Open cell spray foam offgassing 2 years later. How to fix?

Question

My home was built in 2017 and has Accella open cell spray foam in the unvented attic. In spring & summer 2018 we noticed a mild chemical smell coming from the attic on hot and/or humid days. It would enter the 2nd floor living space through the seams of the pull down attic door. It is a mildish sweet chemical smell. Unpleasant but not massively so. It is stronger when standing in the attic.

In the fall/winter the smell is gone from the living quarters. Even standing in the attic during winter the smell is very faint. Still there, but so faint that it isn't offensive.

It is now Spring 2019 and the smell is back again. The strength of the smell seems identical to last year. That is surprising since I figured it would have off gassed by now.

I noticed a large chunk of foam had fallen to the floor so I placed it in a large Ziplock bag and sealed it. A few hours later I opened the bag and smelled the air inside. I did not notice any odor. I then took a saw and cut the piece in half and placed the cut piece to my nose - no smell. Maybe the odor is coming from only certain sections of foam?

I'm concerned about this off gassing and that it hasn't gotten better nearly 2 years after this home was built. The only thing I can think of is to ventilate the attic by forcing air up there and exhausting air from a 2nd floor window.

What could be causing this smell and how can I solve this problem?

Answer 1

This answer was posted by the asker as an update to the question instead of an actual answer.

The problem fixed itself eventually. I followed the solution proposed by Wayfaring Stranger in the comments:

Vent the attic. Push air thru there like crazy all summer. I replaced my attic entry panel with a panel with a strong fan on it. Get a fan with bearings, not bushings. They're the more expensive variety. However, mines been blowing trouble free for 20 years. Every couple years, I blow the dust of the blades with a shop-vac.

If you have no vent, the gas is only going to come out at the rate it diffuses through ceiling and rooftop. If you have access, put a blower fan up there, attached to that white flexible dryer venting tubing. On warm days, run the tubing out a window and turn on the fan. This sounds like a nasty design flaw to me, as every home built this way, with these materials will have this problem. I'd have a good hard look for the name of the stuff that is outgassing, and its long term toxic (or not) effects.

In summer 2019 (The builder spray foamed the new home in July 2017) I bought a powerful contractor grade fan with a long flexible vent so I could force air deep into the attic causing it to become pressurized. The fan was placed in the laundry room and I had other box fans forcing air into that room and exhausting it out the window. The stale attic air returned through the attic hatch then was sucked out the window. I did this a handful of times on hot dry days sometimes for the entire day while we were at work. I guess the theory is that some of the odor is trapped in the foam cycling in/out of it? This seemed to help as the odor was less that summer.

By summer 2020 the odor was almost gone and very rarely was present in the home. It is now 2022 and things are even better still. Still some faint odor when standing in the attic but a fraction of what it once was.

I think the foam just needs time to off gas naturally--a good 3 years.