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Window Energy Efficiency 101

wood grain casement window installationThere are many things to consider when you need to replace your windows. “Energy-efficient” is a buzzword used all the time in the window industry, but what does that actually mean? What questions should you be asking when getting new windows, and what is just “noise” you should avoid?

Today in Windows 101, we are going to dive into “energy efficiency” and cover a wide range of topics, including:

  • U-value
  • CR number
  • ENERGY STAR® guidelines
  • SHGC
  • Surface 4

What To Look For in Windows

In Cleveland, you want to choose a window with the lowest U-value and the highest Condensation Resistance factor (CR).

In a nutshell:

  • U-value can be described as the “insulation level” — the lower, the better, the opposite of R-value.
  • CR is a number between 1 and 100 — the higher, the better. A higher CR will reduce the amount of condensation on your windows.

The best way to achieve the lowest U-value and highest CR is with triple-pane glass. You will spend a little more on triple-pane glass, but it offers many benefits beyond energy savings.

What the Window Numbers Mean, and What They Don’t

When talking about energy-efficient windows and doors, you have to understand the ENERGY STAR® program. These are guidelines on which the federal government rates our windows, such as U-value and Solar Heat Gain Coefficient (SHGC).

A specific Condensation Resistance factor isn’t required for ENERGY STAR but shouldn’t be overlooked for homeowners in a heating-dominated market like Cleveland metro.

SHGC is more important for homes in the southern U.S. or in cooling-dominated markets. If you have a room on the south or west part of your home where you spend evening hours and have to close your blinds because you get too uncomfortable, you will want to consider a solar control product.

Some window and door manufacturers prefer to play games. For example:

When ENERGY STAR guidelines were lowered a few years ago, many manufacturers could not qualify for ENERGY STAR with their current “dual-insulated glass products” (Dual IGU). They were not required to qualify with those products, but many of their competitors could qualify, and their products wouldn’t be competitive.

So instead of spending thousands of dollars and hours redesigning their products to qualify for Dual IGU, they incorporated “Interior surface low-E,” otherwise known as Surface 4 glass. These manufacturers could then qualify for ENERGY STAR with U-values many times lower than the .27 required. What they don’t tell you is their window has a condensation rating just 9% higher than clear glass.

Avoid Surface 4 Glass

Surface 4 glass, also known as I 89 glass, puts a low-E surface on the inside piece of glass you can touch. This low-E surface will bounce the heat back into your home, thus lowering the U-value. Your heat is bouncing off the glass and not warming the surface, so it remains considerably colder than a typical Dual IGU, which has the low-E on the inside of the air space.

Surface 4 or interior surface glass is BAD. Do not purchase windows that use it!  You will experience some if not all of the following if your window has Surface 4 or interior surface glass:

  • Excessive condensation on the inside of your windows, especially on cooler days.
    • This results in poorer views in the morning, but as the day continues, the sun will help
    • The excessive condensation will pool on your sills and damage your varnish
    • It could lead to your sills deteriorating over time
    • It could lead to mold and mildew growth
  • On extremely cold days or long cold spells, ice could form on the glass near your sills. Yes, ICE! On the coldest days, your glass can reach 32 degrees on the inside, and if that happens, you will have ICE.

Questions To Ask a Windows Salesperson

Ask your window salesperson the following:

  • Does this product offer Surface 4 or interior surface glass?
    • Is that what they are quoting you?
    • Does your window meet ENERGY STAR guidelines for the north?
  • What is the U-value and Condensation Resistance factor of the product they are quoting?

Remember that those window companies only use Surface 4 glass because they didn’t want to spend thousands of dollars and time to upgrade their products. Why would you want to buy an outdated and inferior product?

Contact us online if you have any questions, comments, or concerns. We love to help homeowners like you make wise purchasing decisions. 

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