Preventing Condensation from Inward Solar Vapor Drive
Location:
Northern Minnesota
Short Question:
I have condensation in my walls caused by suspected inward solar vapor drive. The walls are currently insulated with fiberglass batt insulation and a poly vapor barrier on the inside face of the wall.
How can I solve this issue? Would I be better off replacing the poly with a smart vapor barrier, replacing the fiberglass batt insulation with closed cell spray foam, or something else entirely?
Details:
Wall Construction: Steel siding, Tyvek building wrap, OSB sheathing, 2×6 studs, R23 unfaced fiberglass batt insulation, poly vapor barrier, 5/8” drywall (not yet installed).
I am converting an uninsulated/unheated workshop at my northern Minnesota home into a heated space. I installed fiberglass batt insulation and poly vapor barrier and began seeing condensation on the inside of the vapor barrier. The condensation occurs after it rains and when the sun shines on the exterior of the building, so I suspect inward solar vapor drive is the issue.
The workshop was built in 2007. It’s not new construction, so this isn’t an issue of moisture being released from new building materials. Is it normal to see significant inward solar vapor drive in this type of wall construction? It seems like any water that finds its way behind the siding should sheet away quickly on the Tyvek and the exterior should remain fairly dry? Do I have an issue with my building wrap?
If I replace the interior poly vapor barrier with Certainteed Membrain would that solve my problem entirely?
Would closed cell spray foam in place of the fiberglass batts be a better solution? Is there a risk of trapping moisture between the spray foam and wood causing mold and/or rot?
Is there another better solution?
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Replies
Is there an air gap between the siding and the Tyvek? It sounds to me not so much that solar drive is the issue as the wall can't dry to either side.
The steel lap siding is nailed directly to the OSB sheathing through the Tyvek, so there's no air gap. It seems like this is fairly typical construction in my area. The steel siding does have weep holes for drainage. Do you think that's insufficient?
Craig,
Inward solar vapour drive only affects permeable siding, and is most commonly seen with materials like brick which are reservoir cladding. Their is no mechanism for vapour to move inwards through metal siding. Something else is in play. probably what DC suggested, or poor interior air sealing. A variable-perm membrane might solve it.