I’ve talked about the Different Types of Glass before and their relative energy efficiency but I wanted to dig into Low-E glass a little today so you can see if it is right for you. I get a lot of questions about making old windows energy efficient and when the temperatures start to dip the questions ratchet up.
Low-E stands for “low emissivity” and has become a standard material for windows and doors since it first arrived on the scene in earnest in the 1980s. Emissivity refers to the amount of heat that the glass is able to emit. So, a low emissivity glass will emit less heat than one with high emissivity. It works by blocking certain wavelengths of light but allowing others through. The sun’s light comes in a few forms and they each have a unique range of wavelengths.
- Ultraviolet Light (UV) Wavelength: 310-380
- Visible Light (VL) Wavelength: 380-780
- Infrared Light (IR) Wavelength: above 780
Low-E glass attempts to block high amounts of UV and IR light while allowing as much Visible Light in as possible. I could get very science geeky here but Low-E glass is basically good at preventing heat from passing through it. In the summer it keeps the heat from getting in and in the winter it keeps the heat from getting out.
The Low-E coating is a micro-thin layer of reflective materials like tin or silver that is applied to the surface of the glass. So knowing which side is coated is imperative to good performance. In single-paned applications you always want the Low-E coating toward the interior of the building to protect it from hazing and premature wear.
In double-paned windows, the Low-E coating is typically applied to the sides of the glass that face each other in the air space so that it is protected from any exposure at all.
You may hear your glass supplier talk about soft-coat and hard-coat Low-E and be wondering which one you need. Soft-coat is usually more effective at blocking heat than hard-coat but it can only be used in double-paned applications. Hard-coat Low-E can be used in single-paned applications and is still very effective for all but the most southern climates. Even if you use a hard-coat Low-E in south Florida or Phoenix you will still enjoy the benefits of decreased heat transfer.
Is Low-E Glass Right For You?
Low-E has some definite benefits whether you live in a hot climate or a cold climate. It is used heavily in double-paned glass applications which if you’ve been a reader for long you know I am not a fan of due to their short lifespan and high failure rate. However, it can be used very effectively in single-paned form as well so that’s what I’ll be focusing on here today.
Low-E Storm Windows
I believe that in historic buildings there are two ways for Low-E glass to be used effectively. The most effective way to use Low-E glass in a historic window is by adding an exterior storm window with Low-E glass. You’ll need the hard-coat Low-E glass for this application and it can be very effective at upping the efficiency of your windows, often even beyond that of new replacement windows with double-paned Low-E!
You don’t necessarily need to build new storm windows. It is often just a matter of swapping out the existing glass with a Low-E substitute and for a very minor cost you have a big gain in efficiency. Easy improvement!
Low-E in Sash
If you don’t have storm windows or your windows won’t accommodate them like with historic steel windows then the next best option is to replace the glass in the windows themselves. I will preface this by saying that I am a huge fan of saving and preserving historic wavy glass. I don’t want it broken out or even swapped if possible, but if it’s a matter or trashing your windows or swapping the glass I see this option as a win because it saves historic windows.
You’ll want a hard-coat Low-E glass for this just like the storm windows and installing it is just a matter of digging out the old putty and glazing points and swapping the glass then re-glazing the window. Here’s a tutorial focused on replacing glass in steel windows.
The last option is probably the cheapest and easiest. Applying a tinted film to your window requires very little work and can be very effective (especially for the cost!). Finding a tint that blocks the heat and doesn’t block too much of the visible light is really the key. Gila makes a good and DIY friendly window film I have use before you can find here.
Make sure you apply your window film on the inside of the glass otherwise your tint will have a very short lifespan.
Remember, Low-E is not just for the summer, it is extremely effective at keeping heat in in the winter too so this is really an anytime of year project. Protect you furnishing from the fading effects of UV light and keep your house warmer in the winter and cooler in the summer, just keep those original historic windows and do it the smart way!
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I love old houses, working with my hands, and teaching others the excitment of doing it yourself! Everything is teachable if you only give it the chance.