I am in the process of replacing an old single hung window with a new nail flange dual pane window. I have removed the old window but I do not know if I need to remove the 1 inch thick wood jamb frame. If I do, the new window will not have any material for the nail flange to nail in to. I am wondering if I need to remove them and expose the bare 2x4 framing studs or if I can just nail them into the existing 1inch thick jambs of the old window.

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    So these "nail flange" windows are face nailed, perpendicular to the plane of the window? If so, from what I know, these are what are normally installed in new construction rather than as replacement windows. If that is what you have, then it seems to me that you would need to leave the 1" thick jamb frame to have something to face nail into. Replacement windows do not have a flange, they are fastened through the inside into the jamb or the rough opening. Then I guess you put trim over the nail flange after flashing. – Jim Stewart Apr 18 '17 at 22:52

Do I have to remove the old window jambs when installing new nail flange window?

The answer is yes; if you use that nail-on window the way it was designed to be used it nails to the underlying wall studs. If you use a "nail-on" window like you describe, face-nailing it to the old window jambs, it will look like crap and will probably leak.

If you do not intend to purchase a window designed for your application (a block frame or "replacement style" window), an option might be for you to cut off the nail flanges so you end up with a window that has what is essentially a box/block frame. Then drill holes through the frame and screw it to the existing jambs, caulking all the way around and finishing it off with aesthetically pleasing trim of some sort. I have done this with success.

enter image description here

The picture above shows frame types, from left to right:

  • nail-on frame for new construction, usually purchased in standard sizes.
  • block frame for various replacement applications, and very suitable for replacing old windows when the jambs and sill are left in place. Usually ordered to specific size needed.
  • "replacement style" window, also used for various replacement applications and includes exterior trim flange to hide old jambs. Usually ordered to specific size needed.
  • Thats a good idea! I can just remove the nail flange. The only question I have about that is drill holes through the frame and screw it to the existing jambs. Wont that cause the crew heads to protrude in the inside of the frame, causing the windows not to close? Surface needs to be flat in order for them to slide open and closed, right? – user2611836 Apr 21 '17 at 20:38
  • Most windows have enough clearance between the sash and frame for pan head screws #8 or #10 – Jimmy Fix-it Apr 25 '17 at 2:27

Windows with flanges are designed to be nailed to the studs. You can get a larger window which equals more glass and light. With "replacement/insert" windows you are placing the jambs of the new insert window inside the jambs of the old window, so you are losing about an inch on all sides: you essentially have double jambs, which equates to less glass, light, and/or visibility.

If your replacement window with the flange does not reach the studs after you rip out the one inch framing/jamb of the old window you can build out the studs by adding the appropriate lumber to the sides and header and nail the flange to that. This obviously does not eliminate the loss of glass/window as it appears the window was sized as if the old window jambs were the studs. The other option was stated by another-remove flange and treat as insert window.

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