spray foam or zip r for cold floor

A homeowner in New England is converting a porch on piers into part of the heated house, and wrote in asking how best to insulate the 2x8 floor joists above the crawlspace. The choice was between two approaches: filling the joist bays with rockwool batts and covering the underside with Zip-R sheathing (plywood sheathing with built-in rigid foam that cuts down on heat loss through the wood framing, while also sealing out air), or using spray foam to seal the bays, rockwool batts for insulation, and a separate layer to deal with thermal bridging and keep pests out.

A building science expert who answered the post said the rockwool-plus-Zip-R option is the one most commonly recommended for this kind of raised floor open to the crawlspace below. The only catch is if the floor sits too close to the ground, access for installation gets difficult. As long as there's enough room to work underneath, this approach holds up well. The expert also suggested lining the underside with hardware cloth, either above or below the Zip-R layer, as extra protection against rodents getting into the insulation.

For anyone with an older home sitting on piers, with a cold floor over a crawlspace or unheated porch, this points to a cost-effective path: rigid foam sheathing paired with mineral wool batts, rather than spray foam, can handle both the insulation and air-sealing job, as long as there's room to get under the house to install it properly.