Learn About Energy Efficient Windows

A window or doors U-Rating represents the insulation value of the total window unit against conductive heat flow. The lower the number the better. This type of rating applies to windows & doors, even in the shade. It can be 25° F or 115° F in the shade. The hot or cold air comes in direct contact with the glass and frame and then heats or chills them until that temperature transfers through. The speed in which that temperature transfers through is the U Factor.

When you touch the glass in the summer, even in the shade, it is warm. Hot and cold air are attracted to each other. Just like a hot front and a cold front, they rush together to cancel each other out, causing high winds and rain. The 55-degree air coming from your AC rushes straight toward the warm glass in the house instead of staying in the middle of the room keeping you cool. The AC is cooling the glass as fast as it can, but the heat is coming in faster than the AC can cool it.

 

Think of a bath tub. Now imagine the tub is your house.

The glass in your windows is the opening in the faucet that the water flows through.

The water that pours out of the faucet is the hot or cold temperature flowing into your home.

If you turn the faucet on full blast, it fills up the tub (your home) very fast. If you slow it down to a trickle, it would take forever to fill the tub (your home) As a matter of fact, by the time it filled the tub, the water wouldn’t even be hot. The heat would dissipate faster than it filled the tub.

The definition of insulation is how long it takes for heat to go from one side to the other. E.g. How fast is the water pouring out of the faucet.

A great U rating slows the heat or cold transference to a trickle so the AC can keep up with it without having to work so hard. Every window, whether in the sun or in the shade, is subject to this transfer 24 hours per day. A lot of people think you just need to change out the windows getting exposed to the sun.

Let’s say you have 10 holes ripped in all sides of the tent and you set up a small cooler in the middle of the tent to help keep cool.

Solar heat gain rating (SHG) is how you measure how much radiant heat or radiation turns into heat once it passes through the glass. In other words, how good of a solar screen the window is. If you put your hand 2 feet away from the glass inside the house while the sun is shining through the glass, the warmth you feel on your hand is radiant heat or SHG. This kind of heat only enters a window 3 or 4 hours a day when the sun is shining on it or reflecting off of something else. It heats up the carpet or blinds and then radiates into the house.

The lower the SHG rating the better. A lot of people think this is the only number that is important because of the Arizona sun, but the U Rating is also important because of the hot air contact with the glass.

The center of glass rating is the pure efficiency rating taken at one pin head point at the exact center of the glass. This number is calculated to notify window manufactures of the efficiency of the glass that can be installed in their windows so the manufacturer can determine the overall efficiency of their window with the glass installed. The overall efficiency of a window can only be determined by having the efficiency numbers of all components used—glass, frame, air infiltration, locks, sashes, etc.

You can use the best glass in an inefficient frame and still not have a window meet even the minimum requirements of ENERGY STAR. The problem is a lot of retail companies use center glass ratings to advertise their windows to make their windows sound more efficient instead of “Total Unit Calculations.” Think of it this way. If one homebuilder advertised the walls of their homes as averaging R-30 and another advertised their walls as R-100, because every 2 ft they had a 2×6 stud that at the center was an R-100, but in between the studs the walls were an R-15. As long as he specifies center of stud measurement when he says R-100, he is accurate, but he is misleading the consumer because the consumer does not understand the disclaimer. It is not the contractor’s fault if the customer doesn’t understand the terminology.

The result is the average consumer chooses the dishonest builder because his numbers sound better, even though they are not comparing apples to apples. Any window manufacture who uses center of glass ratings is trying to confuse the buyer because the only number that is accepted by the governing bodies as true are “Total Unit Calculation,” which is shown on the NFRC sticker that is on the window when it arrives.

The National Fenestration Rating Council is the only organization that rates all windows and doors at the exact same temperature, barometric pressure, and humidity.

There are some companies that offer independent tests other than NFRC numbers to verify the U-Rating and SHG of a window. Those tests may be done at different temperatures or conditions to yield more favorable numbers which again, is an attempt to avoid an apples to apples comparison.

The only numbers accepted by the government is NFRC, and this is the only official rating sticker that is allowed on the window when it is installed. Any other numbers will only be on advertisements to solicit business.