Without Proper ventilation, your attic will trap heat and moisture. Not only will this affect your energy costs, but it can cause damage to shingles, roof decking, insulation, and other areas of your attic and roofing structure. Trapped heat and moisture will attract mildew, and cause decking to warp and sag. Metal components like nails and fasteners will rust, and eventually weaken and fail. Moisture can dissolve the adhesives used in decking and underlayment, and in the winter time, will invite trouble from condensation, dripping, and frost.