Skip to content
Generated on: July 20, 2026 at 09:33 PM

Attic Insulation R-Value Calculator

Inputs

Insulation depth25 cm
R-value per inch3.2
Target R-value49

This uses the US R-value system, where R is the depth in inches times the material’s R-per-inch. Real performance is lower where insulation is compressed, gapped, or settled, and where attic bypasses leak air. Recommended levels vary by climate and code; check your local energy code and the product’s rated value before adding insulation.

Generated with OneCalc · https://onecalc.app/en/construction/attic-insulation-r-value

Construction & DIY

Attic Insulation R-Value Calculator

Convert a measured insulation depth into a US R-value using the material’s R-per-inch, and find how much more depth is needed to reach a target such as R-49.

Inputs

0.1 – 10
0 – 100

Results

Enter a value to see results.

Details

25 cm of insulation gives about R-...; reaching R-49 needs about ... more.

Embed this calculator

Preview

Paste this code into your page to show the calculator.

Share this calculation

Anyone who opens this link sees your values already filled in.

Attic Insulation R-Value

Insulation is rated by its R-value, a measure of thermal resistance, and for a loose or batt material the R-value grows in step with how deep it is laid. This calculator converts a measured depth into an R-value and works out how much more depth is needed to reach a target such as R-49.

Depth times R-per-inch

Every material adds a fixed amount of resistance per inch of thickness, so the total R-value is the depth in inches times that rate. One inch is 0.0254 m, so a metric depth is divided by that constant first:

R=t0.0254 r\begin{aligned} R &= \frac{t}{0.0254} \, r \end{aligned}

where tt is the depth and rr is the R-per-inch. The shortfall against a target RtR_t is whatever is missing, and the extra depth to close it converts back through the same rate:

Ra=max⁡(0,  Rt−R)ta=Rar×0.0254\begin{aligned} R_a &= \max(0,\; R_t - R) \\ t_a &= \frac{R_a}{r} \times 0.0254 \end{aligned}

Worked example

Consider 25 cm of blown cellulose at R-3.2 per inch, aiming for R-49:

R=0.250.0254×3.2=9.84×3.2=31.5Ra=49−31.5=17.5ta=17.53.2×0.0254=0.139\begin{aligned} R &= \frac{0.25}{0.0254} \times 3.2 = 9.84 \times 3.2 = 31.5 \\ R_a &= 49 - 31.5 = 17.5 \\ t_a &= \frac{17.5}{3.2} \times 0.0254 = 0.139 \end{aligned}

So the attic is at about R-31, and roughly 14 cm more cellulose reaches R-49.

Consider 10 in of blown cellulose at R-3.2 per inch, aiming for R-49:

R=10×3.2=32Ra=49−32=17ta=173.2=5.3\begin{aligned} R &= 10 \times 3.2 = 32 \\ R_a &= 49 - 32 = 17 \\ t_a &= \frac{17}{3.2} = 5.3 \end{aligned}

So the attic is at about R-32, and roughly 5.3 in more cellulose reaches R-49.

Choosing inputs

The R-per-inch is the input to get right, because it varies by material and even by how settled a loose fill is: loose-fill fibreglass is about 2.5, blown cellulose 3.2–3.7, fibreglass batts 3.0–3.8, and spray foam higher. Measure the current depth rather than the depth at install, since loose fills settle over time. The target depends on your climate and energy code — US guidance commonly lands between R-38 and R-60 for attics.

To see how added insulation lowers cooling load, see Air Conditioner BTU Sizing Calculator.

Frequently Asked Questions (FAQ)

How do I find my attic R-value?

Measure the depth of the insulation in inches and multiply by the material’s R-per-inch. Blown cellulose at about R-3.5 per inch gives roughly R-35 at 10 inches. If you know the product but not the depth, the bag or batt label lists the installed R-value, and many attics have a marker card stapled to a truss noting the rated R-value at install.

What R-value do I need in my attic?

It depends on your climate. US guidance commonly calls for R-49 to R-60 in cold zones, R-38 to R-49 in mild zones, and somewhat less in hot zones, with local energy codes setting the minimum for new work. More insulation gives diminishing returns, so topping up a thin attic to code usually pays back faster than over-insulating an already-adequate one.

What is R-per-inch?

R-per-inch is how much thermal resistance one inch of a material adds. Multiplying it by the installed depth gives the total R-value. Loose-fill fibreglass is about 2.5 per inch, blown cellulose about 3.2–3.7, fibreglass batts about 3.0–3.8, and spray foam higher. Settling reduces the effective depth of loose fills over time, so measure the current depth rather than the depth at install.

Disclaimer

This uses the US R-value system, where R is the depth in inches times the material’s R-per-inch. Real performance is lower where insulation is compressed, gapped, or settled, and where attic bypasses leak air. Recommended levels vary by climate and code; check your local energy code and the product’s rated value before adding insulation.

Recommended Next

Air Conditioner BTU Sizing Calculator

Estimate the cooling capacity in BTU/hr and kW needed for a room from its floor area, with adjustments for ceiling height, sun exposure, occupancy, and kitchen heat.

 Learn more
Electricity Cost Calculator

Find out what any appliance costs to run — per day, month, and year — from its wattage, daily hours, and your electricity rate.

 Learn more
Solar Panel Sizing Calculator

Enter monthly kWh usage, peak sun hours, and system losses to find the right rooftop solar system size, panel count, annual output, and estimated cost.

 Learn more

200+ calculators · 10 languages · 100% free

Was this calculator helpful?

No

How can we improve this calculator?
Powered by OneCalc ↗