Cementitious fireproofing vs intumescent coating: how to choose

The choice between cementitious fireproofing and intumescent coating starts with the type of fire to be covered: a cellulosic building fire (CAN/ULC-S101, ASTM E119 or UL 263 tests) or a rapid-rise hydrocarbon fire (UL 1709). Then come the desired appearance, exposure, the risk of corrosion under fireproofing and ease of repair. In every case, thickness and system follow from a listed design for that fire type, never from a decision made on site.

Plant framing, a pipe rack, a vessel skirt or an equipment support can require fire protection. Two main families exist: sprayed cementitious products, thick and mineral, and intumescent coatings, thin films that swell under heat. This guide helps facility managers and project managers in Quebec and Ontario understand the differences before starting intumescent coating or fireproofing work.

What is the difference between cementitious and intumescent fireproofing?

The difference lies in how they work: cementitious fireproofing insulates the steel with a thick mineral layer that is already in place, while intumescent coating is a thin film that swells when heated to form an insulating char. Both delay the steel's temperature rise to its critical temperature.

• Cementitious fireproofing. A sprayed cement- or gypsum-based mortar with lightweight aggregates; medium-density versions for interiors and high-density versions for industrial and exterior areas.
• Water-based or solvent-based intumescent. A thin film mainly for cellulosic fires, indoors, often on exposed steel.
• Epoxy intumescent. A thicker, harder film designed in particular for hydrocarbon fires and exposed industrial environments.

Our article on intumescent coating fire ratings explains how thickness is determined.

Cellulosic or hydrocarbon fire: what difference does it make?

It changes which products are acceptable, because the tests do not reproduce the same fire. A cellulosic fire, the building fire, rises in temperature progressively; a hydrocarbon fire, such as a pool fire in a refinery or chemical plant, reaches high temperatures very quickly.

• Cellulosic fire. The standard curve of CAN/ULC-S101, ASTM E119 or UL 263. Most building cementitious products and thin intumescents are evaluated this way.
• Hydrocarbon fire. The rapid-rise curve of UL 1709. Only products evaluated to that test are suitable: high-density cementitious products or epoxy intumescents intended for that service.
• Jet fire. Some facilities also require evaluation per ISO 22899-1, for resistance to a pressurized flaming jet.

A product evaluated only for cellulosic fire must not be used where the risk analysis anticipates a hydrocarbon fire.

How do the two families compare?

The two families compare on eight practical criteria. The table gives general trends; the listed design and the manufacturer's data sheet decide for each project.

In practice, many plants use both: high-density cementitious on columns and skirts at grade, where impact and hydrocarbon exposure are highest, and intumescent on exposed or hard-to-reach members higher up. The split is decided by the project designer from the fire risk analysis, the required rating and access for future maintenance, and the applicator then follows the listed design for each member.

Why is corrosion under fireproofing a risk?

Corrosion under fireproofing is a risk because it progresses out of sight: porous cementitious fireproofing can absorb water and chlorides and hold them against the steel, like wet thermal insulation. The mechanism is similar to the one described in our article on corrosion under insulation.

Usual measures to limit it:

• Compatible primer. The anticorrosion primer under the fireproofing must be part of the listed design or accepted by the manufacturer; a primer that is not accepted can compromise adhesion and fire performance.
• Sealer or topcoat. Outdoors and in damp areas, a compatible topcoat closes the porosity of the cementitious layer.
• Details. Terminations, interfaces with concrete, base plates and supports are detailed so that water cannot get in and stay.
• Inspection. Cracks, disbondment and rust stains signal water ingress to investigate.

The anticorrosion system under the fireproofing is chosen like any protective coatings system, for example from ISO 12944 C3, C4 and C5 paint systems.

How are these systems prepared and applied?

Both systems need clean steel, a compatible primer and thickness-controlled application, but their execution differs. Intumescent is applied like paint in several coats; cementitious is sprayed as a thick layer, sometimes over mesh.

• Preparation. Blasting to the grade the system requires, often SSPC-SP 6 or SP 10 under an epoxy intumescent; see our NACE 2 / SSPC-SP 10 guide.
• Ambient conditions. Temperature, humidity and dew point per the data sheet; cementitious products must not freeze while setting.
• Application. Intumescent in successive passes up to the prescribed dry film thickness; cementitious by spraying, then thickness and density checks.
• Protection. Treated members are protected from impact and water until fully cured.

Our steel structure painting service often coordinates primer, fireproofing and topcoat on the same job.

How are thickness and quality checked?

Thickness is checked member by member and each reading is compared with the design's thickness schedule, then adhesion and integrity are verified. Methods differ by family.

• Cementitious. Thickness and density per ASTM E605, cohesion and adhesion per ASTM E736.
• Intumescent. Dry film thickness measured with a magnetic or electromagnetic gauge, following the measurement plan in the specification or AWCI 12-B.
• Both. Verification of primer, topcoat and the absence of cracks, disbondment and thin areas; recorded in a log.

Our coating inspection service can take these measurements and produce the report.

How are repairs and maintenance done?

Repairs are made with the same product or one accepted by the design, after removing the damaged part, and are then checked like the rest. In a plant, damage comes mainly from impact, mechanical work and water ingress.

• Cementitious. Clean cut around the damage, cleaning, bonding agent if specified, spray or trowel application of the same material, thickness check.
• Intumescent. Feathered sanding, compatible touch-up primer, rebuild in coats to the prescribed thickness, compatible topcoat.
• Compatibility. Any new topcoat over an existing intumescent must be accepted by the manufacturer; see our article on painting over old coatings.

How do you get a quote?

Send the required fire-resistance rating, the fire type, the listed design if one exists, the list of members, the exposure and the condition of the steel. Pricing is on consultation: it depends on the product family, quantity, access, preparation and checks. Our crews and qualified subcontractors work in particular in Montreal and Bécancour. For our intumescent coatings service, send your documents through the request a quote form: reply within 24 business hours.

Frequently asked questions

Can cementitious fireproofing be replaced with an intumescent coating?

Yes, if a listed intumescent design covers the same fire-resistance rating, the same fire type and the same members. The project designer has to approve the change.

Is thin-film intumescent suitable outdoors?

Generally not, unless the manufacturer accepts it with a specific topcoat. For exterior and exposed industrial environments, epoxy intumescents or high-density cementitious products are more common.

Why UL 1709 for a refinery or chemical plant?

Because these facilities carry a risk of hydrocarbon fire, which is faster and more intense than a building fire. UL 1709 reproduces that type of fire; a product evaluated only for cellulosic fire is not enough.

Is a primer needed under cementitious fireproofing?

It depends on the listed design. Some accept bare steel in dry interiors, others require a specific primer. In damp or corrosive environments, an accepted primer is generally specified.