---
title: "Core Foam Vs K-13: The Practical Wall Assembly Guide"
description: "Core Foam vs K-13: one is injected into closed masonry cores, one is sprayed on exposed decks. Identify your wall assembly and the product picks itself."
url: "https://bahlfireproofing.com/core-fill-vs-k-13-wall-assembly/"
---

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Field notes and code guides

# Core Foam vs K-13: The Practical Wall Assembly Guide

- [Ross Bahl](https://bahlfireproofing.com/author/rossbahl/)
- October 21, 2026
- 10:00 am

Core foam vs k-13: the practical wall assembly guide 2

Bahl Fireproofing installs both Core Foam and K-13 sprayed fiber, so we get asked to price them against each other on the same building. Core Foam vs K-13 is not really a product comparison, because the two never occupy the same place in a wall. This article covers what each one is, which assembly each belongs to, and how to tell which assembly you have.

**TLDR:** Core Foam and K-13 are not competing for the same job. Asking Core Foam vs K-13 is almost always a symptom that the wall assembly has not been decided yet, and once it has been decided the product picks itself. One is a foam injected into hollow block cores nobody will see. The other is a fiber sprayed onto a surface everybody sees.

---

## The Wall Section Nobody Finished Drawing

The costly version of this decision is not buying the weaker material. It is pressure-injecting foam into a cavity that was built to drain. Brick veneer over a backup wall with a clear air space is drained on purpose: water that gets past the brick runs down its back face, lands on through-wall flashing, and leaves at the weeps. Fill that space and the water has nowhere to go.

None of that shows on a punch list. What surfaces two or three winters later is efflorescence, spalling at the base of the wall, and staining on the interior finish, and the correction by then is taking brick down. We still get asked for it, and we turn it down. The request usually comes from somebody who read the phrase masonry cavity fill and matched the word cavity to the air space in front of them. Core Foam vs K-13 never gets asked on that wall, because the answer is neither one.

## Core Foam vs K-13: Two Products With Almost Nothing in Common

Core Foam Masonry Foam Insulation is a two-part aminoplast foam from cfiFOAM, Inc. of Knoxville, Tennessee. A spray-dried resin and a foaming catalyst concentrate are mixed by the installer, combined with compressed air, and pressure-injected into the hollow cores of concrete block. cfiFOAM describes it as non-expanding, the ICC-ES listing gives a nominal density of 1.0 pound per cubic foot, the product sheet reports shrinkage under 0.5 percent, and once it is in, it is never seen again.

K-13 is a spray-applied thermal and acoustical insulation from International Cellulose Corporation of Houston, applied wet to a surface and usually left as the finished ceiling. It carries R-3.7 per inch, bonds at greater than 150 pounds per square foot tested to ASTM E736, and runs a nominal 3.5 pounds per cubic foot per UL ER5499-01, revised September 19, 2016. Five standard colors are published, Black, Gray, Light Gray, White, and Tan, plus custom matching.

One naming trap belongs here, since this article cites both. International Cellulose Corporation abbreviates itself ICC. So does the International Code Council, publisher of the building code quoted below. They are unrelated, and submittals that blur them cost review time.

Notice how little of one description touches the other. The separation is structural, and the foam maker draws it: not for horizontal applications, not for flotation, not a waterproofing material, and never injected into a wet wall. The first of those alone rules out the decks and ceilings where sprayed fiber goes.

## What Actually Goes Wrong When Somebody Asks Me Core Foam vs K-13

Running insulation crews across Texas, Kansas, and Oklahoma for more than twenty years has taught me that this question arrives in a predictable shape, and the shape tells me more than the question does. People assume the risk is picking the weaker material. The material is almost never what fails. What fails is an unresolved wall section, with a product question standing in for a design decision.

The tell is the vocabulary. Callers who can describe the assembly do not ask. Nobody who says “8-inch hollow block, cores open, no reinforcing above the bond beam” needs a comparison. The question shows up when a building gets described by what it does instead of how it is built: a warehouse, a gym, a school addition. Several times a year we open a bid set with one line reading spray-applied insulation and a wall schedule holding both filled block and exposed deck.

That is preventable on paper, before a hole is drilled. Someone has to read the wall section and the roof section and say which planes are closed and which stay exposed.

## Both Products’ Performance Is Set by What They Are Attached To

Here is what makes a head-to-head spec sheet misleading. Neither headline number describes what the building gets, and the two are not even the same kind of number.

Start with the foam. R-4.6 per inch at 75F mean temperature is the material figure under ASTM C518-21, per ICC-ES listing ESL-1648. The wall figure comes from the series-parallel method, also called isothermal planes, because the block webs run face to face and carry heat around the foam. cfiFOAM publishes those values in its [Thermal Properties Data Handbook for Core Foam filled block walls](https://www.cfifoam.com/wp-content/uploads/2nd-Edition-cfiFOAM-Thermal-Handbook-May-2025.pdf). One 8-inch unit with three 1-inch webs and Core Foam in the cores calculates at R-8.21 in 85 pound per cubic foot block and R-4.02 in 135 pcf block. Same unit, same fill, and half the resistance went into the concrete. The R-5.0 per inch at 25F mean temperature figure is a cold-side test point, not a design value.

Now the fiber. Absorption is measured under [ASTM C423, sound absorption and sound absorption coefficients by the reverberation room method](https://store.astm.org/c0423-23e01.html), and the result belongs to a mounted assembly, not a material. At 1.5 inches over solid backing, K-13 tests at NRC 0.90. At the same thickness over 1.5-inch metal deck, it tests at 1.05. Over solid backing the curve is not even a straight climb: 0.80 at 1 inch, 1.00 from 1.75 through 3 inches, 1.05 at 4 inches, back to 1.00 at 5 inches.

So remember two numbers instead of two products. Block density moves a masonry wall by a factor of about 2 at one unit size. Deck profile moves sprayed fiber from 0.90 to 1.05 at one thickness. In Core Foam vs K-13 the performance belongs to whatever the material is attached to, which is why the table below maps position instead of picking a winner.

## Core Foam vs K-13 Side by Side

The comparison uses published test data and the manufacturers’ own limits. Read it as a mismatch map, because most rows disqualify one product for the other’s job.

Factor Core Foam K-13

What it is Two-part aminoplast foam: spray-dried resin and foaming catalyst, mixed by the installer Cellulosic fiber and adhesive binder, sprayed on wet

Where it goes Inside the hollow cores of concrete block walls, never the cavity between wythes On the face of steel deck, steel framing, masonry, or gypsum

Concealed or exposed Concealed permanently, behind patched holes Exposed by design; it is the finished surface

New or existing building Both; we fill standing block walls through drilled holes Both; retrofit needs a clean, accessible surface

Installed by whom Installers trained and authorized by cfiFOAM, per its guide specification Contractors licensed by International Cellulose

R-value per inch R-4.6 at 75F mean temperature, and R-5.0 per inch at 25F mean temperature R-3.7

What sets real performance Unit size, block density, web geometry, and grouting Thickness and substrate profile

Acoustic metric and what it measures STC 54 to 60 in 8-inch block walls per the cfiFOAM product sheet, test standard not stated: sound blocked between spaces NRC by thickness and substrate, ASTM C423: sound absorbed in a room

ASTM E84 result Flame spread 25 or less, smoke developed 450 or less, per ICC-ES ESL-1648 Flame spread 5, smoke developed 5

Fire-resistance rating contributed No. The manufacturer says it neither adds to nor detracts from a CMU wall’s rating No. Listed as allowable sprayed fiber over fire-resistive materials

Thermal barrier requirement A foam plastic, so Chapter 26 obligations attach None additional as an exposed interior finish

Temperature limit No extended contact with surfaces above 190F, which is 88C None published in the current product data

Orientation limits Not for horizontal applications, flotation, or waterproofing Overhead routinely; 5 inches maximum in one application without mechanical support

The disqualifying condition The cores are grouted, obstructed, or wet, or the space is a cavity between wythes The surface will not stay exposed

Work the last three rows first. Orientation, temperature, and the disqualifying condition eliminate the wrong material faster than any performance number.

## Which Assembly Do You Actually Have?

Four assemblies cover most of what we walk into, and each answers the question by itself.

Assembly Core Foam K-13

Hollow CMU, cores open, wall closed or closing Candidate Not applicable

Exposed steel deck, framing, or architectural ceiling Not applicable Candidate

Brick veneer over a drained air space No, we do not fill the air space No, there is no exposed interior surface

Fully grouted or solid wall No, there is nothing to fill Only as an exposed face treatment

On row one the fiber is out because no exposed surface is in play, and grouted cells and bond beams are not fillable, so confirm the cores are open before pricing. On row two the foam is out on the manufacturer’s own terms: no horizontal applications and no finish role. A steel stud cavity is a fifth case belonging to neither, and that is spray foam territory. Once the row is identified, Core Foam vs K-13 stops being a question.

## Can Core Foam and K-13 Go on the Same Building?

Yes, and in mixed construction that is usually the right answer. They serve different assemblies on one job: foam inside the concrete block perimeter wall, fiber on the exposed steel deck overhead. Neither affects the other, because they never share a plane.

A single-story school addition is the version we run most. The exterior walls are 8-inch hollow block, and the roof is bar joists over a 1.5-inch acoustic-profile deck the architect left open and painted, because the dropped ceiling came out of the budget and the structure became the design. That building takes Core Foam in the walls and fiber on the deck, two scopes under one mobilization.

What trips general contractors is sequencing, not selection. Core Foam goes in after the wall stands and before finishes close off the drill line, and its submittal is block density, unit size, and open cells. Fiber goes on after the building is dried in and the overhead trades are done, and its submittal is color, thickness, and deck profile. Hang a pipe through cured fiber later and you leave a patch visible from the floor.

## What the Code Does With These Two Materials

The building code does not separate these products the way a data sheet does. Both route through the same section, and it imposes identical surface-burning limits on each. What differs is the installed condition, which is another way of saying the assembly decides.

The current edition is the 2024 IBC, and our three states are not all on it, so name the edition whenever you cite a section. Section 807 covers insulation and sends insulating materials to Section 720, which splits by condition: 720.2 concealed, 720.3 exposed. That split gets described as an exception pair, and it is not one. Both carry the same limits, a flame spread index of not more than 25 and a smoke developed index of not more than 450, and the 720.2 exception covers cellulosic loose-fill, which neither product is. Fiber on an exposed deck lands in 720.3, foam inside block cores in 720.2. The assembly chose, not the chemistry.

Keep tested results separate from code thresholds. Tested to [ASTM E84 surface burning characteristics of building materials](https://store.astm.org/e0084-26a.html), K-13 returns a flame spread index of 5 and a smoke developed index of 5. Core Foam, per ICC-ES listing ESL-1648, has a flame spread index of 25 or less and a smoke developed index of 450 or less. Class A interior finish is a separate threshold at Section 803.1.2 of the 2024 IBC: flame spread in the 0 to 25 band, smoke developed in the 0 to 450 band. Chapter 26 adds separate foam plastic obligations, including separation from the interior at Section 2603.4 of the 2021 IBC. Whether that reaches foam sealed inside masonry cores is a call for the licensed architect or engineer and the authority having jurisdiction, or AHJ.

None of those numbers is a fire-resistance rating. Fire resistance is measured in hours on a complete assembly under ASTM E119 or UL 263, and neither material provides one. K-13 is not a fireproofing product; its manufacturer lists it as allowable sprayed fiber applied over fire-resistive materials. cfiFOAM is equally direct: its masonry foam neither adds to nor detracts from the fire-resistance rating of a CMU wall. Confirm the edition and your occupancy’s finish class with the AHJ.

## What Changes in Texas, Kansas, and Oklahoma

This decision has a thermal half and an acoustic half, and only the thermal half moves when you cross a jurisdiction line. Concrete block is the dominant institutional and commercial wall type in all three states we serve, and exposed steel deck is the dominant warehouse and big-box roof, so the mixed building is the common case here, not the exception.

The energy code is where the states split. Texas enforces the 2015 IECC, effective November 1, 2016 under 34 Texas Administrative Code 19.53(a). Oklahoma’s adopted commercial standard is the 2006 IECC with ASHRAE 90.1-2003 and state amendments, effective September 14, 2022, a 2022 effective date on a 2006 edition. Kansas names the 2006 IECC as its standard for new commercial and industrial structures under K.S.A. 66-1227, and Kansas is a home-rule state, so adoption sits locally. Home-rule cities amend upward: Dallas enforces the 2021 IECC while Fort Worth holds the 2015, so one block wall gets reviewed against two editions twenty miles apart.

A foam-filled block wall carries one more wrinkle. Prescriptive mass wall rows are written in continuous insulation, meaning uninterrupted by framing or webs, and foam inside cores is interrupted at every web, so it generally will not satisfy a prescriptive row alone. Verify the value for your climate zone and assembly against the edition your building official enforces, and read the zone off the county table, not a map graphic. Wichita is Zone 4A, Houston 2A, Dallas-Fort Worth, Oklahoma City, and Tulsa 3A.

None of that touches the ceiling. No energy code sets a sound absorption requirement, so thickness and substrate on the deck come from the owner’s program and the architect’s design.

## Specifying Core Foam vs K-13 Without Guessing

Treat this as a paper exercise first. This is the order we work in, and design teams are welcome to borrow it.

1. **Open the wall section and the roof section before the product literature, and mark every plane closed or exposed.** Closed planes are candidates for injected foam, exposed planes for sprayed fiber. A plane that gets furred later needs a decision, not a product.
1. **For masonry, pull block density off the approved submittal and map which cells are open.** Bond beams, grouted cells, and mortar droppings stop the foam, and a blocked cell leaves a void nobody can see.
1. **For exposed surfaces, get the deck profile before pricing acoustics.** Flute depth moves absorption more than an extra half inch does, and an NRC quoted without a thickness and substrate is unusable.
1. **Run the exclusions before the numbers.** Nothing horizontal for the foam, no wet walls, no extended contact above 190F, and nothing in the cavity between wythes.
1. **Confirm the adopted code edition with the AHJ, then break the scopes out separately in the bid.**

Bring in a licensed architect or engineer wherever the foam’s Chapter 26 obligations, a rated assembly, or a moisture detail is in play. We will not print a per-square-foot figure for either product, because no citable published installed price exists for Core Foam or sprayed fiber in Texas, Kansas, or Oklahoma. Cost turns on unit size, fillable cells, deck profile, and access height, so get a bid.

## Related Reading

- [K-13 Acoustic Performance: NRC Ratings by Thickness](https://bahlfireproofing.com/k-13-acoustic-performance-nrc-ratings/) goes deeper into the thickness and substrate pairs.
- [K-13: The Complete Contractor Guide](https://bahlfireproofing.com/k-13-the-complete-contractor-guide/) is the hub for the exposed-surface half of this work.
- [Commercial Spray Foam Insulation Guide](https://bahlfireproofing.com/commercial-spray-foam-insulation-guide/) covers the steel stud and metal building assemblies that belong to neither.

Our [Core Foam masonry insulation guide](https://bahlfireproofing.com/core-foam-masonry-insulation/) covers the foam in full.

## Frequently Asked Questions

**What is the difference between Core Foam and K-13 insulation?** Core Foam is an aminoplast foam injected into the closed cores of a masonry wall, where it stays concealed. K-13 is a cellulosic fiber sprayed onto an exposed surface and left as the finished ceiling. One fills hidden block cores and the other is an exposed finish, so they do not substitute for each other.

**Can Core Foam and K-13 be used on the same building?** Yes, and in mixed construction that is often correct. A block perimeter wall with an exposed steel deck roof above it takes foam in the wall and fiber on the deck. They occupy different assemblies, so price them as two scopes with separate submittals.

**Can Core Foam be sprayed on a ceiling or an exposed deck?** No. cfiFOAM states the foam is not for horizontal applications, and it is injected into enclosed block cores rather than sprayed onto a surface. It is not a finish material. An exposed overhead surface is a sprayed fiber or spray foam job.

**Does either product give my wall a fire-resistance rating?** No. Both have surface-burning results under ASTM E84, which measures how a surface behaves in a tunnel test. Fire resistance is a different property, measured in hours on a complete assembly under ASTM E119 or UL 263, and a rated wall has to be confirmed against the listed assembly.

**Which one has the higher R-value per inch?** The masonry foam, at R-4.6 per inch at 75F mean temperature under ASTM C518-21, against R-3.7 per inch for the sprayed fiber. That means little alone, because the foam sits inside a block wall whose value is set by unit size and density.

**Can I fill the air space behind brick veneer with Core Foam?** No. We do not fill the cavity behind a brick veneer or between two wythes, in new construction or old. Water passing the brick is meant to run down the back face to flashing and out the weeps, and filling that path traps moisture against the backup wall. Insulation for that wall goes in as board in the cavity while it is built, or on the inside.

## Key Takeaways

**Core Foam vs K-13 is an assembly question, not a product question.**

- One is injected into hollow block cores and never seen; the other is sprayed on an exposed surface and is the finish.
- The question usually signals an unresolved wall section, not a real choice.

**Performance belongs to whatever the material is attached to.**

- The same 8-inch unit with the same fill runs R-8.21 in 85 pcf block and R-4.02 in 135 pcf block, about a 2 times swing.
- The same 1.5 inches of fiber reads NRC 0.90 over solid backing and 1.05 over 1.5-inch metal deck.
- Quote R-value with the unit and density, NRC with the thickness and substrate.

**Two disqualifying conditions decide most projects.**

- The foam cannot go into horizontal applications or a wet wall, or see extended contact with surfaces above 190F.
- Filling a cavity built to drain is a moisture failure, not a thermal upgrade.

**One code section, two installed conditions.**

- In the 2024 IBC, Section 807 sends insulation to Section 720, split into 720.2 concealed and 720.3 exposed, both carrying a flame spread index of not more than 25 and a smoke developed index of not more than 450.
- Tested results are not thresholds, and neither material carries an ASTM E119 or UL 263 rating.

**Three states, three editions, and only half the decision moves.**

- Texas is on the 2015 IECC, Oklahoma on the 2006 IECC with ASHRAE 90.1-2003, and Kansas on the 2006 IECC under home rule, with Dallas amended up to the 2021 while Fort Worth holds the 2015.
- The edition changes the thermal requirement on the block wall and nothing about the ceiling.

If you are holding drawings with block walls on the perimeter and an exposed deck overhead, and you want a crew that reads the wall section before the data sheet, pulls block density off the submittal, maps the open cells, and checks the deck profile before pricing acoustics, that is the work we do throughout Texas, Kansas, and Oklahoma. Call me at 512-387-2111 or email ross@bahlfireproofing.com to walk a wall section, or [Contact Bahl Fireproofing](https://bahlfireproofing.com/contact/) to get both scopes priced. Our [K-13 sprayed fiber insulation](https://bahlfireproofing.com/k13-sprayed-fiber-insulation/) page covers the exposed-surface side, and [our service areas](https://bahlfireproofing.com/service-areas/) page shows where we work.

---

*This article provides general educational information about insulation, interior finish, and masonry wall performance, and it is not engineering, legal, or code-compliance advice. It does not replace project-specific direction from a licensed architect or engineer. Product specifications including R-value, NRC, STC, and density are revised by manufacturers, so confirm current values against the latest published data sheet, and suitability for a given assembly depends on project-specific conditions. Fire-resistance ratings depend on the tested or calculated assembly and correct installation and must be confirmed against the applicable listing. Code citations reference a specific edition and the adopted edition varies by the authority having jurisdiction, and installation must follow the manufacturer’s instructions and be performed by a qualified applicator. Any cost discussion here is general context, not a quote, so obtain a project-specific bid.*

Written for Bahl Fireproofing**Ross Bahl, Owner**

Ross owns Bahl Fireproofing, the commercial fireproofing and insulation subcontractor his family started in 1977. Every code figure on this site is checked against the published source before it goes up. [More about Ross](/ross-bahl/).

[PrevPreviousHotel Insulation: The Essential Spray Foam Guide](https://bahlfireproofing.com/hotel-insulation-spray-foam-hospitality/)

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## Keep reading

### [Hotel Insulation: The Essential Spray Foam Guide](https://bahlfireproofing.com/hotel-insulation-spray-foam-hospitality/)

October 19, 2026

A hotel envelope gets judged in guest reviews before it ever shows up on a utility bill. Every room is its own

 [Read the guide](https://bahlfireproofing.com/hotel-insulation-spray-foam-hospitality/)

### [CMU Retrofit: The Practical Core Foam Guide](https://bahlfireproofing.com/cmu-retrofit-core-foam-existing-buildings/)

October 15, 2026

A CMU retrofit is one of very few ways to insulate a standing concrete block wall without opening it, which is why

 [Read the guide](https://bahlfireproofing.com/cmu-retrofit-core-foam-existing-buildings/)

### [Multi-Family Insulation: The Practical K-13 Guide](https://bahlfireproofing.com/multi-family-insulation-k-13-apartments/)

October 13, 2026

Multi-family developers and property managers call about sprayed insulation for one reason, and that reason is almost always noise. The trouble is

 [Read the guide](https://bahlfireproofing.com/multi-family-insulation-k-13-apartments/)

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