If you convert your vented unconditioned attic to an unvented conditioned attic by installing open-cell spray foam on the underside of your roof sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen.
, you may be surprised to discover that your attic is now the most humid room in your house.
Why? We don't know. Although building scientists haven’t achieved a consensus on the answer, we do have enough information to paint a picture of what’s going on.