Insulation Calculator
Find how thick your insulation must be to hit a target R-value, and how much material that means.
How to use the Insulation Calculator
- Enter the area length and pick its unit.
- Enter the area width and pick its unit.
- Choose the insulation type.
- Enter the target r-value – DOE attic recommendations range from R-30 (warm climates) to R-60 (cold climates).
- Enter the existing r-value – About R-3 per inch for old loose fill; 0 for new construction.
- Press Calculate. The result updates automatically as you type, and you can switch the result unit at any time.
About the Insulation Calculator
R-value measures resistance to heat flow, and R-values of layers in series simply add up. That makes topping up an attic easy to plan: subtract what you have from the target and divide by the R-per-inch of the new material. The US Department of Energy recommends R-30 to R-49 for attics in warm southern zones and R-49 to R-60 in colder northern zones.
Air sealing gaps around top plates, wiring holes and recessed lights before adding insulation often saves as much energy as the insulation itself. Keep insulation away from soffit vents (use baffles) and from non-IC-rated light fixtures and chimneys.
Formula
R-value to add = Target R − Existing R
Thickness = R-value to add ÷ R-value per inch
Volume = Area × Thickness
R-values per inch are typical published values for each material; check the product label, since density and brand change the figure. Blown-in products list a minimum settled thickness and the number of bags per 1,000 sq ft for each R-value.
Example calculation
Using the default values:
- Area length = 40 ft
- Area width = 30 ft
- Insulation type = Blown cellulose (≈R-3.5 per in)
- Target R-value = 49
- Existing R-value = 11
the calculator returns:
- Thickness to add: 10.86 in
- R-value to add: 38
- Insulation volume: 1,085.7 ft³
- Area: 1,200 ft²
Frequently asked questions
How thick is R-49 insulation?
About 14 in of blown cellulose, 15½ in of fiberglass batts, or nearly 20 in of blown fiberglass.
Can I add new insulation on top of old insulation?
Yes, as long as the old insulation is dry and free of mold. Unfaced batts or loose fill can go directly over existing material.
How do I know the R-value of my existing insulation?
Measure its depth and multiply by about R-3 per inch for fiberglass or cellulose, or R-2.5 per inch for older loose-fill fiberglass.
Last updated: · Category: Construction calculators