Installing replacement windows is a project that changes the way a house looks, feels, and consumes energy more than any other renovation short of a new roof. The windows are the eyes of the house. Old windows leak air, rattle in the wind, and fog between the panes when the seal fails. New windows seal tight, open smoothly, and have glass that reflects heat back into the room in winter and back outside in summer. The installation method depends on the condition of the existing window frames. An insert window fits inside the old frame. A full-frame replacement removes the old frame down to the rough opening and installs a new window with a nailing flange. The insert method was covered in the previous guide. This guide covers both methods with an emphasis on the full-frame replacement, which is the more involved of the two and the one that requires exterior work.
The full-frame replacement window has a nailing flange around its perimeter. The flange is nailed to the sheathing around the rough opening. Flashing tape is applied over the flange in a specific sequence, bottom first, sides overlapping the bottom, top overlapping the sides, to direct any water that penetrates the siding out over the face of the wall. The siding, trim, and J-channel around the old window are removed before the old window comes out. The new window goes in. The flashing goes on. The siding and trim are reinstalled or replaced. The exterior of the house around the window looks new because it is new. A full-frame replacement is a siding project as much as it is a window project.
Measuring and Ordering the Right Window
Measure the rough opening, not the old window. For a full-frame replacement, the old window is removed entirely and the new window fits into the rough opening, the framed hole in the wall. Measure the width of the rough opening between the studs at the top, middle, and bottom. Measure the height from the sill to the header at the left, middle, and right. Use the smallest measurement in each direction. The new window is ordered to be slightly smaller than the rough opening, typically half an inch less in width and height, to allow for shimming and insulation. The window manufacturer or retailer will calculate the exact size from your rough opening measurements. Give them the rough opening dimensions, not the window dimensions.
For an insert window, measure the inside of the existing frame as described in the previous guide. The distinction is critical. A full-frame window ordered to insert-window dimensions will be too small for the rough opening. An insert window ordered to full-frame dimensions will not fit inside the old frame. Know which method you are using before you order.
Full-Frame Window Installation Sequence
Remove the interior trim and casing. Remove the old window sashes. For a full-frame replacement, remove the old window frame down to the rough opening. Cut the nailing fins or flanges with a reciprocating saw. Pry the frame out of the opening. The rough opening studs, header, and sill should be exposed. Inspect the rough opening for rot. Replace any damaged wood. The rough opening must be structurally sound, square, and the correct size for the new window.
From the outside, remove the siding, trim, J-channel, and flashing around the old window opening. The amount of siding removed depends on how the siding meets the window. For vinyl siding with J-channel around the window, the J-channel and the cut ends of the siding panels that fit into it are exposed. The new window will have new J-channel installed around it. For wood siding, the siding butts against the window trim. The trim is removed and the siding ends are exposed. The new window trim will cover the cut ends.
Apply flashing tape or liquid flashing to the rough opening sill. The sill flashing extends up the sides of the opening several inches. It is the first layer of the water management system. Any water that reaches the sill will drain out over the flashing, not into the wall.
Set the new window into the rough opening from the outside. The nailing flange rests against the sheathing or the house wrap. Center the window in the opening. Shim under the sill and at the sides to level and square the window. Check that the sashes open and close smoothly. Adjust the shims until the window operates correctly. Nail through the nailing flange at the pre-punched holes. Nail the sides from the bottom up, leaving the top flange unnailed at this stage. The top flange is nailed after the head flashing is installed.
Apply flashing tape over the side flanges, extending past the bottom of the window and over the sill flashing. Apply flashing tape over the top flange, extending past the side flanges. The sequence is bottom, sides, top. Water running down the wall hits the top flashing, flows over the side flashing, and off the bottom edge. The flashing is the water management system. The siding and J-channel cover it. The caulk seals the visible joints.
Install new J-channel around the window if the siding is vinyl. The J-channel receives the cut ends of the siding panels. The J-channel must be installed with the same shingle-lap principle as the flashing: bottom piece first, sides overlapping the bottom, top piece overlapping the sides with drip edges folded down. Install the siding panels into the J-channel. For wood siding, install new window trim, caulk the joints, and paint.
From the inside, fill the gap between the window frame and the rough opening with low-expansion window and door foam. Do not use standard expanding foam. Trim the foam after it cures. Install the interior casing and trim. Caulk the interior joints. Paint.
Frequently Asked Questions
How long does it take to get replacement windows after ordering?
Custom-ordered windows typically take four to eight weeks from order to delivery. Stock windows from a home center are available immediately but are limited to standard sizes. If your rough opening is a standard size, stock windows save time and money. If your opening is not standard, custom windows are the only option. Measure before you go to the store. Know your rough opening dimensions.
Can I replace windows one at a time over several years?
Yes. Replacement windows do not need to be done all at once. The windows are independent units. Replacing one window does not affect the others. The practical considerations are color matching, as window colors and styles change over the years, and the inconvenience of having construction in the house repeatedly. Ordering all the windows at once and installing them over time is more efficient than ordering one window every six months.
Do I need a permit to replace windows?
Most municipalities do not require a permit for like-for-like window replacement, replacing an old window with a new window of the same size in the same opening. Changing the size of the opening, adding a new window where there was not one before, or changing from a window to a door requires a permit. Check with your local building department.
The Bottom Line
Replacement windows are installed by the insert method or the full-frame method. The insert method fits a new window inside the old frame. The full-frame method removes the old frame and installs a new window with a nailing flange and flashing tape. The insert method is faster and preserves the existing exterior. The full-frame method is more thorough and includes new flashing, new trim, and the opportunity to repair any rot in the rough opening. The choice depends on the condition of the existing frames, the condition of the exterior siding and trim, and the budget. A house with sound frames and good siding gets insert windows. A house with rotted frames or damaged siding gets full-frame windows. Both methods produce a window that opens smoothly, seals tightly, and looks like it belongs in the house.
