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Install & Replace

Replacement Window Installation

What is the proper way to install a window going from two single double hung windows to a single unit twin double hung window unit? Is there anything particular that needs to be done to the framing after removing the center? 
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Posted 2016-05-23T14:37:43+0000  by DIYer80 DIYer80

Hello DIYer80 and welcome to the Community.


Your basic window frame has a header on top, jack studs on the sides supporting the Header and stool across the bottom.  These are supported by cripple studs underneath.


In your case you would have an addition set of vertical studs called a mullion.  This is the support framing for between the 2 individual double hung windows.


You will need to remove the mullion.  If the Header and the stool frame are continues pieces and in good condition, you replacement window should be able to be installed, in the now cleared space.  If you have any wood rot this would be the time replace any effected materials.


Most manufactures supply complete installation instructions.  You can usually check those out on line ahead of time.


Reference materials: click on the orange words.

The following is a link to 10 essential steps for a proper window installation.

How To Install a Replacement Window - The Home Depot

Window Installation and Window Replacement

Jeldwen installation video for replacement windows.

Anderson installation guides video.

Anderson installation information by model and type.


Enjoy your new window and thank you for your inquiry.



Posted 2016-05-24T18:37:29+0000  by Char_HD_CHI
Thank you! That information has been very helpful. It appears that when the mullion is removed there is a gap in the header. Should the header be replaced or just leave the gap and install the window? 
Posted 2016-05-25T16:33:34+0000  by DIYer80
Hello Diyer80,

The header is a structural support for the wall above the window and possibly what is above that (second floor). Here is a very good video that shows the importance of a sturdy header. The old mullion should have been installed under the header as a vertical nailing member. If it was a structural support the header should be repaired or replaces.

If you have futher questions please reach out.

Thanks for your inquiry and visiting the Home Depot Community.

Posted 2016-05-26T13:02:27+0000  by Char_HD_CHI
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