The U-value is the single number that tells you how well a wall, roof or floor keeps heat in. Understand it and you can judge any insulation on its real performance rather than its marketing. This plain-English guide explains what a U-value is, how it relates to lambda and thickness, and why installation matters just as much.
By Adam Gough, Founder & Lead Adviser, Insulation Advice Centre
Last reviewed: 18 August 2026. Surface resistances follow BS EN ISO 6946 (BR 443); target U-values reflect typical current Approved Document L guidance. Verify against the current Building Regulations for your project.
Use our free calculator, or ask us for advice. No cost, no obligation.
If you only learn one number when insulating your home, make it the U-value. It is what building control checks, what decides your heating bills, and what product marketing is most likely to blur. The good news is that it is not complicated once you know what it describes. Want to skip straight to the maths? Use our free U-value calculator; the guide below explains what the answer means.
A U-value measures how much heat passes through a part of your building, such as a wall, roof or floor. It is measured in watts per square metre for each degree of temperature difference (W/m2K). The important thing to remember is that lower is better: a lower U-value means less heat escapes and lower bills. As a rough sense of scale, a solid uninsulated wall might be around 2.0, while a well-insulated modern wall should be about 0.18 or lower.
These two are often confused, and the difference matters. Lambda (thermal conductivity, W/mK) is a property of a single material: how easily heat passes through it. Mineral wool has a lambda of about 0.040; PIR about 0.022. U-value is a property of a whole build-up: the wall or roof as a complete element, including every layer and the air films on each surface. You size insulation by working from the target U-value back to the thickness you need, using the material's declared lambda. This is also why you should be wary of any product sold on an "equivalent performance" claim that is not backed by a tested U-value for the actual build-up, multifoil being the clearest example, since its performance depends on air gaps and cannot be captured by a single lambda.
Each layer has a thermal resistance (R), which is simply its thickness divided by its lambda. Add up the resistances of all the layers, plus the standard inside and outside surface resistances (BS EN ISO 6946, often referred to as BR 443), and the U-value is one divided by that total resistance. Two levers lower the U-value: more thickness, or a lower-lambda material. That is why 270mm of mineral wool and a much thinner PIR board can reach the same U-value. Our U-value calculator does this sum for you and compares the result to typical targets.
As a rough guide to typical current UK targets (Approved Document L, and always verify against the current Building Regulations for your project): a loft or roof around 0.16, an external wall around 0.18 for new work, and a floor around 0.18 to 0.25. Many existing homes are well above these numbers, which is exactly where adding insulation pays off. Do not treat any single insulation thickness as automatically compliant in every building, as the right thickness depends on the material and the construction.
A U-value is a calculation for a perfect build-up. In the real home, workmanship decides whether you actually get it. Gaps between boards, insulation compressed under loft boarding, or a quilt pushed into the eaves all let heat leak through and undermine the figure on paper. The lesson: the best product fitted badly can underperform a modest product fitted well, so judge a job on both the specification and the quality of the fit.
A thermal bridge is a path where heat escapes more easily than through the surrounding insulation, for example a timber stud, a steel lintel, or a junction where insulation is interrupted. Thermal bridges lower the real-world performance below the ideal U-value and can also create cold spots where condensation forms. Good design and careful installation keep bridging to a minimum, which is another reason the quality of the work matters, not just the material.
The practical takeaway: get the declared lambda and thickness for the product you are actually being quoted, work out the U-value, and check it against the target for your application before you buy. It takes a couple of minutes with our U-value calculator. If you want a second opinion on a spec or a quote, we are independent and happy to help. Tell us about your project in under a minute, or call us free on 0800 4947163.
The questions UK homeowners ask us most about U-values.
A U-value measures how much heat passes through a building element such as a wall, roof or floor, in watts per square metre per degree (W/m2K). Lower is better: a lower U-value means less heat escapes. It is the reciprocal of the total thermal resistance of the build-up, including the inside and outside surface resistances.
Lambda (thermal conductivity) is a property of a single material, how easily heat passes through it. U-value is a property of a whole build-up, the complete wall or roof including all layers and surface resistances. You use a material's lambda to work out how much of it you need to hit a target U-value.
As typical current UK guidance (Approved Document L, verify against current regs): a loft or roof around 0.16, an external wall around 0.18 for new work, and a floor around 0.18 to 0.25. Many existing homes are well above these, which is where adding insulation pays off.
A lower U-value for the element is better, but it is achieved by the whole build-up, not just the product. Two materials reach the same U-value at different thicknesses depending on their lambda. And a good U-value on paper only helps if the insulation is installed well, without gaps, compression or thermal bridging.
A thermal bridge is a path where heat escapes more easily than through the surrounding insulation, such as a timber stud, a steel lintel or a junction where insulation is interrupted. Bridges lower real-world performance below the ideal U-value and can create cold spots where condensation forms. Careful design and installation minimise them.
Our free U-value calculator lets you enter your material and thickness and see the U-value against current Building Regs targets. If you want a second opinion on a quote, we are happy to help, and if you are weighing up materials, our comparison of multifoil vs mineral wool covers how each performs over time.
Open the U-value calculator →U-value calculator · Insulation comparison · Mineral wool · PIR insulation · Loft insulation · Loft insulation depth · Multifoil vs mineral wool
Insulation Advice Centre