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Bob Vila says that due to the expansive/contractile nature of metal roofs, rubberized underlayments are best for metal roofs. [https://www.bobvila.com/articles/best-roof-underlayments/]
Do folks here think that's true? If it is true, is it true all the time, or only when a wooden lattice is not used between the roofing and the underlayment?
You should not use an underlayment with a granulated or sanded top surface. I would also avoid products where oils may leech and cause the underlayment to stick to the back of the metal panels.
I have not seen issues with synthetic underlayments beneath metal and they have for many contractors been the underlayment of choice beneath metal for 20+ years now. That is my typical recommendation unless building codes dictate otherwise.
The concern with rubberized products is they do not breathe. Covering an entire house with them seals the attic up like a terrarium. Moisture from inside the living space migrates into the attic, carried by convective airflow. (Warm air rises.) If that moisture is not vented out, then it will condense when it hits a cool surface, typically the underside of the roof deck.
If you use a rubberized underlayment you must have really good attic ventilation (intake and exhaust) or else a sealed and conditioned attic.
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