UK retrofit & home energy glossary 2026

UK retrofit speaks in acronyms. Quotes, EPCs, installer reports and grant paperwork all assume you already know what ASHP, SCOP, PAS 2035 or RdSAP mean — and most homeowners don't. This glossary defines the terms you'll actually meet in 2026, written in plain English with UK-specific context. We've prioritised the words that change a decision: what to specify, what to ask an installer, and what to challenge in a survey.

Where a term ties to a calculator or grant we maintain elsewhere on the site, we've linked through. Definitions are kept conservative — we'd rather under-claim than over-promise on payback, savings or eligibility. If a term is missing, tell us and we'll add it.

The terms below are arranged A-Z. Acronyms and short forms are filed under their letter (so SCOP sits in S, not under 'seasonal'). Where two terms are commonly used interchangeably, we cross-reference rather than duplicate. The glossary is written with owner-occupiers, landlords and self-builders in mind; commercial-scale terms are out of scope. Last reviewed against PAS 2035:2023, MCS standards current at May 2026, and the ECO4 final-year delivery rules.

A

ASHP (air-source heat pump)

A heat pump that extracts low-grade heat from outdoor air and upgrades it to a useful temperature for radiators, underfloor heating and hot water. Typical UK installations deliver 2.5-4.5 units of heat per unit of electricity, expressed as SCOP. ASHPs are the dominant technology under the Boiler Upgrade Scheme (BUS) and the default low-carbon heating choice for off-gas and on-gas homes alike. See our heat-pump calculator for an indicative ROI estimate.

Air permeability (Q50)

A measure of how leaky a building envelope is, expressed in cubic metres of air per hour per square metre of envelope at a 50 Pascal pressure difference (m³/h·m² @50Pa). New-build Building Regulations target 8 or below; deep retrofits often aim for 3-5; EnerPHit (Passivhaus retrofit) targets 1.0 ach. Lower numbers mean less uncontrolled draught and a smaller heating load.

Airtightness

The general property of a building envelope resisting uncontrolled air leakage. Distinct from ventilation: an airtight home still needs a deliberate ventilation strategy (extract fans, MVHR or PIV). Improving airtightness without addressing ventilation risks condensation, damp and indoor-air-quality problems — a key reason PAS 2035 mandates a ventilation assessment in any retrofit.

Awaab's Law

UK legislation introduced after the death of Awaab Ishak, requiring social landlords in England to investigate and fix damp and mould hazards within strict statutory timeframes. From October 2025, registered providers must respond to reported emergency hazards within 24 hours. Relevant to retrofit because poorly-specified insulation can introduce condensation risk if ventilation isn't upgraded in parallel.

B

BUS (Boiler Upgrade Scheme)

The Department for Energy Security and Net Zero grant scheme covering up to £7,500 toward an air-source or ground-source heat pump (or £5,000 for biomass boilers in eligible rural properties). Available in England and Wales, claimed by the MCS-certified installer on the homeowner's behalf. Runs to 2028 in current form. Property must have a valid EPC with no outstanding loft or cavity insulation recommendations.

BS 5250

The British Standard Management of moisture in buildings — the canonical UK reference for understanding condensation, vapour control and ventilation in walls, roofs and floors. Cited heavily in PAS 2035 risk assessments. The 2021 revision expanded coverage of breathable construction and retrofit-specific moisture risks.

Battery cycles

One full charge-and-discharge of a home battery counts as a cycle. Modern lithium-iron-phosphate (LiFePO4) batteries are typically warranted for 6,000-10,000 cycles to 80% remaining capacity — roughly 16-27 years at one cycle per day. Cycle count, depth of discharge and round-trip efficiency together determine real-world battery economics.

BREEAM

BRE Environmental Assessment Method — a UK sustainability rating used mostly for non-domestic buildings and large developments. Less commonly cited in single-dwelling retrofit than EPC, SAP or PAS 2035, but you'll see it in mixed-use schemes or local-authority procurement.

C

COP (coefficient of performance)

The instantaneous ratio of heat output to electrical input for a heat pump. A COP of 3.5 means 3.5 kWh of heat per 1 kWh of electricity at that operating point. COP varies with outdoor temperature and flow temperature — it's not the seasonal figure (see SCOP) and shouldn't be used for annual running-cost estimates.

Cavity wall

A wall built as two leaves (typically brick outer and brick or block inner) with a gap between them. Most UK homes built between roughly 1920 and 2000 have cavity walls. The cavity can be filled with mineral wool, polystyrene beads or polyurethane foam to reduce heat loss. Eligibility depends on cavity width (typically 50mm minimum), exposure rating and condition.

Cold bridge

Also called a thermal bridge. A localised area of the building envelope with significantly higher heat loss than its surroundings — typically where insulation is interrupted by a structural element (window reveals, lintels, floor-wall junctions). Cold bridges concentrate condensation risk and undermine whole-house insulation performance. PAS 2035 retrofit assessments specifically test for cold-bridge risk.

Cold roof

A pitched-roof construction where insulation sits at ceiling level (between or over joists) and the loft space above is unheated and ventilated. Contrast with warm roof where insulation sits at rafter level. Cold roofs are the standard UK approach for lofts not used as living space.

C2PA (Coalition for Content Provenance and Authenticity)

A cross-industry standard for cryptographically signing the origin of digital media. Increasingly relevant when reading retrofit case studies and supplier marketing online: C2PA-signed images and reports allow you to verify a photo or document wasn't fabricated or AI-generated. Look for the content-credentials icon in 2026 browsers.

D

DEA (Domestic Energy Assessor)

An accredited assessor qualified to produce domestic Energy Performance Certificates using RdSAP. DEAs gather property data on site and submit it through an accreditation scheme (Elmhurst, Stroma, Quidos and others). They are not retrofit designers — for whole-house retrofit advice, look for a Retrofit Assessor or Retrofit Coordinator.

DNO (Distribution Network Operator)

The company that owns and maintains the local electricity distribution network in your region (UK Power Networks, Northern Powergrid, SP Energy Networks, etc.). Solar PV and battery installations above 3.68 kW per phase need DNO approval (G99); smaller installs use the simpler G98 notification. Heat pumps over a certain size may also trigger DNO consultation.

Defrost cycle

An automatic mode in air-source heat pumps where the outdoor coil briefly reverses to melt frost build-up. Defrosting consumes energy and reduces effective output for a few minutes. Frequent defrosts in cold, humid weather are normal; modern inverter-driven units sequence them efficiently and the impact is already factored into SCOP figures.

Depth of discharge (DoD)

How much of a battery's nominal capacity is actually used per cycle. A 10 kWh battery used between 10% and 90% has a DoD of 80%. Higher DoD shortens cycle life for older lithium chemistries; modern LiFePO4 cells are designed for daily 80-90% DoD without significant degradation penalty. Always compare usable capacity, not nominal.

E

ECO4

The fourth phase of the Energy Company Obligation, the UK government scheme requiring large energy suppliers to fund energy-efficiency measures for low-income and vulnerable households. Runs from April 2022 to March 2026, with ECO5 expected to follow. Funds insulation, heating upgrades and ventilation in qualifying homes. See also LA Flex.

ECO Flex (Local Authority Flex)

A route within ECO4 letting local authorities issue a declaration that a household meets locally-defined fuel-poverty or vulnerability criteria, even where they don't tick the standard benefits-based eligibility boxes. Each council publishes its own Statement of Intent. Often the easiest way for working but cash-stretched households to access ECO-funded measures.

EER (Energy Efficiency Rating)

The headline A-G letter rating shown on a domestic EPC, derived from the SAP or RdSAP score. A is best (92+), G worst (20 or below). Distinct from the Environmental Impact Rating (EIR) on the same certificate, which is carbon-based. Mortgage lenders, MEES regulations and grant eligibility all reference EER bands.

EPC (Energy Performance Certificate)

The 1-page energy rating produced for any UK property when sold, let or significantly altered. Valid for 10 years. Includes EER band, recommended measures, estimated running costs and CO₂ figures. EPCs are produced by DEAs using RdSAP and lodged on a national register. The 2026 EPC reform is expected to add performance metrics (kWh/m²/yr) alongside the cost-based rating.

EWI (External Wall Insulation)

Insulation fixed to the outside face of a solid or hard-to-treat cavity wall and finished with render, cladding or brick slips. Typically 90-150mm thick. Effective at eliminating cold bridges and protecting the masonry, but changes the external appearance and requires planning consent in conservation areas. Specify to PAS 2030 with PAS 2035 oversight.

F

Fabric first

The retrofit principle of reducing heat demand through insulation, airtightness and good glazing before sizing or installing low-carbon heat. A heat pump in a leaky, under-insulated house will run too hot, too long, and never reach its rated SCOP. Fabric-first does not mean fabric-only — it means sequencing the work in the right order.

Flow temperature

The temperature of water leaving the boiler or heat pump and entering the heating circuit. Gas combi boilers historically ran at 70-80°C; condensing boilers run more efficiently at 55°C or below; heat pumps typically design around 35-50°C. Lower flow temperatures mean larger emitters but higher efficiency — the central trade-off in heat-pump design.

FiT (Feed-in Tariff)

The legacy UK scheme paying solar PV owners a generation tariff and an export tariff for systems registered before April 2019. Closed to new applicants and replaced by the Smart Export Guarantee. Existing FiT contracts continue for their 20-25 year term — check your registration date before assuming you're on SEG.

Frost stat (frost thermostat)

A low-set thermostat (often around 5°C) that triggers heating to prevent pipe freezing in unoccupied or unheated spaces. In heat-pump systems, frost protection is typically built into the controller and operates regardless of the main schedule.

G

G98

Engineering Recommendation G98 — the UK process for connecting small generators (up to 16 A per phase, roughly 3.68 kW single-phase or 11 kW three-phase) to the distribution network. The installer notifies the DNO after commissioning. Most domestic solar-PV installs sit under G98.

G99

Engineering Recommendation G99 — the application process for connecting larger generators or storage. Required before installation when system size or type exceeds G98 limits. DNO turnaround can take weeks and may impose curtailment or export limits where local network capacity is constrained.

GSHP (ground-source heat pump)

A heat pump drawing heat from the ground via buried slinky loops, vertical boreholes or pond-loop arrays. Higher SCOP than ASHP (typically 4.0-5.0) because ground temperature is more stable than air, but installation costs and groundworks are significantly higher. BUS grant of £7,500 applies to GSHP as well as ASHP.

Glazing (single, double, triple)

Single glazing has one pane and a U-value around 5.0 W/m²K. Modern Argon-filled, low-E double glazing achieves 1.2-1.4. Triple glazing reaches 0.6-0.8 and is standard in Passivhaus. Frame material (uPVC, timber, aluminium-clad-timber) matters as much as the glass for whole-window performance — always compare U_w (whole window), not U_g (glass only).

H

Heat loss

The rate at which heat escapes through the building envelope, expressed in watts (instantaneous) or kWh (cumulative). A correct heat-loss calculation underpins every retrofit design — radiator sizing, heat-pump capacity, cylinder sizing all follow from it. CIBSE Domestic Heating Design Guide and MCS 3005 set the UK calculation standards.

Heat pump (ASHP / GSHP / hybrid)

Any device that moves heat from a cooler source to a warmer sink using a refrigerant cycle and a compressor. Domestic UK options are air-source (ASHP), ground-source (GSHP) and hybrid (heat pump plus a back-up boiler). Choice depends on heat demand, fabric performance, available outdoor space, and budget. See our heat-pump calculator.

HHCRO (Home Heating Cost Reduction Obligation)

The sub-obligation within ECO4 that targets low-income, vulnerable households with measures aimed at reducing heating bills — typically loft and cavity insulation, first-time central heating, and boiler or heat-pump upgrades. Funded entirely by obligated energy suppliers; no homeowner contribution where the property fully qualifies.

Hybrid heat pump

An installation pairing a smaller heat pump with an existing or new gas boiler. The heat pump handles space-heating demand most of the year; the boiler steps in during cold snaps or for hot water. Useful for poorly-insulated homes where a full heat-pump retrofit isn't yet viable, but does not qualify for the BUS grant in current rules.

I

IAQ (indoor air quality)

A composite description of the air inside a home, covering CO₂, humidity, VOCs, particulates (PM2.5/PM10), radon and mould spores. Tightening the building envelope without upgrading ventilation degrades IAQ — a recurring failure mode in UK retrofits and a focus of PAS 2035 risk assessment. Affordable IAQ monitors (under £150) make this measurable.

Indoor air quality

See IAQ.

Inverter (string, micro, hybrid)

The device converting solar-panel DC to grid-compatible AC. A string inverter handles a series of panels as one unit; microinverters sit behind each panel and isolate shading or fault on individual modules; a hybrid inverter integrates battery storage on the DC side, improving round-trip efficiency vs an AC-coupled retrofit battery.

IWI (Internal Wall Insulation)

Insulation applied to the inside face of an external wall — typically rigid boards, calcium silicate, woodfibre or insulated plasterboard. Cheaper than EWI and doesn't change external appearance, but reduces internal floor area and demands careful detailing at junctions to avoid cold bridges and interstitial condensation. Specify to PAS 2030 / PAS 2035.

K

kWh (kilowatt-hour)

The unit of energy used on every UK domestic energy bill. One kWh equals 1,000 watts running for one hour — a fan heater on full for 30 minutes, an electric kettle boiled three or four times, or roughly one cycle of an A-rated washing machine. Heat pumps are sized in kW (instantaneous power); their consumption is measured in kWh.

kWp (kilowatt-peak)

The nameplate output of a solar PV array under standard test conditions (1,000 W/m² irradiance, 25°C cell temperature). Real-world output is almost always below kWp because UK irradiance rarely hits the test point. A 4 kWp south-facing system in central England typically generates 3,400-3,900 kWh per year. PVGIS provides location-specific yield estimates. Use our solar calculator for site-specific output and payback ranges before signing a quote.

L

LA Flex (Local Authority Flex)

See ECO Flex above. The mechanism letting councils declare wider eligibility for ECO4-funded retrofit measures, beyond the standard benefits-tested route. Each council's published Statement of Intent sets the local rules.

Lambda value (λ)

The thermal conductivity of an insulation material, in W/m·K. Lower is better. Mineral wool sits around 0.035-0.044, PIR boards around 0.022, aerogel and vacuum panels under 0.020. Material thickness and lambda together give a layer's U-value contribution; PAS 2035 designs publish lambda for every specified product.

LiFePO4 (lithium iron phosphate)

The dominant battery chemistry for UK home storage in 2026. Compared with the older NMC lithium chemistry, LiFePO4 is more thermally stable, tolerates higher depth-of-discharge cycling, and uses no cobalt. Typical warranties are 10-15 years or 6,000-10,000 cycles. Slightly lower energy density vs NMC but irrelevant for stationary domestic use.

Load compensation

A boiler or heat-pump control strategy that varies flow temperature based on the room thermostat's measured deviation from setpoint. Less sophisticated than weather compensation alone but cheap to implement. Combining load and weather compensation gives the best efficiency on heat pumps. OpenTherm-capable boilers support both natively.

Loft insulation

Mineral-wool quilt or rigid board laid between and over ceiling joists in a cold-roof construction. Current building-regs guidance is 270mm of mineral wool (roughly 0.16 W/m²K U-value). One of the cheapest and highest-impact measures in any UK retrofit; ECO4-funded for qualifying households.

M

MCS (Microgeneration Certification Scheme)

The UK certification scheme covering installers and products for small-scale low-carbon technologies — solar PV, solar thermal, heat pumps, biomass boilers and battery storage. MCS certification is mandatory for BUS, SEG and most other UK grant or tariff schemes. Every domestic heat pump or solar install routed through grants or feed-in arrangements needs an MCS-accredited installer and an MCS certificate issued on completion.

MEES (Minimum Energy Efficiency Standards)

Regulations requiring private-rented homes in England and Wales to meet a minimum EPC band — currently E. The 2030 target is band C for new tenancies (and all tenancies from 2033, subject to consultation). Landlords must either reach the standard or register a valid exemption. Drives a steady flow of insulation and boiler-upgrade work in the rental sector.

MEV (Mechanical Extract Ventilation)

A continuous low-flow extract fan, typically a single central unit ducted to wet rooms (bathroom, kitchen, utility). Quieter and more reliable than intermittent extract fans. Building-regs Approved Document F System 3. Less expensive than MVHR but doesn't recover heat — appropriate where heat-recovery payback isn't strong.

Modulation ratio

The range over which a boiler or heat pump can vary its output, expressed as a ratio of maximum to minimum. A 6 kW heat pump that can turn down to 1.5 kW has a 4:1 modulation ratio. Higher ratios prevent short-cycling on mild days and matter more in well-insulated homes where shoulder-season heat loads are low.

MVHR (Mechanical Ventilation with Heat Recovery)

A balanced ventilation system extracting stale air from wet rooms and supplying tempered fresh air to habitable rooms, recovering 80-90% of the heat in the outgoing stream through a counterflow exchanger. Building-regs Approved Document F System 4. Most cost-effective in airtight new-builds and deep retrofits where Q50 is below 5; less so in leaky existing fabric.

N

Net zero

The state where a country, organisation or building emits no more greenhouse gases than it removes. The UK's legally binding national target is net zero by 2050, with sector-specific milestones. For homes, net zero typically implies all-electric heating powered by a decarbonising grid, plus a fabric standard sufficient to keep consumption within available renewable capacity. Individual homes targeting net zero usually combine a heat pump, solar PV with battery storage, and a fabric package taking heat demand to roughly 50 kWh/m²/yr or below.

NICE (NICE Guidance NG149)

The National Institute for Health and Care Excellence guideline on indoor temperature and health — relevant to retrofit because cold homes drive excess winter mortality and morbidity. NG149 recommends minimum living-room temperatures of 18°C for healthy adults and higher for vulnerable households. Retrofit measures funded under fuel-poverty programmes are partly justified on the back of this evidence. Useful as a defensible benchmark when arguing temperature requirements with rented-property landlords.

Notification (G98 fast-track)

The post-installation paperwork submitted by an MCS-registered installer to the DNO when commissioning a small-scale generator (solar PV, micro-wind, micro-hydro or domestic battery) under G98 thresholds. No DNO pre-approval needed. The notification typically arrives within 28 days of commissioning and is what triggers the SEG export tariff registration with your supplier.

O

Off-gas

A property without a connection to the mains gas grid — typically rural homes currently heated by oil, LPG, electricity, biomass or solid fuel. Roughly 4 million UK homes are off-gas. Heat pumps usually deliver the strongest economic case in off-gas homes because incumbent fuels are expensive and emissions-intensive.

OpenTherm

An open communication protocol between heating controls and boilers (and increasingly heat pumps) that allows the controller to modulate flow temperature, request specific heat outputs, and read live efficiency data. OpenTherm-compatible thermostats unlock weather and load compensation on supporting boilers and consistently improve seasonal efficiency.

Optimiser (power optimiser)

A small DC-DC converter mounted on each solar panel, delivering panel-level MPPT and isolation in a string-inverter system. Sits between full microinverters and a plain string design — shading on one panel doesn't drag the rest of the string. SolarEdge is the most familiar UK example.

Ofgem

The UK gas and electricity markets regulator. Sets the energy price cap, regulates supplier conduct, oversees ECO4 delivery, administers the BUS and SEG, and publishes the carbon-intensity figures used in SAP and EPC calculations. Effectively the rule-setter for most domestic-energy decisions in this glossary.

P

PAS 2030

The Publicly Available Specification covering the installation of energy-efficiency measures in existing dwellings. Sets out installer competence requirements, on-site quality processes, and product-specification rules. PAS 2030 certification is mandatory for any installer claiming ECO4 funding, and increasingly for MCS heat-pump and insulation work. Always paired with PAS 2035 oversight.

PAS 2035

The companion specification covering the design and oversight of domestic retrofit projects. Mandates a Retrofit Assessment, an Improvement Option Evaluation, a Medium-Term Improvement Plan, a designed package of measures, and monitoring after handover. Run by a Retrofit Coordinator. Required for all ECO4-funded whole-house retrofit and for any project routed through TrustMark.

PIR (polyisocyanurate)

A rigid foam insulation board with a low lambda value (around 0.022 W/m·K) and high compressive strength. Common in floors, warm roofs, internal wall insulation and cold-bridge mitigation. Foil-faced PIR doubles as a vapour control layer when joints are taped — a useful detail in insulating below sloped ceilings.

PIV (Positive Input Ventilation)

A loft-mounted unit pushing tempered, filtered outdoor air into a central hallway, mildly pressurising the home and pushing moist stale air out through passive vents and gaps. Cheap to install, often retrofitted in damp-prone homes. Effective at reducing mould risk in moderately leaky fabric; less efficient than MVHR thermally.

PVGIS

The European Commission's open Photovoltaic Geographical Information System — the freely-available UK-relevant tool for predicting solar-PV yield at any postcode, tilt and orientation. MCS designers and most reputable solar installers use PVGIS (or equivalent) figures as the basis for system-size and payback estimates rather than supplier marketing claims.

R

R32

A common refrigerant in current-generation domestic heat pumps. Lower global-warming potential (GWP ≈ 675) than the older R410A it replaced. Slightly flammable (A2L classification), which influences installer training and indoor-unit siting rules. Being phased toward even-lower-GWP refrigerants under F-Gas regulations.

R290 (propane)

A natural refrigerant with very low GWP (≈3) and excellent thermodynamic performance, allowing higher flow temperatures (up to 75°C) without efficiency collapse. Highly flammable (A3 classification) so use is restricted to outdoor monobloc heat pumps. Now the dominant refrigerant in 2026 retrofit-grade heat pumps.

R454C

A blended low-GWP refrigerant (GWP ≈ 146) used in some heat pumps where R290's flammability is awkward. Sits between R32 and R290 on both performance and GWP. Less common than the other two in 2026 UK domestic kit but appears in some European-manufactured units.

RdSAP

Reduced data Standard Assessment Procedure — the simplified RdSAP methodology used to produce EPCs for existing dwellings, where full SAP isn't practical. RdSAP uses default values for unobservable inputs (wall U-values, glazing performance) based on age band and construction type. Known to systematically misrepresent some fabric upgrades; the 2026 EPC reform is partly a response.

Refrigerant

The working fluid in any heat pump or air-conditioning system, evaporating and condensing through the cycle to move heat. Choice of refrigerant determines maximum flow temperature, efficiency, GWP and installation rules. See R32, R290 and R454C above.

Retrofit Coordinator

The PAS 2035 lead role: a competent person responsible for end-to-end retrofit project oversight — assessment, design, installer coordination, handover and post-occupancy evaluation. Different from a Retrofit Assessor (gathers data) and a Retrofit Designer (specifies measures). All three roles must be present on a PAS 2035 project, though one person can hold multiple competencies.

RHI (Renewable Heat Incentive)

The legacy UK scheme paying ongoing tariffs for renewable heat generation (heat pumps, solar thermal, biomass) for systems registered before March 2022. Closed to new applications and replaced by the upfront-grant Boiler Upgrade Scheme. Existing RHI contracts continue for their seven-year term.

Round-trip efficiency

The proportion of energy that survives a full charge-discharge cycle in a battery. Modern hybrid-inverter LiFePO4 systems are typically 88-92% efficient; AC-coupled retrofit batteries lose more (often 80-85%) because energy makes two AC-DC trips. Multiplies into every payback calculation involving stored solar.

S

SAP (Standard Assessment Procedure)

The UK government's full-data methodology for calculating energy and environmental performance of dwellings — used for new-build building-regs compliance and as the underlying engine for RdSAP. SAP 10.2 is current at time of writing; SAP 11 is in development and changes carbon factors and metric presentation significantly.

SCOP (Seasonal Coefficient of Performance)

The annualised, weather-weighted efficiency ratio for a heat pump across a typical UK heating season. SCOP of 3.5 means 3.5 kWh of heat per kWh of electricity averaged over the year. Unlike instantaneous COP, SCOP is the right number to use for running-cost forecasting. MCS-certified products publish SCOP at standard flow temperatures (35°C, 45°C, 55°C).

SEG (Smart Export Guarantee)

The UK obligation requiring large electricity suppliers to offer a tariff for exported renewable generation from MCS-certified domestic systems. Replaced the Feed-in Tariff for new applications from 2020. Tariff rates vary by supplier (typically 4-15p/kWh) and many are linked to taking the import contract from the same company.

SMETS2

The second-generation UK smart-meter specification — supports half-hourly settlement, supplier switching without re-installation, and richer in-home display data. Required for time-of-use tariffs (Octopus Agile, Cosy, etc.) and for accurate SEG export readings. Older SMETS1 meters are being upgraded by suppliers to function equivalently.

Solar diversion

A device (Eddi, MyEnergi, similar) that detects surplus solar generation and diverts it to immersion-heat domestic hot water before it exports to the grid. Useful when SEG export rates are low relative to the cost of grid electricity for water heating. Less compelling on time-of-use tariffs with high export rates.

SPF (Seasonal Performance Factor)

An older synonym for SCOP — the annualised heat-pump efficiency including all auxiliary loads (controls, circulators, defrost cycles). Some standards distinguish between SPF measured at the heat-pump output and SPF measured at the building-side boundary; the latter is lower and more honest.

Suspended floor

A timber ground floor with a void below, ventilated to the outside through airbricks. Common in pre-1940s UK housing. Insulating between the joists from below (or above, lifting the floor) typically saves 200-400 kWh per year per heated square metre and improves comfort substantially. Maintain underfloor ventilation to avoid timber decay.

T

Tier 1 panel

A solar-PV module from a manufacturer ranked Tier 1 by Bloomberg New Energy Finance — a financial-stability proxy, not a technical performance rating. Tier 1 brands are likely to be around to honour their warranties; they're not necessarily higher-output or more efficient than Tier 2 alternatives. Read the datasheet.

Time-of-use tariff

An electricity tariff with prices that vary by time of day, usually settled half-hourly. Octopus Agile, Cosy, Go and Intelligent are the most familiar 2026 UK examples. Time-of-use unlocks meaningful savings on heat-pump running costs (heat the cylinder overnight) and on battery economics (charge cheap, use peak).

TrustMark

The UK government-endorsed quality scheme for tradespeople working in or on people's homes. TrustMark accreditation is the umbrella requirement for ECO4-funded retrofit and for installers operating under PAS 2030 / PAS 2035. A TrustMark-registered firm is bound by a quality framework and a complaints process with consumer protection backing.

TRV (Thermostatic Radiator Valve)

A self-regulating valve that closes off a radiator when the local room temperature reaches its setpoint, allowing room-by-room temperature differences without zoned wiring. Smart TRVs (eTRVs) add wireless control, schedules and occupancy detection — useful in heat-pump systems for balancing emitter output across rooms with very different heat loss.

U

U-value

The rate of heat transfer through a building element per unit area per unit temperature difference, in W/m²K. Lower is better. The Building Regulations Approved Document L sets target U-values for new and renovated elements; PAS 2035 designs publish whole-element U-values for every measure.

UFH (Underfloor heating)

A heating circuit embedded in the floor structure (in screed in new-build, in low-profile retrofit boards, or clipped to the underside of suspended floors). UFH operates at low flow temperatures (28-40°C) which makes it the natural partner for a heat pump — much higher SCOP than radiator-based systems at the same design conditions.

Ultra-low temperature heating

Heating system designs targeting flow temperatures of 35°C or below — typical of new-build with UFH or of deep retrofits with significantly oversized radiators. Pushes heat-pump SCOP well above 4.0 and reduces electricity consumption in absolute terms by 20-30% versus a 50°C design.

V

Vapour control layer (VCL)

A continuous low-permeability membrane on the warm side of insulation, limiting moisture migration from the heated interior into cold outer layers where it could condense. Critical in internal wall insulation, warm-roof construction and timber-frame retrofits. Specification and continuity of the VCL — including at junctions and service penetrations — is a recurring failure point in PAS 2035 audits. Foil-faced PIR can double as a VCL where joints are sealed, but bonded sheet membranes give a more reliable result on complex geometries.

Ventilation strategy

The deliberate plan for air change in a dwelling, mapped to Approved Document F (System 1 background plus extract; System 2 passive stack; System 3 continuous mechanical extract / MEV; System 4 MVHR). PAS 2035 requires a ventilation assessment whenever airtightness is improved, and the chosen system is documented in the medium-term improvement plan. Skipping this step is the single most common cause of post-retrofit damp complaints.

Volumetric heat capacity

The amount of heat a material can store per unit volume per degree temperature change, in J/m³K. Heavyweight construction (brick, dense block, concrete) has high volumetric heat capacity and stabilises indoor temperature; lightweight timber-frame construction has less. Relevant to retrofit when choosing how much thermal mass to keep inside the insulated envelope — the choice influences overheating risk and how quickly a heat pump can recover setpoint after a long setback.

W

Warm roof

A pitched-roof construction with insulation at rafter level, bringing the loft void inside the heated envelope. Used where loft conversions or heated attic spaces are required, or where cold-roof loft insulation would interrupt eaves ventilation. More expensive to retrofit than a cold roof but enables full use of the upper-floor volume.

Watt peak (Wp)

The peak DC output of a single solar PV panel under standard test conditions. Modern UK domestic panels are typically 400-450 Wp each. Multiply by panel count and divide by 1,000 to get system kWp. See kWp above.

Weather compensation

A controller strategy that varies heating flow temperature in response to outdoor temperature: warmer outside, lower flow; colder outside, higher flow. The single most effective control upgrade for both gas-boiler and heat-pump efficiency. Most modern boilers and almost all heat pumps support weather compensation natively; the limiting factor is usually the room thermostat and how it's been wired.

Z

Zoning

Dividing a heating system into independently-controlled circuits (typically upstairs / downstairs, or living / sleeping). Building Regulations Approved Document L requires at least two zones in dwellings over 150m². Useful for matching heat delivery to occupancy patterns, but in heat-pump systems excessive zoning can reduce SCOP by forcing the unit to short-cycle on small loads — design carefully.

This glossary is maintained alongside the rest of the EcoSaving Hub site and updated as standards, grants and technologies change. If a term you've seen on a quote, EPC or installer report isn't here, it's worth asking the installer to define it on their own letterhead before signing — vague language at quote stage is one of the strongest predictors of a difficult retrofit. For decision-grade numbers on the technologies covered above, work through our heat-pump, solar and insulation calculators rather than relying on supplier marketing figures.

A short note on jargon hygiene. The technical-sounding language in this list does real work: SCOP and COP are not the same number, U-values and R-values measure opposite things, and PAS 2030 and PAS 2035 govern different parts of the same project. Suppliers who blur these distinctions in marketing copy tend to blur them in workmanship too. Read quotes carefully. Ask installers to show you the heat-loss calculation, not just the headline kW size. Confirm the EPC has been re-run after measures complete. None of this is obstructive — it's the basic discipline that separates a retrofit you live with happily for 20 years from one that disappoints in year three.