Closed Cell Foam Insulation
Because it requires professional installation, closed cell foam insulation is pricey, but it offers R-6 thermal value for every inch of thickness. Whereas blown-in insulation works by filling the open joist cavities, closed cell foam installs overhead between the rafters, making it possible to finish the attic for use as living space. Not all foam insulation products are the same. Open cell foam is less expensive, not quite as dense and it has a lower per-inch R-value.
Removing the Old Insulation
While most blown-in insulation contains cellulose fibers, if the house was built and insulated before 1950, the insulation might contain asbestos. Asbestos testing kits are available from hardware stores and lumberyards. If asbestos is present, an asbestos remediation contractor must remove the old insulation. If the insulation is asbestos-free, you can remove it using an insulation-removal vacuum, available from construction rental stores. Protective clothing, a respirator mask and protective eyewear are required during the removal process.
Prepping the Attic
Homeowners should take steps to ensure proper attic ventilation. Although it can vary, ventilation in an unfinished attic often features fresh air intake vents located in the soffits, and one or more exhaust vents, either in gables or on the roof, near the peak. Because installing closed cell foam will seal the attic, an alternate method of ventilation is necessary. This often means installing rafter baffles in every rafter space to permit fresh airflow from the soffits to a ridge vent at the peak of the roof.
The Insulation Process
Select a foam-insulating contractor with care. The process of applying closed cell foam requires the careful mixing of chemicals and spraying at the correct temperature. The contractor directs the closed cell foam, which starts as a liquid and rapidly expands between the rafters. Since closed cell foam is very dense, it might not completely fill the rafter space, but it should be of uniform thickness. For example, if the roof has two-by-twelve rafter framing, and you want R-30 insulation, the contractor will spray the foam 5 inches thick, leaving more than half of the cavity empty. Since the foam expands and hardens almost immediately, you can hang drywall the same day.