Why Are My Windows Drafty? What Minnesota Homeowners Should Know Before Replacing Them

Older windows lose their seal over time. Weatherstripping wears out, wood moves, glazing and storm windows deteriorate, and the hidden sash-weight pockets beside old double-hung windows can leak air.

Many Twin Cities homes were built long before today's expectations for insulation, air sealing and energy efficiency.

Older double-hung wood windows typically used ropes, pulleys and weights hidden in cavities alongside the window. The weights counterbalanced the sash, making it easier to raise the window and keep it open.

It was a clever system.

It wasn't a great system for keeping January air outside.

Now add decades of Minnesota weather. Wood expands and contracts. Paint and glazing compound age. Weatherstripping wears out. Storm windows deteriorate. Joints move and seals fail.

A window that performed reasonably well when the house was built may behave very differently 50, 75 or 100 years later.

That doesn't mean the original builder did anything wrong. The window may have been installed exactly as intended for its time. Building practices, materials, energy codes and our understanding of building science have simply changed.

Not always. Cold air you feel at a window may be coming through gaps around the frame, old sash-weight pockets or a poorly sealed rough opening, or it may be pulled in by air leaks elsewhere in the house.

Air moves through a house. During the heating season, warm indoor air rises. As it escapes through openings higher in the house, colder outside air gets pulled in through openings lower down. Building scientists call this the stack effect.

Windows and doors are common places to notice that incoming air. But the air may be traveling through the gap between the window frame and the wall, through old sash-weight cavities, through a poorly sealed rough opening or through other penetrations in the building envelope.

In other words:

Where you feel the draft isn't necessarily where the problem begins.

Even the Center for Energy and Environment (CEE), the Minnesota nonprofit that runs home energy audits across the Twin Cities, notes that total window replacement is rarely the most cost-effective recommendation to come out of an audit.



That's why we believe in diagnosing a draft before prescribing a solution

Before You Call Anyone: Five Quick Check

Some drafts have simple causes. Here are a few things worth checking on your own:

1. Lock the window. On a double-hung window, the sash lock does more than secure it; it pulls the two sashes together and tightens the seal where they meet. An unlocked window can leak noticeably more air.

2. Close the storm windows. If you have exterior storms, make sure the glass panels are fully closed and weren't left open from summer.

3. Look at the weatherstripping. Is it flattened, cracked, torn or missing?

4. Check the trim. Look for gaps or cracked caulk where the interior trim meets the wall and the window frame.

5. Find the moving air. On a cold, windy day, slowly move the back of a damp hand around the sash, frame and trim. You can often feel exactly where the air is coming in.

For a more complete picture, consider a home energy audit. In the Twin Cities, CEE runs the Home Energy Squad program, provided by Xcel Energy and CenterPoint Energy, which includes measuring your home's air leakage with tools like a blower door. A blower door test makes leaks easier to find and measure, at the windows and everywhere else.

A Drafty Window Is a Building-Envelope Question

Building scientists don't look at a window as an isolated product. Neither should we.

Building Science Corporation founder Dr. Joseph Lstiburek describes walls in terms of four things they must control: water, air, vapor and heat.

A window has to do those jobs too. More importantly, it has to connect properly to the parts of the wall doing those same jobs.

Think about what happens when we cut an opening into an exterior wall. The wall's water management is interrupted. Its air barrier is interrupted. Its insulation is interrupted.

Then we install a window in that opening and have to make all of those systems work together again.

That's why a window's performance involves much more than the numbers on the sticker, and why it matters so much in older Twin Cities homes. It also explains something homeowners often find frustrating:

You can feel a draft at a window even when the glass isn't the problem.

And it's why replacing only the operating part of an old window doesn't necessarily address every source of air leakage around that opening.

Your Window Has an Opening Around It

Every window sits in a framed rough opening, and the gap between the window and the framing needs to be air sealed, not just insulated. If it isn't, that gap can leak air even when the window itself is fine.

This is one of the most important concepts for homeowners to understand.

A window doesn't fit tight against the framing of your house. The framed hole in the wall is called the rough opening, and the window unit is installed inside it, leaving a space between the window frame and the structure.

Modern installation practices handle that space with proper sizing, flashing, water management, insulation and air sealing. The U.S. Department of Energy's Building America Solution Center notes that gaps around window rough openings can let air pass through and water collect, causing drafts, cold spots and moisture problems.

Here's an important building-science distinction:

Insulation and air sealing aren't the same thing.



Insulation slows heat transfer. Air sealing stops uncontrolled air movement. The same DOE guidance points out that rough openings are often left unsealed, or sealed poorly by stuffing fiberglass into the gaps. Fiberglass adds some insulation, but it shouldn't be relied on to stop air.

So when a homeowner asks us:

"There's insulation around my window. How can it still be drafty?"

There may be a perfectly reasonable answer. The opening can be insulated and still let air through.

TCR Building Science Tip: Insulation slows heat transfer. Air sealing controls air movement. A high-performing window opening needs both

Those Old Window-Weight Pockets Matter, Too

They do. The hidden cavities beside old double-hung windows can leak both air and heat, and insulating them through a small access hole often doesn't work well.

If you've ever pulled the interior trim off an old double-hung window, you may have found something most homeowners never see: long cavities running alongside the window where the counterweights traveled.

Those cavities may be empty. They may still hold the original weights. They may have been partly insulated during an earlier remodel. Or someone may have installed a newer insert window without ever opening up what's behind the old frame.

Building Science Corporation's Measure Guideline: Wood Window Repair, Rehabilitation, and Replacement, prepared for the U.S. Department of Energy's Building America program, says abandoned weight pockets allow energy loss through both conduction and uncontrolled air leakage.

It goes further. Even when the pockets are insulated, BSC notes the work is often done in a "blind" manner, by inserting insulation through an access hole in the jamb, "resulting in poor installation quality."

That makes sense. If you can't see the whole cavity, it's hard to know whether it's actually full and sealed. BSC's retrofit guidance instead calls for removing the weights, then air sealing and insulating the cavity along with the voids around the window frame.



Sometimes the conditions you most need to understand are hiding behind the trim

Full-Frame Replacement vs. Insert Windows

A full-frame replacement removes the old window and frame down to the rough opening; an insert fits a new window inside the existing frame. Full-frame lets us inspect and properly seal the opening, while an insert is less disruptive when the existing frame is sound.

When you start shopping for replacement windows, you'll probably run into two terms: full-frame replacement and insert replacement. They're not the same thing, and neither is automatically right for every home.

Full-frame replacement

With a full-frame replacement, the old window and frame come out, exposing the rough opening. From a contractor's perspective, this is often preferable because we get to see the building. We can answer questions like:

• Is there rot or other deterioration?

• What condition is the framing in?

• Are there gaps around the opening?

• Are the old sash-weight pockets still there?

• Are there signs of past water intrusion?

• How was the original window tied into the wall?

• Can the opening be properly flashed, insulated and air sealed for the new window?

Full-frame replacement isn't just about getting a bigger or better window. It gives us access to conditions an insert may leave hidden.

But it isn't the right call for every house. It's more invasive, it usually means more interior and exterior trim work, and it generally costs more.

Insert replacement

An insert window is installed inside the existing frame. When that frame is sound, an insert can be a practical option that's less disruptive to your interior and exterior finishes. The trade-off is that the new frame sits inside the old one, so you typically lose some glass area; Building Science Corporation's guidance lists this reduced vision area as a downside of inserts.

Code can play a role, too. Where an existing bedroom window serves as a required emergency escape and rescue opening, Minnesota's code includes an exception for certain replacement windows. In general, the replacement must be the manufacturer's largest standard size that fits within the existing frame or rough opening, and it must open the same way as the original or provide an equal or larger opening. Changing the size or style of a window in an older home can 

raise questions that didn't exist when the house was built.

That's why the replacement method shouldn't be decided by a blanket rule. The condition of the existing frame, the opening, code requirements, the window's location, how the home was built, your goals and your budget all matter.

A Note on Lead Paint in Older Homes

If your home was built before 1978, the paint on and around your windows may contain lead. 

Under the EPA's Renovation, Repair and Painting (RRP) Rule, firms performing covered renovation work in pre-1978 homes must follow lead-safe requirements. Window replacement is specifically covered regardless of how little painted surface is disturbed, unless the affected components have been properly determined to be lead-free.

That's not a reason to avoid the project. It's a reason to ask any contractor you're considering, including us, whether their firm is EPA Lead-Safe Certified and how they'll contain and clean up the work area.



For the record: Twin City Roofing Construction Specialists is an EPA Lead-Safe Certified firm

Does a Draft Automatically Mean I Need New Windows?

No, but it doesn't automatically mean you don't, either. A draft tells you air is getting in; it doesn't tell you why. The age and condition of the window, its seals and the opening around it determine whether repair or replacement makes sense.

If you have fairly recent replacement windows and suddenly notice air leakage, our first question isn't "Which window should we sell you?" It's "Why is this happening?"

It could be weatherstripping or seals. It could be air leaking around the rough opening. It could be the installation, or something in the surrounding wall. There may be something repairable.

On the other hand, if you have deteriorated, poorly operating, very old wood windows with failing storms and substantial air leakage, replacement may be a reasonable part of improving your home's comfort and performance.



The age of the window matters. So does the condition of the whole assembly.

Sometimes You Can Improve an Older Window

Replacement isn't the only option.

In a 2024 Minneapolis pilot, CEE found that replacing old, leaky storm windows with new ones reduced whole-building air leakage by roughly 10% to 20% in the buildings tested.

That doesn't mean every homeowner should keep their old windows and add new storms. It means something more important:

Diagnose before deciding.

A home with beautiful, serviceable historic wood windows presents a very different decision than a home with windows that no longer operate. Good building science isn't about recommending the most expensive solution. It's about understanding the problem we're trying to solve.

What About Condensation or Ice on My Windows?

Water or frost on the inside of your windows in winter usually isn't a sign that the window is leaking. It's often related to indoor humidity meeting cold glass. That topic deserves its own article, and we'll cover it later in this series.

Indoor humidity, outdoor temperature, glass temperature, ventilation and air leakage can all play a part. And water showing up around a window during a rainstorm may point somewhere else entirely, like flashing or exterior water management.

A homeowner might describe all of these as "My window leaks." But a draft, interior condensation and exterior water intrusion are three different symptoms. They shouldn't get the same diagnosis.

If You Do Replace Your Windows, Installation Matters

Homeowners understandably spend a lot of time comparing brands, frame materials, glass packages, U-factors and prices. Those things matter. But even an excellent window still has to become part of the house.

Remember the four controls? Water. Air. Vapor. Heat.

The opening has to be prepared properly. Water needs a path back outside. Flashing has to integrate with the home's water-management system. The space around the window needs the right insulation and air sealing. None of it is glamorous, and almost all of it disappears once the trim and siding go back on.

At Twin City Roofing Construction Specialists, we typically prefer Andersen window products. 

We also offer more budget-conscious vinyl options, including VinylMax, depending on the home, the opening, your goals and your budget.

But we don't think the first question should be "What's the best window?" The better first 

question is "What's happening with my house?"

Once we understand that, we can figure out whether the right answer is maintenance, repair, 

weatherization, an insert, a full-frame replacement or something else entirely.

The TCR Takeaway: Diagnose the Draft Before You Buy the Window

If you feel cold air around your windows this fall, pay attention. Your house is giving you information.

An old window may have reached the end of its useful life. An old sash-weight cavity may be leaking air. A newer replacement window may have an air-sealing problem around the opening. 

Or the draft you feel at the window may be part of a larger building-envelope issue.

Our goal isn't to sell every homeowner a new window. It's to understand why you're uncomfortable in the first place and fix the actual cause.

Because when a Minnesota winter arrives, you probably don't care whether your window had the fanciest brochure in the showroom.



You just want the draft to go away.

Frequently Asked Questions About Drafty Windows

Next in the TCR Building Science Series

Your AI Remodel Looks Amazing. Can It Actually Be Built?

AI design tools can reimagine your home in seconds. They can also quietly move windows, erase conduit and shrink foundations. Next, we'll show you what happened when we tried it on a real project, and how to turn a beautiful rendering into something that can actually be built.

Sources

• Building Science Corporation (Joseph Lstiburek): BSI-001, The Perfect Wall

• U.S. DOE Building America Solution Center: Air Seal and Insulate Window Rough Openings

• U.S. DOE Building America Solution Center (Building Science Corporation): Insert Replacement Window

• Building Science Corporation (P. Baker), Measure Guideline: Wood Window Repair, Rehabilitation, and Replacement, U.S. DOE Building America, December 2012 (see Section 3.1.2, p. 16)

• U.S. DOE Building America (Building Science Corporation): Window Rehabilitation Guides

• Center for Energy and Environment: Minneapolis Storm Window Pilot (2024)

• Center for Energy and Environment: Home Energy Squad

• Minnesota Rules 7511.1104 (Minnesota State Fire Code): Emergency Escape and Rescue Openings, Replacement Windows

• U.S. EPA: Renovation, Repair and Painting Program for Contractors