Open-cell vs closed-cell Air sealing matters Moisture and code reality Not always the best buy

Spray Foam Insulation: The Straight Story on Cost, Code, Moisture, Air Sealing, and Where It Actually Makes Sense

Spray foam insulation gets talked about like it is either the greatest thing ever invented or the worst idea anyone ever sprayed into a house. Usually, both sides are overselling it. Spray foam is not magic, and it is not nonsense. It is a tool. In the right place, it can solve problems that batt insulation and caulking struggle with. In the wrong place, it can be an expensive way to do a job that did not need to be expensive.

That is the part homeowners rarely hear clearly. People hear “high R-value,” “air seal,” and “premium insulation,” and they assume more spray foam automatically means a better house. That is not how building works. A good wall, roof, or basement assembly is about more than stuffing the cavity with the priciest material you can find. It is about heat control, air control, moisture control, fire protection, detailing, installation quality, and whether the material actually suits the location.

If you are trying to decide whether spray foam insulation is worth it, the smart question is not “Is spray foam good?” The smart question is “Where is spray foam the best answer, and where is it being sold to me because it sounds premium?” Those are very different conversations, and one of them saves you a lot of money.

Fast answer: spray foam is best where air sealing is hard, space is limited, moisture control is critical, or complicated geometry makes other insulation weaker. It is not automatically the smartest insulation choice for every attic, every wall, or every renovation.

Where spray foam shines

Rim joists and band joists
Basement and crawlspace problem areas
Cantilevers and awkward cavities
Unvented roof assemblies designed correctly

Where it gets oversold

Big open attic floors that could use cheaper insulation well
Standard wall cavities with no special detailing need
Houses with unresolved moisture or roof leak problems
Projects where people want “premium” more than a real plan

What matters most

The installer, the assembly design, the moisture strategy, and whether the location really benefits from spray foam’s air-sealing and density — not just the label on the truck.

What spray foam insulation actually is

Spray polyurethane foam is made on site by combining chemical components that react, expand, and cure into insulation. That “made on site” part is one of the reasons quality matters so much. This is not a factory-made batt you slide into place. The material performance depends on the product, the substrate condition, the temperature, the installer, the thickness, and the application itself.

In Canada, medium-density closed-cell spray polyurethane foam is governed by a material standard and an application standard. In plain English, that means both the product and the installation method matter. That should tell homeowners something important right away: spray foam is not a casual commodity product. It is a system that needs to be installed correctly to perform the way people expect. :contentReference[oaicite:1]{index=1}

The other thing people need to understand is that spray foam is doing more than one job. Depending on the type and thickness, it can provide insulation, air sealing, and sometimes vapour control. That can make it powerful. It can also make it dangerous in the hands of someone who does not understand assemblies, because one product doing multiple jobs can either simplify the wall or create a moisture trap.

Builder translation: spray foam is not just insulation. It is often part insulation, part air barrier, and sometimes part vapour strategy. That is why details matter so much.

Open-cell vs closed-cell: do not treat them like the same material

Homeowners often say “spray foam” as if it is one thing. It is not. The two versions most people run into are open-cell and closed-cell, and they behave differently enough that you should stop lumping them together after the first five minutes of the conversation.

Open-cell spray foam

Open-cell foam is lighter, softer, and usually less expensive per inch. It expands a lot, which can help fill irregular cavities. It can be useful where air sealing is important and where drying potential matters. It is often discussed for wall and roof applications, but you need to understand the assembly, climate, and vapour control strategy before calling it “the answer.”

Closed-cell spray foam

Closed-cell foam is denser, more rigid, and gives you more thermal resistance per inch than most other common insulation materials. NRCan’s current guidance also makes an important point people miss in sales talk: fresh-off-the-gun thermal performance is not the same as long-term design performance, so Canadian design values use long-term testing rather than brochure enthusiasm. :contentReference[oaicite:2]{index=2}

Closed-cell foam is often the more practical choice where you need high R-value in limited space, extra rigidity, good air sealing, or moisture resistance. But it is also more expensive, less forgiving of bad application, and easier to misuse in assemblies that need drying ability.

Type	Usually Best For	Main Caution
Open-cell spray foam	Air sealing in some wall or roof assemblies where drying is part of the strategy	Not a one-size-fits-all vapour-control answer
Closed-cell spray foam	High R-value per inch, rim joists, basements, tight spaces, moisture-sensitive locations	Higher cost and easier to misuse in assemblies that need to dry

Where spray foam earns its money

This is the part I wish more homeowners heard early. Spray foam is not usually the best value because it is expensive. It is the best value where its specific strengths solve specific problems.

Rim joists and band joists

This is one of the classic spray foam wins. Rim joists are awkward, leaky, and annoying. They are exactly the kind of place where batt insulation often underperforms because air leakage is the real problem. Spray foam can insulate and air seal in one shot, which is why people who use it intelligently love it there.

Basement problem areas

Basement walls, headers, and transition areas are often messy from an air and moisture point of view. A good closed-cell foam installation can be useful here, particularly where space is tight and a cleaner air seal is needed. That said, spray foam does not solve liquid-water problems. If the basement leaks, fix the leak. Do not bury the evidence under expensive chemistry and call yourself clever.

Cantilevers, bump-outs, and weird cavities

This is where spray foam often separates itself from cheaper insulation. When the geometry is ugly, air sealing is poor, or access is awkward, spray foam can outperform simpler materials because it conforms, seals, and insulates in places where neat batt installation becomes a fantasy.

Unvented roof assemblies done deliberately

Spray foam is often used in cathedral ceilings and unvented roof assemblies because it can help control air leakage and provide high R-value in limited depth. But this is a “done deliberately” situation, not a “spray first, think later” situation. Roof assemblies have moisture consequences, and casual decisions here are expensive.

Where spray foam is often a waste of money

This is where the sales pitch usually gets fuzzy. If you have a big, open, accessible attic floor and no special geometry problems, you do not automatically need spray foam. In many houses, attic air sealing plus blown insulation is a much more economical move. Spending premium money everywhere does not make you more efficient. Sometimes it just makes you poorer.

The same goes for standard wall cavities where nothing unusual is happening. If the wall design is straightforward and space is not extremely limited, there may be other assemblies that perform very well for less money. Spray foam should win because it is the best answer, not because it sounds high-end.

It is also the wrong move where moisture issues are unresolved. Roof leak? Basement water issue? Wet sheathing? Fix the problem first. Spray foam is not an eraser for bad building conditions.

Builder truth: premium insulation does not rescue bad design or deferred maintenance. It just hides it more expensively.

Moisture, vapour, and the part people oversimplify

This is where articles get lazy. People like simple lines like “spray foam is a vapour barrier” or “spray foam solves condensation.” Sometimes those ideas are directionally true. Sometimes they are half-true enough to cause damage.

The real issue is assembly design. Houses need to control heat, air, and moisture, and they also need to be able to dry in sensible directions depending on the assembly. Some spray foam choices reduce drying potential. That may be perfectly acceptable in a properly designed assembly. It may also be a terrible idea in a renovation where old materials, unknown moisture paths, or trapped leaks are part of the picture.

NRCan’s guidance is still useful here because it keeps coming back to the same principle: insulation is not just about R-value. Air barriers, vapour barriers, and where they sit in the assembly matter too. That is exactly why homeowners should be suspicious of contractors who talk about spray foam only in inches and never in assemblies. :contentReference[oaicite:3]{index=3}

Code, standards, and why installer quality matters so much

If you take one practical lesson from this article, let it be this: with spray foam, installer quality matters more than homeowners think. Medium-density closed-cell foam in Canada is tied to CAN/ULC-S705.1 for the material and CAN/ULC-S705.2 for application. That alone should tell you this is not just “some guy with a truck and a hose.” :contentReference[oaicite:4]{index=4}

Ontario code references these standards and also requires proper protection of combustible foam insulation, which is another reason the details around thermal barriers and finished assemblies cannot be shrugged off. Foam can perform very well, but it is not a free pass around fire protection requirements. :contentReference[oaicite:5]{index=5}

So when you are hiring, do not just ask what the price per square foot is. Ask what product is being used, what standard it complies with, how thickness will be verified, what substrates are acceptable, what trimming and cleanup look like, and what is happening after the foam is installed. Those are the questions adults ask before writing cheques.

Health and safety are not optional talking points

Another thing that gets minimized in casual spray foam conversations is exposure. Health Canada has been very clear that MDI-based polyurethane foam products can pose health risks, including breathing problems and skin sensitization. That does not mean “spray foam is evil.” It means uncured chemical exposure and installation safety are real issues, and anybody treating them like drama is not someone I would want working in my house. :contentReference[oaicite:6]{index=6}

This is one more reason I am not a fan of treating spray foam like a DIY toy or a job for the cheapest bidder. The house needs to be prepped properly, the application needs to be controlled properly, and occupancy timing needs to respect the product and manufacturer guidance. Saving a few dollars by being casual about chemistry is usually a bad personality trait in construction.

Cost, value, and what you are really buying

Spray foam usually costs more than batt or blown insulation, so the value question matters. The smart way to look at it is not “How much more per square foot is it?” The smart way to look at it is “What problem is it solving that cheaper insulation will not solve as well?”

If you are spraying a standard open attic floor that could have been properly air sealed and blown full of insulation, the premium may not be the smartest use of money. But if you are solving a rim-joist air leakage problem, insulating a complicated cantilever, or creating a compact roof assembly that needs high R-value in shallow space, the premium can make a lot more sense.

In other words, spray foam should be judged less like a commodity and more like a specialty tool. Specialty tools are worth it when you are actually doing the specialty job.

What to do before you decide

Figure out the real problem first. Is it heat loss, air leakage, moisture, limited depth, or all of the above?
Ask which spray foam is being proposed. “Spray foam” is not a complete answer.
Look at the assembly, not just the product. Walls, roofs, basements, and crawlspaces all behave differently.
Verify the installer and the product standards. This matters more than with simpler insulation.
Compare against alternatives honestly. Sometimes batt plus careful air sealing is enough. Sometimes it is not.

If you are planning broader efficiency upgrades, remember that Ontario’s current Home Renovation Savings Program includes insulation among eligible upgrade categories, though the exact path depends on the stream and what other work is being done. That is worth checking before the project is finalized. :contentReference[oaicite:7]{index=7}

Where I would use it, and where I would not

If I am dealing with rim joists, ugly transitions, basement headers, awkward cantilevers, or a compact roof assembly that genuinely needs spray foam’s strengths, I am very comfortable using it. It is one of the best tools we have for certain jobs.

If I am staring at a big open attic or a normal wall assembly with no special constraints, I am going to compare it honestly against less expensive insulation options before I let anyone sell me on “premium.” Not because spray foam is bad, but because good building is not about collecting expensive materials. It is about solving problems intelligently.

And if insulation is part of a broader mechanical or energy-upgrade plan, it is worth understanding the whole-house load side properly too. That is where OntarioHeatLoss.ca fits in. Insulation choices, air sealing, and heating design are stronger when they are tied to a real building plan instead of guesswork.

The honest conclusion

Spray foam insulation is a very good product in the right place. It is not a religion, and it is not a status symbol. It is best where you need air sealing, high R-value in tight space, moisture resistance, or help with complicated geometry. It is weaker as a blanket “premium everything” strategy for houses that could have been insulated just fine another way.

The homeowners who get the best results are usually the ones who stop asking “Is spray foam the best insulation?” and start asking “Is spray foam the best solution in this part of my house?” That is a much smarter question, and it usually leads to a much better project.

See Ontario attic insulation guidance See our custom home building work

Ontario homeowner questions

Spray Foam Insulation in Ontario: The Big Questions People Ask Before They Say Yes

Spray foam can work very well in the right place, with the right product, installed by the right crew. It can also become an expensive mess when people treat it like magic in a can. Below is a straight-answer Q&A section built for Ontario homeowners who want facts, not sales talk.

Built for Ontario reality

Health & off-gassing concerns
Ontario code & fire protection issues
Attics, basements, and moisture risk
Cost, resale, and installation mistakes

Builder truth: Spray foam is a tool, not a religion. It is excellent in some places, questionable in others, and unforgiving when installed badly.

Safety & Health

Is spray foam safe for indoor use in Ontario homes?

Yes, properly installed and fully cured spray foam can be safe for indoor use in Ontario homes, including basements, garages, and living spaces. The bigger concern is not usually the cured foam itself. The bigger concern is the installation phase, especially if the product is mixed off-ratio, sprayed onto damp materials, or installed without proper ventilation and re-entry precautions.

Homeowners should treat spray foam like a serious chemical installation, not like ordinary paint. Ask for the exact product name, safety data sheet, installer certification, and re-occupancy instructions in writing.

Safety & Health

How long should we stay out of the house after installation?

For high-pressure two-component spray foam, 24 hours is a common minimum if the installer follows the product instructions and ventilates the house properly. Some homeowners choose 48 to 72 hours to be more cautious, especially if the project is large, the house is tight, or someone in the family has asthma, allergies, or chemical sensitivities.

If strong chemical odours remain after the stated re-entry period, do not assume that is normal. Persistent smell can be a warning sign of poor curing, off-ratio spraying, or bad ventilation.

Safety & Health

Does spray foam release toxic fumes forever?

No. Properly cured spray foam is not supposed to release toxic fumes forever. When the chemistry is correct and the foam cures as intended, it is generally expected to become stable. Ongoing odour complaints usually point to a bad installation, not normal long-term behaviour.

If the foam smells strongly for weeks, causes irritation, or appears brittle, shrunken, oily, or crumbly, that should be treated as a defect issue, not brushed off as “new house smell.”

Safety & Health

What about “forever chemicals” (PFAS)?

This is a legitimate concern. You should not assume all spray foam products are PFAS-free, and you should not assume they all contain PFAS either. Product chemistry varies. Some systems and related materials may involve fluorinated compounds or blowing-agent choices that raise environmental concerns.

The right move is to ask for the exact product data, safety sheet, and manufacturer documentation rather than relying on vague “green” marketing language.

Cost & Value

What’s the real cost per square foot in Ontario?

The honest answer is that it depends on foam type, thickness, access, prep work, and minimum job size. Broadly speaking, open-cell is usually less expensive than closed-cell, and closed-cell is often chosen where higher R-value per inch or stronger vapour resistance matters.

In real Ontario projects, homeowners should expect pricing to vary widely. The mistake is assuming one flat internet number applies to every attic, rim joist, wall cavity, or basement. It does not.

Cost & Value

Can I combine spray foam with fiberglass to save money?

Yes. This is commonly called flash-and-batt. A thin layer of closed-cell spray foam handles air sealing and part of the moisture-control job, while fiberglass batts provide additional R-value at a lower cost.

It can be a smart compromise, but the foam thickness still matters. Too little foam in the wrong assembly can leave the structure vulnerable to condensation. This is not something to guess at.

Cost & Value

Is DIY spray foam a good idea?

For large or important building assemblies, usually no. Small one-component cans for little gaps and penetrations are one thing. Spraying major wall, basement, attic, or roof areas with two-component foam is another.

DIY kits can go wrong quickly if temperature, mix ratio, substrate prep, or application thickness is off. When spray foam goes wrong, the correction is messy and expensive.

Code & Compliance

What are Ontario Building Code requirements?

Ontario does not have one simple “spray foam rule” that covers every situation. Requirements depend on where the foam is being used, what assembly it is part of, how fire protection is being handled, and what compliance path the house is using.

The biggest homeowner-level issue is this: spray foam usually cannot just be left exposed in occupied interior spaces. Fire protection and approved finishes matter.

Code & Compliance

Do I need to remove existing attic insulation?

If you are spraying the underside of the roof deck to turn the attic into an unvented conditioned attic, usually yes, or at least mostly yes. Leaving thick insulation on the attic floor can isolate the attic from the conditioned space below and create a confused assembly that is harder to manage from a moisture standpoint.

In a proper roof-deck spray foam conversion, the attic is no longer supposed to behave like a traditional vented cold attic.

Code & Compliance

Can spray foam be left exposed?

Usually no. In many interior residential situations, spray foam needs a proper thermal barrier or another approved protective system. Drywall is the familiar example, but some tested systems may allow other coverings.

Homeowners get into trouble when they assume exposed foam is acceptable in basements, garages, or utility spaces just because “it’s only insulation.” Code does not look at it that way.

Moisture & Mold

Can mold grow on spray foam?

Spray foam itself is not a favourite food source for mold, but that does not mean a spray-foamed wall or roof is automatically mold-proof. Mold can still grow on wood, drywall paper, dust, and debris if moisture is trapped beside or behind the foam.

Spray foam should never be used to cover up a wet substrate and pretend the problem is solved.

Moisture & Mold

Does closed-cell foam trap moisture?

It can if the assembly is wet when sprayed or if the foam fails to adhere properly and leaves hidden gaps. Closed-cell foam is more vapour-resistant than open-cell foam, which can be useful, but it also means drying potential is reduced if something goes wrong.

In a clean, dry, well-designed assembly, that can be a benefit. In a poor retrofit, it can become a hidden problem.

Moisture & Mold

Is it waterproof for basements?

No. Closed-cell spray foam is not a substitute for proper basement waterproofing, drainage, grading, footing drains, crack repair, sump design, or exterior foundation protection.

If liquid water is entering a basement, fix the water problem first. Spray foam is insulation. It is not a miracle basement cure.

Property & Financial Risk

Will spray foam make my home unmortgageable?

Not automatically in Ontario. There is no blanket Ontario rule saying a house with spray foam cannot be financed. What can happen is greater scrutiny, especially if a foamed roof deck makes a buyer, appraiser, or lender nervous about hidden roof condition or moisture issues.

Good documentation matters. Keep product sheets, installer qualifications, photos, and inspection records.

Property & Financial Risk

Can insurance companies deny claims?

They do not deny claims just because spray foam exists in the house. But non-compliant work, hidden defects, fire-safety issues, and moisture-related failures can absolutely create disputes if the installation was poor or the assembly was not done to code.

Property & Financial Risk

What if the installation goes wrong?

Bad spray foam can be brutal to correct. Off-ratio foam, foam that pulls away, foam that smells, or foam that never cures properly often has to be mechanically removed. That means scraping, cutting, grinding, vacuuming, and sometimes replacing adjacent finishes.

This is one of the reasons installer quality matters so much more with spray foam than with simple batt insulation.

Technical Installation

Open-cell vs. closed-cell—which do I need?

Closed-cell is usually chosen where you need more R-value in less space, stronger vapour resistance, added rigidity, or better performance in moisture-sensitive areas. Open-cell is softer, more vapour-open, often less expensive, and can work well where sound control and drying potential matter more.

The answer depends on the assembly, not on whichever product the salesperson happens to prefer.

Technical Installation

How thick should it be?

There is no one honest answer for every application. Thickness depends on the foam type, available cavity space, condensation risk, and the energy target for that assembly. A basement rim joist, a wall cavity, and a roof deck are not all asking for the same thing.

That is why “just spray 2 inches everywhere” is not real design. It is guesswork.

Technical Installation

Should I close my ridge vents when foaming the roof?

If you are intentionally converting the attic into an unvented conditioned attic by spraying the underside of the roof deck, then yes, the traditional vent paths are not supposed to keep working as if the attic were still outside the building envelope.

This should be reviewed before work begins, not after the foam truck leaves.

Technical Installation

Can spray foam damage my home?

Poor installation can absolutely create damage or expensive complications. Small foams can over-expand around windows and doors. Larger spray foam jobs can shrink, pull away, trap hidden moisture, hide leaks, or make future repairs to wiring and plumbing much harder.

The bigger real-world risk is not the dramatic internet horror story. It is concealed defects and expensive correction work.

Environmental & Removal

Is spray foam environmentally friendly?

It can reduce air leakage and improve energy performance, but that does not automatically make it environmentally clean. Spray foam is a petrochemical product, difficult to remove, hard to recycle, and some formulations raise concerns about blowing agents or fluorinated chemistry.

The honest answer is that it can be useful where it solves a real problem well. That is different from calling it universally green.

Environmental & Removal

How do I remove it if there’s a problem?

Usually by cutting, scraping, chiselling, grinding, and making a mess. There is no easy chemical reversal. Bad spray foam removal is labour-heavy, disruptive, and often expensive. That is why homeowners should be much pickier about the installer than they think they need to be.