Open Cell vs Closed Cell Spray Foam in a New York House
Open cell foam has ruptured, interconnected cells that air and vapour move through; closed cell keeps each bubble sealed. Every argument about R-value, water and price follows from that, and in a heating climate the drying path decides it.
One structural difference at the scale of a single bubble decides the R-value, the water behaviour, the price and — in a cold New York winter — whether the assembly can dry.
Open Cell vs Closed Cell Spray Foam in a New York House
Both products arrive in the same truck, come out of the same gun and cure
into polyurethane foam against the same framing. The difference is what happens
at the scale of the individual bubble. In open cell foam the cell walls rupture
as it expands, leaving a soft, interconnected matrix that air and water vapour
can move through. In closed cell foam each cell stays sealed around the gas
that expanded it, producing a dense, rigid material that resists heat and
vapour far more strongly.
Everything else people argue about follows from that one structural
difference. And in New York the argument matters more than it does in a mild
climate, because a cold-winter assembly has to be able to dry in a direction,
and the wrong foam in the wrong place removes that ability permanently.
What the cell structure changes
| Property | Open cell | Closed cell | Why it matters here |
|---|---|---|---|
| Density and feel | Light and soft; compresses under a hand | Dense and rigid; does not compress | Decides whether the material adds anything structurally, and how it behaves when something moves |
| Thermal resistance per inch | Lower | Roughly double, product depending | In a shallow cavity, R per inch is the whole argument; in a deep one it may be irrelevant |
| Vapour movement | Stays vapour-open at any normal thickness | Becomes a vapour retarder once it passes a certain thickness | This is the property that decides most New York jobs, and the one most often left out of a quote |
| Air leakage | Stops air at adequate thickness | Stops air at a smaller thickness | Both are air barriers — this is not a reason to choose between them, though it is often sold as one |
| Liquid water | Absorbs and holds it | Does not readily absorb it | Matters anywhere water can arrive: below grade, at a rim joist, in a flood-exposed space |
| A leak overhead | Water passes through and shows on the ceiling | Water can travel along the deck and appear far from its source, or not at all until the damage is done |
A roof leak you cannot see is a roof leak you cannot fix |
| Expansion | Expands a great deal; fills irregular cavities readily | Expands much less; applied in controlled lifts | Affects how well each fills an awkward bay, and how much trimming follows |
| Cost | Less per unit of installed volume | More per unit of installed volume | Compare cost per unit of R actually achieved in the cavity you have, not per inch or per board-foot |
Take R-values from the specific product’s technical data sheet rather than
from any general figure. Formulations differ between manufacturers, and the
number quoted in a sales conversation is often the best case rather than the
one for the thickness being installed.
The vapour question is what actually decides a New York job
New York spans a wide range of winter severity, from the relatively mild
coastal counties to genuinely cold conditions in the Adirondacks and the
western Southern Tier. What the whole range shares is a heating-dominated
climate, and in a heating climate warm humid indoor air moves outward toward
cold surfaces. Where it meets one below its dew point, it condenses.
An assembly survives that by drying — releasing the moisture it picks up,
in at least one direction, faster than it accumulates. A vapour-impermeable
layer in the wrong place does not stop moisture arriving; it stops it leaving.
That is the mechanism behind almost every foam failure that gets photographed
and shared, and it is why the same product can be the correct answer in one
part of a house and a serious mistake twenty feet away.
Roof decks
The cold-climate approach to an unvented roof assembly is to keep the
underside of the sheathing warm enough that indoor moisture never reaches a
condensing surface, which is what closed cell against the deck does. Open cell
alone in that position lets vapour through to cold sheathing, and in a New York
winter that is exactly the condition to avoid. Assemblies that combine both, or
that use exterior insulation above the deck, exist and are designed
deliberately — they are not something to improvise on site.
Below grade and at the rim
Concrete and masonry are a continuous moisture source; they wick from the
ground and release it inward for the life of the building. Open cell against a
foundation wall absorbs that moisture and holds it against the concrete. Rim
joists and crawlspace walls are the classic closed cell application for the
same reason, and this is the same reasoning that governs
where a basement vapour barrier helps and where it
traps.
Attics
The first decision in an attic is not which foam. It is whether the attic is
being brought inside the thermal envelope at all, or staying a vented space
with insulation on the floor. Those are two different buildings, and foam on a
roof deck is only correct in the first. If the attic stays vented, its
ventilation path has to keep working — a separate question from insulation, and
the one behind when sealing gable vents helps and when it
does harm.
Where each one is the right answer
Open cell earns its place in interior partitions and floors
where sound is the objective, in deep cavities where there is room for enough
inches that R per inch does not constrain the result, in assemblies
deliberately designed to dry inward, and where you want a roof leak to announce
itself on the ceiling rather than travel silently.
Closed cell earns its price at rim joists, in crawlspaces
and against foundation walls, on unvented roof decks in cold parts of the
state, in shallow cavities where the available depth is fixed, anywhere liquid
water is plausible, and where the added rigidity of the assembly is wanted.
Many good New York jobs use both, in different places, for these reasons. A
contractor who proposes one product for the whole house without reference to
the assembly is quoting a material rather than designing an envelope.
Two consequences people are not warned about
A tightened house needs designed ventilation. Foam works by
stopping air movement, and the air that used to leak in through the envelope
was also carrying moisture and indoor pollutants out. Once that path closes,
ventilation has to be provided deliberately — a balanced or exhaust system
sized for the house — or humidity and air quality get worse rather than better.
This should be part of the same conversation and the same quote, not an
afterthought a year later.
Off-ratio foam cannot be repaired by adding more. Both
products are mixed on site from two components in a controlled ratio and at a
controlled temperature. When that goes wrong the foam may never cure properly,
and the symptom is a persistent odour and a material that has to be physically
removed. Ask what the installer does to monitor ratio and substrate
temperature, particularly on a cold-weather install, and what the remedy is in
writing if it happens.
Neither product substitutes for the diagnosis
Foam is expensive and it is permanent, which makes it the wrong thing to buy
before you know where the house is actually losing energy. The sequence that
works is measurement first, air sealing second, insulation third — and the
measurement is a professional assessment with a blower door, not an inspection
by eye. Knowing what the house consumes across a full
year tells you the size of the prize before you commit to the largest
single line in a weatherization project.
It also settles what you should do yourself. Spray foam is squarely on the
contractor side of that line: it involves chemicals with real exposure
requirements, a re-occupancy period, and code requirements for thermal and
ignition barriers over foam plastics that an inspector will look for. Where
that line falls for the rest of the work is
worth thinking through before the quotes arrive.
What to establish before signing
- Which assembly, and why. Ask the contractor to write down,
for each area, which product, what thickness, and what the drying path is. A
proposal that cannot answer the third question is not a design. - The code path. Work in New York City falls under the
city’s own construction and electrical codes, while the rest of the state works
to the State Uniform Code, and which edition applies is a question for the
building department with jurisdiction. Nobody can tell you a particular
installation is compliant from a description of it — the inspector who signs it
off decides that. - Who is doing the work. Programme eligibility usually turns
on the contractor’s status rather than on the product, so establish
what qualification actually means before you compare
prices. - How it is paid for. New York’s support for envelope work
runs through NYSERDA and utility programmes, and the terms change, so read the
current ones rather than a figure quoted anywhere. If financing is part of the
plan, the requirements attached to it are worth
reading before the work is scheduled rather than after. - Ventilation and cure. Get the ventilation plan and the
re-occupancy period in the same document as the foam.
Reduced to one sentence: open cell is the cheaper, softer, vapour-open
material that belongs where an assembly needs to dry inward and where depth is
available; closed cell is the dense, vapour-retarding, water-resistant material
that belongs against cold sheathing, against masonry and wherever the cavity is
too shallow to solve with inches. The choice is a property of the location, not
of the house.
Frequently Asked Questions
What is the actual difference between open cell and closed cell foam?
The cell walls. In open cell foam the bubbles rupture as the material expands, leaving a soft interconnected structure that air and water vapour can pass through. In closed cell foam each bubble stays sealed around the gas that expanded it, giving a dense rigid material that resists heat flow and vapour movement far more strongly. Every other difference — density, R per inch, water behaviour, rigidity, price — follows from that one structural fact.
Which one should I use in a New York attic?
The first question is not which foam but whether the attic is being brought inside the thermal envelope or left as a vented space with insulation on the floor. Foam on the roof deck only belongs in the first case, and in a cold New York winter that generally means closed cell against the sheathing so the deck stays above the dew point. Ask the contractor to state the drying path for whichever assembly they propose.
Is closed cell always the better product?
No. It costs more per unit of installed volume, and its vapour resistance is a liability in an assembly designed to dry inward. It also hides a roof leak that open cell would let show on the ceiling. Where depth is plentiful, where sound is the objective, or where the assembly needs to stay vapour-open, open cell is the correct choice. Many sound New York jobs use both products in different parts of the same house.
Do I still need a vapour barrier if I use spray foam?
It depends on the product, the thickness and the assembly. Closed cell at typical installed thicknesses acts as a vapour retarder itself, and adding a separate impermeable layer on the interior side can create a sandwich with no drying direction at all. Open cell does not retard vapour, so the assembly has to be designed to handle that. This is the question to put to the contractor in writing, because getting it wrong is what causes the failures.
Will spray foam make my house too airtight?
It will make it tighter, which is the point, and that means ventilation has to become deliberate rather than accidental. The leaks that used to remove moisture and indoor pollutants are the same leaks the foam closes. A mechanical ventilation plan sized for the house should be part of the same project and the same quote. Tightening a house without addressing ventilation is how humidity and air quality problems appear.
How do I compare quotes for open cell and closed cell?
Compare cost per unit of thermal resistance actually achieved in the cavity you have, not price per inch or per board-foot. A quote should state the product, the thickness and the area for each part of the house, so the comparison is arithmetic rather than judgement. Take R-values from the product’s own technical data sheet rather than from the conversation, since formulations differ and quoted figures are often best case.
What happens if the foam is mixed wrong?
Both products are combined on site from two components at a controlled ratio and temperature. If that goes out of specification the foam may not cure properly, and the symptoms are a persistent odour and material that has to be physically removed rather than covered. It cannot be fixed by adding more foam on top. Ask how ratio and substrate temperature are monitored, especially for a cold-weather install, and get the remedy written into the contract.
NY Solar Incentives
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Run the arithmetic on your own system
Check the dated program table and your arrangement before counting an incentive. Standard residential NY-Sun blocks are closed; Long Island Affordable Solar has a conditional income-verification route. The NY credit depends on qualified costs and tax liability, while the NYC abatement reduces future property-tax bills. The estimator keeps those timings separate.