Archive for January, 2010
With so many options, ratings and technologies out there, picking a window can be very confusing. Add to that, you will have your own criteria for look, comfort, energy savings and of course price. However, there are many resources out there that can make this quite easy for you. We’ll list them here:
• Energy efficient windows of any type are going to be more expensive than regular windows. This is something you will need to be prepared for. However, this one-time expense really pays off, and even your remodelling contractor will agree. Energy efficient windows can lead to money savings of 7-24% (acc. To the EPA) and in some climates, upto 40% annually. It is proven that in 2-3 years, energy efficient windows pay off for themselves.
• The kind of windows you will need depends largely on the climate in your region. Your climate impacts your heating and cooling requirements, your house design and your energy bills. In order that your windows provide you with an optimal solution, they need to keep comfortable as high as possible while keeping cost as low. Thankfully, you don’t have to do the math. This neat window selection tool on the efficient window collaborative website gives you how much you can save on different window types depending on where you live. Try it out here: http://www.efficientwindows.org/selection.cfm.
• As obvious, go for windows with the Energy Star rating or the NFRC (National Fenestration Rating Council) logo. Starting 2010, EPA has come up with improved energy star ratings for windows, which can get you tax credit of over $1,500. Make sure you check for a red label marked ‘Qualified until March 31, 2010’ which means the window was made according to the old rating system. The NFRC ratings will give you the values for the five criteria mentioned in our first post in this series and are also very reliable.
Sometimes getting the U-Factor, SHGC and VT values right is not enough. Often, the installation of the window itself, i.e. the entire window assembly can have flaws. These flaws result in air leakage – one of the main obstacles in trying to achieve energy efficiency with the help of windows.
If your house is built on the Passivhaus principles; or you are looking at seriously reducing your HVAC requirements (and thus the bill) then air leakages through windows can punch a large through your plan. Some energy efficient windows specify the AL value (expressed in cubic feet of air passing through for every square foot of the window assembly) between 0-1. The lower the value, the lesser the leakage. The industry-accepted value is 0.30.
Another problem for insulation is condensation. If you live in cold areas, you’ve probably seem frost or dew on the edges of the windows. Looks quite ‘Christmassy’ but it only means that much more heating load. Condensation happens when inside warm air comes into contact with cold panes and glass, which essentially means your windows are not doing such a good job insulating.
With condensation and air leakage, there aren’t fixed ratings to go by. However, certain materials and window technologies work better. For example, if you’re opting for multiple pane windows, looks for stainless steel spacers, or thermally improved spacers made from silicon foam or butyl tape. These spacers might be more expensive, but they are better are insulating, sealing and thus preventing unwanted heat transfer, while at the same time allowing from seasonal expansions and contractions in panes. Similarly, opting for energy efficient windows with two or more panes or glazing reduces condensation even at indoor humidity levels of 45-60%. This is especially true of windows with argon/krypton glass filled windows.
It is very evident that the larger your window, the more natural light it will bring in. Depending on the positioning of your windows, and its visible transmittance (VT) value, your windows could provide you with enough natural light to greatly minimize if not eliminate electric lighting requirements during daytime.
VT values are generally given between 0-1. A rating of over 0.70 is the highest you can get with clear glass windows devoid of any coatings or tints. The more the number of panes, coatings and tints the lower the VT value.
A high Visible Transmittance has another advantage – it eliminates the cooling function which is usually required in a house that uses electric lighting. A high VT value is especially important when a home is built using passive house principles or design.
However, one thing to consider when it comes to VT, is that a window with a high VT value, will also admit that much more solar energy (heat) inside the room. Thus, a clear glass window with a VT of 0.8 will also have an SHGC of over 0.60 – something not at all desirable in a warm or sunny climate.
To get around this, one can use Low E coatings, which are invisible, and yet block heat gain considerably. These coatings also protect indoor furniture and upholstery from the damaging effects of UV rays. Another way is to have operable windows that you can open up to air the room out frequently. Needless to say, a good ventilation system is important when you have large windows and a sunny climate.
For windows that are low on SHGC and high on VT, look for a high Light-to-Solar gain ratio. This determines how much more light is let in without adding to the heat.
There are two ways that windows help in heating and cooling – by preventing heat loss from the room, and by taking in heat from the outside. The former is measured by a window’s U-Factor and the latter by SHGC. U-Factor measures how much heat is lost through the window. The actual math gets quite complicated – U-Factor tells you how much heat is lost in one hour for one square foot at a certain temperature. A low U-Factor value (usually of 0.35 or lesser) means less loss and thus better insulation. Conversely, some windows may even mention an R-value, which is a measure of insulation. A low U-value corresponds to a high R-value.
SHGC is a value between 0-1. The lower the value, the less the amount of heat a window admits. Depending on the climate and your heating/cooling requirements you will need to pick a window with the right combination of SHGC and U-value. In cold climates, you want a U-value lesser than 0.35 combined with a high SHGC of around 0.60 or higher. This will considerable reduce the load on your heating system. For warm climates, you want the opposite – a low SHGC of less than 0.40 with a high U-value.
Does this mean you will have to change windows to match the seasons? Not necessarily. A metallic oxide layer (called a low E coating) applied on the outside keep the heat out, lowering the SHGC and when applied on the inside decreases the U-value. Even tints work well, though they only reduce the SHGC value.
Does material matter? Very much so, windows made of wood are not very good on insulation, while vinyl and fibreglass frames have the lowest U-values. Metal should be your last option when looking for insulation.
No, we are not switching to promoting green software – we’re going to stick to good ole’ hardware and home building. For a long time, windows have been thought of just as basic necessities to a house, or as ornamental and status-symbols. However, windows are a big factor in terms of our electricity bills, keep us healthier, make our homes last longer, save energy and essentially, save the planet. However, they can do all this and more.
Eco-friendly windows are essentially energy efficient windows. The make, material, frame, size and positioning of windows has a great impact on interior lighting, indoor temperature, heating & cooling systems and indoor air quality. Energy efficient windows work at maintaining optimal levels of all these aspects and thus significantly reducing our bills and our carbon footprint. And with the new eco-friendly wave taking the construction industry by storm, energy star certified windows are easily available. Also, the certification system is such that it works well for different kinds of windows that are made for different home requirements and weather conditions.
Yet, it can still get quite confusing and most of us are not quite familiar with scientific window terminology. Essentially, when you buy energy efficient windows, you will come across a lot of numbers and ratings. There are five things you need to look at:
1. U-Factor (Level of heat transfer)
2. Solar Heat Gain Co-efficient (Heat capturing capacity)
3. Visible Transmittance (Light Transfer)
4. Air Leakage and
5. Condensation Resistance
And that’s why we have a series of how to understand and buy energy efficient windows – without wishing you’d done more math in high school.