The more evident and more serious blisters are structural blisters. They occur in many forms of deformation and are not confined to the exposed surface. They are caused mainly by the expansion of trapped air and water -vapor or moisture or other gases. Air and moisture trapped within the construction tend to expand during a rise in air temperature or from the heat of the sun, and this expansion causes the plies of the roofing to separate and bulge the roof surface in a balloon effect. The blisters are spongy to the touch, and may occur between any of the layers of roofing felt, or between membrane and deck, or membrane and insulation.