hour square foot Fahrenheit per BTU
Areal thermal resistanceexact by definition
The R-value is the customary US and Canadian imperial measure of insulating performance, in hour square foot Fahrenheit degrees per BTU: how many hours, square feet and degrees of difference it takes to pass one BTU. One imperial R equals 0.176110 m²·K/W, a relation exact by definition from the International Table BTU, the international foot and the 5/9 Fahrenheit degree, though the decimal shown is a rounding of that exact ratio. Higher is better, and R-values of layers in series add.
Watch out: The imperial R-value and the metric RSI differ by a factor of about 5.678, and insulation in Canada is sold with both printed on the bag. R-20 is RSI 3.52. Beyond that, a batt's rated R is the R of the batt alone: once studs, plates and headers are counted, the effective R of the assembly is typically 10 to 25 percent lower, because wood conducts far better than fibreglass. Codes that regulate effective R are asking for the assembly value, not the label value.
| 1 | 0.17611018 |
The R-value became the North American consumer label for insulation because it turns a material property into a single number that adds up. The US Federal Trade Commission's R-value Rule requires it on insulation packaging and in advertising, which is why every batt in a hardware store carries one.
About the hour square foot fahrenheit per btu
An R-value is a thermal resistance per unit area, and the definition is worth spelling out because the unit looks like alphabet soup: , with in Fahrenheit degrees, in square feet, and in BTU per hour. So R-20 means that one square foot of the assembly passes one BTU per hour for every 20 Fahrenheit degrees across it. Turn it upside down and you get the U-value: , in BTU/(h·ft²·°F), which is what windows are rated in.
Two properties make the R-value useful. Layers in series add, so drywall (R-0.45) plus R-20 batt plus sheathing (R-1.3) plus siding gives a nominal total you can just sum. And heat loss is then , so a 1000 ft² wall at R-20 with 60 F° across it loses BTU/h.
Two things routinely go wrong. First, the label R and the assembly R are different numbers. A 2x6 wall with R-20 batts is maybe R-16 effective once the framing is counted, because the studs are R-1.25 per inch against the insulation's R-3.2 and they occupy 20 to 25 percent of the wall. Continuous exterior insulation exists mostly to fix that. Second, the metric confusion: RSI 3.52 and R-20 are the same wall, and the ratio between the two units is 5.678. An imported specification quoting "R 3.5" is almost certainly RSI and describes a wall six times better than an imperial R-3.5 sheet of foam.
One hour square foot fahrenheit per btu in every areal thermal resistance unit
| 176,110 | square millimeter kelvin per watt (mm²·K/W) | |
| 1,761.1 | square centimeter Celsius per watt (cm²·°C/W) | |
| 144 | square inch hour Fahrenheit per BTU (in²·h·°F/BTU) | |
| 1.7611 | tog (tog) | |
| 1.13619 | clo (clo) | |
| 1 | hour square foot Fahrenheit per BTU (R (h·ft²·°F/BTU))this unit | |
| 0.204679 | square meter hour Celsius per kilocalorie (m²·h·°C/kcal) | |
| 0.17611 | square meter kelvin per watt (RSI (m²·K/W)) | convert → |
| 0.17611 | square meter Celsius per watt (m²·°C/W) |