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Replacing a window is one of the easiest projects you can do yourself. In fact, the hardest and most time-consuming part is removing the old window!

Let’s start with a few window-buying basics. Choosing a replacement window is not only about how it looks or how well it fits your budget but also about how much money it will save in the long run! Knowing what the values and factors on the sticker mean can save you big bucks on your energy bill later!

Here is a picture of the sticker that was on one of my windows…

See where it says U-Factor, Solar Heat Gain Coefficient, and Visible Transmittance? The U-Factor is “the number of BTUs (British Thermal Units) per hour that pass through that assembly per degree Farenheit  under a steady rate of heat flow” (taken from the Green Advantage Study Guide). In other words, this is how much heat you’ll feel coming through the window on one of those 110 deg. days we’re sure to have this summer. The lower the U-Factor, the better the insulation. The Solar Heat Gain Coefficient “measures how well a window blocks heat by sunlight” (taken from the Green Advantage Study Guide).  The lower the SHGC, the better the window is at blocking unwanted heat. The Visible Transmittance is “a measure of the amount of light that comes through a window” (taken from the Green Advantage Study Guide).  The higher the number, the more light that passes through. This is great for using natural light without the excess heat the summer sun can cause. 

Start by measuring the window opening. Remove any window trim, jambs, and the sill to get a good measurement of the rough opening. The rule is to take three measurements of the width (top, center, and bottom)  and height (left, center, and right), and use the smallest measurements for your rough opening. When ordering the windows, the rough opening is used in determining the replacement window size unless it is otherwise specified. Generally, the window will be 1/2" smaller than the rough opening. When I ordered my windows, I went a full inch smaller than the rough opening because it is easier to shim it in place than to take away from a window that is too large!

My old windows had aluminum frames with the flanges screwed under the siding. We cut the window away from the flange with a reciprocating saw fitted with a metal-cutting blade. This truly is the hardest part… Not that it’s really “hard” but that the rest of it is so easy!!

Next, I cut a piece of 1x6 for the window sill. The windows sit side-by-side and I made the sill one continuous piece. Once I cut it to length and notched it as it needed to be, I laid it in the window opening and checked for level. Luckily, it was perfectly level! I used a brad nailer and nailed it into place. If the sill were not level, I would have used a few shims underneath to make it level, then nailed it into place.

Then, we placed one window into position. I wanted to make sure the outside of the window was level with the high point of the siding to make it easier to attach the trim. Once it was settled into place, I drilled countersunk holes approximately 2” down from the top (on the side) and 2” from the bottom. The window was shimmed into place, maintaining a level position, and the screws were driven in. 

I like to install the trim on the inside first before using the expanding foam. It keeps the mess outside! Measure the distance from the window to the inside wall. Mine measured 2-1/4” so I ripped 1x3 lumber to 2-1/4” then framed out the top and sides of the window. I used a brad nailer and 1-1/4” brad nails to secure the pieces in place. Add any molding or trim around the window. Caulk around the window and where the framing pieces meet the trim.

The next step was to fill in around the window with expanding foam for doors and windows. Use it sparingly, it will expand a lot! I am the queen of over-filling the voids with the foam and then I end up with a lot to trim away. (But it works!)


I like to use PVC trim on the outside. Do not use nails or screws to secure the outside trim to the siding. Nails or screws on the siding can cause rust, air pockets, and also a way for critters or water to get under the siding. Attach the trim with weather-resistant silicone. It will act as a glue and hold the pieces in place but will remain flexible for movement caused by temperature changes.

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