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condensation on metal roof underside

mmoogie's picture

I've got a problem that I'm not sure how to deal with.

150 year-old-house with a 100-year-old or so standing seam roof that is in good condition. The client has been chasing roof "leaks" for the 13 years she has had the house.

I've been in the attic during heavy rains and observed no leaks. But there is staining all over the attic floor. I went up on a cold day recently and saw wet marks on the floor. Sure enough, every exposed part of the metal roof had frost that was in the process of melting off.

The roof is over wide boards that have spaces of as much as a couple of inches between them (the original roof was cedar shingles. This appears to be the second roof).

I can't do much right now to reduce the humidity levels in the house. The house is on a stone foundation, full cellar under front half, inaccesible crawl under back half with dirt floors. It has bluestone slabs for a floor surface in the cellar half. I'll be able to remedy the crawlspace next year. I hate to cover the bluestone floor in the main cellar, and would like not to have to.

So my question is how best to keep the condensation from occuring on the underside of the roof? The thermal envelope is currently at the attic floor. I'm thinking that short of spraying the underside of the roof deck with foam, there is no good way to isolate the moisture from the metal.

What say the moisture-issue people out there?


Sorry about the long ramble, but wanted to give full info right off.

(post #110618, reply #1 of 31)

howz about fitting ridgid insulation between the rafters? - -

"there's enough for everyone"

(post #110618, reply #2 of 31)

Steve -

How well is the attic vented? Could be it's a simple matter of increasing the air flow to allow the moisture to escape without condensing on the underside of the roofing??


Dennis in Bellevue WA

........... From Beautiful Skagit Co. Wa. Dennis

(post #110618, reply #4 of 31)

Dennis and Green,

It's not vented very well, and can't be without major surgery. It's post and beam and the rafters terminate on the top of the rafter plate, which is an 8x8 beam, so soffit vents are out of the question. Ridge vents on the standing seam are aesthetically out of the question. Gable vents are there, but the back half only has one gable, as the other end abuts the main house.

In the long run I've got to get the moisture levels down. The toilets sweat like crazy in the summer, and it's not a humid climate here.

There was FG stuffed in all the cracks between the sheathing boards before I cleaned out all the insulation to do some structural work. I could foam all the cracks with my gun, but the valleys are a problem. Too large an area to do with a foam gun.


Ridgid foam between the rafters might work for the bulk of it, but again, a main problem is the valleys, as they are hard to get at the undersides of because of the way the roofs overlap.

One thing I'm wondering is would spraying the whole roof deck with foam aggravate any problems? Would I have trouble with moisture between the foam and the roofing?


(post #110618, reply #5 of 31)

It's not vented very well, and can't be without major surgery.

I must be missing something.  It's 150 yrs old?  What you're describing sounds like it wouldn't make it to 50.  Anytime you exceed the dew point you get condensation, on the cold surface.  Ventilation is the normal fix.

Is this water problem only in the last, what did you say, 16 yrs?  Something get changed just before that?  New roof?  Doesn't seem reasonable that this situation could have been around since the War of Northern Aggression and the house still be standing.  I don't think adding inside plumbing did it.

Sweating toilets have as much to do with water temperature as humidity.  Get it cold enough and you'll get condensation. 

PAHS Designer/Builder- Bury it!

PAHS works.  Bury it.

(post #110618, reply #8 of 31)

I am betting that the old cedar roof did not have tarpaper and could breathe through it. When the steel was installed, the condensation problem showed up.

I would foam the bottm side of the roof plane, either by spraying it on or by cuttin 2" dow blue and foaming it in at the edges.

I wopuld also add ventilation, Maybe at gable ends.


Excellence is its own reward!



Oh Well,

We did the best we could...

(post #110618, reply #9 of 31)

I am betting that the old cedar roof did not have tarpaper and could breathe through it. When the steel was installed, the condensation problem showed up.

Sounds right to me.  When something commences, like condensation problems, it makes sense to look at what the change was, to seek a remedy.

I would foam the bottm side of the roof plane, either by spraying it on or by cuttin 2" dow blue and foaming it in at the edges.

Ok, but how does that prevent the condensation?  Just that the foam is a little warmer than the steel roof and it keeps the moist air from getting to the steel?  Certainly no expert roofer here, but the first rule I learned about metal roofs was: ventilate!  Substantially easier than trying to keep the interior moisture from entering the attic.

I wopuld also add ventilation, Maybe at gable ends.

Now we're talking something I can easily understand.  I'd try mechanical ventilation if that was the only venting I could figure out.


PAHS Designer/Builder- Bury it!

PAHS works.  Bury it.

(post #110618, reply #7 of 31)

Continous stud bays from sill to top plate, ie., balloon framing?

Stud bays closed off in attic?

Insulation in walls?

Perhaps those stud bays could be adding to the dilemma by acting as ducts to transfer basement and interior generated moisture to the attic?

Exhaust fan in bathroom and kitchen?

Gas WH or clother dryer not properly vented?

Just thinkin' out loud.

Knowledge is power, but only if applied in a timely fashion.

Knowledge is power, but only if applied in a timely fashion.

(post #110618, reply #3 of 31)

I'm with Dennis. Make sure there is adequate ventilation before trying anything else.

(post #110618, reply #6 of 31)

how about a H.R.V.(heat recovery ventilator).  this is used for taking humidity out of a building exchanging if with fresh air.  costly but effictive

(post #110618, reply #10 of 31)

what type of ridge cap is there? I would just remove it cut back the tops of the panels and just replace or reinstall the cap,install a gable end vent as low as possiable in both ends to make the attic space as cold as the outside.Another opition would be to install two gable end fan wired int o a humdiastat not air temp control

(post #110618, reply #11 of 31)


There's one solution that no one has mentioned yet:  Improve the air barrier at the ceiling plane.  If you can stop air from migrating from the conditioned space into the attic, you will have stopped the migration of the moisture.  Get a home performance contractor in there who is familiar with air sealing techniques.  From the attic, you will need to peel back the insulation to expose the gaps through which air is now leaking.  These gaps can usually be found at chimneys, wiring penetrations, plumbing vent penetrations,  the attic hatch, and partition top plates. 

Martin Holladay

(post #110618, reply #12 of 31)

I think HOLLADAYM is right on in saying that keeping the house air out of the attic is a good step.

Adding ventilation would be the next one. Sofit vents and ridge vents are the best possible combo, even if it's difficult.. Any other type might not work well.

Life is like a grinding wheel. It will either sharpen you up or grind you down, depending on what you're made of.

(post #110618, reply #13 of 31)

To all,

The house is 150 years old. The standing seam is about 100 years old.

I suspect it has been experiencing condensation since the house was first insulated in 1974. Before that, there was probably enough heat getting into the attics to keep things relatively dried out in the winter.

The front half is story and 1/2, timber-framed and the back half, which was probably added in the 1880's is balloon framed, one story. Insulation in the front half consisted only of FG laid on the flat part of the attic ceiling. The sloped side ceilings had no insulation. There is no air-passage from the wall bays to the attic on the eave walls. The gable walls are ballooned, and allowing air from the main house to attic. I will seal those.

The back balloon-framed half is similar. The thermal envelope on the back half was moved from the rafters to the attic floor about ten years ago by a pretty good restoration contractor, so now the back attic stays pretty cold. There are no sloped side ceilings in the back half. The eave walls appear to be pretty well sealed to the attic space, again, the gable wall bays have been capped as well.

There is a lot of water staining in the both attics, but the various plaster and sheetrock beneath the front (timber-framed) attic is pretty sound. The bead-board ceiling that I uncovered in the back (balloon-framed) part is heavily water damaged. The valleys in particular shed a lot of moisture when the condensation gets going.

I know the air-sealing drill backwards and forwards, and will do everything possible, but I still expect there to be too much moisture in the house for another year or more, until I do the foundation replacement on the back half and get access to those dirt crawlspaces.

More venting has been tried on the back half already and failed to mitigate the condensation. A ridge vent was chopped into it about six years ago, and and there is a good-sized gable vent window for outside air intake. It did no good, and the ridge vent was removed a couple of years later and covered over. None of this was my doing. I suppose I could get soffit vents into this section and replace the ridge vent, but I would have pretty serious egg on my face if I convinced to client to go to the expense of trying again what she already tried, and it didn't work again.

I'm not chopping into the 8x8 rafter plate on the timber-framed part to get soffit vents. Part of the story and 1/2 section is going to become cathedral ceiling. I plan to seal those bays off with either sprayed in foam, or ridgid foam, foam-gunned in on the perimeter. The sloped ceilings in the non-cathedral part of the story and 1/2 section I plan to dense-pack with cellulose, air seal as best as possible, and blow in loose cells on the flat section. She may want to make the other half of the story and 1/2 section into cathedral ceilings down the road, but not now.

The back half is more troublesome because of the way the valleys are cut in. I think the only way I could cut off the flow of moisture in that part is to spray the whole underside with foam. I wouldn't think twice about doing it if the interior humidity was going to be under control soon, but I'm just wondering if it could be detrimental under a high-humidity load that may not get fixed for a year or two (or ten).


(post #110618, reply #14 of 31)

I think doc has the right idea.  Fan on humidastat in attic.

If you want to keep moisture out of the attic, not only do you need to seal air leaks, but also apply a moisture barrier paint to ceiling.  Moisture can wick right thru plaster and wall board.

You get out of life what you put into it......minus taxes.


You get out of life what you put into it......minus taxes.


(post #110618, reply #15 of 31)

Bad problem, If you prevent the moisture from getting to the attic, and the source is the basement, won't that simply encourage more mold and mildew down there, increasing the rate of damage to the basement structure?  The house probably has lasted this long because it was loose, hard to heat and the moist heated air easily traveled from basement to attic to the outside in the winter.   


Where are we going, and what's with this handbasket?

(post #110618, reply #16 of 31)

Steve, check out Fixes for Damp, Moldy Houses in FHB 125, pp 74-79. I think Martin's got the right idea.

As to the bluestone, pull it up, add a layer of TuTuf, and re-lay the stone.

Andy Engel, The Former Accidental Moderator

Andy Engel

Never try to teach a pig to sing. It wastes your time and annoys the pig.

(post #110618, reply #17 of 31)

Consider stopping the moisture at its source.

Fix the crawl space now.

What are the other sources of moisture ?  Do you have bathroom vent fans dumping into the attic ?  Do you have bathroom fans at all ?  You should, and they should be properly vented.  Is your clothes dryer properly vented ?


carpenter in transition

carpenter in transition

(post #110618, reply #18 of 31)


I will do the crawlspaces next year. They will be done when I replace the nearly non-existant foundation under that part of the house, but I' can't start it now. Foundation season is over in my neck of the woods. Will be starting the cathedral ceiling section this weekend.

No bath fans, dryer is vented properly. This is a second house for a single Manhattenite. It's sits unused most of the time. And when she's there, she's a very low-impact user. I've gone in on cold days after weeks of no one being there and there is still the frost on the metal. The moisture is not coming from habitation, it's coming from the ground.

Andy, good thought on the bluestone.

I just hate this kind of work; heavy, dirty, and not very glamourous.


Edited 11/25/2003 6:57:30 PM ET by STEVENZERBY

(post #110618, reply #19 of 31)

I've gone in on cold days after weeks of no one being there and there is still the frost on the metal.

Does the furnace have a humidifier?  I would say the humidifier is putting moisture in the air and it migrates to the attic.

Maybe you can get a gauge that measures humidity and take a reading in the basement, main floor, attic and compare the values to see where the moisture is greatest.

You get out of life what you put into it......minus taxes.


You get out of life what you put into it......minus taxes.


(post #110618, reply #20 of 31)

Even if you just put a piece of 6 mil poly over the dirt, for now, would help.


carpenter in transition

carpenter in transition

(post #110618, reply #21 of 31)

Tim has a good idea.  Put poly over all basement floor temporarily and see if it stops the moisture in the attic.  If it doesn't, keep looking for problem.

You get out of life what you put into it......minus taxes.


You get out of life what you put into it......minus taxes.


(post #110618, reply #24 of 31)


Can't get to the crawlspace at all. It's about six inches above the dirt at the near side, framing is on the dirt halfway back, after that I cant even see what's there. It's gotta wait till the foundation is dug out.


(post #110618, reply #27 of 31)


Aside from all the good advice above (deal with the crawlspace, seal the attic floor, etc. all the stuff you already know), there isn't much to add. But one thought I have to deal with the symptom until the problem is solved...

Put a couple of fans in the attic blowing air onto the roof...moving air might dry things up enough.

I remember a customer with a bath that had no ventilation (this was very early in my career as a handyman) and a big problem with water condensing on the walls and ceiling around the shower. A friend of theirs suggested a ceiling fan. That's why I was install the fan.

Worked like a charm. The walls and ceiling stay dry no matter the length of the shower.

Rich Beckman

Another day, another tool.

(post #110618, reply #22 of 31)

This is completely off the wall and I have no idea if this will work or not.

What would happen if you closed off the gable end vents and install a fan in one of them with the fan PRESSURIZING the attic space.

Would that keep moisture from below being forced into the attic by ballon affect.

. William the Geezer, the sequel to Billy the Kid - Shoe

(post #110618, reply #23 of 31)

my $.02: spray icynene or corbond on underside of the board sheathing. has some interesting reading on spraying their foam on the inside of metal buildings.. opened my eyes quite wide.

Sorry if I skimmed the first 24 posts too hastily - but what season is the problem evident?  Heating or cooling?

Warm attic, cold roof plane = sweat. 


(post #110618, reply #25 of 31)

Thanks. I'll check ot that web site.

Heating season.


(post #110618, reply #26 of 31)

First time posting. Read about your problem and it`s just everyday humidity. But does sound like the crawl space needs help. The fix to the problem is not fans or more venting. Here in Canada, we put 2x4 strapping on the rafters on 16" centers, then 15lb perforated felt and then the metal roofing. At this point you are getting! The moist warm air from both inside the house and from outside vent, will travel through the felt. Then it condenses on the underside of the roofing, next, the sun comes out and it`s drip time, but now as "water", runs down the felt and not into the house.It`s in the building code here in B.C. The felt runs parallel with the metal from the ridge to just past the bottom strapping. I`ve done a few fix it up jobs, barns to houses, the best way is to take the roofing off and put the felt on. A few tips, pre-cut the felt, if not, take the roll to the ridge and pull. It`s goes felt-metal-felt-..... . The over lap on the felt is min 3", if the metal is only 2' wide you may want to over lap a little more,so that you are not walking on the felt.The wide boards can stay and you don`t really need the 2x4`s, that`s for when starting on a new roof. Hope this helps.


(post #110618, reply #28 of 31)

Ingenious solution. If it works up there, should work down here. ;-)

Knowledge is power, but only if applied in a timely fashion.

Knowledge is power, but only if applied in a timely fashion.

Condensation on metal roof (post #110618, reply #29 of 31)

I am in the process of building a carriage house, today is a sunny morning after a cool night I found it raining in my new suite.  A metal roof, with Typar synthetic underlayment over 2x4 purlins. There is no source of heat or insulation at this point, everything is still wide open.  There is a significant amount of water falling, any ideas.  

I understand I could spray foam the underside but this would add significant cost.  I installed a metal roof thinking it would be the best for the long term but I'm starting to have my doubts. Help.

Where's the moisture coming (post #110618, reply #30 of 31)

Where's the moisture coming from?  What's the dewpoint compared to last night's temperature?

You get a clear night and a roof will give up heat rapidly and actually become cooler than the surrounding air.  This is more pronounced on a dark roof.  If the temperature of the roof drops below the dewpoint of the air, condensation (dew) will form.  Simple physics.  This phenomenon can be exacerbated by some source of moisture within the structure -- exposed soil in a crawl, wet lumber, wet drywall, etc.

With something like a asphalt shingle roof, you have the shingles and roof sheathing to act as insulators, so the bottom of the roof doesn't get quite as cold.  With a metal roof the bottom will be nearly as cold as the top.

Once the structure is enclosed and heated, the roof will tend to be a degree or two or three warmer (plus sources of added moisture will be reduced), and condensation of this sort will be much less likely.

Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville