Every term you need to know — whether you’re a homeowner planning a project or a tradesperson working in the sector. This guide cuts through that; retrofit jargon explained.
If you’ve ever tried to research home retrofit and come away more confused than when you started, you’re not alone. The industry runs on jargon — and whether you’re a homeowner trying to plan a project or a tradesperson working in the sector, not knowing the language puts you at a disadvantage.
This glossary covers every significant term you’ll encounter in retrofit, organised by category so you can find what you need quickly. Bookmark it, share it, and come back to it whenever something doesn’t make sense.
If you’d rather watch than read — we’ve turned each category into a short explainer video on The Retrofit Hub YouTube channel. Links appear throughout.
The Essentials
These five terms underpin everything else in retrofit. If you only learn five things, make it these.
Retrofit
Retrofit means improving an existing building’s energy performance — not knocking it down and starting again, but working with what you’ve got. It covers everything from adding insulation and draught-proofing to installing heat pumps and ventilation systems. Retrofit is specifically about energy performance; it’s different from renovation, which is about updating a building’s look or layout.
Fabric First
Fabric first is the golden rule of retrofit. It means addressing the building fabric — walls, roof, floor, windows, doors — before you think about changing the heating system. Reduce the amount of heat your home loses before you invest in a new way of generating it. A heat pump in a leaky, poorly insulated house is like running a bath with the plug out. Sort the fabric first, then upgrade the heating.
Airtightness
Airtightness means deliberately sealing a building to prevent uncontrolled air leakage — draughts escaping through gaps, cracks, and poorly fitted windows and doors. Most people assume draughts are a sign of a healthy, breathing house. They’re not. Uncontrolled air leakage is one of the biggest sources of heat loss in UK homes. The goal isn’t to seal a house completely — it’s to control where air moves, rather than letting it happen randomly.
Ventilation
Once you improve airtightness, ventilation becomes critical. A tighter house needs a deliberate strategy for bringing in fresh air and removing stale air and moisture. Without it, you get condensation, mould, and poor air quality. Ventilation and airtightness aren’t opposites — they work together. Seal the gaps you don’t want, then ventilate in a controlled way through the ones you do.
Breathability
Breathability refers to a building’s ability to absorb and release moisture through its fabric. It’s particularly important in older properties — Victorian and Edwardian homes were built with solid walls and lime mortar specifically designed to let moisture move through them. Seal those walls with modern impermeable materials and you trap moisture inside the structure, leading to damp, rot, and structural damage. Understanding whether your home needs to breathe is fundamental to choosing the right insulation and finishes.
🎥 Watch: The Retrofit Jargon Buster — Foundations [YouTube link]
Building Physics
The science behind how buildings perform. You don’t need to be an engineer — but understanding these concepts will help you ask better questions and spot poor advice.
U-value
A U-value measures how much heat passes through a building element — a wall, roof, window, or floor. The lower the number, the better it performs. A well-insulated wall might have a U-value of 0.15–0.3. An uninsulated solid brick wall could be as high as 2.0. When comparing insulation options or getting quotes, U-values give you an objective way to compare performance.
Thermal Bridging / Cold Bridge
A thermal bridge — sometimes called a cold bridge — is a weak point in a building’s insulation where heat escapes more readily than through the surrounding fabric. Common examples include window reveals, structural steel elements, and the junction between a wall and a roof. You can’t see thermal bridges with the naked eye, but they show up clearly on a thermal imaging camera. Good retrofit installers design around them. Poor ones ignore them entirely.
Thermal Envelope
The thermal envelope is the boundary between the heated interior of a building and the outside world — everything that separates inside from outside. Walls, roof, floor, windows, doors. Improving the thermal envelope means improving insulation, airtightness, and glazing across this entire boundary. A good retrofit treats the envelope as a system, not a collection of individual elements.
Thermal Mass
Thermal mass refers to a material’s ability to absorb, store, and slowly release heat. Heavy materials like brick, stone, and concrete have high thermal mass — they heat up slowly but stay warm for longer. Understanding the thermal mass of your building affects how you heat it, when you heat it, and how quickly it responds to temperature changes. It’s particularly relevant when switching to a heat pump, which works best when run steadily at lower temperatures.
Blower Door Test
A blower door test is how airtightness is measured. A large fan is fitted into an external doorframe and used to pressurise or depressurise the building. The rate at which air leaks in or out is measured and expressed as a score — the lower the better. It’s the standard way to verify that airtightness work has been done effectively, and it’s increasingly required as part of grant-funded retrofit projects.
Heat Loss Calculation
A heat loss calculation works out how much heat a building loses per hour, measured in kilowatts. It takes into account the size and construction of the building, the U-values of walls, roof, floor and windows, and the level of airtightness. It’s the essential starting point for sizing a heat pump correctly — and one of the most commonly skipped steps in poor-quality installations.
Heat Demand
Heat demand is the total amount of energy needed to keep a building warm over a given period — usually a year, measured in kilowatt hours. Reducing heat demand is the ultimate goal of fabric improvements — the less energy your home needs to stay warm, the lower your bills and the smaller the heating system you need.
Thermal Conductivity — Lambda or λ Value
Thermal conductivity measures how readily a material conducts heat. It’s the underlying property that U-values are calculated from. Insulation materials are rated by their lambda value — the lower the number, the better the insulating performance. Thin, high-performance insulation boards have a lower lambda value than thicker, cheaper mineral wool — which is why they’re used where space is limited.
Thermal Resistance — R-value
R-value measures the resistance of a material or assembly to heat flow — essentially the inverse of U-value. Higher is better. R-values are most commonly used when specifying insulation thickness: the thicker the insulation, the higher the R-value and the better the performance.
Hygrothermal Performance
Hygrothermal performance describes how a building manages both heat and moisture together. Heat and moisture interact constantly in buildings, and a construction that performs well thermally can still fail if it traps moisture in the wrong place. Hygrothermal modelling is used to predict condensation risk within wall structures before specifying insulation.
Wind-Driven Rain / Driving Rain
Wind-driven rain is rainfall carried horizontally by wind, which can penetrate walls — particularly solid masonry ones. It’s an important consideration when specifying external insulation or renders, as the outer layer needs to manage moisture from the outside as well as the inside.
Stack Effect
The stack effect is the tendency of warm air to rise and escape through the top of a building, drawing colder air in at the bottom. It’s a major driver of uncontrolled air leakage in taller buildings and in homes with loft hatches, ceiling penetrations, and poorly sealed roof spaces.
🎥 Watch: Retrofit Building Physics Explained [YouTube link]
Ventilation
MVHR — Mechanical Ventilation with Heat Recovery
MVHR is a whole-house ventilation system that continuously extracts stale, moist air from the home and replaces it with fresh air from outside — while recovering up to 90% of the heat from the outgoing air to warm the incoming air. It’s the gold standard for ventilation in well-insulated, airtight homes. Done well, it delivers excellent air quality with minimal heat loss.
dMEV — Decentralised Mechanical Extract Ventilation
dMEV uses individual low-power fans — typically in bathrooms and kitchens — running continuously at a low rate to extract moist, stale air. It’s simpler and cheaper than MVHR, doesn’t recover heat, and works well in homes that aren’t highly airtight. A practical and cost-effective ventilation solution for many retrofit projects.
Air Leakage vs Deliberate Ventilation
Air leakage is uncontrolled — draughts through gaps, cracks, and poorly sealed junctions. Deliberate ventilation is controlled — air moving through designed openings, trickle vents, or mechanical systems. The goal of retrofit is to eliminate the former and manage the latter. Confusing the two leads to either draughty, cold homes or sealed, stuffy ones.
🎥 Watch: Ventilation Terms Explained [YouTube link]
Moisture
Vapour Barrier / Vapour Control Layer
A vapour barrier is an impermeable membrane used to prevent moisture vapour from passing through a building element — typically used on the warm side of insulation to stop condensation forming within the wall structure. A vapour control layer (VCL) is similar but allows a small, controlled amount of vapour to pass through. Use the wrong one in the wrong place and you can trap moisture rather than manage it.
Vapour Permeability
Vapour permeability is a measure of how readily a material allows moisture vapour to pass through it. Materials are described as vapour-open (highly permeable) or vapour-closed (impermeable). Lime plaster and wood fibre insulation are vapour-open. PIR foam boards and most synthetic renders are vapour-closed. Matching vapour permeability to your building type is critical — especially in older, solid-walled properties.
Relative Humidity
Relative humidity is the amount of moisture in the air expressed as a percentage of the maximum it could hold at that temperature. In a well-ventilated home, indoor relative humidity should sit between 40–60%. Higher than that consistently and you risk mould, condensation, and structural damage.
Condensation Risk
Condensation risk refers to the likelihood of moisture condensing either on surfaces (surface condensation) or within the structure of a wall or roof (interstitial condensation). Both are driven by the combination of indoor humidity, temperature differentials, and the vapour permeability of materials. Understanding condensation risk is essential when specifying insulation — particularly in solid wall properties.
Dew Point
The dew point is the temperature at which air becomes saturated and moisture condenses. In building physics, it’s used to calculate where within a wall or roof construction condensation might form. If the dew point falls within an insulation layer, you have a condensation risk. Hygrothermal modelling calculates dew point positions to ensure wall constructions are safe.
Interstitial Condensation
Interstitial condensation forms within the layers of a wall, roof, or floor construction — hidden from view. It can cause rot in timber frames, corrosion in metal components, and degradation of insulation over time. It’s one of the most serious risks of poorly specified retrofit insulation, and one of the key reasons why choosing the right insulation system for your property type matters.
Moisture Buffering
Moisture buffering is the ability of a material to absorb moisture when humidity is high and release it when humidity drops — helping to regulate indoor humidity naturally. Materials like wood, clay plaster, and natural fibre insulation have good moisture buffering properties.
Hygroscopic Materials
Hygroscopic materials readily absorb moisture from the air. In building terms this is usually a positive property — hygroscopic materials like lime, clay, and wood fibre help regulate indoor humidity and contribute to breathable, moisture-managing wall constructions.
🎥 Watch: Moisture and Breathability in Retrofit [YouTube link]
Heating
Heat Pump
A heat pump is a heating system that moves heat rather than generating it by burning fuel. It extracts low-grade thermal energy from the air or ground outside — even in cold weather — and upgrades it to heat your home and hot water. Because it moves heat rather than creating it, a well-sized and installed heat pump typically delivers three to four units of heat for every one unit of electricity consumed. It works best in a well-insulated home with a low-temperature distribution system.
EPC — Energy Performance Certificate
An EPC is a rating from A to G that summarises your home’s energy efficiency. Most UK homes sit around D or E. You’ll need a valid EPC to access most grant schemes, and your EPC band affects which funding you’re eligible for. Worth knowing: EPCs are often inaccurate or based on assumptions — a proper retrofit assessment will give you a more reliable picture of your home’s actual performance.
Grant / Boiler Upgrade Scheme
The Boiler Upgrade Scheme (BUS) is the main government grant for heat pump installations in England and Wales, currently offering £7,500 towards an air source heat pump. Other funding streams include ECO4, the Great British Insulation Scheme, and the Warm Homes Plan. Eligibility varies and the landscape changes regularly — always check current guidance before planning your project.
Flow Temperature
Flow temperature is the temperature at which water leaves the heat pump or boiler and circulates around the heating system. Gas boilers typically operate at 70–80°C. Heat pumps work most efficiently at much lower flow temperatures — ideally 35–45°C. This is why heat pumps often require larger radiators or underfloor heating to deliver the same warmth, and why a heat loss calculation matters so much before installation.
COP / SCOP
COP stands for Coefficient of Performance — the efficiency of a heat pump at a given moment. A COP of 3 means you get 3kW of heat for every 1kW of electricity used. SCOP is the Seasonal Coefficient of Performance — the average efficiency across a whole heating season. SCOP is the more useful number for comparing heat pumps, as performance varies with outdoor temperature.
SPF — Seasonal Performance Factor
SPF is the real-world equivalent of SCOP — the actual measured efficiency of a heat pump installation over a season, based on monitored data rather than manufacturer specifications. It’s increasingly used by installers and assessors to verify that a heat pump is performing as expected.
Low-Temperature Heating System
A low-temperature heating system is one designed to work effectively with flow temperatures of 45°C or below — making it compatible with heat pumps. Underfloor heating is inherently low-temperature. Radiator-based systems can be made low-temperature by upsizing the radiators.
Buffer Tank
A buffer tank is a small insulated water tank used in some heat pump installations to smooth out the operation of the system — preventing the pump from cycling on and off too frequently. Not all installations need one. Your installer should be able to explain clearly whether one is required for your specific system.
Emitter / Radiator Sizing
Emitters are the elements that give off heat in a heating system — radiators, underfloor heating pipes, fan coil units. Sizing them correctly for a heat pump system, which operates at lower flow temperatures than a gas boiler, is critical. Undersized emitters mean the system can’t heat the space adequately. A proper heat loss calculation and emitter sizing exercise should be part of every heat pump installation.
🎥 Watch: Heating Systems and Heat Pump Terms Explained [YouTube link]
Materials
Lime Mortar / Lime Render
Lime mortar and lime render are traditional building materials made from limestone-derived binders. Unlike modern cement-based mortars, lime is vapour-permeable, flexible, and slightly self-healing. In solid-walled older properties, lime is often the only appropriate material for repointing and rendering — cement mortars trap moisture and can cause serious damage to historic masonry.
Wood Fibre Insulation
Wood fibre insulation is made from compressed timber fibres. It’s vapour-open, has good moisture-buffering properties, and is well-suited to older solid-walled properties that need to breathe. It also has good thermal mass, which means it slows the movement of heat — helpful for managing summer overheating as well as winter heat loss.
Mineral Wool — Glass Wool and Rock Wool
Mineral wool is one of the most widely used insulation materials in the UK, available as glass wool (made from recycled glass) or rock wool (made from volcanic rock). It’s vapour-permeable, fire-resistant, and relatively affordable. Used extensively in lofts, cavity walls, and internal walls. Not suitable where very high performance in a thin profile is needed.
Rigid Foam Insulation — PIR, EPS, XPS
Rigid foam boards offer high thermal performance in a thin profile, making them useful where space is tight. PIR and XPS are vapour-closed — they don’t allow moisture to pass through. EPS is slightly more permeable. The vapour-closed nature of rigid foams means they need careful specification in older buildings — used incorrectly they can trap moisture and cause damage.
Spray Foam Insulation
Spray foam has become one of the most controversial materials in retrofit. Applied incorrectly — particularly in roof spaces — it can trap moisture, make roofs difficult to inspect, and cause structural damage. Some lenders refuse to mortgage properties with spray foam in the roof. If a property you’re buying or selling has spray foam, get a specialist assessment before proceeding.
Natural / Bio-Based Insulation
Natural insulation materials include hemp, sheep’s wool, cellulose (recycled newspaper), cork, and flax. They’re vapour-open, have good moisture-buffering properties, and typically have lower embodied carbon than synthetic alternatives. Generally used where breathability is a priority — older buildings, timber frames, or where sustainability is paramount.
🎥 Watch: Retrofit Insulation Materials Guide [YouTube link]
Standards & Roles
PAS 2035
PAS 2035 is the British Standard for domestic retrofit. It defines how retrofit projects should be assessed, designed, managed, and installed. Most government-funded retrofit schemes require work to comply with PAS 2035. It introduced the roles of retrofit assessor and retrofit coordinator, and establishes the requirement for a whole-house assessment before work begins. Think of it as the quality framework that separates considered, professional retrofit from a piecemeal upgrade.
Retrofit Coordinator
A retrofit coordinator is a qualified professional responsible for overseeing a retrofit project end to end — ensuring the right measures are specified for the property, in the right sequence, to the right standard. Required under PAS 2035 for funded projects. The coordinator doesn’t usually do the physical work themselves — they manage the process, liaise with installers and assessors, and are accountable for the outcome.
Retrofit Assessor
A retrofit assessor carries out the whole-house assessment at the start of a retrofit project — gathering data about the property, its construction, and its current energy performance. This forms the basis for the whole-house retrofit plan. The assessor role is distinct from the coordinator, though in some projects the same person carries out both.
MCS — Microgeneration Certification Scheme
MCS is the quality framework for the installation of small-scale renewable energy and low-carbon heating systems — including heat pumps and solar panels. MCS certification is required for installers whose customers want to access the Boiler Upgrade Scheme grant. Always check that any installer fitting a heat pump or solar system holds current MCS certification.
TrustMark
TrustMark is the government-endorsed quality scheme for retrofit and home improvement work. It covers a wider range of trades than MCS — including insulation, draught-proofing, and general building work. Grant-funded retrofit work typically requires installers to be TrustMark registered. It provides consumer protection and a route for complaints if work is substandard.
SAP Rating
SAP stands for Standard Assessment Procedure — the government’s methodology for calculating the energy performance of dwellings. It’s the engine behind the EPC. Retrofit assessors and energy modellers use SAP software to model the impact of proposed improvements before work begins.
Passivhaus
Passivhaus is a rigorous international building standard focused on creating buildings with very low energy demand — achieved through high levels of insulation, exceptional airtightness, thermal bridge-free construction, and MVHR. It’s more commonly applied to new build than retrofit, but it sets a benchmark for what high-performance buildings can achieve and informs best practice in retrofit specification.
EnerPhit
EnerPhit is the Passivhaus standard specifically for retrofit projects. It acknowledges that existing buildings can’t always achieve the same performance as new build, and sets slightly relaxed but still highly demanding criteria. Achieving EnerPhit certification is the gold standard for retrofit projects.
Whole-House Plan / Whole-House Approach
A whole-house plan is a document produced at the start of a retrofit project that sets out the recommended measures for a specific property, in the right sequence, based on a full assessment of its construction, condition, and the occupants’ needs and budget. Required under PAS 2035, it prevents the common mistake of doing retrofit work piecemeal without considering how measures interact.
🎥 Watch: Retrofit Standards and Roles Explained [YouTube link]
A Note for Trades
If you’re a retrofit installer, contractor, or designer, this glossary is just the beginning. The Retrofit Hub community is built for trades as well as homeowners — with CPD-relevant content, peer learning, and technical deep-dives that go well beyond what any glossary can cover.
Ready to go deeper?
Every section of this glossary has a companion video on The Retrofit Hub YouTube channel. Whether you want to understand the essentials, get to grips with building physics, or learn how moisture management affects your insulation choices — there’s a short, plain-English video for it.
🎥 Watch the series on YouTube → [link]
And when you’re ready to move beyond the basics — ask questions about your specific project, connect with vetted installers, or access our planning tools and guides — join The Retrofit Hub community.
👥 Join the community → [link]
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