Insulating Beneath Metal Roofs
Often, when a structure is re-roofed, it is seen as an opportunity to gain additional insulation value. However, in so doing, care must be taken to ensure that the goal is met without jeopardizing the integrity of the roof system.
ANALYSIS AND SOLUTION
The best place to achieve insulation on the top of a home is directly on top of the ceilings. Because attic spaces must be properly ventilated, it simply does not make sense to try to achieve insulation on top of the vented attic space (see Technical Bulletin #15). Trying to place insulation boards between a metal roof and the roof deck can be detrimental because it might threaten the integrity of the roof system. When the metal panels are walked on, they can press into the foam and become disengaged or otherwise damaged. Some metal roofs, however, do offer formed insulation backers that are acceptable though such backers are intended more for extra stability or sound deflection rather than for insulation value. On applications such as those over Cathedral Ceilings when extra insulation value is sought but there is no existing attic cavity, it is suggested that an extra chamber for insulation and ventilation be constructed on top of the current roof. In all cases, there should be adequate ventilated airspace achieved between the insulation and the bottom of the roof decking to allow moisture to escape without condensing and causing problems.