Replacing Windows: Options and Features
I love homes with lots of windows. They feel open and bright. But old windows are terribly inefficient. By replacing your windows, you can cut heat loss in half. Here are new window options and features you’ll want to know about.
R-Values, U-Values and SHGC
R-Value: R-value is a measure of a materials ability to resist heat flow. Higher R-values mean better insulation. The less heat lost, the more efficient your windows. This value is based on the center of the window and is lower around the edges. R-values depend on several factors like number of glass panes, size of the air pocket between, glazes used and window frame qualities.
U-Value: U-value indicates how quickly the window will lose heat (transfer energy) due to the indoor and outdoor temperature. Lower U-values mean less heat loss. U-values lower than 0.35 are considered energy efficient.
SHGC Coefficient: SHGC refers to the solar heat gain coefficient. This is a measure of solar radiation admitted through a window. SHGC ranges from 0 to 1. Look for an SHGC rating of 0.4 or lower in hot climates and 0.55 and higher for cold climates. Swing climates should be in-between.
Multiple Glass Panes
Double and triple pane windows are much more efficient. The space between the the two panes can substantially reduce heat transfer. This is achieved by measuring the optimal space between the glass panes to increase R-value. And filling that space with an invisible, inert gas such as Argon. Argon is slow moving, non-reactive gas and great for insulation. The panes are separated by a spacer. Metal (Aluminum) spacers can act as a heat bridge. Spacers composed of less conductive materials such as plastic, foam and rubber are much more efficient.
Energy Efficient Window Glazes
Special glazes can be applied to windows to boost efficiency even higher.
Low-E coating: Low-E refers to a thin metallic coating that will increase insulation and reflect heat. This will reflect solar heat away during summer months. And reduce heat loss during winter months.
UV block: Windows can also be coated to block UV light. This is important as sun light can fade carpet, furniture and more. Check out Todd’s post about sunlight fading his wood floor. Coated windows can block around 75% of UV energy.
Insulated Window Frames
Energy Efficiency shouldn’t stop at the window frame. Frame material and construction is just as important as the window itself.
Wood Frames: Wood frames provide good insulation with an approximate U-value of 0.4. However, they require some maintenance. Wood frames need to be painted or stained. Wood is susceptible to rot, termite and becoming warped. Hybirds, like vinyl clad wooden frames eliminate some problems.
Vinyl Frames: Vinyl frames are a great option. Vinyl is durable and also non-conductive (U-value around 0.4). No need to worry about insects, rot or painting vinyl frames. The only complaint is that vinyl can fad over time. Insulated vinyl frames can be pricey but will drop the U-value to around 0.2.
Aluminum Frames: Aluminum frames are not as popular anymore as they are poor insulators with U-values around 2. However, Aluminum is durable and requires little maintenance.
All this information should be considered specifically for your climate zone. To help determine what factors are most important use this handy Climate Zone Finder from the Energy Star site.
What do you think? Was this article helpful? Have you replaced your windows?
Image courtesy of Christy Gordon
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August 8th, 2008 8:45 am
There are reasons to replace old windows, but there may also be reasons to keep them as well. I personally prefer them, and I think that, while they do take maintenance, with care they actually last longer, and can be made relatively energy efficient.
August 8th, 2008 9:29 am
I, too, prefer old windows– a lot of ours still have wavy glass that can never be replaced. But a little more energy efficiency would be nice on the pocket book! I recently came across an article in which Bob Vila says, if I remember correctly, that old windows with wood storms can be almost as energy efficient as new windows. Because wood conducts neither heat nor cold, the wood storms are ideal, if maintenance hogs. One of my long-term projects for the house is wooden interior storm windows; some of ours are casements that open out, and the old aluminum exterior windows keep that from happening at all.
If I had a newer house, I think I’d happily put in new windows. Unfortunately, the cost of using ones that actually look age appropriate for a house like ours is prohibitive. I’m thinking some good old caulk and a few weekends with the table saw and router, and I can come up with some good new storms…
August 8th, 2008 10:54 am
R and U-values are the same thing. They are just the inverse of each other U-value=1/R. You calculate them the same way. Things like the amount of wind and the still thin air “film” on the interior are also taken into account when you calculate the U-values. Each product has a summer and a winter U-value so you also have to make sure you know which one the manufacturer is specifying (they should be using the winter U-value but some of them are sneaky and don’t). As you mentioned there are different center and edge-of-glass values. The edge-of-glass is actually really important because that is where you loose the most heat and where you get the most condensation as a result. The SHGC & shading coefficient are something you really need to look at if you live in a sunny climate. If you don’t I would spend the money on having a low SHGC on the windows that get the most sun and a slightly higher SHGC on the rest.
We are hoping to replace all of our windows by the time we’re done remodeling. Right now we have original 35 year old single pane energy hogs and each window is 6′x4′. They are completely warped because of their plasma state and many of them don’t open anymore. Plus, most of the storms and screens don’t fit right so they are home to a lot of insects including a wasp nest! We just bought a new patio door so hopefully that will help keep our basement a little warmer this winter.
One other thing I wanted to add is that if at all possible get non-opening windows. We have 4 huge windows in the dining room with the two on the sides operable. We don’t need all of them to open and by doing so we save a lot of energy.
Oh, and to answer your floor question on Todd’s blog. All floors will fade but cherry is by far the most light sensitive wood you can use (excluding some rare exotic woods). Now that cherry has become popular again a lot of people are discovering this and upset. I guess they should have done their homework.
August 8th, 2008 3:16 pm
The cherry floor doesn’t fade..it actually gets darker at the tannins oxidize….just a clarification. Nice post!
August 9th, 2008 8:42 am
@Amalie, Your right about wood being a great non-conductive material. And if your up for the maintenance, they’ll look great. A little caulk work can go a long way to seal up air leaks too.
@Robin, R and U values aren’t quite the “same” thing. They are inversely related as you point out ( U = 1/R). But R values are a measure of insulation, while U values are a measure of heat loss.
That is a great tip about getting windows that don’t open. You’ll loose most of your heat at the seal, so eliminate it if possible. Thanks!
August 12th, 2008 10:41 pm
I was dead-set against replacing all of my old half-rotten windows. I was going to rebuild all 18 of the monsters, I managed to get two of them done. Even with storm windows, they were somewhat drafty and cold. Then I was going to build new storms and screens for all of them. One day I did a reality check and had new double-hung, double pane vinyls installed and I’ll never look back.
I absolutely understand those who want to keep their old windows, I really do and I say go for it. However, the ease of maintenance that comes with vinyl, not to mention added savings due to no more leaky windows, can’t be beat. I could not see myself in 30 years having to climb up a ladder and hoist some big heavy storm window or screen onto every 2nd floor window. I can stand inside my house and completely wash them or replace a screen. If glass gets broken, I quickly remove the entire sash, go to the local window company and they make up a replacement for about $20. I bring it home and click it into place, done!
If something large is going to be brought into the upper floor of the house via lift outside, both sashes come out and I have an opening big enough to accommodate whatever it is.
Vinyl can be made in any size, there are so many different styles and configurations too so you don’t have to lose the personality of your house because they are vinyl. I know I sound like a saleswoman, but I used to look down my nose at replacement windows. Now I sing their praises.
They aren’t for everyone, but you shouldn’t feel bad if this is the route you wish to take.