Slim 1-1/4 in. CI for constrained outboard build-outs
Key features
About R-5.25 layer R from R-4.2/in at 75 °F for 1.25 in. thickness
4x8 sheet module for efficient coverage on large walls
Technical specifications
| R-Value | 5.25 |
|---|---|
| Thickness | 1.25 in |
| Width | 48.0 in |
| Length | 96.0 in |
| Coverage | 32.0 sq ft/pack |
| Material | Rigid stone wool (non-structural continuous insulation / CI board) |
| Fire rating | Noncombustible per ASTM E136; Flame Spread 0 / Smoke Developed 0 per ASTM E84 on USA Comfortboard 80 TDS (07-2025). |
| Sound rating | USA TDS lists NRC 0.75 at 1 in. and NRC 0.90 at 1.5 in. (ASTM C423); 1-1/4 in. is between rows—confirm with ROCKWOOL if required. |
| Density | 8 lb/ft³ actual density per ASTM C303 on USA Comfortboard 80 TDS (07-2025). |
| Thermal conductivity | R-4.2 per inch at 75 °F; RSI 0.74 per 25.4 mm at 24 °C per ASTM C518 (C177) on USA TDS. |
| Framing width | Continuous insulation (not stud-spacing specific) |
| Construction type | Wood or steel frame (exterior CI; non-structural) |
| Application area | Exterior continuous insulation and compatible foundation or slab uses per manufacturer / ICC-ES scope. |
Product details
Not every retrofit has room for 2 to 3 in. of outboard insulation, yet teams still want noncombustible CI and a permeable drying path. A 1-1/4 in. Comfortboard 80 sheet can be an incremental CI step paired with cavity upgrades or interior air sealing.
Thermal resistance scales with R-4.2 per inch at 75 °F: about R-5.25 for this thickness before framing factors.
The USA TDS acoustical table lists discrete thicknesses; 1-1/4 in. sits between the 1 in. row (NRC 0.75, SAA 0.76) and the 1.5 in. row (NRC 0.90, SAA 0.93). For contractual acoustic coefficients at 1-1/4 in., request written values from ROCKWOOL Technical Services rather than interpolating without approval.
Comfortboard 80 is non-structural CI; keep structural sheathing in the load path.
Coordinate WRB transitions at corners and penetrations; thin CI layers still need meticulous air sealing.
Use https://www.rockwool.com/siteassets/o2-rockwool/documentation/technical-data-sheets/residential/comfortboard-80-non-structural-sheathing-continuous-insulation-techdata.pdf (07-2025) as the compliance anchor.
Documents & certifications
Technical data sheet
Manufacturer-tested R-value, coverage, fire notes, and handling—use for submittals, RFIs, and field QA.
Download PDF (opens in new tab)Certifications & compliance
ASTM C612 Type IVB, CAN/ULC S702 Type I, ASTM E136 noncombustible, CAN/ULC S114 non-combustible, ASTM E84 / CAN/ULC S102 flame spread 0 / smoke developed 0 (Note 1 Class A per IBC on TDS), ASTM C303 actual density 8 lb/ft³, ASTM C518 R-4.2 per inch at 75 °F (RSI 0.74 per 25.4 mm at 24 °C), ICC-ES ESR-3773 on USA TDS (Issued 07-2025)
Frequently asked questions
How is R-5.25 calculated?
How is R-5.25 calculated?
Multiply 1.25 in. by R-4.2 per inch at 75 °F on the USA Comfortboard 80 TDS for about 5.25 nominal layer R for the board alone.
Why is NRC not exact for 1-1/4 in.?
Why is NRC not exact for 1-1/4 in.?
The 07-2025 USA TDS prints NRC rows for 1 in. and 1.5 in., not 1-1/4 in. Use manufacturer support for lab-backed numbers if your spec requires a single NRC.
Can I stack two 1-1/4 in. layers?
Can I stack two 1-1/4 in. layers?
Layered CI may be acceptable when joints are staggered and attachment engineering supports combined thickness; follow ROCKWOOL guidance and code paths.
Is a rainscreen gap still required?
Is a rainscreen gap still required?
Many claddings need a drained cavity per manufacturer and code; Comfortboard does not replace drainage detailing behind siding or panels.