Moisture barrier for open porch roof

Alicia Heyman
7/24/2007
This is a very minor problem compared to most I've read but here goes. I have a 60 year old country cottage with an 8'x12' back porch (floor is wood decking) in the back. The roof of the porch is what I believe is standing seam metal(metal with ridges). It is on horizontal battens over 2x4 joists. Sturdy but does not need more weight. In cool weather I get some condensation as would be expected but otherwise I have no problems with the roof. I've had it with mosquitos and would like to screen in this porch and put some chairs/table out there. It will still be very open air. I'm a DIYer and have been renovating for 5 years...doing most of it myself. Before I talk to a contractors (I do the inside...I pay to have the outside done) I need to understand what my options are and aren't. I would like to know what kind of moisture barrier to use. My biggest problem is that this roof is very low (barely code) as it was an add-on to the 7' ceiling'd previous porch which was closed in before I purchased. So I have barely an inch of headroom to spare at the outside edge of the porch roof. My fear is that I'll create a situation where the battens will rot if I use a styrofoam sheet moisture barrier. Someone mentioned using corrugated plastic underneath and screwing it directly to the wood joists. I've used the material before and that seems do-able but I'm not sure how to deal with this where it meets the walls on the lowest end (front of porch where door will be). The moisture may run down inside the plastic but it has to go somewhere when it reaches the wall. Do you think corrugated plastic is a good idea? Is foam insulation board not likely to cause problems? What am not understanding about the foam? I used foam to insulate the ceiling of the front porch that is now my office. Like I said this will remain an open air porch so I'm not concerned about heat/cool loss. I'm fine with the open construction of the roof...I just don't want to get dripped on when I'm lounging out there. Any ideas?
Todd Miller
Classic Products, Inc.
8/3/2007
One good option would be to spray the underside of the metal with icynene insulation. That is a little different than what you're thinking, I realize, but not a bad idea. The vapor barrier would be to prevent moisture from reaching the back of the metal so that it cannot condense. Therefore, if the vapor barrier is complete and there are not other issues, you should not have condensation.
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