Mystery mold around a hot roof…
I have a real head scratcher (for me) that I’m trying to problem solve. Bear with me, there’s a fair amount going on here:
I recently looked at a roof assembly in Cabot, VT (climate zone 6A) that has some very intense molding on the underside of the sheathing, though nothing is rotted out just yet. It’s a steep pitched (I’m guessing a 16in12 or steeper) gable roof that el’s into an attached unheated room over the garage, what would be a “bonus room” in a McMansion, for example. The unvented roof assembly is insulated with roughly 3-4″ of closed cell spray foam, which I think isn’t enough, but maybe that’s besides the point? The thermal enclosure of the roof ends at the roof’s last common rafter before the valley rafter, and the CCSF was installed down the gable end wall, again about 3″ worth. It was installed with Typar house wrap as a backing on the cold side of the wall. Post facto, half the garage was converted into a mudroom. This has foil faced polyiso foam of an unknown thickness over the framing on the interior side with about 10″ of cellulose over it which is a little denser than loose filled, but not “dense packed”, covered by 3/4″ t&g plywood.
Generally speaking, all the work seems to have been done okay, and I wasn’t able to find any obvious point source of moisture that shouted at me as being the dragon’s head of all the mold that was present. There does not appear to be a roof leak, based on the total lack of evidence of dripping inside. Admittedly, the asphalt shingles were covered in snow so I couldn’t see their condition.
The mold appears to stop at the point where the CCSF is in direct contact with the roof deck. The sheathing is moldy into the valley peak and works it’s way down the garage roof deck (it’s a California-style valley) spilling into the garage room and back up the underside of the sheathing in the garage. At first blush one would almost think there had been a fire and it was all soot. There was a half-assed hatch cut with a sharp knife through the foam into the hot roof’s attic that rodents have chewed a couple little tunnels into, but otherwise it fit into it’s hole okay. This seemed to be a point of moisture exfiltration from the house, but I’m inclined to think it’s not leaky enough to justify the quantity of mold. The attic has collar ties that have fiberglass insulation between most of them, kraft paper facing up, which makes me believe they were installed as an afterthought. I don’t understand why they are there, as the roof rafters are insulated up to the peak.
Finally, there were some small puddles of standing water in a fold of the Typar, which was of an unknown origin. Less than a couple cups worth of water, but still, why was it there?
Anyone have any guesses as to what the heck is going on here?
The homeowner wants to blow loose fill cellulose over the attic floor, which I think is not a great idea as it will restructure the thermal enclosure to some degree without addressing venting, or moisture control. Am I missing something here?
Thanks to all who have read this far! I really appreciate any and all feedback as I am coming up empty handed with a solution I feel confidant with.
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