Windows and doors
Not the number one area of heat and cool loss
According to Informed Energy Decisions in some areas, such as the Chicago area "windows account for about 20% of heat loss and 25% of summer heat gain in houses. Replacing windows can reduce some of the unwanted loss or gain." After you have investigated the real source of your efficiency issue or your windows have proven to be old and inefficient, new windows and storm doors might help you considerably. When considering new windows and doors identify the energy efficient terms below.
Energy performance ratings
U-factor -This measures the rate of heat loss by the window assembly. The lower the U-value, the greater a window's resistance to heat flow and the better its insulating value. More
R-value -The insulating value is indicated by the R-value which is the inverse of the U-value.
Solar heat gain coefficient - The SHGC "measures how well a product blocks heat caused by sunlight. The SHGC is the fraction of incident solar radiation admitted through a window (both directly transmitted and absorbed) and subsequently released inward. SHGC is expressed as a number between 0 and 1. The lower a window's solar heat gain coefficient, the less solar heat it transmits." More
Air Leakage -An air leakage rating is expressed as "the equivalent cubic feet of air passing through a square foot of window area (cfm/sq ft). Heat loss and gain occur by infiltration through cracks in the window assembly. The lower the AL, the less air will pass through cracks in the window assembly." Select windows with air leakage ratings of 0.3 cubic feet per minute or less.
Visible Transmittance - This is the amount of sunlight coming through a window. VT is expressed as a number between 0 and 1. The higher the VT, the more light is transmitted.
Condensation Resistance - This measures the resist of the window to form condensation on the interior. The higher the CR rating, the better that product is at resisting condensation formation. CR is expressed as a number between 0 and 100.
Window features and things to look for when shopping
Choose windows based on the climate in your region - In warmer climates
where summer heat gain is high, look for windows with double glazing and low-E coatings that reduce heat gain. In colder climates
choose a U-value of 0.35 or below and double or triple glazing and a low-e coating.
Triple glazing (three panes of glass) - These windows can increase energy savings and comfort over a double pane, low-E window but it also increases the length of the payback period. More
Low-emittance (Low-E) coating - "microscopically thin, virtually invisible, metal or metallic oxide layers deposited on a window or skylight glazing surface primarily to reduce the U-factor by suppressing radiative heat flow. The principal mechanism of heat transfer in multi layer glazing is thermal radiation from a warm pane of glass to a cooler pane. Coating a glass surface with a low-emittance material and facing that coating into the gap between the glass layers blocks a significant amount of this radiant heat transfer, thus lowering the total heat flow through the window. Low-E coatings are transparent to visible light. Different types of Low-E coatings have been designed to allow for high solar gain, moderate solar gain, or low solar gain."Efficient Windows.org
Storm doors - Adding storm doors to existing exterior doors helps can help to reduce energy costs. If your main exterior door is of high quality and energy efficient features adding a storm door might not be helpful. But if the main door is of good quality but is older, leaks a little or has worn weather sealant it might be beneficial to reseal the door and add a storm door. Storm doors are convenient in warmer temperature as well and will help with air flow in the house.
