What Is the Most Common Window Installation Mistake, and How Do You Spot It in Your ADU?

By Buz Artiano, BuildX | October 2026

Watch: Window Delays, Install Fails and What Builders Always Miss w/ Chad Nicoli, BuildX Podcast #25

You are standing in a framed opening looking at a brand new window, and you have no way to know whether it was installed right. The glass looks fine. The trim will cover most of what you can still see. Six months from now, if there's a draft under that window or a stain on the sill, the crew that put it in is working on a job in another town. Between thinking it looks fine and knowing it was done right can be many window problems and almost nobody tells a homeowner what to look for.

My name is Buz Artiano, and I am the founder of BuildX. We've built dozens of ADU projects across Massachusetts, mostly around the South Shore and Plymouth County areas. We are the only team you need: we do the design, permitting, and construction. So we see every phase of a project from the first site visit up until you move in. We evaluate septic constraints, zoning issues, utility connections and the requirements for a permit from the city. And we are also the ones that are responsible for making sure the windows are installed correctly.

Window installation is one of just a few details that a homeowner can inspect without any training at all. We asked questions directly to Chad Nicoli, trade sales manager at Pella Windows and Doors for eastern Massachusetts. His team deals with a territory that goes from the New Hampshire border down to Plymouth. We asked him to name the one most common installation mistake that he sees. His answer was only two words and once you hear it, you can look for it yourself in about ten seconds per window.

What Is the Most Common Window Installation Mistake?

Wrong shimming, or no shimming at all. That was Chad's answer and there was no hesitation. We asked if he meant the whole unit or just one part of it and his answer clarified it even further. The jambs usually in his words if there's an installation error, it's the shimming.

A shim is a thin tapered wedge of wood or plastic, that's put between the window unit and the rough wall opening and its job is to hold the frame straight and square while it's being fastened. The rough opening is a framed hole the window drops into. The jambs are the two vertical sides of the window frame. If the jambs aren't shimmed the right way, the frame will become whatever shape the rough opening happens to be in, and a rough opening framed by hand is never perfectly straight.

That's the whole failure. It's not unusual and it isn't a defect with the window. It's a crew moving quickly through fourteen openings in a single afternoon, and treating the shim like just a formality not an important adjustment.

Why Does a Bad Shim at the Jamb Cost You the Whole Window?

Because the sash has to meet the frame evenly on all four sides, and the weather stripping will only work when it does. The sash is the part that moves and holds the glass. The weather stripping is the compressible seal that closes the gap between the sash and the frame if the window is shut.

When a jamb has a bow in it of even an eighth of an inch, the sash will press hard against the weather stripping on one side but barely touch it on the other side. Nothing will look broken because the seal is just uneven. Air goes in and out through the loose side all year, and on a windy day with rain pounding against that wall, water will go in the same way the air did.

Jambs that have bows in them are so common in the construction field that an experienced builder can see them from across the room. Once a jamb is bowed and the casing is put over the top of it, fixing it means you have to pull off the trim, take out the fasteners and reset the window. That's why it rarely gets fixed.

How Do I Spot a Bad Window Install on My Own ADU?

Look at the reveal which is the gap you can see between the edge of the sash and the frame it sits in, and it should be the same width everywhere. Stand inside the room, close and lock the window, and look down the gap on the left side and then the right, and then across the top and the bottom.

Three things to check at every opening:

• The gap has the same width from the top of the sash to the bottom on both jambs
• The sash is parallel to the frame, not tight in one corner and open at the diagonal one
• No daylight can be seen anywhere around a closed unit before the trim goes on

A gap that begins narrow at the top and gets wider toward the bottom means the jamb is bowed and the window isn't square in the opening. Daylight around a closed window is another way to tell, and it's the one we've seen ourselves on a window unit. Check on the gap before drywall goes up, while a correction will only cost a small amount of time.

The Shim Materials Behind Every Window Jamb

Most builders still use shims that have wood shingles. These aren't a manufactured product or a kit, they are just cedar shingles, the same ones that go on a roof or a sidewall, cut to a certain length and tapped into place. Chad said that his company doesn't supply the shims for the windows at all, and that old-school wood shingles are what's normally used on job sites in this market.

Newer installation systems are starting to do it differently. Some product lines come with molded shim guides on the jamb that show exactly where the shims should go, with plastic shims available to go with them. The guides take the guesswork out of where to place them, and it's a real improvement over a crew having to figure out by feel where the pressure points should be.

The material is less important than the installation. A wood shingle in the right spot, checked for spacing, will keep a jamb straight for the life of the building. A plastic shim in the wrong spot does no good at all.

Who Should Shim the Windows on Your Job?

On most building sites, the framing crew also puts the windows in, because they're already there and they've made the opening. We stopped doing it that way. When we build an ADU, the framers don't shim our windows. We shim the jambs ourselves and we also foam the ADU, so the same people who set the spacing also determine what is sealed around it.

We also don't just check that spacing with our eyes. We use a feeler gauge, which is a set of thin metal blades of a certain thickness that slide into a gap and give the exact width. Checking a gap with the eyes works well enough on a small window but doesn't work on a tall one, because the eye will make a correction for perspective. So a six-foot unit can be off enough to matter at the middle of the jamb but still look fine from four feet away. The gauge doesn't have an opinion on what the window looks like. Either the blade that fits at the top fits the same at the middle and the bottom, or the shim needs to be moved.

This isn't an extra service and it costs almost nothing to do. It's a decision about who cares most about the outcome. A framing crew is paid to frame a building, and the window is just one of a long list of things that have to be done that week. When the window is our responsibility instead of theirs, then the standard changes.

The Failure That Changed How We Set Windows

Years ago, for a house that had a full package of windows from one manufacturer, we put in a window and could see daylight around the entire outside with the sash closed and locked. We called it in as a defective window. The response that we got back was that it was a problem with the shimming.

The service rep arrived and told us the same thing. We'd already measured the gap at both jambs with a gauge before we called, so we handed him the results and asked him to show us where we were off. The conversation stopped abruptly.

What he did next is what's worth remembering. He went out to his truck and came back with a roll of larger weather stripping, took the sashes apart, and then replaced the stripping in every window in the house. It turned out that the gasket on that line was undersized. When we asked why the better material wasn't put in the units to start with, the answer from his side was one word: cost. A technician for the manufacturer of the windows on your ADU runs about a thousand dollars a day, and they still chose to send one instead of upgrading the part.

We don't give the name of the manufacturer here, because the lesson isn't that one company shipped a gasket that didn't fit in one model year. The lesson is that when a window fails, the first thought for everyone is that the installer did it wrong, but sometimes taking a measurement before anyone files a complaint will put a stop to the argument.

Can Your Builder Get Help on the First Install?

Yes, and asking for shows you are a careful builder instead of an inexperienced one. Manufacturers will provide a person to help on the job for the first window. Chad told us the options clearly: a sales rep, a sales rep together with one of the managers, or, with enough notice on the schedule, a project manager on site.

That person will walk the crew through the sequence in order: what materials need to be available before the windows get there, what gets flashed and sealed and the order of it, how the window gets shimmed and what gap to check once it's set. The crew installs the first window with someone watching who installs that product for a living. Then they install the remaining windows the same way.

If your builder is using a crew to install windows for the first time on your ADU, that's a fair question to ask directly. Don't bother with the question of whether they've installed these before, which tells you nothing. Ask whether they plan to have the supplier on site for the first one.

Is an Uneven Reveal Always the Installer's Fault?

No, and we'd be overselling our own methods if we said otherwise. The windows come as a square. Sashes come through this very stiff hardware that can feel like a defect when it's not. Gaskets are sometimes shipped too small, which is exactly what we found on the job described above. An uneven reveal is proof that something's wrong at that opening but it isn't proof of who caused it.

That fairness also applies to framing crews. Plenty of them shim windows well, and taking the task off their list of things to do isn't a judgment of their skill, it's a judgment of their focus and accountability on a busy schedule with four hundred other things. If your builder leaves the windows to the framers and has a way to verify the reveal afterward, that can work. Failure doesn't depend on who holds the shim. It's more about whether anyone is checking.

There's also a limit to what the gap tells you. It verifies the unit is square and holding its shape in the opening. But it doesn't tell you anything about the flashing behind the trim, the sealant at the sill, or the insulation in the cavity, all of which are enclosed by the time you check the room. The gap is the one thing you can check alone but it's not the only check that matters.

Ready to Walk Your Own Window Openings Before Drywall?

It takes ten seconds, no tools, and just one question for whoever is running your job. If the gap has the same width down both jambs, no light shows around a closed sash, and your builder can tell you who sets the shims and how they check the spacing, then the windows on your ADU were installed by people paying attention.

If any of those questions can't be answered correctly, you've found the thing worth pushing on now, while the openings are still open.