage in colder climates. The theory behind no insulation designs relies on using the earth's thermal mass to store heat, rather than relying on a heavy masonry or cement inner structures that exist in a typical passive solar house. This is the exception to the rule and cold temperatures may extend down into the earth above the frost line making insulation necessary for higher efficiencies.