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Acri Windows & Doors / Windows  / How to Replace Exterior Window Trim

How to Replace Exterior Window Trim

Welp, we had some rot. Specifically, the trim around one of our exterior windows was rotting away. How can you tell if window trim is rotting away? It feels like a sponge when you poke it. Wood is not supposed to resemble a sponge. Since winter is coming (don’t laugh, we have embarrassingly short summers!), Mrs. Frugalwoods and I are trying to knock out a few needed projects on the exterior of the ol’ Frugalwoods home and this rotted window trim quickly ascended to the top of the list.


There be rot! Notice the indentation from my finger. Yeah, that’s a bad sign!

Our reasons for replacing it were not entirely cosmetic–rotten window trim can let water infiltrate behind your sheathing (the board, plywood, or OSB layer underneath your siding). If water gets to your sheathing, all sorts of bad things happen mold, rotting insulation, and in the worst cases, rotting framing members.

We’re lucky because the window itself is a relatively new vinyl replacement window. Vinyl windows are awesomely weather resistant so I knew we didn’t have actual window issues. If our windows were wooden frames, we’d likely have a bigger problem on our hands.

Fixing it isn’t rocket science, but the going local rate for this project is $350-$500. Knowing that this cost is almost entirely labor, we figured we could do better, and we did!

Total Cost: $80.33



  • Drill / Driver for screwing the trim to the house. We needed to use screws instead of nails for this project due to the type of trim we were installing. And while a standard cordless drill will do fine for driving screws, an impact driver will freaking change your life. This is the one I have, since it uses the same battery packs as my drill. I put off this purchase for a long time and I kick myself for doing so.
  • Pry bar for pulling apart the existing trim. For the longest time I resisted getting a real one, thinking that I could do just fine with the claws on a hammer. I was dumb. Pry bars are amazing and I now own 2 in different sizes. It makes pulling stuff apart easier, safer, and less likely to destroy things you don’t intend to destroy.
  • Circular Saw for cutting the trim to size. You could use a miter saw, or even a hand saw. But, I have a circular saw so that’s what I used. This project doesn’t need a nice saw, so if you are in the market feel free to get an affordable model. I’d just recommend one that plugs in rather than taking expensive and short-lived batteries.
  • Carpenter’s square for making straight lines. This is another tool that’s easy to ignore, but will make your life easier and more accurate. I have an old one from a garage sale (excellent source of tools!) but the standard stanley speed square is a great new option.
  • Shop vac for cleaning the area during and after the build. Your neighbors and spouse will thank you. Wait for the cheap one to go on sale at Home Depot / Lowes. Unless you are using one really heavily… they’re all pretty much the same. A handheld “dustbuster” can be used in a pinch, but clean it out well or you’ll be getting yelled at!
  • Measuring tape for… I don’t need to explain this, right?


  • Cellular PVC trim. This stuff is magical. Looks like wood, cuts like wood, takes paint like wood… but it’s plastic so it won’t ever rot. This is what you want to use if you have to do trim work in wet areas. And if you’re replacing previous trim because of rot, then you definitely want this. It’s more expensive than normal poplar trim, but you’ll never have to do the job again.
  • PVC compatible screws. PVC trim is great but you need special screws with reverse threads at the top to prevent it from mushrooming out when you drive it in. There are special trim screws for this, but we found the same thing for less marketed as decking screws for composite decks. Might as well save a few bucks here.
  • Spray Foam. There is a special kind made for windows and doors that won’t expand with enough force to bind the window. The “Great Stuff” brand is ubiquitous and just fine.
  • Flashing. I used a flexible and sticky asphalt flashing commonly known as “Ice and Water Shield.” It’s sold in big rolls for roofs but you can also find smaller rolls meant for windows.
  • Caulk. You want exterior UV-rated paintable silicone caulk. The folks in the paint aisle can point you in the right direction from the multitude of caulk tubes. If this is your first time using caulk, you’ll also need a caulk gun. Don’t worry, they are cheap and sold near the caulk.

How To Replace Exterior Window Trim

Step 1: Demo!

Demolition is my favorite part of any home improvement project. Nothing like bashing things with hammers to make me smile! For this project though, you’ll want to be gentle.

First, try and figure out which trim sections are rotten. Our window had both a flat board (sometimes called a “brick mold”) as well as a decorative molding. I originally thought it was only the brick mold that was rotten, but it turns out the molding was also getting soft in spots. Poke around with a screwdriver and you’ll quickly note which components are in need of repair. If it feels like wood, great! If it feels rubbery or spongy… then it’s time to tear it out!


It’s Everywhere!

Once you’ve determined which components need to go, use a small pry bar to start removing the pieces. Your goal is to expose enough of the boards to get accurate measurements. Once you can clearly see what’s where take a photo and measure the dimensions. If you can keep the trim pieces from falling apart as your demo, it will make cutting your new pieces to fit all that much easier.

Now finish removing all trim boards and pray to the god of old houses that you don’t find additional rot as you delve deeper into your wall. If you do find rotten sheathing or even framing… then you have bigger problems. Not insurmountable, but also maybe not a weekend project. If that’s the case, then you should go find another how-to post

Step 2: Shop

Now that you know the quantity and size of trim boards necessary, head to your local home store and make a beeline for the construction supplies. Here’s a pro-tip: Most of the pros are pretty nice. If you are confused, ask the guy in the filthy shirt with giant bulging muscles for advice. Dude will almost always hook you up. In our case it was a very nice Eastern European gentleman with forearms the size of cantaloupes who explained the finer details between brands of cellular PVC trim.

Step 3: Insulate

It’s time to inspect and insulate around the window. If you have a house built in the last 50 years, it won’t look like this. Modern houses use plywood or OSB (oriented strand board) as their sheathing. Besides being cheap, strong, and not requiring the destruction of old growth forests, OSB and plywood also make for very tight window installs.
Old houses like ours have board sheathing. In addition to being expensive, weak, and requiring the wide-scale deforestation of most of the United States, board sheathing tends to be poorly fitted around windows. Case in point:


Here’s how the window looked without any trim. Note the boards as sheathing.

There’s probably a better way to do this, but wow do I love spray foam. The stuff is magical, especially for filling giant irregular voids.

Four extremely important things to know about spray foam:

1. Wear latex gloves! This stuff is uber sticky. Like interior of a hot marshmallow sticky. And it’s not good for your skin. And you are almost certain to get it all over your hands. Wear some gloves and you’ll thank me later.
2. Make sure the area you’ll be foaming is clean. Foam will stick to anything, including the layer of dust and dirt that likely resides around your window. A shop vac makes short work of this.
3. After it’s applied, it continues foaming and expanding. So make sure you lay down a thinner layer than you need and let it expand to fill. You can always add some more on the second pass.
4. Once you break the seal and pull the trigger on the can of foam there’s no stopping. You can’t re-use a can of spray foam and it starts to harden in the tube after about 15 minutes. Sooner if you don’t spray for a few moments. You have to keep moving and spraying if you want to get your money’s worth out of the can.


Me to Mrs. Frugalwoods: Take all the photos you want BEFORE I start spraying…

So have a plan! Move quickly and evenly apply a light layer of foam wherever your voids are deepest. Keep working your way around the window filling cracks, voids, holes, and whatever else gets in your way. In our case, we had giant voids to fill. If you have a newer home you might only have the area immediately adjacent to the window frame to insulate. In either case, insulation is worth the money while you have the window opened up. Might as well do it right.

After you’ve foamed all there is to be foamed (plus some stuff that probably shouldn’t have been, but you know how it goes…) take a break while the foam hardens. It should be fine to continue working after 20 minutes or so.


Just look at that nicely foamed (sort of) window opening!

Step 4: Flash that window!

If your house was built correctly, you won’t need to do this step as you’ll already have flashing. As for us, we needed it.
Flashing a window is a way to seal the area around the window from water infiltration while allowing any water that gets behind the trim boards to exit the wall structure. You can use all manner of materials as flashing: plastic, aluminum, lead (yes, and it works great!). We chose a flexible, sticky asphalt product commonly know in the trade as “Ice and Water Guard.”
This stuff is really easy to use. Just cut your pieces to size, peel the backing paper off, and stick it up.


Pro-tip: Don’t peel the paper off the flashing until it’s in place. This stuff will stick to anything and everything!

You’ll want to apply the various pieces around the window from bottom to top. That is, you want water to be able to travel down the face of the flashing and never find an overlapped seam facing upwards. For the top strip of flashing, try to get it as far underneath the layer of siding above as possible. For the flashing on the sides, try to end the flashing on top of the bottom course of siding. This sounds great in theory, but reality can prevent it from being perfect. Just try the best you can.


Look at that nicely flashed window!

Step 5: Measure and cut trim boards

Now measure for your trim and cut. I like working from the top down, but I have no idea if that’s actually better.

Our old brick mold wasn’t mitered in the corners so I followed the pattern and butt jointed it. Yes, I just said butt joint. It’s a carpenter’s term, for real. To cut a straight line with a chop saw or miter saw is simple. If you are making the cut with a circular saw, draw a straight line on the board with a carpenter’s square to make sure you don’t wander. Don’t be a cowboy, you and I both know you can’t eyeball it.


Use your handy dandy carpenter’s square to make a nice right angle.

While the left side and top of the window was a dead ringer for 1×5, the right side was juuuust too small for me to wedge it in there. Oh the joys of old houses! So I scribed the 1×5 to match the variation in the siding edge and ripped it with the circular saw. This I did mostly eyeball because I knew I would be covering the crappy edge with the decorative molding. Screw the boards in place with your nifty special screws (they are actually pretty neat) and step back and admire how much more like a window it looks already!

Step 6: Miter your decorative molding

Unlike your trim board brick mold, decorative molding is pretty ugly in the corners unless you cut it on a 45 degree angle–aka a miter joint.
When faced with cutting a miter joint, normal folks would go out and buy a miter saw. This makes it really easy to cut perfect miters every time, but will run you a couple hundred dollars.
If you are like me and a member of team “if it can’t be done with a circular saw then you shouldn’t be doing it,” then read on. Cutting a 45 degree miter is not much harder than a 90 degree straight cut. The trick is getting a good line in place, and for that the carpenter’s square is essential. Measure carefully to the outside of the miter (the longest part) and check your marks twice before cutting. It’s really easy to get confused and draw your 45 degree line in the wrong direction (ask me how I know this…).


Steady hands and a good line to follow make cutting a miter with a circular saw not that bad.

Once you have your line in place, carefully follow it with your circular saw. If you go slowly and check your measurements, no one will ever know you didn’t use a miter saw. Now screw in your molding and admire your handiwork!

Step 7: Caulk

Oh you thought you were finished? Not quite. Break out the caulk and run a bead on all of the joints. Smooth each joint with a moist finger (no joke, the best way to get a good caulk line). Once the caulk is dry you can decide whether you want to paint. So far we haven’t, because the new white trim matches our existing trim pretty well.


Don’t forget to add a dab of caulk to each screw hole as well.

Step 8: Victory!

Now you are actually done. Bask in the glory of saving hundred of dollars. Revel in the new confidence you built in your DIY skills. Or just take a shower. You probably need a shower real bad by now.


The finished product!

Source: Mr. Frugalwoods

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