A bare brick wall leaks 1,175 W and the same wall under 15 cm of wool leaks 169. Get the heat flow, the R-value in both unit systems and what one more layer would buy.
What is the R-value of 5 inches of fiberglass?
A bare brick wall leaks 1,175 W and the same wall under 15 cm of wool leaks 169. Get the heat flow, the R-value in both unit systems and what one more layer would buy.
5 inches of fiberglass batt carries a resistance that follows straight from one number: the conductivity of the material, about 0.044 watts per meter per kelvin, which works out at roughly R-3.3 per inch. Multiply that by the depth and you have the R-value of the layer itself, before anything else in the wall is counted.
The figure below adds the two films of still air that cling to the faces of any real surface, worth about R-1 together, because that is how building codes define the number they ask you to hit. US codes commonly want R-13 to R-21 in a wall and R-38 to R-60 in an attic, depending on climate zone, so 5 inches of batt lands somewhere specific on that scale rather than being simply good or bad.
Watts and BTU per hour on the page are worked out for 100 square feet with 70 degrees inside and 10 outside, which is a cold winter day rather than an average one. Change any of those and the heat flow changes with them; the R-value does not, because resistance is a property of the layer and not of the weather crossing it. The comparison table shows what the same depth would give in foam, mineral wool and the masonry it might be sitting against.
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One wall, one afternoon of work, and the difference in watts
A 30 m2 wall of solid brick, 38 cm thick, with 21 °C inside and -5 outside, is losing 1,175.6 W without stopping. Put 15 cm of mineral wool on the outside of it and the same wall loses 169.2 W. Everything else is identical: same bricks, same weather, same house.
Before, brick alone
R-3.8 in total, and R-1.0 of that is the two films of still air clinging to the faces. The brick itself contributes R-2.8 across nearly half a meter of masonry.
After, plus 15 cm of wool
R-26.2 in total. The wool alone is worth R-22.4, eight times what the brick was doing, from a layer a little over a third of its thickness.
That is 1,006.4 W saved, 85.6 percent of the original loss, and 24.2 kWh over a day at that temperature difference. The calculator produces both figures from the same four inputs: what the wall is made of, how thick it is, how big it is and how cold it is outside. Add a second layer and it prints the before, the after and the difference side by side.
How the number is built, layer by layer
- Every layer becomes a resistance. R = thickness divided by conductivity. Fifteen centimeters of mineral wool at 0.038 W/(m K) is 0.15 / 0.038 = 3.95 m2K/W, which is R-22.4 in the imperial units US products are labeled with.
- The air films count as layers too. A thin film of still air clings to each face, worth 0.13 inside and 0.04 outside, R-0.97 together. Building codes quote U-values that include them, which is why the calculator asks whether to count them rather than deciding for you.
- Resistances add up in series. Brick plus wool plus films gives 4.61 m2K/W. This is the only step where the arithmetic is genuinely easy, and it is the reason a thin layer of good insulation beats a thick layer of bad.
- The U-value is one divided by that total. 1 / 4.61 = 0.217 W/(m2 K). Turn it over again whenever someone quotes you a U and you want an R.
- The heat flow is U times area times the temperature difference. 0.217 x 30 x 26 = 169.2 W, and that is the figure your heating has to replace every second of every hour the weather stays like that.
What each extra centimeter buys, and where it stops paying
The same brick wall, the same weather, mineral wool added in five thicknesses. Every row came out of the calculator itself.
| Mineral wool added | Total R | Heat flow | Saved | Gain over the row above |
|---|---|---|---|---|
| Nothing | R-3.8 | 1,175.6 W | - | - |
| 5 cm | R-11.2 | 394.1 W | 66.5 % | 781.5 W |
| 10 cm | R-18.7 | 236.7 W | 79.9 % | 157.4 W |
| 15 cm | R-26.2 | 169.2 W | 85.6 % | 67.6 W |
| 20 cm | R-33.7 | 131.6 W | 88.8 % | 37.6 W |
| 30 cm | R-48.6 | 91.1 W | 92.2 % | 40.5 W, over a 10 cm step |
Read the last column downward and the whole economics of insulation is right there. The first five centimeters kill 781.5 W. The second five kill 157.4 W, a fifth as much for the same material and the same labor. By the step from 15 to 20 cm you are buying 37.6 W. Nothing about that is a diminishing return in the material: resistances add in a straight line, but the heat flow is one divided by their sum, and one over a growing number flattens fast. It is the reason the argument is always about the first layer and never about the last.
R-value per inch, and why one word covers two numbers
A US product is labeled with an R-value in ft2 °F h/BTU, and the rest of the world quotes conductivity in W/(m K) or resistance in m2K/W. One SI unit of resistance is 5.678 imperial ones, exactly, so nothing here is a rule of thumb.
| Material | W/(m K) | R per inch | What that looks like on site |
|---|---|---|---|
| PUR or PIR foam | 0.023 | R-6.27 | The thin option, for when depth is the constraint |
| XPS foam board | 0.033 | R-4.37 | Below grade and under slabs, because it resists water |
| EPS foam board | 0.035 | R-4.12 | The white bead board sold as rigid foam sheathing |
| Mineral wool | 0.038 | R-3.80 | Slightly worse per inch than foam, and it does not burn |
| Blown cellulose | 0.040 | R-3.61 | Attics, where depth is free and access is awkward |
| Fiberglass batt | 0.044 | R-3.28 | Six inches of it is the R-19 batt sold by the roll |
| Aerated concrete block | 0.120 | R-1.20 | Structure and modest insulation from one material |
| Softwood | 0.130 | R-1.11 | The stud that bridges your insulation, in one number |
| Solid brick | 0.770 | R-0.19 | Half a meter of it is worth about half an inch of PUR foam |
| Concrete | 1.700 | R-0.08 | Structural, and thermally close to a hole in the wall |
| Steel | 50 | R-0.003 | A lintel or a balcony slab, carrying heat straight out |
Six surfaces, and what the calculator says about each
Of that resistance, 97.7 percent is the two films of air on the faces. The glass itself contributes 0.004 m2K/W, which rounds to nothing. A window insulates because of the air trapped near it and, in a double unit, the gas between the panes. Never because of the glass.
The steel's own resistance is 0.00002 m2K/W. Take the films away and the flow goes to 1,000,000 W, which is the calculator telling you that a bare metal sheet is not a wall. Any steel that crosses your insulation is a hole in it.
Nearly two kilowatts through one wall, which is a fan heater running permanently. Concrete is structure, not insulation, and every argument about its thermal mass is about when the heat arrives, not how much of it leaves.
Eight times the total resistance of the concrete wall above, and nearly eighteen times the resistance of the concrete itself, from a material that still carries the roof. It sits right at the R-13 that milder climate zones ask of a wall assembly, and short of the R-20 the cold ones want.
This is the standard R-19 batt with the air films added on top of it. Fine for a wall in a cold zone, thin for a ceiling, where the same codes ask for R-38 to R-60 because heat leaves upward and the depth is free.
Over half again the area of the ceiling above, and half the heat loss. Depth in an attic costs almost nothing beyond the material, which is why this is the one place where R-50 and beyond is routine rather than extravagant.
Seven answers about walls that lose more than they should
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Reviewed by: Patryk Matyjasik