Passivhaus diaries: Keeping it all in

Making period homes highly energy efficient is one of the most challenging jobs currently facing the industry. Robert Prewett, partner at Prewett Bizley Architects, has taken on the task of refurbishing a terraced home in Balham, south-east London, to near Passivhaus standards. This means a leaky, 100-year-old, three-bed, solid-walled property will be heated with the equivalent of a single towel rail. To achieve this, the house had to be highly insulated and airtight; the team has managed to achieve an airtightness figure of 0.8m³/hr/m², which is over 12 times better than the Building Regulations demand. Prewett has been posting a regular diary on progress at building.co.uk. Now, he completes his journey …

The external insulation

Work at 64 Midmoor Road is now completed. Last week I had the pleasure of handing the keys over to the new occupants.

The project overran a little, thanks to seasonal difficulties and our realisation that testing all systems would require longer than originally planned. So how did it all come together?

The approach to insulating the rear walls at Midmoor Road has been very different to the front, where we carefully lined the internal faces with phenolic insulation, overlaid with oriented strand board to form an airtight layer. This was driven by the need to avoid impinging on what is an attractive and well-built streetscape. At the rear, however, we were presented with a much more workaday elevation. The internal dimensions of the rooms within the rear part of the house were also much tighter than at the front, so taking any space away for insulation would have affected the usability of those spaces. Also, the amount of external envelope at the rear far exceeds the almost flat front of the house. To offset the effect of this large surface area we were keen to strive for the lowest possible U-value we could achieve.

All the points above led us to pursue a rendered insulation solution but there were some practical issues that needed solving to make this feasible. The soil stack was fixed to the outside of the rear elevation, in other words, in the same plane as the insulation. Simply burying it within the insulation would have made maintenance almost impossible and would have compromised the insulation. So we moved the stack to the inside face of the wall, which also made the new plumbing connections easier and reduced the number of times the stack passes through the airtightness plane.

The other big issue was how to stop the insulation at the top of the wall. Victorian houses often have shallow, clipped eaves and verges that offer no overhang to protect any insulation below. We could have adjusted the roofing in these instances but that would have meant stripping the roof tiles in order to extend the rafters at the eaves and sprockets at the verge. That would have cost half as much again as the wall insulation, and besides, the roof had been re-slated only a few years before.

So we came up with idea of making a super-wide gutter that could be fixed at the eaves and would act as a giant stop bead for the insulation. A flashing/gutter piece at the verge was also feasible. Working with Brian Cherry of JWD rainwater products, this idea was turned into reality using powder-coated folded aluminium sections, coloured to match the existing man-made slates.

The external insulation is a Sto product and was chosen as it was the only system that could provide a very low U-value, 0.12W/m2K, that used a simple expanded polystyrene (EPS) system.

The 280mm-thick blocks are 1m long and 0.5m high and fitted in a giant brick bond fashion. These can be easily cut and shaped on site in order to accommodate irregularities in the wall and to achieve special shapes and profiles.

The surface irregularities of the wall at Midmoor Road meant some areas of wall needed to be dubbed out a little with a levelling compound. The blocks were fixed to the external brickwork using an expanding foam adhesive. Each block also received one mechanical fixing as a “belt and braces” measure to ensure long-term stability.

The mechanical fixings were counter sunk into the blocks to minimise thermal bridging, with the hole being plugged by expanding foam. All gaps between the blocks and behind them have been similarly filled to prevent any air movement within the insulation that could reduce its effectiveness.

The freezing temperatures in December prevented the render being applied to the external insulation, which in turn prolonged the scaffold, which in turn prevented completion of the external surfaces. When conditions were right the system was rendered with a standard, just off-white flexible mineral render. The reveals to the windows (which have been splayed for better day lighting) were finished in a special smooth, bright white render to present a slightly more refined appearance
reminiscent of the treatment on Victorian and Georgian reveals.

The insulation laps over the fixed frames of the windows to reduce the visual bulk of the frames and alleviate the thermal bridge here. In spite of the difficulties our installers, Drens, completed the works extremely well and we are particularly pleased with the splayed reveals and cills.

Finishing off the interior

Works to the inside of the house, on the other hand, were nearing completion by late November. Dave Manby and Ray Weston, who have formed the core building team throughout the job, were installing the boiler and thermal store at that point and making a very nice job of what at first sight looks like a complicated network of pipes. Why a thermal store, I hear you say. While others may beg to differ, we think this system offers a number of advantages. The boiler runs only a few times a day at maximum efficiency and banks the energy in the well-insulated store. When hot water is required it is instantly available from the store. The heating system similarly draws heat (albeit very small amounts) when required to top the house up to the chosen temperature. We believe this has two advantages:

• It maximises the temperature stability
• It allows heat to be drawn off in very small amounts, which is particularly useful as part of heating system works by pre-warming supply air being fed to the living spaces. 

Completing works to the ventilation system required some final tweaks. When we came to fine tune the supply and extract rates, two issues became apparent: one of the fans in the MVHR was making a slight ticking noise and the installation of the main ducts to the unit had not worked out quite as planned, resulting in a some very tight bends on some ducts. Andrew Farr, of the Green Building Store, acting beyond the call of duty, organised a repaired fan within days and some additional components to re-rig the ducts into the machine. Once reassembled, the unit was running almost inaudibly by late December, by which time the heating had been on for most of that month. 

How the house performs

Finally, we could feel how well the house was performing. The very cold weather now became an advantage as it meant we could test the house to temperatures that don’t get much lower in the UK. What was striking was just how stable and even the temperature remained during that period of snow in December - so much so that returning to my flat at the end of a day on site was less than enticing.

Striking, too, was how long the snow remained on the roof of the house compared with the rest of the street. When the ventilation system was finally switched on the sense of freshness and humidity stability was remarkable.

And particularly satisfying was that the hot water delivery to kitchen and bathroom is both very quick and at just the right temperature - all thanks to the engineered pipe sizing and compact layout as described in previous blogs.

Getting a feel for all of these issues and having the time to test how well the controls work has been very useful and informative. The controls available to the tenant were deliberately kept very simple, consisting of:

• Buttons to boost ventilation when required (when cooking or showering)
• A thermostat in the hallway that controls the heating system.

Monitoring the house

When activated by one of two buttons, located conveniently near bathroom and cooker, the boost ventilation provides air at a rate one-and-a-half times the normal rate for 20 minutes and clears excess humidity from showering or cooking very effectively.

Setting the thermostat to the desired temperature brings the house to that temperature within an hour and thereafter the temperature remains very stable. Fine adjustments to each room, depending on the activity and occupant demand, can be made by setting the thermostatic radiator valves.

While the system components and approach are hardly groundbreaking, what is pleasing is how responsive the system is and hopefully how intuitive it will be for the
new tenant.

Having established that house was working in line with our expectations, we were finally able to invite our monitoring installation subcontractor, Microwatt, to the site to fit a comprehensive system of sensors. This enabled us to see remotely what the house is doing and build up a picture of how much energy is being used and where. Madison West and her colleagues fitted sensors measuring temperature, humidity, gas use, hot water use and CO2 levels throughout the house as well as electrical power use for each circuit. This equipment will record information for two years as part of our contract with the Technology Strategy Board.

Over the coming months and seasons, we will be able to build up a very detailed picture of energy use for direct comparison with our design model. Importantly, we will also get feedback from the tenants about how the house feels and how easy it is to modify their environment.