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.
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.
you need to know is that you can convert from U-value to R-value simply
by dividing 1 by the U-value.
If you're building your
dream house and you don't want to see water pooling on your window
it gets cool, get very involved with the selection of your windows.
Insulation-Guide has seen many a disappointed homeowner with
windows that condense moisture. It is not as simple as just
picking a good brand, the glass and the
the edge spacers need to insulate as much as possible. Find
out more by reading below.
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
can also choose to block or allow solar gain based on the type of
nitrogen, argon, krypton (the best), xenon (highest density).
The goal is to get a gas that is less resistant to
currents by being more viscous and less conductive, this leads to more
insulation value. If the seal leaks
over time then the benefits diminish.
trapped air spaces = more insulation value. This can be done
of glass or stretched suspended films between panes. The more
better. Double pane windows are common, triple and quadruple
less common, very expensive and have weight considerations.
film windows can answer the weight considerations but are
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.
conductance edge spacers to hold the panes or films- Aluminum is traditional for
dimensional stability but very conductive. Other materials
steel) are less conductive. Some spacers are coated and/or
reduce thermal conductivity. The effect of edge spacers on total
greater the smaller the window. The most important reason to
edge spacers is for condensation control.
Window type - Casement and awning windows are
usually more air tight when closed than vertical or horizontal sliders.
windows are the best. The quality of the latches, the action
window, the long term dimensional stability of the frame, and the
the weather-strip affect the air and water tightness of the window.
weeps reduce airtightness, and are most common on slider type windows
get water caught in the tracks. Factory
airtightness data can tell
you the quality of the window you are considering, if they choose not
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.
that the window you choose must be foam sealed to the rough opening to
air leakage down.
Guide- Try to
get windows that have the National Fenestration Rating Council (NFRC)
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
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
down. It can get cold back there
between the insulating
shade and the window so the warm air may cool and deposit moisture on
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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