Insulation R-Value & U-Value Calculator

U-Value and Insulation Guide

What should I know about R-Value and U-Value Explained?

Thermal resistance (R-value): R = thickness (m) / thermal conductivity (λ W/mK). Units: m²K/W. Higher R = better insulation. R-values of multiple layers ADD together. U-value (thermal transmittance): U = 1 / R_total. Units: W/m²K. Lower U = better insulation. The U-value tells you how many watts escape through 1 m² of construction for every 1°C temperature difference. A wall with U = 0.3: if inside is 20°C and outside is 0°C (ΔT = 20°C): heat loss = 0.3 × 20 = 6 W/m².

What should I know about UK Building Regulations Part L (2022)?

Minimum U-values for new builds (England, Part L 2021): External walls: 0.18 W/m²K. Roofs (pitched, insulation at ceiling): 0.16 W/m²K. Ground floors: 0.13 W/m²K. Windows and doors: 1.4 W/m²K. Existing building extensions: walls 0.28, roofs 0.18, floors 0.18. Typical uninsulated cavity wall: U = 1.5 W/m²K. With 100mm mineral wool cavity fill: U ≈ 0.45 W/m²K. With 100mm PIR full-fill: U ≈ 0.22 W/m²K. For Part L 2021 compliance, 150mm mineral wool in a cavity wall achieves approximately U = 0.18 W/m²K.

What's the key thing to understand about Thermal Conductivity (λ) Values?

Common building material conductivities: brick: 0.77 W/mK. Concrete block (dense): 1.5 W/mK. Concrete block (aerated): 0.18 W/mK. Timber: 0.13 W/mK. Plasterboard: 0.25 W/mK. Render: 1.0 W/mK. Air cavity (unventilated 25mm): R = 0.18 m²K/W. External surface resistance: Rse = 0.04 m²K/W. Internal surface resistance: Rsi = 0.13 m²K/W. These surface resistances must be added to get total R of a complete construction — they represent the thermal film on surfaces.

What's the key thing to understand about Avoiding Thermal Bridges?

A thermal bridge is a path of significantly lower thermal resistance through a construction — a 'cold spot'. Common thermal bridges: structural timbers within an insulated frame (timber has λ = 0.13, much higher than insulation). Concrete columns in insulated walls. Lintels over windows. Floors meeting walls. Roof/wall junctions. Thermal bridges can dramatically reduce the effective U-value below the theoretical value. Continuous insulation (no breaks) prevents thermal bridging. Structural thermal breaks — specialised materials inserted at junctions like balconies or steel beams — are used in new-build construction specifically to interrupt these bridges.

What do I need to know about Combining Layers of Different Materials?

Total thermal resistance for a multi-layer construction, such as a wall or roof build-up with several different materials, is simply the sum of the R-values of each individual layer, including any air gaps, which is why this calculator allows multiple layers to be added together rather than treating the construction as a single material. This additive principle holds because each layer resists heat flow independently, in series, so a plasterboard layer, an insulation layer, and an air gap each contribute their own resistance to the total, and the overall R-value is simply their sum. This is useful for understanding upgrade options: adding a second layer of insulation, for example a 50mm board on top of an existing 100mm layer, increases the total R-value by the R-value of the new layer alone, not by recalculating the whole construction from scratch. It also explains why thin, high-performance insulants, such as rigid PIR boards with a lower thermal conductivity than standard mineral wool, can match the R-value of a much thicker, cheaper material in situations where the available depth is limited, such as a loft conversion floor or a thin internal wall upgrade.

What should I know about Moisture and Insulation Performance?

Insulation materials rely on trapped air (or another low-conductivity gas) within their structure to resist heat flow, and most common insulants lose a meaningful amount of their rated performance if they become damp, because water conducts heat far more readily than trapped air and effectively short-circuits the insulation's resistance. Mineral wool and similar fibrous insulants are particularly vulnerable to this if they get wet from a roof leak, rising damp, or interstitial condensation, and performance doesn't always fully recover once dried out, since fibres can compress or mat together. This is why a vapour control layer, a membrane positioned on the warm side of insulation in a wall or roof build-up, matters as much as the insulation thickness itself: it's there to stop warm, moist internal air migrating into the colder parts of the construction where it could condense inside the insulation. Getting the vapour control layer on the wrong side, or omitting it where the construction needs one, is a common cause of hidden damp problems in insulated lofts, timber-framed walls and insulated rooms-in-roof, even when the insulation thickness itself was calculated correctly.

How Do Mineral Wool, PIR and Natural Fibre Insulation Compare?

The three most common insulation material categories differ meaningfully in thermal performance per unit thickness, cost, and other practical factors worth weighing alongside R-value alone. Mineral wool (glass wool or rock wool) is the most widely used loft and cavity insulation material in the UK, offering solid thermal performance at relatively low cost, along with good fire resistance and sound insulation properties, though it needs more thickness than some alternatives to achieve the same R-value and can lose some performance if it becomes compressed or damp. PIR (polyisocyanurate) rigid foam boards achieve a notably higher R-value per unit thickness than mineral wool — often requiring roughly half the thickness for an equivalent thermal performance — which makes them popular where space is limited, such as in floor or roof insulation retrofits where thickness is constrained, though PIR boards cost more per square metre and require careful detailing at joints to avoid thermal bridging through gaps. Natural fibre insulations — sheep's wool, wood fibre, hemp — generally sit between mineral wool and PIR on thermal performance per thickness, cost more than mineral wool, but offer better breathability (useful in older solid-wall buildings prone to damp issues if sealed with less breathable materials) and a lower embodied carbon footprint, appealing to those prioritising sustainability alongside thermal performance. The right choice depends on the specific application, available space, and whether breathability or raw thermal performance per centimetre is the priority for that particular part of the building.

Typical R-Values by Material (Reference Table)

R-value (thermal resistance) depends on both the material and its thickness. The figures below are broadly typical for common UK insulation products at commonly installed thicknesses — actual values vary by manufacturer and product, so always check the specific product's declared R-value where precision matters.

Worked example: a loft currently has 100mm of mineral wool (R ≈ 2.5) and is being topped up to the current recommended depth of around 270mm. R-value scales roughly linearly with thickness for a given material, so the additional 170mm layer contributes approximately (170/100) x 2.5 ≈ 4.25 of extra resistance. Because insulating layers in series simply add together, the new total comes to roughly 2.5 + 4.25 ≈ 6.75 m²K/W — in line with the typical reference value for a 270mm depth shown above. This same series-addition principle is also why a thin, squashed layer of loft insulation performs worse than its rated R-value: compressing mineral wool under stored boxes or boarding reduces the trapped air gaps that give it most of its insulating effect, so its real installed R-value can fall well short of the manufacturer's figure measured at full, uncompressed thickness. Similarly, loft insulation that has absorbed moisture from a roof leak or poor ventilation loses a significant part of its resistance, since the still air pockets that insulate are partly displaced by water.

R-Value, U-Value and Building Regulations Part L

R-value and U-value describe the same physical property from opposite directions: U-value is simply the reciprocal of total thermal resistance, U = 1/R. A higher R-value (more resistance to heat flow) always means a lower U-value (less heat lost), and vice versa. This simplified U=1/R relationship is useful for quick comparisons, but it's worth knowing its limits: a real U-value calculation for a wall, roof or floor (to the method in BS EN ISO 6946) also accounts for the internal and external air-film resistances at each surface and for any thermal bridging (timber studs or joists interrupting the insulation layer), so the U-value of a complete wall or roof build-up is not exactly 1 divided by the insulation material's R-value alone — it's 1 divided by the total resistance of the whole construction. UK Building Regulations Approved Document L sets minimum (limiting) and notional target U-values for new work and major renovations, and as a general pattern the standards are tightest for roofs, looser for walls, and loosest for windows and doors — reflecting both what's achievable and where the most heat is typically lost. Exact limiting figures have been revised over successive versions of Part L and differ for new-build versus retrofit/renovation work, so anyone relying on a specific figure for Building Regulations compliance should check the current Approved Document L (or an equivalent SAP/RdSAP assessment) for the exact standard that applies to their project, rather than assuming a figure from a general guide.