Condensation with Metal Roofs

[Gatorboy]

Help! I am still in the final planning stages of my Pole Barn, and have been reading on metal roofs, and the condensation thing has really gotten me worried. I had planned to put a metal roof right on the purlins -- but to avoid the inside "rain", I think I should put up plywood first. Does anyone know if trusses 8' O.C. with 2x4 purlins on edge 24" O.C. have enough strength to handle plywood being installed on it? If so, I have started to think, gee, if I've got the plywood up, why not just put up shingles.

Cowboydoc over on TBN had talked about insulating a metal roof. Does that mean putting some corning fiberglass between the purlins? I am confused -- pictures on how this is done would be very helpful, if anyone has the answers for me.

My Pole Barn plan is getting so many daily revisions, I don't know when I'll be finalized so I can start setting my posts. [img]/forums/images/icons/crazy.gif[/img]

[AndyF]

Your materials supplier should be able to get you the ceiling insulation you'll need for the pole barn. The insulation is either 24" or 48" wide depending upon your truss spacing and has a vinyl face which faces inside. If you have an 84 Lumber store near you, take a look at their ceiling as this is what they usually use.

Your truss spacing is really too large to deck with plywood. If you really want a plywood deck build the roof like a house roof with the trusses on 24" centers and no purlins. Check with your neighbors to determine if condensation is really an issue in your area.

[SteveB]

Generally with a metal roof you would use a type of faced insulation that goes over the purlins and under the metal sheets. I used 4X8 1/2" R-4 rigid sheets sheets (TUFF-R) for my roof insulation. Performance is excellent but screwing down the roof was interesting because the sheets had a tendency to compress. I would not recommend this method for that reason. I brought up this issue on TBN (there is a hole thread on it there) and CowboyDoc said there is special insulation for this purpose. It is plastic faces and comes in a roll. You just roll it out under the sheet you're puting down, staple it tight (I'm guessing), and slap your metal panel down on top of it. He said it compresses down to nothing between the purlin and metal sheet. This avoids the problem that I had.

Steve and Andy are right. It's too far apart for plywood. You need 24" on centers for plywood. The roll out insulation is very easy to do. It's also very easy to put your metal tin right on top. If you can't find it I will get the exact name of it for you.

[Gatorboy]

Does this insulation get rolled out on top of the purlins?

Here is a picture on TBN of the felt paper in your barn.



I don't see any insulation up there. If you don't have it, then where would this insulation be placed -- between the metal roofing and the purlins?

[fenneran]

Last year I finished the second floor of our Cape-Cod style house. It was built around 1925 and has a metal roof. The metal was right on the purlins. We found some thin foam panels at Lowe's made for this purpose. They are about 2 1/2 - 3 feet across and in the general shape of a flat "W"
Kind of like:

__

We stapled these under the purlins, then stapled the regular bat insulation under that. This way, if there is any condensation, it is channeled away from the insulation and down to the bottom of the roof-line. Although I have never seen so much as a drop come down, it may have during the middle of the winter.

Don't know if this applciation would help in the barn project, but thought I would share.

Frank

[egon]

Dave:
The 2x4 purlins are probably too small to span 8 ft. It may be advisable to have the roof design checked and ensure it confirms to your areas codes.
Longer purlin spans require larger purlins which in turn requires a truss that will support more load. The sizing of one is dependent on the other.
The truss suppliers have all the information.
Egon

[Phred]

Gatorboy,

As mentioned above you need the roll insulation. I think it has an R value of about 13. In the attached picture you can see the white vapour barrier on the walls and celling that is the inside lining of his type of insulation. This is standard for metal buildings which all use the metal roofs. These buildings also use it for the walls. The bag in the right hand part of the photo contains a bunch of scrap pieces. Its basically a few inches of fiber glass with a platic vapour barrier.

Fred

[Phred]

Here is a shot of the raw material, pilled up out front, prior to the roof going up.

Fred

Gatorboy,
I've got several barns and that one doesn't have it. That's one way I know you want insulation because that one drips all over everything, esp. with animals in it. I will get you some pics tomorrow of what it looks like in the the other barns.