How to Replace A Window

Window replacement isn't difficult if you understand the steps. Replacing old windows can reduce your electric bills and make your house more energy efficient and secure. We'll walk you through the steps of learning how to determine your rough opening and replacing a window.

Determining Rough Opening Measurement

When replacing windows, one must first know the rough opening measurement. This is usually referred to as the "R.O." on window spec charts. Your replacement window must fit into the rough opening measurement or you will have to reframe the window. If the rough opening is smaller, it is usually easy to fill in, but a larger rough opening would require you to cut the existing rough frame. To determine rough opening of a window, you can usually take off the trim around the window on the inside of the house. Be careful not to damage the sheet rock or what ever the trim is nailed to. A couple of flat bars are very useful tool when removing trim. A small, thinner flat bar will get the trim loose enough to get the wider, thicker one started. Start on one corner and work your way loosening as you go all the way around the window. Once you have all the trim loose go back and remove the individual pieces. Now you will be able to measure the R.O. of your existing window.

Buying the Correct Size of Replacement Window

Your new window will need to be a half inch less in height and width than the existing R.O. This gives a quarter inch clearance on all sides to allow for any inconsistency in the existing framing. On really old windows, the bottom seal might be sloping down towards the outside, and you might have to reframe for a flat bottom seal to make a modern window slide in. It is easier to cut the inside down to the existing outside height, than to come up with siding to match the exterior to build it up to the inside height. If you have a sloped bottom seal you might need to allow enough space to put in a 1.5 inch thick board to make the seal flat. Sometimes the studs are cut at an angle and the bottom seal nailed right to them.

Siding and Exterior Considerations

Now, for the hard part, depending upon what kind of siding you have and upon how the currant window is trimmed, you must determine how best to match your outside trim. Most all new windows will come with a two inch or greater flange that runs all the way around the window. This molding will have nail slots to secure your window and seal out the elements. I have said all of this to say, you will have to expose the sheeting under you siding at least as wide and preferably a little wider than the nail flange that runs around the window. Often this means cutting back your siding all the way around the window, carefully so as to keep your cuts straight, and to not crack the siding further away from the window. You will then need to rip boards to put back on as final trim, and calking around them. If you have vinyl siding you have to carefully take loose the j trim, and the siding all the way around your window, enough to slip the new one in. If you have brick or masonry then often times there is wood attached to the brick that you can nail to, or, you will have to attach wood yourself with the use of a hammer drill and some manor of wood to masonry fastener, such as tapcon screws , or bent pins and appropriately sized bits. Remember that is generally easier to make a smaller window fit into a larger R.O. than it is to make a R.O. larger, however you will then have to deal with how make the outside trim match your new window size and perhaps fill in with some additional siding.

Setting the Window

Once you have the siding on the outside prepared, dry fit the window to make sure it fits easily into your opening with an eight to quarter inch clearance all around. Make sure the window is not upside down (most widows have visible weep holes for water to run out of after it has passed through the screen and hit the glass of a closed window). It is much easier to have two people for this next step. One person is on the inside, and one is on the outside to hold the window. If it fits, remove it and put caulk (I prefer silicone for this application) all the way around the R.O on the exterior opening edge or sheeting. This will seal the tab that runs all the way around the window. This is a vital step. Keep moisture from entering your house by making sure the calk line has no gaps, and that the lines of calk are placed where they will make contact between the sheeting and the tab. Insert the window again, this time using shims to help hold the window centered (top to bottom and left to right) in the R.O. To center the window just split the difference of the gap from top to bottom and from side to side. Once the inside person has adjusted and centered the window into place, the person on the outside holds the window in place while someone opens the window to see if upon closing, the reveal is the same from top to bottom (on a side sliding window), or from side to side (on a vertical sliding window). Adjust as needed. Now temporarily nail the four corners of the window through the outside tab and check again for smooth operation and proper alignment. To finish, nail every six to four inches in the provided slots all the way around the window's exterior tab. Roofing nails have a nice big head, and are flat enough to not mess with the trim, and are galvanized, so work well. Screws can also work. Note that some window instructions say not to nail along the top tab to allow for settling.

Finishing and Further Sealing

After nailing the tab securely, if you have enough room, it is good to put on some rubberized window seal. This usually comes in rolls six inches wide or so and really helps seal the window to the sheeting and covers the nail heads. You place it over the nailing flange up against the window, and out onto the sheeting. Window seal is very sticky on one side, and will not come loose if stuck to itself, so take care when cutting and placing it. Start at the bottom, with a strip that is wider on both sides of the window as the width of the rubberized seal and then do the sides, so they run from the bottom of the rubberized seal up past the top of the window the width of the rubberized seal. Finish with the top running to the out side edges of the vertical strips. Why install in this manner? Because water runs down, so water should always be able to run on top, not under.

Finish Exterior Siding and Interior Trim

Now, all that is left is to measure and rip the trim boards, or to carefully re-nail the vinyl siding. Remember to calk any cracks in between the window and trim boards, and between the siding and the trim boards. It is a good idea to calk between the vinyl J trim and window also. Remember that you need to use paintable calk in this application; most pure silicone caulk is not paintable.

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