forming a semi-permeable vapor barrier. (N.b., vapor barriers are usually required by the Building Codes, regardless of the type of insulation used. Check with the local authorities to find out the requirements for your area.) Closed-cell foams are superior insulators. While open-cell foams typically have R-values of 3 to 4 per inch (RSI-0.53 to RSI-0.70 per inch), closed-cell foams can attain R-values of 5 to 8 per inch (RSI-0.88 to RSI-1.41 per inch). This is important if space is limited, because it allows a thinner layer of insulation to be used. For example, a 1-inch layer of closed-cell foam provides about the same insulation factor as 2 inches of open-cell foam. Closed-cell foam is very strong, and structurally reinforces the insulated surface. By contrast, open-cell foam is soft when cured, with little structural strength. Open-cell foam requires trimming after installation, and disposal of the waste material. Unlike open-cell foam, closed-cell foam rarely requires any trimming, with little or no waste. Advantages of open-cell over closed-cell foams Open cell foams will allow timber to breathe. Open cell foams are incredibly effective as a sound barrier, having about twice the sound resistance in normal frequency ranges as closed-cell foam. Open cell foams provide a better economical yield. Open cell foams often have a low exothermic reaction temperature; will not harm coatings on electrical wiring, plumbing or other building components. Types Cementitious foam One example is AirKrete, at R-3.9 (RSI-0.69) per inch and no restriction on depth of application. Non-hazardous. Being fireproof, it will not smoke at all upon direct contact with flame, and is a two-hour firewall at a (or normal stud wall) application, per ASTM E-814 testing (UL 1479). Great for sound deadening; does not echo like other foams. Environmentally friendly. Non-expansive (good for existing homes where interior sheathing is in place). Fully sustainable: Consists of magnesium oxide cement and air, which is made from magnesium oxide extracted from seawater. Blown with air (no CFCs, HCFCs or other harmful blowing agents). Nontoxic, even during application. Does not shrink or settle. Zero VOC emission. Chemically inert (no known symptoms of exposure per MSDS). Insect resistant. Mold Proof. Insoluble in water. Disadvantages: Fragile at the low densities needed to achieve the quoted R value and, like all foams, it is more expensive than conventional fiber insulations. In 2010, the Ontario Building Code Commission ruled that AirKrete did not conform to requirements for a