The best way to properly seal around rough openings of window frames is to remove trim on both the inside and outside of the window frame. You should now be able to see any filler between the wooden window frame and the rough wall opening. Now is also the best time to adjust any settling of the window in the wall and re-square the window if you have any abnormally overt problems. Remove any of the filler. depending on how old the home and windows are and the building system used at that time, there may be celotex strips in there, cellulous (it looks like mouse nest packed in there), fiberglass, news paper, or if recently done, foam.
Remove any loose or inferior, packing and using a 'plumb bob' to guage square; you can make a simple guage by using a piece of string, thumb tack and a good size metal washer for weight and tie the washer to the bottom end of the string and with a loop on the top end, thumb tack the string in the top left corner of the window frame so the washer hangs down without touching the sill. With a ruler, measure from the top of the string to the left side of the window frame and again near the bottom of the string to the left side of the window frame. If the difference is less than an eighth of an inch, I wouldn't change the 'setting' of the window in the rough opening.
You may also use a small topedo level to check both side and sill for square.
If you require 're-setting' the square of the window frame in the rough opening, remove any of the anchors that have been used to attatch the window frame to the rough opening. In a wood frame building, there are usually nails driven through the window frame and into the studs of the rough opening and should have some kind of shims or spacers holding the frame out to create 'square'. However don't freak if they are not there as this may be why the window isn't square in the first place after you test and determine its not square. Windows should never be installed tight into the opening but shimmed in to securely 'float' in the rough opening so that there is room to adjust the window to square and to pack the gap to insulate and seconarily soft hold the window frame and shimming further in place. With your plumb bob in place, this permits you to regularly take measurements to check for square during the 're-setting' until you have the window where you want it. There should be two nails on each side about 1/4 the way from the top and bottom. Tapered cedar shims work best but have seen broken lath used. Don't hammer on the window frame with the sashes in place unl;ess you want to re-glaze your window panes too. Drive your nails in with shimming in on both sides adjusting the amount of shim stock on one side or the other until you establish square, then nail the nails in flush but no further as drawing the nails in will drive the setting off.
If the building is block or concrete, it may be bolted or nailed in with older nails designed for concrete called steel cuts. If bolted, loosen bolts and insert shimming as needed until square then follow the repacking instructions above.
If the windows are installed with steel cuts, we suggest using blue masonary screws with matching pre-drilling bits designed to permit the screws to cut into the concrete as they are screwed into the drilled holes. Have your local hardware or lumber yard help you select the proper screws and drill bits. Drill into the wood window frame with a wood bit first. Then drill through the hole and into into the concrete or block with the masonary bit. Any shims should be pre-drilled with your wood bit first also. Insert your masonary screw into the wood frame shallow, insert the shims bewteen the frame and concrete wall and line up the pre-drilled holes in the shims so the screw will pass through the shims when the screw is pushed the rest of the way through. The screw should have between a half and three quarters of an inch but not more than two inches of screw left to drive in to be flush with the window frame surface. Your pre-drilled hole in the block or concrete should be at least a half inch if not more deeper than the remaining screw length. Bottoming out of the screw in a shallow hole will cause the "tapping" of the screw in the pre-drilled hole to fail and the screw will no longer Anchor the frame in place.
Once the window is Square and Set in place, the old method is to use a putty knife or screw driver and push or pack pieces of newspaper, cellulous, or fiberglass insulation into the gaps to close off the passage of air around the window frame.
The modern and most efficient method, which as a restoration contractor, I am not opposed to as it is concealed, is to use an A/B componant foam or canned expansion foam such as Great Stuff or something comparable. Shake the can well, turn it upside down as most directions require, insert the straw or semi rigid hose like nozzle into the opeing and push the trigger, discharging the cans contents into the opening allowing it to expand and fill to seal the gap. Let the foam cure for 4 to 6 hours and then using a razor knife, trim the now dry foam flush with the wall surface both inside and outside. You may now re-install the trim boards and caulk around the trim boards where they seat flush up against the wall surfaces and wipe clean with a damp cloth. This will give you a nice clean joint and will look great when painted. Hope this helps.
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