Installing a New Construction Window

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Installing a new window can be as simple as: remove old window, add new window in existing hole, make sure it’s plumb and level, install new trim inside out (don’t forget to add a drip cap), caulk, done.

But, if you hadn’t noticed it in our window blinds turned serving tray craft, we really like to make our tutorials exhausting exhaustive.

Brace yourselves and let’s get to it!


This tutorial is particularly for new construction, vinyl windows, installed in houses with wooden siding (cedar, hardboard, etc). If you have vinyl siding, you have to remove all the adjacent siding cautiously – so it doesn’t get damaged – before you proceed with the removal and installation.

Also, if you live in Greece or any other place that builds walls with concrete and cinder blocks/bricks, this tutorial is quite useless to you (sorry!).

The materials and tools needed for this project, are at the end of the post.

New construction windows are easily distinguished by “replacement windows” by their “nailing flange”. If it has a nailing flange, it’s “new construction” – it’s that simple.




How to Install a New Window

Before We Start:

Point zero: Check your area’s building codes (in USA they differ per state), make sure everything is legal and sturdy, get permits if required, get professionals to help or test your installation. Be safe, be legal, etc.

Ok, let’s start.

1. Remove the old window. Clean up the opening from any insulation and debris.

2. With a utility knife, cut the excess drywall (if any) flush to the opening.



2. Check your window opening (frame) for:

  • any rot or serious water & insect damage. If you find any, you should repair the problem before you install the new window. The damage can be as little as a rotten window sill (easy to replace) or as far as molding sheathing.

In our case, water had seeped under the siding (that rot at the edges) but while it also seeped in the frame, it did not rot that part (which was lucky) or the sheathing. There is a very simple way to test that: Knock the discolored area with a hammer or screwdriver (gently); if wood chips easily, it’s rotten. Ours was not budging one bit, so – surprisingly – it was still in good condition.




  • plumbness and level-ness. Depending on how much off-level the opening is, you will need shims and perhaps planks of wood (if the opening is way off). It is not an expensive job but it can be tedious.

3. Dimensions: Make sure your new window is either a perfect match for the opening, or a little smaller. (I will show you how to add a much bigger window in another post, but for now, let’s assume we will have only small adjustments to make to our opening.)


Assuming our opening is decently level and square:

1. Adjusting for size

If you didn’t buy a window that fits perfectly in the opening (usually a custom one), you can adjust by adding a plank/stud piece. In my opinion, if you have to get a smaller window, try to aim for shorter – rather than narrower. It is easier to simply add a piece of wood on the bottom than supporting side studs.

How To:

Stud walls are usually made with 2 x 4s OR 2 x 6s studs. Check what your wall’s depth is, cut a stud of that size to match your opening and screw a matching piece of sheathing (check photo bellow). Sheathing is usually out of plywood, OSB (oriented strand board) or wood planks. Get the same material AND at the same thickness as your existing sheathing.

Measure and cut the size you need in both stud and sheathing. With a framing nail gun, nail the sheathing piece to the stud piece:



The purpose here, is for our add-on piece to seamlessly match the existing walls, excluding the drywall and siding: we only size-match the studs and sheathing.


(that piece of drywall was already broken when we bought the house)

2. Dry fit the window, check for fit and – with a pencil – mark where to cut the siding – so the flange fits. Cut the excess siding (if any) with a dremel or a skill saw (adjust the depth of the blade so it cuts ONLY the siding and not the sheathing underneath it).



3. Adding flashing

Super important! Really. We used tar paper and this is how to install it properly:

  • Starting at the bottom, cut stripes of tar paper (you will need 4 pieces: bottom, top and two for the sides) and slide it between the sheathing and the siding starting from the bottom (sill).
  • Then add both sides and finally add the top. The order matters as you want the flashing to overlap so water glides away.
  • Once all pieces are in, wrap the flashing (from bottom, to sides, to top) around the studs and staple everything in place.
  • Cut any excess on the interior, with a utility knife.


In reality….



Employ several choice words trying to persuade the flashing to slide in place.

Use a flat screwdriver to gently lift the siding. Employ more choice words as this will barely improve the situation.

Realize there are siding nails blocking your tar paper and remove them.

Persevere. Teach that siding that resistance is futile and finish the job! Once job is finished, feel free to gesture – victoriously – at the siding.


This is how it should look once done properly:



3. Time to put our window in place: Apply a silicone bead across the sill (bottom of the window opening) and up 2 inches on both sides.



Put the window on the sill in an angle and gently but firmly, push in place.


4. This is the second most important part for me: checking if the installed window is level and plumb. Because if it isn’t, it will be ALL I will see and think about, for years to come.

Nail one galvanized roofing nail in one of the nailing flange’s corner holes, to keep it in place and using a construction level (though I much prefer the laser ones), place it underneath the window and on one side (on the exterior side). Fix any slope with shims.


5. Finish nailing the window in place.


Well, almost…

Our window is installed but, we should make it pretty and finish water proofing around it.


6. Seal with primer & paint – the exposed cuts of the siding.


7. Screw in the exterior trim/brick mold. We added a simple vinyl brick mold, with mitered cuts. Cut back the siding as needed, to precisely fit your brick mold, using a skill saw or dremel (we used a dremel):



And naturally, as it usually is the case when we install windows, it started raining and we did not finish adding the brick mold that day…If that happens to you, too, temporarily cover the window with a plastic sheet and duct tape. Ain’t pretty but works.


8. Add a drip cap edge.


9. Final waterproofing (almost done!): Use exterior grade, waterproof and paintable (or clear) silicone and apply it:

  • Above and around the drip cap edge
  • All around the trim/brick mold where it meets the siding
  • All around the window casing, where it meets the brick mold
  • All the screw holes and all the mitered cuts of the brick mould

Silicone tends to sink a bit once it dries. You may want to add some more before you paint it.



10. Fill the gaps between the window casing and the wall with expandable spray insulation (or any other insulation you prefer). If you use the spray, be frugal. The foam expands and can even bend the casing. Use a little and let it swell.


11. Finish the interior side, by adding the interior decorative trim of your choice and even more caulk (here is how to do that and how to make perfect seams).



Materials & Tools

(Affiliate links included, please review our full disclosure here.)

New construction window

Brick mould and trim (to finish up the installation)

Construction Level

Tar paper (or flashing of your choice)

Heavy Duty Stapler & Utility Knife

Galvanized (for protection against rust) roofing nails & a Hammer

Skill saw (circular saw) or Dremel (like this one)

Screws & Screwdriver

Silicone & Caulking gun (jump to this post for really good tips on caulking guns)

Probably a ladder

Optional: Framing Nailer (only if you need to add a stud piece – we used this one)


ps. There are 3 new construction windows that posed for the photos of this tutorial, in case you wonder why the walls (and the caulking jobs) look different. Especially concerning caulking, there has been a vast improvement on my part.

If you have any questions or noticed that I missed something, let me know! 🙂

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16 thoughts on “Installing a New Construction Window

  • October 23, 2018 at 11:01 pm

    Thanks for the tips for installing new windows. I like how you walked through even getting rid of the old windows. To be honest, I think that hiring a professional to help put in our new windows is what we’ll do. It just seems safer.

  • October 24, 2017 at 9:48 am

    Helpful post! Such a good thing for those wanting to change their windows on a budget, why not DIY? Thanks for sharing this.

  • May 3, 2017 at 3:34 am

    Oh, you are killing me with your humor, not to mention the projects and the Instagram pics! :)))
    Where have you been hiding? Anyway, I sooooo glad I found you. You rock!

  • April 30, 2017 at 9:11 pm

    Dione, Here’s my feedback & hope it’s helpful:
    – You always want new window to be slightly smaller than rough opening, so you can use shims to get the window level (rarely would I expect rough opening to be level or plumb).
    – Don’t be afraid to order custom windows & be sure to learn how to measure them so you get exactly what you want. If you’re doing a ton of renovations, try to register as a contractor with your local building supply house that sells custom windows.
    – Didn’t see any mention of the groove on the underside of the window sill, called a drip cut to prevent water from getting into your house (see video,
    – Installing your window flashing is more complicated & you need to be especially careful around corners where sides & horizontal pieces overlap. Here is more detail about this … FineHomeBuilding’s website, It looks like they used a peel & stick flashing membrane which is much, much better as it won’t move & it seals around your nails (similar to Ice ‘n Water Shield used on roofs).
    … so getting a bit nervous about all this, so think I better write an article myself pulling all the pieces together – will let you know when done

    • May 1, 2017 at 12:00 am

      Good suggestions Tina and thank you for your comment!

      This is a new construction window, the link you gave is for replacement windows. I don’t see a difference in the flashing part besides the material used. I don’t see them use any silicone between the flashing and the window (perhaps it’s unnecessary with replacement windows)?
      Our flashing is stapled in so many places (by me – the guys said I was overdoing it), I doubt it will move in the next…decades.
      I think the reason you don’t see how well it overlaps (which is identical to the link you gave), is likely due to my very poor photography skills. I could swear I had more photos of it but alas, no.

      I’ll definitely see if there is enough room under our sills to add a drip cut! Anything that adds water resistance and helps my windows last longer is a winner in my book.

      And…you shouldn’t get nervous. You provided a heartfelt opinion and is well appreciated. 🙂

  • August 1, 2016 at 4:02 am

    This project will really be money-saver 🙂

    Your tips are really helpful for a new DIY-er like me.

  • July 29, 2016 at 5:57 am

    Nice blog! I’m about to ask someone to remove some old windows in my house, but after reading this post I think I will do it myself. Thank you!

  • July 15, 2016 at 7:07 am

    You lucky girl, you saved a ton of cash by DIYing and installing your own windows! We unfortunately have a block house, and even I won’t attempt to replace ours. Plus they have to be hurricane proof which equates to big $ 🙁 I know you are loving your new windows..and next time just go for it and use the nail gun! You can do it! LOL

    • July 16, 2016 at 12:56 pm

      I did, I did (save)! We had to replace all the windows (12) and that would be quite the labor cost.

      If I had to install hurricane proof ones, I think I’d cry.

  • July 14, 2016 at 1:33 am

    You’ve just made me not want to change our windows ever. I think I’d rather just pay someone else to do all the choice word sounds for me LOL. Looks fantastic!

    • July 14, 2016 at 10:44 am

      Hahah, darn, I made it sound harder than it is.
      It takes a lot of words to explain but the process is just a few hours of pushing and nailing. 🙂

  • July 11, 2016 at 4:40 pm

    They look really great! And you’re saving a ton of money. I’ll be pinning this for future use because I predict new windows in my future.

    • July 13, 2016 at 3:06 pm

      They are a huge improvement even just for the fact they close and open easily and fully (unlike the old windows)!
      And yes, we are saving a few thousands (insert big smile here ^^) by DIYing the installation.


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