Spray Foam Insulation Guides

For sealing a few gaps, a can or a small kit can be a reasonable DIY job. For insulating a whole attic, wall or crawl space, hire a trained installer: the U.S. Consumer Product Safety Commission says the high-pressure system used for that work is a professional system and should only be used by a trained applicator.

Updated October 26, 2026

Spray foam doesn’t create moisture, but it changes where water can go and how fast a house can dry. Closed cell foam slows water vapor and doesn’t soak up water. Open cell foam lets vapor through and can hold liquid water. Most moisture trouble with spray foam comes from four things: foam sprayed over wet wood, roof leaks that go unnoticed, humid air reaching a cold surface, and sealed attics or crawl spaces with no way to remove moisture. Fix water problems first, pick the right foam for each spot, and make sure any space you seal up is dried by your HVAC system, a fan or a dehumidifier.

Updated October 26, 2026

Spray foam is made on site. Two liquids, an “A side” and a “B side,” are pumped through heated hoses, mixed at the gun and react into foam on your walls or roof deck. When that reaction goes right, cured foam is considered relatively inert. When it goes wrong, the usual causes are the installer’s process, not the product: the wrong mix ratio, passes sprayed too thick, temperatures outside the manufacturer’s range, a wet or dirty surface, or skipping the required fire protection. The good news is that nearly every one of these is avoidable if you hire a trained crew and ask the right questions before they start.

Updated October 26, 2026

Ask for proof of license and insurance, who will actually spray the foam and how they were trained, the exact product, the inches and R-value you will get, and how long you must stay out of the house. Walk away from a quote with no thickness or product named, pressure to sign today, or anyone who says you don’t need to leave during spraying.

Updated October 6, 2026

Open cell spray foam gives about R-3.7 per inch and closed cell about R-6.5, so divide the R-value you need by those numbers. A 2x4 wall filled with open cell reaches about R-13, and a 2x6 wall about R-20. Closed cell needs about an inch and a half before it also seals air and blocks moisture.

Updated September 23, 2026

Closed cell is denser, insulates about 75% more per inch, holds up to water, and costs about two and a half times as much per inch. Open cell is the budget choice where there is room for thickness, like an attic roofline. Pick closed cell anywhere the foam could get wet or space is tight.

Updated September 23, 2026

Put spray foam under the roof deck when your ducts or HVAC unit sit in the attic — it brings them inside the insulated part of the house. With no ducts up there, sealing and insulating the attic floor usually costs less. Either way, compare quotes by the R-value they reach, not by inches.

Updated September 23, 2026