What’s Inside an Insulated Glass Unit, and Why It Fogs
The insulated glass unit is the part of a window you look through, and it does most of the work. It’s two or three panes of glass, an invisible coating, a sealed pocket of gas, and a thin edge system that has to hold it all together through decades of Ohio summers and winters.
Open the Glass Lab below to take one apart layer by layer, use the Fog Finder to read the moisture on your own windows, and bring the five-question checklist to your next window quote.
Insulated Glass Unit Basics
The U.S. Department of Energy describes insulated window glazing simply: two or more panes of glass, hermetically sealed, with an insulating space between them. An insulated glass unit is that sealed package, built in a factory and set into the window sash.
Each layer solves one problem. Glass blocks wind and rain. The coating handles radiant heat. The gas slows heat moving across the gap. The spacer, desiccant, and seals keep the space dry and the gas inside.
If you’re new to window terms, our window terminology guide shows where the insulated glass unit sits inside the sash and frame, and R-value vs. U-factor explains how its performance is measured.
The Insulated Glass Unit Lab
Pick a layer to light it up, then hit Explode to pull the unit apart.
Glass panes
Two or three sheets of glassThe panes are the walls of the unit. Double-pane is standard; a third pane adds a second insulating space for more comfort and quiet. Compare both in our Series 8000 Triple Pane post.
Low-E coating
A microscopic metal-oxide layerAn invisible coating on one of the inner glass surfaces reflects heat back toward its source. The U.S. Department of Energy says low-E coatings can cut energy loss by as much as 30% to 50%.
Gas fill
Argon in the sealed spaceThe space between the panes is filled with a dense, harmless gas that slows heat flow. DOE notes argon is most common because it’s inexpensive and performs well in a typical 1/2-inch space. More in Argon Gas.
Spacer
Holds the panes apartThe spacer runs around the edge and sets the gap. Warm-edge spacers lower the window’s U-factor and reduce condensation at the edge of the glass. Read Why Settle for Less for how spacer materials compare.
Desiccant
Beads that keep it dryDesiccant inside the spacer soaks up any stray moisture sealed in during manufacturing, so the space between the panes stays dry in a January cold snap.
Edge seals
Keep gas in, moisture outSealants bond the glass to the spacer and stop gas and moisture leaks. When the seal fails, humid air gets in, and you see fog you can’t wipe away.
Drawing is simplified and not to scale. Coating position and gap width vary by window.
Is Your Insulated Glass Unit Failing? Find the Fog
Where the moisture sits tells you almost everything. Pick the spot that matches your window.
Usually good news
Dew on the outside of the glass on a cool morning is common with efficient windows. The glass is keeping your heat inside, so the outer pane stays cold enough for dew to form. It burns off as the sun warms it.
Indoor humidity
Fog or frost on the inside surface means the room air is humid and the glass is cold. Lower the humidity, run bath and kitchen fans, and open blinds for airflow. If it’s heavy every winter, see Window Condensation in Ohio.
The seal has failed
Fog you can’t wipe off from either side means the insulated glass unit has lost its seal. Moist air has gotten in, and the gas fill is likely leaking out. The window won’t clear on its own.
A failed insulated glass unit doesn’t leak water into the house, but it does lose its insulating gas and its clear view. If several windows are fogging, they’re likely the same age and build, and more will follow. Run the room-by-room tests in Need New Windows? before you decide.
How the Insulated Glass Unit Edge Got Better
Most of the progress in the last few decades happened at the edge, where the panes meet the spacer. That’s where heat escapes fastest and where seals fail first.
- Then
Aluminum spacers
Strong and cheap, but aluminum conducts heat fast, so the edge of the glass ran cold and wet in winter.
- Next
Warm-edge spacers
Less conductive materials kept the glass edge warmer and cut condensation along the bottom of the sash.
- About 2014
Stainless steel: Endur
The windows we installed starting around 2014 used Cardinal’s Endur glass, with a stainless steel spacer and two seals. See the Endur page.
- Today
TPS warm-edge
The windows we install today use a TPS spacer, applied as one continuous bead with no corner joints. Learn more in TPS Replacement Windows.
Why does the edge matter so much? Metal moves heat. Stainless steel is about 1/15 as conductive as aluminum, and rubber and thermoplastic spacers conduct far less than that, according to GreenBuildingAdvisor. A cooler edge means a colder strip of glass along the bottom of the sash, which is exactly where you see condensation and frost first on a January morning. Swapping the spacer material won’t change the middle of the glass, but it changes how the whole window feels from your chair.
5 Insulated Glass Unit Questions to Ask Before You Buy
Two windows can look identical and perform very differently. The insulated glass unit is where the difference hides, so ask about each part by name.
- Spacer typeAsk what holds the panes apart: aluminum, warm-edge, stainless, or TPS.
- Gas fillConfirm argon (or krypton in narrow triple-pane gaps) is included, not optional.
- Low-E coatingAsk which coating and on which surface. It sets the U-factor and solar heat gain.
- NFRC labelThe label lists the U-factor and SHGC for the whole window, not just the glass.
- Seal warrantyRead what the window warranty says about seal failure and fogging, in writing.
For Ohio homes, check the label against ENERGY STAR criteria for the Northern zone. Our ENERGY STAR 7.0 windows post lists the current numbers, and Choose Vinyl Windows explains why vinyl frames pair well with a high-performance insulated glass unit.
Windows Installed by the Family That Answers the Phone
Wilbur Thrush founded Thrush & Son in 1960. W. Allen Thrush owns the company today, and Corey Thrush is the third generation. More than 31,000 Miami Valley homeowners have trusted us, from our Brookville headquarters and offices in Troy and Lebanon.
We install Nobility vinyl windows, built in Ohio by Vinylmax, in the Diamond and Platinum collections. Every window we install is set by our own crews. Browse the project gallery, read homeowner reviews, or see our replacement windows.
Insulated Glass Unit Questions, Answered
What Miami Valley homeowners ask us most about the glass in their windows.
What is an insulated glass unit?
An insulated glass unit is two or more panes of glass held apart by a spacer and sealed around the edges, with an insulating space between them. In most replacement windows that space is filled with argon gas.
Why is there fog between my window panes?
Fog between the panes means the seal on the insulated glass unit has failed and moist air has gotten inside. It can’t be wiped away, and the window won’t clear on its own.
Can just the glass be replaced?
Sometimes. If the frame and sash are in good shape, a new glass unit may be an option. If the window is older, drafty, or hard to operate, replacing the whole window usually makes more sense.
Is triple-pane worth it in Ohio?
It adds comfort and quiet, especially on north walls and bedrooms near traffic. Many double-pane windows already meet ENERGY STAR requirements. Compare both in Series 8000 Triple Pane.
Does argon leak out over time?
A small amount can over many years. A strong edge seal slows that loss, which is why the spacer and sealants matter as much as the gas itself.
Fog Between the Panes? Let’s Take a Look.
We’ll check your windows, tell you whether it’s a seal, humidity, or nothing to worry about, and give you a written quote if replacement makes sense. Planning ahead? See window replacement savings in Ohio and our financing options.
Call 937-833-5136 · Office: Mon–Thu 9–4, Fri 9–2 · Michelle, our AI assistant, answers 24/7
