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Site Visit to Low Energy Student Housing Project in Cambridge
April 24, 2013 @ 2:00 pm - 4:00 pm
Venue: Site is at Girton College, Huntingdon Road, Girton, Cambridgeshire CB3 0JG
Best to follow the signs to the Mansells site rather than the College main entrance (or contact the Porters Lodge at the college for directions, 01223 338 999) Visitors are requested to bring their own safety footwear.
This project aims to be one of the most energy efficient halls of residence in the country, together with refurbishing adjacent facilities. An extremely demanding energy budget has been specified and the performance will be a massive departure from that of the current Grade II listed 100-year-old building stock.
Although the building is long and narrow with generous (triple) glazing, a very tight limit of 16.5 KWh/m2/year for heating & ventilation will be met by rigorous passive design and incorporation of mechanical ventilation with heat recovery. The Passivhaus Planning Package (PHPP) has been used as a valuable design tool. See http://www.fluidstructures.com/html/energy/girton.html and http://www.girton.cam.ac.uk/supporters/a-great-campaign/ash-court
This event is to be hosted by Ralph Swallow of AECB members Fluid Structures, a practice of structural engineers with an in depth understanding of low energy building. This has been built up over years of evaluation and practical application of this import part of the sustainable building team.
This will be a joint event by the London Group and the East of England Group but all AECB members are welcome to attend. Places are limited so please RSVP your interest to the Local Group leaders Anna Carton and Paul Bourgeois directly or c/o email@example.com
Please let us know if you would be willing to offer a lift to a fellow member as part of the Lift-Share scheme – either from your area or from a station close to the venue.
Lifts on offer: James Livingstone is offering a lift from the Norwich area
Feeback from the meeting (by Ralph Swallow) . . . .
On a refreshingly sunny Wednesday afternoon in late May Fluid Structures hosted a tour of the new residences under construction at Girton College. Fluid Structures have been working on the project as structural engineers since early 2011, whilst the architects Allies & Morrison were appointed some three months earlier. Mansells started on site at Easter 2012 and the new halls are due to open for this Autumn term, so the site was very much a live construction site. On the day of the visit work was underway both externally on the facades and internally, but with most areas not yet “closed up” so still available for scrutiny.
The scale of the new halls is quite modest (50 rooms) by comparison with other student residences but the ambition of the project is noteworthy in terms of quality. The College had the foresight to prepare a commendably clear and ambitious brief for the project. This included demanding requirements for energy (16.5kWh/sqm/yr heating), design life (100 years), BREEAM (Excellent) and a number of other points. Allies & Morrison had been appointed on the basis of their competition winning proposal for a linear three storey highly insulated building with a PV clad pitched roof. This main building and it’s external colonnade will form the fourth side of Ash Court, enclosing a landscaped garden and modest gravelled parking area. The project also incorporates a number of other challenging elements such as total refurbishment of the college swimming pool, and replacement of boiler plant to the existing accommodation adjacent, but the tour focussed on the new residences.
During Stage C design a key issue had been the selection of the most appropriate construction method for the envelope and structure. As is often the case with low and medium rise construction, the structural engineering design and the envelope design are tightly interwoven. On this project this was particularly true given the energy and design life requirements and the evolving air tightness targets. The client, as owner and user of the buildings, had some clear opinions as well in terms of robustness and maintenance. Here, I have to confess that I have been firmly in the sceptics camp when it comes to using cavity wall construction for low energy buildings (all that messing around with lintels, wall ties, wind posts etc etc) but that is exactly where we ended up. A fairly detailed and project specific scoring matrix, which had encompassed seven systems from the proprietary “Beattie Passive” method to CLT and to Coolvault, had brought us ultimately to thermally exposed RC slabs combined with cavity masonry walls. The selection of concrete for the floor plates was driven by future climate predictions and the aim to mitigate the need for future cooling, a strategy supported by Max Fordhams’ thermal modelling. The slabs provide significant thermal mass now and embedded cooling pipework which may be commissioned in future.
The first step on the tour was therefore to answer the various questions arising in relation to the wall construction, thermal bridging, U values etc. The majority of the walling is a 300mm Rockwool filled cavity wall, with dense block inner and brick outer leaves, not dissimilar to Passivhaus builds such as Denby Dale, or even back to BedZed or the Vales’ Autonomous House. This walling achieves a U-value of around 0.1, but only by dint of using non-conductive basalt ties from Magmatech. Not only are these excellent thermally they also easily cope with the wide cavities from a structural point of view and are readily available in long lengths. For this project Fluid worked with Magmatech to prove / approve a new face fixed version of the tie (numerous samples proffered by Ben on site!), which crucially enabled Mansell to get the inner leaf up well ahead of stonework and brickwork. The brickwork outer leaf sits on a course of insulating bricks bearing onto the toe of the warm ground slab. Workmanship of course remains a key aspect of thermally effective cavity wall construction, even with the generally site-tolerant ‘squishy’ rockwool, and the sharp eyes of our visitors were everywhere.
It’s cool at the top
We then scaled the scaffolding to enter the attic, previously a cavernous space (and subject of much associated comment), but now filling up rapidly with MVHR units and ducting, electrical trunking, and hot water circuits for DHW and the small rads in each student room. The environment in the loft space was much cooler than the warm exterior, which bodes well in terms of temperature stability when the building is occupied. A conscious decision had been made by the design team to take the thermal envelope and airtightness line up over the roof pitches (rather than across the top floor ceiling), partly to achieve this thermal buffering to the top floor rooms, and partly to keep as much M&E kit as possible within the thermal line. This approach has dramatically reduced the number of service penetrations through the airtightness line, and should maximise the efficiencies achieved with the MVHR systems. The clean roof structure of insulated I-joists also avoids the need for truss ties/braces which might have massively compromised the air tightness. A number of comments were made about the airtightness detailing, and in general the group felt the right decision had been made in keeping the membrane exposed and visible on the loft soffits. The building has been designed to achieve an airtightness of 1.0, with a negotiated contractual backstop of 3.0cum/sqm/hr at 50Pa (a long story, not enough space to record here!).
Back down on the ground we headed inside and inspected a selection of the student rooms, some bare plaster and RC slabs above and below, others fitted out with services and their raised timber floors, and the sample room fully furnished and decorated. The sample room itself was commendable for it’s good light and soffit height, quietness (particularly with the triple glazing closed) and general solidity. In general all have been designed and detailed to keep services and associated plaster penetrations off the external walls, again to preserve airtightness.
So overall a successful visit and one which seemed to conclude that the imminent air tests ought to go well, and that the project should provide a fine example of a very low energy building that avoids looking like the stereotype of one.
If there is interest then we may be able to arrange a further visit to the completed building in the Autumn. As this could perhaps happen in the early evening (rather than in site hours) it may enable a number of members to attend who otherwise would not be able.
Thanks go to Mansell and to Phil Pordham in particular in accommodating us, and to the visitors for coming along and for providing useful comments and insights.
Main Contractor – Mansell; Project Manager, Cost consultant – CDM Sweett Group; Architect – Allies & Morrison;
MEP, Acoustics, BREEAM – Max Fordham; Structural Engineering – Fluid Structures; Air tightness testing & consultancy – Stroma
About Ralph Swallow and Fluid Structures . . . .
These Engineers are constantly evaluating their approach to structures and the the solution to each project is found with due regard to emerging materials and systems. To this end the practice invites guests from various fields to join them for their regular lunchtime get togethers where they share news, ideas and innovations that enable the company to provide current solutions to projects (rather like the AECB Local Group meetings!) http://www.fluidstructures.com
From the Fluid Structures website: “Very often money is wasted installing thick insulation in walls and roofs without ensuring the detailing will make the insulation effective. Badly fitted rigid foam boards are a classic example, where thermal bypass is often a real problem. Rather than relying on thermal imaging at the end, it is better to specify a realistic and buildable system”
For those with the 2009 Spring Edition of Green Building Magazine, you may have read the article “Aiming for AECB ‘Gold’ Standard – What can be achieved at reasonable cost to minimise energy consumption of a typical family by thoughtful extension and refurbishment? Will the gas and electricity meter spin any slower? Ralph Swallow, records his first hand experiences, lessons learned, and post occupation monitoring.”