Case Study – Passivhaus Retrofit2018-08-30T09:23:40+00:00

Hiley Road – Passivhaus Retrofit Project

This Victorian mid terraced house was in need of a revamp, and the owner decided this was the opportunity to upgrade to a highly thermally efficient, comfortable eco-house, showcasing what can be done to this common house type throughout Britain. This solid brick walled house with old and leaky windows (some single glazed) was cold, draughty and generally uncomfortable, with mould growth in some areas. Peter thought if you are going to improve the thermal performance, why not go the full way and upgrade to full Passivhaus.

This obviously has some restrictions – the orientation is fixed with the front facing west which is not ideal, and the windows need to remain similar to retain the street scene. The foundations have not been built with the thermal bridge free details of a new Passivhaus, and cannot easily be changed. However, the house is mid terrace and so the heat losses either side are minimal (assuming these houses are heated to a similar temperature), reducing the heat loss area.

The following strategy has been taken to make the building highly efficient, and incredibly comfortable:

1. Add a tea cosy

The house is being wrapped in a blanket of insulation: 250mm of insulation on the front and back walls; 270mm of insulation between and above the rafters in the roof, and the floor has been excavated and filled with 300mm of insulation below a new polished concrete slab. Our original planning application to add insulation to the outside front of the house was refused, so the plan was to demolish the wall and move it in to then add insulation to the outside and render to bring it back to the level of the original house.

We had rejected internal insulation, as the amount we could add was limited due to the risk of interstitial condensation, and this would not allow us to meet full Passivhaus. However we successfully won permission under permitted development to leave the existing front wall intact and add insulation and render to the outside, and we obtained a certificate of lawfulness for this from the planning authority. The planners have informed us that there is no maximum thickness, but the building cannot get bigger internally.

2. New windows

All the windows are being replaced with high performance triple glazed timber windows from the Green Building Store. These will be carefully fitted with additional insulation around the frame (the weakest area in the building) to reduce the area of heat loss. They are also carefully taped to ensure a completely draught free installation and feature double seals, again to reduce draughts to zero. The front door has a U value of just 0.59, 3 times better performing than most front doors, and incorporates a vacuum insulation panel.

3. Reduce the heat loss areas

The work to the house has enabled a redesign of the interior making it more suitable to current lifestyles, and additional floor space has been gained by removing the chimneys internally (which were also thermal bridges), adding a second floor loft room and removing the recess at the front door. These all enabled a more efficient ratio of usable floor area to external heat loss area.

4. Attention to detail

To reach the Passivhaus standard needs incredible attention to detail, not only to ensure it is draught free but also to ensure that the insulation is installed well and without gaps that can leak heat. Cold bridges are areas that use materials that conduct heat well, bypassing the insulation. This can cause cold patches on the inside, and potentially condensation and mould. Passivhaus requires these to be reduced to a minimum, which can be particularly difficult in old houses, as structural materials like brick and concrete conduct heat well. In this project we have done a lot of thermal analysis of the details, particularly at the junctions with the neighbouring properties and the ground where it is more difficult or impossible to add insulation.

4. Attention to detail

One particular area of cold bridging which is often overlooked is the load bearing internal walls that bypass the floor insulation to the ground, and have a route on both sides of the wall, doubling the heat loss. Our solution to this is to make the cold bridge path as long as possible, to maximise the limited thermal resistance of the brickwork by increasing thickness. We have achieved this through adding additional insulation to the base of the walls, covered with a larger than normal skirting board. Other difficult areas have included making the wall behind the existing staircase airtight, and where the front wall adjoins the neighbour’s wall.

5. Eliminate Draughts

Draughts not only make a house uncomfortable, but also take with them large amounts of heat; reducing these to a minimum is a fundamental requirement of Passivhaus. In new, well insulated buildings, the losses in heat due to draughts can be as much or more than the losses through walls, floor roof and windows put together. Making a building airtight (draught free) is not easy, especially in existing buildings. The house is being completely stripped back and a new airtightness layer formed on the inside, in some places with ordinary plaster or concrete, and in others with intelligent breathable membranes. The junctions between elements are critical, as this is where shrinkage and movement cracks can appear, and these will let air out. In these locations, specialist airtightness tapes are being used that have been designed to last. We are aiming for an airtightness of less than 0.6 air changes per hour at a pressure of 50 Pascals, whereas building regulations has a backstop equivalent 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 will provide the maximum performance with a very low level of noise – less noisy than the fridge!

7. Reduce hot water demand

In a Passivhaus the hot water demand of the home becomes a big percentage of the fuel bills, as much as the heating if not more, so reducing the amount of hot water usage can significantly reduce the fuel bills. In this house, low flow shower heads and taps will be installed, using aerators to add volume to the water whilst minimising water use, particularly hot water. Also below the bathroom will be added a hot water recovery system that pre-warms your shower water with the heat from the waste water from the shower, which can be up to 60% efficient. The heat going the other way up the soil vent pipe has also been considered, this can act as a thermal chimney leaking heat out of the building and was calculated at 1.5kWh/m2a (10% of the total heat losses allowed in a Passivhaus!), and even more if it is uninsulated. We have eliminated the need for the vent pipe, thus removing this loss.

8, Renewable energy

The property will also feature Photovoltaic (PV) panels on the sloping roof facing west and on the flat roof of the loft, tilted slightly east. Conventionally it has been thought that the optimum is to face panels south for maximum gain, however the peak demands of electrical demand are in the mornings and afternoons, not at mid-day when the sun is in the south, and as the meets the demands closer an east-west roof may be preferable! This also allows a larger area of panels.

9. Efficient appliances

Once we have an efficient building, attention turns to the appliances, including at least an A++ Fridge Freezer and other efficient white goods. This will ensure that energy usage is kept to a minimum.

Passive House Plus Magazine have published an article on this project

Click here to read it