Spray foam insulation has become a popular choice for home projects and renovations, well-known for its energy efficiency, air sealing, and ability to fill hard-to-reach spaces. When applied correctly, it performs exceptionally well, forming a durable, airtight barrier that can last for decades.
But spray foam is also chemistry in motion. It relies on two reactive components that must be mixed at precise ratios and temperatures to form a stable, cured material. If that chemistry drifts even slightly off-balance, the results can include persistent odors, soft or tacky foam, and poor indoor air quality.
OFF – RATION MIXING: THE ROOT OF MOST SPRAY FOAM FAILURES
A Carrier HVAC unit installed in an attic surrounded by spray polyurethane foam insulationSpray polyurethane foam (SPF) depends on an exact chemical balance between its two liquid components — the A-side (isocyanate) and the . When these are properly proportioned, they react through an exothermic chemical reaction, to create a dense, cross-linked polyurethane matrix that hardens within seconds.
If that ratio is off — even slightly — the reaction may not go to completion. Too much A-side can leave unreacted isocyanates, which are potent respiratory sensitizers capable of lingering in the foam and off-gassing over time. Too much B-side, on the other hand, can trap excess polyols and amine catalysts, producing a persistent “sweet” odor and soft, under-cured foam.
According to the U.S. Environmental Protection Agency (EPA) and the National Institute of Standards and Technology (NIST Technical Note 1921), off-ratio foam can release elevated levels of amines, aldehydes, and residual isocyanates — all of which affect both the foam’s structural performance and indoor air quality.
HOW TO IDENTIFY OFF-RATIO FOAM
Common field indicators include:
- Odor: A lingering chemical or sweet smell that persists beyond the first few days after installation.
- Texture: Foam that remains soft, tacky, or crumbly when touched or cut.
- Appearance: Uneven color, glossy patches, or large, irregular cell structures.
- Instrument readings: Elevated VOCs detected by a photoionization detector (PID) or other air quality meters.
To confirm, professionals may perform VOC sampling using EPA Method TO-15 or collect a foam core sample for laboratory analysis.
HOW TO FIX IT
If off-ratio foam is confirmed, remediation should focus on removing the defective material and restoring proper conditions for re-application:
- Mechanical Removal
- Cut out and remove all visibly soft, sticky, or uneven foam down to clean substrate.
- Avoid re-spraying over compromised areas; trapped, uncured foam will continue to emit VOCs.
- Re-Application Under Controlled Conditions
- Recalibrate the proportioning equipment (verify temperature, pressure, and feed rate).
- Perform a small test spray to ensure proper density, texture, and odor.
- Apply the foam in thin layers so each section can cure fully before adding more.
- Ventilation and Monitoring
- Increase ventilation during and after re-application using an ERV/HRV or temporary negative air machines.
- Monitor air quality after curing, especially if occupants report odor or irritation.
- Verification
- Optional post-remediation testing (VOC or formaldehyde sampling) can confirm that emissions have returned to safe baseline levels.
BOTTOM LINE
Spray foam is an efficient and effective insulation — but it’s also a chemical reaction that demands precision. When mixed off-ratio, it can turn from a performance upgrade into an indoor air liability. The fix isn’t just cosmetic; it’s chemical. Identifying, removing, and reapplying under proper conditions ensures both the longevity of the foam and the health of the indoor environment.
For more on spray foam types and their chemical makeup, check out the EPA’s guide: EPA Spray Polyurethane Foam Product Types (PDF).
If you’re unsure about your home’s air quality or suspect chemical odors after a renovation, Full Spectrum Environmental can help. Our certified specialists provide detailed air quality testing and clear recommendations so you can breathe easy with peace of mind.
