Ventilation and Airtightness of Housing
Design of housing for the North must take into consideration the climatic extremes encountered during the winter. Warm moist indoor air exfiltrates and cold dry outdoor air infiltrates the building. The leakage of indoor air can result in problems such as: moisture accumulation in the building envelope. The infiltration of outdoor air causes cold drafts and increases the energy consumed to heat the building. Proper design and installation of ventilation systems and air barriers can help to minimize air leakage.
There are three main causes of air leakage in a building.
A wind barrier is an air barrier located behind the exterior cladding. It is used to stop wind from blowing through or around the insulation. This is needed because in many cases the exterior cladding is not airtight. A wind barrier is any air-impenetrable layer placed between the insulation and the exterior cladding. The harsh climate of the North necessitates a wind barrier of a material which will not shrink or shatter. Rigid materials like plywood, fibreboard, or foamed plastic sheathings may be more suitable than thin membrane materials.
Control of air flow requires control of either the driving force or the opening through which the air flows, or both. Air barriers prevent air from moving through the building envelope. One critical requirement of an air barrier is that it be continuous. It must also be sufficiently strong and durable to withstand differential air pressures. This is particularly the case in the North where exposure to strong winds is common and the long cold winters increase the impact of the stack effect. To ensure long-term performance and to help reduce heating costs, air barriers should be designed and installed to last for the life of the house.
The successful performance of air barriers requires particular attention to detail during installation. Some important procedures to follow are listed below.
Because of extreme weather conditions in the North, some special design features and weatherstripping procedures are recommended for buildings in this region. Some of these are listed below.
FIGURE 2. Simple Box Vent
Installation Problems Encountered in the North
There are several features of northern housing that make the installation and maintenance of air barriers particularly difficult.
Traditionally, houses have received an adequate supply of fresh air through uncontrolled air leakage and operable windows. However, in the quest to reduce the problems associated with air infiltration, this uncontrolled natural ventilation is being eliminated. Opening windows often become inoperable in the winter. They can be difficult to seal and may be impossible to close due to ice build-up, causing excessive heat loss. Fresh air for ventilation and combustion must be supplied to the house by other controlled means.
Air is typically exhausted by means of bathroom and kitchen exhaust fans. The fans can be installed in partition walls instead of the ceiling. In this way, the ductwork can be run down through the stud space and under the floor where it will pass between the joists to exit the building through the exterior wall, instead of running up through the ceiling and roof or out through the soffit or end gable. This prevents warm air from escaping through the system by stack effect when the fan is not running. Condensed moisture and frost do not collect in the ductwork since it is kept warm within the dwelling instead of in the cold attic where it would need to be insulated. This technique may, however, require a longer duct run and larger fan motor. Even with this improved alternative, ice and condensation will build up at the exterior wall cap, and when the fan is shut off, exterior air may enter the building through the ductwork and fan. Recirculating kitchen fans using charcoal filters help to recondition the air without wasting energy, and also reduce the number of penetrations through the building envelope. However, they may result in poorer air quality, and possibly moisture problems, since they do not remove moist air from the kitchen.
In addition to exhaust fans, there should be a source of fresh air supply to the building. This can be achieved in one of several ways.
If the house has a forced-air heating system, ventilation can be provided by a fresh air intake connected by an insulated duct to the return air plenum of the furnace. The fresh air is mixed with the return air and circulated through the house. In order to incorporate an exhaust with this type of system, the furnace blower could be controlled by a dehumidistat and interlocked with an exhaust fan or a heat recovery ventilator. Both fresh air and exhaust air would then be provided when required.
Kitchen and bathroom exhaust fans can be linked to an air-to-air heat exchanger. The heat exchanger can provide fresh air for the house provided the intake and exhaust openings are at least 4 m apart. However, there have been problems with condensation and ice formation with heat recovery ventilators operating in cold conditions.
Wind pressures can adversely affect the performance of the fans and can force air and snow through the openings if the openings are in side walls and roofs. If the house is raised, intake and exhaust openings can be placed through the floor, under the building. They should be placed in the centre portion of the floor area to avoid turbulence from wind which occurs at the perimeter. Intake and exhaust ducts should be at least 2 m apart and be located on opposite sides of a protruding beam. They should be vented to the side of the building to prevent condensation of moisture underneath the flooring. There should be no skirting around the base of the house, except for expanded metal or welded wire fabric, and the space between the ground and house should be reasonably uniform over the whole area.
It should be noted that if fresh air is supplied, but exhausting of air is not balanced, then the house will be under a positive pressure. This will force the warm moist air out through the building envelope, causing moisture damage. Controlled exhaust should be provided in order to avoid this situation.
Combustion air should be provided separately for oil and gas burning appliances. The combustion air intake must be designed and located to prevent cold winter air and snow from entering the house. Some suggestions include: placing the combustion air opening through the floor in houses without basements and having a motorized damper interlocked with the burner.
Remote and Northern Energy-Efficient House Design Catalogue; prepared by REIC Ltd.; prepared for The Remote Community Demonstration Program of Energy Mines and Resources Canada; March 1989.
Examples of Housing Construction in the North; by Burdett-Moulton, Architects and Engineers, Northwest Territories, Don Jossa and Associates, Yellowknife, Northwest Territories, and Wayne Wilkinson, Consultant, Whitehorse, Yukon; for Canada Mortgage and Housing Corporation; April 1987.
Design Guidelines and Technology for Northern Housing Construction; Indian and Northern Affairs Canada, Technical Services and Contracts; April 1983.
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