Extreme low carbon dwelling refurbishmentFebruary 2010
While we fret about making new homes zero-carbon, its easy to forget about the refurbishment market. James Parker visits 89 Culford Road in London to see what can be done to make existing dwellings low carbon.
If you look through any estate agents window you can always find a house in need of modernisation. In most cases this means a new kitchen or bathroom suite, and maybe a lick of paint over horrible wallpaper from the 1970s.
Never does it say opportunity to become zero-carbon. At least, not yet. New houses may be being built to more stringent energy and environmental targets, but refurbishment is where the emphasis should truly lie. Of the housing stock that will exist in 2050 (the date at which the UK intends to reduce carbon emissions by 80 per cent), over two thirds have already been built.
The British love of Victorian villas and Georgian terraces means that the straightforward approach of demolition and rebuilding is not an option. In fact in many areas it would be virtually impossible to get planning permission to do this. This means extreme low energy refurbishments are tomorrows new build, and should become the definition of Estate Agents modernisation.
A pioneer in attempting extreme low carbon refurbishment is Camco technical director Dr Robert Cohen. Robert, with his partner Bronwen Manby, chose a three-storey Victorian town house in the very conservative De Beauvoir Conservation Area of Hackney, North London. The aim was to refurbish an old house in dire need of modernisation and in so doing reduce its carbon emissions by 80 per cent.
The micro double-glazed windows retained their sash features, albeit replaced with new frames with improved seals (four draught seals per sash) to help with airtightness.The design, by architect Robert Prewett of Prewett Bizley Architects (a runner-up in Building magazine's Sustainability Awards), involved major work to the fabric of the building, while retaining the front façade. While it was never intended to change the structural elevation facing the street, the same cannot be said of the rear of the house which has been completely rebuilt.
There were various motives for this, but notably it was driven by the need to increase and improve the existing accommodation space, which had significantly been reduced by the addition of high levels of thermal insulation and other internal changes. The new rear wall was also built to provide a small extension for the ground and first floors along with a small roof extension.
Despite the fact that the rear of the house was not viewable from the street, it caused the most problems with the planners, with worries about overlooking neighbouring properties for example. Ironically, had the house been in the next street the story may have been very different. The London Borough of Islington is much more engaged in the low carbon refurbishment agenda, even offering grants. By contrast, Hackneys planning practices are running somewhat behind its policies on carbon dioxide emission reductions.
While the front elevation was kept original, an internal frame was constructed inside the front wall to support rebuilt floors and also to eliminate thermal bridges. The gap between the existing façade and the internal frame is filled with 140 mm of insulation, with a small ventilated space
The original back wall was completely removed and rebuilt with reused brick and 200 mm of insulation. Rainwater from the roof is captured by copper drains. Standard triple-glazed windows were acceptable as the rear was not governed by conservation rules.between the façade and the insulation. Extra insulation was also added to the floor (100 mm) and to the roof (180 mm).
Passive measures sorted, attention then turned to the buildings airtightness. Surprisingly, for such an old building, the team managed to get a very low air permeability rate of 1.1 air changes per hour (ach) prior to plastering - a remarkable achievement. This creditable performance started with the architect lecturing the building team on the importance of airtight detailing. This led to more care being taken with wall, floor, window and roof junctions.
The initial target of 1.5 ach was always going to be a challenge, as the first test proved with a result of 5 ach. Taping and sealing brought this down to 3.5 ach, with a third and final push (before plastering) bringing it down to 1.1 ach. The final value will be a fraction of that.
However, an airtight house with negligible ventilation would soon become unliveable. Kitchen fumes (which may contain NOx if using gas hobs) and humid air from the bathroom would combine with volatile organic compounds released from materials in the house, and of course natural smells from the human (or animal) occupants to produce a very unpleasant environment.
The high level of airtightness therefore justified the use of a mechanical ventilation and heat recovery unit (mvhr). The product chosen was the Itho HRU Eco 4.
Table 1: Emissions before and after refurbishment. (click image to zoom)This plant, installed on the top floor adjacent to what will be Bronwens home office, will suck out stale air from the bathroom and kitchen, transferring the heat to outside supply air, then distribute it throughout the house via a system of flexible ducts run down an interstial gap adjacent to the party wall.
As the mvhr manufacturer claims an heat reclaim efficiency of up to 91 per cent, the Cohens accepted the fan power penalty as the mvhr should considerably reduce the dwellings heating requirements. (See tables 1-3 for energy savings and paybacks.)
Table 2: Annual energy costs before and after recfurbishment. (click image to zoom)For Robert Cohen, the ventilation strategy was the key feature of the refurbishment in terms of reducing energy demands. Looking at the figures calculated for before and after the work was carried out, it is clear where most of the savings will come from.
A reduction of 210 kWh/m2 per annum or 92 per cent is outstanding, and would not have been possible without the mvhr/airtightness combination. In fact, this saving is three times the total predicted energy use of the house.
The build team is so confident of the thermal capability of 89 Culford Road, that they have only installed underfloor heating on the lower ground floor, with two small towel rails in the bathroom to make sure towels dry out.
Table 3: Breakdown of annual energy consumption by end use. (click image to zoom)Glazing
In addition to the walls, careful choices had to be made with the windows. To complement the rest of the house, they had to have very good seals and very low U-values. This was not easy to fit into a façade in a strict conservation zone. The solution was micro (or slim) double-glazing. These have a small argon-filled gap of only 4 mm. This gave a glazing profile very close to the original. Heat retention is improved with a low-e coating that reflects heat back into the room.
The front windows also kept the sash opening, albeit reproduced with improved seals (four draught seals per sash) to help with the airtightness. This gives the front of the house a virtually original appearance. The rear of the house, without the strict levels of conservation to observe, is equipped with a more traditional low energy window: triple glazing with double edge seals.
This all sounds quite ground-breaking in the UK, but really its nothing new. In general Robert Cohens approach follows the guiding principles of Passivhaus, developed in Germany by the Passivhaus Institut in the 1990s. Indeed, the architect Robert Prewett used the Passivhaus Institutes PHPP software to predict the performance of 89 Culford Road as the buildings design developed.
Due of the nature of Passivhaus, the PHPP tool is much better at estimating the effects of things like window design, cold bridging and risk of summer overheating than the UKs SAP calculation tool.
The many extreme low carbon features of 89 Culford Road. The dwelling has all-new floors, and the staircases were reversed to enable better use of the space. (click image to zoom)No self-respecting low energy house would look complete without some kind of renewable energy technology. But having said that, with this development there was an agreement from the outset to avoid overt eco-bling. This was not only because of the conservation zone, but also because the team believed that they were neither appropriate nor cost-effective. That is not to say that there arent any.
The original plan was to have installed a heat pipe solar-thermal system. However, to be effective it would have been visible from the street - a consequence of the lack of space to hide it. But luckily for the Cohens the government announced feed-in tariffs that benefited micro-generation. They quickly jumped on this and with a few calculations found that installing solar photovoltaics could be the answer.
The feed-in tariff of 36p/kWh reduces the payback period of photovoltaics from 30 to 40 years down to just 10 to 15 years. The photovoltaics at 89 Culford Road, were installed at a cost of around £5,000. They are calculated to provide 1.2 kWp and to meet around 30-40 per cent of the electrical load of the house.
Refurbishment verses new build
Spot the difference. Despite being a conservation area, the house next to 89 Culford Road has standard double-glazed units, whereas Robert Cohen retained the original style sash windows.How does 89 Culford Road compare to a new house? The current benchmarking system for new homes is the Code for Sustainable Homes. However, it is important to note that this system can only assess new dwellings. At present the only (BRE-backed) assessment tool that can be used for refurbished homes is Ecohomes, although this hasn't been updated since 2006. BRE is in the process of developing a domestic refurbishment assessment scheme, but this is not yet available.
Looking at the Code for Sustainable Homes and making small adjustments (not possible in a real assessment), the house would achieve a rating of Level 4. While a lot of the Code looks at the periphery of the construction process, the key elements of a rating are the energy and water categories. These have mandatory levels to reach to receive a rating at a particular level.
The 80 per cent reduction of energy use at 89 Culford Road has helped score highly here, but to reach the Level 5 rating a 100 per cent improvement would be needed, and even more for Level 6. The water issue would also pass the requirements of the higher levels of the Code because of the rainwater harvesting strategy.
The AECB Carbonlite silver and gold standards were also used, with the house falling in between the two levels. These standards use the German PHPP software to calculate energy usage as it is more capable of predicting energy use accurately in low energy buildings compared to the current version of SAP.
Leaving aside the assessment methods (which are only tools of prediction), 89 Culford Road will be monitored to provide a real measure of performance. Its lessons can then be rolled out across the domestic sector, with hopefully the anticipated energy savings achieved in practice. The first set of monitoring equipment has been installed by the insulation manufacturer Knauf.
Sensors have been installed into the walls, particularly on the front wall, and at the interface between different materials to give a temperature gradient through the wall. This should provide an evidence base and allow Knauf to make recommendations as to how its product could be better used. The house could also be the subject of MSc theses.
What 89 Culford Road illustrates is the sheer degree to which the UKs existing domestic housing stock must be improved if the country is to have any chance of meeting its carbon targets. The effective refurbishment of our existing housing stock is non-negotiable. Its a standard that must apply to all domestic dwellings, not just the small fraction of new homes that will be built - probably sporadically given normal economic cycles of growth and recession - between now and the next 40 years.