Thermal Performance Units: W/mK, R-value and U-value Decoded

Three units describe how a building holds onto its heat, and an extension lives or dies by them. They turn up on every insulation board, in your building regs targets, and on the calculation a thermal assessor hands to building control. The trouble is they look almost identical, the letters reshuffle into each other, and one of them runs the opposite way to the other two. Mix them up and you can specify a board that fails the element without anyone noticing until the inspection.

The three are W/mK, m2K/W and W/m2K. Each measures one specific thing, they connect in a fixed order, and Part L sets its target against the last of them.

The three units at a glance

Note the trap in the "better when" column. Lambda and U-value are both losses, so a low number is good. R-value is a resistance, so a high number is good. The R-value running the opposite way to the other two is the single most common mix-up, and it is why a board with a small lambda carries a large R-value.

W/mK: thermal conductivity (lambda)

Thermal conductivity, written W/mK and called the lambda value, is a property of the material itself. It says how readily heat passes through it, with no reference to how thick the board is. A low number is a good insulator.

The spread between materials is enormous, which is the whole reason a thin sheet of foam can out-insulate a thick masonry wall. PIR rigid foam, the silver-faced board you see most on extensions, sits at 0.022 W/mK. Mineral wool, the soft non-combustible quilt, sits higher at 0.032-0.044 W/mK, and expanded polystyrene at 0.032-0.038 W/mK. A lightweight aircrete block, the kind used for the inner leaf of a cavity wall, comes in around 0.15 W/mK. A dense aggregate block is far worse at roughly 0.9 to 1.3 W/mK, an order of magnitude leakier than the foam.

That last comparison is worth holding onto. The foam at 0.022 and the dense block at 1.3 are doing the same thing, conducting heat, but the foam does it dozens of times more slowly. Lambda is the number that decides how much of any material you need.

m2K/W: R-value

Thickness is missing from lambda, and the R-value puts it back. The R-value, written m2K/W, is the thermal resistance of one layer once you account for how thick it is. The formula is simple:

R-value = thickness in metres / lambda

So a 100mm PIR board is 0.100m divided by its lambda of 0.022, which gives an R-value of 4.55 m²K/W. Halve the board to 50mm and the resistance halves to 2.25 m²K/W. Take it up to 120mm and it climbs to 5.45 m²K/W. Same material, more thickness, more resistance.

Higher is better here, because resistance is what slows the heat down. The R-value is usually the big headline figure printed on the face of an insulation board, which is convenient, because it already folds the thickness and the lambda together into one number you can compare across products.

W/m2K: U-value

The U-value, written W/m2K, is the figure that actually matters to building control. It measures heat loss through a whole element, the complete build-up of a wall, floor, roof or window, including every layer plus the still air on either face. A low number means little heat escapes.

You get there by adding up the R-values of all the layers in the element and inverting the total:

U-value = 1 / (sum of all the layer R-values)

The inversion is why a high R-value gives a low U-value. Pile up resistance and the transmittance drops. This is the number Part L sets its targets against, and it is what a thermal assessor or a SAP calculation produces from the lambda figures listed on each datasheet. A homeowner with a calculator can get a rough version, but the real number also accounts for thermal bridging through studs and joists, air leakage, and the perimeter of a floor, so a hand calculation is always optimistic.

How the three connect

Read top to bottom, the chain is straightforward. A lower lambda or a thicker board gives a higher R-value, and a higher total R-value gives a lower U-value. Thicker insulation always pulls the U-value down.

The worked example that decides most extensions is the solid floor. A 100mm PIR board on its own carries an R-value of 4.55. Dropped into a typical floor build-up of slab, screed and surface resistances, the whole floor lands at about 0.17 W/m2K, which sits under the extension floor target of 0.18 and passes. Take the same floor and swap the board down to 75mm and the whole element rises to roughly 0.22 W/m2K, which is over the limit and fails. Same material, 25mm thinner, no longer compliant. That is the kind of substitution that gets caught at the structure inspection and forces a board change after the slab is already down.

Part L 2021 target U-values

These are the maximum U-values Part L allows for the elements of a domestic extension in England. Lower than the figure is fine; higher fails.

Two caveats sit around that table. The 1.4 figure for windows is the limiting maximum, the line an extension window must not cross, while 1.2 is the design target you would aim for on a new dwelling, so treat 1.4 as the backstop rather than the goal. Scotland runs tighter under its own technical handbook: floors and roofs both target 0.13 W/m2K rather than the England figures. And if your plans were submitted before 15 June 2022, they fall under the older Part L 2013 standard, where the limits were looser, a wall at 0.28 and a floor at 0.22, which is exactly why an old set of drawings can specify insulation that no longer passes today.

Why the small print matters

The notation is where two boards that look identical quietly differ. A board with a lower lambda reaches the same U-value in less thickness, so a phenolic board can hit a target that a PIR board only meets when it is fatter, and that extra thickness changes your cavity width and every detail that follows. A wrong-spec substitution is worse, because it fails the element without looking any different on site. A 75mm board where the calculation assumed 100mm, a dense block where the design wanted aircrete, an EPS board swapped for PIR at the same thickness: each one looks like insulation, and each one quietly pushes the U-value over the line.

The habit that protects you is short. Read the lambda on the datasheet, check the thickness against the drawing, and confirm the U-value the calculation produced before you sign the element off. The three units are the only way to tell a compliant build-up from one that will be condemned at inspection.

For the full set of notation you meet across an extension, from kN and N/mm2 to falls and flow rates, see the master decoder: Units and symbols: a quick-reference decoder for your build.