A.Martin Holladay, editor of Energy Design Update, responds: Even though thicker is always better with any type of insulation, applying a thin layer of spray foam is a good way to get air-sealing benefits at considerable cost savings over full-thickness spray foam.

Some spray-foam contractors dismissively call the technique "flash-and-dash"; they point out that fiberglass batts may fail to remain in contact with the spray foam, creating an air space and the potential for convective air currents through the insulation. But I think this is a relatively insignificant problem, particularly if the...

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