Air Sealing and Moisture Control

How condensation gets inside a wall

Warm indoor air holds a lot of water vapor. When that air leaks into a cold wall or roof cavity it cools, and cooling air cannot hold as much water. The surplus condenses out onto the first cold surface it meets - the back of the sheathing, usually.

In a Maine January that condensation can arrive as frost. It melts in a thaw, soaks the insulation, and the cycle runs again the next cold night.

What that does over thirty years

• Wet fiberglass conducts heat, so it stops insulating long before it looks bad
• It gets heavy, sags, and pulls away from the surface it was meant to be touching
• Damp sheathing and framing are where mold and rot start
• None of it is visible until something is opened up

Stopping the air stops the moisture

Water vapor does not mostly diffuse through materials - it mostly rides along with moving air. Close the air paths and the great majority of the moisture problem closes with them.

Spray foam is a solid cell structure adhered to the framing, so air does not move through it or around it. Closed-cell goes further and blocks vapor itself, which is why it is what we use in crawl spaces, basements and against foundation walls.

Which is why foam does not need venting

Venting a roof exists to carry moisture out of fibrous insulation so it can keep working. If moisture cannot get into the insulation, there is nothing to vent off. That is why closed-cell cathedral ceilings and roof decks do not need a vent channel and can be filled to full depth. More on attics and roof decks.