Insulation R-Value Calculator
Work out the R-value of an insulation layer, the thickness needed to hit a target R-value, and the total for a layered assembly.
How to use this calculator
- 1Choose whether you know the target R-value and want a thickness, or know the thickness and want the R-value.
- 2Pick the material — or choose "Other" and enter the R per inch printed on the packaging, which is always more reliable than a typical figure.
- 3If adding to existing insulation, enter what is already there so only the shortfall is calculated.
How the calculation works
R = thickness (in) × R per inch. Thickness = R ÷ R per inch. U = 1 ÷ R- R
- Thermal resistance in ft²·°F·h/BTU — how well the material resists heat flow
- R per inch
- The material's resistance for each inch of thickness
- U
- U-factor, the reciprocal of R — the rate heat passes through
R-values add in series, so layers stack: two inches of R-5 board over R-13 batt gives R-23 in total. This is why continuous exterior insulation over a framed wall is such an effective upgrade.
U-factor is the reciprocal of R and is what actual heat-loss calculations use, because heat flow is proportional to U rather than to R. Windows and doors are conventionally rated in U-factor while insulation is rated in R, which is why the two cannot be compared directly without converting.
The US R-value used here (ft²·°F·h/BTU) is about 5.68 times the metric RSI value (m²·K/W) for the same material. A product rated R-13 in the US is roughly RSI-2.3.
Worked example
Reaching R-49 in an attic with fibreglass batt
- 1.Fibreglass batt is taken here at a typical R-3.2 per inch.
- 2.Thickness needed: 49 ÷ 3.2 = 15.31 inches.
- 3.U-factor: 1 ÷ 49 = 0.02041.
- 4.Across 1,000 sq ft that is 1,000 × 0.02041 = 20.41 BTU per hour for each 1 °F of temperature difference.
- 5.Closed-cell spray foam at R-6.5 per inch would reach the same R-49 in 7.54 inches — less than half the depth, which is why it is used where space is tight.
Result: 15.31 inches of fibreglass batt for R-49
What R-value measures
R-value is thermal resistance: how strongly a material opposes the flow of heat through it. Higher is better, and the scale is linear in thickness — doubling the depth of the same material doubles the R-value. That linearity is what makes the arithmetic here simple, and it is why R-values of stacked layers simply add together.
The reciprocal, U-factor, is what heat-loss calculations actually use, because the rate of heat flow is proportional to U rather than R. Insulation is conventionally sold by R-value while windows and doors are rated by U-factor, which means the two cannot be compared without converting one to the other first.
Why nominal R-value overstates real performance
The R-value on the package describes the insulation in isolation. A real wall or roof also contains studs, joists and rafters, and wood conducts heat far better than insulation does — roughly R-1.25 per inch against fibreglass's R-3.2. Heat takes the path of least resistance, flowing around the insulation through the framing in what is called thermal bridging.
- Wood framing — typically costs 10–25% of the nominal assembly R-value, depending on stud spacing and how much of the wall area is framing.
- Steel framing — far worse, because steel conducts heat hundreds of times better than wood. Steel-framed assemblies can lose more than half their nominal R-value without a thermal break.
- Continuous exterior insulation — the standard fix — a layer of rigid board over the outside of the framing, uninterrupted by studs, so there is no bridging path at all.
- Air leakage — not captured by R-value in any form. A well-insulated but leaky building can underperform a less-insulated airtight one, which is why air sealing is usually the higher-value first step.
Choosing between materials
Higher R per inch is not automatically better value — it matters when depth is constrained and matters much less when it is not. An attic usually has unlimited depth available, so cheap blown cellulose or fibreglass at R-3 per inch is the economical choice; a cathedral ceiling or a basement wall with only a few inches to work with is where paying for closed-cell spray foam at R-6.5 per inch makes sense.
Other properties often decide the choice regardless of R-value: closed-cell spray foam acts as its own vapour retarder and air barrier, mineral wool performs far better in fire, EPS tolerates ground contact and moisture better than most, and polyiso loses R-value as temperatures drop — which specifically undermines it in the cold-climate roof assemblies where it is often specified.
How much is enough
Recommended levels vary by climate zone and by which part of the building is being insulated. Attics carry the highest recommendations because heat rises and attic space is usually cheap to fill — commonly R-49 to R-60 in cold climates and R-30 to R-38 in mild ones. Walls are constrained by cavity depth, so R-13 to R-21 is typical without exterior insulation.
There are diminishing returns. Going from R-0 to R-13 eliminates roughly 92% of the conductive heat loss through that path; going from R-13 to R-26 removes only about 4% more of the original. Beyond the recommended level for a climate, money usually does more good spent on air sealing or windows than on further depth.
What this assumes, and where it stops
Assumptions
- R-values per inch are typical published figures for each material and vary between products, densities and manufacturers.
- Calculates the insulation layer only — framing, sheathing, cladding, air films and drywall all contribute to a real assembly and are not included.
- US R-value units (ft²·°F·h/BTU) are used throughout, not metric RSI.
Limitations
- Does not account for thermal bridging through framing, which reduces real assembly performance by 10–25% for wood and considerably more for steel.
- Does not model air leakage, moisture movement or radiant heat transfer, all of which affect real-world performance and none of which R-value captures.
- Polyiso's R-value falls at low temperatures; the single typical figure used here does not reflect that temperature dependence.
- Loose-fill insulation settles over time, reducing installed depth and therefore R-value below what was originally blown in.
Common questions
How thick does insulation need to be for R-49?
It depends entirely on the material. At a typical R-3.2 per inch, fibreglass batt needs about 15.3 inches. Closed-cell spray foam at R-6.5 per inch reaches the same R-49 in about 7.5 inches. That trade-off between depth and cost per inch is the main reason to choose one material over another.
Do R-values of different layers add up?
Yes — thermal resistances in series add directly. Two inches of R-5 rigid board over an R-13 batt gives R-23 total. This is exactly why continuous exterior insulation is so effective: it adds to the cavity insulation while also bypassing the thermal bridging through the framing.
Is more insulation always worth it?
No, because returns diminish sharply. Going from R-0 to R-13 removes about 92% of conductive heat loss through that path; doubling again to R-26 removes only around 4% more of the original. Once you reach the recommended level for your climate, air sealing and window upgrades generally return more per pound spent than additional depth.
Formula and content last reviewed on .
Results are estimates for information only, not professional advice.
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