Replacing an existing window in your home with a replacement vinyl window unit isn't as difficult as you'll think. With minimal tools and the ability to use a measuring tape, you can change the average window in about one hour. The first item of business is to live the prevailing window opening to work out the dimensions unit you would like. Once the measurements are taken you'll order the replacement unit. Most companies take about three weeks approximately to deliver custom-sized vinyl windows so leave this point to pass.
To determine what size window you need, open both the upper and lower sash halfway. Using your tape measure, measure the exact distance between the inside of the tracks where the existing sash run. It will measure slightly wider than the sash themselves. Take one measurement at the top of the window and one at the bottom. They are seldom the same. Now close the sash and take yet a third measurement at the mid-point of the window or the top of the bottom sash. Again, this measurement may be different from the other two. You will use the narrowest measurement you have to order the new replacement window. Unless there is a huge difference in the three measurements the small difference between the largest and smallest measurements will be minimal but still important for fitting the new unit into the rough opening.
Once the new window arrives, carefully unpack the unit to check there is no damages. In the package will be a loose piece of aluminum angle measuring approximately three quarter by three quarter inches and it is color matched to the new window. Set this aside for later. Remove all masking tape from the header piece, as once the window is installed, it will be impossible to remove.
Starting on either side of the existing window, remove all window trims including the sill and apron pieces. If reusing the trims, strip all the nails and set aside. Now carefully remove the casing trims that are holding the lower sash in place. Once one side is removed, the lower sash can be removed and discarded. Now with a sharp chisel or a flat screwdriver, remove the parting beads that separate the upper and lower sash. These will both be discarded so if you destroy them during removal it is not a problem. Paint and age can make them a bear to remove. Now remove the upper sash and on older homes, remove all sash weights, ropes and pulleys. Save the sash pocket doors so they can be reinstalled. With all the accessories removed, use loose insulation or non-expanding spray foam to completely insulate the sash pockets. These sash pockets are a huge source of cold air and wind leaks. Do a good job.
You are now able to install the new window unit. Slide the unit into the rough opening you ready and push it out until it comes fully contact with the outer trim or wood bead on the frame. I always put a bead of excellent quality silicone caulk during a matching color round the top and sides to assure a decent seal. Now using the most important carpenters hand level which will fit, check the top and jambs to assure they're plumb and level. Use some wood trim shims to amend any alignment problems.
You need the unit to sit as close to the center of the opening as possible when you are done. Remove the four plastic side pieces in the side channels at the top and bottom of the window to reveal the four holes for your attachment screws. Slide the header channel hard up against the window frame. Again, place a good bead of caulk between the header and the wood frame. Install you four anchoring screws through the side jambs being careful not to over tighten the screws and pull the new window out of plumb. Using two small screws provided, anchor the header channel to the window unit thereby preventing it from sliding back down. Place the screws as near to the wood frame as possible so you may conceal them behind the trims when they are reinstalled.
On the exterior of the window, place some loose insulation in the space between the new window and the existing sill. Using the piece of three-quarter-inch angle and two small screws provided, close off the space between the new window and the sill. Caulk the angle and wipe all surfaces clean.
Reinstall all your interior trims but you may well find that they are large at this point as the vinyl window is thicker than the old sash. You will have to rip the trims and sill to fit. The face trims and apron are a direct renewal. Set all your nail heads and a coat of paint will build it look like it was never touched.
Most building departments do not require approval for replacements that do not alter the structural integrity of the framing. If any re-framing is involved at all, a building permission is required. Ask the building inspector before you start to be sure either method.