Re: OT - Metal Roof Insulation

Posted by John in La on December 26, 2015 at 05:51:13 from (96.33.136.54):

In Reply to: OT - Metal Roof Insulation posted by Bill VA on December 26, 2015 at 03:43:10:

More info than you may want to know but here goes.

I have been doing a lot of research on metal roof/siding as I am in the process of building a pole barn/garage.
Note; I live in a subtropical humid area and severe wood deterioration zone so this may not apply to you and your area.
Others may say they live in a humid area but they really do not know what really humid is.

You can insulate metal roofs many ways. Spray form; ridged boards; and a thin layer of insulation with foil on each side. The foiled backed insulation is the most common and should be available where you buy your metal. It comes in a roll and you just roll it out before you put on the metal. Many metal buildings built around here with no condensation problems using this foil backed insulation.

But lets look at what is happening......
Metal will condensate with differences in temperature.
Some say it is poor ventilation.
Some say it is ground water coming up threw the slab.
Some say it is the difference in temperature from the bottom and top of the metal.
I say B/S to all of the above

I know a friend at work that has a solid plywood wooden porch on the front of his house about 2 feet off the ground with a painted rib metal roof over the porch. No sides except for the 1 year old house sitting next to it.
It rains from his roof onto the porch due to condensation drip in the right conditions.
No ground water gets to it due to the porch. Has plenty of ventilation due to 3 open sides. No temperature difference as the bottom and top are exposed to the same outside conditions.

So why does this roof have condensation problems?
Then one day he told me the condensation was frozen. "Frost"
But wait it is only 35 degrees outside and he has a thermometer on the porch to verify it.
So this got me to thinking why the metal would frost when the temperature is above 32 and I remembered windshields on cars.
A windshield on a car will frost over even thought the air temperature is above 32.
This is because the glass radiates heat away at a faster rate and actually becomes colder than the air around it.

So it's not that the temperature is different on either side of the metal that always causes condensation like a cold soda can in a hot room. It could be that the metal is radiating heat away at a different rate than the air around it like a windshield and as it crosses the dew point it sucks the moisture out of the humid air causing condensation to form on the bottom side of the metal.

So how can we stop this?
We can heat the area to keep the metal above the due point. Heck good tight insulation may hamper this effort.
Or we can seal the building in a effort to keep out moist air. Not really possible on a pole barn or porch.
Heck even sea going containers condensate around here and they are sealed pretty good.
So I see no real way to stop condensation other than totally sealing "and I mean totally" the moist air from getting to the bottom of the metal or changing the metal structure to where it does not radiate heat away faster than the air temperature around it changes. I feel putting insulation or plywood or what ever you want under the metal is not stopping the condensation. It just stops it from dripping on your head. This kind of barrier is got to lead to problems down the road due to moisture on the barrier.

So I started looking around and asking questions about metal structure and condensation.
I found one neighbor that has one of those metal frame carports that he says never condensates so this makes me believe metal structure or it might be what it is painted with.
I have also found one roofing supply house that says he has the answer in a paint can.
The jury is still out on this as my friend just bought this paint this past weekend and he has not had time to see if it really helps his condensation problem on his porch roof.

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