Open Foundation Insulation Ideas

This is a homeowner forum post, not a program announcement, so there's no rebate or deadline news here — just one owner's question about insulating the floor of an old cabin. The setup: a 100-year-old, 38-by-20-foot wood cabin near State College, Pennsylvania, sitting on an open foundation (no walls enclosing the crawlspace) with 2x6 floor joists and only 10 to 30 inches of clearance underneath. The owner already air-sealed the walls and penetrations, and heats mainly with a mini-split heat pump, backed up by a vent-free propane heater and an occasional wood fire.

The owner tried reclaimed rigid foam board wedged between a few joists, but on hot, humid days condensation forms on it, raising concerns about moisture and mold given how damp the ground stays after rain. Planned next steps include exterior drainage work already done, laying a plastic ground cover to cut moisture rising from the soil, treating the underside of the floor with a mold-inhibiting spray, and then choosing an insulation method — foam wedged between joists, foam board fastened underneath, or spray foam across the whole underside — possibly protected with a wood panel layer. The owner is also asking whether the decorative skirting around the base should come off to let more air move through the crawlspace.

For homeowners with similar older, un-enclosed foundations, this is a reminder that insulating a crawlspace floor without controlling ground moisture and airflow first can trade a cold floor for a damp, mold-prone one.