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Sustainable Building Envelope Applications Improved Performance Reduces Operation Costs
April, 2008
By Kevin Day
Upgrading the building envelope is often an effective method of achieving better performance while reducing operational costs and ensuring that systems and assemblies surpass their intended service life. Developers seeking to make their buildings more sustainable should select systems that result in low operational and maintenance costs. Using materials with low embodied energy and/or renewable resources is ideal.
GLAZING AND CURTAN WALLS
As the gross glazing area of an exterior wall assembly increases, it becomes increasingly difficult to mitigate the impact of unwanted solar gains (summer) and thermal losses (winter). Thermal efficiencies of windows are limited when compared to opaque wall areas incorporating insulation.
Curtain wall with glass and metal panel spandrels are not much better than the vision glass areas, even when insulated behind, and can't match the thermal resistance of most other claddings. Therefore, well-insulated, airtight wall assemblies with strategically placed fenestration will optimize potential solar gains and daylighting, while reducing excessive thermal losses and gains.
Glass thermal performance values (referred to as U-values - the measure of heat conductivity in the form of watts per m2 per oC) are often referenced by designers and builders without accounting for the overall thermal conductivity of glazing spacers and framing. For example, a well insulated glazing unit with a rated 0.25 U-value can end up in an assembly with an overall 0.50 U-value - this means the glass ends up in an assembly that is half as efficient as the glazing by itself.
Selection factors for fenestration assembly U-values include:
* Glazing type - consider warm edge spacers and argon gas-fill
* Increase spacing between structural members - this is more limited for window-walls
* Select higher quality (more substantial) thermal breaks in the fenestration framing
* Triple glazing for enhanced thermal performance and occupant comfort
* Close off curtain wall systems that extend above soffit areas and over parapet walls to reduce air leakage and condensation risk
Heating typically accounts for 50% of a building's total energy consumption so the glazing portion has a huge influence on the operational costs, which is in proportion to the amount of energy consumed and carbon dioxide emitted. Advanced thermal modeling of curtain wall and window systems can be money well spent to evaluate the threshold conditions for adequate occupant thermal comfort, condensation risks, daylighting, and heat losses/gains (impact on space heating/cooling requirements).
U-values are not the only consideration for fenestration. Spectral selective glazings can maximize visual light with minimal solar heat gain, and low-E coatings reduce the effect of radiant heat loss. In a cold climate, the coating would be, ideally, located on the #3 glazing surface, which is the outside face of the innermost glass lite.
STEEL FRAMED WALL CONSTRUCTION
During the last 25 years, exterior steel stud wall construction has become increasingly more common, but not without some problems along the way. More robust assembly and moisture control measures has become a necessity for long-term service, but steel stud wall assemblies are vulnerable to corrosion due to condensation and water penetration, and the assembly strength significantly diminishes when fasteners and bracing are not properly installed.
Key requirements for steel stud framing include: redundant framing connections (don't rely on just one screw when two will do); increasing exterior insulation (reducing stud cavity insulation); and installing a moisture resistant (even waterproof) exterior sheathing system. Adopting a low risk approach to moisture contact with steel framed wall assemblies is the best choice to maximize service life.
EIFS & STUCCO CLADDINGS
Asking for stucco in eastern Canada typically results in exterior insulation and finish systems (EIFS) - a thin synthetic stucco applied overtop foam insulation that is easily sculpted to create interesting and ornate decorative effects. In the last six years, the Ontario Association of Architects has issued practice directives to its members indicating EIFS over framed-wall construction must incorporate a secondary moisture control layer and drainage.
The obvious benefit of EIFS is the improved thermal efficiency - best achieved when drainage is subtle and strategic, minimizing air movement behind the cladding. EIFS is ideally suited to incorporate an airtight moisture barrier that can transition with all other claddings and windows, however, this benefit is only realized when the construction sequence is properly devised.
Traditional stucco is more common in western Canada, but whenever used, the moisture control layer and drainage strategy should meet local code requirements and this is not always understood. A key requirement for both EIFS and stucco is to avoid exposing the systems horizontally to rain and snow accumulation. Aesthetics aside, the long -term durability of exposed complex and ornate features are at risk so do not discount strategic placement of metal flashings.
PRECAST CONCRETE
Architectural precast is a robust cladding, typically configured as monolithic panels with two-stage sealant joints. Precast often provides the air barrier, albeit sometimes unintentionally.
Batt insulation should be avoided for the entire cavity to reduce the likelihood of condensation forming on the back of the panels and/or on the anchors. It is best to place 50 mm of rigid closed-cell plastic insulation sealing the board joints, or spray-applied polyurethane foam up against the inner face of the precast. Then build the steel frame and insulate (with batt or otherwise). This approach is more thermally efficient with a lower condensation risk.
When precast panels are configured with well-insulated anchors protected from the risk of condensation and rain water, the performance life can be substantial, provided the concrete has a good mix and finish design to resist the rigors of changing weather.
BRICK VENEER
Historically, a well-proven cladding material, clay brick masonry provides a durable enclosure when moisture and movement are properly managed. Solid multi-wythe (layer) clay brick evolved into clay brick with block backup, then to brick veneer over steel stud.
Proper spacing of control joints for brick veneer is key - best using a soft joint (sealant and backer rod) rather than a solid mortar fill since the latter typically cracks. Anchorages must be properly spaced, especially important at brick terminations, such as at control joints and abutments to curtain wall.
It's fairly common to leave a 10 mm space between shelf angles and brick veneer below. The placement of the sealant must only be adhered to the underside of the shelf angle so the brick above will weep properly.
Don't place brick vents at the upper edges of parapet walls unless the cap flashing is designed to cover these over. This prevents heavy rain from driving into the cavity. Install building corner closures in the cavity to reduce wind washing and enhance pressure moderation (venting during heavy rain fall).
Ensure all vertical terminations in brick veneer have end dams for the through-wall-flashing. This common deficiency accounts for many water penetration problems in the last 30 years.
Also ensure that the brick cavity can drain and vent the wall. The nominal 25 mm cavity can be inadvertently filled with mortar, but mortar blocking devices are available and many tradespeople know some tricks to keeping the cavity clear.
Remember that the brick anchors are a thermal bridge, which some studies have shown reduces the effective brick cavity insulation value by more than 10%. Account for this difference when calculating the thermal resistance (R-value) of the wall.
BALCONIES
Most of the residential high-rises in Ontario have concrete balconies, and these require significant repair, typically in a cycle of 15 to 30 years. Eliminating the typical drip-edge cast into concrete is one very simple change that would extend this repair cycle. The problem occurs when reinforcing steel in the slab is not properly covered with concrete in these locations. A better option is to form the drip edge outward, rather than as a slot, or to install architectural trim.
Balconies suck the heat out of a building in the wintertime. Changing the design to glazed solariums can dramatically improve the thermal efficiency. New technology for thermally broken balconies exists, but these designs are more expensive and not yet proven in the Canadian climate.
Waterproofing exposed cantilevered slabs (not over living space) rarely results in a good return on investment, and long term repair cycles are not typically affected by this practice.
ROOFING OPTIONS
A sustainable roof, ideally, reflects solar radiation for reduced heat-island effect and controlled rainwater run-off, thus reducing the load on the local storm sewers. One of the simplest upgrades is increased insulation thickness, provided it the increased thickness is part of a tested assembly.
White roof membranes such as TPO and PVC (thermoplastic polyolefin and flexible polyvinyl chloride) are one approach. Alternatively, ballasted roof membranes can employ cement pavers or stone ballast, either of which can provide comparable solar light reflection after about three years of exposure. Cold-applied white membranes and white-coated modified bitumen provide additional options. It is important to look at the service life of the roof system. Typically, the cheaper membranes don't last as long.
A green roof doesn't always mean planting on the rooftop. Plant growth, such as prairie grass, may contravene fire ratings for roof level and fire separations may be required. Use a code consultant to get clear direction and check with municipal building officials about local requirements.
Plant life requires a cistern for the first year's irrigation, ideally using water captured and stored on-site. Keep plant life away from all service penetrations, roof expansion joints, and perimeter flashing. Roof drains must be equipped with ballast guards that will prevent soil from washing down and clogging the drainpipes. Be sure to anticipate the load of plant growth at maturation (part of structural design).
Reducing rainwater runoff can provide an evaporative cooling effect during the summertime, but reducing the storm water burden is probably of greater value. Protected membrane roofs should be equipped with flow-control devices for the roof drains, thereby providing comparable performance to having plant life but at a lower cost. To achieve this, evaluate the water dissipation required to ensure the optimum amount of water is stored.
LEED Canada Credit MRc8 for a Durable Building
The intent of this credit is to minimize material waste caused by premature failures of buildings and components. Using CSA-S478 as a guide, develop a Building Durability Plan (BDP) during the design phase of the project.
The key objective is to ensure the building is designed and constructed to exceed the design service life. Sounds easy, until you assign the design and predicted service lives to the building's major structural and building envelope components - some debate between project team members will likely ensue.
The benefit of seeking this credit is to ensure the building's long-term performance life is discussed thoroughly, and documented during the design stage. The bonus of the LEED durability credit is the lower risk of performance problems such as moisture and thermal comfort issues.
Kevin Day is a building science specialist with Halsall specializing in best practice principles for cladding design and restoration. He can be reached at kday@halsall.com. The preceding article is reprinted from CondoBusiness, April 2008.
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