Selecting Energy Efficient Windows for your New Home
For folks building a new home, the job of selecting the Energy Efficient windows can be daunting. Based on the nature of the contract, the architect may or may not be involved in the window selection. Their contract may end with assisting the home owner with obtaining the permit. In such cases the homeowners may rely on the general contractor to guide them or ask friends who may have been through the experience before, in addition to researching on the internet. My goal here is to provide a summary of things to watch out for when selecting windows – a one-stop-shop blog that has all you need to know without getting too detailed.
First and foremost, it’s important to understand that every window is essentially a hole in your wall insulation. Each time you cut open the wall for a window, the insulating value of your wall drops from at least R19 to about R1 (‘R’ is the measure of thermal resistance). So it’s important to select the most energy efficient windows you can afford, not just to help your utility bills, but for your thermal comfort. With climate change in our future, we cannot always expect low energy prices and a reliable power supply. Stormy weather and disasters like earthquakes can cause long term blackouts. During times like these, you will be much more comfortable in your home if you have energy efficient windows, since they have such a huge impact on the heat loss and gain through the walls of your home.
Energy Star Windows
Look for the energy star label on the window. This will enable an apple-to-apples comparison between all the windows you are considering. The values to look for on the label, are whole window “U value”, “SHGC” and “VT” or “VLT”. “U Value” denotes the rate of heat transfer through the whole window (averaging for glass and frame). “SHGC” is the ratio of the solar heat transmitted through the window to the heat incident on it. VT is visible transmittance and VLT is visible light transmittance. Either of these may be used and they both mean the same thing. They denote the ratio of sunlight transmitted through the window to the amount incident on it. It can be expressed as a ratio or a percentage.
Here in the San Francisco Bay Area the recommended values to target for the windows are U value in a range of 0.20 to 0.30, the lower the better. For SHGC, recommended values are in the range of 0.30 to 0.40, again the lower the better. VT or VLT values are best kept around 0.50 to 0.60 or 50 to 60. Below 50, the window will start to feel a little dark and below 45 it will be noticeably so.
Do not make the mistake of thinking that the more sunlight streaming in (i.e. higher VT) the better. Remember that when you let in sunlight, you also let in solar heat. While this may not be a problem on the north side of your home, it’s a problem pretty much everywhere else. The most critical orientations are the South and West when selecting energy efficient windows.
Energy Efficient Windows for the South and West Sides
Unless your home has been designed with awnings over the windows angled correctly to keep the summer sun out and let the winter sun in, (which is rare), you had better select the best possible glass for your windows on the S and W sides. For these 2 sides, its best to select “spectrally selective” glass for your energy efficient windows. Spectrally selective glazing incorporates an additional layer of low-e film (usually on the inside face of the exterior glass pane or the outside face of the interior glass pane). This additional layer keeps out not just the UV but also most of the infrared spectrum letting in mostly visible light, so you can have your cake and eat it too! This marvelous glazing also offers a fairly high VLT of 70+. Low-e 366 by Cardinal is an example of a high performance spectrally selective glass. The other 2 sides – East and North can have “regular” low-e coating and that will work fine.
While triple glazed windows are an overkill in our climate here (unless your home is aiming for net-zero energy or passive house certification) dual glazed is a must and Argon gas fill (between the glass panes) in lieu of an air gap is also a cost effective upgrade.
The 2 panes of glass should be separated by a “warm edge spacer” made of rubber or silicon (as opposed to conductive metals like stainless steel or aluminum). Window frame choices range from vinyl, fiberglass, wood to wood clad with aluminum. There are other choices too, but these are the most common ones. Hollow vinyl is the cheapest and very energy efficient. Often times the vinyl extrusion is filled with foam making it even more energy efficient. But durability and a high coefficient of thermal expansion are its Achilles heels. Fiberglass addresses both these issues, being very durable and energy efficient. However, fiberglass windows are harder to come by and do not offer the wide range of frame styles that can be found with wood and wood-clad windows. Hence wood-clad windows tend to be very popular as they offer a good balance between cost, durability, energy efficiency and aesthetics.
The normal warranty on the glazing tends to be 10 years and the frame carries a 20 year warranty.
I would strongly recommend sacrificing some aesthetics and opting for windows that are not “truly divided”. When the glass is “truly divided” the glass pane is broken up into many individual panes for to give a “paneled” effect. Whereas when the divided look is simulated, the glass pane remains intact and the dividers are affixed to the surface of the glass. The simulated look results in a window unit that is much more energy efficient and longer lasting, while also costing less. Opt for window units where the dividers are affixed to the outer surface rather than being located in the space between the dual glazing.
My final tip for selecting energy efficient windows is to not make a window operable unless it needs to be. Fixed windows are much cheaper and more efficient than operables. Amongst operables, casements are more efficient than single hung which in turn are more efficient than double hung and sliders.
Keep these tips in mind and you can expect an energy efficient AND comfortable home!
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