Thermal breaks prevent heat loss

by nithya_caleb | December 27, 2018 11:08 am

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by Brent Chancellor, PhD, P.Eng.

Emperor penguins can survive Antarctica’s frigid climate because—much like efficient building envelopes—their bodies contain an air and moisture barrier, continuous insulation (ci), and thermal breaks preventing heat loss into the environment.

Air and moisture barrier seals out weather

The only animal to inhabit the open ice of Antarctica during the winter, emperor penguins withstand wind chills of up to –60 C (–76 F) and blizzards up to 200 km/h (124 mph). Scale-like feathers shield their bodies from harsh wind, ice, snow, and water, while their skin provides the final barrier to transmission of moisture and vapour.

Similarly, modern building envelopes prevent the migration of moisture from the outside environment to the inside. Some envelopes employ a combined moisture/air barrier, while others utilize a rainscreen.

Continuous insulation retains heat, conserves energy

The emperor penguin’s thick layer of continuous blubber serves as its primary insulation against bitter temperatures. Additional insulation is provided by its feathers, which trap a layer of air against the skin.

While the penguin’s ci is dictated by natural selection, the ci of commercial building envelopes is now being dictated by code requirements. Many provincial and local codes already require structural thermal breaks (STBs) when taking a prescriptive path, and others are not far behind in the adoption of more stringent standards.

Thermal breaks prevent heat loss into the environment

Emperor penguins prevent the escape of heat through their feet into the ice and water by means of thermal breaks in their legs: a vascular adaptation thermally separates their rounded, well-insulated bodies (low surface-area-to-volume ratio) from their flat, poorly insulated feet (high surface area-to-volume ratio).

Similarly, the balconies of commercial buildings—cantilevered structural extensions of commercial building floor slabs—quickly equalize to cold exterior temperatures. As a result, balconies continuously transfer heat from the interior floor slab to the exterior environment, unless thermal breaks are installed where the balcony interrupts the ci of the building envelope.

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Traditional balcony construction wastes heat

Conventional concrete and steel balconies are designed as cantilevered extensions of steel or concrete floor slabs because of loadbearing requirements. Thus, they not only create a thermal bridge in the ci of the building envelope, but also rapidly conduct heat from the interior slab through the cantilevered balcony and into the environment.

Prior to 2010, wasted heat and cold interior floors were accepted as unavoidable outcomes in North American buildings with balconies. Problems of mould growth arose when builders began wrapping exteriors with airtight vapour barriers.

Before airtight envelopes, most commercial buildings leaked air profusely, causing humidity levels inside buildings to equalize with low exterior humidity levels during winter months. Forced hot air (typically vented at or near balcony doors and windows) further ensured interior humidity remained too low at the cold balcony penetration to reach dewpoint, form condensation, or support mould growth.

However, as mentioned, the advent of airtight vapour barriers to prevent leakage of heated air during the late 20^th century had a major unintended consequence: increased mould growth.

As a building becomes more airtight, it requires less heat and retains more moisture from evaporation and human off-gassing. This can increase interior humidity by 35 to 40 per cent—the target level for human comfort, which can also create a danger zone for condensation when the thermal conductivity of balconies drops the temperature to the dewpoint on the interior side of the envelope at the point of penetration.

With nowhere to go, the moisture will condense if interior temperatures drop to the dewpoint, especially within cold wall cavities adjacent to uninsulated balconies and other envelope penetrations. The resulting condensation leads increasingly to mould growth on gypsum board, studs, and insulation on the inside of the building. Mould can grow and cause respiratory problems in building occupants years before it becomes visible on interior ceilings and walls.

By then, remediation will, at minimum, require removal and replacement of gypsum board. However, mould is likely to recur since high interior humidity and cold envelope penetrations in existing structures are unlikely to be corrected due to difficulty and cost.

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STBs eliminate energy loss

An STB is a fabricated longitudinal assembly about as wide as the exterior building wall and as high as the floor slab. It creates a structural insulated foam break between the interior floor and the exterior balcony to minimize thermal conductivity between the two masses, while optimizing loadbearing capacity.

STBs for concrete construction contain engineered stainless steel rebar for casting into the concrete slab on the interior side and the concrete balcony on the exterior side, yielding structural strength equivalent to that of conventional reinforced concrete extensions of floor slabs.

STBs for steel construction are equipped with flanges and bolts for fastening to steel floor joists on the interior side. On the exterior side, they are fastened to cantilevered balcony supports or to other steel connections, such as canopies, signage, sunshades, rainscreens, roof-mounted equipment, and fencing.

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In this author’s experience, when compared to non-insulated connections, STB achieves a 90 per cent reduction in thermal conductivity in the connection area for standard loadbearing scenarios. This translates into average annual reductions of 14 per cent in energy use and carbon footprint for the overall building.

The reduction in energy required to heat the building also allows corresponding reductions in heating system size/capacity, resulting in savings on capital equipment and ongoing operation and maintenance of mechanical systems.

Lastly, thermally isolating balconies using STBs improves comfort for occupants and value for developers by increasing the warmth and usability of interior floor space.

STBs help comply with tighter codes

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STBs help buildings comply with tightening building codes mandating higher energy efficiency and ci.

The American National Standards Institute/American Society of Heating, Refrigerating and Air Conditioning Engineers/Illuminating Engineering Society (ANSI/ASHRAE/IES) 90.1, Energy Standard for Buildings Except Low-rise Residential Buildings, was revised in 2016 to require energy-efficiency modelling for uninsulated assemblies in building envelopes, including balconies and perimeter edges of floor slabs and parapets.

The 2016 update of ASHRAE 90.1 offers three paths to compliance: the prescriptive method, the energy cost budget (ECB) method, and the performance rating method (PRM).

The prescriptive method specifies details of building elements such as:

• ci;
• specific R-values for components depending on construction type and geographic region; and
• glass-to-opaque-wall ratios limited to 40 per cent of wall surface.

The ECB method is a performance-based system for determining compliance and is available as a free web program. ECB compares two models of a building: the proposed building as designed and the budget building design (a structure of the same size and constructed to minimum ANSI/ASHRAE/IES 90.1 prescriptive requirements), calculating costs and identifying areas needing change. The program simulates the building’s proposed energy costs, comparing them with those of the code-compliant structure and indicating whether the proposed costs are less than or equal to the baseline, and thus compliant. ECB can also summarize a building’s energy performance as a percentage of the ANSI/ASHRAE/IES 90.1 standards.

In PRM, the project team applies software modelling tools to prove the building will perform at least as well as under the prescriptive requirements with an equal or lower annual energy cost. PRM is also used for ‘beyond code’ programs such as Leadership in Energy and Environmental Design (LEED) or the International Green Construction Code (IgCC).

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Vancouver in the vanguard

Canadian codes have imposed higher standards than in other North American jurisdictions. The 2017 National Energy Code for Buildings (NECB) requires all new buildings to be “net-zero energy ready” by 2030. Vancouver takes energy performance particularly seriously since available space for new construction is limited and property is expensive. Most of the projects found there are infills, redevelopments, and construction of condos on parking lots.

These types of projects need rezoning, and application approval requires efficient construction such as Passive House certification or meeting a new metric called thermal energy demand intensity (TEDI). Measured in kWh/m² annually, TEDI is the amount of heat required to keep a building warm after offsets from building envelope losses and heating of ventilation air, regardless of how efficiently the heat source is produced. This metric directly reflects the building enclosure performance.

[7]Vancouver building officials, aware of the performance challenges facing their densely built environment, have instituted a green buildings policy. It requires new rezoning applications for high-rise buildings to meet standards for “near-zero emissions buildings” (equivalent to Passive House or the International Living Future Institute’s [ILFI’s] net-zero certification) or “low-emissions green buildings,” which address building envelope performance.

Conclusion

Problems related to thermal conductivity of conventional balcony designs worsened when airtight building envelopes became a code requirement. In addition to the heat loss and cold interior floors inherent with traditional construction, higher interior humidity levels then caused condensation to form on the interior side of cold penetrations, resulting in mould growth. By breaking the thermal bridge between the cold exterior balcony and warm interior floor, STBs eliminate 90 per cent of energy loss at the balcony and increase the comfort of interior living space, while minimizing the developer’s exposure to mould-related liability.

[8]Brent Chancellor, PhD, P.Eng., is the New York City and mid-Atlantic regional sales manager for Schöck. A former professor at Bucknell University, Chancellor worked on the design of tall buildings throughout the world before joining Schöck. He can be reached at brent.chancellor@schock-na.com[9].

Source URL: https://www.constructioncanada.net/thermal-breaks-prevent-heat-loss/

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