When it comes to replacing windows, homeowners seem to give the least amount of thought to the ones in the basement. In many of the older homes I work on, the original wooden sashes on these windows are rotting away and no longer operate. Often, the sashes are nailed shut, making the basement damp and musty from lack of fresh air. Additionally, the inefficient single-pane glass in the windows is frequently dingy and loose with the putty falling out, letting in cold winter air but not letting in much daylight.

The typical fix is just sliding a replacement window into the existing wooden frame and running a bead of caulk around the outside perimeter. But for bigger problems, that’s just a Band-Aid. Instead, I always suggest replacing the whole window unit—frame and sash. A modern-day double-glazed window is more energy efficient, and the thinner, modern frame lets in more light with less air leakage, while offering better security and easier operation. Recently, a client needed eight basement windows replaced in an older home with a rubble foundation, and here’s how I approached the job.

I always cut the pieces I need from PVC sheets because I can make them any size I need with minimal waste. Using a Trac Saw, I ripped all the larger widths, labeling them as I went.
I always cut the pieces I need from PVC sheets because I can make them any size I need with minimal waste. Using a Trac Saw, I ripped all the larger widths, labeling them as I went.

Measure and Order Window

I started by measuring the openings of all the existing windows around the outside perimeter of the frame. I then deducted for the frame, the sills, and the space needed for insulation, plus another 1/2 inch to deal with any out-of-square situations I might encounter in the openings. These measurements gave me the window sizes I needed.

With the windows ordered, I put together a list of material that I’d need to complete each window unit. I added up the widths and lengths of all the necessary pieces and figured that I could get them all out of two 4x8 sheets of 3/4-inch PVC. I also rounded up pocket screws, deck screws, PVC glue, foam insulation, caulk, and some mortar to complete my materials list.

Frame Fabrication

After the windows arrived, I used them to determine the exact sizes of the parts for the window frames. The original window sills extended only about 1/2 inch beyond the foundation, which concerned me. So I decided to make them about an inch wider to better deflect water away from the foundation.

I always cut the pieces I need from PVC sheets because I can make them any size I need with minimal waste. Using a Trac Saw, I ripped all the larger widths, labeling them as I went. For smaller rips, I used a table saw. I then cut the sides of the frame to length on the miter saw, including the 12-degree angle needed for the sloped sill.

After cutting the sill stock to a rough length, I glued and screwed a 2-inch-wide piece onto the outer edge to double the thickness where the sill extended beyond the foundation.
After cutting the sill stock to a rough length, I glued and screwed a 2-inch-wide piece onto the outer edge to double the thickness where the sill extended beyond the foundation.

After cutting the sill stock to a rough length, I glued and screwed a 2-inch-wide piece onto the outer edge to double the thickness where the sill extended beyond the foundation. After gluing the two pieces together, I ran the assembly through a table saw with the blade set at a 12-degree angle to make the outer edge plumb. To help keep water from entering under the sill through capillary action, I cut a saw kerf 1/2 inch in and about 3/8 inch deep along the bottom outer edge of the sill. Finally, I stacked the head jamb and sill together and cut them to the same length with one pass on a miter saw.

After gluing the two pieces together, I ran the assembly through a table saw with the blade set at a 12-degree angle to make the outer edge plumb.
After gluing the two pieces together, I ran the assembly through a table saw with the blade set at a 12-degree angle to make the outer edge plumb.

To assemble the frame, I drilled pocket holes along the bottom edges and secured them to the sills with weather-resistant pocket screws and PVC glue. I finished the frame by gluing and screwing the head jambs to the sides with deck screws to form simple butt joints.

To assemble the frame, I drilled pocket holes along the bottom edges and secured them to the sills with weather-resistant pocket screws and PVC glue.
To assemble the frame, I drilled pocket holes along the bottom edges and secured them to the sills with weather-resistant pocket screws and PVC glue.

Windows Go Into the Frames

Before I could put the new units into the frames I’d built, I needed to install the interior stops for the windows to fit against. Taking the lengths directly from the frame, I marked each stop piece and cut it to length. After predrilling the stops and inserting screws in my holes, I screwed the stops into place, using a spacer block as a guide to space them evenly from the inside edge of the frame.

After predrilling the stops and inserting screws in my holes, I screwed the stops into place, using a spacer block as a guide to space them evenly from the inside edge of the frame.
After predrilling the stops and inserting screws in my holes, I screwed the stops into place, using a spacer block as a guide to space them evenly from the inside edge of the frame.

Each window came with a head expander that slides up to fill the gap at the top of the window. I applied polyurethane low-expansion foam to each expander and slid it onto the top of a window. Next I squeezed out a bead of caulk (I used DAP Dynaflex 230, an elastomeric latex sealant) onto the front side of the stops and pressed the window against the stops, leaving equal space along both sides for insulation.

Each window came with a head expander that slides up to fill the gap at the top of the window. I applied polyurethane low-expansion foam to each expander and slid it onto the top of a window.
Each window came with a head expander that slides up to fill the gap at the top of the window. I applied polyurethane low-expansion foam to each expander and slid it onto the top of a window.
Next I squeezed out a bead of caulk (I used DAP Dynaflex 230, an elastomeric latex sealant) onto the front side of the stops and pressed the window against the stops, leaving equal space along both sides for insulation.
Next I squeezed out a bead of caulk (I used DAP Dynaflex 230, an elastomeric latex sealant) onto the front side of the stops and pressed the window against the stops, leaving equal space along both sides for insulation.

Opening the windows let me access the holes for securing the units to the frames, which I did with four screws short enough not to go through the frames. I then checked the frame for square by taking diagonal measurements. If adjustments had been needed, I would have used the attachment screws to tweak the fit.

Opening the windows let me access the holes for securing the units to the frames, which I did with four screws short enough not to go through the frames.
Opening the windows let me access the holes for securing the units to the frames, which I did with four screws short enough not to go through the frames.

At the bottoms of the windows, I attached a sill expander that I had ripped to width on a table saw. After applying low-expansion foam to the bottom of the window, I tapped the expander into its groove, starting at one end of the window and slowly working my way to the other end until it was secure.

Next, I measured and cut the exterior stops for the top and sides. Then, installing one piece at a time, I filled the gap between the window and the frame with low-expansion foam and installed the stop to keep the foam from oozing out. A bead of caulk applied to the back of each stop piece sealed it against the window sash before I screwed it into place. I finished by filling the screw holes with OSI Quad sealant.

I filled the gap between the window and the frame with low-expansion foam and installed the stop to keep the foam from oozing out.
I filled the gap between the window and the frame with low-expansion foam and installed the stop to keep the foam from oozing out.

Prepping and Installation

Removing the rotten and dilapidated old windows was pretty easy. They were no match for my pry bar and reciprocating saw. When all the pieces were removed, I cleaned the opening with a brush.

As I was removing the windows, I noticed that some of the openings needed additional attention. The old rubble stone foundation hadn’t been maintained through the years, and a few stones were loose. After vacuuming up any loose debris, I applied a bonding agent that helped the new mortar adhere to the stones and to the old mortar. Using Type-N mortar, I secured the loose stones and then applied a coat of new mortar to the entire opening, brushing it smooth.

Using Type-N mortar, I secured the loose stones and then applied a coat of new mortar to the entire opening, brushing it smooth.
Using Type-N mortar, I secured the loose stones and then applied a coat of new mortar to the entire opening, brushing it smooth.

The next step was installing the new windows and frames in the openings. Fastening the frames to the old stones with masonry anchors would have been extremely difficult, if not impossible. So after sliding the frames into the openings, I secured them to the house sill through the head jamb of the frames. To keep the units from moving in the openings and to air-seal around them, I sprayed low-expansion foam between the frame and the foundation.

After the foam had cured, I applied mortar around the perimeter of the frames, both inside and outside.
After the foam had cured, I applied mortar around the perimeter of the frames, both inside and outside.

After the foam had cured, I applied mortar around the perimeter of the frames, both inside and outside. When the mortar had cured a few days later, I cleaned the mortar residue off the stones around the opening with muriatic acid.

The impact of the new windows was immediate. Opening the windows quickly improved the musty smell of the basement, which was much brighter with the added light. And the new double-glazed basement windows made of rot-proof material should easily perform better and last far longer than the original windows.

All photos by Emanuel Silva