Metal Roof Insulation and Ventilation in Charleston, SC

A vented attic pulls outside air in low and lets it out high. The 2021 International Residential Code asks for net free vent area of 1/150 of the vented attic area, or 1/300 when 40-50% of that area sits in the upper part of the attic (within 3 feet of the ridge) and the rest in the bottom third. Each attic space needs cross ventilation, vent openings are screened to between 1/16 and 1/4 inch, and insulation has to stay at least an inch clear of the sheathing at the eave vents. On a metal roof, the ridge vent is a metal part too, and one that needs wind testing on a FORTIFIED roof (see Hurricane Metal Roofs).

The FORTIFIED checklist also asks for temporary storm protection to be on hand for gable-end wall vents, since a gable vent is an open hole in a hurricane. If your attic breathes through gable vents, ask about covers, or about moving the ventilation to the soffits and ridge during the re-roof.

The other route seals the attic and brings it inside the house. The code permits an unvented attic when the space sits inside the building's thermal envelope, no Class I vapor retarder is placed on the attic floor, and the insulation follows its rules. With air-impermeable insulation such as closed-cell spray foam, that means foam in direct contact with the underside of the roof sheathing. Done right, the ductwork in the attic also ends up inside conditioned space. The trade-off is visibility: foam covers the underside of the deck, so a roof leak can travel a long way before it shows. Ask for the roof work to be finished and checked before any foam goes on.

When fluffy (air-permeable) insulation goes under the deck in an unvented attic, the code wants rigid board insulation on top of the sheathing to keep the deck warm enough to avoid condensation. In climate zones 1 through 3 the table minimum is R-5. This only makes sense during a full re-roof, when the deck is bare. Ask the panel maker how the clips and fasteners attach through the board.

Under metal panels, the code lets underlayment follow the panel maker's instructions where wind design isn't required. Where it is required, the underlayment has to be ASTM D226 Type II or ASTM D4869 Type III or IV, with self-adhered ASTM D1970 membrane allowed as an alternative. A full peel-and-stick deck is one way to seal the roof for wind, and the IBHS FORTIFIED checklist notes that manufacturers stress good attic ventilation when it covers the whole deck. Metal panels get hot in summer sun, so ask whether the membrane is rated for high temperatures.