OT - Metal Roof Insulation

Been reading some posts regarding metal roofs and condensation. Sounds like you can put some insulation under the tin and solve the problem?

I'm asking because we are going to put up 2 or 3 metal carports around the place for parking equipment and storing hay next year. I'm concerned about the condensation. Sounds like some insulation might be a good idea. Don't know if that's a sheet type insulation or some kind of spray foam that would also cover the rectangular tubing structure under the roof.

Please tell me about this insulation, thickness, how it's applied, where one typically buys it, is it specific for metal roofs?

Thanks!
Bill

When we put up our shop, we put a layer of house wrap over the purlins and then put the sheet metal on. Any condensation would drip on the house wrap and then run down to the outside. Has seemed to work weel here for the last 12 years. YMMV.

If you dont have walls and ends covered just a roof shouldnt be much of a problem. When building is total enclosed they use a layer of thin foam comes in rolls it works well whether dirt floor or cement.

Bill,
I am planning a building and came across "Dripstop" http://www.dripstop.com/
I don't know how effective it is.
K-Mo

Closed cell spray-on foam insulation about 1 inch thick has worked well for me. Cost was around 75 cents per square foot. Not cheap but effective.

Beagle

It looks like a thin sheet of bubble wrap about 1/4 to 3/8 in thick and silver on one side white on the other. comes in rolls about 10 feet wide. Recommended under metal roofs in enclosed buildings. I agree, if there is one or more open sides it is probably not necessary.

I have two open sided sheds with metal roofs and they both will drip condensation. The frost forms overnight and when the temp goes up over freezing down comes the rain.

The two most commonly used systems to prevent condensation and the resulting dripping on/from the underside of metal roofs are the bubble type material and the newer felt type coating that is actually attached to the metal.

I have buildings with each and have had not a drop from either.

Dean

Condensation under a carport is another animal. My sister in Florida has a carport and roof over boat dock . Everything under the roof is full of condensation in the morning because of fog. I saw the same thing in Indiana one foggy morning. So depending on what part of the country you live, all the insulation in the world won't prevent condensation all the time.

Recently in Indiana it was raining, about 65. Everything inside pole barn was about 50. After I opened the door, it didn't take long before I saw condensation on everything inside pole barn. Even the plastic quart oil bottles sitting on a shelf. When I drove the truck outside it has stopped raining, the cold window glass instantly fogged up on the outside.

So depending on where you live, if you have foggy mornings, good luck with carports.

Condensation occurs when the dew point temp is greater than the metal temp.

The systems in my post below prevent condensatyion on the underside of the metal roof and subsequent dripping. Neither is intended to prevent condensation on anything stored beneath.

Dean

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.

The most common insulation is single layer fiberglass with the white vinyl vapor barrier. It is put up before the roof goes on the purlins. With a carport, might be a good idea to put chicken wire first to protect it from wind.

Another consideration for carports, instead of using C or Z channel purlins, use rectangular tubing and have no bracing, wiring, lights in the middle of the storage area. Leave no place for birds to roost.

That subject is argued on here ever 6 months or so. I think a lot depends on what part of the country you are in. We build at least one new building a year here on the farm. For the last 10 years or so have been using a product called micro-foil or most folks call bubble wrap. I like the aluminum foil on both sides but aluminum on one side and white plastic on the other works. About 1/4 in thick and solves MOST condensation problems. We have used it in fully closed buildings to some just roof structures.

I had a new building built this spring & on the roof sheets I used what is called Drip Stop. It is a felt type material attached directly to the bottom side of the roof tin. It works!!!!!!! Not one drip any where in the building.

On my main pole barn I have wood with shingles on the roof. I have a 30 ft lean-to I added onto the back. They are both enclosed with steel sides. I get allot of condensation on the steel roof but very seldom have it under the wood roof.

For that reason I went with wood and shingles on the small shed I just built. One guy suggested I put steel on top wood on the roof instead of shingles. I just finished the shed. I haven't found if its going to sweat yet.

I did put allot of roof vents in the main pole barn and none in the lean-to. That may help in making the difference.

I used that 1/4 inch thick pink foam under my steel on the house.Mostly as a cushion from the missed nails when we stripped the roof. I also think it would tend to give some sort of vapor barrier for condensation. I guess I don't get that worried about it in the shed since the poles will rot off before the rest of the building is going to go bad.
Where the old wood shingles were left on I didn't use the foam under the steel.

Dean,
Like I said, fog is another animal. So if you live on a lake or near a large body of water, good luck. Water dripps off the under side of boat dock metal roof. Fog is a rain cloud that hit the ground. So a carport without sides, will only keep rain off hay providing the wind isn't blowing. Geo.

Sorry, but I'm a mile as the crow flies from the Ohio River (much larger body of water than the Wabash). My buildings are about 200 yards from a tributary that always has about 100' (wide) of standing water since 1962 when the gates were closed at Markland dam.

I've lived with fog all or my life. Never a drop from either metal roof.

Dean

John in LA: Nice dissertation! Condensation is a lesson in basic physics.

1 Any material will radiate to the night sky. If it is a clear night the radiation will be greater. Grass does it, windshields, steel, wood, shingles etc. The radiation depends on the difference of the fourth power of the temperatures (absolute temperature of the material squared then squared again) and the dark night sky is close to absolute zero so heat will leave the item looking at the sky. Insulation on top of the roof does not reduce the radiation but it helps keep the material underneath warmer.

2 Condensation only happens when the material temperature is lower than the dew point of the air in contact with it. If you have a cool day and warm moist front comes by everything will get wet except those things in a closed building where the warm moist air does not get. Open a big door and the moisture will enter a cold building and wet what it touches until the dew point of the air is lowered by condensing water out of it, or the air is cooled enough to lower the dew point. If you walk outside with cold glasses they will fog up only if the dew point of the air is higher than the temperature of the glasses. Any open roof will drip if warm moist air moves in after a cold night. The bottom of cold insulation will still get wet if high dew point air contacts it.

3 Under roof insulation works because most of the time the roof does not get cold enough long enough to chill the bottom of the insulation. You don't get much moisture between the insulation and the metal because the air can't get to it. The little bit of air trapped between the layers will condense a small amount of moisture until a few hot days (all summer) drives the moisture out of the gap. If there is no air flow between the insulation and the steel that moisture will not be replaced. Felt or porous material will tend to absorb the moisture until the temperature comes up and then it can evaporate into warm air instead of dripping.

4 No insulation works in open sheds in many climates because the dew point is low overnight and only a little moisture condenses on the bottom of the roof, not enough to drip off come morning. We don't usually have a problem in Oklahoma, but my hay barn has rained many times this fall, for a few minutes. Then it warms quickly enough the water on the hay and equipment evaporates with no damage.

5 If you have something you absolutely need to keep moisture off, air conditioning to keep the dew point below the material temperature is probably necessary. Bagged feed in a well insulated building will get wet when the door is opened on a warm day after a long cold spell if a warm front came through bringing high dew point air in. one way to condition the air is with heat, you can also condition the air by cooling it to condense the moisture then reheating it slightly (what we normally think of as an "air conditioner")

Dean I would really like to know if you feel the condensation problem has been fixed as in no condensation;
OR
Could it be possible that the metal is still has condensation but just not able to drip because of the barrier you have between you and the metal.

Some are recommending drip stop.
Read the tech sheet on drip stop.
It says it does not stop condensation; it only traps it in the pores of the drip stop so it will not drip on your head.

I feel putting insulation against a metal that does not eliminate the condensation is asking for trouble down the road.
But then again with wood preservatives today that common lumberyards sell the post will most likely rot out first anyway.

Best bet (done it MANY times) is use Tyvek (house wrap) under the metal - keep it tight. HTH

I used a product called "insulboard" under the tin. Then blew in foam on top of the white tin ceiling inside.

(quoted from post at 10:28:22 12/26/15) Dean I would really like to know if you feel the condensation problem has been fixed as in no condensation;
OR
Could it be possible that the metal is still has condensation but just not able to drip because of the barrier you have between you and the metal.

Some are recommending drip stop.
Read the tech sheet on drip stop.
It says it does not stop condensation; it only traps it in the pores of the drip stop so it will not drip on your head.

I feel putting insulation against a metal that does not eliminate the condensation is asking for trouble down the road.
But then again with wood preservatives today that common lumberyards sell the post will most likely rot out first anyway.

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I know my post was too long, but the issue of condensation between the metal and insulation is just not an issue unless it continues over and over. After one summer the air in there will be dry, even in Louisiana (wife's uncle lived in Houma), and there is no way to get the humidity back between the steel and insulation.

John in La,
John you have a very good point. A jail in the county south of me had to remove a 35 year metal roof. The metal looked good on news. There was rust on the under side of metal proving your point. The metal was put over plywood. They now have a mold issue in the insulation. Water caused damage to electrical too, $200,000 repair cost. So was it condensation or something else? Geo.
jail roof repair

Who cares.

Neither building has dripped a drop, though things stored inside do, indeed, sweat.

Older building was built in 2009.

Dean

Bingo.

Nor does condensation run down the underside fo the metal and drip off at the eves, etc. Neither on the bubble type of material nor on the felt type adhered to the underside of the metal.

FWIW: My old 100+ year old hay barn with cedar shingles and two layers of asphault shingles beneath the galvanized steel roof installed in 1976 never dripped a drop either (aside from the occassional leak). The barn blew down when the remnants of Hurricane Ike came through here in 2009(?). Though the roof metal had been painted on the top side twice since 1976, it was as good as it was the day it was installed in 1976 on the underside.

Dean

"5 If you have something you absolutely need to keep moisture off, air conditioning to keep the dew point below the material temperature is probably necessary."

Bingo!

This is why those with expensive classic cars, etc., keep such in temperature AND humidity controlled storage.

FWIW, My neighbor has a pole building about 1/4 mile from mine in which he keeps his collection of vintage Ford tractors. His building has a bare metal roof with nothing on the underside and it regularly drips like a rain forest when conditions dictate.

Dean

Sorry.

This post was intended to respond to that of George, TH, IN, below.

Dean

Here in Minnesota we use a product call peak proof it is a white plastic and paper product with fiber glass insulation glued on the top side with the fiber glass insulation having a r valve of 6 you can get a heavier r value but it is harder to work with. It comes in different widths of 3' and 4' the most common so the fiber glass insulation is 3 or 4 feet and the there is an extra 3 inch tab of the plastic/paper product that has double face tape on it.
This is how you use it: It goes on top of your purlins the same way your steel will lay on your purlins it goes on with the plastic paper side down and the exposed fiber glass insulation facing upward you start with a 4 foot wide piece with a 3 inch wide tab, you lay it out and pull it somewhat tight and put your piece of steel over it and screw it down then you remove the cover paper on the double face tape and add a 3 foot wide piece of peak proof so that the insulation is side by side and the 3 inch is on the other side for next time
It is called peak proof because the birds do not peak through the plastic/paper surface, This is used on all roofs and even side wall in almost all livestock sheds in Minnesota it cost about .55 per square foot for R-6 insulation I have used this a few times and it is very nice to work with and my brother also has in his free stall barn and parlor, here in MN we have temps as low as -30 degrees and it will keep the manure from freezing to the manure alleys also

(quoted from post at 11:41:43 12/26/15) John in La,
John you have a very good point. A jail in the county south of me had to remove a 35 year metal roof. The metal looked good on news. There was rust on the under side of metal proving your point. The metal was put over plywood. They now have a mold issue in the insulation. Water caused damage to electrical too, $200,000 repair cost. So was it condensation or something else? Geo.
jail roof repair

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News story says it was due to weather, wear and tear, and started to leak. Probably had some open joints. But contributing could have been a lack of vapor barrier and lack of attic ventilation allowing humidity from human occupation to saturate the insulation, plywood, and rust the roof. Humans give off a lot of water, and their activities, like cooking and bathing give off even more. Bottom of most of the metal sheets didn't look bad, just a few spots. If it was condensation under the metal (and how does the air get there to condense?) they all would have been rust.

The metal building insulation I use has a white vinyl face to it and is 3" thick and 5' wide. If all you are worried about is condensation you could use a lot of different things. You could use a vinyl tarp or house wrap or even foam sheathing.

If it is open there might not be a lot of condensation. I have a 12 X 20 carport that I enclosed and made it into a garage. There is condensation on the roof. I didn't really insulate it but I did put up some sheets of fanfold insulation to keep the moisture off of the car. That did take care of it.

My sons shop has hard foam sprayed on just after it was built. It encloses everything. Walls are covered also. And insulation under concrete floor also. No condensation ever. Hardly needs any heat in winter. Cool in summer also. But cost. 1.70 sq. ft.