72%less heat loss through the assembly at one inch of closed-cell foam. Ninety percent at three.
The most common question we get is some version of “how many inches do I need?” The honest answer is that the first inch does most of the work, the second and third inches finish the job, and everything past that is buying single percentage points at full price. Here is the data behind that.
The test
Architectural Testing Inc. of York, Pennsylvania ran a guarded hot-box test to ASTM C-236 on an assembly insulated with closed-cell spray polyurethane foam at increasing thicknesses. A hot box measures the actual heat flowing through a whole wall section, not the theoretical R-value of the insulation alone, so it captures what foam does that fiberglass cannot: it stops the air movement that carries heat around insulation.
Reading the curve
| Thickness | Heat loss reduction | Gain from the previous step | Nominal R-value* |
|---|---|---|---|
| 1" | 72% | — | R-7.4 |
| 1.5" | 81% | +9 pts | R-11.1 |
| 2" | 86% | +5 pts | R-14.8 |
| 2.5" | 88% | +2 pts | R-18.5 |
| 3" | 90% | +2 pts | R-22.2 |
| 4" | 92% | +2 pts | R-29.6 |
| 5" | 93% | +1 pt | R-37.0 |
| 6" | 94% | +1 pt | R-44.4 |
| 7" | 95% | +1 pt | R-51.8 |
Between one and two inches you pick up 14 points. Between three and seven inches, four more inches of foam, you pick up five. The R-value keeps climbing in a straight line, but the heat actually leaving the building does not, because by three inches the assembly is already air-sealed and the foam is a vapor retarder. What remains is conduction through a material that is already very good at resisting it.
When more is right
- Code minimums. Pennsylvania is Climate Zone 5. Prescriptive ceiling values are high enough that roof decks in living space often need 5 inches or more of closed-cell, or a hybrid of foam plus batt.
- Condensation control on the underside of a roof deck. Enough foam to keep the sheathing above the dew point through January matters more than the R-value on paper.
- Structural stiffness. Closed-cell adds racking strength to metal panels; thicker foam adds more.
*Nominal aged R-value per ASTM C518 for Heatlok HFO High Lift, R-7.4 per inch (R-26 at 3.5 inches). The hot-box results are for the tested assembly; your building's savings depend on its construction, existing air leakage and how it is used.