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Once spraying begins, it’s a good idea to check foam regularly for quality, thickness, and yield, particularly when temperatures are cool or the humidity high. Quality-control samples — taken periodically during the job by either the installer or the GC — can indicate problems with foam thickness, adhesion, and cell structure, and can help document that a foam installation meets specifications.

Lifts for closed-cell. To avoid problems, closed-cell foam should be installed in lifts less than 1 1„2 inches thick, with breaks of about 10 to 15 minutes between lifts to allow exothermic heat to dissipate. A certain amount of exothermic heat is needed to properly cure SPF foam, but excessive heat can result in cracks and shrinkage. Unusually strong odors after foam application can be an indication of this problem.

A good rule of thumb is to size the spray gun’s mixing chamber and nozzle so that passes overlap by more than 60 percent. This will allow the foam to grow at a uniform rate from one side to the other during a lift. When spraying studwall and ceiling cavities, for example, the installer should picture-frame the cavity, allow the foam to set a few minutes, and then fill in the middle, spraying parallel to the direction of the studs.

On a masonry wall, an experienced installer should be able to spray 2 or 3 inches of closed-cell foam in two or three lifts within a 1/4-inch tolerance. In framing cavities, the variation is typically greater due to the thicker application against rafters or studs.

One shot for open-cell. Half-pound SPF is typically installed in a single lift, starting at the bottom of a studwall or ceiling cavity and working up. To prevent gaps and voids, hard-to-reach areas or spaces behind the studs are usually picture-framed first.

You can expect considerably more thickness variation with open-cell foam due to its greater expansion rate. A 1/2-inch tolerance when spraying 3.5 to 5.5 inches of foam would be considered exceptional, while a more typical 1/2-pound foam installation would have a 3„4-inch to 1-inch tolerance between studwall cavities. As a result, trimming is usually necessary after the foam has been sprayed.

Job-site protection. SPF insulation can drift a few hundred yards on a windy day if the doors and windows have been left open, and it sticks tenaciously to any surface it lands on. Therefore, every job should have a comprehensive overspray plan that includes careful masking and the removal of any items in the immediate vicinity that might be damaged by overspray. And since the fumes and mist produced by an installation can be harmful, SPF contractors are responsible (per OSHA regs) for a written respirator plan for employees, and for a safety plan that protects all nearby occupied areas and prevents nonworkers from entering the work area during spraying. (GCs are responsible for a safety plan as well.)

Fumes and mist usually dissipate within a few hours to a few days, depending on the amount of ventilation available. Since off-spec foam can create odors that linger much longer, any odors that last longer than a week may indicate a problem with the foam and should be investigated.