Controlling Moisture With OSB Sheathing and Steel Roof / Siding
Dear Martin, Thank you GBA for this great resource. I built a 700 sq ft house. Zone 5. Steel siding and steel roof. The roof has a 3/4 inch air gap from strapping over the tar paper and osb. The siding is attached directly to the tar paper and OSB. We’ve had some condensation issues, I think, especially on the north wall around outlets. I believe the mistake we made was not including exterior insulation or better air sealing from the interior. Roof has no Kraft paper, side wall has Kraft paper vapor retarder. After reading Joe’s study with fiberglass insulation in a cathedral roof , I realized that I didn’t have 40% percent exterior insulation after the sheathing. Oddly after pulling an osb board from the peak, there wasn’t any sign of moisture damage, but I have replaced several sheets of the north wall siding because of moisture damage. I’m trying to fix this situation and all that I can think of is adding rigid insulation to the exterior siding and roof. I am worried that the foam board on the siding will stop drying potential to the outside while having a Kraft face vapor retarder on the insulation. The roof doesn’t have Kraft paper and it could dry better interiorly with rigid foam on top. Is exterior ridgid foam the right approach in this case? Thank you, John
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Replies
John,
Martin is retired and only posts occasionally. Did you include a ventilation gap in the cathedral ceilings? Do you have soffit and ridge vents?
If you have gaps that allow air movement between the interior and the exterior, those should be caulked or foamed.
Hi Steve,
There isn’t any soffit or ridge vent and we did take care to spray foam cracks and gaps before the insulation was installed. The ventilation that it does have is from 3/4” furring strips under the ribbed metal roof. We are more like zone 5, north central Nebraska. The roof picked up some moisture over winter but appeared to dry out fast enough that it didn’t cause damage, we had a -26 degrees low which isn’t very common. The house was finished last August so it’s only had one winter. There aren’t any furring strips under the siding metal panels. There are outlets indoors just below the wall top plate and that is where I see signs of moisture problems when I replaced some osb panels from the outside.
Consider measuring for a problem so you don't fix things that are working OK.
Also consider exterior Rockwool sheets, which don't block vapor and are beneficial at any thickness.
Always air seal well, preferably with blower testing.