Key Takeaways
- 1The first question in insulation brick selection is not temperature, it's position: does the brick sit in the hot face, or purely as backup behind another lining? CFI is backup-only; HFK and Porosint can both carry a hot face.
- 2CFI (Cold Face Insulation, Group 16) is a backup-only brick rated to 900°C, not designed for direct flame contact, at 0.75–0.80 g/cc and 1.5 MPa minimum cold crushing strength.
- 3HFK (Hot Face Kyanite) is kyanite-based, converts to mullite on firing, and is rated for direct hot-face duty: HFK 135 to 1350°C and HFK 140 to 1400°C, at 2.5–3.9 MPa cold crushing strength.
- 4Porosint is a type of insulation brick whose generic grade name is IFB (insulating fire brick); CUMI's Porosint, known as CUMILAG, is an 8-grade family spanning 1260°C to 1850°C, changing base material as the range goes up, calcined clay/alumina at the low end, fused mullite in the middle, bubble alumina for the two premium grades above 1750°C.
- 5Above 1400°C, HFK is out of range and Porosint's mid-to-upper grades are the only option; on a pure backup layer, CFI is normally the more economical choice.
Position First, Then Temperature
The question that actually decides between CFI, HFK and Porosint insulation brick is not "what temperature," it's "where in the wall does this brick sit." CFI is built exclusively for backup duty behind another lining. HFK and Porosint's mid-to-upper grades are built to carry a hot face directly. Picking a backup-rated brick for hot-face duty, or over-specifying an expensive hot-face grade where a cheaper backup brick would do, are the two most common selection mistakes, and both are avoidable once the position question is answered first.
Quick answer: CFI is backup-only, rated to 900°C, and never sits in the flame path. HFK is kyanite-based and rated for direct hot-face duty: HFK 135 to 1350°C, HFK 140 to 1400°C. Porosint (generic grade name IFB; CUMI's is CUMILAG) is an 8-grade family from 1260°C to 1850°C that can serve as hot face or backup depending on grade. Decide position first, then pick the brick whose temperature range covers your zone.
CFI — Cold Face Insulation (Group 16)
CFI is a highly porous, low-density backup brick. It is never placed in the flame path, its job is purely thermal: cut heat loss through the shell and reduce the heat stored in the lining.
- Max safe working temperature: 900°C
- Apparent porosity: 70%
- Bulk density: 0.75–0.80 g/cc
- Cold crushing strength: 1.5 MPa (15 kg/cm²) min
- Hot face capable: No, backup/cold face only
HFK — Hot Face Kyanite (HFK 135 and HFK 140)
HFK is a kyanite-based insulating brick, meaning it can sit in the hot face itself rather than behind another lining. Kyanite converts to mullite on firing, which gives it better dimensional stability at temperature than a plain clay-based insulating brick, the property that actually allows hot-face use.
- Rated temperature: 1350°C (HFK 135), 1400°C (HFK 140)
- Apparent porosity: 60–66%
- Bulk density: 0.95–1.20 g/cc
- Cold crushing strength: 2.5–3.9 MPa (25–40 kg/cm²)
- Hot face capable: Yes, direct hot-face duty within the grade rating
Porosint (IFB, CUMI CUMILAG) — The Wide-Range Family
Porosint is a type of insulation brick, graded generically as IFB (insulating fire brick). CUMI's Porosint, known as CUMILAG, is not a single classification but an eight-grade family running from 1260°C to 1850°C. The base material itself changes as the range goes up:
- 1260–1550°C grades: calcined clay and calcined alumina based
- 1650–1750°C grades: fused mullite based
- 1800–1850°C grades: bubble alumina, the two premium grades for the most demanding zones
Because the range spans such a wide temperature window in one family, Porosint is often the practical choice where a single specification needs to cover multiple zones of the same furnace at different temperatures, rather than mixing separate brick families.
Side-by-Side Comparison
| Property | CFI (Group 16) | HFK 135 / 140 | Porosint / IFB |
|---|---|---|---|
| Temperature range | Up to 900°C | 1350°C / 1400°C | 1260–1850°C (8 grades) |
| Base material | Alumino-silicate (28% Al₂O₃ min) | Kyanite (converts to mullite) | Calcined clay/alumina → fused mullite → bubble alumina |
| Hot face capable | No | Yes | Yes (grade-dependent) |
| Apparent porosity | 70% | 60–66% | Grade-dependent |
| Cold crushing strength | 1.5 MPa min | 2.5–3.9 MPa | 1.2–12.7 MPa across the range |
| Typical role | Backup layer only | Single-layer hot face to 1350–1400°C | Hot face or backup, wide span |
How Do You Choose Between CFI, HFK and Porosint?
| Situation | Specify |
|---|---|
| Backup layer behind a dense working lining, under 900°C | CFI |
| Single-layer hot face, up to 1350°C (HFK 135) or 1400°C (HFK 140), no separate working lining wanted | HFK |
| Hot face or backup above 1400°C, up to 1850°C | Porosint, grade selected by temperature |
| One furnace, multiple zones at different temperatures | Porosint across grades, or HFK plus CFI as a two-layer build below 1400°C |
SAPL Supply for Insulation Brick
Why this matters: most furnaces don't run a single temperature end to end, they have zones. Carrying CFI, HFK and the full Porosint range under one roof means SAPL can spec a complete multi-layer wall build across all three brick families for one furnace, instead of you sourcing each zone's brick from a different supplier.
Shanker Agencies supplies CFI, HFK and the full CUMI CUMILAG (Porosint/IFB) grade range with batch test certificates, and can specify a complete multi-layer wall build across all three where a furnace has more than one temperature zone. See our CFI, HFK and Porosint/IFB product pages, or contact our engineering team with your furnace zones and operating temperatures for a grade recommendation.
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Frequently Asked Questions
What is the difference between CFI and HFK insulation bricks?
CFI (Cold Face Insulation, Group 16) is a backup-only insulating brick rated to 900°C, not designed for direct flame or hot-gas contact. HFK (Hot Face Kyanite) is a kyanite-based insulating brick rated for direct hot-face duty: HFK 135 to 1350°C and HFK 140 to 1400°C. The core difference is position, not just temperature: CFI sits behind a working lining, HFK can be the working lining itself in lower-temperature applications.
Can Cold Face Insulation (CFI) bricks be used on the hot face?
No. CFI is manufactured and tested as a backup brick, sitting behind a dense working lining or a hot-face insulating brick like HFK. Using CFI directly in the flame path or hot gas stream is outside its design intent: it was never engineered for that duty.
What temperature is HFK insulation brick rated for?
HFK 135 is rated for direct hot-face service to 1350°C and HFK 140 to 1400°C. Permanent linear change is held to ±1.5% (HFK 135) and +0.5 to -1.8% (HFK 140) after 5 hours at 1350°C, which is what confirms the brick holds dimensional stability at temperature rather than just surviving it once.
What is Porosint and how is it different from CFI and HFK?
Porosint is a type of insulation brick, and its generic grade name is IFB (insulating fire brick). CUMI's Porosint is known as CUMILAG, an eight-grade family spanning 1260°C to 1850°C. Unlike CFI and HFK, which are each a single classification at a fixed rating, Porosint changes base material as the range goes up, calcined clay and calcined alumina at the lower grades, fused mullite in the middle grades, and bubble alumina for the two premium grades above 1750°C. It is the range to specify when a single brick family needs to cover a wide temperature span.
Do I need a separate working lining in front of HFK or Porosint?
Not necessarily. Both HFK and the calcined-clay/alumina and fused-mullite grades of Porosint are rated for direct hot-face exposure, so a single-layer wall build is possible within their temperature rating. Above the highest grade's rating, or in applications with mechanical wear or slag contact, a separate dense working lining is still required, insulating brick is not a substitute for a working lining under abrasive or corrosive service.
What is HFK insulation brick made from?
HFK is kyanite-based. Kyanite (Al2SiO5) converts to mullite plus a siliceous phase when fired, which gives the brick better volume stability at temperature than an ordinary clay-based insulating brick, allowing it to hold a hot face at up to 1400°C (HFK 140) while keeping bulk density low.