Choosing Windows that Insulate

Here are some points to consider for choosing an efficient and durable window or skylight.  Efficiency is certainly the main point but you may also want to be concerned with potential for condensation and solar heat gain.   

Insulation-Guide- Window insulation values are described in terms of U-Values not R-Value.  U-value is the inverse of R-value so the conversion is U-value = 1/R-value.  This also means that the lower the U-value the better the insulation level, whereas the higher the R-value the better the insulation.  The reason for using U-values rather than R-values is to better represent the complication of a window.  Remember, a window is not just glass, it is spacers, a frame and sometimes more.  The U-value represents all those pieces as one unified value of heat transfer.  Anyway, what you need to know is that you can convert from U-value to R-value simply by dividing 1 by the U-value.   

Insulation-Guide- If you're building your dream house and you don't want to see water pooling on your window sills when it gets cool, get very involved with the selection of your windows.  Insulation-Guide has seen many a disappointed homeowner with high-end windows that condense moisture. It is not as simple as just picking a good brand, the glass and the frame and the edge spacers need to insulate as much as possible.  Find out more by reading below.

Frame- Vinyl, wood, aluminum, aluminum clad over wood, steel, fiberglass.  Conductivity, dimensional stability and weight are the main factors to consider.   Some materials are less conductive like wood or vinyl but not as dimensionally stable as aluminum or steel. Fiberglass frames fits all the criteria with low conductivity and high dimensional stability but, you guessed it, fiberglass frames tend to be more expensive.    

Coatings- Reduce thermal radiation from the warm pane to the cold pane of glass by coating the face towards the gap, can also choose to block or allow solar gain based on the type of coating you choose.

Gas filled- Air and nitrogen, argon, krypton (the best), xenon (highest density).    The goal is to get a gas that is less resistant to convection currents by being more viscous and less conductive, this leads to more insulation value.  If the seal leaks over time then the benefits diminish.

Panes or films- More trapped air spaces = more insulation value.  This can be done with panes of glass or stretched suspended films between panes.  The more the better.  Double pane windows are common, triple and quadruple panes are less common, very expensive and have weight considerations.  Suspended film windows can answer the weight considerations but are expensive.

 

Insulation Guide- A word on the R-value of windows. Many people are disappointed to discover that most "insulated" windows are only R2 to R3 and premium windows are only R5 to R7. Cutting edge windows today are R11 to R20 but the average joe can't even comprehend the price of those. That's the sacrifices we make for outside light, most windows are nowhere near the R-value of a typical wall. The good news is that windows allow in sunlight which can help heat your home, walls just can't do that. Somewhere there is an optimal balance of price, r-value, and solar heat gain that comprises the perfect window.

Low conductance edge spacers to hold the panes or films- Aluminum is traditional for dimensional stability but very conductive.  Other materials (stainless steel) are less conductive.  Some spacers are coated and/or shaped to reduce thermal conductivity. The effect of edge spacers on total U-value is greater the smaller the window.  The most important reason to consider edge spacers is for condensation control. 

Window type - Casement and awning windows are usually more air tight when closed than vertical or horizontal sliders. Fixed windows are the best.  The quality of the latches, the action of the window, the long term dimensional stability of the frame, and the quality of the weather-strip affect the air and water tightness of the window. Window weeps reduce airtightness, and are most common on slider type windows which can get water caught in the tracks.   Factory airtightness data can tell you the quality of the window you are considering, if they choose not to list it ask why.   It should be from .1 to .3  cubic feet of air flow per minute per square foot (CFM/sf), the lower the better.  Don’t forget that the window you choose must be foam sealed to the rough opening to keep the air leakage down. 

Insulation Guide- Try to get windows that have the National Fenestration Rating Council (NFRC) rating label on them. The labels list the key facts about the window in one place: U-value, the solar heat gain coefficient, visible transmittance, air leakage and condensation resistance.  

So far I’ve discussed just the windows themselves.  You can also add insulated window coverings that can increase the effective R-value of your windows and decrease heat loss at a reasonable cost.   The only thing you need to be aware of is condensation.  Some window coverings are better than others at keeping warm air in the house from getting behind the shade when it is down.   It can get cold back there between the insulating shade and the window so the warm air may cool and deposit moisture on the window.  This is less of a concern if you live in a low humidity climate. If the condensation is light each morning it is probably nothing to worry about, most building materials will tolerate slight amounts of water with no problem. If you have high amounts of condensation you will either need to consider airtight interior storm windows, airtight shades, or new windows with a good condensation rating.  

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