Isocyanates and Spray Foam in the Homes You Enter

Why this matters

Spray polyurethane foam (SPF) is now in a large share of the attics, crawlspaces and rim joists that HVAC, plumbing, electrical and pest techs work in, and most of the time it is harmless to crawl past. The exception is the week it goes in. Freshly applied foam releases isocyanates, and isocyanates are one of the leading causes of work-related asthma. The harm is not a bad afternoon: a person who becomes sensitized can react for the rest of their career to amounts too small to smell, which can end a trade. The tech at risk is usually not the foam installer, who wears supplied air. It is the next trade through the door, who was never told.

What is actually in the foam

SPF is mixed at the gun from two liquids:

• The A-side is the isocyanate, in residential foam almost always methylene diphenyl diisocyanate (MDI) or its polymeric form. This is the sensitizer.
• The B-side is a polyol blend carrying catalysts (often amines, which give fresh foam its fishy smell), blowing agents, flame retardants and surfactants.

When the two meet, the isocyanate reacts with the polyol and the foam rises and hardens. The isocyanate is consumed by that reaction, but not instantly and not completely while the foam is fresh, and the high-pressure spray that applies most whole-house foam throws an aerosol of mixed and unmixed droplets into the air of the space.

Open-cell foam (soft, light, usually on roof decks) and closed-cell foam (rigid, dense, usually on rim joists, crawl walls and some roof decks) share the chemistry. One-component foam from a can and low-pressure two-component kits are smaller exposures by volume but contain the same class of chemical, so a homeowner's weekend kit job is not automatically benign.

Sensitization: the one-way door

Sensitization is the immune system learning to react to a chemical. The first exposures may cause nothing, or a cough and watery eyes that clear up. Once a person is sensitized, later exposure at far lower levels can trigger asthma attacks, chest tightness and, in severe cases, life-threatening reactions. It does not reliably reverse, and the usual medical answer is permanent removal from exposure.

Two routes matter:

• Inhalation of aerosol or vapor during and soon after spraying is the main route to respiratory sensitization.
• Skin contact with uncured foam or overspray can cause skin sensitization, and NIOSH notes evidence that skin exposure may contribute to respiratory sensitization as well. A glove is part of the control, not an extra.

The vocabulary for dose and route belongs to its own card. See related: Dose, Route and the Exposure You Cannot See.

The limits, and why they do not help you much

OSHA has no isocyanate-specific standard. MDI appears in the air contaminant tables at a ceiling of 0.02 ppm (29 CFR 1910.1000 Table Z-1 for general industry, 29 CFR 1926.55 for construction). NIOSH recommends an 8-hour limit of 0.005 ppm with a 10-minute ceiling of 0.02 ppm. OSHA has run a National Emphasis Program on isocyanate exposures since 2013 because these numbers are hard to monitor and easy to exceed.

The practical problem is that a sensitized person can react below any of those numbers, and nobody on a service call has a direct-reading isocyanate monitor. So the field control is not measurement. It is time, ventilation and not being there.

Fresh foam and cured foam are different hazards

The re-occupancy window is the time after spraying before people other than the applicator crew should re-enter, with the space ventilated throughout. It is set by the foam manufacturer for that product and application method, and appears in the installer's instructions and product documentation, not in a code. For high-pressure two-component residential foam, 24 hours is a common figure, and some manufacturers specify longer; EPA's guidance is to follow the manufacturer and the installer. Ask for the figure. Do not use the common one.

Off-ratio foam is foam mixed with the wrong proportion of A-side to B-side, usually from equipment trouble. It may never cure properly, can stay sticky or crumbly, and can off-gas odors for weeks. A strong chemical or fishy odor well past the window, or foam that is tacky to a gloved touch, is a defect for the installer and the homeowner, not something to work around.

Cutting and heating cured foam

Cured foam has two ways to hurt you that fresh foam shares:

• Cutting, trimming or grinding it to run a line set, a drain or a cable makes dust. The dust is a particulate respiratory and eye irritant, and freshly cut foam that is not fully cured inside can expose new material. Cut with a knife or a hole saw rather than a grinder, wear safety glasses and gloves, and for more than a few cuts in a tight space wear at least a filtering facepiece respirator. Whether the shop's respirator program has to cover that is its own question. See related: How to Decide Whether a Task Needs a Respirator.
• Heating it is the more serious hazard. Polyurethane foam is combustible, and decomposing or burning foam releases isocyanates, hydrogen cyanide and carbon monoxide. No torch, heat gun or brazing flame within reach of exposed foam. Codes require foam in attics and crawlspaces to be covered by a thermal or ignition barrier unless the product is specifically tested to go without one (IRC Section R316, as adopted by your jurisdiction), and a tech who brazes next to foam that barrier was never installed over has the fuel and the ignition in the same square foot.

What to ask before anyone goes in

The office can find out most of this when the call is booked, which is cheaper than finding out at the attic hatch.

OFFICE:   Has any spray foam insulation gone in at the
          house recently? Attic, crawlspace, rim joists?
CUSTOMER: Yes, they did the crawl on Tuesday.
OFFICE:   Do you know what time they finished, and did
          they leave you any paperwork on re-entry?
CUSTOMER: Around four. There's a sheet on the counter.
OFFICE:   Could you read me the part about when people
          can go back into the space, or send a photo?
CUSTOMER: It says 24 hours with the fans left running.
OFFICE:   Thanks. Is there still a strong smell down
          there, or is it mostly gone?
CUSTOMER: Faint. Their fans are still running.
OFFICE:   Okay. We'll book after four on Wednesday at
          the earliest, and the tech will look before
          going in. Who installed it, in case we need
          to ask them anything?

The questions that matter: whether foam went in, where, when it was finished, the manufacturer's re-entry time in writing, whether ventilation ran through the window, whether odor is still strong, and who installed it. The answers go on the ticket.

A worked example

A plumber is scheduled to replace a cracked cast-iron cleanout fitting in a crawlspace. Closed-cell foam was sprayed on the crawl walls and rim joists. The customer reads the installer's sheet: re-occupancy 24 hours after application with exhaust ventilation running (illustrative; read the real sheet). Application finished at 4:00 p.m. Tuesday. The plumber's first open slot is 9:00 a.m. Wednesday.

Is 9:00 a.m. inside the window? 4:00 p.m. Tuesday to 9:00 a.m. Wednesday is 17 hours. The window is 24, so 9:00 a.m. is 7 hours short. The office moves the call to Wednesday after 4:00 p.m., and the plumber's first slot after that is 8:00 a.m. Thursday, 40 hours after application.

At the hatch on Thursday. The installer pulled the ventilation fans Wednesday evening, after the 24 hours had run. The plumber notes a faint odor and checks the foam with a gloved finger at the hatch: hard, not tacky. That is cured foam behaving normally, so the crawl is entered with the plumber's usual crawlspace kit: coveralls, nitrile gloves, knee pads, safety glasses, and the crawlspace's own hazards (biological and confined-space considerations) handled as the shop does any crawl.

The work that touches foam. The cleanout sits behind a sprayed rim joist where foam overspray has bridged onto the pipe. The plumber trims about a square foot of foam away with a utility knife, bags the trimmings, and does not use the oscillating tool, which would throw dust in a space with the plumber's face a foot from the cut. The repair is a mechanical coupling, so there is no heat. Had the joint been copper needing a soldered repair against the foam, the plumber would have cut the section out and made the joint outside the crawl, or shielded the foam and kept an extinguisher at hand, rather than putting a torch near foam with no barrier over it.

What would have changed the answer. If the odor at the hatch had been strong and the foam tacky 40 hours out, the plumber does not enter, photographs what is visible from the hatch, and the office calls the installer and the customer; the cleanout can wait, and the customer's family has the same exposure problem the plumber does. If the customer had no paperwork and could not reach the installer, the office does not guess a number; it books after the installer has been reached or the customer has the manufacturer's sheet in hand. If a tech already has asthma or has reacted to foam before, that tech is not sent into recently foamed spaces at all, whatever the window says.

References

• NIOSH, isocyanates topic page and recommended exposure limits for MDI
• EPA, Spray Polyurethane Foam (SPF) guidance for homeowners and contractors on re-entry and ventilation
• OSHA National Emphasis Program on Occupational Exposure to Isocyanates; 29 CFR 1910.1000 Table Z-1 and 29 CFR 1926.55
• International Residential Code Section R316, foam plastic, as adopted by your jurisdiction
• See related: The Attic Entry the Day After the Spray Foam; The Hierarchy of Controls on a Service Call