Envelope · Thermal

Wall and roof U-value calculator

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 and you will see the insulation 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.

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

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
Unventilated air cavity, 25 mm	R = 0.18 m²K/W

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.