Sonder Maisonneuve

Montreal Extended Stay Hotel Achieves Sustainability Goals, Prevents Thermal Bridging with Structural Thermal Breaks

Sonder Maisonneuve, an upscale extended stay hotel at 1500 Maisonneuve in downtown Montréal, is operated by Sonder, a hospitality company with a mission to provide a “thoughtful, exceptional stay” without sacrificing style in over 35 cities around the world.  

In 2020, Sonder established a corporate responsibility and sustainability function to further its “commitment to the environment and minimizing our footprint,” by working with “world-class architects and designers to create truly remarkable spaces—ones that transcend hospitality and enrich lives” while using technology to create sustainable and affordable spaces. The design team for 1500 Maisonneuve comprises Le Groupe Architex, Pomerleau Construction, L2C Structural Engineers and Desjardins Experts Conseils consulting engineers. The building is owned by Prime Properties, a property management company owned by international firm, Rumpf.

The Team instituted sustainability measures well in advance of requirements set by the National Energy Code of Canada for Buildings (NECB) 2015 and ahead of Montréal’s 46-point action plan established in 2016 targeting carbon-neutrality by 2050. As part of Canada’s commitment to reducing the energy consumption and greenhouse gas emissions, the NECB calls for such measures as electricity usage monitoring, lighting power density reduction, air ventilation heat recovery, and continuous insulation of the building envelope. In the case of Sonder Maisonneuve, this included insulating exterior walls to R25, and insulating wraparound balconies on 17 floors using 1.6 km (5,248 ft) of structural thermal breaks at the envelope.

Mechanical/electrical efficiencies were achieved in part using condensing hydronic boilers with 96% efficiency for central domestic hot water distribution throughout the building, according to Jean Desjardins, founder and president of Desjardins Experts Conseils consulting engineers.

Each apartment is fitted with energy recovery air exchangers for fresh air requirements. For common areas such as corridors, fresh air comes from a central gas fired high efficiency modulating air handling unit on the roof.

An underground garage ventilation system controls CO/NOx from car exhaust with in-line fans dedicated to each CO/NOx sensor, saving energy by reducing how often the exhaust fans and fresh air louvres turn on.

HVAC serving the 156 dwelling units is provided by high efficiency variable refrigerant volume heat pumps located on the roof. The heat pumps produce heat with a coefficient of performance for heating of 2.8 at an exterior temperature of 17⁰ F (-8⁰ C). For cooling they provide a seasonal efficiency rating of 17. (The higher the SEER between 13 and 21, the higher the energy efficiency and comfort level.)

Thwarting thermal bridging at wraparound balconies

1500 Maisonneuve’s 156 furnished studios and one- and two-bedroom apartments feature floor-to-ceiling double-glazed window walls leading onto continuous balconies that encircle the building.

While visually striking, the 2,788 m² (30,000 sq ft) of balconies and 1600 linear metres (5248 ft) of window walls posed a risk of thermal bridging, particularly where the concrete floor slabs penetrate the insulated building envelope. With relative humidity of 40-50% typical for occupant comfort, the design team was concerned not only that thermal bridging would cause heat loss but that condensation could form in chilled interior cavities adjacent to the balcony connections, leading to mould growth. 

“During the winter, the balconies outside are very cold, so we have a big risk of condensation inside, because it’s the same concrete slab inside and outside the balconies,” states Felix Daoust, Project Manager for Pomerleau, the first construction company to become a member of the Canada Green Building Council. The same slabs support the window walls.

“Also, because the building had a lot of window wall, the contractor wanted to make sure there wasn’t any thermal bridging leading to condensation in the window walls as well as inside the units,” notes Jean-René Larose, Co-Founder/Senior Partner with L2C and Structural Engineer for the project.

Thermal breaks specified to prevent condensation, mould 

After consulting with UL (Underwriters Laboratories), Pomerleau installed Schöck Isokorb^® concrete-to-concrete thermal breaks at the balconies. By reducing thermal bridging between balconies and the interior slabs supporting them, structural breaks retain heat inside apartments, eliminating areas where temperatures fall below dew point, forming condensation that would support mould growth. Larose was familiar with thermal breaks and supported their addition to the design.  

“I’ve used them for almost 10 years. We’re structural engineers, so we understand all the strain, the bending, the transfer of shear into the product. So, for us, it was not a big challenge,” he recalls. “This building was the first in Quebec of this scale to use the Isokorb^® thermal break system,” he adds.

Design, function and installation of thermal break modules 

Construction of the mostly concrete structure began in late 2019. Concrete-to-concrete thermal breaks were installed at the insulated envelope of the 17 floors constructed with continuous balconies.

The thermal break module's expanded foam block is approximately 98% less conductive than concrete, while the stainless steel rebar penetrating it is approximately one-third as conductive as carbon steel rebar, reducing heat loss at balcony penetrations by up to 90 percent. In addition to mitigating heat loss and carbon emissions, insulating balcony penetrations during cold winter months prevents heated interior structures from reaching dew point, forming condensation and supporting mould growth, while increasing the warmth and comfort of interior floor slabs next to balconies.

Key to the structural thermal break's design is its ability to insulate while simultaneously supporting loads equivalent to conventional monolithic balcony extensions of interior floor slabs.

Each module consists of a graphite-enhanced expanded polystyrene block with stainless steel rebar running through the insulation for tension and shear resistance. Stainless steel reinforcing bars are positioned at the top of the insulation body to accommodate tension forces imparted by the cantilevered slab. Compression concrete modules placed at the bottom of the assembly transfer compressive forces from the cantilevered slab, which in conjunction with the tension bars accommodate the moment at the connection. Additional stainless steel reinforcing bars run diagonally through the insulation body to address shear loads at the connection. The reinforcing bars extend on both sides of the thermal break and are simply wire tied to the reinforcing cages of the balcony and floor slab prior to conventional pouring of concrete.

At 1500 Maisonneuve, the interior floor slabs contained exhaust air ducts (from each unit’s kitchen, laundry, w.c.), which Jennifer Giron, Architect with Le Groupe Architex, says limited where the thermal breaks could be placed. However, Schöck had worked with in-slab ducts many times in the past, so it was not a problem to accommodate ducts and breaks alike. The ducts—coated with polyurethane insulation—were installed at the same time as the thermal breaks. Likewise, concrete for the interior and balcony slabs was poured simultaneously.  

In total, 1,590 concrete-to-concrete breaks were installed. Almost all the thermal breaks were concrete-to-concrete Isokorb connections. Also installed were smaller quantities of thermal breaks designed for corners, supported balconies, load peaks with supported balconies and earthquake resistance. 

Insulation continuity is assured 

The exterior walls were insulated with 9 cm (3.5 in.) of sprayed polyurethane. The double-glazed window walls were positioned at the interface of the thermal breaks above and below each balcony after the concrete was cured. Electric baseboard heaters were installed near the window walls to further limit condensation and provide extra heat on the coldest days.

Thermal breaks were not needed on the roof “because it was a completely insulated structure,” says Daoust.  Adds Giron, “We have inverted roofs with approximately R30 insulation. The window walls are R4.”  The shading coefficient of the tinted windows is 0.26, reducing cooling requirements.

Protected from condensation and mould and assuring occupant thermal comfort, Sonder Maisonneuve opened its doors to guests in the fall of 2021.  

Installation meets expectations

The installation of the concrete-to-concrete breaks proceeded smoothly as expected, based on Larose’s experience and expertise with them, and met Daoust’s expectations in reducing condensation. 

“I think it’s a good technology,” states Daoust, adding that building codes are becoming stricter when it comes to reducing energy use and heat loss, making balcony breaks all the more appealing. “All projects for the next few years will probably have to consider using them,” he says.