Civil engineers study energy efficiency for older apartments

A team of civil engineering researchers is working on a plan to increase the efficiency of aging apartment buildings by creating an outer perimeter whose temperature varies between the inside and outside temperature.

Nested Thermal Envelope Design, or NTED, is essentially a building within a building, said Marianne Touchie, a PhD candidate in civil engineering at the University of Toronto.

Touchie delivered a presentation on NTED and how it applies to highrise residential buildings to an audience of about 120 at the recent Building Envelope Solutions conference in Toronto.

NTED, developed by Touchie, U of T civil engineering professor Kim Pressnail and Ryerson University professor Russell Richman, is designed to control air flow, heat flow and moisture transfer.

Touchie said NTED allows the temperature of a perimeter to drift between the core and exterior temperature, using a heat pump between the perimeter and core space.

“Not only are heat losses moving from the core to that exterior captured in that perimeter zone, we’re also able to take advantage of passive solar gains from the exterior,” Touchie said.

“By transferring that heat by an interior heat pump, we can use those losses and that potential solar gain to heat the core space.”

Studies have shown energy savings in a single family home of 74 per cent using NTED, she said.

In highrise apartment buildings, the team is aiming to increase that, though she emphasized this is an ideal situation.

“We would like to see the energy use of these buildings reduced on the order of 80 or 90 per cent if we can start to meet the energy demands of these buildings using renewable energy sources,” she said.

About a third of Torontonians live in post-war apartment buildings, most of them constructed between the 1950s and 1980s when energy costs were lower.

“Now that these buildings are entering their fourth or fifth decade, we’re seeing building envelopes deteriorating, mechanical systems reaching the end of their useful life,” she said.

“But, structurally speaking, these buildings still have decades left of service life in them.”

In older highrise apartment buildings, there is often little or no insulation in the walls and HVAC systems sometimes operate at 50 per cent efficiency, Touchie said.

Using NTED in an apartment building, the perimeter would be created using the balconies.

They can also improve the air tightness of a building and remove thermal bridges from the structure.

Pressnail said air tightness can be a problem, but the amount of ventilation to each unit can be varied using a dedicated outdoor air system (DOAS).

A sensored DOAS can have 83 per cent energy savings, he said during the conference.

There is a tendency to increase efficiency in highrise buildings by trying to seal the suites, which in the past were ventilated by increasing the pressure in the corridors, he said.

One key question is how much air is needed to provide adequate ventilation.

“In these centralized systems, if you go through the calculations — and I’m trusting the mechanical engineers on this — their conservative estimates are that they need 20 to 70 per cent more air supply in the hope that they will end up getting it where they want it,” Pressnail said.

“We get higher ventilation rates because we are attempting to compensate for poor delivery systems.”

Building engineers can increase efficiency by installing carbon dioxide sensors, reducing ventilation to unoccupied rooms and increasing ventilation only when people are present, Pressnail said.