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Mold on attic gable wall , non obstructed ridge / soffit vent in place and well insulated?

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Help! I am Dumbfounded !!!!! Newley constructed 20 x 22.6  2 story addition, Roof is a 6/12 pitch vent chutes for soffit in every bay , with full run of Cobra ridge vent as well as temp controlled gable vent (for hot summers) R38 faced insulation for 2nd floor cieling.

There is a green/Black mold issue on lower section of gable attic wall and on underside of roofing a couple bays out from gable end as well. there are no obstructed chutes and no roof leaks. Any idea's on how to rectify?????
asked in Attic Area by woodrogue (160 points)
edited by darekrudy
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1 Answer

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Hi Woodrogue,
There are a few things you can investigate:
  • Check the attic when the ambient temperature drops well below freezing and look for frost covered surfaces (most likely where the mold appears on the decking surface). Frosted surface could be a sign of a poorly ventilated section (clogged ridge or soffit vent corresponding to this section).
  • There might be leaks in those mold covered areas caused by nails penetrating surface, improperly installed wall (loose siding, gaps between bricks ) / roof finishing material (asphalt shingles, tiles, wood shingles).
  • There might be no house wrapping material between the plywood / OSB (or its sections) and the exterior wall finish. House wrap protects walls from damaging wind and rain / moisture that can penetrate through the exterior cladding and cause mold growth.
  • Installed plywood / OSB sheets may not be designed for unconditioned space installation / not resistant to moisture.
  • There might be some gaps in the wall’s top plate (right beneath the mold covered gable wall) or under the insulation in that area that permit air penetration from inside the house (i.e., drilled holes for electrical conduit). There may also be some missing insulation inside the wall cavity directly beneath the mold covere spot.
  • Installed boards might have been contaminated or wet prior to the installation which resulted in mold growth
There’s nothing else I can think of… let me know if this helped or if you have more questions.
answered by darekrudy (21,730 points)