spaces (like loose-fill, but superior). Provides acoustical insulation (like loose-fill, but superior). Expands while curing, filling bypasses, and providing excellent resistance to air infiltration (unlike batts and blankets, which can leave bypasses and air pockets, and superior to some types of loose-fill. Wet-spray cellulose is comparable.). Increases structural stability (unlike loose-fill, similar to wet-spray cellulose). Can be used in places where loose-fill cannot, such as between joists and rafters. When used between rafters, the spray foam can cover up the nails protruding from the underside of the sheathing, protecting your head. Can be applied in small quantities. Cementitious foam is fireproof. Disadvantages The cost can be high compared to traditional insulation. Most foams, with the exception of cementitious foams, release toxic fumes when they burn. According to the US Environmental Protection Agency, there is insufficient data to accurately assess the potential for exposures to the toxic and environmentally harmful isocyanates which constitute 50% of the foam material. Depending on usage and building codes and environment, most foams require protection with a thermal barrier such as drywall on the interior of a house. For example, a 15-minute fire rating may be required. Can shrink slightly while curing if not applied on a substrate heated to manufacturer's recommended temperature. Although CFCs are no longer used, many use HCFCs or HFCs as blowing agents. Both are potent greenhouse gases, and HCFCs have some ozone depletion potential. Many foam insulations are made from petrochemicals and may be a concern for those seeking to reduce the use of fossil fuels and oil. However, some foams are becoming available that are made from renewable or recycled sources. R-value will diminish slightly with age, though the degradation of R-value stops once an equilibrium with the environment is reached. Even after this process, the stabilized R-value is very high. Most foams require protection from sunlight and solvents. It is difficult to retrofit some foams to an existing building structure because of the chemicals and processes involved. If one does not wear a protective mask or goggles, it is possible to temporarily impair one's vision. (2–5 days). May require the HVAC system to have a source of fresh outside air, since the structure may not refresh inside air without it. Advantages of closed-cell over open-cell foams Open-cell foam is porous, allowing water vapor and liquid water to penetrate the insulation. Closed-cell foam is non-porous, and not moisture-penetrable, thereby effectively