cover board
An insulation board used over closed cell plastic insulation (e.g., polyisocyanurate) to prevent blistering when used in conjunction with hot bituminous membranes.
cover board: protection layer for foam under hot tar
A cover board is a rigid sheet installed directly above rigid foam insulation on a flat roof, sitting between the foam and the hot bituminous membrane. It serves as a thermal and physical barrier, preventing the foam from absorbing moisture and heat that would cause it to blister, delaminate, or off-gas when exposed to temperatures above 100°C during torch-down or mopped asphalt application.
Cover boards are typically made from mineral fiber, fiberglass, or perlite composite materials in thicknesses of 0.5 to 1.5 inches. They must be rigid enough to span roof framing without deflection, yet porous enough to allow vapor transmission. The mineral fiber variants are most common in North America and are often faced with kraft paper or fiberglass tissue on the top surface to provide a stable base for the bituminous layer.
The critical job is preventing closed-cell foams, particularly polyisocyanurate (polyiso), from reaching temperatures at which they soften or release blowing agents. Without a cover board, the hot bitumen can transfer enough heat into the foam to cause visible blistering within days or weeks of installation. This blistering compromises the membrane's seal and the foam's insulating properties. Open-cell foam insulation typically does not require a cover board because it is more vapor-permeable and less prone to blister formation.
Installation and overlap
Cover boards are laid over the foam in staggered rows and mechanically fastened or adhered with hot bitumen. The bituminous membrane is then applied directly to the cover board surface. The board's top face must be dry and clean for proper adhesion. In high-wind zones, fastening density and fastener placement are critical to prevent blow-off during installation or service.
Although cover boards add cost and labor, they are often mandatory by building code when polyiso or other foam with a low softening point is used under built-up roofing or torch-down systems. They may be omitted if single-ply membranes (EPDM, TPO, PVC) are used instead, since these apply without heat. The presence of a cover board also slows thermal cycling and extends membrane life by evening out temperature spikes across the roof assembly.