We install both. That matters, because a shop that only owns one product will find a reason to sell you that one. Here is how the two foams actually differ and how we choose.
Side by side
| Property | Closed-cell | Open-cell |
|---|---|---|
| Density | ~2.0 lb/ft³ | ~0.5 lb/ft³ |
| R-value per inch (aged) | R-7.4 | R-3.5 to R-3.8 |
| Air barrier at | about 1 inch | about 3 to 4 inches, product dependent |
| Vapor permeability | Low: a Class II vapor retarder at roughly 1.5 to 2 inches | High: typically Class III at full cavity depth; needs a separate vapor retarder in Zone 5 |
| Bulk water | Rejects it; FEMA-listed as flood damage-resistant | Absorbs it |
| Structure | Adds racking strength to the assembly | Negligible |
| Sound | Fair | Good; the open cells absorb sound |
| Cost per inch | Higher | Lower |
| Cost per R | Similar, once thickness is equalized | Similar, but needs the cavity depth |
The vapor question is the whole question
Erie and the rest of Western Pennsylvania are in IECC Climate Zone 5A. In winter, warm indoor air carries moisture toward the cold side of the wall. If that moisture can pass through the insulation, it condenses on the first cold surface it finds, usually the back of the sheathing or a metal panel, and stays there. The building code (IRC R702.7) requires a Class I or II vapor retarder on the interior side of framed walls in Zone 5 for exactly this reason.
Closed-cell foam at two inches is that vapor retarder. Open-cell is not; it is vapor-open, which is fine in a hot climate that needs to dry inward, and fine in Zone 5 only when a separate retarder or a smart membrane is added and the assembly is designed around it. Open-cell sprayed straight onto the inside of a metal building in our climate is a wet wall waiting to happen.
Where open-cell is the right answer
- Interior partition walls where sound control is the goal and there is no temperature difference to drive vapor.
- Deep, dry attic cavities in a vented assembly, where thickness is free and moisture is handled elsewhere.
- Budget-driven whole-house jobs where a vapor retarder will be installed on the warm side anyway and the cavity is deep enough to hit the R-value.
Where closed-cell is the right answer
- Metal buildings, pole barns, shops and any assembly with a cold impermeable skin.
- Rim joists, basement walls, crawlspaces, anywhere near bulk water or ground moisture.
- Unvented roof decks, where the foam has to keep the sheathing warm enough not to condense.
- Anywhere the cavity is shallow and the R-value target is high.
When we walk your project we will tell you which one it needs, even when the answer is the cheaper foam.