OSB needed under metal roof on pole barn?

Hi! I am from southern Ohio and have been looking around at pole barn kits from both dealers and lumber companies and there seems to be a varying opinion of what is best as far as the roof is concerned...Some of the places that I talk to want to use purlins 2' OC, 7/16" OSB and roof felt underneath the metal roof of a pole barn while other places just have roof purlins 2' OC and insulation (the insulation with the silver backing) underneath the metal roof. Is one approach any better than the other? Since this will be a horse barn, I would like a decent moisture barrier on the roof to help to diminish "sweating" during the summer. The OSB seems to add a little over a $1,000 to the cost of a barn so I want to make sure that it's worth it before going that route. Thanks for any advice, recommendations, etc.

Our pole barn has purlins with the roofing directly attached, no insulation or osb.

How would the roofing sweat during the summer? I understand heat and moisture coming off the horses, but the roof will be hotter with the sun hitting it, making it unlikely to condense moisture, right? I could see it happening during the winter more with the cold roof and warm moist horses underneath it, I guess.

Also, you really need to keep the building ventilated regardless of season as horses kept in a tightly enclosed barn are more likely to have breathing issues/illnesses.

Ok this will be showing my bias, but keep in mind that horses left out on pasture year round stay healthier than horses kept in a barn. We have a couple stalls in our barn but never keep our horses in them. I would if they needed doctoring or had an injury or condition that required limiting their activity of course. Not to mention much less work with no stalls to clean, no buckets to fetch water to, etc. [img]/forums/images/icons/smile.gif[/img]

- Tim

The sweating is more likely to occur during the winter as the warmer air on the inside of the barn condenses on the metal that has been cooled by the outside air. If you have large open areas (3/4 of one side of one of my barns) is open so the air can circulate there should be no problem. However if it is closed up, it will condense.

We used a type of roll foam insulation with metal backing between the purlins and the metal, and it seems to work just fine on another barn that is closed up pretty tight. Sorry I can't tell you the name of it right off but as soon as the senior moment subsides I will let you know. [img]/forums/images/icons/crazy.gif[/img] .........Low-E Insulation...........Duuuuh!

You probably experience similar temperature ranges as we do in Pennsylvania..... I can tell you that we do not have anything under our metal roofing. Our barn is a 2 story with a 1 story addition and neither shows any signs of sweating. We do have fairly good ventilation since we leave all doors(cross ventilation is good) open except in the most extreme weather and we live in an area that gets a fair amount of wind..I'm not sure how many animals will be in your enclosure, we have 4 horses mainly in a 20x30 area and the addition is normally only 1 horse which is apprx 20x15.

I never had anything other than the purlins under the sheet metal. I'm not sure how well the felt will last there but it would help get the water off the OSB. I have seen roofing applied over sheet metal (5V crimped tin) attached to purlins which were in turn attached to the roof system i.e., nailed thru the tin roofing material and into the roof decking and that worked just fine. And I have seen the reverse of that Tin over roll roofing! The secret is having air moving in the voids between the two roofing systems!
Leo

Our barn (built last year) has a metal roof with the silver backed insulation between the metal roof and the purlins.

We've had no problems at all with condensation with the roof built like this. We decided to add the insulation after reading about a few condensation problems on the web.

We did add a full length roof vent, and vented soffit on all overhangs for additional ventilation.

I would not use osb, plywood or any other sheathing on a metal roof. That would just drive up the cost of the roof significantly and serve no useful purpose.
IF you use sheathing it will require that the roof trusses be spaced 24'' OC as opposed to 48'' centers for a metal roof.
By the time you pay for twice as many trusses as you need and buy the sheathing that you don't need it's a considerable unnecessary cost serving no useful purpose.

There are several options for handling the condensation situation.
All of them in addition to proper standard ventilation techniques
1*Cover the rafters with plastic or house wrap before nailing on the 2x4 purloins.OR you could use celotex type foam sheets instead.
2*Naill celotex type foam sheets on the under side of the purloins after the metal roof is installed or staple plastic or house wrap there.
3* Have the under side of the metal roof covered with spray foam type insulation. This would be applied on the metal between the purloins the Max thickness would be 1.5 inches which is the thickness of the purloins. There is no need to cover the purloins