In March, I addressed NRCA recommendations for the use of rigid board, faced, polyisocyanurate insulation as components of membrane roof systems. Included in those recommendations is the use of a suitable cover board layer. Following is some background information and NRCA's rationale and current recommendations for the use and selection of cover boards.
NRCA first recommended the use of cover boards in an interim alert bulletin (NRCA Technical Bulletin No. 4) dated Aug. 10, 1978. The bulletin cited concerns about the growing incidence of blistering of roof membranes installed over rigid board polyurethane roof insulation. Installing a thin layer of wood fiberboard, perlite board or fiberglass board insulation was recommended with joints staggered from those of the polyurethane insulation layer below.