Urea-formaldehyde foam insulation was developed as an improved means of insulating difficult-to-reach cavities in house walls. It is typically made at a construction site from a mixture of urea-formaldehyde resin, a foaming agent and compressed air. When the mixture is injected into the wall, urea and formaldehyde unite and "cure" into an insulating foam plastic. The chemistry of UFFI and SPF are very different, as SPF contains no added formaldehyde.