The Toronto Method + Riseborough Residence (Or How I Learned to Love Exterior Insulation)

Written by: Nicholas Discenza

Sustainable is literally pushing the building envelope. Proposing a forward-thinking, environmentally responsible, and energy-efficient wall assembly that improves the durability of your home to ensure improved occupant health and comfort.

THE TORONTO METHOD

Typical home construction in Ontario tends to meet the bare minimum in all respects: upfront cost, quality, and performance. Because of this, their long-term operating costs increase significantly, and the non-quantifiable aspects of a home such as the Health and Comfort of the occupant tend to suffer.

The concept of putting insulation within wall cavities originated as an afterthought: “the structure’s already here, we’ve got space between the studs… let’s fill it with fluffy stuff!”

Code-Minimum Wall

This approach does very little for the durability of the building, and can have serious negative consequences on a building’s longevity such as mould and structural failure.

Changes to the building code and new energy requirements take steps to improve this situation, namely the prescription of continuous exterior insulation.

Meeting the new energy code will be impossible using conventional 2x6 stud wall construction. Furthermore, conventional 2x6 stud framed walls are structurally unnecessary—2x4 wood stud framing is more than adequate for residential new construction.

RDH Perfect Wall

The work of John Straube and RDH (a building science and engineering consulting firm) proposes the idea of the “perfect wall” assembly, which consists of (from inside to out): structure, control layers, and exterior finish.

Taking the idea of the “perfect wall” assembly one step further, Sustainable considers the material composition of every element that goes into the wall: durable exterior cladding; recycled industrial waste mineral wool insulation; and sustainably forested wood structure—the lumber of which acts as a carbon sink sequestering CO2 from the atmosphere as the tree grows.

Toronto Method Wall

The Toronto Method approach of placing 9” of mineral wool insulation on the outside of the structure eliminates thermal bridging almost entirely. A continuous air barrier is “vapour open”, allowing the wall assembly to dry in both directions, depending on the seasonal vapour drive. Being able to dry in both directions means the conventional layer of 6mil poly on the interior of a wall assembly can be eliminated.

Aside: mineral wool, such as Roxul Comfortboard IS (Insulated Sheathing) is highly regarded by Sustainable for its various performance features: unlike fibreglass insulation, it will retain its shape and insulative properties after getting wet; mineral wool is fire resistant; has good sound attenuation qualities; is made from recycled industrial waste… etc.

Being a relatively novel concept, pushback from builders is expected, as anything that challenges the status quo can only make things more complicated and (in the short term) more expensive.

To prove that the Toronto Method was more than just a theoretical exercise, the architects and building scientists at Sustainable, along with students and professors from Ryerson University and a forward-thinking contractor, built a test structure to implement the construction method and to measure its performance over time.

Toronto Method Test Structure

The test structure is fitted with temperature, moisture, and humidity sensors, and currently resides on the grounds of the Evergreen Brickworks for its first year trial. The results thus far are promising, with a thermal imaging camera identifying only slight thermal bridging at the screw attachment and above-average air-tightness and hygrothermal performance (relating to moisture and humidity transmission through the assembly).

As with any experiment, there is always room for improvement. Although the assembly performs significantly better than the Ontario Building Code currently prescribes, a balance must be struck between construction cost increase, and energy use savings.

Identifying the optimal area of the energy curve will allow our clients to obtain the most bang for their buck, without breaking the bank. Opting for 6” of exterior insulation still allows for many of the benefits of the Toronto Method while keeping cost in check.

Cost Savings

RISEBROUGH RESIDENCE

Sustainable is currently constructing a home for clients with a specific list of desires: As extreme weather events occur more and more frequently, they want a home that will sip energy, endure for generations, and remain comfortable at all times.

Blueskin on Risebrough

To begin every project Sustainable employs the Trias Energetica approach: Reducing the initial energy demand by making smart, low-cost design choices with the highest impact on energy-efficiency; Reusing the energy nature provides for free through passive means; and Recycling nature’s energy through active strategies such as solar photovoltaic panels to convert the sun’s energy into electricity. This last step has the highest cost for the lowest impact on efficiency.

Trias Energetica

In this Risebrough Residence, the architecture does the heavy lifting. The staggered rooflines accommodate high windows that admit daylight naturally; while convection currents passively vent warm, stale indoor air through these upper windows and draw in cooler, fresh air from the garden level during the shoulder seasons.

Building Section

To reduce energy demand, the home is wrapped in a thick blanket of insulation: 6” of mineral wool derived from the lessons learned from the Toronto Method test structure, and 3½” of mineral wool insulation within the stud cavity. This balance allows many of the benefits of the Toronto Method, while being more familiar to contractors and trades in it’s application.

Wall Comparison

Mechanically, an air-tight building envelope reduces unwanted air-leakage, meaning the two, fully-ducted energy recovery ventilators (ERV’s) handle controlled ventilation efficiently: ensuring the fresh incoming air is efficiently preheated and humidified by the outgoing exhaust air.

As heating and cooling loads have passively been reduced by 80% over a typical home, the reduced heating requirements are easily handled by a hydronic in-floor radiant heating system powered by a super-efficient natural gas boiler.

As expected, being a relatively novel approach, pushback from builders was expected, as anything that challenges the status quo would only make things more complicated and (in the short term) more expensive.

Fortunately Mike Manning from GreenBilt homes was up to the task. GreenBilt understands better than most contractors the benefits of a high-performance buildings while remaining within a client’s specified budget range. The construction cost for the sustainable features included in the Risebrough Residence fall well within the range of a typical new Toronto home. Unlike a typical new Toronto home, the Risebrough residence is targeting 80% heating and cooling energy use reduction.

Blower Door Testing

To ensure these lofty goals are met, Sustainable performs periodic site reviews to ensure construction is proceeding as intended. A Blower-Door test was performed to review the building envelope for air leakage to ensure a continuous air barrier. From a pre-inspection reading of 4.30 ACH50 (Air Change per Hour at 50 Pascals), Sustainable was able to pinpoint air leakage, and improve air-tightness to a final reading of 1.7 ACH50—which translates to a 35% reduction in heating and cooling energy use from air-tightness alone.

In a world where “smart” technology strives to make our lives easier, investing in complicated systems that are susceptible to failure is a risky proposition. Passive design is the future, and Sustainable’s Toronto Method and Risebrough Residence are proof that ‘Simple is the New Smart’.

Exterior Rendering

Dining Room Rendering

Kitchen Rendering