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Don’t Be Fooled by Surface 4 Glass

wooden windowWhen ENERGY STAR® guidelines were updated a few years ago, many manufacturers could no longer qualify for ENERGY STAR with their “dual insulated glass products” (Dual IGU). Of course, they were not required to qualify with those products, but many of their competitors could qualify and they would no longer be competitive.

So instead of spending thousands of dollars and hours redesigning their products to get their Dual IGU products to qualify, they incorporated “interior surface Low-E,” otherwise known as Surface 4 glass.

These manufacturers could now qualify for ENERGY STAR with a U-value many times lower than the required .27. However, they don’t tell you that their window has a condensation rating just 9% higher than clear glass.

What Is Surface 4 Glass?

Surface 4 or interior surface 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 heat back into your home, thus lowering the U-value.

However, your heat is bouncing off the window 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 #2 or #3).

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 leads to 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.
    • Your sills could deteriorate over time.
    • You could see mold and mildew growth.
  • On extremely cold days or long cold spells, ice could actually form on the glass near your sills. Yes, ICE! It is possible on the coldest days for your glass to get to 32 degrees on the inside, and if that happens, you will have ICE.

It’s important to know this isn’t something you could claim under warranty. The window has a condensation resistance factor number, which isn’t required by ENERGY STAR or even to appear on the NFRC label.

Remember that most S4 products come with a CR number under 50, which is only 9% better than clear glass.

The Window Professor’s Take

In summary, The Window Professor says, “Please don’t buy a product from a window supplier who is offering you a product with surface 4 or interior surface glass for your home in Cleveland metro, which is a heating-dominated market.”

Ask your window salesperson these questions about the window they are offering you:

  1. Does it meet the current ENERGY STAR guidelines for Northeast Ohio?
  2. Does it incorporate Surface 4 or interior surface glass?
  3. Do they know the U-value and CR number of the window?
  • U-value of .27 or lower = Good.
  • CR number of 50 or lower = Bad. This is Surface 4 glass.
  • Clear glass has a U-value of .46 but a CR of 43.

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

Check out an article that an industry friend of ours wrote several years ago on this topic.

Contact us today with any questions, comments, or concerns. We love helping homeowners like you make wise window purchasing decisions. 

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