I am almost reluctant to enter into this subject because of all the information already out there that is already confusing. I will try to find a few links to, what I believe, are the most accurate instructions for installing windows and doors.
It is, perhaps, a little early to approach this subject, as we haven’t really got into the details on any other part of house building yet. It might, however, influence your choice of windows, and you may have to order them as much as 90 days ahead, and certainly 45 days, to ensure timely delivery.
Don’t attempt to install windows by yourself.
For even a small window, you should have some one inside to make certain the window is properly centered in the opening.
Make certain the rough opening allows about 1/2 inch of free space on all sides of the window. I have found that you can not rely on the manufacturers to give you the right rough opening size. They often try to second guess you. The best is to actually measure the window, or insist that the supplier give you the exact measurements of the frame.
Windows are constructed in so many ways, that it is not possible give instructions that are applicable in all cases.
The first consideration is building codes, and of course these must be followed when installing windows or doors. They may simply say that windows are to be installed according to manufacturers instructions. Secondly, the manufacturers instructions should be adhered to, as closely as possible.
Both windows and doors for new construction are commonly supplied with nailing flanges with which they can be attached to the framing of the building from outside.
Those supplied with a brick mold attached are usually intended as replacement windows. A brick mold is a molding of softwood or other material specifically used to case around the outside of a door or window. This does not mean they are not interchangeable, if you happen to get a deal. Those with flanges are attached with 1.5 inch roofing nails, or with screws. I prefer the nails, as they are less likely to interfere with subsequent finishing.
Wooden brick mold should be attached with 2 inch or longer finishing nails applied through the face and set. A vinyl brick mold may have a recess for screws. Again, follow the manufacturers instructions.
It is my opinion that nailing flanges alone do not secure a window quit well enough. It may allow enough flex to affect interior and exterior seals, if there are sudden changes in air pressure from outside or inside. Loud noises, or slamming doors, can cause pressure differentials which can be substantial on a large surface such as a window. On some windows, you may be able to fasten though the frame, but you probably would not want to. I think that the best solution all around is to use low expansion foam, specially formulated for windows and doors, both for insulation, and to secure the window. Carefully applied, it also provides an air and moisture barrier, although I wouldn’t rely on this alone, for that purpose. Of course, if the foam is to secure the window, it must make direct contact with both the window frame and the framing of the house. There should not be house wrap or flashing interfering with this bond. The exception is the pan flashing at the bottom of the rough opening.
Care must be taken when applying the foam, as to not warp the frame and making the window inoperable. When spray foam first became available in aerosol cans, it generally wasn’t available in low expansion formulations. A friend of mine, who owns a glass shop, experimented with it when replacing the windows in his own house. When the job was completed, he was unable to open any of his windows. To this day, he is leery of spray foam.
I have not seen much damage to framing by water infiltration from the exterior, but then we have a fairly dry climate. I have seen a lot of rot in wooden window frames that were not properly maintained with caulk, and paint, or exterior cladding. I did, in one case, have to replace an entire wall below a window. It was destroyed by rot, caused by moisture infiltration from inside. The window was placed at about head level above a bathtub that had a shower installed. Of course, there was no exhaust fan in the bathroom. Bad design first and poor sealing second.
A better job of applying house wrap will result, if it is applied before installing windows and doors. Cut the openings in an X diagonally from corner to corner, and fold into the opening on both sides and the bottom. Fasten near the outside edge, and cut off all but about 1.5 inches. This will leave bare wood for spray foam adhesion. Fold back a flap at the top to fold down once the window and the flashing is installed
Make and install a pan flashing, or purchase and install a ready made one.
You can make corners, purchase them, or the material you use may be flexible enough that they are not required. Extend this flashing a foot on either side of the opening. Caulk the top and side edges of the opening with a butyl type caulk before installing the window. Try to center the window in the opening with a ½ inch space all around. Use bottom shims on larger windows that have more than one panel. Make certain the window is plumb and square, and determine that it will operate before fastening permanently.
Some may say that a flashing is not necessary when installing windows with nailing fins, but I prefer to see the extra layer of protection. You could even add yet another layer of metal flashing for added insurance, but fasteners may cause some unwanted penetrations. As I said before, follow the manufacturers recommendations.
I like the idea of self adhesive and self sealing rubberized flashing strip, but it does not seem to be available in all areas. You can use strips cut from the material used for roofing, but the manufacturers of it may not condone it for this purpose, for liability reasons. You can also use asphalt impregnated building paper for flashing material. Use strips about one foot wide. Apply flashing down the sides covering the nailing strip and extending a few inches above the window and covering the pan flashing. If the top of the window is more than a foot below an eave, add a rain cap along the top at this point. Place the upper flashing so it covers the nailer and the ends of both side flashings. Pull the flap of house wrap down, and caulk and tape it in place. If you are using brick mold, at this point caulk the edges. If you are applying J trim for vinyl siding, caulk this as well. Seal from the interior with spray foam, using two or three passes, a few minutes apart. Make certain the window is well sealed against moisture penetration from the interior when insulating and applying vapor barrier.
Follow nearly the same procedure when installing prehung exterior doors, as for installing windows. Caulk the bottoms well. Consider placing a strip of sill gasket underneath.
To square a door, first make sure the base is level or slightly higher on the hinge side. If not, shim the bottom. Shim the hinge side of the door at the hinges to center the door, and to make it plumb. Make certain both sides are plumb in both directions, before they are fastened. Fasten with #10 screws, long enough to penetrate into the framing, through one hole in each of the hinges. Do not over tighten.
Use the door as a square by ensuring equal clearances on the top and lock side. Shim and fasten with 2 inch finishing nails placed at the shims. place them so they will be hidden by the closed door or the seal. Three or four should be sufficient. Screws may be used if you drill first to prevent possible splitting. In fact, I prefer to drill first for large size finishing nails as well. Install the lock set and dead bolt. Flash, and foam in place, in the same manner as a window.
For security, it is a good idea to use screws long enough to reach into the framing when attaching striker plates, at least for the deadbolt. It may be worthwhile to purchase a heavier plate which could be attached with more screws. I will mention again, if there is a window within reach use a deadbolt that is keyed from both sides. Shim the frame behind them to prevent distortion.
I once owned a mobile home that had a dead bolt surface mounted on the interior. I came home one day, and not realizing the door was bolted, simply opened it and walked in. It had been so poorly installed that I did not even notice the resistance. I think we should look for better security than that.
If any of the above conflicts with building codes or manufacturers instructions then go with the codes or instructions to avoid possible warranty problems.
If that is clear as mud it might get worse as you study further. Every builder uses slightly different methods, and has different opinions. Even the manufacturers rarely agree on best practices. Some methods may be unnecessarily complicated, and some may be not quite adequate. From all this you have to sort out what seems to be the best approach with the available materials.
The thing to remember; the main purpose of flashing when installing windows or doors is to stop air infiltration, and to keep moisture away from the framing and insulation. Work from the bottom up, as water runs down, and you want to overlap the layers.
I am not including any pictures with this post, but I believe the links provide enough visuals.
Installing a Window with Building Paper on OSB over Wood Frame Wall Buildingscience.com How to install a window This Old House How to install windows in a new house Ask Rob Basic window installation Sawdac Installing and flashing windows correctly FineHomebuilding