A crawl space can be insulated in two ways:
As a general rule, treat heated crawl spaces as short - sometimes very short - basements and renovate them as described earlier in this chapter.
Fix any water leaks and remove sources of water infiltration as noted in Control of moisture flow.
Remember, as with a basement, never vent a clothes dryer into a crawl space.
Insulating the walls is recommended to avoid having to insulate and protect all plumbing pipes and heating distribution systems. Walls can be insulated externally to reduce the internal moisture problems that can develop in damp crawl spaces and to keep the soil below the footings warm. It is also usually easier to insulate the walls than the ceiling above, especially in tight crawl spaces or where joist spacing is uneven. Walls tend to require fewer materials than ceilings.
If there is no moisture barrier on the crawl space floor, add one. The minimum barrier should be 0.10 mm (4 mil) clear or opaque polyethylene overlapped, caulked and taped at the seams. Although more difficult to find, white opaque polyethylene brightens the space, shows areas of leakage or vermin entry more readily, and hides moisture or mould that may be on the other side of the plastic.
Mechanically fasten the moisture barrier to the walls and all obstructions that it cannot go over such as floor support posts. It should also be sealed to any moisture barrier added to the walls. To prevent the plastic from billowing, as may happen occasionally, hold it down with a few old boards or some smooth scrap material. If there is likely to be any traffic, protect the polyethylene with a length of plastic floor mat. Do not use sand or gravel.