Retrofit: Breathing Space

As low-energy retrofit slowly emerges onto the construction stage, many architects are asking themselves whether it is something that they may be interested in. It is fair to say that this is not seen as the glamour end of the industry.

David Marks and Julia Barfield, partners in Marks Barfield Architects, have recently completed extensive works to their 1840s home in Stockwell, south London, and in so doing have gone some way to demonstrating that retrofit work can be so much more than energy calculations and insulation specification.

Back in 2008, their home of some 30 years occupied an irregular site at the end of a terrace with ample space to the side on which to extend. As is often the case, it was this opportunity to extend and reconsider that provided the impulse to investigate measures to reduce energy use.

The way the house has been reorganised is deceptively simple. Viewed from the street, extensions to the side and at second floor continue the Georgian language but develop it into a double frontage composition. Not only does this give rise to an improved form factor (the ratio of external envelope to internal floor area), but it also delivers a more attractively proportioned elevation. The increase in scale lends the house a feeling of a bookend to its neighbours, a bold approach within a conservation area but one that secured planning approval within the statutory eight weeks.

At the rear there is a new, rather more monolithic structure out of which larger openings have been carved. Adopting a more abstract form has allowed the designers to slip the proportional constraints of the original building. The extensions create a four-bedroom family home with a very generous 400 square metres of internal floor area.

The original end-of-terrace structure has been left more or less intact, save for some upgrading of the sash windows and a modest amount of insulated plaster board in place of the original thick layer of lime plaster.

The new structure has been fabricated using a concrete frame wrapped in a cavity wall construction, faced with either stock brick or lime render on block. The concrete provides an abundance of thermal mass to the interiors, which the architects hope will moderate the environment from extremes in weather.

Looking at the summary table (below), we can see that U-values for the new wall, roof and floors have all been pitched substantially below the regulated thresholds. Works to the existing walls, however, were less gung-ho due to the desire to preserve features such as cornices and shutter window surrounds.

The finely made six-over-six sash windows contain individual double-glazed units between slim astragal glazing bars.

They are beautiful, but the resultant Uvalue remains stubbornly high. In conservation areas, this remains a seemingly intractable problem and while tripleglazed sashes are now emerging onto the market, the thick edge of the glazing units precludes the use of real astragal bars. The larger window openings to the rear (also double-glazed) have much

improved U-values due to larger spaces between panes and reduced frame-toglazing ratio. Reflections like these on how architectural detail and expression relate to low-energy building aims make this project particularly interesting.

The design of the rear extension was less constrained by a fixed architectural language and the expression seems to emerge from the practical issues of insulation and ventilation. Deep window reveals lined with Portland stone not only express the thick insulated wall depth, but deliberately exaggerate it. The sense of articulation combined with the subtle textural play between stone and render enrich an otherwise simple volume. This is taken a further step forward in response to natural ventilation needs in the summer. The design team has engineered a system that resolves a common conundrum: the need to open buildings for ventilation in the summer while keeping intruders out. Screens of vertical Portland stone fins cover internal Passivent louvres which are automatically triggered by temperature sensors to open when cooling is required. While similar approaches have been developed by others, in this instance the design of the components dovetails very beautifully with the architecture of the project.

In line with accepted wisdom in low energy design, ventilation during the cooler months is provided exclusively by a mechanical ventilation unit with heat recovery (MVHR). No data has been collected on air quality but on my visit the house had that fresh feeling I recall from study trips to Passivhaus buildings. Energy for heating and hot water is provided by an air-source heat pump, requiring 2.5-3.5 kW input to produce up to 9kW output. Environmental consultant Loren Butt was responsible for planning the M&E installation. His coordination work is evident throughout the executed works and it is a delight to see such a well-installed system in a domestic setting.

He has also been collecting data from sub-meters for all energy consumed in the house and his continued participation beyond completion is helping to ensure that any commissioning errors – such as one that occurred with the heat pump – are spotted and corrected swiftly.

The data from December 2011 to November 2012 makes for interesting reading, and I hope the analysis of it will be published at some point in the future. The metered circuit for heating and hot water indicates just 38.5kWhr/m2/year consumption. Of course that equates to a much higher actual delivery of energy from the pump to the house, of perhaps around 100kWh/m2/year (assuming a coefficient of performance of say 2.5). Looking at the energy demand for the summer (hot water supply only) and extrapolating back would indicate that there is a rough 50-50 split between heating and hot water production, suggesting actual respective demands for these of around 50kWhr/m2/year each. For the heating side that would appear relatively good and about right for a house with this level of insulation.

For the hot water production however, a demand of 50kWhr/m2/year seems rather high, with best practice arguably closer to 20 kWhr/m2/year. It is interesting to note that the hot water supply is run as a continually pumped circuit to provide instant warm water at the tap or shower. Depending on the pipework design, systems like these can suffer from energy loss either as useful gains in the winter or as wasted energy in the summer. Loren Butt hopes to optimise the timing of these circuits to reduce the energy required for the system, and I look forward to finding how that went.

This is a project that provides a number of useful lessons for architects wishing to work within the retrofit sector. First and foremost is that fundamental fabric measures such as compact form and good levels of thermal insulation do indeed yield homes with lower heating demands (in this case close to new Part L recommendations for new build). Second, it shows how important it is to check how well buildings perform after they have been constructed, so that problems can be identified, dealt with and knowledge taken forward to the next project. This approach is still off the radar in the UK, but until we engage with our work in this way, real low-energy success may remain elusive.

Finally, this project makes it clear that there are interesting opportunities for architectural expression in the field of low-energy design, and it is heartening to see a well-known practice rolling up its sleeves and getting its hands a little dirty.

View the Priory Grove project here.