UFFI, or urea-formaldehyde foam insulation, is a synthetic type of insulation that is injected into the walls of a house. It was used extensively in homes in the 1970s, until it was discovered that it produced some adverse health effects from formaldehyde vapour emitted during the curing process, affecting the eyes, nose and respiratory system. Because of these health concerns, UFFI is no longer used in home construction in Canada and hasn’t been since at least the 1980s.
Because of these health concerns, which unlike asbestos can happen even if it is undisturbed, banks do not like to lend money to buy houses with UFFI unless you have a plan in place to remediate it fairly quickly after closing. When you buy a house and get a mortgage, you will be required to sign a document that says that there is no UFFI in the house to the best of your knowledge and belief, and when you sell, you will be required to warrant this same thing in the agreement.
UFFI is pretty rare to find these days, but if you do find it, you will need to deal with it right away.