Resistance to heat getting through
An R-value is a measure of how much a material resists heat passing through it. The R is for resistance, and the direction is the intuitive one: a bigger number is better insulation.
It behaves the way electrical resistance does, which is what makes it a useful unit rather than just a rating. Stack layers up and their resistances add: board plus insulation plus sheathing is the sum of the three, so you can build up a wall on paper and get a number for the whole thing.
Its opposite is the U-value, and they are the same fact upside down
A U-value measures how readily heat passes through, so lower is better. R and U are reciprocals: one over the other, in matching units. Nothing separates them but which way round you find it convenient to think.
In practice they attach to different things. An R-value usually describes a material or a layer, which is why insulation is sold by it. A U-value usually describes a whole finished element, a complete wall or roof with everything in it, which is why building regulations are written in it.
The trap: there are two numbers both called R
This is the one that costs people money, because both numbers are printed as R and neither says which system it belongs to.
American R-values are in imperial units. Metric R-values, properly RSI, are in metric ones. The conversion is a factor of about 5.68, and Australia, which uses the metric version, sells batts labelled R2.5 that sit right in the middle of the American R-13 to R-15 range.
So the reading order matters. Multiply an American R by 0.176 to get metric RSI.Divide a metric R by 0.176 to get the American figure. And one divided by an RSI gives you the U-value British regulations are written in.
A modern British wall target of U 0.18 comes out as RSI 5.56, which is American R-31.5. Somebody comparing "R-31" against "0.18" with no conversion has no idea they are looking at the same wall.
Where the number stops describing your building
R-values add up in series, and heat does not have to travel in series. It will happily go around the insulation through anything that conducts better, and a timber stud does. That is why the R figure on the packet and the R figure of the wall you built are two different things, and it is the whole subject of the R-value calculator, which works out the whole-wall figure once the framing is counted.
Two more things R-value is silent about. It says nothing about air leakage, which on a draughty building moves more heat than the fabric does, so insulating a leaky house disappoints people who did the sums correctly. And it says nothing about moisture, which decides whether the assembly is still doing its job in fifteen years.
Putting a number on your own wall
For a build-up, add the layers and remember the studs: the R-value calculator handles the framing factor and the climate zone tables, which is where the required figure actually comes from. British readers working in U-values want the insulation and U-value tool instead, which is the same physics with the arithmetic the other way up.
If the wall is being built rather than assessed, the stud wall explainer covers what is in the void and why an internal partition is usually insulated for sound rather than for heat, which is a different reason to fill it and a much cheaper standard to hit.
Common questions
Is a higher or lower R-value better?
Higher. R stands for resistance, and what it is resisting is heat moving through the material, so more resistance means less heat escaping. That is the opposite of a U-value, which measures how readily heat passes through and where lower is better, and it is the single most common confusion between the two systems. If a number is called R, more is better. If it is called U, less is.
What is the difference between an R-value and a U-value?
They are reciprocals of one another, so one is simply one divided by the other, expressed in matching units. R measures resistance and U measures transmittance. The other practical difference is what they are usually applied to: an R-value normally describes a material or a layer, while a U-value normally describes a whole finished element such as a complete wall including its studs, cavity, sheathing and finishes. So adding up R-values of layers and inverting the total is roughly how you get to a U-value.
Why is an Australian R2.5 batt not the same as an American R-13?
Because they are, near enough, exactly the same batt with the number printed in different units. American R-values use hours, square feet, degrees Fahrenheit and BTU. Metric R-values, properly called RSI, use square metres, kelvin and watts. One American R is 0.176 metric, so multiply an American figure by about 0.176 to get metric, or divide a metric figure by 0.176 to go the other way. R2.5 divided by 0.176 is 14.2, which is why an Australian R2.5 sits between an American R-13 and R-15.
Can I just add R-values together?
For layers in series, yes, and that is the useful property of the unit: plasterboard plus insulation plus sheathing is the sum of their individual resistances, plus small allowances for the air films on each face. Where it stops working is anything that lets heat go around the insulation instead of through it. A timber stud bridges the cavity and conducts far better than the insulation beside it, so the real performance of a wall is well below the sum of its middle-of-the-bay layers.
Does R-value tell me everything about insulation?
No, and treating it as the whole answer is how people end up disappointed with an expensive upgrade. R-value describes resistance to conducted heat and says nothing about air leakage, which on a lot of buildings moves more heat than the fabric does. It also says nothing about moisture behaviour, about whether the material will slump in a cavity over twenty years, or about performance changing with temperature, which some foam products genuinely do.
Does the UK use R-values at all?
Rarely on their own, which is why British readers find American insulation advice hard to follow. British building standards specify a U-value for the finished element and insulation products are usually sold quoting lambda, their thermal conductivity, from which an R-value for a given thickness can be worked out by dividing thickness by lambda. So a British reader looking at an American R figure has two steps to make: convert the units, then invert to get to the U-value the regulations actually ask for.