Partnering With Gresford Architects

This project is a collaboration with Gresford Architects, an award-winning Oxford-based practice led by Tom Gresford. Founded in 2006, Gresford are RIBA-chartered and widely recognised as one of the leading Passivhaus and EnerPHit design practices in the UK. Tom’s own home — a 1960s bungalow in Oxfordshire retrofitted to the EnerPHit standard, clad in timber harvested from the site itself — has been featured in national architectural press and stands as a genuine example of what’s possible when retrofit is done with ambition and care.

Gresford handled the full architectural design for Riverside Road. We’re delivering the build. It’s the kind of partnership that consistently produces the best results in retrofit work: a design team with deep expertise in building physics, thermal modelling, and Passivhaus Planning Package (PHPP) analysis, working hand-in-glove with a construction team that understands how to translate those specifications into reality on a live building site.

That relationship between design and construction is critical in EnerPHit work. When you’re chasing airtightness targets of 1.0 air changes per hour at 50 Pascals — compared to the 5–15 range that standard building regulations allow for new homes — there is no margin for miscommunication between the architect’s intent and what gets built. Every junction, every penetration, every transition between old fabric and new needs to be right. The design has to be buildable, and the builder has to understand why every detail has been specified the way it has.

The Thermal Envelope: External Insulation and Render

The single most impactful measure in any EnerPHit retrofit is the thermal envelope — wrapping the existing building in a continuous layer of insulation that dramatically reduces heat loss through the walls and roof. On Riverside Road, the existing masonry walls are being externally insulated with a render finish system.

External insulation is the preferred approach for most solid-wall retrofits, and for good reason. It wraps the building from the outside, which means the existing masonry sits within the warm, insulated envelope rather than being exposed to temperature swings. This eliminates cold bridges at junctions — the points where floors, walls, and roofs meet — which are typically responsible for a disproportionate share of heat loss in older homes. It also avoids the reduction in internal floor area that comes with internal insulation, and sidesteps many of the interstitial condensation risks that internal insulation can introduce if not detailed extremely carefully.

One detail that’s easy to overlook but makes a significant difference: the construction drawings specify that all wall penetrations — every pipe, duct, and cable that passes through the thermal envelope — must be insulated with a minimum of 150mm of insulation along the entire length of the duct, from the internal unit to the external termination. These penetrations are weak points in any airtight envelope. Left untreated, a single uninsulated duct run can create a thermal bypass that undermines the performance of the entire wall. Getting this right requires careful coordination between our insulation team, the plumber, the electrician, and the MVHR installer. It’s one of those details where having all trades employed directly rather than juggling subcontractors makes a real difference.

The roof insulation follows the same principles. Whether we’re dealing with the existing pitched roof structure or the new flat and mono-pitched roof sections over the extensions and dormer, the insulation layer must be continuous — no gaps, no thin spots, no compression at awkward junctions. Intumescent fire barriers are specified at the head and base of all ventilated cavities, addressing fire safety without compromising the thermal line. The section drawings show the insulation wrapping over the top of the existing walls and into the roof zone, so the entire building sits inside an unbroken thermal blanket.

Triple Glazing and Controlled Daylight

Every window and external door in the house is being replaced with high-performance triple-glazed units. In an EnerPHit retrofit, the windows aren’t simply about keeping heat in — they’re part of a carefully calculated balance between thermal loss, solar gain, and daylight.

On the ground floor, the rear wall opens through large triple-glazed sliding doors onto a new stepped terrace, flooding the open-plan kitchen, dining, and living space with natural light and connecting the interior directly to the garden. These aren’t small units — the drawings show openings approaching three metres wide — but the triple glazing means they perform thermally far better than even the best double-glazed alternatives. For the cool-temperate climate zone that covers most of the UK, EnerPHit-compatible windows typically need to achieve U-values of around 0.85 W/m²K — roughly three times more thermally efficient than standard double glazing.

At first floor level, the bedrooms have new triple-glazed windows sized to suit each room, with a high-level window to the landing that brings light deep into the plan without compromising privacy. In the new loft floor, a triple-glazed rooflight supplements the dormer window, drawing daylight down into the centre of the house. Even the existing window openings that no longer serve the new layout have been carefully infilled rather than simply boarded up — maintaining the integrity of the thermal envelope at every point.

The window positions and sizes aren’t arbitrary. In a PHPP-modelled design like this, each opening has been calculated to balance heat loss against useful solar gain. South-facing glazing can contribute meaningful passive heating in winter, but only if the overall envelope is insulated well enough to retain that warmth. It’s one of the reasons the fabric-first approach works — you insulate the shell to the highest standard, seal it tight, and then the glazing strategy can deliver as it’s intended to.

Reconfiguring Three Floors

This isn’t a skin-deep retrofit. The interior of the house is being substantially reconfigured across all three floors to create a home that works as well spatially as it does thermally.

Mechanical Systems: ASHP, MVHR, and Solar

Once the thermal envelope is performing at EnerPHit levels, the mechanical systems needed to heat, ventilate, and power the house can be dramatically downsized compared to a conventional home. That’s the fundamental logic of fabric-first design: reduce the demand before you think about supply.

An air source heat pump provides all the heating and hot water. In a home with this level of insulation and airtightness, the heat demand is a fraction of what an unimproved house of the same size would require. The Passivhaus Institute targets a maximum heating demand of 25 kWh per square metre per year for EnerPHit certification, compared to typical consumption of 100–200 kWh/m²/yr in older UK homes. That means the heat pump can be smaller, run more efficiently, and cost significantly less to operate than it would in a leaky building.

The MVHR system — mechanical ventilation with heat recovery — is essential in any airtight building. When you seal a house to EnerPHit levels, you can’t rely on draughts and trickle vents for fresh air anymore. The MVHR extracts stale, moist air from bathrooms and the kitchen, passes it through a heat exchanger that recovers 90% or more of the warmth, and uses that recovered heat to temper the incoming fresh air supply to bedrooms and living spaces. The result is a house with consistently excellent air quality, no condensation problems, and no cold draughts — something that’s hard to appreciate until you’ve experienced it.

The drawings show the MVHR unit located on the ground floor alongside the heat pump cylinder, with ducted distribution running up through the building to supply and extract points in every room. The duct routes have been carefully planned to minimise both length and the number of bends — both of which affect airflow efficiency and noise. Every duct penetration through the thermal envelope is insulated to the 150mm specification. A non-return valve on the incoming water supply prevents backflow. These are the details that determine whether an MVHR system works brilliantly or becomes an underperforming disappointment — and they need to be right on every single run.

Solar panels on the new roof extension complete the energy picture. In an EnerPHit home with radically reduced demand, even a modest PV array can cover a meaningful proportion of the annual electricity consumption — including running the heat pump. The panels are positioned on the south-facing roof slopes of both the dormer and the rear extension, maximising generation across the available roof area.

Building in a Flood Zone

One of the distinctive features of this project — visible in the section drawings — is a flood void beneath the ground floor. Riverside Road sits close to the water in Botley, and the design has been developed with flood resilience built in from the outset. The ground floor is raised above the flood level, with a void beneath that allows floodwater to pass through without reaching the living spaces. A pre-cast concrete lintel spans the flood void opening, with a proposed flood grille providing access for maintenance while maintaining structural integrity.

The drawings note that the flood void requires periodic inspection — head height within the void is limited, and maintenance access is essential. The hatch for flood void access has been designed for maintenance to be carried out by suitable professionals with risk assessments undertaken.

The flood void also shapes the foundation strategy. The drawings show mass concrete trenchfill foundations to the structural engineer’s specification, with new sleeper walls supporting the raised ground floor slab. Existing ground is retained against the original rear wall. New pad foundations support the rear extension, and the existing foundations — assumed but to be confirmed — are left undisturbed where the structural engineer has specified “do not underpin.”

Combining flood resilience with EnerPHit performance is unusual — most retrofit projects deal with one challenge or the other. It adds complexity to the thermal envelope strategy, because the floor construction has to manage both insulation continuity and the structural demands of a raised floor over a void. It’s the kind of design challenge that requires close coordination between architect, structural engineer, and build team — and it’s another reason this project benefits from the Gresford Architects / Lynch Brother Homes partnership.

Materials and External Finishes

The external appearance of the house is being transformed alongside its performance. The existing walls receive a clean render finish over the external insulation system — a significant visual upgrade that also serves as the weather protection layer for the insulation beneath. The rear extension and dormer are finished in standing seam metal — a material that’s durable, low-maintenance, and gives the new elements a distinct contemporary character that reads clearly against the rendered original house.

On the first floor, the window sill over the ground floor extension roof is formed from EPDM roofing membrane on an 18mm external structural grade plywood substrate, with an upstand in standing seam zinc cladding on a timber frame to form the shaped opening. The zinc back gutter drains towards the south onto the loft roof. It’s a detail that shows the level of care in the design — every junction between old and new, between different materials, has been resolved both thermally and aesthetically.

The stepped terrace to the rear uses external tiles laid to a fall of 1 in 80 away from the building, ensuring rainwater drains cleanly away from the newly insulated envelope. Cement board sheathing faces the dormer cheek on the party wall side. The rainwater goods — pipes and guttering — are positioned to handle the altered roof geometry without creating water management issues at the insulation line.

Why EnerPHit Matters for Oxford’s Housing Stock

Oxford is full of homes like this one — solid, well-built houses from the mid-twentieth century that were never designed with energy performance in mind. Victorian terraces in Jericho, Edwardian semis in Summertown, post-war housing in Botley, Headington, and Cowley. They’re structurally sound, often architecturally valued, and thermally terrible. The EnerPHit standard, developed by the Passivhaus Institute, exists precisely for buildings like these — targeting energy savings of 75–90% while accepting that the constraints of an existing structure mean full Passivhaus new-build performance may not always be achievable.

For the homeowner, the benefits are tangible and immediate: dramatically lower heating bills, consistent temperatures in every room throughout the year, excellent indoor air quality from the MVHR system, elimination of draughts and cold spots, reduced condensation and mould risk, and a home that’s genuinely comfortable in every season. These aren’t marginal improvements — the difference between living in an uninsulated house and an EnerPHit-certified one is transformational.

For the wider picture, retrofitting existing homes at this level of performance is one of the most effective steps the UK can take toward decarbonising its housing stock. The Climate Change Committee has been clear that reaching net zero is impossible without addressing existing homes. The UKGBC estimates that approximately two low-energy home retrofits are needed every minute for the next 25 years to meet our targets. Projects like Riverside Road demonstrate that it’s technically achievable and practically deliverable — the limiting factors are skills, knowledge, and the willingness to do it properly.

The Team Behind the Build

A project like this demands more than standard construction skills. The tolerances are tighter, the sequencing matters more, and understanding why each detail is specified — not just what to build — makes the difference between hitting the standard and falling short. Every junction is a potential thermal bridge. Every service penetration is a potential air leak. The build team needs to think in terms of the continuous thermal envelope at every stage, from foundations to ridge.

That’s one of the reasons we’ve recently invested in specialist training. Our Project Manager Philip Franklin-Slattery and I both completed the Passivhaus Retrofit Delivery course run by Coaction Training this month — an advanced programme accredited by the Passivhaus Trust that focuses specifically on the practical challenges of delivering deep fabric retrofits on site. It covers building physics, airtightness strategy, moisture management, and the sequencing of retrofit works — taught by practitioners who’ve personally delivered certified EnerPHit projects.

Our team of 50+ directly employed tradespeople means we control quality at every stage. Carpenters, bricklayers, plasterers, roofers — all employed by us, all working to the same standards, all coordinated on the same site. When the insulation installer, the window fitter, and the MVHR duct installer all work for the same company, the careful sequencing that EnerPHit demands happens naturally. There are no gaps between trades where accountability gets lost.

Combined with a design team as capable as Gresford Architects and the independent rigour of the EnerPHit certification process, the result should be a home that performs exactly as it was designed to — verified by testing, not assumed.