Urea-formaldehyde foam is an effective and relatively inexpensive insulant, and it has brought substantial benefits in energy conservation, but its use in inappropriate circumstances or without due care has resulted in a limited number of people suffering discomfort and irritation from the formaldehyde vapour given off as the UF foam cures. This short paper reiterates the correct procedure to follow when using the foam (see BS 5618) and describes the types of construction for which this form of wall insulation is not appropriate.