According to US DOE EERE (US Department of energy, energy efficiency and renewable energy), inefficient windows, doors and skylights are responsible for over 25% of heating and cooling cost in a typical home.
Windows are an essential and expensive component of home construction that ensures ventilation and fresh air to perimeter rooms but also keep out wind, rain or snow, allow daylight to brighten the interior of the house and with good design will enhance the aesthetic beauty of your home.
Basic terminology:
The frame is the main structural element of a window. Aluminum frames are all but long gone from the market because they readily conduct heat and cold making them very inefficient. Metal clad wood windows, vinyl and fiberglass are the main choice available today.
The sash is the moving part of a window.
The U-value measures the rate of heat transfer through an element, the lower the number the more efficient the window is.
The R-value measures the thermal resistance of an element in the construction industry and is the reciprocal of the U-value. The bigger the number, the better the insulation effectiveness is. Using the International System of Units (SI) R= 1/U. The R-value in North America uses the imperial unit system and requires a conversion factor to turn it into (SI) R-value.
The solar heat gain coefficient (SHGC) is a fraction that shows how much of the sunlight hitting a window gets inside the room as heat. For cold climates the highest SHGC is best, in temperate climates a coefficient of 0.55 to .40 is ideal and for hot climates look for 0.40 or less.
Visible transmittance (VT) measures how much visible light get through the window. The higher the VT, the more exterior light you get inside.
Window types commonly available on the market:
CASEMENT windows are hinged at the side and crank out to open. They can be positioned to catch passing breeze. Due to its design, there are size limitations to individual casement windows.
AWNING windows are hinged at the top and crank or swing outward. They can be left open when it is raining because they deflect rain during storms. They are a good option for controlled natural ventilation.
HOPPER windows are opposite of awnings and are hinged at bottom. They open usually inward or sometimes outward when installed above a door or another window protected by eaves. They are also commonly used in basements.
Those three types of windows are energy efficient due to the window sash pressing against the frame on closing and thus are high performing in terms of air leakage. Eventual insect screens are placed on the inside. Opening and closing can be slightly more difficult than sliding and hung windows because most of them use cranks that must be operated manually. They also are more difficult to clean on the outside due to accessibility of exterior side of glass panel.
FIXED windows are used where lighting but not ventilation is important. They are airtight and thus most efficient as they do not open.
SINGLE HUNG windows open from the bottom only.
DOUBLE HUNG windows have a top and bottom sliding sash allowing warm air to escape at the top and cold air to enter the room through the bottom opening. HUNG windows have a traditional look that makes them popular. Only half the glass area can open. Screen installation is easy. They are not energy efficient and are more prone to leakage.
SINGLE SLIDER windows only have one sash that slides horizontally.
DOUBLE SLIDERS (GLIDING) have two horizontally sliding sash.
Sliders are easy to open, close, clean, less obstructive and can be used in big sizes. Screens installation is simple on the outside of the window. However only half the area can be opened and they lack the compression seals thus they are not energy efficient compared to Casement, Hopper and Awning types.
Consumer Reports rates highest for metal clad wood windows and fiberglass windows but they are about twice more expensive than Vinyl windows.
Vinyl windows are inexpensive and convenient but tend to leak air and water more readily in cold climate. They do not have the same visual appeal as wood and fiberglass which can be stained or painted.
Window Performance is defined by testing for air and water leakage, wind resistance and resistance to forced entry, condensation resistance, screen strength and ease of operation.
Choose your windows to match your local climate.
Check warranty given by window manufacturer and compare.
GLASS PANES:
The type of glass used is crucial for boosting energy efficiency in windows, glazed doors or skylights.
The energy efficiency of a glass pane is measured by the U-factor or R-value and SHGC coefficient.
Single clear glass is highly inefficient by causing too much energy to be lost in winter and too much sunlight to enter in summer.
Single clear glass U-value is 5.4 R-value is R-1
Ordinary double glazing U-value is 2.7 R-value is R-2
Low E double glass U-value is 1.8 R-value is R-3
Low E with argon gas U-value is 1.6 R-value is R-3.4
(All values are approximations, based on average of available results from different source tests)
Double glazing can halve heat loss thru windows.
The optimum space between two panes is about ¾” or 2cm.
Double glazing using Low-E glass rates equally to normal triple glazing without the 50% increase in weight.
Low-E glass or low emissivity glass is best for cold and temperate climate. It involves the application of silver, zinc or tin coat to glass in a vacuum. This coating highly reduces heat loss by reflecting heat back to the inside or outside source.
In hot climates, where cooling costs are higher than heating costs, lowest solar heat gain coefficient (SHGC) is the goal.
To obtain this, solar control glass called Low-E2 or spectrally selective glass is the best option to block infrared and some ultraviolet rays.
Energy saving is further improved if an inert gas (Argon) is enclosed between the double pane glass instead of air as long as the gas is not allowed to escape through a leak.
Using Low-E glass reduces heating bills, carbon dioxide emission and also reduces condensation on your window frames internally.
ENERGY EFFICIENT DOORS
In colder climates windows, doors and skylights are selected to keep heat inside.
In temperate climates they are designed for heating in winter and cooling in summer.
In warmer climates they are selected to keep heat outside.
Fiberglass and steel entry doors with well insulated polyurethane panels with R-value ranging from R-5 to R-6 are much more efficient than wood doors or hollow panels construction.
Low-E coating glass for colder climates and Low-E2 glass for hot climates boost the energy efficiency of glazed doors.
Magnetic strip as a door seal is crucial to control air leaks.
Regularly check the seals and caulking around the door to ensure air tight condition when closed. Use foam caulking to seal the door frame to the threshold and rough opening.
Sliding glass doors are not energy efficient as the weather stripping eventually wears down and air leakage occurs.
Swinging glass doors is the best option to save on heating and cooling costs.
Screen doors are great to create natural ventilation and keep the bugs out.
Insulated garage door is a must to improve your house energy efficiency, if the garage is attached to the house.
Storm doors are secondary doors that fit over the primary exterior door. They can reduce air leakage and create an insulated air space in between the main door and this secondary door.
However a storm door does not significantly improve the energy efficiency of a well insulated door.
SKYLIGHTS and LIGHT TUBES
Skylights are mainly used for day-lighting interior rooms and at times they are used for ventilation and views. They are horizontal windows placed at the roof plane. For maximum efficiency, they are better located so to flood a light color interior wall rather than at the center of a room. It is crucial to use Low-E or Low-E2 glass
(Depending on climate) to somewhat control the solar heat in summer and heat loss in winter. Using interior shade or openable glass also helps to avoid heat collection in the skylight well.
Skylights are not energy efficient as they create a solar furnace in summer and then lose more solar gain than it captures in winter.
Skylights transmit direct sunlight with UV ray that damage carpet, flooring and furniture. They are not appropriate for low ceilings and can be a security concern.
Use only latest technology skylights to avoid air and water leakage, condensation problems. Professional careful installation is recommended.
Light Tubes or solar tubes are placed on the roof and transport and distribute daylight with much flexibility to specific areas in the home using reflective tube lining material. Because they have less exposed area they do not allow as much heat transfer as skylights do, however the visual contact with the exterior environment is eliminated.
Light transmission efficiency is best with short straight tubes as longer, angled tubes may lose part of the light intensity.
To maximize the use of solar or moon light, install a heliostat that tracks the sun or moon movement in the sky at the top of the tube within a dome or cupola and use a diffuser at the bottom inside end to spread the light in the room.
Fiber-scope (optical fiber) can also be used for day-lighting in a building. However it needs a parabolic collector to track the sun and concentrate the light in the fiber core.
