Underlayment for sheathed metal roof in pole building
I am in climate zone C4. I have recently finished building a home with a lot of good advice from all of you no this site. I am now paying a contractor to throw up a pole building that I will outfit as a shop. The building will have a OSB sheathed metal roof. I will eventually be trying to insulate the ceiling, but that is later down the road. What kind of underlayment is best for this roof. This building will have a shed roof that will be difficult to ventilate well, so it seems that the breath-ability of felt paper would be good and would be more likely to be installed properly by a pole barn contractor, but I have heard that the felt is a poor choice for metal roofs because of the heat and expansion of the metal. Any thoughts?
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Asphalt felt is always a safe roofing underlayment. If you are tempted to use a synthetic roofing underlayment, you should be aware that most manufacturers of synthetic roofing underlayment forbid their products to be used on unvented roof assemblies.
Pole barns are hard to air seal and insulate (based on reading comments here on GBA).
Have you consider using reclaimed foam? You might want to bite the bullet now while you still have the option of placing some or all of the insulation above the OSB.
Thanks for the feedback. I have heard a lot of talk about Asphalt felt heating up and sticking to the bottom of metal roofs and then tearing when the metal expands. Is there any truth to this? I have also read some comments about oils from the asphalt deteriorating metal. Is this all just hype from manufactures?
I read about the concerns around felt paper and heat. In a discussion here a few years ago I think posted about them. However I haven't seen any further cautions about using roofing paper, either in the installation manuals of suppliers or more generally. I'm beginning to think there wasn't much to worry about.
I've used asphalt felt under metal roofing in Vermont for years, and never had any problems. It's possible, I suppose, that problems occur in hotter climates.
A wide variety of synthetic roofing underlayments are available, including some that are touted for use in "high temperature" situations. There's no reason you couldn't use one of those if you want to. In most cases, you won't really be expecting your roof sheathing to dry outward, so the vapor permeance of the underlayment isn't very important in your situation.