UK vs US Building Performance Terms: A Practical Comparison

If you have ever come across building performance standards, guidance or technical information from across the pond, whether you are based in the UK or the US, it can sometimes be difficult to understand exactly what you are looking at.

The underlying building physics may be the same, but different terminology, units and assessment methods are often used. For example, in the UK you are likely to come across the g-value of glazing, while in the US you are more likely to see SHGC, or Solar Heat Gain Coefficient.

This can make things confusing when working between the UK and US, reading overseas guidance, reviewing product information or trying to translate information from one system into another. That is why we have created the tables below, bringing together some of the most common differences in building performance terminology, measurements, standards and assessment methods.

The UK predominantly uses metric or SI units, while US building practice still makes extensive use of US customary units. US building energy regulations also vary by state and local jurisdiction, so not every US term or requirement will apply everywhere in the country.

Building Fabric & Envelope

Building performance concept	UK	US	Notes
Building exterior	Building fabric / building envelope	Building envelope	Building envelope is also used in the UK, but building fabric is particularly common in UK practice.
Thermal transmittance	U-value, W/m²K	U-factor, often Btu/h·ft²·°F	These describe the same underlying heat-transfer property. ASHRAE also recognises U-factor in SI units.
Thermal resistance	R-value, m²K/W	R-value, commonly h·ft²·°F/Btu	Same underlying principle, but US R-values use different units and feature much more prominently in US insulation specifications.
Thermal conductivity	λ-value / lambda value, W/mK	k-factor / thermal conductivity	ASHRAE uses the term k-factor for thermal conductivity.
Solar heat gain through glazing	g-value	SHGC	SHGC stands for Solar Heat Gain Coefficient. US NFRC window ratings use SHGC alongside U-factor and visible transmittance.
Visible light through glazing	Light transmittance / VLT	Visible Transmittance / VT	NFRC uses Visible Transmittance, or VT, as one of its window performance ratings.
Low-emissivity glazing	Low-E glazing	Low-E glazing	Essentially the same terminology.
Airtightness measurement	Air permeability, commonly m³/(h·m²) at 50 Pa	ACH50 / CFM50	Related measurements, but not directly interchangeable. ACH50 is air changes per hour at 50 Pa, while CFM50 describes the airflow measured at 50 Pa.
Airtightness test	Air pressure test / airtightness test	Blower door test	Blower door is particularly common terminology in the US.
Reducing air leakage	Airtightness / draught-proofing / air sealing	Air sealing	The underlying aim is the same, although air sealing is particularly common US building-science terminology.
Vapour control	Vapour control layer / VCL / vapour barrier	Vapor retarder / vapor barrier	Similar concept, with different terminology and US spelling.
External insulation	External insulation / continuous insulation	Continuous insulation / ci	Continuous insulation is a common US energy-code term.
Thermal bridging	Thermal bridging	Thermal bridging	Essentially the same terminology.
Window performance rating	U-value, g-value, light transmittance and product data	NFRC U-factor, SHGC and VT ratings	NFRC provides a standardised US system for rating fenestration products.

Building Services & Lighting

Building performance concept	UK	US	Notes
Airflow	L/s or m³/s	CFM	CFM means cubic feet per minute.
Temperature	°C	°F	The UK primarily uses Celsius, while Fahrenheit remains standard in US building practice.
Heating and cooling capacity	kW	Btu/h	US HVAC equipment frequently expresses capacity in British thermal units per hour.
Cooling capacity	kW	Btu/h / tons of cooling	A ton in US HVAC is a measure of cooling capacity, not equipment weight.
Fan energy performance	Specific Fan Power, SFP, commonly W/(L/s)	Fan power, often expressed in W/cfm	The concepts are closely related, but the compliance metrics and units are not always directly interchangeable. ASHRAE 90.1 provisions use fan-power metrics including W/cfm.
Duct airtightness	Duct leakage rates / leakage classes	CFM25 and other duct leakage metrics	US residential testing commonly measures duct leakage at a pressure of 25 Pa.
Ventilation guidance	Approved Document F / CIBSE guidance	ASHRAE 62.1 / 62.2	These are not direct equivalents, but ASHRAE 62 standards are major US references for ventilation and indoor air quality.
Thermal comfort	CIBSE guidance / TM52 / TM59	ASHRAE Standard 55	ASHRAE 55 sets criteria and calculation procedures relating to thermal comfort.
Overheating assessment	Part O / TM52 / TM59	No single direct equivalent	US projects may use ASHRAE 55, building energy modelling and state or local requirements depending on the project.
Boiler efficiency	Seasonal boiler efficiency / UK product efficiency metrics	AFUE	AFUE means Annual Fuel Utilization Efficiency and is widely used for US furnaces and boilers.
Heat-pump seasonal heating performance	SCOP	HSPF2	HSPF2 is a US seasonal heating performance metric used for relevant heat pumps.
Air-conditioning seasonal efficiency	SEER / seasonal efficiency metrics	SEER2	SEER2 is used for relevant US air-conditioning and heat-pump equipment.
Illuminance	Lux	Foot-candles / lux	Foot-candles are genuinely used in US lighting practice, although lux is also recognised. The Illuminating Engineering Society provides recommendations in both formats.
Lighting efficacy	lm/W	lm/W	Lumens per watt is used in both the UK and US.
Lighting power density	W/m²	W/ft²	Same concept, but different floor-area units.
Colour temperature	Kelvin, K	Kelvin, K	Same.
Colour rendering	CRI	CRI	Same.

Energy, Metering & Renewables

Building performance concept	UK	US	Notes
Building energy intensity	kWh/m²/year	EUI, commonly kBtu/ft²/year	EUI means Energy Use Intensity. ENERGY STAR calculates EUI by dividing annual energy use by gross floor area.
Floor area	m²	ft²	A basic but important difference when comparing building sizes or energy-intensity figures.
Electricity consumption	kWh	kWh	Same.
Electrical power	kW	kW	Same.
PV array rated capacity	kWp	kWdc / DC nameplate capacity	UK solar installations are commonly described in kWp. US solar analysis often explicitly describes PV array capacity as DC capacity.
PV inverter / AC capacity	kW	kWac	US PV documentation frequently distinguishes between PV capacity in kWdc and inverter or AC capacity in kWac.
PV energy generation	kWh	kWh	Same.
Battery energy capacity	kWh	kWh	Same.
Battery power	kW	kW / sometimes explicitly kWac or kWdc	The distinction depends on where the power is being measured in the system.
Operational energy benchmarking	kWh/m²/year and UK operational-energy schemes	EUI / ENERGY STAR Portfolio Manager	Portfolio Manager can benchmark building energy use and eligible building types can receive a 1 to 100 ENERGY STAR score.

Weather Data & Energy Modelling

Building performance concept	UK	US	Notes
Typical annual weather file	CIBSE TRY	TMY / Typical Meteorological Year	Both are used to represent typical weather for annual modelling, although they should not be assumed to be created using identical methodologies. CIBSE recommends TRY files for annual energy assessment.
Hot-weather / overheating weather file	CIBSE DSY1, DSY2 and DSY3	No direct DSY equivalent	CIBSE DSYs represent different hot-weather events and are intended for overheating assessment.
Weather file format	EPW and other formats	EPW and other formats	EPW files are used internationally, so EPW itself is not a UK-versus-US distinction.
Heating degree days	HDD, commonly using °C-based conventions	HDD, commonly using °F-based conventions such as HDD65	US climate data frequently uses a 65°F base.
Cooling degree days	CDD, commonly using °C-based conventions	CDD, commonly using °F-based conventions such as CDD65	Always check the base temperature before comparing degree-day figures between datasets.
Energy modelling	Energy modelling / Dynamic Simulation Modelling, DSM	Building Energy Modeling, BEM	DOE uses Building Energy Modeling, or BEM, as the general US term for physics-based building energy simulation.
Building simulation software	IES VE, TAS, DesignBuilder, EnergyPlus and others	EnergyPlus, OpenStudio and other BEM tools	There is significant international overlap in software. EnergyPlus is DOE’s open-source whole-building energy modelling engine.
Regulatory non-domestic modelling	NCM using SBEM or an approved DSM	No single national equivalent	For non-domestic EPCs in England, the approved methodology uses SBEM or an approved DSM. The US regulatory system is more decentralised.
Performance-based code modelling	Part L / NCM compliance modelling	IECC and ASHRAE 90.1 performance approaches	These serve related functions but are not direct equivalents. ASHRAE 90.1 sets minimum energy-efficiency requirements for most buildings other than low-rise residential buildings.
Simplified commercial code compliance	SBEM in the UK NCM system	COMcheck	COMcheck performs a related compliance function for supported US energy codes, but it is not an American version of SBEM.
Residential compliance calculation	SAP / HEM and associated UK methodologies	REScheck and other code-compliance approaches	Again, these perform related functions rather than being direct equivalents.

Regulations, Ratings & Professional Standards

Building performance concept	UK	US	Notes
Building regulations	Building Regulations	Building codes	US building codes are generally adopted and enforced at state or local level rather than through a single direct equivalent to the UK Building Regulations.
Building energy regulation	Part L	IECC / ASHRAE 90.1, depending on jurisdiction and building type	Part L covers energy efficiency requirements in England. ASHRAE 90.1 is a major US commercial-building energy standard.
National energy calculation methodology	NCM	No single direct equivalent	The UK NCM is a nationally approved methodology for calculating building energy performance.
Non-domestic calculation methodology	SBEM / approved DSM	No single direct equivalent	SBEM or an approved DSM is used for relevant non-domestic EPC calculations in England.
Residential calculation methodology	SAP / HEM	No single direct equivalent	SAP has historically been used for Part L compliance and domestic EPCs in the UK regulatory system.
Energy Performance Certificate	EPC, generally A to G	No single nationwide US equivalent	The US has a number of different building-rating and benchmarking systems rather than one national EPC regime.
Residential energy rating	EPC	DOE Home Energy Score and other systems	DOE’s Home Energy Score rates homes on a 1 to 10 scale. It should not be described as simply “the American EPC”.
Commercial operational benchmarking	DEC / other operational energy benchmarking	ENERGY STAR Portfolio Manager	Portfolio Manager can track energy performance and eligible buildings can receive a 1 to 100 ENERGY STAR score.
Sustainable building assessment	BREEAM	LEED	BREEAM originated in the UK and LEED in the US, but both are now used internationally. LEED describes itself as a globally recognised green-building rating system.
BREEAM in the US	BREEAM is widely established in UK practice	BREEAM is also available in the US	BREEAM International New Construction is used for new developments in the US and Canada.
Professional sustainability credential	BREEAM AP	LEED AP	Both are professional credentials associated with their respective sustainability assessment systems, although they are not direct equivalents.
Building services professional institution	CIBSE	ASHRAE	A useful practical comparison rather than a direct institutional equivalent. CIBSE is a global professional body for building professionals, while ASHRAE is an international society focused on building systems, HVAC, energy efficiency, indoor air quality and related areas.

Discipline

Building performance concept	UK	US	Notes
Discipline concerned with the physical performance of buildings	Building physics	Building science	Broadly comparable terms. Both cover areas such as heat, air, moisture, building envelope performance, energy and indoor environmental conditions, although their professional usage and emphasis can differ.

Same Physics, Different Language

Not every building performance term needs translating. Low-E glazing, thermal bridging, kilowatt-hours, lumens per watt, Kelvin and CRI, for example, are already used on both sides of the Atlantic.

The purpose of this guide is not to suggest that every UK term has an American equivalent, but to make it easier to understand building performance information when moving between the two systems.

If you have any questions about this content, or there is a UK or US building performance term you think we should add, let us know here.