What can I do about “impossible” mold on the spray foam insulation in my sealed attic?
This spring I bought a 10-year-old home in New York State. The attic is a narrow crawl space with thick, open-cell spray foam insulation on the roof, no insulation on the floor, and no ventilation. It appears that there is little air exchange between the attic and the house, and presumably even less between the attic and the outdoors.
Summers here are warm and very humid; winters are quite cold.
A month after moving in, I climbed into the attic for the first time, and found a decent amount of mold growing on the foam insulation.
It has since been looked at by a mold remediator and two assessors, all of whose reaction was “huh, I’ve never seen that before!”
The assessor suggested adding lots of ventilation into the attic, and insulating the floor instead of the roof. Several articles on this site suggest that’s rarely a good idea, especially when (as in my case) there is duct work in the attic.
Since my local “experts” seem mystified by this problem, what on earth am I supposed to do? How did the mold get into this supposedly mold-resistant attic, and how can I prevent it from coming back?
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Open cell foam can harbor quite a bit of moisture, and unless there is small air exchange between the attic and CONDITIONED SPACE AIR (not outdoor air) wintertime moisture can build up in the roof deck over a winter, but stays in the attic for much of the summer. There are often cases of "ping-pong" moisture in summer, where the relative humidity in the attic soars during the day when heated by the sun, then drops at night when the roof deck cools. Even a tiny amount of active ventilation with conditioned space air fixes that.
Often it's just a matter of adding a small register to supply duct to deliver 50-100 cfm of dry air conditioned air whenever the AC is running, and that's usually the simplest thing to try first. Alternatively, a continuous 20-40 cfm of room air delivered by a tiny ventilation fan on one end of the attic, with a small ceiling grille on the other to guarantee the air exchange will do it.
Monitor the temperature and relative humidity in the attic to see if it's working. At temperatures over 60F a relative humidity north of 70% is edging into the high mold risk region. Under 50% RH at any temperature is low risk. Autumn on into early winter is typically the driest period, since that's when it's had time to shed last winters' accumulation, and as the nights grow colder more of the moisture ends up staying in the roof deck. Spring into early summer is when the attic RH is likely to peak, unless given some air flow to/from the AC to dry it out.
If it's possible to install a broad-sheet membrane type vapor retarder in a reasonably air tight manner over the foam once the roof deck & foam are dry, it will mitigate the total volume of moisture over the winter. If that's not possible, a shot of 1" of CLOSED cell foam on the open cell foam brings it's vapor permeance down an order of magnitude, enough to prevent excessive humidity cycling, and is protective of the roof deck.
More here:
https://www.greenbuildingadvisor.com/article/high-humidity-in-spray-foam-attics
Thank you for a GREAT answer! I was skeptical when I first read it (ok, I tend to be pretty skeptical of just about everything) but after reading the linked article, then diving down the rabbit hole and reading a dozen more articles on the topic, and even tracking down the building codes, it's clear that you are absolutely right - about the cause of the problem, about the vapor retarder, and about the need and techniques for conditioning the attic.
Of course, in my case, it turns out there was another part to the problem - I found a rather large hole in the exhaust vent from a bathroom fan. (So frustrating that none of the professionals I've had in have thought to check for this.) So for 10, years every time someone has taken a shower they've been blowing humid air into the attic! That explains why most of the mold is centered in one area of the attic.
In the area where I live, most people don't have AC systems - it's just not worth it for how few really hot days we get. But outdoor humidity this summer was often 90-100% at night and 70-90% during the day. I don't know how everyone else prevents mold in these conditions without AC. My house has an ERV system, which as far as I can tell just keeps the indoor humidity at the average of the outdoor humidity level. So I've had a free-standing dehumidifier running nearly full-time, and the ERV seems to be doing a decent job of distributing the dried air around the house. I guess I'll need to start blowing that dehumidified air into the attic now, too.
"another part of the problem."
It sounds like you may have found the entire problem! That is crazy it wasn't identified by the mold remediator (assessors are pretty useless for that level of detail, in my experience).
>"It sounds like you may have found the entire problem!"
Naw. The obvious stuff is just the first 90% of the problem.
The vapor permeance of the foam and lack of air exchange is the next 90% (and can contribute to other, even less obvious and longer term issues north of Westchester County).
If experience is any guide there is probably a third 90% of the problem (for an even 270%)- we just don't know what that is yet. :-)
>" My house has an ERV system, which as far as I can tell just keeps the indoor humidity at the average of the outdoor humidity level."
That's what happens when there is no AC or other mechanical dehumidification going on indoors, or when the ventilation rates are set too high. An ERV is not a dehumidifier- it has a humidity exchanger between the incoming and outgoing air streams, but it's not perfect exchange. Only a fraction of the humidity will be swapped in or out, and at a much lower percentage compared to the sensible heat exchange efficiency.
> small register to supply duct to deliver 50-100 cfm of dry air conditioned air whenever the AC is running... Alternatively, a continuous 20-40 cfm of room air
A dehumidifier in the attic will be even better.
+1 on monitoring the humidity at the foam.
Dana said: "adding a small register to supply duct to deliver 50-100 cfm of dry air conditioned air whenever the AC is running, and that's usually the simplest thing to try first. Alternatively, a continuous 20-40 cfm of room air delivered by a tiny ventilation fan on one end of the attic, with a small ceiling grille on the other to guarantee the air exchange will do it"
I'm intrigued by the option of a continuous venting of conditioned room air into the attic. I may be about to seal my attic to get the ducts in the envelope. I've got un-removable R-19 attic-floor batts (under plywood decking), so I'll probably need to have some way of actively moving conditioned air into the attic, but adding a supply duct and return will add to the cost because I think I'd want a pro if it involves cutting into metal ducts.
A low-power continuous vent could take air from the peak of a cathedral ceiling at one end of my gable-roofed house and deliver it to the attic, while a grille at the opposite end could give the extra attic air somewhere to go. Drywall cutting I can do!
Also, here in zone 2A, we're about to enter a time of year from about October to April when the central air and heat don't necessarily run a whole lot, and we open windows in the daytime, and I'd think a continuous supply of room air would do a better job of dehumidifying than an occasional burst from ductwork, at least in the Gulf South. Is it still a better idea to open a duct in the attic for some reason I'm missing?
Alternatively, does it make sense to use a similarly small fan (20-40 cfm), perhaps humidity/temperature controlled, to vent attic air to the outside and suck in conditioned air from the living space when the humidity or temp gets too high? FWIW, attic is gabled with about 600 sq ft of floor space.