Almost every spray foam decision starts with the same question: open-cell or closed-cell? They are applied the same way and both seal air, but they behave differently enough that picking the wrong one can cost you money or, in a damp space, cause a real problem.
The short answer: use closed-cell wherever moisture, exterior exposure or limited cavity depth is involved — basements, crawl spaces, roof decks, metal buildings, vans. Use open-cell for interior walls and ceilings where the space is dry, the cavity is deep, and sound control matters more than R-value per inch. Closed-cell is not simply the better product; it is the more specialised one.
Open-Cell vs Closed-Cell at a Glance
| Open-cell | Closed-cell | |
|---|---|---|
| R-value per inch | ~R-3.5 | ~R-5.66 |
| Density | ~0.5 lb/ft³ — soft, spongy | ~2 lb/ft³ — rigid |
| Expansion | Very high (~100x) — fills deep cavities cheaply | Moderate (~30–40x) |
| Water & vapour | Permeable — absorbs water, dries out | Acts as its own vapour barrier at sufficient thickness |
| Sound absorption | Better — the open structure traps sound energy | Weaker at absorbing; blocks better only when paired with mass |
| Structural effect | None — stays flexible | Adds rigidity to the assembly it bonds to |
| Typical installed cost | $0.50–$1.50 per sq ft | $1.50–$3.50 per sq ft |
| Best for | Interior walls, ceilings, sound control | Basements, roofs, exteriors, metal buildings, vehicles |
What the Difference Actually Is
The name describes the cell structure. In open-cell foam the tiny bubbles are not fully enclosed, so air can move between them. That makes the foam soft, light, and permeable — and it is exactly why it absorbs sound well and why it will take on water.
In closed-cell foam each bubble is sealed. Air and moisture cannot pass through, which is what gives the material its higher R-value per inch, its rigidity, and its ability to function as a vapour barrier without a separate membrane.
The Mistake People Make
Because closed-cell has the higher R-value and the bigger price tag, it gets treated as the upgrade — the one you buy if you can afford it. That is wrong in two specific situations:
- Sound control. If your goal is a quieter bedroom, home office or media room, open-cell is the better material. Its open structure absorbs sound; closed-cell reflects more of it. Paying more for closed-cell here buys you a worse acoustic result. See our guide to spray foam for soundproofing.
- Deep interior cavities. Open-cell expands roughly three times as much. In a deep, dry interior cavity it fills the space for considerably less money, and the extra R-value per inch of closed-cell does not help once the cavity is full either way.
The reverse mistake is just as costly: using open-cell in a basement, crawl space or against a roof deck. It will absorb moisture, lose performance, and can hold water against the structure.
Where to Use Each
Choose closed-cell for
- Basements and crawl spaces — moisture is the defining condition. See basement insulation.
- Roof decks and attics — especially unvented assemblies, where vapour control matters. See attic insulation.
- Metal buildings — condensation on steel is the main failure mode, and closed-cell stops it.
- Vans, RVs and trailers — thin walls mean R-value per inch is at a premium, and the added rigidity is a bonus.
- Any exterior-facing assembly — anywhere the foam sits between conditioned and unconditioned space.
Choose open-cell for
- Interior partition walls — between rooms, where there is no moisture drive and sound is the real goal.
- Ceilings between floors — footstep and voice transmission is the problem, not heat loss.
- Deep, dry cavities — where its expansion does the work at lower cost.
- Home theatres, offices, nurseries — anywhere acoustics matter more than thermal performance.
Cost: Why Closed-Cell Costs More
Closed-cell is roughly two to three times the price per square foot. Two reasons: it uses substantially more material for the same volume because it expands less, and the chemistry itself is more expensive.
That said, most of a professional quote is labour rather than material either way — which is why a DIY kit changes the maths considerably. For the full breakdown, see the cost of spray foam insulation.
How Thickness Changes the Answer
R-value is per inch, so the type you choose determines how deep you have to go. To reach roughly R-21 in a wall you need about 3.7 inches of closed-cell or about 6 inches of open-cell. If the cavity is only 3.5 inches deep, open-cell physically cannot get you there — and that constraint, not preference, often makes the decision for you.
For target R-values by area and climate, see how thick spray foam should be.
Frequently Asked Questions
Which is better, open-cell or closed-cell?
Neither is better in general. Closed-cell wins on R-value per inch, moisture resistance and rigidity. Open-cell wins on cost, expansion and sound absorption. The right answer depends on whether the space is damp, how deep the cavity is, and whether you are chasing warmth or quiet.
Is closed-cell spray foam waterproof?
It is highly water-resistant and acts as a vapour barrier at sufficient thickness, but "waterproof" overstates it. See is spray foam waterproof for the distinction.
What happens if open-cell foam gets wet?
It absorbs water. It will dry out if the source is fixed and airflow is adequate, but while wet it loses insulating performance and can support mould growth. This is why it does not belong below grade or against a roof deck.
Can I use both in the same building?
Yes, and professionals often do — closed-cell on the roof deck, rim joists and below-grade walls, open-cell on interior partitions. Choosing per assembly rather than per building is usually the most cost-effective approach.
Does closed-cell foam really add structural strength?
It bonds rigidly to the surfaces it contacts and measurably increases racking resistance in framed assemblies. It is a genuine secondary benefit, not a substitute for structural design.
Which one do DIY kits use?
Most consumer kits, including ours, are closed-cell — it is the more broadly applicable choice for the jobs homeowners typically tackle, and the higher R-value per inch matters most where space is tight.
Making the Call
Work through three questions in order. Is moisture involved? If yes, closed-cell, and the decision is made. Is the cavity deep and dry? If yes, open-cell will fill it for less. Is the goal quiet rather than warmth? If yes, open-cell absorbs sound better.
If you are ready to start, see the ixirpro Spray Foam Insulation Kit.