Hiley Road – Passivhaus Retrofit Project

Now a full Passivhaus Retrofit, this Victorian mid-terraced house had been in need of a revamp. The owner seized the opportunity to upgrade to a thermally efficient, comfortable eco-house, showcasing possibilities for similar homes across Britain. The solid brick-walled house, with old leaky windows, was cold, draughty, and uncomfortable, with some areas affected by mold. Peter decided to upgrade to Passivhaus to maximize thermal performance.

As an existing property some restrictions existed. For example, the orientation faces west, which isn’t ideal. Also, the windows had to retain the same character tot match the street scene. The foundations weren’t built with Passivhaus thermal bridge-free details and can’t be easily changed. However, the mid-terrace position minimizes heat loss, assuming neighboring houses are similarly heated.

The following strategy was taken to make the building highly efficient, incredibly comfortable and ensure that it achieve Passivhaus certification:

1. Add a tea cosy

We wrapped the house in a blanket of insulation: 250mm on the front and back walls. 270mm between and above the roof rafters. 300mm beneath a new polished concrete slab after excavating the floor. Our initial planning application to add insulation to the outside of the front wall was refused. So we planned to demolish the wall, move it inward, add external insulation, and render it to match the original house level.

We rejected internal insulation due to the limited thickness we could add and the risk of interstitial condensation, which would prevent meeting full Passivhaus standards. However, we won permission under permitted development to keep the existing front wall intact, add external insulation, and render it. We obtained a certificate of lawfulness from the planning authority for this option. The planners informed us there’s no maximum thickness, but the building cannot grow internally.

2. New windows and a Passivhaus door

We replaced all the windows with high-performance, triple-glazed timber windows from the Green Building Store. The windows use a double seal to eliminate draghts. We carefully detailed fitting with additional insulation around the frames, the building’s weakest point, to minimize heat loss. The windows are meticulously taped to ensure a completely draught-free installation.

The front door, with a U value of just 0.59, performs three times better than most front doors and includes a vacuum insulation panel.

3. Reduce the heat loss areas

The house redesign made the interior more suitable for modern lifestyles. We also gained additional floor space, by removing the chimneys (which were thermal bridges), adding a second-floor loft room. The front door recess was also eliminated, creating much needed space by the front door. These changes produced a more efficient ratio of usable floor area to external area, reducing the heat loss areas.

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4. Attention to detail

Reaching the Passivhaus standard needs incredible attention to detail. Not only to ensure the property is draught free but also to ensure that insulation is installed well, without gaps that can leak heat. A thermal bridge is a weak spot in the insulation where heat is allowed to pass through more easily. It occurs when a conductive material spans from the inner to the outer face of the insulation. They can cause cold patches on the inside, and potentially condensation and mould. Passivhaus requires thermal bridges to be reduced to a minimum. This can be particularly difficult in old houses, as structural materials like brick and concrete conduct heat well. To minimuse thermal bridges we conducted extensive thermal analysis.

4. Attention to detail

This included junctions with neighbouring properties and the ground where it is more difficult or impossible to add insulation. One often overlooked cold bridging area is load-bearing internal walls that bypass floor insulation. These bridges double heat loss as heat loss occurs on both sides of the wall. Our solution, to make the cold bridge path as long as possible, to maximise the limited thermal resistance of the brickwork by increasing thickness. We achieved this by adding extra insulation to the base of the walls, covered with a larger skirting board. Other difficult areas have included making the wall behind the existing staircase airtight. Also, where the front wall adjoins the neighbour’s wall.

5. Eliminate Draughts

Draughts make a house uncomfortable and cause significant heat loss, so minimizing them is crucial. It’s also a fundamental requirement for Passivhaus certification. In well-insulated buildings, heat loss from draughts can equal or exceed losses through walls, floors, roofs, and windows combined. Achieving airtightness, especially in a Passivhaus retrofit, is challenging. We completely stripped the house back and created a new airtightness layer inside, using ordinary plaster or concrete in some areas and intelligent breathable membranes in others. Junctions between elements are critical, as shrinkage and movement can create cracks that let air escape. We detailed specialist airtightness tapes designed for durability in these locations. We achieved an airtightness of less than 0.6 air changes per hour at 50 Pascals, far exceeding the building regulations backstop of approximately 10.

6. Efficient MVHR

Once the house is draught free, there is a need for ventilation to provide fresh clean air to the occupants and to expel stale air and excess moisture. This could be achieved by making holes in the fabric (vents and trickle vents), or by using a ventilation system that extracts heat from the exhaust air and pre-heats the fresh intake air – Mechanical Ventilation with Heat Recovery (MVHR). These can be up to 92% efficient at transferring the heat, and provide filtered air at calculated rates and locations. This also ensures a healthy environment, and has been shown to improve the health of the occupants. A high quality Paul Focus 200 unit has been specified, which provides the maximum performance with a very low level of noise – less noisy than the fridge!

7. Reduce hot water demand

In a Passivhaus, hot water demand can equal or exceed heating costs, so reducing hot water usage significantly lowers fuel bills. We specified low-flow shower heads and taps with aerators to increase water volume while minimizing usage. Additionally, a hot water recovery system was specified below the bathroom to pre-warm shower water using heat from the shower’s waste water, achieving up to 60% efficiency. We also addressed heat loss through the soil vent pipe, which can act as a thermal chimney. For this home calculated at 1.5kWh/m²a (10% of the total heat losses allowed in a Passivhaus!). To eliminate this loss, we removed the need for the vent pipe entirely.

8. Renewable energy

Photovoltaic (PV) panels were added to the sloped west-facing and flat loft roofs, tilting the flat roof panels slightly to the east. Although it’s traditionally thought that south-facing panels provide maximum gain, peak electrical demand occurs in the mornings and afternoons, not at midday when the sun is south. Therefore, an east-west roof configuration may be preferable as it maximizes energy gain during peak usage when use is higher. It also allows for a larger area of panels.

9. Efficient appliances

Once the bulding has been optimised we turned our focus to appliances. This requires installing at least an A++ fridge freezer and other energy-efficient white goods to minimize energy usage.

Hiley Road has been featured in the Passive House magazine and on the Passivhaus Trust website.