Best Underlayment for Metal Roof Over Rigid Foam
I’m planning to replace an ancient slate roof in northwest Massachusetts (Zone 5) with the following plan: remove slate and keep original board sheathing. Put down peel and stick membrane over original sheathing, then 7 inches of recycled polyiso rigid foam insulation, 5/8 plywood, underlayment and finally standing seam metal roof. There are a ton of underlayment options: what would be best in this situation? Does it need to be made for high temperatures? Should it be vapor permeable? Any specific brand suggestions? Any and all advice most appreciated.
What brand and type of metal roof are you planning to use? They should have instructions and be able to inform on what underlayment properties are required.
They also may have requirements for the roof assembly to include venting.
ABC SL-16 Galvalume
FT Synthetics platinum paper and ice and water is the best we’ve used. It’s high temp rated for standing seam. Staples only, no button caps. They will emboss through the panel.
From the instructions: https://www.abcmetalroofing.com/Products/Panel-Profiles/SL-16-/ SL-16 manual.
WATER/VAPOR CONTROL (MOISTURE BARRIER UNDERLAYMENT)
When warm moist air contacts a cold surface such as the underside of a metal roof panel, the water vapor contained in it condenses.
The continuous presence of moisture is detrimental to many things, including metal. The use of suitable moisture/vapor barrier and
insulations protect the metal and your investment. The minimum acceptable barrier would be 30# builders felt. Other breathable barriers
such as “Shingle Mate®” or “StormGuard® HT” from GAF Materials Corporation provide exceptional long term protections when applied
as directed by the manufacturer. Whatever barrier you decide to use must be applied in a smooth continuous manner, free from holes
or rips. Fasteners used to attach the barrier must be covered to protect the underside of the panels. Fasteners must be flush with the
surface of the barrier or they will cause unwanted distortion of the panel surface. It is further recommended that peak and eave or gable
vents be installed to provide adequate air flow in trapped air spaces such as attics. Consult a qualified design professional for venting
For your application, you might want to consider adding a layer of strapping between the insulation and roof decking and provide ventilation of this cavity (between the strapping). (eave and peak)