Crawlspace Insulation Calculator

If the crawlspace is dry and can be sealed, insulating the perimeter walls and encapsulating with a vapor barrier performs better — ducts and pipes end up inside the conditioned envelope. Insulate the floor above only if the crawlspace stays vented.

Yes. Bare dirt pushes gallons of moisture into the house every day. Use 6 to 20 mil sheeting across the whole floor, lapped 12 inches and run up the walls.

It depends on whether the crawlspace is vented. A vented crawlspace is outside the thermal envelope, so insulation goes in the floor above. A sealed and conditioned crawlspace is inside it, so insulation goes on the perimeter walls — which is usually less area and brings the ductwork and pipes inside the envelope too. Doing both is wasted money; doing the wrong one traps moisture.

On bare earth, yes, and it does more good than the insulation. Ground moisture evaporating into a crawlspace is the source of most crawlspace problems, and a sealed ground cover lapped up the walls stops it at source. Insulating a damp crawlspace without covering the ground first generally makes the damp worse.

They are common in floor assemblies but they are unforgiving there — batts installed under a floor sag over time as their supports fail, and a sagging batt with an air gap above it does very little. If you use them, support them properly with wire rods rather than relying on friction, and expect to inspect them.