Insulation comes with its own vocabulary — U-values, cold bridging, PAS 2030, vapour control — and quotes often assume you already speak it. This glossary explains every term you're likely to meet in plain English, grouped by theme. Use the contents list to jump to what you need.
Building science
- U-value
- A measure of how much heat passes through a building element, in watts per square metre per degree (W/m²K). Lower is better — a well-insulated wall might reach 0.30 W/m²K versus 2.0+ for an uninsulated solid wall.
- Lambda value (λ)
- The thermal conductivity of an insulation material — how well it conducts heat. A lower lambda means a thinner board achieves the same performance. Phenolic boards (≈0.018) outperform mineral wool (≈0.040) for the same thickness.
- Cold bridging
- Also called thermal bridging: a path of higher heat loss through a structure — around a window reveal, a floor junction or a steel lintel — where insulation is interrupted. Cold bridges waste heat and are the most common cause of localised condensation and mould.
- Interstitial condensation
- Condensation that forms inside a wall or roof build-up, where warm moist air meets a colder layer. It is the main risk when internal wall insulation is specified incorrectly, and is controlled with the right vapour strategy.
- Breathable (vapour-open)
- A construction that lets water vapour pass through and evaporate rather than trapping it. Older solid walls are vapour-open, so heritage insulation uses breathable materials such as wood fibre to keep them dry.
- Vapour control layer (VCL)
- A membrane fitted on the warm side of insulation to limit how much moist indoor air reaches colder layers, preventing interstitial condensation. Essential detail in rigid internal wall and warm-roof systems.
- Thermal envelope
- The continuous insulated boundary separating heated indoor space from the outside — walls, roof, floor and openings. Good retrofit aims to make this envelope as unbroken as possible.
Construction types
- Solid wall
- A wall built as a single masonry leaf with no air gap — typical of pre-1930s homes. Solid walls can lose 30–40% of a home’s heat and need external or internal wall insulation, as there is no cavity to fill.
- Cavity wall
- A wall built as two leaves with a gap between, standard from roughly the 1930s onwards. The cavity can often be filled with insulation relatively cheaply where it is clear and suitable.
- Suspended timber floor
- A ground floor of floorboards on joists with a ventilated void beneath, common in pre-1950s homes. Insulated from below where there is access, or by lifting boards — always keeping sub-floor ventilation.
- Cold roof
- A roof insulated at ceiling (joist) level, leaving the loft space above at outside temperature. The standard, low-cost approach for lofts used only for storage.
- Warm roof
- A roof insulated at rafter level, bringing the loft inside the heated envelope. Used for converted or habitable lofts; needs careful ventilation and condensation control.
- Room-in-roof
- A loft already converted into living space. It needs rafter-line (warm-roof) insulation rather than insulation over the floor, so it stays usable year-round.
- Air brick
- A perforated brick that ventilates a sub-floor void to keep timber dry. Floor insulation must never block air bricks — the airflow is maintained and insulation sits above it.
- Damp-proof course (DPC)
- A barrier built into a wall to stop rising damp. Insulation systems are detailed to respect the DPC line so moisture is not bridged.
Insulation measures
- External wall insulation (EWI)
- Insulating board fixed to the outside of external walls and finished with weatherproof render. The most thermally effective option for solid-wall homes, with no loss of internal space.
- Internal wall insulation (IWI)
- Insulated board or a breathable system fitted to the inside of external walls. The right choice for listed buildings, conservation areas and flats where the exterior cannot change.
- Loft top-up
- Adding insulation over an existing under-insulated loft to reach the current 270mm standard. The cheapest measure with the fastest payback.
- Draught-proofing
- Sealing gaps around doors, windows, floors and skirtings to stop uncontrolled cold air. Cheap, and especially effective paired with floor insulation.
Materials
- Mineral wool
- Glass or rock fibre insulation supplied as rolls or batts. The standard for loft top-ups and a good non-combustible option; lambda around 0.034–0.044.
- PIR board
- Rigid polyisocyanurate foam board (brands include Celotex and Kingspan Therma). High performance for its thickness; widely used in roofs and floors.
- Phenolic board
- A premium rigid board (e.g. Kingspan Kooltherm) with very low lambda (≈0.018), giving the best performance for the least thickness — useful where space is tight.
- EPS
- Expanded polystyrene — a cost-effective rigid insulant often used in external wall systems where budget matters and thickness is less constrained.
- Wood fibre
- A breathable, vapour-open board made from compressed wood fibres. The go-to for internal insulation of older, solid-wall and listed buildings because it manages moisture safely.
- Thin-coat render
- The silicone or acrylic finish coat applied over external wall insulation. Available in a full colour and texture range; durable and breathable.
- Multifoil
- A thin, multi-layer reflective insulation used where build-up depth is very limited. Performance is space-saving rather than best-in-class.
Standards & schemes
- PAS 2030
- The British standard governing how retrofit insulation must be installed. PAS 2030-19 certification, which we hold, means installations are independently audited against it.
- PAS 2035
- The companion standard covering retrofit design and assessment — how a whole-house plan is put together before measures are installed.
- TrustMark
- The UK government-endorsed quality scheme for tradespeople. A TrustMark-registered firm (we are licence 2259022) has its work and standards independently checked.
- Competent Person Scheme (CPS)
- A scheme that lets a registered installer self-certify building-regulation compliance for their work, avoiding a separate council application.
- EPC
- Energy Performance Certificate — the A–G rating of a home’s energy efficiency. Insulation is one of the most reliable ways to lift an EPC band, which feeds into property value.
- Permitted development
- Works allowed without a full planning application. Most insulation is permitted development, but conservation areas and listed buildings need consent — checked during the survey.