Install plywood or oriented-strand board (OSB) to act as structural roof sheathing. Lay the plywood perpendicular to the roof joists and fasten it in place using galvanized or stainless steel roofing nails.
Layer building paper over the sheathing to help protect against moisture and leaks. Overlap each sheet of building paper by six inches and fasten it to the sheathing using nails or staples.
Install sheets of rigid foam insulation over the entire roof surface. Butt the sheets closely together but do not allow them to overlap.
Seal the seams between the sheets of insulation with duct tape to further protect against moisture. The duct tape will also help prevent air leaks and maximize the insulating ability of the roof.
Install the metal roofing panels over the insulation in accordance with the manufacturer’s installation instructions.
Insulating an Existing Metal Roof
Examine the underside of your metal roof from inside the attic. You will likely be able to see a layer of plywood roof sheathing supported by some type of framing system. Small horizontal framing members are known as purlins, while larger angled beams are known as joists.
Place fiberglass batts into any gaps you can find between the purlins and the sheathing. Fiberglass insulation tends to be less effective if it is compacted too tightly, so try to fit it loosely into any available space. Joists are usually set tighter to the roof, so it is often not possible to fit insulation behind them.
Use rigid foam insulation to cover wider spans of roof sheathing between joists. Cut each sheet to the desired size using a utility knife, then fasten it in place using staples or adhesive. Avoids nails or screws, which could penetrate the metal roof and cause leaks.
Insulating with Spray Foam Insulation
Purchase or rent a spray-foam insulation kit from a metal roof expert, including a sprayer and a supply of closed-cell liquefied foam. These materials can be found at most home improvement and hardware stores.
Cut a small hole in your ceiling so you can access the roof framing. Use the instructions included with your sprayer to determine where to cut the hole and how big it should be.
Check around the ceiling to ensure your metal roof is supported by plywood or OSB sheathing. Some metal roofs are installed without this sheathing. If your metal roof does not have plywood installed underneath, you should not attempt to install spray foam yourself. It could spread through the seams in the roof and cause significant damage.
Spray the foam insulation into the hole carefully according to the manufacturer’s instructions. The foam will expand to fill the roof structure. Patch and repair the drywall to complete the installation.