Envelope Upgrades Support Energy Efficiency ASHRAE 90.1/2007 Emphasizes Air Barrier Effectiveness
March, 2008

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By Tony Woods

The relationship between the HVAC system and the building envelope can mean the difference between a comfortable, safe, energy efficient building and a drafty, uncomfortable building with poor indoor air quality, high energy bills and a lot of complaints from occupants. If the performance of the building envelope is compromised, the HVAC system pays the price. Uncontrolled air leakage means conditioned air escapes and the HVAC works overtime to maintain the indoor environment.
 
The 2005 publication of the National Institute of Standards and Technology (NIST) report, Investigation of the Impact of Commercial Building Envelope Airtightness on HVAC Energy, found that uncontrolled air leakage has a significant impact on energy efficiency. Study results showed that the inclusion of an air barrier system in four sampled types and sizes of building can reduce air leakage by up to 83%, representing a large reduction in energy consumption and operating costs.

This translated to potential gas savings of greater than 40% and electrical savings of greater than 25%.
Based on these findings, the American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE) started a process to include air barrier requirements in its widely cited Standard 90.1 Energy Standard for Buildings Except Low-Rise Residential Buildings. The recently published 2007 version of ANSI/ASHRAE/IESNA Standard 90.1 contains 47 addenda implementing changes to the 2004 standard, including Addendum z, which establishes performance requirements for air leakage of the opaque building envelope.

"One of the best ways to reduce building energy consumption is to reduce, or eliminate, the cooling or heating loads. By doing so, the systems installed in buildings become smaller and use less energy," says Mick Schwedler, Chair of the Standard 90.1 committee. "For example, on a hot, sunny day, having more insulation in the roof and better glass on the southern and western façades of a building reduce the air conditioning necessary as well as its resultant energy use. Two of the ASHRAE addenda do this by enhancing the insulation and fenestration (or window) requirements for the building exterior."

BUILDING SYSTEM LINKAGES

The new ASHRAE 90.1 standard is expected to have wide ranging implications for the adoption of air barriers. This reinforces existing evidence of the relationship between HVAC performance, energy efficiency and the building envelope.

For example, building envelope upgrades performed on 15 school buildings for the Liverpool Central School District in Syracuse, New York in the mid-1990s saved $52,000 per year in energy costs, for a simple payback of five years. A similar project performed in at Gravenhurst High School in Ontario's Muskoka region cost $5,777 and saved almost $4,200 in the first two winter months.

Elsewhere, air sealing of roof/wall joints in single-storey Toronto public schools delivered 17% average electricity cost reductions. Perimeter air sealing of high-rise apartment buildings in Ottawa and Toronto showed an average of more than 10% reduction in electrical demand and about 9% reduction in electricity consumption. Significant savings were realized in a smoke control and thermal comfort retrofit project at an apartment complex in Toronto's North York community, delivering unexpectedly large energy cost savings and a shortened payback period.
Performance contractors and ESCOs have recognized the real contribution that can be made by a high-performance building envelope. Although typically focused on upgrade strategies such as heat pumps, high efficiency boilers, building automation, and perhaps heat recovery, they also recognize that building envelope upgrades allow for downsized, more efficient heating and cooling systems.

AIR LEAK PHYSIOLOGY

Air leaks 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 underlie leakage. First there must be a hole, gap, crack or leak from one side of the envelope to the other. Second, there must be an air pressure difference, which is generally caused by wind, stack effect and/or 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% of total leakage and 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 sucks in air.
The third pressure comes from the mechanical system itself. Mechanical engineers and on-site managers often choose to bring in makeup air to increase pressure and overcome infiltration at the base of the building. However, this increases pressure at the top, causing greater exfiltration problems in that area.
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.
Flawed ventilation design can result in excessive indoor pressure in northern climates, and, conversely, negative pressure in southern climates. In the typical Canadian climate, excessive pressurization causes condensation or moisture damage within building envelope cavities.
A discordant interface between mechanical systems and the building envelope can cause other indoor air quality and comfort issues. In winter, dry indoor conditions, annoying static electricity and poor indoor air temperature control indicate the building is leaking too much air. In summer, leaking air can result in heat and moisture loads too great for the cooling equipment.
Simply using ventilation to improve comfort conditions does not prevent moisture damage nor improve energy efficiency. Upgrading the air tightness of a building is a cost-effective way to improve comfort, increase energy savings and extend building durability.

DETECTION & CORRECTION

First an assessment is required to detect the air leaks. Cracks, gaps, leaks and holes are easily made visible with an air leakage detector or smoke pencil. A large-scale depressurization fan can also be used to create negative pressure in the building and increase the visibility of leaks, or infrared thermography from outside the building can show patterns of air leakage. Energy saving potential is then analyzed using energy-use calculation software.
Once the air leakage pathways have been identified throughout the building, including exit and entry points, an air barrier continuity plan can be devised to addresses air sealing in five critical areas. These are: the top of the building; the bottom; the vertical shafts; the outside walls and openings; and internal horizontal air leaks.
Examples of typical pathways that need to be sealed include:

Top of the building:
* Roof/wall intersections
* Mechanical penthouse doors and walls
* HVAC equipment
* Various roof penetrations

Bottom of the building:
* Underground parking access doors
* Exhaust and air intake vents
* Soffits and ground floor access doors
* Pipe, duct, cable and other service penetrations into core of the building
* Sprinkler hanger penetrations, inspection hatches and other holes
* Core wall to floor slab

Vertical shafts:
* Stairwell fire doors
* Fire hose cabinets
* Plumbing, electrical, cable and other penetrations within service rooms
* Elevator rooms, cable holes, door controller cable holes, bus bar openings
* Garbage chute perimeter and access hatches
* Hallway pressurization grille perimeters
* Elevator shaft smoke control grille
* Service shafts

Outside walls and openings:
* Weatherstripping on windows, doors, balcony and patio doors
* Window trim
* Exhaust fans and ducting
* All service penetrations
* Baseboard heaters
* Electrical receptacles
* Baseboards

Compartmentalization:
* Vented mechanical rooms
* Garbage compactor room
* Emergency generator room
* High voltage rooms
* Shipping docks
* Elevator rooms
* Workshops

Tony Woods is President of Canam Building Envelope Specialists Inc., and a past president of the Ontario Building Envelope Council. He has served on more than 10 Standards Committees dealing with air leakage control, air barriers and ventilation. For more information, see the web site at www.canambuildingenvelope.com.