Lancaster Cohousing Project - 44

The Lancaster Cohousing project is a certified Passivhaus/Code for Sustainable Homes, level 6 and Life Time Homes, affordable community housing project. It has evolved through a participatory design process with the individual householders and Eco Arc Architects. In this article Andrew Yeats and Graham Bath provide an overview on the wall construction, and first floor construction, with particular regard to the integration of Passivhaus detailing. Work on the largest certified Passivhaus cohousing project in the UK has progressed well since the article in the previous issue of Green Building magazine. The project, when complete, will consist of forty one individual households, ranging from one bed flats to three bed family houses, along with shared community facilities.

The Lancaster Cohousing project, from its conception, has aimed to be a cutting edge example of sustainable design and living. The decision to design and certify all homes to Passivhaus standard ensures a rigorous approach to the energy performance of the buildings, with attention to detail to ensure continuity of the insulation throughout the external fabric with minimum cold bridges at junctions of elements or penetrations through the fabric for doors and windows etc.

Super insulated wall construction types that were considered at the outset of the project

Eco Arc has been building 300mm wide super insulated cavity walls (originally with Peter Warm and David Tasker with imported Danish wall ties) since 1992, initially at York Eco Centre & Heeley City Farm. From the same time period we have been building 300mm wide super insulated Masonite l-beam timber frame constructions, with the first one being David's House in Wales. Our projects have been featured in previous issues of this magazine. However, we had not built to the exacting Passivhaus standard before. We decided to go back to basics and prepared eight wall type construction options (each described/illustrated below) for project team review.

1. 500mm wide masonry cavity wall with 300mm insulation in cavity with render or timber boarded external finish.

2. 200mm solid stud timber frame (type A) over clad with Driffutherm & render external finish.

3. 300mm timber I-beam stud timber frame (type B) with timber boarded finish.

4. 300mm plywood web timber frame outer leaf & 140mm blockwork inner skin / render finish.

5. 300mm plywood gusset timber frame outer leaf and 140mm blockwork inner skin / boarded finish.

6. 300mm plywood gusset timber frame wall detail/ boarded finish.

7. 300mm adhesive applied external insulation & render finish with 140mm masonry blockwork inner skin.

8. 425mm solid clay block wall with Perlite integral insulation and 40mm external insulated render.

The wall types finally selected

After much deliberation and discussion (with some strongly held views by various parties) within the project team of the pros and cons of the eight wall options on the table, along with a thorough cost review and program review of the consequences of each option, we settled on the traditional cavity wall. Interestingly, with one of the timber frame options we would have saved 10 weeks in the overall construction program, but even allowing for the reduced contract preliminaries it would have cost £80,000 extra to the total projected contract sum.

The contractor was particularly keen on the cavity wall option as the north of England seems to be dominated by traditional masonry trades. Graham Bath had watched Bill Butcher's Denby Dale video several times and gained the confidence he needed to train his team to deliver the same Passivhaus exacting standard in Lancaster.

The cavity wall option would not frighten off the locally available tradesmen, it would allow us to build in some thermal mass and it was the cheapest option on the table, allowing us to deliver more affordable homes to the client group. We also understood the key disadvantages; relating to construction quality control for good performance being hard to check and manage on site, and the need to work hard to design out traditional thermal bridges, with having some structure inside and some outside.

Although the cavity wall was generally agreed upon (Figs. 1 and 2), it was clear the wall facing the river to the south elevation was going to be mostly door or window and the small gaps between would be best as timber frame, so a 9th option was developed with Ramboll, the project engineers, for a 38mm wide, 300mm deep, Kerto structural timber frame panel system, insulated between the studs and externally insulated with Pavatherm Plus wood fibre insulation and clad with Operal fibre cement/ cellulose board (see Fig 3).

The key details to note, that enhance this conventional cost effective cavity wall detail up to Passivhaus standard, are described below.

Fig. 1. 300mm insulated masonry wall construction/externally insulated window head detail.

300mm wide cavity, full filled with Dritherm 37 (or in some houses Dritherm 32) recycled glass, soft mineral insulation. To give an effective wall U-value of 0.12W/m2K and 0.10W/m2K respectively. Initially we started with three rolls of 100mm wide insulation, with staggered joints but had expansion problems with the roll distorting the green block work over night in the damp air. We changed to two layers of 150mm which alleviated the problem.

Basalt Teplo wall ties, which surprisingly don't transfer heat across the cavity and don't figure as a cold bridge in PHPP.

Independent and separated internal and external lintels over openings. We looked at GRP combined lintels and cavity closers, but this simple separated detail worked out much less expensive.

Partial 18mm WBP ply box to close the cavity to the back of the window head. Interestingly both the engineer and contractor wanted to take the ply box right across the cavity to tie both leaves together but Alan Clarke and Nick Grant calculated in PH PP/Therm it would amount to 1.0kWh/m2yr heat loss a year through the linear cold bridge and would cause us to fail the Passivhaus target for certification. Air tight tapes seal the back face of the window head to the ply box, which is then concealed with the skimmed plaster board soffit.

Externally over insulating the window head and window reveal with 75mm EPS insulation up to the front face of the window unit, combined with the partial ply box, reduced the cold bridge Psi value down to a good value of 0.01, which was acceptable in PHPP.

Fig. 2. 300mm insulated masonry wall construction/externally insulated window cill detail.

Setting the window unit back 165mm from the face of the wall was the optimum location in terms of reduced shading for the soffit over hang, whilst still being towards the middle of the insulation zone, and being partially isolated from the cold outer leaf wall elements.

The use of high performance Passivhaus certified externally insulated/aluminium clad window frames provided by Greensteps, using the German Gutmann window alu frame components with 48mm and 52mm triple glazed low E, argon filled glazing with a glass U-value of 0.60W /m2K, with an Insulated Thermix Spacer PSi value of 0.036, giving a window frame U-value of 0.80W /m2K, and a total unit installation U-value of 0.9W /m2K. Initially we had problems with the Secure By Design requirements, which required laminated glass to all ground floor windows which both reduced U-value performance and the g-value of the glass, but the police ALO relaxed his requirements in some areas due to the high level of neighbourhood watch provision inevitable within a cohousing scheme.

The windows have been tested to 1350 Pa (equivalent to force 14 and 102mph wind speed) for water-tightness and they weren't leaking when the test was stopped, which indicates the units will be extremely air tight under normal conditions.

A clever Wetherby Render APU rail allowed for a wind and water tight, flexible seal at the junction of the external through colour render and the external face of the aluminium frame to the window units.

As above the partial 18mm WBP ply box was used to close the cavity to the back of the window cill. This was combined with insulating below the window cill with Pavatherm Plus insulation up to the front face of the window unit. This reduced the Psi value down to a good value of 0.016, which was acceptable in PHPP. Air tight tapes seal the back face of the window cill to the ply box, which is then concealed with the window board set in to a rebate at the back of the window.

Down to DPC level around all the house perimeters, below a consistent window cill dado line, the external wall render was substituted with Eternit Cedral weatherboard cladding on battens. To ensure the cavity insulation remained in a wind tight void to avoid thermal bypass, the external air porous blockwork was protected with a wind tight barrier of Proclima Solitex Wall Wrap.

The infill timber frame walls to the south elevation was developed as a 38mm wide x 300mm deep Kerto structural timber frame panel system, with OSB sheathing, fully insulated between the studs and externally insulated with 100mm Pavatherm Plus wood fibre insulation and clad with battens & Operal/Cedral fibre cement/cellulose cladding board.

Over insulating the window head and window reveal with 100mm wood fibre insulation up to the front face of the window unit reduced the cold bridge Psi value down to a good level, which was acceptable in PHPP. Air tight tapes seal the back face of the window unit to the Proclima Intello vapour control layer over the Kerto structural frame with the taped joint concealed with the skimmed plasterboard reveal.

Setting the window unit back 160mm from the face of the wall was the optimum location in terms of reduced shading for the soffit over hang, whilst still being towards the middle of the insulation zone, and being partially isolated form the colder external elements. To avoid any services penetrating though the internal air tight barrier, or the insulation zone, a 25mm battened out service void was created behind the plaster board inner skin.

One of the requirements of Passivhaus design is 'thermal bridge free' details. The heat loss through poorly designed junctions can exceed that through the actual floor, roof and walls when they are insulated to Passivhaus levels.

Fig. 4 shows the thermal performance predictions at the wall junction (head and reveal) developed for this project. The problem we have is the transfer of heat from the warm inside through the weak link in the connection details around the window to wall abutments. The solution was to bring the window head inboard in to the depth of the cavity insulation zone and over insulate the window unit with 75mm EPS insulation. Although not often seen in the UK this is a standard Passivhaus detail on the continent. The Therm analysis provides an accurate prediction of the heat loss through the junction using detailed numerical analysis to 'solve' the steady state of heat loss and temperature throughout the construction, hence the 'isotherms' of equal temperature on the diagram. Using the results of the analysis we calculated the thermal bridge factor in terms of watts/m/K, and added in the estimate of the total heat loss in PHPP.

As most energy conscious designers/builders will know by now, supporting the first floor joists by bedding them in to the inner leaf of a cavity wall is a cardinal sin and a guaranteed way of creating multiple air leaks around the perimeter of the building. At Eco Arc we have been using perimeter ledger plates bolted to the wall for 20 years as an alternative, but not realising air can still escape behind the ledger plate through the porous holes in the block work in to the cavity. At Denby Dale, Bill Butcher finally nailed the detail in an air tight robust Passivhaus manner by parging behind the ledger plate first to seal the porous surface of the block wall, and ensuring the fixing bolts are stopped before fully penetrating the inner leaf of block work in to the cavity. On this project we adopted this tried and tested detail.

As with any Passivhaus we needed to accommodate extensive MVHR duct work. Using open web posi -joists to form the intermediate floor gave us more scope for routing ducts, cables, and soil pipes through the floor without having to have bulk head boxings to the ceiling below, or core drilling the webs of every I-beam floor joist.

Interestingly as shown in Fig 5 the structural formation of the steel webbed joist allowed us to cut away the bottom flange in critical locations to allow us to gain the required fall in a waste pipe without impairing the structural integrity of the joist.

Problems discovered and overcome on site

Originally the project design included for open cathedral ceilings within the insulation zone on the slope of the pitched roof from eaves up to the ridge level. As part of the inevitable post tender value engineering phase, we reluctantly agreed to drop the vaulted ceiling to flat ceilings to realize a cost saving of £177,000 across the project to get back on budget. In the original design, the project engineers at Ramboll had sensibly allowed for a single 100x100 RHS gable wind post within the inner leaf of block work from ground floor slab level to ridge line at the end of each terrace.

It was only with the erection of the first terrace on site, with the new design of flat ceiling, did we realize (in horror) that the old wind post was still in the engineer's design, penetrating through the flat ceiling insulation zone, creating a terrible cold bridge. After sweating palms for a while and Alan, Nick and Peter Warm running several Therm analysis trials through various parts of the cold bridge, did a mitigation solution emerge? With the problem exposed the structural engineer was able to design out the wind post above the insulation zone on future terraces, and Whittle Construction duly cut down the remaining un-installed wind posts left on site to make sure it did not happen again.

Airtight tapes not sticking in wet conditions

Once we were wind and water tight with the first new house in terrace A, Paul Jennings & Mike Neat set up an airtightness induction training day for the project team and key Whittle Construction operatives on site. It had became clear early on in preparation for the day that many of the ply window boxes had been taped when wet and the junction of the block walls to the ground floor slab had also been taped in damp conditions. Consequently, the stickiness of some of the tapes were starting to fail. Some even had to be removed and Mike Neat invested in a room heater and a hair dryer to locally dry out the substrate before re-taping and resuming the door air blower tests on a house by house basis.

Article by Andrew Yeats and Graham Bath

Green Building, 24. Summer 2012

The builders' experience so far

Following our selection as preferred contractor for the Lancaster cohousing project we were embraced by the client and their consultants into the project team at an early stage. Whilst the fundamental design principles had been established, we were able to contribute to the practicality of the detail. design throughout a regular series of team meetings during the 12 months prior to commencement on site.

These project meetings enabled us to understand the philosophy of the client and their design team to achieve Code for Sustainable Homes, level 6 and Passivhaus accreditation. Whilst we had carried out various schemes for housing associations throughout the Northwest to CSH level 4, the project brought new and exciting challenges, particularly due to the utilisation of masonry construction, rather than the more usual timber frame or prefabrication solutions. One of the primary requirements of the Passivhaus Standard is to achieve an airtightness level of 0.6m3/hr/m2 @ 50Pa and with the current Building Regulations at l0m3/ hr/m2 @ 50Pa this was seen as the major challenge on the project. We had, for some time previously, been achieving level below 5.0 and as low as 2.2 but the Passivhaus requirement set new standards.

In conjunction with the design team we selected an air testing company and appointed an 'airtightness champion', both With Passivhaus experience, to advise and assist in achieving this rigorous standard.

The Airtightness Champion is employed full time on site to install all air barrier membranes, taping, and to carry out air leakage tests at critical stages of the construction. He also oversees the activities of all other trades during this process to ensure and maintain the integrity of the air tightness and thermal barriers.

We are presently approaching plaster stage on the first properties so we await our first full preliminary air test, while will be carried out as soon as one house is plastered, but air leakage checks previously carried out indicate that we are on the right track. The traditional two coat wet plastering provides the primary air tightness barrier with pre-installed proprietary tapes and seals at all junctions, changes of direction and entries. Extensive parging is also being carried out to the blockwork behind services, timber supports, bearers etc which overlaps with the plaster basecoat.

One other defining aspect of the progress on site has been the process required to resolve day to day queries which often require the consideration of various members of the design team to ensure that Passivhaus standards are maintained, particularly with regard to thermal breaks airtightness etc... Whilst this has affected progress on the early plots we are confident that the steep learning curve experienced will prove beneficial during the construction of the remaining properties.

Graham Bath of Whittle Construction Ltd