Short answer, if you're in a hurry
If your facility has any moisture cycling at all — humidity swings, condensation, leak risk — order foam board. Even at 2–3× the unit cost of fiberglass. If your space is genuinely dry and climate-controlled, fiberglass still wins on cost-per-R-value.
But that's not the comparison most people are actually making. The two specs that decide whether your order survives year one are long-term moisture performance and the fire rating that applies to your actual installation — not the R-value printed on the marketing sheet.
Why take my word for it
I've been handling packaging and industrial material orders for 8 years. I've personally wasted somewhere around $22,000 on mistakes I've written down, and I keep the list because telling other people what I got wrong is genuinely cheaper than letting them repeat it.
In March 2022, we ordered $4,200 of fiberglass roll for a Midwest warehouse retrofit. Price was fine, R-value was fine, nobody asked about moisture. Six months later, roof condensation bled through, the fiberglass was toast, and we redid the whole thing with XPS foam board. Total cost: $9,600. Plus a late delivery to the client, which doesn't show up on a P&L but shows up in the relationship.
The 'budget option' looked smart right up until it didn't.
The three checks that actually decide it
Before you ask foam board vs fiberglass, ask these questions first.
1. What's the real moisture exposure?
Fiberglass loses roughly 40% of its rated R-value once it absorbs about 1.5% moisture by weight (ASTM C553 covers the moisture absorption test method). Foam board — XPS, EPS, polyiso — barely absorbs water at all. If the space ever sees condensation, has any leak risk, or cycles through humidity, foam is the only version of this that survives long enough to be worth the invoice.
2. What does 'fire rating' actually mean here?
'Class A' means different things for foam versus fiberglass, and it also depends on whether you're looking at ASTM E84 or NFPA 286. Foam almost always needs a thermal barrier — typically gypsum board — in occupied spaces. Fiberglass usually doesn't. Most buyers don't know that distinction until an inspector points at the ceiling and hands them a correction notice.
3. What can the install crew actually handle?
Fiberglass needs real PPE. 3M nitrile gloves cover most of it, but if your installers are burning through a pair inside a day, they're working long shifts in itchy material and the schedule math is already wrong. Foam board cuts with a knife, seals with tape (3M Temflex 2155 rubber splicing tape is what we keep on hand for the critical electrical bonds on retrofits), and goes up behind drywall. Faster install, less waste, different skillset.
On the efficiency side of this — we moved our spec-verification off paper checklists and into a shared digital sheet with photo confirmation back in 2023. Error rate dropped by roughly 60%. Not because the software was magic, but because it forced us to photograph the moisture map instead of 'trusting that we remembered to look.' Cutting manual steps out of spec approval cuts the errors that came with those steps.
When you probably shouldn't choose foam board
Honest limits, because I don't think everyone should order the same thing I do:
- Dry interior partition walls with a hard budget cap: fiberglass wins on cost-per-R-value, and pretending otherwise doesn't help anyone.
- Open-cell foam: absorbs water in humid conditions. Closed-cell vs open-cell sounds like a technicality until it isn't. Check the spec sheet, not the sales pitch.
- ESD-sensitive electronics assembly areas: generic foam board accumulates static. Unless you're ordering ESD-rated sheet, something like ESD polycarbonate is the right material choice — not just any insulation panel.
- Aircraft-adjacent applications: foam board in a plane interior needs aviation-certified material, not construction-grade board from a general supplier.
One more thing on the green side: per the FTC Green Guides (16 CFR Part 260), claims like 'recyclable' or 'eco-friendly' need evidence behind them. A lot of foam board manufacturers use the word 'recyclable' loosely, and what they mean is 'theoretically recyclable in a facility that may not exist near your job site.' Ask for documentation. If they can't produce it, that tells you something.
Bottom line from the person who already paid this particular tuition: our checklist has caught 47 potential errors over 18 months. Two of those would have been $4,000-plus redos. If you take one thing from this, let it be that — before the PO goes out, write it down and take the photo.