What's the actual difference between open and closed cell?
Both are two-component polyurethane foams sprayed in place. The difference is the cell structure the chemistry leaves behind. Closed-cell cures into a dense, rigid foam of sealed cells: roughly 6.5 R per inch — the highest of any common insulation material — vapor-resistant, and stiff enough to add racking strength to what it coats. Open-cell cures soft and spongy with interconnected cells: roughly 3.7 R per inch, vapor-permeable, a strong air barrier, and an excellent sound absorber.
Closed-cell builds up in roughly 1-inch lifts so the exothermic cure can shed heat; open-cell can go on in thicker single passes. Both are covered application-by-application on the spray foam hub — this page is the decision between them.
Which works where in a Denver home?
Closed-cell territory: rim joists, where it air-seals and insulates the leakiest framing in the house in one pass; crawl space perimeter walls, where it tolerates masonry contact and incidental moisture — the crawl space conditioning guide covers that whole assembly, and the crawl space service page covers the project; vaulted and cathedral ceilings; and unvented roof assemblies, where code wants air-impermeable insulation at the deck doing the vapor work.
Open-cell territory: interior walls where sound dampening is the goal, and certain humid-climate roof applications that are uncommon in Denver's cool-dry climate. In Climate Zone 5B, open-cell at a roof deck or exterior wall without careful vapor design is a condensation risk — soft foam, cold sheathing, and vapor-open structure are a bad combination when indoor moisture finds the cold surface.
The pattern worth internalizing: closed-cell where the home meets the outdoors or the ground; open-cell where one conditioned room meets another.
How does each handle Climate Zone 5B vapor rules?
The 2021 IRC (R702.7) requires a Class I or II vapor retarder on the interior side of frame walls in Climate Zone 5, and Denver assemblies have to answer that requirement somehow. Closed-cell foam answers it with its own body: at roughly 1 perm per inch, around 1.5 to 2 inches of depth it performs as a Class II vapor retarder — insulation and vapor control in the same layer. That's why unvented roof decks and rim joists lean on it.
Open-cell is far more permeable and never provides this. That isn't a defect — vapor-open assemblies that can dry are often the more forgiving design — but it means open-cell placement has to be somewhere the assembly's vapor strategy doesn't depend on the foam. One trap to know in both directions: adding interior polyethylene over closed-cell foam creates a double vapor barrier with no drying path. The vapor barrier guide walks the full Class I/II/III logic for 5B if your project touches walls or roof decks.
What does each cost installed?
Closed-cell typically runs $2.50 to $5.00 per square foot of treated surface in Denver; open-cell runs $1.50 to $3.50. A typical rim-joist project — 100-200 square feet of treated surface — commonly lands at $400 to $1,200, and most crews carry a $400-$800 minimum charge that makes very small jobs price like medium ones.
Per square foot, closed-cell looks half again as expensive. Per unit of R, the gap narrows — 6.5 versus 3.7 R per inch means closed-cell buys more insulation per inch of depth, which matters where cavity depth is the constraint. The full pricing anatomy, including what makes foam quotes swing wider than blown-in quotes, is in the spray foam cost guide; for a foam-versus-blown-in decision on an attic floor, the comparator tool runs your numbers, and the cost calculator frames the whole-project budget.
When is spray foam the wrong answer entirely?
More often than foam marketing admits. On an open attic floor, blown-in at $1.50-$3.00 per square foot delivers the same R-49 for a fraction of closed-cell's price — the flat, accessible field is exactly where fibrous insulation is at its best and foam's air-sealing superpower can be replaced by targeted air sealing plus depth. Foam over an unresolved problem is worse than no foam: a roof leak, an unsealed ceiling plane, or active knob-and-tube wiring needs fixing before anything gets sprayed over it, because foam makes whatever it covers permanent.
And in a tight post-2010 home, foam anywhere is usually money spent on a problem the house doesn't have. The honest frame: spray foam is a specialist material for assemblies that need air sealing, vapor control, or rigidity in the same layer as the insulation. Where those needs don't exist, cheaper materials do the job. The cellulose vs fiberglass guide covers the materials that win the open field.
What should I ask an installer before signing?
Six questions separate crews that engineer from crews that spray:
- Which foam, and why for this assembly? The answer should reference vapor behavior or structure, not just price.
- What thickness, verified how? R-value lives in installed inches. Ask how depth gets documented at the walkthrough.
- What's the lift schedule? Closed-cell in roughly 1-inch lifts with cooling time between — a crew that hot-stacks lifts is cutting cure quality.
- What's the cure window and re-entry plan? 24 hours unoccupied is standard, with active ventilation; 48-72 for sensitive occupants.
- Is a thermal barrier in scope? Exposed foam in living spaces needs ignition protection — a quote that skips it isn't cheaper, it's incomplete.
- What's the minimum charge? If your job is small, ask whether bundling the rim joist with other work spreads the truck roll.
A crew that answers all six without flinching is the crew you want. The broader homeowner guides live on the resources hub.
Sources
What the data says
According to the International Residential Code, “the 2021 IRC (Section R702.7) requires a Class I or II vapor retarder on the interior side of frame walls in Climate Zone 5, with Class III permitted only under the specific cladding and insulation conditions listed in the section tables.”
According to the Building Science Corporation, “air leakage transports far more moisture into building assemblies than vapor diffusion does — which is why air sealing comes before vapor-retarder adjustments in retrofit work.”
According to the ENERGY STAR, “Climate Zone 5 homes (which includes Denver) need attic insulation rated R-49 to R-60 for optimal performance.”
Take the next step
Want the assembly answered, not the marketing?
Open or closed cell is an assembly decision — rim joist, crawl perimeter, roof deck, and interior wall each have a right answer. The free in-home estimate names which foam belongs where in your home, where blown-in wins instead, and what the honest scope costs.
Get a quote
Tell Us About Your Home — Get a Quote in Hours, Not Days
30 seconds to fill out. Free quote, no high-pressure follow-up.
We've Got It. Here's What Happens Next.
We've got your info. A local pro is reviewing it now. Expect a call within a few hours, or by tomorrow at the latest. While you wait, here's what to look for in the quote you receive: (1) R-value target — current Colorado code is R-49 to R-60 for attics, anything less is under-spec. (2) Air sealing scope — insulation alone does nothing if air leaks aren't sealed first. (3) Rebate handling — Xcel rebate paperwork should be handled for you, not by you. (The federal IRA Section 25C credit expired in 2025 and Colorado HEAR closed for the Front Range — Xcel programs are now the active rebate stack.) (4) Removal scope — pre-1990 homes often need old insulation removed before new install. If a quote skips all four, get another quote.
Frequently asked
What do Denver homeowners ask about open and closed cell foam?
How long until I can re-enter after spraying — does it off-gas?
Off-gassing during cure is real, and the standard Denver practice handles it: vacate the home for 24 hours after install with active ventilation running, and confirm with the crew that cure is complete before re-entry. Occupants with respiratory sensitivities or chemical concerns reasonably extend that to 48-72 hours. After full cure, modern spray polyurethane foam is inert — no ongoing off-gassing. The window is a scheduling cost of the project, not a permanent condition.
Can I just use a DIY spray foam kit?
For sealing a gap around a pipe, sure. For anything structural — a rim joist run, a crawl perimeter, any wall or ceiling — no. Two-component foam is temperature-sensitive chemistry: wrong substrate temp, wrong mix ratio, or wrong lift thickness produces foam that shrinks, pulls from framing, or never reaches rated R-value, and PPE during application is non-negotiable. The failure mode isn't "slightly worse" — it's tearing out bad foam, which is miserable work. Pro rigs exist because the chemistry punishes shortcuts.
Can closed cell foam crack if the structure moves?
It's rigid, so seasonal framing movement is a fair question. In practice, properly applied closed-cell flexes enough for normal wood movement, and its adhesion means hairline stress shows up as cosmetic surface checking, not failed insulation. Large structural movement — settling foundations, truss uplift — can crack any rigid material, but at that point the foam is the messenger, not the problem. Substrate prep and correct lift schedule during install are what prevent the avoidable cases.
Does open cell foam absorb water?
It can hold bulk water when soaked — the interconnected cells that make it vapor-open also make it sponge-like in a real wetting event, which is one reason it doesn't belong against masonry, below grade, or anywhere with bulk-water exposure. It dries out, but slowly. Closed-cell tolerates incidental moisture and masonry contact, which is exactly why crawl perimeters and rim joists default to closed-cell in Denver work.
What's the R-value per inch for each type?
Closed-cell roughly 6.5 R per inch — the highest of any common insulation material. Open-cell roughly 3.7, about the same as blown-in cellulose. The per-inch difference is the whole reason closed-cell owns depth-constrained assemblies: a 2x6 cavity or a shallow rim joist simply can't fit enough open-cell to hit the same number. Where depth is unlimited — an open attic floor — the per-inch advantage stops mattering and cheaper materials win.
My home was built after 2010 — is spray foam worth it for me?
Almost certainly not. Post-2010 code homes are already tight and insulated near current targets; foam's premium buys air sealing and R-value the house already has. If a newer home has a cold room or a high bill, diagnose first — duct balance, a builder defect under warranty, or a localized gap is far more likely than a whole-assembly insulation deficit. Spend money on a diagnosis, not a material.
Can foam be sprayed over my old attic insulation?
No. Foam adheres only to a bare, clean deck — spraying over existing loose-fill or batts means the foam bonds to debris instead of structure and never seals. Removal of the old material comes first, which adds $1.50-$3.50 per square foot to foam and hybrid attic scopes. That removal cost is a real part of why whole-attic foam rarely beats blown-in on value — the comparison is run honestly in the spray foam vs blown-in comparator.
Related guides