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Window Construction 101

Know What You're Getting Before You Buy

If the subject of window construction sounds a bit "dry" that's because it is. Unless of course you're a fenestration expert OR someone who's interested in making an informed window choice.

If you fall into the latter category you won't be disappointed because understanding how a window is made will go a long way toward helping you choose good windows for your home. True, a primer on window construction doesn't have the same appeal as a romantic thriller but hopefully you'll walk away with a better grasp on what to look for when it's time to buy new windows.

Let's be honest. Buying windows takes a bit more technical savvy than choosing a new sofa. That's because of the complex role windows play. A window may seem like it's just plugging up the holes in the wall but it's being asked to do a lot. It (should) keep you comfortable and dry while offering a nice breeze now and then.

At the same time it needs to withstand the harshness of the outside environment while keeping the inside as pleasant as can be.

And it needs to do all this as efficiently as possible (or as efficiently as you choose, depending on the decisions and choices you make).

Some windows do this better than others and to some degree, they're designed that way. They're made to meet variable budgets and utilize different materials to achieve different goals.

So before you choose your next set of windows, find out how they're put together. You'll make a better window choice.

The Important Parts Of A Window

Shopping for new or replacement windows without knowing much about the product is sort of like buying a new car without doing any prior research. It'll roll down the road but it may not meet your needs. You might even end up with a lemon.

The same is true for new windows. They're a bit more complex than you might think. Gaining an understanding of a window's construction, it's important parts and how they impact it's performance is key to making good buying decisions. At a minimum you'll be better equipped to discuss the topic with window salespeople.

Window construction is made up of many individual pieces and components but the parts listed here are the important ones to get familiar with.

parts of a window


The window frame is the part of the window that forms the "foundation" of the entire window structure. It's also the part of the window that identifies the material the window is made from. In other words, when you hear the term "aluminum window" or "wood window", those descriptions are based off of the material that the window's frame is made from.

The window frame and the material it's made from is important because it plays a big role in the window's cost, performance, and how easy or difficult it is to maintain. See the section below for more information about window frame materials and their pros and cons.


The sash is the movable part of the window. In a casement window it's the part that swings out. The sashes on a double hung window slide up and down beside each other.

The sash is also a "frame" in that it surrounds and retains the glass. That frame is made up of the "stiles" (the vertical parts of the sash frame)

window cross section

and the "rails" (the horizontal frame members). The only thing of real note here is that an operable window is one that has movable sashes (in other words, you can open the window) whereas a fixed window has an unmovable sash.


The glazing is just another word for the glass within a window. When window manufacturers refer to "glazing options" they're talking about the various kinds of glass you can get with your windows.

The glazing is important because it, along with the frame, has a lot to do with the overall performance of your window. The type of coatings on the glass as well as how many pieces of glass that are in the window impact how quiet, efficient and easy to clean your windows will be.

Single glaze windows use only one piece of glass; double and triple glazed windows use 2 and 3 sheets of glass respectively and are more energy efficient. The cutaway shown in the picture above is an example of a double glazed window.


Window spacers are the small "bars" that separate the panes of glass in double and triple glazed windows. In addition to serving the obvious role of forming a uniform space between the panes, they also have an impact on the window's performance.

When shopping for windows you might come across the term "warm edge spacers" and in fact it's something you should look for. This refers to a particular type of spacer bar that's resistant to the transmission of heat and cold across it.

Here's another way to look at it: a window's spacer is like a bridge that crosses between the panes of glass. Better (more efficient) spacers do a more effective job of preventing the cold on the outside pane of glass from "crossing the bridge" to the inside pane. That helps reduce condensation at the outer (and usually the lower) edges of the window, where humid inside air contacts the cold window pane and condenses into liquid water.

The bottom line on window spacers? Look for windows that boast spacers with better insulating properties. Avoid straight aluminum. Metal is a good conductor of temperature (in other words, a bad insulator) so spacers made from other materials or that employ some technology to stop that temperature transfer will be a better choice.

Muntins (AKA Grilles or Grids)

Window muntins are traditionally the framing pieces that support individual panes of glass (these individual panes are also known as "lites" or "lights"). They were necessary on older windows because glass could only be manufactured in small pieces and in order to make larger windows a number of small panes of glass would be joined together using this type of framework.

Modern windows can be made with large pieces of glass which don't need all that support structure. However the look of a window with multiple panes is still desired by many which is why the style is with us today.

The key point here is that you can buy windows that truly have individual panes of glass set in a muntin frame or windows simulated to look that way. Simulated window muntins are sometimes referred to as window "grilles" or "grids".

Note: In truth a window "grille" is really a metal or wood lattice-like structure attached outside the plane of the window, sort of like a window guard. However over time, and with the help of window makers, the term has been borrowed as another name for window muntins. So if you can't beat 'em. . .

Regardless of what you call them, here's what's important to know about window grilles: They're available in a variety of styles and each style has an impact on how the window looks, what the window costs and how easy it will be to clean. Here are your options --

True Divided Lites

True divided lites are built like the windows of old - individual panes of glass are enclosed in a muntin framework. This means there's a separation in the glass at the locations where the glass meets the muntin framework.

true divided lite window true divided lite window
Pros Cons

Great for historical windows that need an authentic look

Provide the "truest" look of a traditionally-made window

Looks authentic from any perspective/angle

Multiple panes make for a real tedious window cleaning chore

Individual panes aren't as efficient as windows with fewer panes or no separations in the glass at all

Usually more expensive than other simulated grille options

Simulated Divided Lite

Simulated divided lite windows use simulated (ok, fake) muntins on the inside and outside of the window. In a double glazed window with simulated divided lite muntins there's nothing in between the glass panels. The muntin framework is usually adhered to the glass (non-removable).

simulated divided lite window section simulated divided lite window section
Pros Cons

More efficient because there are fewer window edges (fewer individual panes of glass)

The muntins are permanently affixed to the glass and the adjoining sash frame which typically makes them more authentic looking (save for the gap between the glass)

Usually a less expensive option than true divide lites

When viewed close up from almost any position you can see that there's nothing between these simulated muntins

Just like with true divided lites, the more individual panes the more tedious the cleaning job

Simulated Divide Lite with Spacers

This is the same configuration as a simulated divided lite window except for the addition of spacer bars sandwiched between the panes of glass on multi-glazed windows. This style fools the eye somewhat by filling in the gap between the inside and outside muntins, giving an appearance more closely resembling a true divided lite window.

simulated divided lite with spacer simulated divided lite with spacer
Pros Cons

Provides a more realistic appearance

Greater efficiency associated with a simulated divided lite window (but see the cons on the right)

May be a happy medium for those who want the look of a true divided lite but better cost and efficiency

Usually more expensive than simulated divide lite

Spacers may become a "bridge" between the panes of glass that transmit heat/cold and add to condensation around edges of window panes (depends on specific window construction and brand)

Just like with true divided lites, the more individual panes the more tedious the cleaning job

Grilles Between The Glass

This configuration is used on double glazed windows. On this type of window the grille is placed in between the panes of glass. It's also known as "grilles in the airspace". Some manufacturers offer the option of different colors on each side of the grille to match the interior and exterior color schemes.

grille in between glass window section grille in between glass window section
Pros Cons

More efficient option than true divide lite because there's no break in the glass

Much easier to clean the window since there's no physical obstruction on the outer surface of the glass

The 'fake factor' runs pretty high with this style - you can see that the grille is on the inside of the glass

This artificial look is exacerbated by the lack of any interruption of reflection in the window, which obviously occurs with an external muntin

No option for changing the color of the grille if you opt to change your interior or exterior window trim color

Removable (External) Grilles

Removable window grilles are usually installed on the inside of the window and attach to the sash frame by some form of resistance clip or similar means.

external window grille external window grille
Pros Cons

Makes for an easy to clean window - just unclip the grille to remove it

Allows for a more efficient window construction than true divide lite styles

Offers a fairly convincing look from the inside of the window (depending on the specifics of how it's made and attaches to the window)

Offers the least expensive method of getting an efficient window with the 'suggestion' of divided lites

Not a convincing look from the outside since window reflection isn't "interrupted" like it would be with a true or simulated divide lite window

How cheap and/or flimsy they look is somewhat subjective but removable grilles sometimes exhibit those traits

Removable grilles have a greater tendency to break since they're handled more and if they're not very robust

Yes that was a fairly lengthy discussion just on window muntins but you can probably see now the impact that they have on window style, it's performance and cost. This is just one set of options to consider when choosing a window and one that can swing the price one way or the other.

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Window Materials & Their Pros/Cons

When you're faced with choosing new windows one of the main considerations is the material choice. And as was stated earlier in this article, a window's material is governed by what the window frame is made from.

Each material has it's pluses and minuses and the available choices each fit a specific niche in the consideration of performance and cost.

Aluminum Window

Don't be confused here by products that are aluminum clad that use an aluminum cover over a material like wood. Aluminum windows have an aluminum frame. In simplistic terms the frame is essentially a box with hollow chambers or channels on the inside. These channels are designed to provide a "thermal break", or some insulative capacity against temperature transfer through the frame. Because aluminum is a poor insulator these windows are best used in mild climates.

Pros Cons

An economical window and among the lowest in cost relative to other window materials

Very strong and rigid which allows for larger window designs (more glass, less frame)

Doesn't require a lot of ongoing maintenance like period scraping & painting

Available with an anodized (chemically treated) or factory-painted surface for additional durability and resistance to the elements

Not a good insulator nor an efficient window material - readily conducts heat/cold

Expands/contracts at a faster rate than glass in response to temperature swings which challenges the joints where the glass meets the frame (which could lead to glass/frame seal failure over time)

Limited color choices and DIY painting of aluminum will probably yield poor results

Limited options for interior looks too - you don't have the warmth of wood nor the ability to paint/stain to the color of your choice

Vinyl Window

Vinyl windows share a similar construction to aluminum windows in that the frame is typically a box construction with hollow chambers that help increase insulation. The obvious difference is that the material is PVC, or vinyl for short.

One thing to keep in mind about vinyl is that there are two types of PVC that can be used: plasticized and unplasticized (or uPVC). Plasticizers are additives that give the vinyl flexibility. Some drawbacks of that include a lack of rigidity. Unplasticized vinyl (which some window makers advertise using) is more rigid. However the down-side here is that it becomes brittle and can crack and break.

Pros Cons

Economical - among the lower priced window materials

Provides an easy to clean and maintain surface

Somewhat better insulator than aluminum windows particularly if other materials like foam are used inside the frame

Has the greatest expansion rate relative to the glass which over time could degrade the glass/frame seal

Very few colors available which limits your decorative options

Not the strongest material when compared to other window frame materials

Wood Window

Wood windows are where it all started and they have their benefits. The main question is whether you're up to the amount of maintenance involved in keeping them "healthy".

Pros Cons

Good insulating values

Wood frames are solid wood (in other words, no open internal cavities)

Limitless decorative options - you can stain and paint them any color you want

Still a moderately priced window - not the cheapest but not the most expensive either, all other factors being equal

Wood requires maintenance because paint and stain deteriorate over time

Wood that's not maintained will deteriorate over time causing poor performance and ultimately requiring replacement

Clad Windows

Cladding refers to the covering of some material by another. In the case of windows it's usually a wood window that's clad in aluminum or vinyl or fiberglass.

Aluminum clad windows come in two forms: extruded and roll-formed. An extruded aluminum cladding is usually thicker and more robust than roll-formed aluminum. The thicker material will be stronger, with less tendency to bow or warp.

Pros Cons

Good option for getting the beauty of wood on the interior and the benefit of easily maintained surfaces on the exterior

Aluminum clad wood is a better insulating choice than straight aluminum (the wood provides some insulating value)

Aluminum clad windows are available in a range of colors

Can be some of the most expensive windows available (though reduced maintenance cost and longevity should be factored in as well)

Vinyl cladding doesn't offer many color choices

Fiberglass Windows

Fiberglass windows have a lot to offer based on the material properties of the fiberglass itself. You'll pay more for them than for vinyl, wood or aluminum but the benefits may be worth it to you.

You may see the term "pultruded fiberglass" used in association with fiberglass windows. "Pultruded" is just a fancy way of describing how the fiberglass material is formed into the window framing members (it's heated and pulled through a die to form the frames).

Pros Cons

Material is strong and isn't prone to flexing like vinyl

Dimensionally stable - has a thermal expansion rate that's equal to glass so it expands and contracts with the glass, minimizing any stress on the glass-to-frame seals

The least conductive among all the window types and can be made even more efficient if foam is added inside the frame

Fiberglass can be painted/re-painted so you can choose your colors

Low-maintenance material

More expensive than most other window types save perhaps for clad windows.

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What To Look For In A Window

Virtually any product made for the home is open to buyer subjectivity and windows are no different. What works for one homeowner just won't cut it for another. Such is the nature of the wide variety of products available to us.

That being said, everyone still wants a good quality product, whether it's the best money can buy or sufficient enough to serve the intended purpose. To that end here's a list of what to look for when buying new windows to ensure you're on the right track toward getting a satisfactory product.

  • Solid Construction Techniques
    It sounds obvious but the devil is in the details. Look for features that provide increased strength and durability. Examples include "welded" corners on vinyl windows for the best seal against moisture intrusion; extruded aluminum vs. roll-form on clad products (look for thicker rather than thinner material); metal reinforcements inside the hollow channels of vinyl window frames.

    What are "welded" corners? A vinyl window is put together like a picture frame using 4 pieces of material. They can be joined at the corners in two ways: mechanically (using screws, clips, etc.) or welded. On welded corners the two pieces of vinyl material are fused together. This provides strength and eliminates a potential leak path for water.

    window cutaway

    There's really no substitute for actually seeing and touching real window samples to see how they compare to one another. Cross sectional samples are one good way to witness a window's construction. Once you've seen a few examples from various window makers you'll begin to see the differences and get a feel for products that meet your quality standards.

  • Low Maintenance Materials
    Unless you enjoy doing regular maintenance on your windows look for materials that will maximize your investment by minimizing the effort required to maintain them. On average, aluminum clad and vinyl windows will require less maintenance (like scraping/painting) to maintain their looks and longevity.

    That doesn't mean that wood windows aren't good windows. Just remember that they will take more maintenance and if maintenance is neglected they may not last as long as windows made from other materials.

  • Note: the items listed below pertain to a window's efficiency. You can learn more about how to choose an efficient window at this article on window efficiency.

  • Efficient Materials For Your Environment
    At a minimum you should choose double glazed insulated glass windows. Single pane windows are just too inefficient. Triple glazed windows are effective in very cold climates. Look for windows with LowE (low emissivity) coatings. This helps reduce heat loss in colder climates and heat gain in warmer climates. The space between the glass in an insulated glass window should be filled with argon or krypton gas for greater insulation properties.

    Fiberglass window frames are efficient because they have low thermal conductivity (they don't translate the heat/cold from the outside to the inside) and they do it better than aluminum or vinyl. If you choose vinyl windows look for products that use thermal "breaks" - technologies and/or features that help insulate the frame such as foam inside the frame channels.

    The important point is to educate yourself on what kinds of efficient materials make the most sense in your particular environment. You may not need windows with a very low solar heat gain if you live in a northern climate. Conversely, a triple-glaze window (3 panes of glass) might be overkill in a moderate climate that doesn't experience hot or cold extremes. Choose window options that will work best in your situation.

  • Use Energy Star Windows
    For overall energy efficient windows look for those with the Energy Star rating. These windows meet a minimum criteria for energy savings. Energy Star windows are identified by the National Fenestration Council, an independent, non-profit, third-party that tests and certifies windows that offer better energy performance.

    Windows that are Energy Star qualified are more than likely going to include some or all of the efficient materials noted above. However it makes sense to look for those features so that you fully understand the quality of the window you're buying. And there's nothing that says you can't exceed Energy Star performance criteria either.

  • Good Air Leakage Ratings
    Air leakage (AL) or air infiltration is the amount of air that passes through your window from the natural crevices, seams and cracks in it's structure. It's measured when a simulated 25mph wind pressure is applied to the window. The AL number is expressed as a decimal and represents the amount of air in cubic feet per minute per the square footage of the window (example: 0.3 CFM/sq-ft).

    Air leakage is important because it's tied to the energy efficiency of the window. You don't want cold air blowing into your warm house or hot air into your air-conditioned space.

    According to the Efficient Windows Collaboration a window with an AL rating of 0.30 or lower is acceptable. Keep in mind however that there are windows with lower AL stats and lower numbers mean better (tighter) windows. Another thing to know is that not all window companies tell you what the air leakage rating is in their product specs or literature. Air leakage testing is currently voluntary among window manufacturers. Some provide it, some don't. Your best bet is to contact the window maker directly if their window's AL rating isn't specified.

    So consider the window's air leakage as part of your evaluation on a window's efficiency. And keep in mind that the tightest window in the world isn't worth a darn if it's not installed correctly (so pay attention to who installs your windows).

  • Warm Edge Spacer Technology
    Windows that use spacers designed to minimize temperature transfer between the panes of glass in an insulated glass unit are preferable. What you don't want are simple aluminum spacers which do a poor job of insulating against the cold, increasing the tendency to develop condensation.
  • An Established Manufacturer
    This may seem like it has nothing to do with good window construction but the reputation and track record of the window company you buy from has a lot to do with your ultimate satisfaction. Check to see how long the company has been in business. Some companies come and go while others have survived for a reason.

    Check the warranty that company has; really read it and use it as a comparison to other window makers' products. (Go here for more about window warranties and what to look for).

    This doesn't mean you have to go with one of the well-known national brands since there are successful smaller local window makers. Just do your homework on the company. Check the Better Business Bureau for any performance issues.

Publisher's Comments - My Advice

Buying new windows is really an exercise in compromise, unless you have money to burn. That's mainly because windows are designed with a variety of features and options, some that might not be necessary for your particular application.

When it comes to window construction it really pays dividends to become familiar with real-life hardware so that you can make informed decisions about which brands, materials and methods of construction suit your needs.

When I researched windows for my own home remodel I reviewed several different brands. I looked at real windows at home shows. I studied the cross sectional models that the dealers have on display to "see" how they're put together. I saw differences among the various products and was able to make a choice about which kind of windows I wanted.

If you aren't familiar with window terminology and what constitutes a good window you'll be at a disadvantage when talking with a sales representative. And because there's a wide range of available choices, you'll help yourself by knowing what it is you really want and don't want. The more you find out (meaning the more window-educated you become) the better you'll get at refining your preferences. And in the end that'll make it easier to make those inevitable tradeoffs between what you need and want and what you can afford.

It's analogous to buying a house; you really don't know what you "want", given what you can afford, until you've seen more than one house and can reconcile the kind of house you'll get for your budget. After you've seen 3, 5 or even more houses you have a pretty good idea what it is you like and don't like.

The question of "what are the best windows" may be universal but the answer is less definitive and more subjective. It's not necessarily the brand that has the most effective marketing department. There's a strong component of "what works best for you and your situation" involved in there too. A vinyl window may be perfect for one homeowner whereas another might think it incredulous to buy anything but the latest fiberglass, triple-glazed model.

I can't tell you what the best window is because A) I don't know (haven't studied every single one and don't want to try either) and B) what I think is a good window might not square with your position.

What I will say is this (drum roll please. . .) - my strongest recommendation is to get the best window your money will buy, even if that means pulling back on other parts of your budget. As was mentioned above windows truly do "plug up the holes in your wall" and you want to be sure you get the most efficient and effective windows that you can. Along with that you want windows that will be durable and provide good service for a long time.

Aesthetics are important but a window is a part of your house that "performs" and you want it to perform well. It should keep your house comfortable and be easy to maintain.

So my advice is don't go cheap. When my own home was remodeled we pushed our budget a bit on the windows and scaled back somewhat on other discretionary items. I'll use the same philosophy when it comes time to replace the remaining ancient windows in the rest of the house.

Start with a firm grasp of those characteristics noted above that make a good window. Then take a look at the other facets of choosing windows that include the types of windows you can choose from and window efficiency, to round out your window education. I've listed the other articles below.

How To Choose Windows - this breaks down the process of choosing new windows by looking at where you should start, brand considerations, warranties and where you can go for estimates.

Window Types And Styles - if you're not sure about the type of window you want (or don't know what all the types are), this article can help you out. It also talks about "style" and how (in my opinion) the style of window is a separate consideration from the "type".

A window isn't any good if it doesn't keep your house comfortable while saving on energy bills. Find out what features make a window efficient (or not) and what you should consider in this article on window efficiency.

P.S. If the thought of having to find window specialists in your area sounds like another task you just don't have time for, consider having them come to you instead. Click here to learn how you can do this.

Good window hunting!

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