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U-value and R-value: how to convert U-factor to R-value (and RSI)
If you compare insulation, windows or wall assemblies from different sources, you will meet four related numbers: R-value, RSI, U-factor and U-value. They describe the same physics in two directions and two unit systems, and converting between them takes one division and, at most, one multiplication.
Quick answer. U-value and R-value are reciprocals: R = 1 ÷ U and U = 1 ÷ R, as long as both are in the same unit system.
- Imperial U-factor (BTU/h·ft²·°F) → imperial R-value: R = 1 ÷ U. Example: U-0.30 → R-3.33.
- Metric U-value (W/m²·K) → RSI (m²·K/W): RSI = 1 ÷ U. Example: U 0.25 → RSI 4.00.
- Metric U-value → imperial R-value: R = 5.678 ÷ U. Example: U 0.25 W/m²·K → R-22.7.
R-value to U-factor and RSI conversion table
U = 1 ÷ R RSI = R ÷ 5.678
Worked example: an R-13 wall cavity has U = 1 ÷ 13 = 0.0769 BTU/h·ft²·°F and RSI = 13 ÷ 5.678 = 2.29 m²·K/W. Going back, R = 1 ÷ 0.0769 ≈ 13.
| R-value (IP) | U-factor (BTU/h·ft²·°F) | RSI (m²·K/W) |
|---|---|---|
| R-13 | 0.0769 | 2.29 |
| R-15 | 0.0667 | 2.64 |
| R-19 | 0.0526 | 3.35 |
| R-30 | 0.0333 | 5.28 |
| R-38 | 0.0263 | 6.69 |
| R-49 | 0.0204 | 8.63 |
| R-60 | 0.0167 | 10.57 |
Source for the R-values: U.S. Department of Energy, Energy Saver: Tips on Saving Money & Energy at Home, "Recommended Total R-Values for New Wood-Framed Houses" (ranges such as R13–R15 for wall cavities and R30–R60 for attics). The U-factor and RSI columns are our arithmetic, not DOE figures. These are conversions, not code minimums.
Fiberglass vs spray foam: R-value comparison
| Material | R-value | U-factor if 1 inch (1 ÷ R) | Source |
|---|---|---|---|
| Fiberglass batt, 6 in | ≈ R-19 (about R-3.2 per inch, 19 ÷ 6) | ≈ 0.31 | Figure already used in the attic section of this page; per-inch value is our division |
| Open-cell spray foam | R-3.6 per inch (aged, varies within range) | 0.28 | DOE Building America, Which Spray Foam Is Right For You? |
| Closed-cell spray foam | R-6.1 per inch (aged, varies within range) | 0.16 | Same DOE guide |
Example: 3.5 in of open-cell foam gives about 3.5 × 3.6 = R-12.6 (U ≈ 0.079), while 2 in of closed-cell foam gives about 2 × 6.1 = R-12.2 (U ≈ 0.082). The two foams reach a similar R-value at different thicknesses; closed-cell packs more resistance into each inch. This page has no energy.gov per-inch figure for fiberglass, so it does not quote one; use the manufacturer's labelled R-value for the product you are considering. Cost, air sealing and moisture behaviour are separate questions this table does not cover.
Planning a project? Use our paint coverage calculator, drywall calculator, roofing calculator or concrete calculator to size the materials.
Two directions: resistance and transmittance
- R-value is thermal resistance: how strongly a layer resists heat flow. Higher is more insulating.
- U-factor / U-value is thermal transmittance: how much heat passes through per unit area per degree of temperature difference. Lower is more insulating.
U = 1 ÷ R and R = 1 ÷ U (in matching units)
Window labels in North America usually show a U-factor, because a window is rated as a whole product (frame, glass and spacers together). Insulation is usually sold by R-value; in the United States, R-value claims for home insulation are covered by the Federal Trade Commission's R-value Rule (16 CFR Part 460).
Two unit systems
| Quantity | Imperial (IP) | Metric (SI) |
|---|---|---|
| Resistance | R-value, ft²·°F·h/BTU | RSI, m²·K/W |
| Transmittance | U-factor, BTU/h·ft²·°F | U-value, W/m²·K |
RSI = R(IP) × 0.1761101838
R(IP) = RSI × 5.678263
U(SI) = U(IP) × 5.678263
U(IP) = U(SI) × 0.1761101838
The factor comes from the unit definitions (BTU, foot, degree Fahrenheit) and is listed in the SI/IP conversion tables of the ASHRAE Handbook—Fundamentals. A temperature difference of 1 K equals 1.8 °F, which is why no offset (such as 32 °F) appears in these conversions.
Metric sources sometimes write the metric R-value simply as "R" (for example "R 2.5 m²·K/W"). If the number looks about 5.7 times smaller than you expect, it is probably metric. The converter shows all four forms at once.
How to convert R-value to U-value and U-factor to R-value, step by step
Follow these steps to convert between thermal resistance (R-value, RSI) and transmittance (U-factor, U-value):
- Find the unit. A U-value in W/m²·K is metric (common on European and many international data sheets). A U-factor in BTU/h·ft²·°F is imperial (used on North American window labels). If no unit is printed, find it in the source document before converting: the imperial and metric numbers differ by a factor of about 5.7.
- Take the reciprocal. 1 ÷ U gives the R-value in the same system: imperial U-factor → imperial R-value; metric U-value → RSI (m²·K/W). Conversely, convert R-value to U-value using U = 1 ÷ R.
- Change systems if needed. RSI × 5.678 = imperial R-value; imperial R × 0.1761 = RSI. Or divide 5.678 by metric U-value to get imperial R-value directly: R = 5.678 ÷ U.
- Check the basis. A whole-window or whole-assembly U converts to a whole-window or whole-assembly R. It is not comparable with the nominal R-value of a single insulation product (see comparing like with like).
Going the other way, from R-value to U-value, is the same in reverse: U-factor = 1 ÷ R (imperial), U-value = 1 ÷ RSI (metric), or U-value (W/m²·K) = 5.678 ÷ R (imperial).
R(IP) = 1 ÷ U(IP)
RSI = 1 ÷ U(SI)
R(IP) = 5.678263 ÷ U(SI)
U(SI) = 5.678263 ÷ R(IP)
To skip the arithmetic, enter your value in the R-value, RSI & U-factor converter and choose its unit; it shows all four forms.
U-value to R-value conversion tables
These rows are calculated with the factors above and rounded for reading. They are unit conversions only — not product ratings and not code minimums. For exact figures, enter your own value in the converter.
Imperial R-value to RSI and U
| R (IP) ft²·°F·h/BTU | RSI m²·K/W | U-factor (IP) BTU/h·ft²·°F | U-value (SI) W/m²·K |
|---|---|---|---|
| R-1 | 0.18 | 1.0000 | 5.678 |
| R-3 | 0.53 | 0.3333 | 1.893 |
| R-5 | 0.88 | 0.2000 | 1.136 |
| R-8 | 1.41 | 0.1250 | 0.710 |
| R-10 | 1.76 | 0.1000 | 0.568 |
| R-13 | 2.29 | 0.0769 | 0.437 |
| R-15 | 2.64 | 0.0667 | 0.379 |
| R-19 | 3.35 | 0.0526 | 0.299 |
| R-21 | 3.70 | 0.0476 | 0.270 |
| R-30 | 5.28 | 0.0333 | 0.189 |
| R-38 | 6.69 | 0.0263 | 0.149 |
| R-49 | 8.63 | 0.0204 | 0.116 |
| R-60 | 10.57 | 0.0167 | 0.095 |
Imperial U-factor (e.g. window labels) to metric
| U-factor (IP) BTU/h·ft²·°F | U-value (SI) W/m²·K | Equivalent R (IP) | Equivalent RSI |
|---|---|---|---|
| 0.15 | 0.85 | 6.67 | 1.17 |
| 0.20 | 1.14 | 5.00 | 0.88 |
| 0.25 | 1.42 | 4.00 | 0.70 |
| 0.30 | 1.70 | 3.33 | 0.59 |
| 0.35 | 1.99 | 2.86 | 0.50 |
| 0.50 | 2.84 | 2.00 | 0.35 |
| 1.00 | 5.68 | 1.00 | 0.18 |
Metric U-value (e.g. European data sheets) to imperial
| U-value (SI) W/m²·K | U-factor (IP) BTU/h·ft²·°F | Equivalent R (IP) | Equivalent RSI |
|---|---|---|---|
| 0.15 | 0.0264 | 37.9 | 6.67 |
| 0.18 | 0.0317 | 31.5 | 5.56 |
| 0.25 | 0.0440 | 22.7 | 4.00 |
| 0.35 | 0.0616 | 16.2 | 2.86 |
| 1.0 | 0.1761 | 5.7 | 1.00 |
| 1.4 | 0.2466 | 4.1 | 0.71 |
| 2.0 | 0.3522 | 2.8 | 0.50 |
Metric RSI to imperial R-value and U
Useful when a metric source writes "R 2.5" and you need the US figure.
| RSI m²·K/W | R (IP) ft²·°F·h/BTU | U-value (SI) W/m²·K | U-factor (IP) BTU/h·ft²·°F |
|---|---|---|---|
| 0.5 | R-2.8 | 2.000 | 0.3522 |
| 1.0 | R-5.7 | 1.000 | 0.1761 |
| 1.5 | R-8.5 | 0.667 | 0.1174 |
| 2.0 | R-11.4 | 0.500 | 0.0881 |
| 2.5 | R-14.2 | 0.400 | 0.0704 |
| 3.0 | R-17.0 | 0.333 | 0.0587 |
| 3.5 | R-19.9 | 0.286 | 0.0503 |
| 4.0 | R-22.7 | 0.250 | 0.0440 |
| 5.0 | R-28.4 | 0.200 | 0.0352 |
| 6.0 | R-34.1 | 0.167 | 0.0294 |
| 7.0 | R-39.7 | 0.143 | 0.0252 |
US Attic Insulation Requirements & Climate Zone Standards
In residential construction and energy retrofit projects across the United States, attic insulation accounts for the largest potential reduction in building heating and cooling energy loss. Because heat rises in winter and the sun radiates intensely onto roof planes during summer, thermal resistance requirements for ceilings and attics are significantly higher than those for exterior walls.
The International Energy Conservation Code (IECC) and the US Department of Energy (DOE) establish prescriptive minimum R-values and overall U-factor maximums across eight US climate zones:
| IECC Climate Zone | Representative US Metros | Recommended Attic R-Value | Metric RSI Equivalent m²·K/W |
Ceiling U-Factor BTU/h·ft²·°F |
|---|---|---|---|---|
| Zone 1 | Miami FL, Honolulu HI, San Juan PR | R-30 to R-49 | 5.28 – 8.63 | 0.0333 – 0.0204 |
| Zone 2 | Houston TX, Phoenix AZ, Orlando FL, New Orleans LA | R-38 to R-49 | 6.69 – 8.63 | 0.0263 – 0.0204 |
| Zone 3 | Atlanta GA, Charlotte NC, Dallas TX, Las Vegas NV | R-38 to R-49 | 6.69 – 8.63 | 0.0263 – 0.0204 |
| Zone 4 | Washington DC, St. Louis MO, Seattle WA, Louisville KY | R-49 to R-60 | 8.63 – 10.57 | 0.0204 – 0.0167 |
| Zone 5 | Chicago IL, Denver CO, Boston MA, Detroit MI | R-49 to R-60 | 8.63 – 10.57 | 0.0204 – 0.0167 |
| Zone 6 | Minneapolis MN, Milwaukee WI, Burlington VT, Helena MT | R-49 to R-60 | 8.63 – 10.57 | 0.0204 – 0.0167 |
| Zones 7–8 | Duluth MN, Fargo ND, Fairbanks AK, Anchorage AK | R-49 to R-60 | 8.63 – 10.57 | 0.0204 – 0.0167 |
When upgrading an attic that already contains insulation, you can add the R-values of existing and new continuous layers directly in series. For example, an older attic floor with existing 6-inch fiberglass batt (approximately R-19) topped with blown-in insulation (approximately R-30) reaches a combined nominal rating of R-49 (RSI 8.63 m²·K/W), satisfying modern code standards for Zones 4 through 8.
Worked examples
The figures below are illustrative inputs chosen to show the method; they are not ratings of any real product. Each "Check in the converter" link opens the tool with the same numbers filled in.
1. An insulation label: R-13
A product is labelled R-13 (imperial):
- RSI = 13 × 0.17611 ≈ 2.29 m²·K/W
- U-factor = 1 ÷ 13 ≈ 0.0769 BTU/h·ft²·°F
- U-value = 1 ÷ 2.29 ≈ 0.437 W/m²·K
A common mistake is to read an "R-13" from a US source and an "R-2.3" from a metric source as very different products. They are about the same resistance in different units. Always check the unit before comparing.
2. A metric wall specification: U = 0.28 W/m²·K
A wall assembly documented in metric units at U = 0.28 W/m²·K:
- RSI = 1 ÷ 0.28 ≈ 3.57 m²·K/W
- R (IP) = 3.57 × 5.678 ≈ R-20.3
- U-factor (IP) = 0.28 × 0.17611 ≈ 0.0493 BTU/h·ft²·°F
Because this is an assembly U-value, it can be compared with a US whole-assembly figure of about R-20 — but not with the nominal R-value printed on a single insulation product (see thermal bridging).
Check U 0.28 W/m²·K in the converter
3. A window label: U-factor 0.30
A North American window label shows a U-factor of 0.30 BTU/h·ft²·°F:
- U-value (SI) = 0.30 × 5.678 ≈ 1.70 W/m²·K
- Equivalent R = 1 ÷ 0.30 ≈ R-3.3 (RSI ≈ 0.59)
Compare that with the R-20 wall in example 2: per square foot, this window lets through roughly six times as much heat as the wall. That is why the share of glazing on a facade matters — see the window-to-wall ratio guide.
4. From thickness and conductivity to R-value
Data sheets for a uniform material often give thermal conductivity (λ or k) instead of R. For one homogeneous layer:
RSI = thickness (m) ÷ λ (W/m·K)
R (IP) = thickness (in) ÷ k (BTU·in/h·ft²·°F) = thickness (in) × R per inch
Example with illustrative figures: a 100 mm (0.100 m) board with λ = 0.035 W/m·K gives RSI = 0.100 ÷ 0.035 ≈ 2.86 m²·K/W, which is about R-16.2. Use the conductivity and thickness from the manufacturer's documentation; some materials' R per inch changes with temperature or age, and the label figure is the one to use.
Check RSI 2.857 in the converter
5. What a U-factor means for heat flow
U-factors make heat-flow estimates simple:
Q = U × A × ΔT
Illustrative case: 100 ft² of wall, of which 20 ft² is a U-0.30 window and 80 ft² is an R-15 wall section (U ≈ 0.0667), with a 40 °F difference between inside and outside:
- Window: 0.30 × 20 × 40 = 240 BTU/h
- Wall: 0.0667 × 80 × 40 ≈ 213 BTU/h
The window is one fifth of the area but carries more than half of the heat flow. This is a steady-state snapshot; it ignores solar gain, air leakage and changing temperatures, which energy modeling handles.
Adding layers in series
When heat passes through layers one after another, their resistances add:
R(total) = R₁ + R₂ + … + Rₙ
Example with hypothetical figures: if documentation lists one continuous layer at R-5 and another at R-10, the series total for those two layers is R-15, and the U-factor for that path is 1 ÷ 15 ≈ 0.067 BTU/h·ft²·°F. Note that you add R-values, never U-values.
Add R-5 + R-10 in the converter
Surface air films
Assembly calculations usually include the still air film on the inside surface and the moving air film on the outside surface. Commonly used design values for a wall (horizontal heat flow) are:
| Surface | R (IP) | RSI | Source |
|---|---|---|---|
| Inside surface, still air | 0.68 | 0.13 (ISO) / 0.12 (from IP) | ASHRAE Handbook—Fundamentals; ISO 6946 |
| Outside surface, 15 mph wind (ASHRAE) / ISO standard conditions | 0.17 | 0.04 (ISO) / 0.03 (from IP) | ASHRAE Handbook—Fundamentals; ISO 6946 |
Values differ slightly between methods and depend on surface orientation, heat-flow direction and surface emissivity; use the values your calculation method or designer specifies. Adding the ASHRAE films to a hypothetical continuous R-15 layer: 0.68 + 15 + 0.17 = R-15.85 (U ≈ 0.063).
Add films + R-15 in the converter
Why the whole wall is lower than the insulation rating
Real walls are not uniform. Studs, plates, fasteners, slab edges and window frames conduct heat around the insulation. These thermal bridges act as parallel paths, so the effective R-value of the whole assembly is lower than the series sum of the insulation layers. That is one reason continuous insulation (a layer not interrupted by framing) is discussed so often in sustainable building.
The parallel-path method averages the U-factors of each path, weighted by the share of area each path covers:
U(average) = f₁ × U₁ + f₂ × U₂ + … (f = area fraction, Σf = 1)
R(effective) = 1 ÷ U(average)
Illustrative figures only: suppose the path through the cavity insulation totals R-15 and the path through the framing totals R-7, and framing covers 25 % of the wall area.
- U(average) = 0.75 × (1 ÷ 15) + 0.25 × (1 ÷ 7) ≈ 0.0500 + 0.0357 = 0.0857
- R(effective) = 1 ÷ 0.0857 ≈ R-11.7 — about 22 % below the R-15 cavity path
Notice the averaging is done on U, not on R: averaging the R-values (0.75 × 15 + 0.25 × 7 = R-13) would overstate performance. Real framing fractions and material values come from the project drawings and documentation; the more detailed methods in the ASHRAE Handbook—Fundamentals (or ISO 6946 in metric practice) are best done by your designer or energy modeler.
The series sum in the converter is therefore a "clear-field, continuous layers" figure. You can convert a parallel-path result there too: open U-0.0857 in the converter.
Comparing like with like
| You see | What it usually describes | Compare it with |
|---|---|---|
| Insulation product "R-…" | Nominal resistance of that material at the stated thickness | Other insulation products at their labelled thickness |
| Whole-window U-factor (NFRC label) | Frame, glazing and edge together | Other whole-window U-factors, not centre-of-glass values |
| Centre-of-glass U-value | The glazing unit only, away from the edges | Other centre-of-glass values |
| Assembly U-factor / effective R | A whole wall, roof or floor including framing and films | Other assembly figures calculated the same way |
Frequently asked questions
How do fiberglass and spray foam differ in R-value?
Using the figures on this page, a 6-inch fiberglass batt is about R-19 (roughly R-3.2 per inch), while the U.S. Department of Energy's Building America spray foam guide gives open-cell foam about R-3.6 per inch and closed-cell foam about R-6.1 per inch. Closed-cell foam therefore delivers more resistance per inch; check the manufacturer's label for any specific product.
What is the difference between U-value and R-value?
R-value measures thermal resistance (how strongly a material or assembly resists heat flow); higher is more insulating. U-value measures thermal transmittance (how much heat passes through per unit area per degree of temperature difference); lower is more insulating. For the same element they are reciprocals: U = 1 ÷ R and R = 1 ÷ U in matching units.
How do I convert a U-value to an R-value?
Divide 1 by the U-value. A metric U-value in W/m²·K gives RSI in m²·K/W: U 0.25 → RSI 4.00. To get the imperial R-value, multiply RSI by 5.678 (or divide 5.678 by the metric U-value directly): U 0.25 W/m²·K → R-22.7.
How do I convert a U-factor to an R-value?
A North American U-factor is in BTU/h·ft²·°F, so its reciprocal is the imperial R-value directly: R = 1 ÷ U-factor. A window with a U-factor of 0.30 has an equivalent R-value of 1 ÷ 0.30 ≈ R-3.33 (RSI ≈ 0.59).
How do I convert an R-value to a U-value?
Divide 1 by the R-value. An imperial R-20 gives a U-factor of 1 ÷ 20 = 0.05 BTU/h·ft²·°F. For the metric U-value, divide 5.678 by the imperial R-value: 5.678 ÷ 20 ≈ 0.284 W/m²·K. From RSI, U-value = 1 ÷ RSI.
Is U-factor the same as U-value?
They are the same quantity, thermal transmittance. In North America "U-factor" is the usual term and is normally given in BTU/h·ft²·°F; elsewhere "U-value" is common and is normally given in W/m²·K. The numbers differ by a factor of 5.678, so always check the unit: U-factor 0.30 BTU/h·ft²·°F equals U-value 1.70 W/m²·K.
How do I convert RSI to R-value?
Multiply RSI by 5.678 to get the imperial R-value; divide an imperial R-value by 5.678 (or multiply by 0.1761) to get RSI. RSI 3.0 ≈ R-17.0, and R-13 ≈ RSI 2.29.
Can I add U-values together?
No. For layers that heat crosses one after another, add the R-values and then take the reciprocal of the total to get the U-value: R-5 + R-10 = R-15, so U = 1 ÷ 15 ≈ 0.067. U-values are only combined by area-weighted averaging for side-by-side paths, such as framing and cavity in the same wall.
Why is the conversion factor 5.678?
1 W/m²·K equals 0.1761 BTU/h·ft²·°F, so 1 BTU/h·ft²·°F equals 5.678 W/m²·K. The number comes only from the definitions of the BTU, the foot and the degree Fahrenheit (a 1 K difference is a 1.8 °F difference), and it is listed in the SI/IP conversion tables of the ASHRAE Handbook—Fundamentals. The same factor converts RSI to imperial R-value.
Can I compare a window U-factor with a wall R-value?
Yes, once both are in the same form. Convert the window U-factor to an equivalent R-value (R = 1 ÷ U) or the wall R-value to a U-factor (U = 1 ÷ R). A U-0.30 window is about R-3.3, while a wall whose whole-assembly value is about R-20 has a U-factor of 0.05, so per square foot the window passes roughly six times as much heat. Compare whole-window figures with whole-assembly figures, not with the nominal R-value of an insulation product.
Is a higher R-value or a lower U-value better?
Both point the same way: a higher R-value and a lower U-value each mean less heat flow through the element. Which level is appropriate for a project depends on climate, cost and the energy code that applies where you build; this guide does not set requirements.
What is R-value and what does it measure?
R-value measures thermal resistance: the capacity of an insulation material or building component to resist conductive heat flow. The higher the R-value, the better the thermal insulating performance. In North America it is expressed in ft²·°F·h/BTU; international metric standards use RSI in m²·K/W (where 1 RSI ≈ 5.678 R-value).
What is the formula to calculate R-value?
To convert from thermal transmittance, use R = 1 ÷ U (imperial) or R = 5.678 ÷ U (from metric W/m²·K). For a homogeneous material layer from thickness and thermal conductivity, R = thickness (in) ÷ k-factor (BTU·in/h·ft²·°F), or in metric RSI = thickness (m) ÷ λ (W/m·K).
How do I calculate total R-value for multiple layers?
For layers in series where heat passes through sequentially (e.g., siding, continuous insulation, sheathing, cavity batt, drywall), add individual layer R-values: R_total = R₁ + R₂ + ... + Rₙ, including still interior (0.68) and exterior (0.17) surface air films. Do not add U-values for series assemblies.
What is the difference between nominal R-value and effective R-value?
Nominal R-value is the rated thermal resistance of an uninstalled insulation product alone (e.g. R-15 batt). Effective (or whole-wall) R-value accounts for thermal bridging through framing studs and plates using area-weighted U-factor averaging (U_avg = f₁·U₁ + f₂·U₂; R_eff = 1 ÷ U_avg), typically reducing performance by 15% to 25%.
What are the US attic insulation requirements by climate zone?
Under the International Energy Conservation Code (IECC) and Department of Energy (DOE) benchmarks, recommended attic insulation levels range from R-30 to R-49 in warm climates (Zones 1 to 3) and R-49 to R-60 in cold climates (Zones 4 to 8). In metric RSI, this translates to RSI 5.28 to RSI 10.57 m²·K/W.
How do I convert imperial R-value to metric RSI for US building plans?
Multiply the imperial R-value by 0.17611018 to obtain metric RSI (in m²·K/W), or divide the R-value by 5.678263. For example, standard US wall batt R-13 equals RSI 2.29 m²·K/W, and attic insulation R-38 equals RSI 6.69 m²·K/W.
What is the difference between prescriptive R-value and U-factor compliance in US building codes?
The prescriptive R-value path specifies minimum thermal resistances for individual insulation layers (such as cavity batt and continuous exterior insulation). The U-factor alternative evaluates the overall heat transmission of the complete assembled wall or roof, accounting for thermal bridging through studs and joists.
What R-value is required for residential exterior walls in the US?
Under the IECC, residential wood-framed exterior walls typically require R-13 to R-20 cavity insulation in mild climates (Zones 1 to 3), and R-20 cavity or R-13 cavity plus R-5 continuous insulation (ci) in colder climates (Zones 4 to 8) to reduce thermal bridging.
How much does attic insulation reduce heat loss in a home?
Because the ceiling plane represents a major convective and conductive boundary, upgrading attic insulation from R-19 (typical of older unrenovated homes) to R-49 or R-60 reduces ceiling heat transmission by 60% to 68%, substantially reducing heating and cooling equipment loads.
Using these numbers responsibly
- Take layer R-values and window U-factors from the manufacturer's documentation or an independent rating label.
- Compare products in the same unit and on the same basis (centre-of-glass vs whole-window, nominal insulation vs whole-assembly).
- Minimum insulation levels are set by energy codes that vary by location and edition. This guide does not state requirements; check with your local building department or a qualified professional.
Next step: convert your own values or add up layers in the R-value, RSI & U-factor converter.
Related: window-to-wall ratio shows how much of the facade uses the window U-factor instead of the wall's. Explore our residential building codes (IRC) guide for complete home building standards.