IGS vs. IGU vs. storm window
An insulating glass system, an insulating glass unit and a storm window all use a layer of air to slow heat loss. They get that layer in different ways, and that changes what you keep, what can fail and what it costs to maintain.
The three, in plain terms
Insulating glass system
A glass panel that mounts over your existing window. The still air between the two windows is the insulating layer, and the added glass is usually Low-E coated. Your original window stays where it is.
Insulating glass unit
Two or more panes of glass sealed together at the edge as one factory-built unit, with the air space inside. It is the glass inside a replacement window, and the old window is removed to make room for it.
Storm window
A second window mounted over or inside your existing one. Most are a single pane of clear glass in a frame, which adds a layer of air and blocks wind. Low-E glass is an upgrade on some models.
How an insulating glass system works
An insulating glass system works with the window you already own. It mounts over the existing window, and the still air trapped between the two windows becomes the insulating layer. The added glass is typically Low-E coated, a thin coating that reflects heat back toward where it came from.
Heat moves slowly through still air, so a second window with a gap between it and the first cuts the heat that passes through. Low-E glass adds to that by reducing the heat that radiates across the gap.
The panel is made to the size of your opening, and a snug fit matters, so measure carefully.
Nothing sealed to fail
An insulating glass unit keeps its air space, and often an inert gas such as argon, sealed inside the unit. That works well until the edge seal wears out. Then moisture gets between the panes, the glass fogs, and the unit loses the performance it was built for. Failed seals are a common reason replacement windows get replaced.
An insulating glass system has no sealed cavity and no gas fill. Its air space is the gap between the system and your existing window, so there is no seal to fail. If the panel is ever damaged, it can be repaired or replaced on its own, and your original window is untouched.
Keep what you have
Original windows are often well made, and in older homes they are part of the character. An insulating glass system lets you keep them and still improve how the window performs.
Blends in
Low-profile designs sit close to the window and use slim frames, so they do not take over the look of the wall. Pick a frame color near your trim and it fades into the background.
Side by side
How the three compare. Individual products vary, so treat this as a general guide.
Where the air space is
- IGS
- Between the system and your existing window.
- IGU
- Sealed inside the unit, between its own panes.
- Plain storm window
- Between the storm window and your existing window.
Glass coating
- IGS
- Typically Low-E coated.
- IGU
- Often Low-E coated.
- Plain storm window
- Usually clear glass; Low-E on some models.
Gas fill
- IGS
- None. The air space is ordinary air.
- IGU
- Often argon or another inert gas.
- Plain storm window
- None.
Edge seal that can fail
- IGS
- No sealed edge.
- IGU
- Yes. A failed seal lets moisture in.
- Plain storm window
- No sealed edge.
Your existing window
- IGS
- Stays in place.
- IGU
- Removed and replaced.
- Plain storm window
- Stays in place.
Look from the street
- IGS
- Low-profile frames that blend in.
- IGU
- A new window in the opening.
- Plain storm window
- Varies by style; a visible added frame.
Installation
- IGS
- Mounts over the existing window.
- IGU
- Part of a full window replacement.
- Plain storm window
- Mounts over or inside the existing window.
Which one fits
If your windows are worn out or rotted beyond repair, replacement with an insulating glass unit may be the answer. If your windows are sound and you want better comfort and lower bills without losing them, a system that works with them is worth a look.
Either way, compare independent ratings rather than claims, and make sure whatever you add fits the opening well.
FAQs
What is an insulating glass system?
An insulating glass system, or IGS, is a glass panel that works with the window you already have. It mounts over your existing window, and the still air space between the two windows is the insulating layer. The added glass is typically Low-E coated.
How is an IGS different from an insulating glass unit?
An insulating glass unit, or IGU, seals two or more panes together as one piece, with the air space inside, and is used inside a replacement window. An IGS makes its air space between itself and your existing window, so there is no sealed edge and no gas fill to wear out.
Why does a failed IGU seal matter?
When the edge seal on an IGU fails, moisture gets between the panes. You may see fog or condensation that cannot be wiped away, and the unit no longer performs as it did. It is a common reason replacement windows get replaced.
Is an IGS a storm window?
It is closely related. Both mount over an existing window and use the air space between the two. The difference is mostly in the glass and the frame. An IGS is typically Low-E coated and designed as a low-profile frame, while many traditional storm windows use clear glass.
Do I keep my original windows with an IGS?
Yes. Your existing windows stay in place, so you keep their frames, trim and character. That matters for older and historic homes where the originals are part of the look.
Does an IGS use argon gas?
No. There is no sealed cavity, so there is no inert gas fill. The insulating layer is the air between the system and your existing window.
Will an IGS look out of place?
Low-profile frames are made to sit quietly on the window, so from the street they tend to blend in. Choose a frame color close to your trim.
How do I compare performance?
Look for independent ratings. U-factor, solar heat gain and air leakage show how a window performs, and rated products can be compared on the same terms. See our guide to AERC ratings.
What is AERC?
The Attachments Energy Rating Council (AERC) is an independent, nonprofit organization that rates the energy performance of window attachments, a group that includes storm windows. Its ratings are published in a public directory so products can be compared on the same terms.
What is the difference between AERC and ENERGY STAR?
ENERGY STAR sets a bar that a model either meets or does not, for a climate zone. AERC does not pass or fail a product; it rates how the product performs, with numbers you can compare. A window can carry one label, both or neither.
Are storm windows eligible for tax credits or rebates?
The federal Energy Efficient Home Improvement Credit ended for property placed in service after December 31, 2025, so there is no current federal credit for windows. Some utilities, states and local programs still offer efficiency rebates, and many ask for ENERGY STAR certified or AERC-rated products. Programs and amounts vary, so check with yours before you buy. We cannot promise any specific rebate, but we can supply certification details for a model on request.
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