Spray foam is a chemical product created by two materials, isocyanate and polyol resin, which react when mixed with each other and expand up to 30-60 times its liquid volume after it is sprayed in place. This expansion makes it useful as a specialty packing material which forms to the shape of the product being packaged and produces a high thermal insulating value with virtually no air infiltration.
Spray foam is a very specialized packing material, often required for use in shipping valuable fragile items. Engineered packaging principles are designed to protect sculptures, vases, large fossils, lamp bases, busts, computers, furniture, chandeliers and other objects of unusual shape. By virtue of the liquid foam expanding by up to 30-60 times the volume of its liquid state, it efficiently protects almost any size, form and weight.
The custom fit of the molds, top and bottom, securely and uniformly cushions the object. There are many types of alternative materials that can be used to handle more specific needs.
"Spray foam" is also an informal term used to refer to various plastic foam materials that are used in building construction to provide thermal insulation and minimize air infiltration. Polyurethane and polyisocyanurate are two types of foam used in this application.
There are two main types of spray foam insulation, open and closed cell. Open cell insulation can be crushed in your hand and has a lower insulation value. Closed cell, is rigid to the touch and each air cell is completely sealed. While closed cell is the superior product it is more costly to buy. 
Another type of use quite familiar in South East Asian countries is applying the pu or pur foam by spraying it against the bottom of roof tiles under high pressure with a spray gun. A hard but flexible layer of rigid foam is then created and seals all the tiles to each other and to the steel structure.
This spraying method, especially very popular in Thailand, is used not only against heavy leaks but helps also as insulation against the enormous heat the roofs constantly face.
This tropic heat actually is the cause that the steel structure, which most roofs in that part of the world are built on, expands and contracts all the time. By doing so, the sun-battered steel structure causes the roof tiles to dislodge and slowly creates small openings between the roof tiles, through which rain water can run into, creating leaks that can cause much damage to plastered ceilings and electric wiring.
Polyurethane Foam provides a complete, seamless building envelope, creating a more comfortable indoor environment for the life of your home.
To avoid the expense re-roofing a house that is simply suffering from nail fatigue, and to obtain optimum insulation, an 85mm layer of polyurethane foam can be sprayed onto the underside of the slates or tiles of a pitched roof. This stabilises and weatherproofs the roof by fixing the nails, battens and roofing felt. As the foam is a superior insulant, coupled with just 100mm of insulation at joist level, your roof will also meet the stringent thermal performance standards required for a new home. A typical roof can be insulated using the polyurethane treatment for around half the cost of re-roofing.
The insulation at eaves level will leave your loft area comfortable for use and free from condensation. By installing the insulation at rafter level, it keeps the loft void warmer and prevents condensation build-up, which can otherwise occur when increasing insulation at ceiling level. The warmer, drier, cleaner roof space eliminates the risk of pipes and tanks freezing.
In the West
Some 150,000 properties have been treated with polyurethane foam insulation in the UK. There are systems which have British Board of Agreement approval for use in existing and new build applications and can show compliance to Building Regulations.
In addition to Europe, the United States has adapted to using sprayfoam insulation and a new technology called Wall Injection to retrofit existing wall construction by drilling small holes between wall studs in the structures framing and filling the void with a less aggressive expanding water based foam. This allows existing home and business owners to conserve energy by creating a thermal envelope in their existing structure.
- "Closed-cell spray foam tops home insulation options". Montreal Gazette. Retrieved 29 January 2013.