What to put under metal roofing?

[N80]

I am building a post and beam, open carport down at my cabin. It is shown below. I will use 2x4 (actual) purlins. It will have a 5v crimp metal roofing.

My much larger tractor shed that i built about 15 years ago has the same metal roof set up; metal on purlins. When the conditions are right water condenses on the underside of the roof and it practically rains under there. This is causing the purlins to be moldy and they'll eventually rot (although I'll be tearing it down before long to make room for a shop).

Anyway, I'd prefer not to have this condensation problem. My initial thought was to use 1" thick boards as 'sheathing'. I have a sawmill and have plenty of it. So the sheathing would sit on the purlins, then tar paper on top of sheathing then the metal roof. Some folks have suggested that there would still be a condensation problem and the 1" pine boards would rot.

I've thought about OSB sheathing but it would be ugly from the underside and probably also rot if the boards would.

So I'm looking at the insulating stuff used in this capacity. Usually white from what I've seen. Sometimes silver. I don't know how much it costs or how to install it. I don't know how well it would hold up over time either. I've seen barns and sheds with it hanging down in tatters and I don't want that.

I am looking for any thoughts or recommendations on this issue.

 

[Fuddyduddy1952]

Right before Covid I had a 38x64 horse stable built. They used this "double bubble" insulation before screwing down steel roof. Same under a shed overhang which is fantastic to me, weatherproof and reflective.

 

[Fuddyduddy1952]

To install is easy, just cut length you want on ground, take it up and use hammer tacker staple it on. It's clean & lightweight to work with.

 

[Hay Dude]

How about T&G PVC planks?
Attractive on underside, won’t rot.

 

[5030]

Had the contractor do what HD posted. Just had the side shed on the big barn roofed.

 

[caver]

My cabin plans call for 17mil Epilay Protectite Platinum roof underlayment. (It's included in my kit) I told them I wanted to use some type of metal roof. The roofing will be attached to osb that is glued to 10" styro panels (like SIPS) and they call it a nailbase panel. Platinum Synthetic Roofing Underlayment

 

[JJT]

How about a pair of ceiling fans? A neighbor's pole barn rained like you said, installed a pair of ceiling fans to move the air and that cured the indoor rain issue.

 

[grsthegreat]

The fabric insulated underlayment gets eaten up by birds pecking at insects and squirrels looking for nesting materials. Ask me how i know. I had a company tear off my barn roof, remove that nasty rolled insulation, install cdx plywood, tar paper and reinstall metal roof using longer, thicker screws. The tar paper will channel any moisture. When they use metal roofing on houses, they do the same thing.

 

[Jstpssng]

^^^^
Exactly!
I had a shed with metal roofing. When I moved here I stored a lot of things in it and lost anything which wasn't plastic.

I'm looking at the same problem and like you, have a sawmill and plenty of trees. Put your boards lengthwise up the roof so that any moisture will run down them; they used to build barns that way with no outside covering. Then put furring strips lengthwise to hold the metal screws. That will create an air gap and should eliminate any mildew problems.

I plan to use asphalt shingles instead.

 

[CalG]

30# felt