Most windows are plastic, most windows are sliders and most window replacements are complete tear workouts rather than installed inside the old frame.
This is supposed to be pretty manageable. You will find differences with various materials and types of windows, and thus don't try to extrapolate.
Step 1: Ordering Your Windows
Take off the casings (moldings) enclosing the windows. If you feel you can measure the window without taking the casing off, you are mistaken.
Step 2: Removal of Old Windows
When the windows arrive, inspect them. First assess the impact window installation and be certain there's not any damage and they are the ideal size. You do not need to find out that as soon as you've removed the old window.
The idea here would be to remove the old windows with as little damage to the home as you can. The construction paper probably got mangled if you're removing the window. Trim it back and repaper as best you can. The purpose is to protect the wood.
You can use a peel and stick product, it works really well. You are limited though because in some circumstances the siding is still in place.
Step 3: Installing
The window I say two places since shims should be used in pairs, so assuming that the framing is relatively right, and it frequently is not. Rough sills are rarely flat and you will need to adjust your own shims to acquire a level surface to rest the window.
While someone holds the window in place for you, get the bottom frame member of this window level. Drill holes through the side frame pieces about eight inches from the top of the window and run screws in to hold it in position, not tight.
As an alternative, you can drive claws on all sides of the window. You're only trying to prevent it from falling out while you're working on it. You'll have to include shims at strategic points under the base frame member.
Step 4: Insulating
Use polyurethane expanding foam that expands a little. Leave the 3X expanding foam onto the shelf. Install the expanding foam sparingly. If you use too much foam you'll distort the window forcing the framework inward.
You can see why, in case you haven't used any screws to fasten the shims in place, the shims become useless. Use the minimum quantity of material.
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