Canadian Woodworking

How to replace an old window

Author: Nick Reaume
Photos: Rob Brown
Published: April May 2020

Upgrading an old window can be a great way to improve the look of your home, and help keep heating and cooling bills down.


For this project, we had two identical window openings with old Pearson-style sliding windows that needed upgrading. The first step was to decide on what they would be replaced with, and we settled on a basic white vinyl framed window from North Star.

The next step, and perhaps the most critical step, was to order the correct size of window. As a full-time builder and renovator, I have seen many times how costly it can be when a mistake is made at this stage. In our case, the new window size was determined by the existing J-trim of the exterior siding that bordered the old window frame. We did not want to change the siding detail at all, so the windows were measured such that the new brick mould would fit nicely inside the existing J-trim. Additionally, some newer windows had already been installed on other parts of the house, so we also wanted to match the detail of these other windows. To accomplish this, we simply subtracted 1/4″ from the height and width of the J-trim opening and ordered the windows based on the outside dimensions of the brick mould. This gave us 1/8″ all the way around the perimeter of the window. This is not necessarily the most common method to size new windows, but it was the most effective for this installation.

Rough and Worn
An old, weathered window not only looks awful, it allows water and cold air to enter your home. Sometimes it's just a matter of some fresh paint, but often it's a situation where replacing your windows is the only option.

Get it Out
 Removing any screws, and using a reciprocating saw to cut through any nails, is a good place to start when it comes to removing a window.

Pesky Nails
 Ensure all nails and other debris that may cause the new window to not sit properly in the opening are removed. Now is also a good time to check for any rotten framing members, and replace them.

A Foot to Stand On
 Shims under the two outer corners of the window, as well as shims under the center of a larger window, will start to level the window in the opening.

Remove a Strip
The new window very likely has a removeable strip of vinyl that can now come out (left). Screws can be driven behind this strip, then it can be put back into position to hide any screw heads (right).

Another Removable Strip
 It may be possible to remove a wide strip across the lower section of the new window in order to hide a few screws behind it. If you're careful taking it off nobody will ever know it was ever removed.

Add Some Foam
 Proper window foam will go a long way to keeping cold air out, and will also assist with keeping the window positioned where it is. Once cured, it's surprisingly strong.

First Line of Defense
 A quality exterior-grade caulking will last a long time, and is the first line of defense in keeping the elements out of your home.

Out with the old

Once the new windows arrived, it was time for demolition. There are several types of windows out there, so different amounts and methods of demolition may be required depending on what you are starting with and how the new window is being finished. In our case, the entire window frame needed to be removed so we would have a large enough rough opening.

First, the individual panes of glass were removed (as is the style of older Pearson-style sliding windows). Next, the wood frame was removed by using a reciprocating saw to cut the nails between the window frame and the rough opening framing. Note that in some cases it may be possible to simply back out the screws that hold the window frame in place, but being an older window, our frame was fastened in with hand nails. It may also be necessary to cut the window frame itself in one or more spots with a reciprocating saw to be able to remove one side of the frame at a time.

When all of the necessary wood and nails were removed, the rough opening was thoroughly cleaned with a shop vac along all sides.

In with the new

With the openings prepared, we were ready to install the new windows. These windows were not particularly large or heavy so they were easily managed by two people. Remove the screens from the windows and set them aside somewhere safe. Lift the window into place and have one person remain outside (to prevent it from falling out), and the other person should head inside to fasten it in place.

An important note about the installation here is that because we opted to have a window with brick mould, there was very little ability to plumb the window in the vertical plane of the wall. It is important that your wall is plumb if this type of installation is used. Also, by using brick mould in this way, it’s impossible for your window to tip inwards too far because the brick mould will contact the surface of the wall and not fall all the way through the rough opening. This can be helpful if both people are needed to lift the window without needing someone inside to catch it (assuming it was measured right).

Start with shims

The most common way of fastening a window in place is by using shims and screws. Normally cedar shims are just fine, but in some cases composite shims may be required. Start by shimming underneath the bottom of the window. Our windows were small enough that shims were only needed in each bottom corner, but larger windows may need a support shim in one or more spots along the bottom. Shim the bottom corners until the sill is level and the gap between the window frame and rough opening is the same across the top and bottom. If you are working with a rough opening that is out of square, or out of level, this may not be possible and you will have to get these measurements as close as you can. In most installations, this gap is nominally 1/2″. The window is usually ordered such that the frame is 1″ smaller than the rough opening in both directions, leaving you with 1/2″ on each side, but the perfect rough opening does not always exist.

Next, place shims along the bottom left and right hand side of the window, again, trying to achieve the same size gap on each side. Push shims until they are snug and don’t fall out, but do not force shims in too much as they can distort the window frame. Continue to place shims in the upper left and right hand corners, again trying to maintain equal spacing on both sides.

For the windows in this installation, shims were not needed across the top. On larger windows, when the top is likely to sag under it’s own weight, it may be necessary to fasten the window frame to the wood framing and you therefore will need to shim at these points to prevent the window frame from being bowed.
When the shims have all been installed, double check that the window is plumb, level and straight. The longer a window dimension is, the more easily it can be bowed. It might read level from corner to corner, but have a sag in the middle that needs correcting. Always use the longest level possible for any given side for best accuracy.

Screws are next

To fasten the window frame to the rough opening, it is usually necessary to temporarily remove a piece of the vinyl frame to get a screw in a position where the head will not be visible. In some cases, this is not possible and the screw must be counter-sunk through the frame and a proper size plug is used afterwards to fill the hole. This is often the case for casement windows.

For our installation however, two small vinyl plates in the upper two corners were easily removed and a yellow 3″, #8 Robertson screw was used in each upper corner. Just before the screws are driven, have the person on the outside apply firm pressure to the window frame so that the brick mould is making solid contact with the wall surface. Do not over tighten the screws when installing. With shims holding everything in place it is unlikely that any part of the window will distort, but you only to drive them until they are snug.

The same screws were used in the bottom corners as well, but a longer vertical piece of the vinyl frame needed to be removed in order to access the bottom corners of the window frame and drive a screw. In total, four screws were used to fasten each window in place. Once more, the window was checked for plumb, level and straight, and we confirmed the spacing between the frame and the rough opening all the way around. I also made sure the window operated properly. Note that this method may not work on all vinyl windows, or for other window frames made from different materials (i.e. wood or fibreglass). Contact the window manufacturer for installation questions about your particular window.

Expanding foam

At this point, it’s appropriate to apply expanding foam around the outside of the window. I generally like to foam across the bottom first, then both sides, and the top last. Use window and door approved spray foam. Other types of expanding foam, such as big gap/crack filler, can expand too much and bend the window frame. Close and lock the window before applying any foam and leave the window this way until the foam is fully cured. When foaming, hold the nozzle close the brick mould, if there is enough room for it, so the foam expands all the way to the brick mould and you do not trap pockets of air.

The last detail of the installation to perform on the inside is to cut the shims and cured foam back flush with the window frame. The foam normally cures in a couple of hours, but sometimes it’s necessary to leave it overnight. If it’s still a little tacky when you try to cut it, just wait a little longer. There is no need to rush it and your knife blade will thank you.

Move to the outside

For completing the installation on the outside, it was simply necessary to apply a high quality caulking between the brick mould and the existing J-trim of the exterior siding. For this project, the type of caulking used was Supra by Mulco, which is available in a large assortment of colours.


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  2. I would have put in casement windows
    an expert I talked with a few years ago told me sliders wear out much faster
    Nother thing with a screwdriver he would be inside a slider within 10 mins
    not so with casements

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