AEE Canada East

Written by: Lorrie Rand

Construction is currently under way on Halifax’s first Passive House retrofit, or Enerphit project, designed by Habit Studio with builder Root Developments. The building is a two storey 1850s era house on Falkland Street in Halifax’s North End. When renovations are complete it will be used as office space for the Habit Studio team.

A Passive House relies on superinsulation, airtightness, and high performance windows and ventilation to achieve exceptional levels of energy efficiency and comfort. Performance is modelled using a Passive House Planning Package (PHPP) model. The performance targets for the Enerphit standard are slightly relaxed from new build Passive House targets because working with existing building geometry and construction details is more challenging than new construction.

The building is an uninsulated timber frame structure, built on a fieldstone foundation, with single glazed wood windows, including some with original sashes and frames. The building has a simple form, which is ideal for a passive retrofit, however with an initial blower door test result of 24 air changes per hour and heating demand of 450 kWh/m² per year, meeting the Enerphit target is not guaranteed.

In undertaking a Passive House retrofit the simplest approach to improving thermal performance and airtightness is to wrap the exterior in insulation. This can be challenging on tight urban sites and is impossible at Falkland, with the street facade and one side wall built on the property lines and a shared party wall. For this building as for many urban, heritage type buildings an internal retrofit is necessary.

The existing wood shingles and board sheathing are being retained, so it’s important that the retrofitted walls are vapour open to facilitate drying. The perimeter walls are being furred out, using a staggered stud approach, to make a 7” cavity. The wall will be filled with dense packed cellulose insulation held in place with Intello Plus, a variably permeable vapour control membrane.  Habit Studio is committed to using materials with low embodied carbon, and choosing cellulose, which is made of recycled newsprint, is a way to store carbon in the retrofitted structure. The low pitched roof is also being furred down to increase depth for insulation and to simultaneously reinforce the structure to permit the possibility of a future third floor addition.

The basement walls will be left uninsulated for the short term so that existing conditions can be monitored to ensure any moisture issues are resolved appropriately before covering up the stone. Ultimately the basement walls will be insulated with 6” of Rockwool. The space has a very low ceiling and it will be used for storage and utility use only, however the space will be heated and ventilated to ensure air quality and to protect the timber floor structure from deteriorating.

To offer flexibility in meeting the technical challenges of a Passive House retrofit, there are two pathways to achieving an Enerphit project, the energy demand method and the component method. The energy demand approach is closely aligned to classic Passive House, with specific targets for heating and cooling demand and primary energy based on climate zone. For Halifax the heating demand target is 25 kWh/m²a. The component method outlines performance targets for individual building components and does not specify a maximum heating demand target. Both options have a minimum airtightness target of 1 ACH. Habit Studio is following the component method, which offers a more relaxed target for walls with internal insulation.

Working with the character of the original home, the windows and most new doors are wood framed and aluminum clad triple pane Elite 92 windows supplied by Vetta Windows with a u-value of 0.5 W/m²K  and a g-value of 0.53. The main entry door is not a Passive House product, it is a wood door with transom and sidelights sourced from Norwood to closely emulate the original entrance screen. Sourcing the door locally meant that a local team from Metro Windows and Doors could visit the site to measure the original entry to ensure the best fit. This decision was one of the main challenges in meeting the Enerphit targets as the Norwood door specs do not meet the targets, however the average performance of all windows and doors in the building just meets the component target.

Excellent ventilation is critical to meeting the efficiency targets in a passive house and it’s also essential for the wellbeing and comfort of the building users. The ventilation unit has not been finalized for the project, mainly because of supply chain concerns with preferred options. The selected model will be an ERV, to provide humidity control as well as heat recovery. Heating will be provided with a dual head minisplit heat pump, with one head dedicated to the basement and one installed inline with the ventilation ducting. Energy modelling predicts an 87% reduction in annual heating after work is completed, with electrical bills projected to be approximately $500 per year at current Nova Scotia electric rates. Habit Studio is an 8 woman design team that specializes in designing custom new build Passive Houses and sustainable whole home renovations. This is their first Passive House retrofit.