A
ACH (Air Changes per Hour)
How many times the entire volume of air inside your home gets replaced in one hour. A draughty Victorian terrace might have 10+ ACH, while a modern airtight home aims for under 3. Lower ACH means less heat escaping, but you need proper ventilation to keep air fresh.
Air Permeability
A measurement of how much air leaks through the fabric of a building at a given pressure. Tested using a blower door and expressed as cubic metres per hour per square metre of building envelope. The lower the number, the more airtight the building.
Airtightness
How well sealed a building is against uncontrolled air leakage. Good airtightness stops warm air escaping through gaps around windows, doors, pipes, and junctions. One of the most cost-effective energy upgrades, but must always be paired with proper ventilation.
ASHP (Air Source Heat Pump)
A heating system that extracts heat from the outside air, even when it is cold, and uses it to heat your home and water. Works like a fridge in reverse. Eligible for the Boiler Upgrade Scheme grant in England and Wales.
B
BER (Building Energy Rating)
The Irish equivalent of an EPC. Rates a building's energy performance on a scale from A to G. Sometimes referenced in UK contexts when comparing international standards.
Blower Door Test
A diagnostic test where a powerful fan is fitted into an external doorway and pressurises the building to measure how much air leaks out. The results tell you how airtight your home is and help identify where draughts are coming from.
BUS (Boiler Upgrade Scheme)
A UK government grant that helps homeowners in England and Wales replace fossil fuel boilers with heat pumps. As of 2026, the grant is £7,500 for air source heat pumps and £7,500 for ground source heat pumps.
C
Carbon Footprint
The total amount of greenhouse gases produced by a building's energy use, including heating, hot water, lighting, and appliances. Measured in kilograms or tonnes of CO2 equivalent per year.
Cavity Wall
A wall made of two layers with a gap between them. Most UK homes built after the 1930s have cavity walls. The gap can be filled with insulation to reduce heat loss, though the type of fill matters.
Cold Bridge
See Thermal Bridge.
Condensation
When warm, moist air meets a cold surface and the moisture turns into water droplets. Common on single-glazed windows and cold walls. After retrofit, condensation can get worse if ventilation is not upgraded to match improved airtightness.
COP (Coefficient of Performance)
A measure of how efficiently a heat pump works at a specific moment. A COP of 3 means for every 1 kWh of electricity used, the heat pump produces 3 kWh of heat. Higher is better. COP varies with outdoor temperature.
D
dMEV (Decentralised Mechanical Extract Ventilation)
Small individual extract fans fitted in wet rooms that run continuously at a low speed. They pull out moist, stale air and let fresh air in through trickle vents. A simpler, cheaper alternative to MVHR.
DPC (Damp Proof Course)
A barrier built into a wall near ground level to stop moisture rising up from the ground. Usually a strip of plastic, slate, or bitumen. If bridged by raised ground levels or render, damp can bypass it.
Draught-proofing
Sealing gaps around windows, doors, letterboxes, floorboards, and other openings to stop cold air getting in and warm air getting out. One of the cheapest and most effective energy upgrades.
E
ECO (Energy Company Obligation)
A UK government scheme that requires large energy suppliers to fund energy efficiency improvements in eligible homes. Usually aimed at low-income households or those on certain benefits.
Embodied Carbon
The carbon emissions produced during the manufacture, transport, and installation of building materials - before the building even starts using energy. Increasingly important in whole-life carbon assessments.
EPC (Energy Performance Certificate)
A rating from A to G that tells you how energy-efficient a building is. Required when selling or renting in the UK. Includes estimated energy costs and recommendations for improvements. Valid for 10 years.
EWI (External Wall Insulation)
Insulation boards fixed to the outside of a solid wall, then covered with render or cladding. Wraps the building in a thermal blanket without losing internal floor space. Can need planning permission.
F
Fabric First
A design approach that prioritises the building's physical structure over bolt-on technology. Get the insulation, airtightness, and ventilation right first, then you need a smaller heating system. Widely considered the smartest approach to retrofit.
Feed-in Tariff
A now-closed UK scheme that paid homeowners for electricity generated by solar panels and other renewables. Replaced by the Smart Export Guarantee. Existing agreements continue to pay out until they expire.
Flow Temperature
The temperature of the water leaving your boiler or heat pump and heading to your radiators. Heat pumps work most efficiently at low flow temperatures (35-45°C). Older boilers often run at 60-80°C.
G
GSHP (Ground Source Heat Pump)
A heating system that extracts heat from the ground via buried pipes. More efficient than air source heat pumps because ground temperature is more stable, but installation is more expensive and disruptive.
H
Humidity
The amount of moisture in the air. Relative humidity above 60% increases the risk of condensation and mould. After making a home more airtight, humidity can rise if ventilation is not improved. Ideally kept between 40-60%.
I
IWI (Internal Wall Insulation)
Insulation fitted to the inside of external walls. Useful for solid-walled properties where external insulation is not possible. Reduces room size slightly and needs careful detailing to avoid condensation behind the insulation.
K
kWh (Kilowatt-hour)
A unit of energy. One kWh is what a 1,000-watt appliance uses in one hour. Your energy bills are charged per kWh. A typical UK home uses about 12,000 kWh of gas and 3,000 kWh of electricity per year.
L
Loft Insulation
Insulation material laid between and over the joists in your loft space. One of the cheapest energy upgrades. Current Building Regulations recommend at least 270mm of mineral wool. About 25% of heat in an uninsulated home escapes through the roof.
Low-e Glass
Glass with a microscopically thin metallic coating that reflects heat back into the room while still letting light through. Standard in modern double and triple glazing.
M
MCS (Microgeneration Certification Scheme)
A quality assurance scheme for renewable energy installations in the UK. Your installer must be MCS-certified for you to qualify for the Boiler Upgrade Scheme or Smart Export Guarantee.
MEES (Minimum Energy Efficiency Standards)
UK regulations setting the minimum EPC rating that rental properties must meet. Currently Band E, with proposals to tighten to Band C by 2030.
MEV (Mechanical Extract Ventilation)
A centralised system that uses a single fan unit connected by ducts to extract stale, moist air from kitchens and bathrooms. Simpler than MVHR as it does not recover heat.
Mineral Wool
A type of insulation made from rock or glass fibres. Comes in rolls or batts. Non-combustible, breathable, and relatively cheap. Good all-round insulation but lower performance per thickness than PIR.
MVHR (Mechanical Ventilation with Heat Recovery)
A whole-house ventilation system that continuously extracts stale air from wet rooms and supplies fresh filtered air to living spaces. A heat exchanger recovers up to 90% of the heat from the outgoing air. Best suited to airtight homes.
N
n50
The number of air changes per hour at 50 Pascals of pressure, measured during a blower door test. UK Building Regulations typically require below 10. Passivhaus requires 0.6 ACH at n50.
Net Zero
A building or target where total carbon emissions are balanced by carbon removal or offset. In practice for homes, it usually means very high insulation, renewable energy generation, and clean heating. The UK aims for net zero by 2050.
O
Overheating
When indoor temperatures become uncomfortably or dangerously high, usually in summer. Building Regulations Part O requires new homes to be designed to avoid overheating. Highly insulated homes with large south-facing windows are most at risk.
P
Part F
The section of UK Building Regulations covering ventilation. Sets minimum ventilation rates for new builds and renovations. If you improve airtightness, Part F may require you to upgrade your ventilation system.
Part L
The section of UK Building Regulations covering energy efficiency. Sets minimum insulation levels, heating efficiency, and overall energy performance for new builds and renovations.
Part O
The section of UK Building Regulations covering overheating (introduced 2022). Requires designers to limit solar gains and provide adequate ventilation to prevent overheating in new residential buildings.
PAS 2030
A British Standard specifying requirements for installing energy efficiency measures in existing buildings. Installers must be PAS 2030 certified to carry out work under government schemes like ECO.
PAS 2035
A British Standard for the assessment, design, and coordination of whole-house retrofit projects. Requires a Retrofit Coordinator to oversee the work. Mandatory for government-funded retrofit.
Passivhaus
A rigorous building standard from Germany focused on extreme energy efficiency. Requires very high insulation, exceptional airtightness (0.6 ACH at n50), MVHR, and minimal thermal bridging. Heating demand must be below 15 kWh/m² per year.
PIR (Polyisocyanurate)
A type of rigid insulation board with high thermal performance for its thickness. Good for walls, roofs, and floors where space is tight. Not breathable - needs careful detailing to avoid trapping moisture.
PSV (Passive Stack Ventilation)
A ventilation system that uses warm air's natural tendency to rise to draw stale air up through vertical ducts and out through the roof. No fans needed. Less controllable than mechanical systems.
PV (Photovoltaic)
Solar panels that convert sunlight directly into electricity. Output depends on roof orientation, pitch, and shading. Typically paired with a battery for maximum self-consumption.
Q
q50
Air permeability measured at 50 Pascals, expressed as cubic metres per hour per square metre of building envelope. The standard metric used in UK Building Regulations for airtightness testing. Lower is better.
R
Radiator
A heat emitter that warms a room by circulating hot water from a boiler or heat pump. Traditional radiators are designed for high flow temperatures. For heat pumps to work efficiently, you often need larger radiators or fan-assisted models.
Retrofit
Upgrading an existing building to improve its energy performance. Can include insulation, new windows, airtightness, ventilation, and heating system changes. The key challenge is doing all these things in the right order.
S
SAP (Standard Assessment Procedure)
The UK government's method for calculating the energy performance of homes. Used to produce EPC ratings. RdSAP is the reduced version used for existing homes.
SCOP (Seasonal Coefficient of Performance)
The average efficiency of a heat pump over an entire heating season. More useful than COP for comparing heat pumps because it accounts for varying outdoor temperatures. A good SCOP for UK conditions is 3.0 or above.
SEG (Smart Export Guarantee)
A UK scheme requiring licensed electricity suppliers to offer payment for surplus renewable electricity exported to the grid. Replaced the Feed-in Tariff. Rates vary between suppliers.
Smart Meter
A digital gas and electricity meter that sends readings to your supplier automatically and shows real-time energy use. Useful for tracking the impact of energy efficiency improvements. Free to have installed.
Solar Gain
Heat from sunlight entering a building through windows. Beneficial in winter (free heating) but can cause overheating in summer, especially in well-insulated homes with large south-facing glazing.
Solar Thermal
Panels that use sunlight to heat water directly (not to generate electricity). Can provide 50-70% of hot water demand in summer but much less in winter.
Solid Wall
A wall built as a single thickness of brick or stone, with no cavity. Common in UK homes built before the 1930s. Loses heat much faster than cavity walls. Can be insulated externally (EWI) or internally (IWI).
T
Thermal Bridge
A point or area in the building fabric where heat transfers more easily than through the surrounding structure. Common at wall-floor junctions, window reveals, and lintels. Causes cold spots, condensation risk, and increased heat loss. Also called a cold bridge.
Thermal Imaging
Using an infrared camera to see heat patterns on a building's surface. Shows where heat is escaping and where insulation is missing. Best done in cold weather.
Thermal Mass
The ability of a material to absorb, store, and release heat. Heavy materials like brick, concrete, and stone have high thermal mass. Useful in passive solar design.
Trickle Vent
A small ventilation slot built into a window frame that allows a controlled amount of fresh air into a room without opening the window. Required by Building Regulations in most new and replacement windows.
Triple Glazing
Windows with three panes of glass and two gas-filled gaps. Better insulation than double glazing. Standard in Passivhaus projects. Whether it is worth the extra cost depends on the rest of the building.
TrustMark
A UK government-endorsed quality scheme for tradespeople. Required for installations under government-funded retrofit schemes. Check before hiring anyone for energy efficiency work.
U
Underfloor Heating (UFH)
A heating system where warm water circulates through pipes embedded in or under the floor. Provides even heat at low flow temperatures, making it ideal for heat pumps. Slower to respond than radiators.
U-value
A measure of how quickly heat passes through a building element. Measured in W/m²K. Lower is better. A solid brick wall might have a U-value of 2.0, while an insulated cavity wall could be 0.3.
V
Vapour Barrier (VCL)
A membrane that stops water vapour passing through the building fabric. Placed on the warm side of insulation to prevent moisture reaching cold surfaces where it would condense. Getting the position wrong is one of the most common retrofit mistakes.
Ventilation
The process of providing fresh air to a building and removing stale, moist air. Can be natural (windows, trickle vents), mechanical (fans, MVHR), or a combination. After improving airtightness, controlled ventilation becomes essential.
W
Whole House Approach
Planning energy improvements as an integrated package rather than one measure at a time. Considers how insulation, airtightness, ventilation, and heating interact. Required under PAS 2035 for government-funded work.