    |
Designing House Envelopes for the North
Technical Series 94-203Key Messages
|
Introduction
The goals of building low-energy housing are to use less heating fuel than conventional housing and to construct houses that are more comfortable and durable. Low-energy houses are designed for better control of the flows of heat, air and moisture through the house shell or envelope (the exterior walls, roof and floor). This approach makes for more comfortable houses, saves on operating costs and also increases the durability of the structure.
One of the keys to building durable housing for the North is the control of water in all its phases: solid, liquid and gaseous. The building envelope provides protection from rain, snow, wind the sun. How well the envelope achieves this task will help determine the long-term performance and durability of the whole house.
Design of low-energy housing must recognize that all aspects of the house are inter-related: the building envelope, the mechanical systems and the occupants. An understanding of the relationship between these three factors is critical to a successful design. For instance, higher insulation levels mean the house will require a smaller heating system, and a more air-tight house may require mechanical ventilation.
Heat Loss
Heat flows from warm areas to cold areas. The greater the temperature difference, the greater the driving force behind the flow of heat and the greater the potential for heat loss.
All materials conduct heat, some better than others. Insulation materials are poor conductors and so slow heat flow through the building envelope. Each material is rated on its ability to resist heat conduction as identified by an RSI value: the higher the RSI, the greater the ability to slow down heat flow.
Insulation can only work effectively if three conditions are met:
- It must be kept dry.
- It must form an even blanket around the building. There must be no air pockets or gaps.
- There should be no places where materials penetrate across the insulating layer, forming thermal bridges, which short-circuit the insulation.
Air Movement
The control of air movement through the building envelope is critical to reduce heat loss and prevent moisture buildup. Exfiltrating air carries both heat and moisture(in the form of water vapour) to the outside. Lost heat increases operating costs. Water vapour (as carried in the air) can condense within the building envelope and is a primary cause of structural failure in housing. Air flow accounts for the largest source of water vapour transport.
Envelope design must also control the flow of cold infiltrating air, which causes uncomfortable drafts and dry building interiors.
There are three main driving forces for air movement in a building.
- Warm air rises and can escape through openings in the upper parts of the house. Cold air is drawn in around floors and baseboards to replace the escaping warm air (Figure 1). This is referred to as the 'stack effect'.
FIGURE 1. Stack Effect - Wind pressures also influence air leakage, forcing cold air in through cracks on the windward side and warm air out on most of the rest of the structure (Figure 2).
FIGURE 2. Wind Pressures - Mechanical and passive ventilation systems intentionally exchange indoor air with 'fresher' outdoor air. Pressurized systems blow air into the building, depressurized types blow air out and balanced systems bring in as much as they push out (Figure 3).
FIGURE 4. Barriers Needed to Control Air and Vapour Movement Within the Building Envelope
A wind barrier is used to stop the wind from blowing through or around the insulation. This is needed because in many cases the exterior cladding is not airtight. One example of a wind barrier would be a wind proof layer between the insulation and the exterior cladding. This type of barrier can also be deemed a weather barrier since it will help to prevent rain or snow from penetrating the building envelope (Figure 4). The harsh climate of the North necessitates a weather barrier of a material which will not shrink or shatter. Plywood, or rigid foam sheathings may be more suitable than thin membrane materials. It is also important to ensure that the weather barrier will not act as a vapour barrier, trapping moisture inside the envelope The weather barrier should be wind proof but should allow the passage of water vapour.
Moisture Damage and Condensation
Moisture penetration of the building envelope can cause a variety of problems. Wood rot will occur if the fibres are saturated and the surface temperature stays above 10 C for any length of time. Metal corrosion may occur in wet conditions. Wetting and drying cycles change the dimensions of many building components and this often leads to physical damage. Water stains on finish materials look unsightly and create maintenance problems. Mold and fungal spores flourish in high humidity and wet conditions, sometimes creating serious indoor air quality problems. In extreme cases, ceilings may collapse under the weight of ponded water and saturated insulation.
The Theory
Air can hold different amounts of moisture depending on the air temperature. Warm air can hold a lot more moisture than cold air. Relative Humidity (RH) is a term that describes the moisture content of the air as a percentage of the maximum amount of moisture the air can hold at that temperature. As the temperature drops, the air's ability to hold moisture also drops. A 5% RH at 21 C represents roughly the same mass of airborne moisture as 100% RH at -20 C. During cooling, excess moisture that was present at a higher temperature condenses to liquid on the nearest cool surface.
FIGURE 5. Cycle of Air Movement Into and Out of a Building Enlarged Image
How it Works in Buildings
Outdoor air at -20 C and 80% RH is drawn into the building and heated to 21 C. At this temperature it has a RH of roughly 5%. The indoor air is much more humid, with a typical RH of 40% at 21 C. High humidity levels are produced indoors by many household activities including cooking, washing and drying clothes and dishes, bathing, exercising, indoor drying of wood and other activities. Once inside, the outdoor air warms and its ability to hold moisture increases, eventually reaching 40% RH. As the warm, moist air escapes through openings in the walls and roof its temperature drops and the RH climbs. At an RH of 100%, the air cannot hold any more moisture. As the temperature drops further, moisture must now condense out of the air and is deposited inside the envelope. This continues until the air temperature reaches the outdoor temperature.
Moisture Transport
Moisture moves through the building envelope in two ways: air leakage and vapour diffusion. Air leakage is by far the greatest source of moisture in the building envelope as the warm air escaping the building carries moisture with it. Air leakage through a hole 2 cm2 in size can carry 30 litres of water into the building envelope over a heating season (Ottawa example). Methods to control air leakage are discussed in the next section.
Vapour Diffusion
Vapour diffusion through building envelope components is not of great concern in warm climates. However, it should be accounted for in designing house envelopes for the North because of the long periods of cold. Diffusion occurs due to differences in vapour pressure which result from differences in the concentration of water vapour between two locations. When there is a difference in vapour pressure, water vapour will move through materials. In winter conditions, this vapour movement carries water vapour through the building envelope where it can condense on cold surfaces.
Control of Vapour Diffusion
Vapour barriers are used on the interior side of the envelope to prevent the moisture in the warm interior air from reaching a cold surface and condensing (see Figure 6). All materials allow water vapour to pass through them to some degree. Some form better barriers to water vapour than others. Polyethylene, plywood, foil and some paints form effective vapour barriers and are used to prevent diffusion.
Bologna HotelsFIGURE 6. Wall with Vapour Barrier
Controlling Moisture Flow
The easiest method to control damage from water vapour and moisture is to reduce the amount generated. Moisture can be reduced by simple practices, such as covering pots when cooking, and by drying clothing and firewood outside whenever possible. Moisture laden air can also be exhausted using small kitchen and bathroom exhaust fans vented to the outside.
|