Fort St. John 50 Unit Passive House

Project Description

THE PROJECT

The Fort St. John 50-Unit Passive House apartment project originated from a partnership between BC Housing, British Columbia’s Housing Management Commission, and BC Hydro, to develop a highly energy-efficient affordable rental housing building in Fort St. John, British Columbia. The project is intended to initially provide housing for the Site C Dam workforce, later reverting to affordable housing for the community.

WCPG Construction Ltd responded to this Design-Build RFP by assembling a team experienced with Passivhaus and affordable housing projects. Low Hammond Rowe Architects was the lead designer and coordinating professional. The project was supported by a full team of engineers, landscape architects, energy specialists and passive house design consultants. WCPG Construction managed the design process, construction and delivery of this unique project, designed to meet the International Passivhaus Standard and achieve certification, the largest Passive House building of its kind in Canada at time of construction.

This six-storey wood frame building consists of two- and three-bedroom suites, common interior and exterior amenity spaces, bicycle storage, outdoor playground and landscaped rain gardens, serving families living and working in Fort St. John.

DESIGN RATIONALE

Site Strategy
The integrated design team explored numerous siting and massing options for locating this six-storey apartment building, analyzing physical site conditions, sun paths, prevailing winds, relationships to adjacent buildings, and local design guidelines. The design team compared north-south and east-west orientations to optimize performance using the Passive House Planning Package energy modelling tool. While maximizing southern orientation is normally recommended, the energy demand increase to implement a north-south orientation was 2 kWh/m2a. The team believed this was an acceptable cost to provide each apartment direct sunlight and to realize an optimum use of the site with quality outdoor space.

The building plan is a simple double-loaded corridor scheme with inset stairwells, central elevators and a 9-degree cranked plan, symmetrical through the center of the building. This simple gesture accentuates the main entrance when viewed from the street, orients half the building façade further towards south, creates a natural break in the west façade for exterior balconies off the common amenity rooms on each floor and creates a v-shaped landscaped setting for the ground-oriented suites on the east side.

Façades
The building façade is designed to express the simple principles and qualities of this high-performance Passive House building. Composed using a variety of materials, colours and textures, each façade responds to unique solar orientation, and creates a cohesive and distinct character among the non-passive house multi-family housing stock in the community. Large earth-toned coloured panels divide the 6-storey building into layered proportions, punctuated with vertical accent colours reflective of warm natural landscapes. The west façade incorporates vertical sunshades to shade the late afternoon summer sun while providing a rich texture of light, colour and shadow.

The thickness of the Passive House wall system is articulated at the narrow ends of the building and above the main entrance, intensifying the expression of the simple bent form. The south façade incorporates larger windows into the living spaces and are shaded with horizontal sunshades. The north façade mirrors the south in colour only, where north facing windows are minimized.

Mechanical Systems
Heating + Cooling Systems:
The building is heated and cooled by a heat recovery variable refrigerant volume air source heat pump system, designed to -36C heating design temperature and 26C cooling design temperature. The condensing units operate in ambient conditions down to -20C, below which natural gas based supplemental heating is provided. Domestic hot water is provided by condensing natural gas heating tanks.
Heating + Cooling Equipment:
Daikin VRV-IV + Heat Recovery
AOSmith BTH-400A Mxi x 2 Domestic Water Heating

Ventilation Systems: The suites are ventilated by a central ERV, providing both fresh air supply and exhaust air extraction. The main floor auxiliary rooms have smaller dedicated ERVs providing ventilation.
Ventilation Equipment: Swegon and Zehnder PHI-certified Energy Recovery Ventilators

CHALLENGES:

I. The soil conditions on the site were poor, consisting of clay that tends to expand when in contact with ground water. The project required a complex field of steel helical piles on suspended grade beams with suspended concrete floor slab, all of which had to be wrapped with EPS insulation and air barrier. Each steel connection point of contact between steel pile and concrete grade beam was factored into the overall thermal losses for the building, utilizing Therm modeling. Due to the potential for soil expansion of the native clay material, additional void-form insulation was installed below the thermal insulation, to avoid damage to the concrete slab. Special detailing was necessary to secure the insulation to the underside of the slab when the concrete floors and grade beams were poured, to ensure the insulation does not fall away from the slab if the void-form shifts. In addition, it was necessary to incorporate ventilation pipes under the suspended slab and grade beams to remove any build-up of Radon gas that is present in the soil throughout Fort St. John.

II. The building is 6 storeys high and constructed of wood. Until recently, the BC Building Code only allowed 4 storeys of wood frame construction for this building type. Special detailing and structural measures were necessary to accommodate the concentrated loads on the first and second levels, consisting of closely spaced wood studs, manufactured wood sill plates that minimize shrinkage and additional acoustic measures to compensate for more solid wood in the walls.

III. Due to the pending onset of winter, occurring in late October-November at this high latitude, it was imperative that the design team fast-track the design process to allow for early building permits and construction of the foundations before winter to maintain the owner’s occupancy schedule.

ASSEMBLIES:

The main exterior cladding consists of Hardie Panel and Hardie Plank cementitious rain-screen system, with deep recessed Duxton triple glazed vinyl windows and metal flashing surrounds. The roof consists of a mono-sloped wood truss system with two-ply SBS roofing membrane and 275mm of XPS roof insulation, which is built continuously under the mechanical penthouse to reduce the surface area of the Passive House envelope. The single sloped roof is drained to the east through four large scuppers connected to rainwater leaders aligned flush with the exterior cladding system, avoiding unwanted roof penetrations and diverting roof water directly to landscaped rain gardens below.

The original concept for the exterior wall design was to use a double stud system consisting of an inner structural wall and an exterior TJI vertical frame to house the 200 mm of exterior insulation. As the details were developed and reviewed with the design-builder, it became clear that this wall system would pose serious problems for the construction schedule, due to the doubling work. In order to achieve the insulation thickness and install the exterior strapping to support the cladding, the design team settled on increasing the thickness of the exterior plywood sheathing. The sheathing joints were sealed with air sealing tape to form the air barrier, and the exterior strapping was screwed through the insulation and exterior sheathing. The additional thickness of wood allowed for screws to be supported as needed, rather than only where internal stud supports occurred. This gave ultimate flexibility for placement of cladding strapping to suite the cladding joint patterns and colour changes, perfectly aligning with window openings. Not only did this construction method improve the construction schedule, the ease of maintaining the air barrier produced a 0.2 ach. result in final pressurization test.

Penetrations through the building envelope were taped with SIGA Wigluv to the exterior plywood sheathing and correctly integrated into the exterior weather barrier. SIGA Wigluv was chosen as it provides excellent permanent adhesion to many substrates without the use of primers and can be applied in temperatures as low as -10C. SIGA Wigluv is also vapour-permeable so it will assist with the wood frame wall’s ability to breathe to the exterior. Windows and doors were tied into the air barrier with a combination of SIGA Wigluv and other SIGA tapes. The careful and thorough taped sheathing air barrier approach led to the excellent air leakage testing results:
0.20 ACH50 / 0.17 cm2/m2.

Thermal Envelope

Mechanical System