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Spray foam insulation is an alternative to traditional building insulation such as fiberglass. A two-component mixture composed of isocyanate and polyol resin comes together at the tip of a gun, and forms an expanding foam that is sprayed onto roof tiles, concrete slabs, into wall cavities, or through holes drilled in into a cavity of a finished wall.
Polyurethane, the most common type of spray foam insulation, was developed and used by the military in the 1940s and applied to airplanes. It wasn't until the 1970s that it started to be used as foam insulation.
Various systems are used to apply the spray foam. The two component high pressure system is generally used in new home construction. It is a quick expanding type of spray foam. The two component low pressure spray foam is another system that is used primarily for remodel jobs where there are existing walls with drywall already in place. This is also known as a slow rise formula and often referred to as injection foam.
Spray foam insulation can be categorized into two different types: open cell and closed cell.
Open cell foam insulation
Open cell is a type of foam where the tiny cells are not completely closed. Open cell is less expensive because it uses fewer chemicals. It is a very good air barrier but does not provide any type of water vapor barrier. It is much more sponge-like in appearance. It is often used for interior walls because it provides sound reduction. It is not recommended for outdoor applications.
Closed cell foam insulation
Closed cell foam insulation is much denser than open cell. It has a smaller, more compact cell structure. It is a very good air barrier as well as a water vapor barrier. It is often used in roofing projects or other outdoor applications, but can be used anywhere in the home.
Spray foam insulation saves on energy costs and lowers utility bills. Studies by the US Department of Energy show that 40% of a home's energy is lost as the result of air infiltration through walls, windows and doorways. Buildings treated with spray foam insulation typically insulate as much as 50% better than traditional insulation products.
Insulation that is sprayed in buildings protects against moisture, which provides the benefit of reducing the chance of harmful mold and mildew. Eliminating mold growth reduces the likelihood of rotting wood in a home, and allergic reactions to mold spores.
In addition to building temperature and moisture control, spray foam insulation is often used to reduce noise. Foam insulation serves as a barrier[disambiguation needed] to airborne sounds, and reduces airborne sound transfer through a building's roof, floor and walls.
R-value is the term given to thermal resistance to heat flow. The higher the R-value of an insulation product, the more effective the insulation properties. 1.8-2 pound polyurethane foam has the highest R-value of readily available insulation used in homes and buildings.
Polyurethane is a closed-cell foam insulation material that contains a low-conductivity gas in its cells. As a result of the high thermal resistance of the gas, spray polyurethane insulation typically has an R-value around R-5 to R-6 per inch. In comparison, blown fiberglass typically has an R-Value of only R-2 to R-4 per inch.
Foam insulation blocks all three forms of heat transfer:
Isocyanates are powerful irritants to the eyes and gastrointestinal as well as the respiratory tracts. Direct skin contact with isocyanates can also cause marked inflammation. Some people say that their eyes feel like they have sand in them at the onset of problems. Some break out in a rash on their arms, chest, and neck.
Overexposure to isocyanates can sensitize workers, making them subject to asthma attacks if they are exposed again. Respiratory irritation may progress to a chemical bronchitis. Additional exposures can make the onset easier with less isocyanate necessary to start the attack.
Sporadic cases of hypersensitivity pneumonitis (HP) have also been reported in workers exposed to isocyanates. The symptoms may seem like the flu, with fever, muscle aches, and headaches. Other symptoms may include a dry cough, chest tightness, and difficult breathing. Individuals with chronic HP often experience progressively more difficult breathing, fatigue, and weight loss. Individuals with acute HP typically develop symptoms 4-6 hours after exposure.
Foams in recent years have nearly eliminated some or most of these problems.
Spray foam insulation is typically non-toxic only after it has cured. While curing spray foam emits a gas that causes blurred vision and trouble breathing. Using full face and respiratory protection while applying the product is recommended. After being sprayed, it expands to roughly 30-100 times its original volume. As a result, it is able to fill vacant air gaps, and will expand and contract in relation to the building.
Though spray foam insulation is an excellent insulation material that helps homeowners and business conserve energy and reduce fossil fuel usage, there are two ways that spray foam insulation can negatively impact global warming. The first is the embodied energy, the energy that it takes to produce and ship the insulation material. The second is the blowing agents (used to propel the foam out of the spray gun) are often made of hydrofluorocarbon (HFC) agents that are very potent greenhouse gases.
It is possible to calculate the "Lifetime GWP" by calculating the length of time it will take for the energy savings from insulation (at a specific R value) will save, in a building, over its life v. the GWP of the embodied energy plus the blowing agents of the chosen insulation (at a specific R value). 
Expanding foams are in some cases able to exert considerable force on anything that tries to contain them.
Foam has often been applied to the underside of timber framed roofs, primarily as a cure for slate slippage on old roofs with decaying nails. But this can cause serious structural problems. Water reaching the timber structure is no longer able to drain & evaporate away, and timber can saturate and rot. The foam makes inspection of the timber structure impractical.
The adhesion between foam and slate also makes later reuse of the slates impossible, adding significant extra cost to the eventual reroof. Because of these two problems, lining timber-framed roofs with spray foam is now regarded as bad practice in the UK.
Asphalt roof manufacturers recommend against using spray foam directly on the under-side of roof sheathing called a hot roof. The lack of ventilation allows high heat buildup which is the primary cause of deterioration in asphalt shingles.
Due to propellant gas emission using spray foam in small rooms is highly dangerous. After the use of only few cans without venting the room there is enough gas (propane/butane) to be lit by the sparks of a drill or vacuum cleaner causing a heavy explosion.
Insulation of all types stop a good deal of energy loss. Some types including spray foams also seal air leaks. Insulation can also save energy in hot climates by reducing air conditioning use.
Spray foam insulation levels vary, with some providing up to an R-value of 6.7 per inch.