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Window and Door Types There are two types of windows: those that open and those that don't, called operable and fixed. Use as many fixed windows as codes allow, keeping in mind that floors with bedrooms need at least one operable window for emergency exit if there is only one exit from that storey, above or below the main floor. Fixed windows are more efficient because of their better air tightness characteristics. They also offer the most safety and security. Of the operable units, there are many forms: awning, casement, hopper, horizontal slider, vertical slider (either single- or double-hung) and turn-and-tilt. There are two ways of sealing operable windows to minimize air leakage: with a compression or a sliding seal. Windows with compression seals are generally the more airtight of operable types and should be the window of choice whenever possible. Casement, awning, hopper and turn-and-tilt windows, for example, should have a closure/locking mechanism that pulls the unit tight against the seal. Make sure the gasket is a compression, neoprene rubber type. Doors are a little less complicated. They are either: solid, solid with an insulated core, solid with window(s), or solid with an insulated core and window(s). Patio doors operate like a large horizontal sliding window. Hinged French doors, with a solid centre post to close against, or rolling doors with a compression-fit like aircraft door, are more energy-efficient. Some materials reduce heat flow better than others. Solid wood doors, for example, are not as good as metal-clad, insulated core doors, depending on the style of door and insulation material used to fill it. Otherwise, doors have a frame, sill, optional glazing, and rough frame opening in a wall as do windows. Like windows, some doors are even installed in the frame and sill system while still at the factory. Glazing is the generic term for the transparent or sometimes translucent material in a window or door. A window may be divided into one or more sashes, some of which may move and others which may be fixed. For example, a double-hung window generally has two moveable sashes, while a single-hung window has just one moveable sash. A sash may be divided into two or more lights (panes of glass) held in place by mullions and muntins. When we speak of windows, we tend to use the terms single-, double- or triple-glazed. These terms simply refer to the number of panes of glass incorporated into the window unit: single-glazed, one pane; double-glazed, two panes; triple-glazed, three panes. All windows in Canada should be at least double-glazed. To determine the number of glazings in a particular window, hold a light next to the glass and count the reflections. In a double-glazed window, for example, you'll see two main reflections, corresponding to the number of glazings. If you look carefully at each reflection, there are actually two reflections very close together, bouncing off both surfaces of each pane. Most window manufacturers offer several types of glazing, which affect the insulation value of the window and the likelihood of condensation forming on the glass. Sometimes, transparent plastic films are placed inside the glazing unit between the panes of glass to increase energy efficiency. A variety of coatings on the glazing surfaces, plastic films, or inert gases between glazings, for example, can increase the insulating value of a double-glazed window to more than equal that of a standard triple-glazed window. Coatings are often used with gas fill. Most windows now incorporate sealed, insulated glazing (IG) units in which two or more glazing layers are sealed around the outside edge to prevent air or moisture from entering the air space, eliminating dirt and condensation between glazings. If moist air finds its way into the sealed air space, condensation may form between the glazings. This is usually caused by a faulty seal and cannot be corrected except by replacing the IG unit. If you look between the window panes in a conventional double-glazed window where the glass meets the frame, you will probably see a strip of material, known as a spacer. The purpose of spacers is to maintain a uniform separation between the panes of glass. Spacers have traditionally been made of hollow aluminum, containing a drying agent or desiccant designed to absorb the initial moisture present at the time of manufacture in the space between the glazings. Metal spacers conduct energy easily and are a significant source of heat loss and poor window performance. The best insulating spacers are often made from non-metallic materials. There are also hybrid spacers made out of metal and non-metallic materials. These materials do not conduct nearly as much heat. A window consists of a glazed unit, a frame and a sash. The glazed unit fits into the sash, and the frame holds the sash. The frame and sash may occupy as much as one third of the total window area. Both the frame and the sash can be major sources of heat loss. This heat loss is a result of conduction through the material. Heat loss can also result from air leakage, sometimes increased by expansion and contraction or warpage of a window's frame or sashes. Highly conductive materials used in the construction of the frame and sash must have thermal breaks incorporated in order to reduce heat loss. A large amount of heat loss through the sash and frame will result in the formation of condensation and frost on interior window surfaces. Frames and sashes are manufactured from a variety of materials; aluminum, fiberglass, vinyl, wood, and combinations of these materials. Each material has benefits and drawbacks in terms of insulating value, strength, durability, cost, aesthetics and maintenance requirements. Good quality windows can be made using any of these materials. Use the Energy Rating (ER) to compare energy performance. The ER for windows takes into account the thermal performance of the frame, sash and the glass. Check the following points on a full-sized window; some features may be visible only on a cutaway sample. Look carefully at the product literature and ask a sales representative to explain the features to you. • Check for strong, tight-fitting sealed joints to prevent air and water leakage. • For windows that require thermal breaks, inspect a corner cutaway sample of the frame and the sash. Ensure that the exterior and interior surfaces are separated by an insulating material (thermal break). • For larger and heavier windows, inquire if additional reinforcement is required and how this might affect the thermal performance. • The insulated glazing unit is held in the sash using various sealing methods to prevent water from entering the interface between the glass unit and the sash. Check the sealing system on both the interior and the exterior surfaces for an effective continuous seal. Trapped water in the sash is probably the greatest cause of failure of the glazing unit. Some manufacturers incorporate a drainage system into the sash; this feature allows any water that may otherwise be trapped to drain away. • A price and quality comparison is essential. Low-priced, poor quality windows are not a good, long-term investment. Energy-efficient, durable and low-maintenance windows will provide energy savings, increase comfort levels, reduce or eliminate condensation on the interior surface of windows and last longer. Aluminum frames are strong and durable. Aluminum maintains its strength, offers low maintenance and resists warping. Aluminum frames and sashes must be designed with thermal breaks to reduce conduction heat loss. Use the ER number to assess the effectiveness of the thermal break. Fiberglass frames rate highly in terms of energy performance. In some designs the hollow sections of the frame and sash are filled with foam insulation to further decrease heat loss. Fiberglass frames offer good structural strength and durability, and require minimal maintenance. Vinyl extruded frames provide good thermal performance, and are easy to maintain. Large vinyl windows may be reinforced to increase their strength; some reinforcing materials may increase conductive heat loss. Use the ER number to assess the effect of the reinforcing material on thermal performance. Thermally welded corners can prevent air and water leakage if the welds are continuous. Hollow sections of vinyl frames can be filled with foam insulation to improve thermal performance. Solid wood frames have a good insulating value and structural strength but require protection from the weather. Low-maintenance, factory-applied claddings and finishes are available. They must prevent water from becoming trapped behind them. Look for well-sealed corners as well as gaskets between the cladding and the glazing. Heat- conductive cladding materials should not touch the glazing or extend in from the outside of the window towards the interior. When installed in this manner, they can cause condensation problems and lower the thermal performance of the window. The combination window frame or sash combines two or more materials, such as aluminum, fiberglass, wood and vinyl. The objective is to obtain the best features of each material so that the frame and sash will provide good thermal performance, durability and strength, and require minimal maintenance. Combination windows use different materials separately in appropriate areas, while windows made out of composite materials have frames and sashes made out of materials such as fiberglass and wood that have been "blended" together through a manufacturing process. Composite materials essentially adopt the positive attributes of the materials from which they are made, in a single unified form. No matter what frame and sash materials you consider, remember that the ER number is your best indicator of window energy performance. Window hardware includes the hinges, casement cranks, handles, latch plates, etc., of operable units. The quality of hardware and hardware placement can affect the performance of the weather stripping. Weather changes can affect the durability of the hardware design, its attachment, and window members to which it is attached. During a cold winter, opening a window may cause problems if ice builds up, making it difficult to close the window tightly. Windows should use durable, flexible gaskets to make an airtight seal between the operating sash and the window sill and frame. An airtight seal is also needed between a door and its frame. The air tightness of the joint between operable sashes and a window frame or between a door and frame depends on the type of weather stripping used and the amount of pressure that can be applied on the joint. Compression seals, which can be squeezed tightly between the moving sash and the window sill and frame, and which are resilient over many years and not subject to cracking or other deterioration, are better than sliding seals with brush-type weather stripping. You may not be able to avoid brush-type seals, especially if you are selecting horizontal sliders. On this type of window, look for thick brush seals with a thin flexible plastic flange embedded in the brush to minimize air leakage. How Windows Perform Before making a decision about which windows to buy, it is useful to review how windows perform, in terms of how they allow a home to gain energy from the sun, and how they affect energy loss when the sun isn't shining. Factors Affecting Gains There are several factors affecting the ability of windows to capture solar heat. They include: 1) placement and orientation; 2) design of the window unit (and the amount of clear window opening);" 3) the type of glazing used; and 4) the amount of interior and exterior shading. Placement and Orientation Placement and orientation of the window with respect to the sun will be the primary determining factor affecting solar gains, although some gain is possible in all directions from diffuse sky radiation. During the winter, the sun's low elevation in the sky at midday enables it to shine through south-facing window. These solar gains can help reduce your heating costs during the winter. In the summer, when the sun is much higher at midday, very little sun actually strikes a south-facing window. And what sun does reach the window is at such a low angle that it reflects off the window. Awnings or a modest eave overhang can be used to shade south windows in the summer to minimize these unwanted heat gains even further. Properly placed, these shading devices shouldn't interfere with winter solar gains. Overheating in summer tends to occur more from un-shaded west-facing windows and, to a lesser extent, east windows. Well-placed deciduous trees will reduce summer overheating while permitting desirable winter solar gains. Window Design The design and heat gain factor of a window will also have a bearing on its ability to capture solar heat. A window with a wide frame and numerous small lights separated by mullions and muntins has less glazing area available to capture solar energy. By contrast, a window in the same rough opening with a thin frame and one large light will have a greater proportion of glass to frame area and so will allow more sunlight into the living space. The number of glazing layers will also affect solar gains. For example, a triple-glazed window with ordinary glass reduces solar gain by 20 percent compared to a single-glazed window with the same glazing area. A double-glazed unit reduces solar gain by about 10 percent. Glazing coatings and tints also make a difference. Clear glass transmits the most solar energy into a building. Tinted glass and glass with special insulating low-E coatings can reduce solar gains by up to one third. For example, a double-glazed window with a low-E coating on one glass surface may transmit up to 20 percent less solar heat to the interior, compared to a double-glazed window of similar area with standard glass. Different types of low-E coatings vary greatly in terms of their effect on solar gains. Some that are designed for southern climates are not appropriate for use in Canada. The shading of windows, either from interior drapes and curtains, or from exterior landscape elements such as trees, will also influence the amount of solar gain. On sunny days during the winter, keep the drapes open to admit as much solar gain as possible. Remember that the types of trees and shrubs you plant near your windows may affect the winter solar gain potential of the windows. Select deciduous trees with thin branching characteristics for southern exposures. They will provide shade in the summer but will lose their leaves in the fall and allow more sunlight through. Factors Affecting Heat Losses There are several processes at work which influence rates of heat loss through window components. These processes follow a basic law of nature: heat energy tends to move from warmer areas to colder areas. There is no way to get around this fundamental principle; all we can do is slow the processes down. The principal heat transfer processes in windows are radiation, conduction and convection. In addition, air leakage is responsible for a significant portion of heat loss. Radiation, Conduction and Convection Absorbed by the inside pane of a double-glazed window, heat moves to the cooler outside pane and is released to the outdoors. This heat loss through windows takes place through the glazing (by radiation);" across the spacer material which separates the two glazing layers at their edges and through the frame of the window (by conduction);" through the movement of air in the space between the two glazings (by convection);" and between the moveable or operable frame components (by air leakage). Radiation losses through the window glass represent about two thirds of the total heat loss in a standard window. Because ordinary glass readily emits heat to colder surfaces (i.e., has a high emissivity), radiation losses can be reduced by lowering the emissivity of the glass (hence the term low emissivity low-Eglass). Conduction losses in windows occur primarily through the edges and frames of the units. Advances in materials and designs that more effectively use insulating materials have dramatically reduced these losses. Convection losses occur due to air movement between the spaces of multi-glazed windows. If the space is too small, conduction through the air is significant. If the air space is too large, the still air will begin to rise as it is heated on the warm interior side, and fall as it is cooled on the cold exterior side of the window. This convection movement of the air passes heat to the exterior. The best spacing to minimize convection losses is 12 to 16 mm (one half to two thirds of an inch) between the glazings. Other gases (argon, krypton) are often used to reduce convection heat loss. Optimum spacing for these gases can be different. Air leakage is a significant contributor to energy costs during both heating and cooling seasons. Most of the air leakage of operable (i.e., open able) windows occurs between the window's sash and frame, or at the meeting rails of a sliding sash. Bigger windows tend to leak less air per area. Air leakage can also occur in poorly constructed fixed windows between the insulated glass unit and the frame. (Remember: even in these types of windows, holes are required to effectively drain rainwater.) Windows with the lowest leakage rates, regardless of type, tend to be fixed windows, that is, windows you can't open. Operable or openable windows come in many types. The operable windows with the least rates of air leakage are awning, casement and similar types with a closure mechanism which pulls the sash against a compression gasket. Air leakage can also be a big problem if the windows are poorly or carelessly installed in the rough opening. If the space between the outside perimeter of the window frame and the rough opening isn't sealed with either caulking or foam insulation, air will leak through it. This space should be insulated and sealed before the window trim is attached. Balancing Gains and Losses As we have seen, there is a great deal of two-way "traffic" passing in both directions through windows. South windows often gain more solar energy during the day than they lose at night through convection, radiation and conduction losses. North windows are usually net losers of energy, while east and west windows tend to be neutral during the heating season. However, during the summer, west windows may be net gainers of energy, posing an overheating problem. High-performance window technology is pointing the way to significant improvements in this balancing act between gains and losses; maximizing gains when needed, while at the same time minimizing heat transmission as never before. Condensation and Windows The occupants of a house rightly feel that condensation on the inside surfaces of windows is not good. They immediately think of obstructed visibility, reduction of the intensity of natural lighting and, above all, deterioration of interior finishes (rings, stains, peeling paint) and mould. Superficial condensation occurs when the surface temperature of a solid (glass, sash, and frame) is lower than the dew point of the humid air in its immediate vicinity. The moisture, naturally present in the air in the form of vapour, changes into liquid water on contact with these cold surfaces. The resulting droplets form a film of water and run down the glass when the condensation is heavy or does not evaporate fast enough. In the case of windows, condensation will often occur at the edge of the glazing because of conduction through the spacer and air convection within the glazing cavity. Such condensation can be decreased or eliminated by raising the inside surface temperature and/or decreasing the relative humidity of the indoor air. Prevention Techniques Reducing or eliminating condensation often means using several complementary techniques. These techniques concern the window itself, the method of installation, the interior window accessories (curtains, blinds, and valances), and the arrangement of heat sources (hot air registers, baseboard heating, convection heaters) and the relative humidity of the indoor air. The overall condensation resistance of a window depends on each of these factors. Window Installation To minimize condensation on windows, follow these guidelines during installation: • position the window as close as possible to the interior finish; • insulate the space between the window frame and the rough opening around the perimeter of the window; and • seal the joint between the frame and the rough opening on the interior side. Controlling the Relative Humidity of Indoor Air Replacing old windows with new, more efficient ones generally results in a significant improvement in the air tightness of the building enclosure, and thus substantial energy savings and improved comfort for the occupants. However, when living habits and the production of humidity in the house (showers, baths, cooking activities, plants, etc.) remain unchanged, and there is a significant reduction in the rate of air exchange, the resulting relative humidity at certain times during the winter may become excessive and cause condensation on the inside surface of new windows. Reducing the amount of humidity in the house may enable you to limit or eliminate the problems caused by humidity. Some simple but effective measures can be applied. • If your heating system is fitted with a humidifier, or if you use portable humidifiers, disconnect them. • Avoid hanging laundry inside to dry, and make sure that the exhaust from the clothes dryer is vented outside. • If you have a crawl space under your house, cover the beaten earth with 0.15 mm (6 mil) polyethylene. The crawl space may have to be ventilated during the summer. • Make sure that your basement is well drained and protected against excess moisture. Also, make sure that gutters and the slope of the land around the house drain water away from the house. • Try not to produce too much humidity. Plants, laundry, showers and cooking without lids are major sources of water vapour. • Avoid drying firewood in the house. A cord of wood can release more than 270 liters (60 gallons) of water. If the signs of excessive humidity persist, you should increase the ventilation of your house. When the frequency of condensation is low (once or twice during winter), you can reduce or eliminate the problem by briefly opening two windows located on opposite walls or by turning on the kitchen or bathroom exhaust fan. If the frequency of condensation is unacceptable, you should install a controlled mechanical ventilation system. Systems incorporating a heat recovery unit and a relative humidity control are preferred. Roger Frost is a professional home inspector for Barrie and Simcoe County in Ontario Canada. Visit his site at http://napoleon.cc the "Barrie Home Inspector".
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