Thoughts on stained spray foam vs full replacement?

I ran into a nearly identical situation when I was converting my workshop back in 2019. I had those same dark, ugly patches on the closed-cell foam in the gables. Like you, my moisture meter showed 0% and the new roof was solid. I initially panicked thinking I needed a $5,000 remediation crew, but after talking to a local inspector, I took a more measured approach. Instead of ripping it all out, I used a spray bottle of Concrobium Mold Control to treat the surface. It’s designed to crush the spores as it dries rather than just bleaching them. Since spray foam is non-porous, the mold ussually stays right on the surface anyway. I scrubbed the worst spots with a stiff nylon brush, let it air out for a weekend with a heavy-duty fan, and then checked for any odors.

If it truly is bone dry and the source is fixed, you aren't dealing with an active colony. I ended up just sealing it back up with 5/8 inch Firecode X drywall. It’s been five years and I haven't had a single issue or musty smell. As long as you don't have moisture trapped behind the foam against the roof deck, pulling it all out is probaly just burning money.

I have to disagree with the idea of just leaving it be or doing a light surface cleaning. I’ve spent fifteen years in home restoration, and I can tell you that spray foam, expecially open-cell, is incredibly porous. If that mold is from an old leak, the spores aren't just on the surface; they are likely embedded deep within the structure of the foam itself. Just becasue it measures dry today doesn't mean those spores won't reactivate the second you have a high-humidity summer day or a tiny bit of condensation behind your new drywall.

When I did my own carriage house, I found similar black spotting. I tried using a Concrobium fogger first, but it didn't give me any peace of mind because I coud still see the staining deep in the crevices. I ended up using a stiff wire brush and a HEPA vacuum to mechanically remove the top half-inch of the affected foam untill I saw clean material. It was a messy, miserable weekend, but it’s much cheaper than having to rip out finished drywall two years from now because the garage smells like a damp basement. Don't risk sealing a potential respiratory irritant into your living space just to save a few days of labor.

Look, identify if you’ve got 2lb closed-cell or that half-pound open-cell sponge first. If it’s high-density stuff like BASF Walltite, that mold is strictly topical. I’ve seen this on dozens of jobs. Instead of a full demo, I’d hit it with Fiberlock ShockWave to kill the spores, then skin it with Foster 40-20 Fungicidal Protective Coating. It’s a pro-grade sealant that locks everything down. If the foam density is still firm and not delaminating from the rafters, you’re golden. No sense in a $10k rip-and-replace for a detached garage if the source is dead.

I'm totally new to this, so sorry if this is a dumb question, but how do I tell if I have the 'sponge' open-cell or the hard closed-cell stuff? I'm looking at my rafters and it all just looks like bumpy yellow popcorn to me. If I try to poke it with a screwdriver and it's soft, does that mean I have to rip it out like the restoration guy said? I'm realy hoping to avoid a full demo, but I'm worried about just painting over something that might be a 'biohazard' deep inside.

Oh wow, I'm so glad I found this thread! I'm just starting my first DIY garage project and reading all this is making me feel way better about the wierd spots I found too. Totally random though, but speaking of spray foam, does anyone know if it helps with soundproofing? I'm planning on setting up a drum kit in my finished space and I'm wondering if the foam will keep the neighbors from losing thier minds, or if I need to add somethig else before the drywall goes up!

Quick update—I tried the screwdriver test someone mentioned and it’s definatly hard closed-cell foam. The ShockWave spray worked wonders on the surface spots, but I’m still seeing deep, dark staining where the foam meets the wood. Is it safe to just coat those spots with an antimicrobial primer now, or do I need to scrape those corners down to the bare rafters to be sure?