Replacement windows install in the existing window frame - where the sashes move up and down - so you need to measure the opening carefully. You'll be removing everything from the window between the outside stop (the piece of wood that keeps the window from falling to the exterior of the house) and the inside stop (the trim pieces on the inside that keep the window from falling into the house). That will include the sashes (the top and bottom parts that move up and down, as well as any tracks, springs, dividers, ropes & pulleys, or whatever). The inside stops will be removed to install the windows, then replaced to hold them in. Don't touch the outside stops. Generally speaking, you'll do the whole installation (minus some caulking and paint touch up) from the inside.
The instructions for my windows said to order them 1/4" less than the width of the opening (the height is less critical, since there's a head expander you move up to fill the space). I did my windows in two batches - the first I tried to get almost exactly the size I thought they should be, and they were difficult to get in the openings. Once I did, there was very little room for shims and insulation. The next batch I ordered a bit smaller, and they fit in easier. The extra room is filled up with shims and non-expanding foam anyway, so it's every bit as tight when complete as the others, and it will be covered up with the inside stops/trim anyway.
I was fortunate, in that my old windows were almost perfectly square and plumb (all the corners were right angles, and the vertical sides were straight up-and-down). This simplifies both measurement and installation - but don't worry, you can still install windows if your old ones are a little off. If your old window frames are in very poor condition, or if they are rotted or leaking, you should not use replacement windows - you'll need to remove the old windows, frame and all, and replace them.
Measure in at least 3 locations horizontally and vertically, and DON'T assume all windows will be the same size. You should be able to measure your windows accurately without removing the old sashes - just take your time, and don't take shortcuts. Remember, you're measuring the area where your current windows slide up and down.
You also need to check to make sure your openings are square and plumb. The most accurate way to determine if they are square is to measure the diagonals - if they measure the same from corner to corner, the opening is square (assuming the length and width is also consistent). Check for plumb with a level on each side. It's important that your replacement window sits level in the frame, so if your opening is off, you'll need to order the window a little smaller to allow you to adjust it.
Most of the time, your windows will be custom made - so you won't be able to return them if you measure wrong. When you order them, you will typically tell them your "rough opening" size, and they will adjust the actual window size to fit. On the order form, it should give you window dimensions, so make sure it will fit - it is your responsibility to get it right.
One other thing you need to check is the depth or thickness of your old windows. Compare this to the dimensions of your replacements - if they're the same, you're in luck. If the new windows are thinner than the old (not likely), you can put some shims on the outside stop to make it work. If the new windows are thicker, as they were in my case, you'll need to move the outside stops or make adjustments to the inside trim. My old windows were only 31/4" thick, compared to 31/2" for the replacements, so I elected to fix the inside stops. I bought a cheap table saw, and was able to trim 1/4" from the inside edge of each board. Unfortunately, I also had to cut 1/4" from the window stool (the ledge on the bottom of the window trim). I was able to do this by cutting the nails holding it in place with a Dremel Multi-Max, removing the piece, and trimming it in the table saw. Most people don't have this step.
When your windows arrive, you'll want to remove the plastic wrapping, and store them standing upright (you don't want them laying flat on the floor. Avoid extreme heat for storage. Check all the dimensions carefully - you don't want to find out they're wrong after you've removed the old window! If they're not all the same size, mark each window with it's intended location.
Do yourself a favor, and install your first window in the least noticeable location (I started in a spare bedroom). If you have storm windows, remove screws or other hardware and take them down. You won't use storm windows now. Go inside and carefully remove the window casing (the trim around the window), as well as the inside stops that hold the sashes in place. At this point, you should be able to remove the sashes, and any hardware in the opening. Remember to leave the outside stops in place - they're what the replacement window sits against. Take a stiff putty knife, and scrape the opening to remove loose paint, caulk, etc.
Now is the time to see if you measured properly! Dry fit your window into the opening - set the bottom in place, and tilt it up and back until it rests against the outside stops. You should have a little room on each side (less than 1/4" on each side), and a bit more at the top. Pull the window out and set it aside. There should be a couple of pieces with the window. A channel that sits on the top, that will lift up to fill the space at the top of the window, and a "sill angle", that attaches on the bottom to match up with the existing window sill. Cut some fiberglass insulation, and put it on top of the window, then cover it with the channel. The sill angle usually slips into a slot on the bottom of the window.
Next, you'll need some "window and door caulk", to match your windows. Put a nice bead of caulk around the exterior stop, so your window will seal against it when installed. Also caulk against the stool (that's the ledge that is commonly on the inside) - if you have it. Don't go overboard on the caulk - just a nice even bead - too much just makes a mess.
Fit your window into the opening, and check, with your level, that the bottom is level, it's pushed tight against the outside stops, and that both sides are plumb. Slide the head extender up so it contacts the top of the opening. Install shims top and bottom, check again for plumb and level, and install screws through the installation holes. Don't tighten the screws at this time - they're just holding the window in place. The window should be solidly in the opening, and you can relax for a second.
When you're done relaxing, check to make sure you have shims in all 4 corners, and that the window is level and plumb. Tighten the screws just enough to get them snug, but not enough to distort the window. Add shims where needed - particularly in the center of each side, making sure it's still square and even. Operate the window and make sure it opens and closes properly.
You'll want to fill the empty space between your new window and the old frame. If you've got room, use "Door & Window, Non-Expanding Foam". Don't use the regular expanding foam - it expands too much, and will actually put pressure on the windows, bowing them so they don't operate properly. In any case, don't go overboard on the foam - even the non-expanding stuff will expand enough to fill the void.
If your windows are very tight in the opening (like my first batch were), you may have to fill the void with caulk or fiberglass insulation. I slid a little insulation in the crack with a putty knife, then caulked for a good seal.
Once you've sealed the window, all you have to do is re-install your interior stops and trim. In my case, this was complicated by having to trim 1/4" from each piece, but I didn't have to remove any of the window casing. If you can, use a pneumatic trim nailer - otherwise, you'll need a hammer and nail set. Fill the nail holes with matching putty, caulk outside between the exterior stop and the window, touch up any interior and exterior paint or stain, and you're done!
I read all the articles that said you'd take about an hour per window to swap them out - mine took quite a bit longer. My first window took over 6 hours from start to finish. I went very slowly, since I didn't want to mess something up. The second one took about 4 hours. After that, I would do two at once (since most of them were installed in pairs), in between 4 and 5 hours. One thing that slowed me down was I had to remove and re-install the old blinds and/or curtains - that's never accounted for in the articles. Of course, having to trim the inside stops (which also required re-cutting some miters), slowed me down even more. I also continued to move very slowly, trying to make sure I didn't make a mistake (I still made a few). Without those extra tasks, I'm certain I would have been able to average less than 3 hours per pair of windows, and might have gotten to 2 hours per pair (1 hour per window).
As far as cost, here's how I ended up:
- $2,912 - Windows, with screens (American Craftsman 8500, tan, low E, Argon filled)
- $ 284 - Tools (Table Saw, Dremel Multi-Max, Blades, Wonder Bar, Misc Drill Bits, etc.)
- $ 136 - Supplies (Paint, Caulk, Insulation, Fasteners, etc.)
I'm very glad I did this project myself. I saved a bunch of money, and I believe my finished project was of at least as high a quality as what I would have gotten had I paid for it. I also have a couple of new tools left from the project, too, and I used both of those in my room addition project at my lake home (more about that in the future). If you're not sure if you can do this or not, try one window. At the worst, you'll mess it up and have to pay someone to fix it. I'm guessing you'll find out it's not that tough, and you'll do just fine.
