Metal Roof Condensation ?

[jimbob44]

I am planning to build using a 4/12 pitched trussed roof.
On the trusses I was planning to use 2x4 purlins and then the metal panels over the purlins.
The ceiling would be drywall with R-40 cellulose insulation.
Roof ventilation would be perforated soffit inlet and ridge vent outlet.
The question is; with this arrangement, could I still get condensation on the underside of the metal roof panels ?
If that's a possibility, I would probably go with felt covered osb over the trusses and then the metal panels over that unless there is a better solution.
The climate is mid Arkansas.
(On a polycarbonate panel patio cover that I built, I have got condensation on the underside of the panels a time or two when the conditions were just right.)

[n74tg]

Where in mid Arkansas?  I'm in Hot Springs.

[glenn-k]

Metal panels are pretty famous for raining inside under the right conditions.  Tule fog here in the San Joaquin valley will cause inside rain quite often each winter.

[benevolance]

Wouldn't you just put down some felt/tar paper under the metal roof? wouldn't that create a barrier to keep the wood safe and dry?

Allow in the design a place for the water to run down and drain at the eaves...

Couldn't something simple like tyvek or felt paper cure this problem?

[Okie_Bob]

My experience in central east Texas, on a lake seems to work well and I doubt you have any more humidity than we do here.
I used 1X4 fir strips on top of my trusses then screwed the metal panels to the fir strips. I used the Hardiboard soffet panels and a vent strip at the top of the roof which is also 4:12 pitch. I then had Icynene sprayed under the panels and completely covered the underside leaving an opening at the bottom for air flow up thru the soffetts and into the attic. That was three years ago and have had NO problems with condensate.
Okie Bob

[glenn-k]

Wouldn't you just put down some felt/tar paper under the metal roof? wouldn't that create a barrier to keep the wood safe and dry?

Allow in the design a place for the water to run down and drain at the eaves...

Couldn't something simple like tyvek or felt paper cure this problem?

Tyvek is near impossible to work on on any kind of slope - the chemicals in it are very slippery until dirt or some other surfactant deactivates them then the remaining cloth is not very effective.  Also it works with water vapor but not water well and will run it through any penetration into the house.

Felt would be better - seals holes pretty well when penetrated by nails - deforms when wet making drainage channels.

If there is not enough damp air under it, moisture won't form on the panels.  If it doesn't reach the dew point, condensation won't form.  Metal buildings here commonly sandwich 2" white vinyl covered fiberglass insulation between the panels and the purlins.  The pinch point is not well insulated but overall it seems to stop condensation similar to Okie Bob's Icynene.

Vegetable storage under an uninsulated metal roof in farming operations is inviting disaster - ask me about it.

A sweet potato farmer ordered an uninsulated building for storage of his crop.  He tried to sue me because it sweated the moisture back down on his stored crop.  I think we added vents or something but he is the one who ordered without insulation so he didn't get a judgement against us.  All in all a bad situation.  He also had a long history of suing anybody that worked for him and his nephew was his lawyer.  A lawyer that couldn't type or spell.  His letters were like something out of "Psycho".

[Okie_Bob]

Glenn, it's a good thing you live so far away! If you were closer, I'd buy the beer and we'd have us a good old time sitting around a campfire and me listening to you tell your tales. And I especially like the ones about physco lawyers! Ones that can't spell are the best. Now if we could just get Jonsay up from down under, I could write a book about you guys stories and make enough money to afford the beer and 'guana'? (Spelling?)
Okie Bob

[benevolance]

dont know what guana is...Hope it is not bat droppings

But I am game for the beer...Campfire and laughing until our sides ache

[Okie_Bob]

Bene, it's something that they eat down under. Jonsay claims it tastes just like chicken but, I don't know that I trust ole Johsay that much. At least not likd we all do Glenn!
Okie Bob

[glenn-k]

Well, whenever you are this way I expect you to stop in and I'll try to think up some good ones.  Did I ever tell you about the customer who loved our work in the morning but then proceeded to get drunk every morning the give us hell in the afternoon...

or the one who decided not to pay any progress payments until the building was completed....

I'll get the beer.

Throw a Goanna on the Barbie, Jonesy.  We're having company.



Venomous: Dr Bryan Fry with an Australian goanna.

[ Photo: Alexia Fry ]

http://uninews.unimelb.edu.au/articleid_3009.html

Note- Jonesy just got moved in and now has his high speed net.  As soon as he gets caught up a bit after the big move of the marmalade, I expect him to drop by more often.

[jimbob44]

Let me clarify my intended method.
The trusses would be conventional 'W' trusses  and the purlins would be 2x4's laid flat across the trusses. The metal panels to be screwed to the purlins. (There would be no support for a layer of felt over the purlins unless I were to first put down a solid roof on the trusses which I would rather not do because of the expense). A drywall ceiling would be attached to the bottom of the trusses with about R40 worth of cellulose insulation on top of the drywall. There would be a continual attic vent flow from the soffit to the ridge.
The question is, even with venting throughout the entire length of the attic, would there still be a chance of condensation on the underside of the metal panels ?

[JRR]

I doubt if you will have any condensation problems with your insulation and ventilation plan.  If you did have the occasional dripping, I expect the water would later harmlessly evaporate.

But taking extra precautions makes sense.  A layer of felt would probably guarantee dryness.

You could glue felt to the bottomside of the metal panels before installation.  If the felt is "tiled" carefully, you would have a good drainage plane as well as a bit of insulation.

[glenn-k]

I don't think there would be a condensation problem in this situation either- warm moist air should stay below the sheetrock - vapor barrier on the heated side of the insulation.  I sandwiched 30 lb felt on my RV storage garage running it vertically from eave to ridge.  I also had purlins I think on 18" centers.  It was a bit of a pain doing it but I managed.  My recycled corrugated was full of nail holes so I used the felt for added leak prevention.  The 30 lb is heavy enough -to span it under the sheets.  My concern was rain leakage.  I don't think vapor moisture would be a problem - look at all of the old barns etc. built that way -but not as big a concern there with moisture.  

[Amanda_931]

I occasionally get a bit of condensation coming off the underside of the barn roof.  Doesn't seem to hurt much, but then the barn is pretty open.

Probably a bunch of it ends up just rolling down to the outside--even if it's not corrugated and there's not all that much that doesn't stay in contact with the purlins.

[Okie_Bob]

Jimbob, why are you using 2x4's for 'purlins'? Usually the trusses are on 24" centers and 1x4's are plenty good enough. You do need to do some internal bracing of the trusses with 2x4's running diagonally the length of the run. (do you know what I'm talking about?) A good framing book will show you details on this and it is necessary in my opinion.
Okie Bob

[JRR]

I used 2x4's as purlins under metal roofing without sheathing.  They are stiffer than 1x4's.  The additional cost was minimal, and the 2x4's allowed me to extend the roof overhang to nearly 2' without additional bracing.

I also ran diagonal bracing.  Makes me feel more secure when the wind blows big,  but it probably wasn't really necessary.  

[Amanda_931]

We went with 2x4 purlins at my barn as well.  More depth to screw the screws into.

[glenn-k]

I also used 2x4 on the RV garage.  I used some of my crappier lumber - obviously I must have forgot to call a grader. :-/

[Okie_Bob]

Over the weekend we had what one weatherman said he had never heard or or seen. We had stright winds up to 60mph and the dust was so thick you could not see beyond a couple of hundred feet across the lake.
Point is, my roof with stood the blow without problem. I've seen many roofs with only 1 X 4 fir strips and don't know of any failures as a result. Agreed, the cost difference isn't that great and I'm usually the first to build stronger than needed. Just don't see the benefit on this one.

Oh, and I was working on the inside of the new house while the wind was blowing and had left a window open in the garage apartment...sorry to say I had a huge coat of dirt on everthing! They say the dirt came from West Tx and that is a long way.
Okie Bob

[Amanda_931]

another Dust Bowl?  I hope not.  

The new septic tank approver told me that a lot of the silt I have for soil here is from the 30's Dust Bowl.  I'm not absolutely sure I believe him, but it makes a good story.

[glenn-k]

Actually I think it came from a bit South of you , Okie BoB.  As I see it, nearly all of the dirt generated in the the US right now is coming out of Crawford, Texas.