"In climates where low temperatures threaten the integrity of plastic sprinkler pipe in residential attics, it is important that adequate protection be provided to prevent freezing. A common protective method is to cover the sprinkler pipe with insulation while allowing convective heat from conditioned spaces below to keep the fire protection water in a liquid state. It is important that the insulation be installed tightly against the joists so cold air doesn't migrate from the unheated attic to the pipe. In areas where attic temperatures drop below 0 °F (-18 °C), an additional layer of insulation should be provided. In all cases, the insulation should be installed in accordance with the insulation manufacturer's recommendations."
Coffee Break Training - Fire Protection Series No. FP-2006-51
United States Fire Administration: http://www.usfa.fema.gov/