Why use this calculator
Most older American homes have far less insulation than current guidance recommends, and the attic is usually the cheapest place to fix that. The trouble is the numbers come in three different forms: a recommended R-value that depends on climate zone and location in the house, an R per inch that depends on the product, and a depth that you can actually measure with a ruler.
This calculator joins them up. It tells you the target for your zone, how much you need on top of what is already there, how deep that is for batts, loose fill or spray foam, and how much material that means across your area. It also gives the metric RSI figure, so an Australian, New Zealand or European product label can be compared directly.
Recommendations follow the ENERGY STAR and US Department of Energy tables, which are built on the International Energy Conservation Code.
How to use it
- Pick your IECC climate zone from the list, using the ENERGY STAR or Department of Energy map if you are unsure which county falls where.
- Choose whether you are insulating the attic, the walls or a floor over a crawlspace or garage.
- Choose the material you plan to add, which sets the R per inch used for depth.
- Enter the existing R-value, found by measuring the current depth and multiplying by about 3, or by reading the batt label, and enter zero if there is none.
- Finally enter the area in square feet, measured as the attic floor or the net wall area.
What each result means
- Target R-value
- The recommended total R-value for your climate zone and location, at the upper end of the ENERGY STAR range.
- Additional R-value needed
- How much insulation value to add on top of what is already there. Zero means you already meet the recommendation.
- Depth to add
- How many inches of your chosen product give the additional R-value. Loose fill settles, so blow slightly deeper than this.
- Depth to add in millimetres
- The same depth in millimetres, handy when working from metric product data or an Australian or European label.
- Material volume
- Area multiplied by the added depth in cubic feet. Bag coverage charts convert this into a number of bags.
- Target in metric RSI
- The target converted to metric R-value in m²K/W, which is how Australian, New Zealand and European products are rated.
- Existing level against target
- What share of the recommended R-value your current insulation already provides, a quick way to rank which areas to fix first.
The formula
Additional R = max(0, target R - existing R)
Depth in inches = additional R / R per inch of the chosen material
Depth in millimetres = inches x 25.4
Material volume in cubic feet = area in square feet x depth / 12
Metric RSI in
m²·K/W = US R-value / 5.678
Targets use the upper value of the ENERGY STAR range: attic R-49 to R-60, walls R-13 to R-25 including continuous insulation, floors R-13 to R-30.
How this number is used in practice
The attic is almost always the first job. It is accessible, loose fill can simply be blown over the top of old batts, and heat loss through an under-insulated ceiling is large in both winter and summer. ENERGY STAR recommends R-49 to R-60 for attics in most of the country, which is 14 to 19 inches of fibreglass.
Walls are harder, because the depth is fixed by the stud cavity: a 2x4 wall holds about 3.5 inches and a 2x6 wall about 5.5 inches, so reaching R-20 or more in a wall usually means dense-pack cellulose, spray foam or continuous board insulation outside the studs during a re-side.
Air sealing matters as much as R-value. Gaps around downlights, plumbing and wiring penetrations and the attic hatch can move more heat than the insulation stops, and insulation laid over leaking gaps performs well below its label. The Department of Energy recommends sealing first and insulating second.
Federal tax credits under the Energy Efficient Home Improvement Credit and many utility rebates cover part of the cost, so check before buying.
Outside the US, the same physics applies but the numbers are metric. Australia's National Construction Code sets total R-values in m²K/W, typically R4.1 to R6.3 for ceilings depending on climate zone, and a metric R4.0 batt is about US R-23. Divide a US R-value by 5.678 to compare.
Worked examples
Zone 4 attic, 1,000 sq ft, R-13 already there, topping up with fibreglass batt
Target R-60, add R-47, which is 14.7 inches (373 mm) and 1,224 cu ft of batt
Zone 6 attic, 1,200 sq ft, R-19 existing, blown cellulose
Add R-41, 11.7 inches of cellulose, about 1,171 cu ft of material
Zone 3 walls, 900 sq ft with no insulation, closed-cell spray foam
Target R-20, which closed-cell foam reaches in 3.1 inches, inside a 2x4 cavity
When people use it
- Deciding how much loose fill to blow into an existing attic
- Comparing the depth needed for batts against spray foam where space is tight
- Checking whether a house bought with old insulation meets current recommendations
- Converting a US R-value to metric RSI to compare with an imported product
- Estimating material volume before getting quotes from installers
Tips
- Seal air leaks before insulating. Gaps around lights, pipes and the hatch can undo much of the benefit.
- Keep loose fill clear of soffit vents with baffles so the attic still ventilates.
- Do not compress batts into a shallow space. A compressed batt loses R-value in proportion.
- Blow loose fill about 10 per cent deeper than the target to allow for settling.
- Check for federal tax credits and utility rebates before buying materials or booking an installer.
- Keep insulation clear of recessed lights unless they are IC rated, and away from flues.
What it assumes, and where it stops being accurate
- Targets use the upper value of the ENERGY STAR recommended range for each zone. Local code minimums can be lower or higher.
- Wall targets for zones 4 and up assume cavity insulation plus about R-5 of continuous insulation.
- R per inch values are typical mid-range figures. Your product's label is the authority.
- Existing insulation is assumed to be in fair condition. Wet, compressed or rodent-damaged insulation performs well below its nominal value.
- Thermal bridging through studs and joists is ignored, so a whole-wall R-value is lower than the cavity figure.
- No cost is calculated because installed prices vary widely by region, access and product.
Terms used on this page
- R-value
- Thermal resistance of insulation. In the US it is measured in ft²·°F·h/BTU.
- RSI
- Metric R-value in m²·K/W, equal to the US R-value divided by 5.678.
- Climate zone
- One of eight IECC regions based on heating and cooling degree days that set insulation targets.
- Continuous insulation
- Insulation board running unbroken over the framing, which cuts heat loss through studs.
- Thermal bridging
- Heat flowing through framing members that bypass the insulation between them.
Questions people ask
- What does R-value mean?
- R-value is resistance to heat flow. Higher numbers insulate better, and layers add together, so R-19 over R-13 gives about R-32. US R-values are in ft²·°F·h/BTU, while Australian and European products use metric RSI, which is the US figure divided by 5.678.
- How do I find my climate zone?
- The ENERGY STAR and Department of Energy maps assign every US county to one of eight IECC climate zones, from zone 1 in south Florida and Hawaii to zone 8 in interior Alaska. Your local building department can also confirm the zone for your address.
- Can I put new insulation over the old?
- Usually yes, provided the old insulation is dry and free of mould and pests. Lay unfaced batts or blow loose fill over the top, running batts at right angles to the joists to cover them. Never add a second vapour retarder facing on top.
- Is more insulation always better?
- Returns diminish. Going from R-11 to R-30 saves far more than going from R-49 to R-60, because heat loss falls with the inverse of R-value. Beyond the recommended level, the extra material rarely pays back within its life.
- How does this compare with Australian R-values?
- Australian R-values are metric. Divide a US R-value by 5.678 to get the metric equivalent, so US R-38 is about R6.7 metric. The National Construction Code sets Australian ceiling targets of roughly R4.1 to R6.3 depending on climate zone.
Use this with
Insulation R-Value Calculator answers one part of the question. These pick up where it stops: HVAC BTU Calculator, Power Bill Calculator, Roof Pitch and Rafter Calculator and Solar Panel Savings Calculator.
Where these figures come from
Figures and rules on this page follow United States federal rules.
- ENERGY STAR: Recommended Home Insulation R-Values. Recommended R-values by climate zone and location
- US Department of Energy: Energy Saver. Insulation, air sealing and climate zone guidance
- International Code Council: IECC. International Energy Conservation Code prescriptive R-values
Insulation R-Value Calculator produces an estimate for general information. It is not financial, tax, medical, legal or engineering advice. read the full disclaimer. Spotted a wrong number? Report it and it gets fixed. The form picks up this calculator and the inputs you used.