How Should Your ADU Windows Be Flashed? (Two Experts Disagree, and What to Ask Your Builder)

By Buz Artiano, BuildX | August 2026

Watch: He Said "I Don't Like Your ADU Plan." Then Buz Invited Him on the Show. Stephen Chung Architect

Window flashing is the part of your ADU that you won't see again. After the siding is put on, the thin layers that keep water out of the walls around the windows are sealed up inside the house for the rest of its life. You can't inspect it or take a picture of it, and you won't know if it's failed until you get a stain on your drywall or a window sill begins to rot. So the question that a lot of homeowners are left with is how do I know if the builder's window details will hold up if I can't look behind the siding?

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. 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 requirement to get a permit from the city. After building dozens of ADUs we have opinions about window flashing that we can back up. Before BuildX, we owned and operated two fire and water damage restoration companies. While doing that job, we took apart somewhere in the neighborhood of 18,000 houses, and many of them came apart because the windows were flashed wrong. Since we do the design, permitting, and construction that means the people flashing your windows are responsible to us, just like the company that pulls your permit.

Not long ago, an architect named Stephen Chung emailed us and said he didn't like one of our ADU plans. Instead of ignoring him, we asked him to come on our show to talk about it. He walked through the finished ADU and on camera we ended up disagreeing about one thing, which is how the bottom of a window should be flashed. He thinks it should be sealed all the way around and we leave it bottom at the bottom edge. Neither one of us changed our mind and we aren't going to tell you who won because that's not the point. The point is that two experienced people can disagree about the same thing and that means you can't trust this decision to a rule book. But what you should do is ask your builder one certain question and you will learn a lot from how they answer it.

Why Do Two Experts Disagree About Window Flashing?

Flashing is layers of thin materials that goes around the opening of the window and keeps water away from the framing and insulation behind your siding. Everyone in the building trade agrees on that goal. Where experienced builders and designers disagree on is how to deal with the very bottom edge of the window, and that's what the disagreement on our show was about.

Stephen Chung, the architect who toured our work, describes his method like this: he seals the window all the way around. In theory, a window that's sealed all the way around should not get any water behind it. It's a simple position and many buildings are built just that way.

We do the opposite at the bottom and leave that bottom edge open on purpose. Our reasoning on the show had to do with pressure and weather. On a very windy day, with the house facing a certain way, the back side of the building can be under negative pressure and will draw moisture toward the window opening. Because water can be pushed into that space, we want to give it a way to come back out instead of a place to collect in. Two experienced people, two logical answers, and we aren't going to pretend the question is settled because it's not.

The Detail You Can Never Inspect Again

Most of the choices you make on an ADU can be checked. You can walk the floors, open the cabinets, test the faucets, and look at the energy paperwork. But window flashing is different because it's installed early and it gets covered by housewrap and then by siding, and from then on it's sealed inside the wall for the life of the ADU. There isn't an inspection that opens the wall back up to make sure it was done right.

That's why it's such a useful thing to ask about before you sign. A flashing failure does show up by itself. It can be years later as a water stain shows through the inside paint, or there's a soft spot under a window sill or the musty smell of framing that has been damp for a long time. By then you can't fix it with a tube of sealant. It means pulling siding, cutting into the wall, and replacing framing and insulation that rotted while everything looked fine from the kitchen.

This isn't a reason to panic. But it's the reason to make sure that you treat window flashing as a real evaluation point, the same way you'd think about a builder's foundation or roof details. The homeowners who avoid this problem are hardly ever lucky. The lucky ones are those who asked how it would be done, listened to the answer, and then made a choice.

What Does Leaving the Bottom Edge Open Actually Do?

It seems like the idea of sealing a window on all four sides seems safe but that idea is why there is a disagreement. A wall isn't a sealed box, it's a drained system that assumes some amount of water will always get to the surfaces behind the siding. It could be from rain driven by wind, a small gap, or the differences in pressure that build up on a windy day. The job of good flashing isn't just to keep out water. It's to give the water that does get in a controlled path to get back out.

This is where our experience matters more than any diagram. Before BuildX, we owned and operated two fire and water damage restoration companies, and while doing that work we took apart about 18,000 houses. A lot of them came apart at the windows, and the pattern was the same. When water got into a window cavity that had been sealed shut at the bottom, it didn't have anywhere to drain so it stayed against the sill and the framing and slowly caused damage out of sight. To leave the bottom edge open is our solution to that problem. If a small amount of water ever gets to the opening, it leaks out at the bottom instead of pooling inside the wall. The window still gets rid of the vast majority of the water at the top and sides. The open bottom releases the small amount that gets past everything else.

For you, the benefit is a wall that can dry and drain instead of trapping and holding. A builder who installs the flashing this way isn't being careless by leaving something open. They are building for the reality that water will eventually show up and building the wall so a bad day won't turn into a rotted sill in the future. Whether or not a builder agrees with our specific method, that's the kind of thinking you want with the window details you will never get to see.

Does Sealing a Window All the Way Around Make It Wrong?

No. Here's the part where you'd think we are going to tell you that our way is the only right way since we just made a case for it. But we aren't going to do that. Sealing a window on all four sides is a legitimate method used by skilled builders and respected designers, including the architect who discussed it with us and didn't agree. A window can be built to last that has fully sealed flashing when the rest of the wall is designed to keep water away from it in the first place.

The real divider isn't sealed bottom versus open bottom. It's whether the builder doing the work understands why they made the choice. A builder who seals all the way around and can tell you how the rest of the assembly keeps water out has thought it through. A builder who leaves the bottom part of the window open and can explain the drainage logic to you has also thought it through. The one you should worry about is the builder who just does either one out of habit, can't give you a reason and hasn't ever had to take a wall apart to see what happens when it goes wrong.

So the reason for asking isn't to see if your builder will give you the wrong answer. There may not be a single right answer. The goal is to find out whether there's real logic behind their method or whether it's just how they have always done it.

What Should You Ask Your Builder Before You Sign?

Here's the question, and it's short. How do you flash the bottom of my windows, and why? That's it. You aren't trying to become a building-science expert. You're giving your builder an opening and seeing if they walk through.

A good answer will be specific. The builder tells you what they actually do with the window, whether it's leaving the bottom open to drain or it up, and they connect it to how the whole wall handles water. Good builders will have a lot of detail on this, the kind of information that only comes from doing it and from seeing what fails. They'll be able to talk comfortably about where water goes, not just where it is blocked. You don't need to grade their answer, you just need to know that the reasoning is real.

The warning signs are just as clear. Be careful with a builder who dismisses the question and acts like it's a strange thing to ask. Or only gives you a brand name and no method. A phrase like "we use good windows" isn't the answer you want about flashing. Windows and flashing are two different things, and the water gets in around the window, not through it. If a builder can't tell you how they keep water out of the wall at the place it's most likely to get in, then that tells you something about the rest of the ADU building you also aren't able to see.

You're probably going to interview more than one builder before you make a commitment. Be sure and ask every one of them this question. You'll be surprised how quickly it separates the builders who've thought hard about the wall and the ones who haven't.

Ask the Question Before the Siding Goes On

Window flashing is one of the few issues with your ADU that you can't check after the fact and can't easily fix once it fails. Two experienced professionals may disagree about the right way to do it, which means the smartest thing is not to choose a side from a blog. Just ask your builder how they handle the bottom of the window and why, and then listen for real reasoning behind the answer.

We're glad to be one of the builders you ask that question of. If you want to see how we handle it rather than just read about it, here's the next step.