Replacement Window FAQ and Terminology

When you start shopping for replacement windows for your home, you’ll need to understand some new terms and window vernacular. It can be confusing. We’ve created this list of replacement window terminology in order to help our clients make informed decisions.

Window FAQ – Style Terminology

Replacement Window Terminology
Cut out Solar Heat Gain with Vinyl Replacement Windows

Double-hung: This style of window operates by sliding up and down vertically. The lower sash is normally the main operator, but the upper window can also be opened for venting from the top. Many doublehung windows also feature tiltin sashes, making the windows much easier to clean from inside the home. Safety stops are also included so that the windows can be gapped/opened a couple inches for a secure nighttime venting of your home.Single-hung: Single hung windows operate vertically like the doublehung windows, but only the bottom sash operates. These windows have fewer moving parts and are likely to be less expensive than doublehung units. Extra tall “hung” windows may also require that the upper sash be larger than the lower sash due to weight of the sashes. If the sizes are unequal between the upper and the lower sash, the term “oriel” applies.Slider: Sliding windows operate horizontally. Depending on the feature set and the offerings from the manufacturer, a singlesash may be the operating sash and the other side will be permanent. When ordering this type of window you will need to decide which side (right to left slider or left to right slider) to specify on the order. Many sliders offer the option to operate both sashes.Casement: Casement windows are crank-operated and generally open by hinging out from one side. Many homeowners like casement windows due to their open area of unobstructed glass, and the fact that they are hard to pry open on the outside, adding an extra security factor. Depending on the size of the window opening, casement windows can be manufactured into multiple units with other casements or with picture windows.Picture: Picture windows are stationary windows. They do not open. They are used in living rooms and other rooms where you just need windows for view and light. They are never used in bedrooms (see egress) but can be connected (mulled) to many other styles of windows to accommodate large openings. Geometric: As you may have already guessed, these windows feature  geometric designs (shapes other than squares and rectangles). Some available shapes include hexagons, pentagons, octagons, ovals, and rounds. They are usually non-opening picture windows, and can be combined (mulled) to other windows as well. Roundtop oriel singlehung sliders (that’s a mouthful) is one popular example.Replacement Window FAQsAwning: Similar to casements, awning windows are hinged and have a crank or other type of resistance mechanism. The main difference is that awning windows are hinged from the top and open up like an awning.Hopper: Hopper windows are hinged on the bottom of the sash and most likely will open into the home’s interior. You see a lot of hopperstyle windows on factories and warehouses.Twinned/triple: Manufacturing windows always have some height and width limitations. This varies from manufacturer to manufacturer, but Houston Window Experts tackles extra wide or extra tall openings by splitting the opening up into two or more windows.These windows are usually either identical or mixed styles. One popular style is a picture window in the center with two operating windows flanking the sides.Mulled: A mullion is a special reinforced frame. The adjoining windows are screwed into the mullion, making the unit one strong window. Window mulling can be performed at the factory or job site.

Types of Glass and Treatments

Annealed glass: You probably already have annealed glass in your current windows. It’s untreated glass that breaks very easily, usually in very sharp shards of glass. Most singlepane, aluminumframe windows in homes around Houston are made from annealed glass.Double-strength glass: This glass is the new standard in all replacement windows. It’s thicker than annealed glass and is heattreated to increase strength. The windows are more durable and resistant to accidental breakage.Tempered glass: This glass is further heat-treated to increase durability. Also called safety glass, tempered glass breaks into small, blunt pieces when broken. Building ordinances require tempered glass be installed in doors and windows near exits or lower than about 18 inches from the floor. The regulations vary from city to city, but some HOAs may have further requirements.Laminated glass: When you take two sheets of doublestrength glass and laminate them to an inner poly vinyl sheet, you get laminated glass. This glass has several benefits: It won’t fall out if broken because it’s glued to the inner sheeting; it offers additional UV protection; and it adds a very effective soundreducing barrier to your windows.Hurricane windows: Hurricane windows are required in Texas’s immediate coastal areas in case of extreme storms—and for good reason. These window frames are internally reinforced with aluminum, and the mounting specifications require more screws of longer length. The glass is doublepane, but constructed using tempered glass on the outside and laminated glass on the inside. The construction and mounting specs increase the design pressure of the window.Window FAQLow-E glass: In order to block out damaging UV sun rays, low emissivity (Low-E) glass is coated on the inside with a thin, invisible layer of aluminum oxide. There are two popular specifications for Low-E coatings: 2/70 and 3/66. The first number is the number of coats of aluminum oxide the glass receives (either 2 or 3 coats), and the second number is the percentage of visible light allowed through. The two Low-E coatings are slightly different—the 66% coating will be slightly darker than the 70% coating.Single: This style of window has just a single pane of glass, making it the leastinsulating and lowestperforming window available. However, they’re also the least expensive, which is why so many new homes have them.Double-pane: Doublepane windows have two panes of glass separated by an 1/2 to 1 inch air gap. Most new replacement windows have a gap of about 3/4 inch, and are typically filled with either argon or krypton gas for the best insulation. Double-pane windows usually contain higher-density, or double-strength, glass compared to brittle singlepane windows.Triple-pane: These windows are commonly used in Northern climates where extreme cold is common. The third pane of glass and the second air gap increase thermal efficiency.

Replacement Window FAQ – Energy Ratings

Solar Heat Gain: Solar Heat Gain Factor (SHGF) and Solar Heat Gain Coefficient (SHGC) are ratings developed and provided by the National Fenestration Rating Council (NFRC) to indicate how well windows, doors, or skylights reflect and transfer the sun’s heat. Low-E coating and other measures increase efficiency and thereby lower the SHGF number. Numbers in the .26 range are a good minimum rating, and lower numbers (.18 for example) are even better.U-Factor: The U-Factor is an NFRC rating that measures a window’s ability to seal a home from heating and cooling loss. Just as with fiberglass batt insulation, lower numbers are better—the average between .30 and .25. Air gap: The space between panes of glass that usually measures around 3/4 of an inch.Spacer: Located around the edge of the glass, this device creates a space or air gap and keeps panes insulated. The spacer (also called a super spacer) is normally made from either butyl rubber, vinyl, or aluminum depending on the manufacturer and quality. Argon gas: Argon gas is pumped into the air gap. Since argon gas is heavier than air, it increases the efficiency of the window as a sealed unit. The glass, spacer, and air gap are collectively referred to as the Insulated Glass Unit or IGU. Krypton gas: Another type of inert gas commonly used in Insulated Gas Units. Krypton gas is more of an upgrade because it’s heavier than argon.