Properly sealing an attic with foam

Q: My question is about foam insulation. I went to a meeting about insulating attics, and the speaker said that you could seal up the entire attic with foam with no ill effects. He would spray the foam until it covered every air leak, between the rafters and covering the gable ends. This was mainly for older homes. He said it was like putting a hat on your house. This goes against everything I have learned about insulating and ventilating. Is this a good idea? Also, do the new foams have any chemicals that they release into the building?

A: Foaming an attic is recommended for both older homes as well as new construction. By sealing the attic with foam, air leaks are stopped. Plus, most foams have a higher thermal resistance per inch of product than other types of insulation.

With an older home, all gaps that might leak must be closed before the liquid foaming agent can be applied. Gaps such as soffit vents, roof vents, gable vents, chimney openings and plumbing vent stacks must be tightly sealed. Otherwise, the foaming agent will ooze out through the gaps and expand outside the home. The foam is a great sealing agent, but it can be unattractive if exposed outside.

It is recommended that a heating/cooling vent be installed in the foamed attic to condition the space. Expanding foam should also be used to completely seal crawl-space foundation walls, the ends of the floor joists (rim/band joists) and the band joists in basements. Low expanding foam can be injected into the existing wall cavities of older homes if you are sure there is no older knob and tube wiring in the walls. Sealing knob and tube wiring might cause the wiring to overheat, which could become a fire hazard.

Also be aware that older homes used fire blocking (a 2-by-4 block installed horizontally between each wall stud), with many older homes using two 2-by-4s or more fire blocks in each stud cavity. To properly insulate the exterior walls, the foam would have to be injected at two or three different elevations between each stud.

Insulating and air sealing the home will lower the amount of energy used, but a tightly sealed home may need to have a fresh-air supply added to improve the indoor air quality.

As for the chemicals released or the off gassing of foams, each product is different and information can be obtained from the manufacturers. Contact a certified energy rater before sealing and insulating the home to ensure safe and effective installation.

Dwight Barnett is a certified master inspector with the American Society of Home Inspectors. Write to him with home-improvement questions at d.Barnett@insightbb.com. Sorry, no personal replies.