How to read the chart
Each column is one type of insulation and each row is a target R-value. The cell shows how thick that material has to be to reach it:
Depth (in) = R-value ÷ R per inch
Under each material name you'll see λ (lambda), its thermal conductivity in W/m·K. That's the metric way of saying the same thing: the lower the λ, the less thickness you need. To switch between the two, R per inch ≈ 0.1442 ÷ λ. The R-value to U-value converter does this for any thickness.
| Column | R per inch | λ (W/m·K) |
|---|---|---|
| Fiberglass batts (wall grade) | 3.7 | 0.039 |
| Fiberglass batts (attic and floor) | 3.2 | 0.045 |
| Mineral wool batts | 4.2 | 0.034 |
| Blown cellulose | 3.65 | 0.040 |
| Blown fiberglass | 2.92 | 0.049 |
| Polyiso board | 6 | 0.024 |
| XPS board | 5 | 0.029 |
| EPS board | 3.9 | 0.037 |
| Open-cell spray foam | 3.7 | 0.039 |
| Closed-cell spray foam | 6.5 | 0.022 |
Fiberglass batts appear twice because wall batts and attic batts are made differently. Wall batts are made to pack more R into a 3.5 in or 5.5 in stud cavity.
Worked example: how thick is R-38?
Reading across the R-38 row:
| Material | Depth for R-38 |
|---|---|
| Closed-cell spray foam | 5.85 in |
| Polyiso board | 6.33 in |
| XPS board | 7.6 in |
| Mineral wool batts | 9.05 in |
| EPS board | 9.74 in |
| Wall fiberglass or open-cell foam | 10.3 in |
| Blown cellulose | 10.4 in |
| Attic fiberglass batts | 11.9 in |
| Blown fiberglass | 13 in |
Now compare with real products. The R-38 fiberglass attic batt in our data is 12 in thick. GreenFiber's chart puts R-38 of cellulose at 10.4 in settled, installed at 11.2 in. Knauf's chart puts R-38 of blown fiberglass at 13 in. For R-38, chart and products line up closely.
When to trust the label instead
The chart uses one R per inch for each material. Real products vary a little, and for anything you're buying, the label or coverage chart wins.
- Batts come in fixed sizes. The R-49 attic batt in our data is 14 in thick, not the 15.3 in the 3.2-per-inch column suggests, because the maker packs more R into each inch of the thicker batt. Buy by the R on the label.
- Loose-fill settles. The chart shows settled depth. Cellulose is blown deeper and settles down to it: for R-60, GreenFiber's chart says install 17.3 in and it settles to 16.1 in.
- Coverage charts aren't perfectly linear. For R-60, Knauf's chart gives 19.75 in of blown fiberglass, a little less than the 20.5 in in our column. The blown-in insulation calculator uses the manufacturer chart, so its numbers can differ slightly from this page.
Checking what you have now
Old insulation doesn't perform like new product. To estimate the R-value of what's already in your attic, ENERGY STAR gives these per-inch values: fiberglass batts 3.2, loose fiberglass 2.5, cellulose 3.7, rock wool 2.8 and vermiculite 2.7. For example, 6 in of old loose fiberglass is about R-15, even though new blown fiberglass rates 2.92 per inch.
If what you find is pebbly vermiculite, don't disturb it. The EPA warns it may contain asbestos.
Thickness isn't everything
- Compression cuts R. A batt squeezed into a shallow space ends up below its label.
- Gaps and air leaks let heat bypass the insulation. Seal before you insulate.
- Framing conducts more heat than insulation. The R-values here are for the insulation alone, the way US targets are written.
Once you know the depth, plan the quantity with the attic insulation calculator, and check your zone's target on insulation R-values by climate zone.
Sources
- ENERGY STAR: DIY checks and inspections (depth multipliers)
- U.S. DOE: Guide to Home Insulation
- Owens Corning PINK Next Gen Fiberglas fact sheet
- ROCKWOOL Comfortbatt R-value fact sheet
- GreenFiber SANCTUARY attic coverage chart (Rev K)
- Knauf EcoFill Wx blowing insulation product data
- Owens Corning FOAMULAR NGX product sheet
- Johns Manville AP Foil-Faced polyiso
- Menards: foam board insulation (retail prices)
- U.S. EPA: Asbestos-contaminated vermiculite insulation