This guide explains what a whole house retrofit involves, why the order of works matters more than any single product, what a proper survey finds, which materials belong in older houses, and what realistic timescales and outcomes look like — drawn from retrofits we have actually delivered across Oxfordshire.
What a whole house retrofit is — and what it is not
Most energy improvements in this country are piecemeal: a loft top-up one year, a new boiler the next. Each measure is judged on its own, and the junctions between them — where one trade’s work meets another’s — are nobody’s job.
A whole house retrofit inverts that. The building is assessed as a single thermal envelope, a plan is designed for the whole of it, and the works are sequenced so that each measure makes the next one possible. Insulation, airtightness, ventilation and heating are treated as one system, because in physics they are one system: seal a house without planning its fresh air and you trade draughts for condensation; fit a heat pump before insulating and you buy a machine sized for a building that is about to stop existing.
The houses that need this thinking most are the ones Oxfordshire is full of: Victorian and Edwardian solid brick, inter-war semis, village stone houses — none with a cavity you can simply fill, all managing moisture in ways modern sealed-up construction does not.
Fabric first: why walls, roofs and floors come before the heat pump
“Fabric first” is the discipline of cutting the building’s heat demand — through insulation, better windows and airtightness — before changing how it is heated. There are three hard reasons for it.
First, the heat pump must be sized against the finished building. A heat pump’s size, its radiators and its running cost all follow from a room-by-room heat-loss calculation. Run that calculation on an uninsulated house and you install a bigger, more expensive machine that will be oversized the day the insulation goes in. At Orchard House, near Banbury, the air source heat pump was designed last, against the improved envelope: heat losses were recalculated on post-retrofit values, and all sixteen existing radiators were photographed, logged and checked against low-flow-temperature output — replaced only where they would genuinely fall short.
Second, low temperatures need low losses. Heat pumps run most efficiently delivering water at low temperatures — which only keeps a house comfortable if the house holds its heat. On Howard Street the old gas boiler came out only after walls, roofs, floors, windows and ventilation were done, and the register now rates the heat pump and its zoned controls “very good”.
Third, the trades physically depend on each other. This is the part no product brochure tells you, so here is the actual sequence from Orchard House, a detached 1970 stone house we retrofitted over about six months:
- Electrics first. Full electrical renewal before anything else — consumer unit relocated, degraded wiring replaced, and dedicated supplies run for the heat pump and ventilation that did not yet exist. The telling detail: back boxes were specified deep enough for the coming 60 mm insulation zones, so sockets would sit cleanly on the new wall build-up without piercing it.
- Ventilation routes decided before the windows were made. The house was getting humidity-controlled whole-house extract ventilation, and the background-air strategy was settled before the windows went into production — so air inlets were designed into the fabric, not bored through finished work afterwards. The wet-room windows carry no trickle vents at all; their air leaves through the extract terminals, under control.
- Windows and their junctions. New glazing went in with each frame sealed to its opening with pre-compressed airtightness tape, and the cold reconstituted-stone mullion reveals insulated with thin calcium-silicate board — done concurrently with the wall insulation so reveal, tape, plaster and wall build-up meet as one junction rather than four trades’ guesses.
- Fabric: walls, sloping ceilings, floors. Room-by-room internal wall insulation in breathable materials; nine separate sloping-ceiling zones opened up and insulated between the rafters with a ventilation gap preserved above; each zone inspected before it was re-lined, because work concealed is work verified first.
- Heat last. Only then did the oil boiler, its tank and its cylinder leave the site, replaced by an air source heat pump — with the old distribution pipework that ran through the uninsulated ground slab abandoned, channels excavated, and new insulated runs laid so the heat travels to the rooms, not the earth.
Get that order wrong and you fit windows twice, cut holes through finished render, or bury services behind insulation. On our Church Way project in Iffley, the top-floor windows were deliberately held back until after the re-roof, because the new insulated roof build-up changes the size of the roof-level openings. Fit them first and they would have been fitted twice.
What a proper retrofit survey finds
A whole house retrofit starts with knowing exactly what you have — measured, not assumed. On our own projects, the survey stage has produced findings that changed the entire design:
- Walls that are not what they look like. Orchard House appears to be a traditional stone house; the survey found a hybrid — rubble-stone outer leaf, concrete-block inner leaf, an irregular unfilled cavity, roughly 410 mm thick overall. That construction cannot be cavity-filled predictably, so the whole insulation strategy moved inside.
- Measured air leakage, not guessed. A pre-works pulse test at Orchard House recorded air changes ranging from 1.4 in the large sitting room to 5.6 in one bedroom — a house leaky in the wrong places, yet with no deliberate background ventilation anywhere.
- Hidden defects that would sabotage the works. At Howard Street, a borescope survey found the rear extension’s 50 mm cavity part-blocked with builders’ rubble — a damp risk in its own right, cleared well below damp-course level before any fill went in.
- The forgotten surfaces. Sloping ceilings — the roof slopes that pass through bedrooms, called skeilings — are routinely uninsulated even in houses with a topped-up loft. At Orchard House, nine separate skeiling zones totalling about 28 m² sat over the bedrooms and bathrooms with almost nothing behind them, while the main loft carried 300 mm.
- Where condensation already lives. Cold reveals, uninsulated stone surrounds, wardrobes against external walls — the survey maps the surfaces where moisture already condenses, because those are the details the design has to solve.
A survey that consists of a walk-round and a standard quote is not a retrofit survey. Ask what will actually be measured.
Materials: why older houses need breathable systems
Solid masonry walls — brick or stone — handle moisture by absorbing and releasing it. Insulate them with impermeable plastic foams and cement-based finishes and you can trap that moisture in the wall, which is how internal insulation gets its bad name: damp, decay and mould behind the boards.
Our whole-house projects use vapour-open, capillary-active systems instead: wood-fibre boards, lime plasters, cork in wet rooms, calcium-silicate board where depth is scarce or damp risk is highest. At Orchard House the insulation was specified room by room — wood fibre at 30–60 mm where depth allowed, cork with lime in the wet rooms, calcium silicate at the stone reveals — every layer able to buffer and release moisture, and the moderate thicknesses deliberate: most of the benefit at far lower moisture risk than aggressive build-ups. The full reasoning is in our natural insulation materials guide.
Ventilation is not optional
Every whole house retrofit makes a building more airtight, and an airtight house must breathe on purpose. On every project we deliver, a designed ventilation system goes in as part of the works — typically humidity-controlled continuous extract, which runs quietly in the background and ramps up automatically when cooking and showers raise moisture. At Church Way in Iffley the system was engineered for the specific house: a 275 m² dwelling, a calculated moisture generation rate of 82.5 litres per second, two central extract fans, humidity- and presence-sensing terminals in the wet rooms, fully commissioned at handover.
Skipping this step is how retrofits create the mould problems they were meant to solve. The physics is explained plainly in our damp, mould and condensation guide.
What outcomes look like — evidence, not promises
We quote two kinds of outcome, and we keep them separate: design targets (what the specification aims for) and verified results (what the public register records). Two of our whole-house retrofits have register-verified outcomes:
- Howard Street, East Oxford — a late-Victorian solid-brick mid-terrace, the most common hard-to-heat house type in Oxford. Certificate before: E (49), dated November 2013. Certificate after: B (89), dated January 2022, with estimated emissions down from 5.2 to 0.7 tonnes of CO₂ a year — a cut of roughly 87%. The design model predicted a high C; the finished house beat its own target.
- Orchard House, near Banbury — a detached 1970 stone house, heated by oil a year earlier, certified EPC B in December 2023.
Both certificates are on the government’s public energy-certificate register; nothing above relies on our own measurements. A third project, an 1880s Victorian semi on St Margaret’s Road in North Oxford, was certified EPC C in March 2022 and has since gone entirely gas-free on a heat pump — its owners now open the house to the public at Oxford’s green open-home events, validation no brochure can manufacture.
Just as important is what does not change. At Orchard House, nothing is different from the street: the stone, the mullions and the roofline read exactly as before. The retrofit is in the performance, not the appearance.
How long it takes
Orchard House took around six months on site — honestly longer than first hoped, as the opened-up building revealed more scope — with the owners away for the five most intensive months and home for Christmas. That is a truthful benchmark for a detached house. Terraces can move faster, particularly empty: at Howard Street the owners moved out, several trades ran at once, and the programme shortened accordingly.
The owners of Orchard House put it better than we can: “You really have to go into a project like this with your eyes wide open.” Deep retrofit is disruptive. Honest programmes are part of doing it properly.
Choosing a contractor
Whole house retrofit lives or dies on coordination, so the questions worth asking a contractor are about sequence and junctions, not just products:
- Who designs it, and who owns the junctions? One accountable design-and-build team, or five trades and a hope? Ask who details the point where window frame, airtightness tape, reveal insulation and wall build-up meet.
- Will the heat pump be sized after the fabric works, from a room-by-room heat-loss calculation? If the heating quote arrives before the insulation design, the sequence is already wrong.
- What will the survey measure? Wall construction, airtightness, existing ventilation, moisture — or just square metres for a price?
- What is the ventilation design? If the answer is “extractor fans in the bathrooms”, the airtightness has not been thought through.
- Are the materials right for the wall? Anyone proposing foam boards and gypsum against a Victorian brick wall has not understood the building.
- What evidence stands behind past projects? Ask for register-verified outcomes and named build-ups from real jobs, not stock photography — ours are at our projects page, with materials, measurements and reasoning on every one.
One coordinated contract also has a practical economy: on our Church Way project, a single weather-protected scaffold served the re-roof, four chimney repoints, the rooflights, the rainwater goods and the window installation together — one mobilisation instead of four.