Energy saving ratings for windows are discussed in different areas and presented in different ways. This article is intended to put into simple understanding how these different values affect your windows and their energy efficiency. In-depth technical explanations will not be offered in this section. Rather, it is intended that this section create an easily relatable understanding to the subject matter. This section will only deal with the ratings most important to energy efficiency of the windows.
It is a given that to have any sort of organization requires some structure. The structure of energy efficient windows and products is given through the rating system. Mandatory ratings and guidelines are set down by the National Fenestration Ratings Council to keep a uniform approach to products. Though it was founded by industry interests, it operates as an unbiased non-profit organization so that consumers can know the NFRC is not shilling for any particular company.
Heat loss rate is indicated by the U-Factor of the window. The lower the rating, the better insulator that the window is. The normally recognized rating by the NFRC is for the entire window. Some windows may have a "Center of Glass" U-Factor which only deals with the actual window panes rather than the entire assembly. High-quality triple-pane windows can have U-Factors as low as 0.15. Exceptional double-paned window will feature a U-Factor of about 0.30 or less.
What this means for energy savings is that this value will have an affect on the insulating qualities of the window. If you live in a hot or cold climate; this value will help keep the interior at a given temperature. It can be thought of much the same way as walls and their own insulating R-Value. In the past, windows have been portals to allowing easy energy transfer because they were nothing more than glass sheets in wood frames. With the many options available, a low U-Factor window can offer the same kind of insulating value that your walls have.
The SHGC is stated to be the rate that the window blocks heat from entering through the window from the exterior. This is a separate value from Visible Transmittance which is the light that is allowed through. The SHGC is expressed as a value between 0 and 1, with lower permitting less solar heat transfer.
This value is an important one to look at depending on the area you live in. For areas that feature cold climates, it is a good idea to have a higher SHGC to permit more natural sun to help provide warmth to your home. In areas that have higher heat, it would be better to have a lower SHGC to keep the sun's radiant energy out of the home. Otherwise one will need to run their air conditioning more often.
The final rating that is important to look at for energy efficiency is the window's Air Leakage. This value is expressed in how many cubic meters of air pass through a square foot of area on the window. So, what does that mean exactly? Everything is porous unless it is an airtight container. A certain amount of air will always permeate a given object unless it is sealed up tightly. Windows feature seals to help prevent this transfer in addition to coatings, multiple panes, and inert gasses to help further defeat it. Regardless, some air will always find its way through the windows. It could be through the glass or through the seams of the window.
A lower AL is the preferable rating for maximized energy efficiency. It means that less air is moving back and forth through the window itself which will make the interior easier to maintain. In hot weather, one will not be losing air conditioning to the great outdoors. In cold weather, your heat is more likely to stay right where it needs to be to help keep your home comfortable. This increased efficiency means less running of your air conditioner, fans, furnace, or space heaters in general.
Though there are other window ratings, these three are the most important to look at when it comes to energy savings for your replacement windows.