What is a good U-value for walls?

For UK new-build walls, Part L (Approved Document L) requires U ≤ 0.26 W/m²K. Passivhaus standard targets ≤ 0.15 W/m²K. Existing solid brick walls typically range 1.8–2.1 W/m²K without insulation, 0.28–0.45 W/m²K with a modern insulation system. In the US, ASHRAE 90.1 targets R-13 to R-20 depending on climate zone, equivalent to approximately 0.20–0.35 W/m²K. This calculator computes the exact value for your specific layer build-up.

How do I check for interstitial condensation risk?

Interstitial condensation risk is assessed using the Glaser method (ISO 13788). You enter the construction layers from inside to outside, the internal occupancy humidity class, and the monthly external climate. The method calculates whether vapour diffusing outward through the construction will reach saturation and condense at any interface. This calculator performs that analysis automatically for all 12 months.

What is the Glaser method?

The Glaser method (BS EN ISO 13788:2012) is a steady-state calculation that checks whether water vapour diffusing through a construction element will condense at any internal interface. It plots the saturation vapour pressure (psat) and the actual vapour pressure (p) across the construction. Where the vapour pressure line intersects the saturation curve, condensation occurs. The method is a screening tool — it does not model capillary transport or hygroscopic buffering, but it is the internationally recognised first-pass assessment for building designers.

What is a vapour control layer (VCL) and where should it go?

A vapour control layer (VCL) is a membrane with a high vapour resistance (sd value typically 50–200 m) placed on the warm side of insulation to prevent water vapour from diffusing into the construction and condensing on cold surfaces. It must be on the warm (inner) side of insulation — placing it on the cold side traps moisture and causes failure. This calculator demonstrates the effect clearly: add a VCL as the first layer after the plasterboard and observe the condensation verdict change.

What does 'transient — dries out' mean?

'Transient — dries out' means the construction experiences condensation in some winter months, but the accumulated moisture evaporates completely before the next winter cycle. The construction dries out in spring/summer. This is generally acceptable for robust materials (masonry, concrete), but the peak accumulated mass should be checked against manufacturer guidance for moisture-sensitive materials such as OSB, plasterboard, or timber-based insulation.

Is this calculator suitable for building regulation compliance?

This calculator implements the Glaser steady-state method per ISO 13788:2012 and ISO 6946:2017, which is the method referenced in UK Building Regulations Approved Document C and CIBSE guidance for condensation risk assessment. The results can support a compliance argument, but building control and formal design submissions should always be verified by a qualified engineer. For complex or marginal constructions, a full hygrothermal simulation (BS EN 15026 / WUFI) is recommended.

Why does climate location matter for condensation calculations?

The Glaser method uses monthly mean external temperature and relative humidity to determine how much vapour pressure drives moisture through the construction each month. A wall that passes in mild maritime London may fail in continental cold-dry Berlin or cold-humid Glasgow. This calculator fetches real 20-year climate normals for your specific location via Open-Meteo, so results are tailored to where the building actually is.

How accurate is the live climate data?

Climate normals are computed from ERA5 reanalysis data via Open-Meteo, averaged over 2000–2023. ERA5 is the ECMWF global climate reanalysis dataset, widely used in building physics and CIBSE TRY derivations. Monthly means are accurate to within 0.5–1°C and 3–5% RH for most locations.

How do I convert U-value to R-value (US imperial)?

Multiply the SI R-value (m²K/W) by 5.678 to get the US R-value (ft²·°F·hr/BTU). For example: a wall with U = 0.26 W/m²K has R_total = 1 ÷ 0.26 = 3.85 m²K/W, which equals 3.85 × 5.678 = R-21.8 in US units. To convert a US R-value back to SI U-value: U = 1 ÷ (R_US × 0.1761). This calculator has a built-in SI / US toggle — click 'US (BTU)' in the results panel to switch all thermal values to imperial units automatically.

What R-value should insulated walls have in the United States?

ASHRAE 90.1 and the IECC specify minimum assembly R-values by climate zone. Zone 1–2 (hot/humid — Miami, Houston): R-13 nominal cavity. Zone 3–4 (mixed/marine — Los Angeles, Seattle): R-20 cavity + R-5 continuous. Zone 5–6 (cold — Chicago, Denver): R-20 + R-10 continuous. Zone 7–8 (very cold/subarctic): R-21 + R-12 continuous. This calculator builds the exact assembly layer-by-layer and shows the whole-assembly effective R-value, which is more accurate than relying on a single layer nominal rating.

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U-value & Interstitial Condensation Risk Calculator

Calculate thermal transmittance (U-value / R-value) and assess interstitial condensation risk using the Glaser method — ISO 13788:2012 / ISO 6946:2017 / ASHRAE 90.1. Enter any location worldwide for live 20-year ERA5 climate normals. Toggle between SI (W/m²K) and US imperial (BTU/hr·ft²·°F). .

ISO 13788:2012ISO 6946:2017ASHRAE 90.1BS 5250Open accessSI & US imperialAny location worldwide

Material Library

Building Elements— tap a card to switch · click the name to rename

Wall

2.69 W/m²KVery low

Now editing:External Wall

Element type

Humidity class

Internal temp (°C)

Climate

UK — London (built-in normals)

#	Layer (inside → outside)	Thick. (mm)	sd (m)	R (m²K/W)
—	Internal surface (Rsi)			0.130
1			0.100	0.050
2			1.632	0.132
3			0.380	0.019
—	External surface (Rse)			0.040
	TOTAL	134 mm	2.112	0.371

Units

U-value

2.69

W/m²K

Very Poor

Condensation Risk

VERY LOW

Very low risk — no interstitial condensation predicted in any month under the modelled conditions.

Total R

0.371

m²K/W

Total sd

2.112

m vapour resistance

f_Rsi

0.491

Surface temp factor

Peak mass

None

Monthly overview

JanDec

ISO 13788:2012 · ISO 6946:2017 · BS 5250:2021
ASHRAE 90.1 (US) · NCC (AU) · NZBC H1 (NZ)
Displaying: SI metric (W/m²K)

Glaser Vapour Pressure Diagram

January: ext. 4.7°C, RH 85%, Δp = 715 Pa, p_i = 1441 Pa, p_e = 726 Pa— No condensation

Temperature Profile — January

12-Month Analysis(Class 3 (dwelling), θi = 20°C)

Month	θe (°C)	RH%	Δp (Pa)	p_i (Pa)	p_e (Pa)	Condensation	Rate (g/m²·mo)	Cumulative (g/m²)
January	4.7	85	715	1441	726	—	—	—
February	4.8	82	713	1418	705	—	—	—
March	7.2	77	664	1446	782	—	—	—
April	9.4	73	620	1480	861	—	—	—
May	12.6	73	555	1619	1065	—	—	—
June	15.5	74	496	1799	1302	—	—	—
July	17.8	72	450	1916	1467	—	—	—
August	17.6	74	454	1942	1489	—	—	—
September	14.8	78	510	1823	1312	—	—	—
October	11.2	83	583	1687	1104	—	—	—
November	7.5	84	658	1529	870	—	—	—
December	5.0	85	709	1450	741	—	—	—
Peak accumulated mass								0.000 g/m²

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Got a borderline result?

Try our Enhanced Hygrothermal Calculator — uses 8,760 hourly ERA5 data points and hygroscopic buffering to give a more accurate verdict for marginal constructions.

Try Hygrothermal →

U-value & Interstitial Condensation Risk Calculator

Glaser Vapour Pressure Diagram

Temperature Profile — January

12-Month Analysis(Class 3 (dwelling), θi = 20°C)

Understanding the Calculation

Frequently Asked Questions