"Yes" says Tony Wood. "Especially when your project includes an upgrade to the building envelope."

The relationship between high performance building envelopes and improved energy efficiency, especially lower energy operating costs per square foot, has been accepted for some time by most engineers in the building envelope fraternity, but forecasting the energy and cost savings has been at best an inexact science.

Recently, three factors have helped improve forecast accuracy: recognition and acceptance of the step-by-step assessment and calculation procedures being used to predict savings, new understanding of the relationship between performance of the building envelope and performance of the building's mechanical/HVAC systems and proven savings from a growing inventory of documented projects, especially ESCO contracts.

Before we get into the dollars and cents, we need to take a brief look at the science. Air leakage through the building envelope can lead to unnecessary heat loss in winter or heat gain in summer, condensation and moisture damage in hidden cavities, rain penetration, poor indoor temperature and humidity control, not to mention excessive energy consumption.

Air can leak directly through roofs and exterior walls, but most often it travels through the joints of assemblies such as roof/wall junctions, parapets, low level soffits, the intersections of different cladding systems, and through numerous internal vertical and horizontal pathways.

Two conditions are needed for air to leak. First there must be a hole, gap or crack from one side of the envelope to the other. Second, there must be an air pressure differential, for which there are three causes: wind, stack effect and the HVAC system.

Wind pressurizes the windward side of the building and depressurizes the back, sides and roof. It can account for up to 25 per cent of total leakage; it cannot be controlled, only reduced by plugging the holes in the envelope.

Stack or chimney effect is a buoyancy phenomenon where warm inside air rises through the building and exerts continuous pressure against the roof and upper parts of the exterior walls. The resulting lower pressure at the bottom of the building actually sucks in air.

The third source of pressure differential is the mechanical system itself. Mechanical engineers and on-site managers often choose to bring in makeup air to increase pressure and overcome this infiltration at the base of the building. Unfortunately, this increases pressure at the top, causing greater exfiltration problems in that area. This over-pressurization at the top of the building cannot be controlled at the same time as controlling infiltration at the base of the building. The only solution is to seal air leaks at the top and the bottom of the building. When the building envelope allows air leakage, the mechanical system has to take the brunt.

If a large amount of the conditioned air is escaping from the building, the heating system is working overtime and energy costs are increased, says Dean Brigham, business development manager for Comstock Canada Ltd., a national design-build construction, multi-trade contracting and facilities management firm. A building with proper envelope sealing can benefit from a heating system that is 20-30 per cent smaller, and usually less expensive, than one with uncontrolled air leakage. Eliminate the stack and you get a typical four to five year payback in natural gas savings coming directly out of reduced heating demand, with the option to upgrade the plant with smaller, more efficient capacity which yields annual net savings.

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