Project Overview:
Background:
In 2011, the Mildmay Centre rose to prominence as the UK's first non-domestic retrofit to be certified to the full Passivhaus standard. It was originally built in the late 19th century as a power station for London’s tram network but its use gradually declined to the extent that it was facing dereliction in 1973. At that point, it was taken over to be run as a community centre but a combination of solid walls, poor insulation and limited accessibility made it cold and uncomfortable. These conditions prompted a large-scale refurbishment project in 2011.
The Brief:
The architects, Bere, together with the structural engineering company Conisbee, developed a plan for a thorough refurbishment that would see the property brought up to the very highest standards of sustainability. The scheme assumed a fabric-first approach and called for a variety of measures, including the installation of triple-glazed windows, a heat recovery system, solar PV panels, solar water heating, a ground source heat pump, sun-shading to prevent overheating in summer and, importantly, the use of external wall insulation over the entirety of the building’s solid brick walls.
Design and Specification:
The conversion of the centre to a fully certified Passivhaus required the entire building fabric to be made airtight and insulated to a very high degree, with significant effort made to eliminate thermal bridging at junctions and interfaces.
Working closely with the architect, PermaRock produced a specification for the external walls to meet the rigorous requirements of the Passivhaus standard. PermaRock’s LA Parge Coat was used to treat the entire external masonry surface to improve air tightness by reducing air leakage through the old, weather-worn external fabric. This was integrated with seals at all structural openings and penetrations.
The application of PermaRock External Wall Insulation (EWI) systems to the walls (above and below the injection DPC level) provided the high level of thermal resistances required to enable the building to meet the Passivhaus performance requirements. For most of the wall areas above DPC level, PermaRock’s EPS EWI system incorporated 290 mm insulation, adhesively bonded and mechanically fixed to the walls. Each of the fixings was recessed into the insulation and covered with a cylinder of the insulation material to minimise point thermal bridging.
For the southern upper wall areas, the insulation thickness increased to 320 mm – again adhesively bonded and mechanically fixed, and below DPC level, the PermaRock EPS-Plinth system with 200 mm of high strength EPS250 insulation was utilised.
The insulation was covered with a PermaRock reinforced basecoat followed by the decorative renders. Above DPC level, PermaRock Silicone K-Finish (with Clean Concept Technology) was specified, with a 1.5 mm grain size and in two contrasting grey shades: the very pale Granit 60 and mid-toned Granit 40. The plinth area, below DPC level, was finished with PermaRock’s hard wearing Stone Chip Render in the near-black ‘Carbon’.
Performance: Energy Saving
The architects continued to monitor energy usage within the building for two years after its completion. Writing about the scheme, the Passivhaus Trust noted that the “project presented an excellent opportunity to monitor energy use and evaluate the first certified non-domestic Passivhaus refurbishment of a medium sized building in the UK climate.”
In its final report, Bere listed some headline reports as follows:
- “A remarkable 85.5% overall energy saving was achieved after its Passive House retrofit… The size of this saving in energy and similar saving in carbon emissions is all the more remarkable in the light of the heavily increased occupancy of the building post-retrofit, including several new servers and many new computers.”
- “The basement office space has no heating whatsoever, yet indoor temperatures in the space have never dropped below 19.75 degrees Celsius... Other spaces hardly ever use the low temperature radiators dotted around the building.”
- “The retrofit will save the local community at least £1 million in energy bills over the next 50 years.”
The architect’s more detailed project analysis report can be found here in PDF format.
Result:
The project was a great success, achieving the required Passivhaus standard and winning a series of awards. In 2011 and 2012, these included the Best Public Building category in the 3R Awards; the Leisure category winner in the Greenbuild Awards; the Retrofit category winner in the Passivhaus Trust UK Passivhaus Awards; and the Building Performance category winner in the Constructing Excellence Awards. The Passivhaus Trust also made the project the subject of a 2015 special video feature, which can be viewed here on YouTube.
In the years that have followed, the external wall insulation system has performed faultlessly and the render finish has remained bright, attractive and colour-stable.