Author Topic: Metal roof sweating?  (Read 13098 times)

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Offline Bill Houghton

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Metal roof sweating?
« on: December 14, 2008, 01:00:08 PM »
A lot of the homes I see here are using metal roofing.  How do you keep them from sweating?  Is that just a cold climate problem?  I see quite a few where the metal is attached right over the 2 x 4's running at 90's across the rafters with nothing in between the metal and the framing.  There is no way that would fly here.  It is almost like rain inside the building if there is no moisture barrier between the metal and the framing.

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Offline brian_nj

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Re: Metal roof sweating?
« Reply #1 on: December 14, 2008, 01:17:03 PM »
it is all a matter of ventilation to keep the sweating to a minimum. I have seen houses with traditional shingled roofs have the nails that penetrate the sheathing sweat bad enough to make you think it was raining in the attic. The attic of a home should be close to the outdoor temperature this will reduce the condensateion that forms due to the temperature difference between the two sides of the roof surface.
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Offline glenn kangiser

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Re: Metal roof sweating?
« Reply #2 on: December 14, 2008, 02:40:14 PM »
The inspectors have been requiring felt over the purlins directly under the sheathing or sprayed in foam as Christina did in Georgia. 

They will seriously sweat nearly anywhere there is warm moist air under and cool outside them.
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Offline cordwood

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Re: Metal roof sweating?
« Reply #3 on: December 14, 2008, 04:06:48 PM »
 It's all in the venting. A fiberglass/asphalt shingle roof wont last as long if it don't have good ventilation. With eave vents and ridge vents the air circulation dries any moisture that would normally accumulate in an unvented area.
 It's not just with metal roofs, It's just with a slick metal roof you can see the moisture. Roll roofing will sweat just as bad but you don't see it because of the sheathing under it. And after the plywood rots out someone will say the roof must have leaked,.......NOT ALWAYS! Poor venting causes it too not to mention the mold that will grow in there here in the south. d*
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Offline n74tg

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Re: Metal roof sweating?
« Reply #4 on: December 14, 2008, 08:09:13 PM »
Vented or not vented, it won't matter.  Moisture barrier or no moisture barrier, it won't matter.  If the metal surface temperature (or the moisture barrier temp - if one is present) goes down below the dew point temp for the relative humidity of the air inside the attic... condensation WILL occur.   And as others above have said it also happens on sheathed roofs with asphalt shingles. 

If the air outside is moist and you bring that air inside the attic (via vents) and then the temp goes down (like it does most every night); if it goes down below the dew point temp, condensation will occur.  How much condensation; it depends on how far below the dew point the air temp went.  Will that condensation drip down on anything else; depends on how saturated the condensing surface is with moisture.  Fully saturated; it will drip (or at least run down the sloped surfaces of the rafters or trusses or sheathing or metal roof material) once the water drops get large enough to overcome surface tension.

To prevent condensation you have to keep the temperature of the condensing surface above the dew point temp.  Just swapping the inside (the attic) air for outside air via passive or forced ventilation won't accomplish anything unless the outside air is significantly drier (and it seldom is) or significantly warmer.  The simplest way to prevent condensation is with insulation.  Expanding foam (like Christina used) works well because it is pretty impermeable to moisture.  Next most effective would be Dow blue or Owens Corning pink foam sheet insulation, but you have to seal the joints well.  They too are pretty impermeable to moisture.  Unfaced fiberglass batts aren't impermeable to moisture, so if installed directly under the metal roofing, the moist air will still come in contact with the bottom of the metal roof; and when the temp goes down below the dew point; voila, you got your condensation.  Paper faced batts stapled to the rafters or trusses won't insulate the actual rafter or truss surface which becomes the new condensing surface. 

All this stuff is part of the field of psychrometrics and is right in the middle of the larger field called "building science".  There are a lot of websites out there that discuss it at length.  BuildingScience.com is just one of them, and yes, they have lots written about this subject. 

Now, that being said, I'm still putting a metal roof on my house; and spraying the underside of it with expanding insulation, just like Christina did.  Through a friend I also know a guy in Dallas (pretty humid there) who built a home with a metal roof insulated this way in an unvented attic.  Yes, I said unvented.  In the summer his attic temps are never more than 10 degrees warmer than the rest of the house, nor more than 10 degrees colder in the winter.  His house is going on three years old and there is no mold in the attic.  I just hope my roof performs this well. 

« Last Edit: December 14, 2008, 10:42:31 PM by n74tg »
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Offline desimulacra

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Re: Metal roof sweating?
« Reply #5 on: December 15, 2008, 05:14:10 AM »
I used Reflectix Foil (or equiv.) in my shop. I can't say enough good about it. A friend used fiberglas insulation and the difference is dramatic. I can walk into my shop under a blazing July sun and it will be no hotter than outside. His shop will have to be opened and aired out before we can work. I have not had any problem with the metal roof sweating, .. none. I also used fiberglas on a commercial building in the 80's,  hated it. It would sweat next to the roof and sag down. then tear. (Humid Tennessee)
 The foil is super easy to use and install, fairly inexpensive esp. when you factor time.
 The story of raining reminds me of a house in our town. The people built it and it was a beautiful large plank style ! However they did nothing under the tin roof and it would sweat like it was raining under the right conditions. They had to spray it with foam but for some reason it still sweated some? They finally sold it, dunno how the new owners are making out. ???

Mark
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Offline JRR

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Re: Metal roof sweating?
« Reply #6 on: December 15, 2008, 06:25:24 AM »
We have a steel roof on the porch of our weekender.  I have never observed sweating underneath the bare metal roofing ... but that's mostly due to my not taking time to inspect ... but haven't yet noticed it "raining" down on me.  On the main roof there is an air/drain space under the metal and over a roll roofing membrane.  If the metal gets wet on the bottom, it should be no worse than being wet on top ... since it can drain and dry off.

Offline Arky217

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Re: Metal roof sweating?
« Reply #7 on: December 15, 2008, 05:22:27 PM »
I'm planning on using plywood sheathing over the trusses, then 30 # felt over the plywood, then metal over the felt.
Ventilation to be conventional soffit to full length ridge vents. Insulation to be cellulose above the ceiling drywall.
Is this arrangement likely to suffer from condensation getting on the truss wood ? (Very humid here in Arkansas)
If so, should I plan on putting something like 2"x2" nailers between the felt and metal ?
Also, if nailing strips are used, since they would have to be perpendicular to the run of the metal, it seems like they would have to be perforated with holes to allow ventilation air movement under the metal.
And would condensation still occur ? I'm thinking it probably would based on the metal carport that I built (it 'rains' under the carport roof under the right conditions).
What about 1/2" foam insulation between the metal and the felt ?
I just don't know; what IS the best way to do a roof when you have the insulation in the ceiling ?

Offline Ernest T. Bass

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Re: Metal roof sweating?
« Reply #8 on: December 15, 2008, 05:40:50 PM »
We have a steel roof on the porch of our weekender.  I have never observed sweating underneath the bare metal roofing ...
A roof like that, open to the air top and bottom is not at all likely to run into that problem. The problems arise when there is an enclosed space directly under the metal, like an attic.

Actually, I have noticed condensation under our exposed metal porch roof on rare occasion, usually on a cold morning when the sun hits it. Never any dripping though, just some moisture. The wood strapping doesn't show any sign of water damage, so I think that it can handle the occasional moisture as long as it's well ventilated.

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Offline n74tg

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Re: Metal roof sweating?
« Reply #9 on: December 15, 2008, 06:21:16 PM »
Arky, the way you describe what you want to do seems to me to be a guaranteed source of condensation.  Bringing outside air into your attic there will be virtually no way to keep the underside of the sheathing above dew point temperature.  The cellulose above the ceiling sheetrock will further insure the attic gets as cold as possible, especially at night.

I'm only about 100-150 miles to your southwest (Hot Springs), so we're in the same humid climate region. 

If you do go with the plan you described above, I'm going to be real interested in hearing how it performed. 
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Offline Arky217

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Re: Metal roof sweating?
« Reply #10 on: December 15, 2008, 07:52:56 PM »
n74tg,
You may be right as 'rain' (no pun intended).
So what WOULD be a good way to do the roof when you want to use cellulose above the ceiling and metal for the roofing ?
( I've heard that people sometimes just put nailing purlins directly over an existing asphalt roof and then the metal on the purlins; seems like that would be about the same deal)
Appreciate all advice,
Arky

Offline glenn kangiser

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Re: Metal roof sweating?
« Reply #11 on: December 15, 2008, 08:00:42 PM »
Arky, just an idea but, felt over the plywood, then 1x4 spacers about 6" to a foot long over each truss then a  skip with a solid 1x4 purlin continuous over that to fasten the sheets to.  That would allow air movement through the 3/4 inch space between the blocks above the purlins- just an idea as I haven't worked in the high humidity you deal with.
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Offline JRR

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Re: Metal roof sweating?
« Reply #12 on: December 16, 2008, 05:02:52 AM »
I think a system such as Glenn describes is the "gold standard" for metal roof installation over sheathing:
 
.Felt or roll roofing over the sheathing.
.Spacer strips centered on (each) rafter or truss.  (I like 5/4" decking strips here.)
.Purlins crossed atop the above strips.  (I like 2x4's spaced approx 16", or less, for this.  1x4's would work also, but 2x4's make a very simple stiff overhang if the roof overhang is carried 2 feet or so beyond the building.)
.Metal attached to the purlins, using the special screws made for this purpose.

However, I have read that some folks have had lasting roofs, when the metal goes down directly on top of the felt.  I believe that the "gold standard" above, however, has to be the quieter of the two installations ... and the one that would reject the most sun heat gain, a concern in my neck of the woods.

Offline Arky217

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Re: Metal roof sweating?
« Reply #13 on: December 16, 2008, 08:43:09 AM »
I like Glenn's idea with the spacers, especially if the spacers and purlins were treated wood, but oh boy, that would sure take a lot of extra work and materials.
I'm wondering if there could be a less troublesome way.
What if roll roofing were used over a light (15#) layer of felt, and then the metal on top of the roll roofing.
Would that be any better, or would I still likely get condensation. And if so, where would it likely occur, under the metal.
If it occurred there, would that cause a problem or would it just drain off of the roll roofing ?
Or could condensation also occur under the felt/roll roofing so as to contact the truss wood ?

Thanks,
Arky

Offline NM_Shooter

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Re: Metal roof sweating?
« Reply #14 on: December 16, 2008, 08:43:58 AM »
If the metal roofing is applied over spacing strips to keep it off of the deck, does that make it more susceptible to damage from walking on it?  Seems like that would get it bent up in a hurry.

I have only had decked (OSB) roofs. 

I've always used the pre-shaped foam sealing strips under the metal edges to seal them up to keep critters out.  Seems like that foam does a pretty good job of keeping airflow out from underneath the metal.  I would bet that sealing the edges like that, and applying the metal right on top of the paper over the deck removes the threat of sweating (for all practical purposes >= 30 years life).

In looking at the original question, it seems that the question had to do with applying metal roofing directly over purlins (no decking).  In a humid environment, I think that is going to sweat, no way around it. 

I'd use decking.  As cheap as OSB is now, why not?
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Offline glenn kangiser

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Re: Metal roof sweating?
« Reply #15 on: December 16, 2008, 08:30:44 PM »
Most metal roofing is the same as used on steel buildings and we used to walk in the low rib flat area (high rib will buckle) all of the time.  This was steel roofing to steel purlins and 4 foot purlin spacing without damaging the sheeting.  Anything at that or less on spacing should be fine without damage to the sheets.

Personally I think the steel over 30 lb felt directly is fine but your experience may be different.  The felt will wick the moisture to the rib area to dry out.  At the most it only has about 5 inches to go to the rib area from center.
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Offline JRR

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Re: Metal roof sweating?
« Reply #16 on: December 17, 2008, 08:59:32 AM »
Years ago 24 ga was the norm for corrugated roofing metal, it was pretty tough .... now its 26 ga or even 29 ga.  Of course, years ago I was much lighter and maybe that's the reason I don't remember bending the metal when I walked or kneeled on it.  But if I walk on a metal roof today, I use rubber mats or plywood walk-boards to avoid leaving dimples behind.  I don't think having sheeting, versus purlins, underneath makes much difference ... but it might with a standing edge style metal.