Welcome to our glossary of everything windows, interior storm window inserts, and energy efficiency! If you don’t see a definition here that you need, check out our storm window FAQ. If it’s not there either, please let us know. We’re here to help!

interior storm window inserts



A-weighted decibels. Measurement of sound as perceived by the human ear. Decibels relate to all sound possible (that can be perceived by say, a dog). A-weighted decibels are a scale of measurement for all sound that can be heard by humans, starting with zero as the lowest threshold of human hearing and going up to 110 dBA for jet fly-over/indoor rock band.


STC Rating

Sound Transmission Class rating. The higher the rating, the less noise penetration. A single pane window yields an STC rating of 26-28, meaning normal speech can be heard clearly. Dual-pane is rated 33-35, insulated walls come in at 34-39. Indow Accoustic Grade window inserts combined with double-pane windows are rated 42-45. Learn more about STC rating.


CFM (Cubic Feet Per Minute)

When a part of your home isn’t completely air sealed, there will be a leakage rate calculated using Cubic Feet Per Minute. The higher the CFM, the more rapid the air leak is.

Single-pane window has a CFM of .5
Double-pane window has a CFM of .1 – .3
Single-pane window + Indow insert has a CFM of .024


Energy Efficiency


R-value measures the thermal resistance (how much heat transfers through your walls, windows) of walls, windows, and other parts of a building. The greater your R-value of your windows, the greater the insulation they provide.

An Indow window insert combined with a single pane window provides 94% of the thermal insulation of a standard double pane window.

Single-pane + Indow window insert = R-1.87
Double-pane = R-2.0
Single-pane = R-1.0


Building Envelope

The walls, ceiling, and floor that together enclose the living space. The building envelope is basically the seal we put between ourselves and the outside elements: weather, noise, air. It’s the structure of our home that keeps us comfortable.



Out of Square

Because homes settle over time, most windows are not square. Even if your windows look square, they probably aren’t. In fact, 95% of windows are slight trapezoids or parallelograms. That’s why many traditional window inserts do not create a near airtight seal and why we use laser measurement to ensure precise fit in each window frame.


Frame Depth

The distance between your original window and the end of the flat surface extending from it. This is where your insert will be placed, so if the window sill protrudes past the wall, do not measure that part.


Frame Step

A flat surface extending out from your window which is parallel to your floor. Your window may have multiple frame steps leading up to your window. This is where your insert will be installed. When we ask you to measure a frame step or for you to install an insert on a frame step, we are asking for the largest continuously flat surface leading to your original window.




A mullion divides a window (or window insert) into smaller, more manageable units. Mullions help support glass either vertically or horizontally. If you have older windows, this could be from when large sheets of glass were impossible to manufacture. Window inserts require mullions for support when placed in extremely large window frames (using two window inserts to cover the area). Multiple window inserts with a mullion support can make daily removal or cleaning easier.


Acrylic Glazing

Acrylic is a highly transparent material (more light transmissive than glass) and has a high strength to weight ratio.


Safety Chains

Small ball chains attached to the back of some window inserts. Can be connected to window frame via bracket hidden behind insert frame. Used to keep window insert in place in cases of severe venting (rare) or when one desires access to open and close window.



High wind pressure from the outside of leaky exterior windows and window insert can cause Indow insert to be pushed out of window frame in one or more corner. This is rare and can be solved by pushing window insert back into place.


Compression Tubing

Our window inserts are lined in silicone compression tubing. Our compression system is patented for the way it fits snugly in the window frame and fills any gaps to create a near airtight seal. The silicone is resilient and durable so that long exposure to sun does not cause fading and multiple removals do not cause damage.


Accessories (Window Insert)

Accessories for your window insert can help keep it in place if your insert is going in a skylight, is especially large, or your frame depth is especially deep or shallow. Here are some window accessories that can help:

Grip Strips: provide a channel for your insert to keep it in place in the window frame. Use if your frame depth is very deep, too shallow, or made of slick material.
Frame Swivel Clips: mount on the face of your window frame to hold the window insert in place. They are small plastic clips, held in place with a screw, that can be swiveled in front of the insert. Use if you have an especially large window insert, if you are concerned about venting, or are using an insert in a skylight.
Retaining Pins: pins inserted into pre-drilled holes in your window frame to keep insert in place. Pins can be placed either in front of or behind insert. Pins extends into the window area. Use if you have an especially deep frame or if you are concerned about venting.
L Brackets: a bracket that is L shaped attached to the inside of your window frame to keep your window insert in place. These provide a backing for your insert so it does not move too far into the frame. Use if you have an especially deep frame, are concerned about venting, or are using an insert in a skylight.

Both Swivel Clips and Retaining Pins automatically come with your skylight insert order.


Legacy Insert

Our legacy inserts are those that came before the addition of our ABS carrier. They are still used for odd-shaped windows that need a non-rigid frame, which will be flexible to fit non-rectangular shapes.


ABS Carrier

Acrylonitrile Butadiene Styrene plastic carrier. The plastic frame between the acrylic glazing and silicone tube. The ABS carrier is rigid and holds both the acrylic glazing and silicone tube in place. It is present on all standard rectangular window inserts. It is replaced by a flexible silicone carrier on odd-shaped windows so that the window insert can bend to meet its shape.


Window Attachments

Window attachment can refer to anything added to your window to enhance or protect it: storm windows, window inserts, even a planter box. When we use it, we’re generally referring to window inserts, but may use the term gernally to refer to all storm windows.


Old-Growth Wood

Old-growth wood comes from old-growth trees that were allowed to grow for a long time. The rings of the tree are closer together (compared to younger trees used in modern construction) and are therefore more dense. This makes the wood more resistant to noise, bugs, and rot. Old-growth wood cannot be replaced because the resources are not available or should not be extracted.

interior storm window inserts

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bedroom with window with indow inserts installed for soundproofing in san francisco