How do you calculate a U-value?

Add up the resistance of every layer, its thickness in metres divided by its conductivity λ, plus the two surface resistances (Rsi 0.13 and Rse 0.04 m²K/W for a wall), and take one over the total: that is EN ISO 6946. A wall of 10 mm render, 115 mm brick, 100 mm mineral wool, 100 mm aerated block and 13 mm plasterboard has R = 3.651 m²K/W, so U = 1 / 3.651 = 0.27 W/(m²·K).

What U-value does 100 mm of mineral wool give?

With λ 0.037 and only the two surface resistances of a wall, 0.348 W/(m²·K): 1 / (0.13 + 0.100 / 0.037 + 0.04) = 1 / 2.873. With the render, brick, aerated block and plasterboard of the worked example around it, the same 100 mm gives 0.27. Reaching 0.18 takes 200 mm.

How thick does insulation need to be for a U-value of 0.18?

Counting only the insulation and the surface resistances of a wall, to EN ISO 6946: 200 mm of mineral wool (λ 0.037), 184 mm of XPS (λ 0.034) or 119 mm of PIR board (λ 0.022). The masonry barely helps: a 100 mm brick leaf with render and plasterboard saves about 8 mm.

How do you calculate the insulation thickness for a target U-value?

d = λ × (1/U − Rsi − Rse − R of the other layers). For a wall Rsi is 0.13 and Rse 0.04 m²K/W, so for U 0.18 with mineral wool: (1/0.18 − 0.17) × 0.037 = 0.199 m, which is 200 mm.

Envelope · Thermal

U-value calculator for walls, roofs and floors

A construction's U-value is one divided by the sum of its resistances, and in an insulated wall one layer supplies almost all of it. Add up d/λ for every layer, plus the two surface resistances, and invert: U = 1 / (R_si + Σd/λ + R_se). Build the wall below, with mineral wool, EPS, XPS, PIR or wood fibre as the insulation, and you will see it carrying eighty or ninety per cent of the total while the brick, the block and the plaster fight over the scraps.

—

U-value

— W/(m²·K)

Total resistance R

— m²K/W

Total thickness

— mm

Insulation to hit target

— mm

Where the resistance comes from

Windows are the other half of the envelope, and they are always the weak side: a wall at 0.25 sits next to glazing at 1.1 or worse. Work that out in the window U-value calculator.

Insulation thickness for a target U-value

How much insulation a wall needs to reach a given U-value, counting only the insulation and the two surface resistances (Rsi 0.13, Rse 0.04 m²K/W). That is the safe number, since anything else in the wall can only help. Worked out by the calculator above to EN ISO 6946 and rounded up to the next millimetre. In mm:

Insulation, λ W/(m·K)	U 0.35 R-16 US	U 0.30 R-19 US	U 0.25 R-23 US	U 0.18 R-32 US	U 0.15 R-38 US	U 0.11 R-52 US
Mineral wool, 0.037	100	118	142	200	241	331
EPS, 0.038	103	121	146	205	247	339
XPS, 0.034	92	108	131	184	221	304
Wood fibre board, 0.040	108	127	154	216	260	357
PIR board, 0.022	60	70	85	119	143	197

The rest of the wall barely counts. A 100 mm brick leaf with 15 mm of render and 12.5 mm of plasterboard saves only 8 mm of mineral wool at U 0.18 (192 instead of 200 mm). A roof needs at most 2 mm more than a wall, because heat flowing up meets a smaller inside surface resistance. The R-values are for the whole construction, in US units; for inches, divide the millimetres by 25.4.

Studs, rafters and wall ties cut through the insulation and make it worse. This table, like the calculator, treats the insulation as one continuous layer.

How EN ISO 6946 does it

R_total = R_si + Σ (d / λ) + R_air + R_se U = 1 / R_total d in metres, λ in W/(m·K)

Surface resistances	R_si	R_se
Wall — heat flowing horizontally	0.13	0.04
Roof — heat flowing upward	0.10	0.04
Floor — heat flowing downward	0.17	0.04

Unventilated air cavity	Wall	Roof or floor
10 mm	0.15	0.15
20 mm	0.17	0.16
50 mm and over	0.18	0.16

The resistance of an unventilated air cavity, in m²K/W, from table 2 of the Spanish support document DA DB-HE/1 (January 2020), with a straight line between its rows as the document allows. The calculator takes a cavity from 10 to 300 mm with ordinary faces, not foil. The document gives roofs and floors one column; heat flowing down does not set the air moving, so a floor's cavity resists a little more than that, and the calculator stays on the safe side.

Note what those numbers mean in practice. A wall's two surface resistances add up to 0.17 m²K/W before you have laid a single brick — which is roughly what 6 mm of mineral wool would give you. In a badly insulated wall that is a real fraction of the total; in a good one it is noise. The calculator shows both effects as you build.

Typical conductivities, to get you started

Material	λ W/(m·K)
PIR / polyurethane board	0.022
XPS	0.034
Mineral wool	0.037
EPS	0.038
Wood fibre board	0.040
Aerated concrete block	0.18
Softwood	0.13
Plasterboard	0.21
Hollow clay brick	0.44
Fired clay brick	0.77
Cement render	0.90
Dense concrete	1.75
Glass wool slab	0.035
Stone wool slab	0.036
Cellulose, blown	0.040
Hardwood	0.18
Plywood	0.13
OSB	0.13
Chipboard	0.14
Gypsum plaster	0.4
Sandstone	2.3
Granite	2.8

These are ordinary published figures for orienting a design. For anything you submit, use the declared λ from the product's own datasheet — insulation in particular varies a lot between grades, and the difference between 0.022 and 0.038 is the difference between 100 mm and 170 mm of it.

Worked example you can check by hand

A wall: 10 mm cement render, 115 mm fired brick, 100 mm mineral wool, 100 mm aerated block, 13 mm plasterboard.

R_si = 0.130 render 0.010 / 0.90 = 0.011 brick 0.115 / 0.77 = 0.149 wool 0.100 / 0.037 = 2.703 block 0.100 / 0.18 = 0.556 board 0.013 / 0.21 = 0.062 R_se = 0.040 ------ R_total = 3.651 m²K/W U = 1 / 3.651 = 0.27 W/(m²·K)

The mineral wool is 74 % of the total resistance from 29 % of the thickness. Everything else in that wall — 238 mm of masonry and finishes — contributes less than a quarter.

What this page does not do

No thermal bridges. This is the U-value of the plain build-up. Timber studs, mortar joints, wall ties, lintels and junctions all bypass the insulation, and on a timber-frame or steel-frame wall that correction is large — EN ISO 6946 has a whole method for it that is not here.
No condensation check. Where the dew point falls inside the construction is a separate calculation, and it is the one that decides whether the wall rots.
No ground contact. Floors on the ground follow EN ISO 13370, not this.
No ventilated cavities. A well-ventilated cavity and everything outside it is discounted entirely, which changes the answer a lot.
Typical λ values are typical. Use the product's declared figure for anything real.

Read this. A design and comparison aid, not a compliance calculation. A U-value that goes into a building regulations submission needs the thermal bridging, the declared product data and, usually, accredited software.

Questions people ask

How do you calculate a U-value?: Add up the resistance of every layer, its thickness in metres divided by its conductivity λ, plus the two surface resistances (Rsi 0.13 and Rse 0.04 m²K/W for a wall), and take one over the total: that is EN ISO 6946. A wall of 10 mm render, 115 mm brick, 100 mm mineral wool, 100 mm aerated block and 13 mm plasterboard has R = 3.651 m²K/W, so U = 1 / 3.651 = 0.27 W/(m²·K).
What U-value does 100 mm of mineral wool give?: With λ 0.037 and only the two surface resistances of a wall, 0.348 W/(m²·K): 1 / (0.13 + 0.100 / 0.037 + 0.04) = 1 / 2.873. With the render, brick, aerated block and plasterboard of the worked example around it, the same 100 mm gives 0.27. Reaching 0.18 takes 200 mm.
How thick does insulation need to be for a U-value of 0.18?: Counting only the insulation and the surface resistances of a wall, to EN ISO 6946: 200 mm of mineral wool (λ 0.037), 184 mm of XPS (λ 0.034) or 119 mm of PIR board (λ 0.022). The masonry barely helps: a 100 mm brick leaf with render and plasterboard saves about 8 mm.
How do you calculate the insulation thickness for a target U-value?: d = λ × (1/U − Rsi − Rse − R of the other layers). For a wall Rsi is 0.13 and Rse 0.04 m²K/W, so for U 0.18 with mineral wool: (1/0.18 − 0.17) × 0.037 = 0.199 m, which is 200 mm.