The Ultimate Guide to

Window Wall Design

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What is a Window Wall?

The simple answer is that window walls are glass elements cut into walls like windows and doors. The difference between a window and a door is one of semantics.  If the window/door element goes to the floor it is a door, and if it stops short of the floor it is a window.

Holes in the Wall

Single windows and doors are called punched openings and multiples of windows and doors added together form window walls.

Punched Openings are either:

    • Fixed Windows
    • Operable Windows
    • Doors

Big Holes in the Wall

Window walls are when the wall is almost entirely a combination of fixed glass, windows and/or doors in a limitless combination.  It is a larger more complex “punched opening.” Window walls create drama and offer the opportunity to mix and match all of the functional varieties that are considered for punched openings.  The major combinations making up window walls are:

    • All Fixed
    • Windows + Fixed
    • Doors + Fixed
    • All Doors

These groupings are important to consider because they represent a range of costs and satisfy different design goals.

What is Your Goal?

There are several reasons you would insert a window wall but for the purposes of simplifying the decision-making process we are going to limit the discussion to these three:

    • Vision
    • Movement
    • Ventilation

One of the first things you need to do is define just how important each of these things is to you. All of your design decisions will be made based on these goals.


Vision is all about view and light.  Here the motto is the more glass the better. Size of the opening minus the area taken up with framing material yields the best view and most light that can stream into the room.

The number of mullions, especially the vertical ones and their spacing impair the seamlessness between the inside and outside visually.  Also, the wider the spacing and the thinner the framing material, the less of an interruption they are.  If vision is your main goal remember that thin doors are better than fat doors.

Once you have reduced the number of interruptions, the face dimension of the mullions needs to be minimized.  In addition to mullions the framing face widths of operable windows and doors also need to be minimized to maximize the view/light quotient.

Of course, the width of the operable sashes and frames is an engineering factor of the product itself and has an inverse relationship to the size of the operable unit.  If the size of the operable windows and doors is limited, the number of them increases. Thus this negatively affects the net amount of glass.  The design effort is always a tradeoff battle between competing needs or desires.


Movement is the ability to pass from one side of the wall to the other freely and easily; movement means doors.  Within a window wall, doors are mostly glass doors in keeping with the theme.  A single door strategically placed to conform with the room flow might be adequate with the rest of the window wall being fixed.  Single units allow for vision AND free passage so they are the smallest element that will satisfy the movement goal.  French doors might be considered the smallest window wall configuration where movement is an important factor.

Joining the interior space seamlessly to the outdoor space on a grand scale requires a French door up to eight feet wide or a sliding door with 2 or more panels or a folding door to make the full connection. These are the most engaging movement options.


Ventilation is most easily accomplished with windows because they are easier to apply insect screens.  Window walls often incorporate a rather small awning or hopper window within one of the vertical sections defined by the vertical mullions which remain mostly fixed glass.  These windows are also most often added to the extreme top or bottom of the band to reduce the likelihood of visual interference.

Doors can also have screens added, but the screen reduces the free mobility of inside to outside.  Need occasional ventilation? The screen door is not an issue.  For a larger event where the room and outdoor space encourage mingling, they are not used and the flies just go where they want for that limited time.

Can Window Walls Move?

Sure, why not!  Some people think of window walls as giant walls of fixed glass.  However, in the extreme, there is a movement toward movable glass walls”  These are often folding or sliding doors that completely open since all of their elements can move.  This is what the indoor-outdoor living movement is all about.

The range of movement can range from all fixed to adding a few operable windows, to fixed+swing doors, to full opening folding or sliding doors.  Your design goals will help you determine the correct mix of elements is right for you.

Window elements can be added primarily for ventilation.  Door elements can be added primarily for movement.  They can be either projected or sliding and the decision depends on location within the wall and personal preference.

Projected (Swing) Windows & Doors

With projected windows, the glass within the operable sash moves off of the primary wall plane.  They swing in or out from the top, bottom or side.  Each of these has a special name:

    • Casement windows & doors swing in or out and are hinged on the sides.  Both windows and doors can come as single elements or in pairs which are called French windows and French Doors
    • Awning windows swing out and are hinged on the top
    • Hopper windows swing in and are hinged on the bottom

Folding Doors & Windows

Relatively new to the scene are Folding or Bi-Folding Glass Doors (and windows).  Unique to folding doors is that every sash moves.  They are hinged together in pairs and hinged pairs are joined to other pairs or singles with hinges that connect to a trolley that is able to move left or right.  The hinged pairs stack up to the extreme left and/or right leaving the glass wall when closed become a full opening as though there was no glass wall there at all.  They can create a most dramatic transition from inside to outside.

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Sliding Windows & Doors

Sliding windows remain on the primary glass plane of the wall and just move over other elements vertically or horizontally.  They are either:

    • Hung windows sliding up and down
    • Sliding windows & doors sliding side to side

Single hung windows have the lower sash slide up over the upper sash that is fixed in position. Double hung windows differ in that both sashes move up and down.  

The sashes of all sliding windows have glass on at least two planes and sometimes more.  Multiple planes allow movement passed one another.  Having the offsetting planes is a disturbing visual effect to some people because of the irregular reflections.

Sliding doors which most commonly stack in a telescoping manner on top of one another leave part of the opening obstructed by the “stack” when fully open.  However, with careful planning of the wall beyond the sliding door, the panel stacks can tuck away out of sight into wall cavities leaving a full opening.

How is a Window Wall Divided Up?

The short answer is YES!  The way of doing it is through the use of mullions and muntins. 

Both words are used in the window and door industry and describe dividing and joining glass and frames.  This idea of both dividing and joining has always struck me as peculiar, which is it?  Well, it is a matter of perspective. 

What is a Mullion?

The use of the word mullions dates to the Middle Ages, and they were exclusively vertical members.  As reinforcement, they absorb wind-load from the glass/frame surface where the opening is intrinsically weak and transfer it to the head and sill of the structure.  While window and door people usually use the term mullion to describe both vertical and horizontal members, the term transom is the preferred name of horizontal “mullions”.

Another useful thing about mullions is that they can join dissimilar items together.  Fixed and operable windows can be joined to doors of any variety within the same opening by incorporating mullions.

What is a Muntin?

Muntins on the other hand divide, reinforce and join glass within a single window or sash frame.  These are the small vertical and horizontal bars that change large pieces of glass into small “divided lites”.  Their origin is also very old and made necessary because glass could not be made in very large sizes without breaking.  To fill a larger frame with glass, small pieces needed to be joined together with muntins.  Since the glass joints were also weak relative to a large frame opening, they sometimes needed to also add depth to resist wind load.

Essentially, muntins are a specialized mullion. I often work with architects who say that they need mullions in the window when in fact they are talking about muntins; I often correct them because I am a wiseguy.  It isn’t a cardinal sin, but irritating accuracy is an engineer’s plight.  So here is the deal; mullions divide openings while muntins divide glass.  It’s that simple.

What are the Engineering Challenges?

The only reason we call them challenges is that everytime you focus on optimizing one of them it affects the performance of one or more of the others.  Here are a few of the big things to think about from an engineering perspective.

Wall Opening

If the structure is being built from scratch it not such a big deal.  Once framing has started, the trick is that the top of the opening has to have an engineered “header” that can support the roof and other floors that are above it.  The header is a heavy wooden beam or steel member supported on the ends that supports the weight in a downward direction and windload in a horizontal direction.

When a blank wall or wall with a few “punched” windows or doors is being planned to open up to a window wall, there is usually no such header in place.  This means that the hole has to be carefully cut out while the wall above it temporarily supported while the new header is put in place.  This is a big deal and brings many “open the wall up” projects to a screeching halt once the challenge is discovered.

Profile Material

With most windows walls the goal is to maximize the glass area, but the non-glass material making up the windows, doors and, mullions is also important.  These are often called the profiles.  The most common profile materials are:

    • Wood
    • Aluminum
    • Vinyl
    • Steel

Wood comes in many species from pine to mahogany with each serving a different durability and aesthetic objective.  Wood ofter requires continued maintenance with painting and it can swell and shrink over time.  That being said, wood remains one of the most common materials for the profile because it is a natural insulator and relatively less expensive.

Aluminum is the most common metal profile material because it can be easily extruded into intricate shapes.  It is most common in commercial buildings and for making sliding and folding doors. The addition of thermal breaks in the 1970’s overcame the major objection which was poor thermal performance.  It has better stability than wood and it stronger than any profile material other than steel

Vinyl is the plastic windows that dominates the replacement window market in the U.S.  It has very good thermal values.  It’s relatively weak structural performance can be improved with multiple chambers within the shape and though the addition internally with steel reinforcements.  Another advantage is that the corners are thermally welded which is stronger than wood.  The corners are also more stable than mechanically joined aluminum corners and seal better.  Color choices are usually limited with vinyl.

Steel Window & Door profiles date back to the 1800’s when hot rolling of steel was perfected for the creation of railroad tracks.  Steel is naturally stronger than any of the other profile materials.  This makes steel very desirable for ultra-modern structures that feature glass area at all cost.  With steel being a better insulator than aluminum, thermal breaks in steel sections has only become popular in the last several years.  Steel profiles are painted after welding so color selection is limitless.  Rust is the enemy of steel, but modern finishing techniques have gotten this issue under control

Thermal Performance

We’ve talked about thermal performance for the various profile systems above, but the profile is usually only 10-25% of the window wall opening.  The glass is the larger element affecting the overall thermal value of the wall.

Insulating value is rated in “R” values which are the reciprocal of thermal conductivity.  Single glass is about R=1 while nominal insulated glass with two panes of clear, uncoated glass having R=2.  Triple paned insulated glass improves the thermal performance even more.

Low emissivity (Low-E) coatings can be added to one or more surfaces of either the inside or outside pane of insulated glass to improve thermal performance.  The space between the panes is not a vacuum as some people think but just simple dehydrated air.  The air can be replaced with an inert gas like argon to improve the performance even more.

Maximized insulated glass (IG) with all of the tricks utilized can bring the overall IG thermal performance down to R=5, but the cost starts to really go up and clarity of the glass goes down as the thermal performance improves in most cases.

Sill Design

Especially when doors are involved, there is also the battle over water infiltration and trip hazard as it comes to sill design. 

Door sills that seal best stopping air and water usually require a compression sealing gasket that allows the moving sash to sandwich a flexible rubber material between itself and the fixed frame to stop both.  Compression seals are only on swinging or projected window and door elements.  Sliding elements need sliding gaskets which by nature don’t seal as easily.

Compression seals and most sliding seals require a step over.  If the door opening is being used to join the room to the all outdoors as when large sliding or folding doors are used, the most desirable solution is to have “zero” saddle conditions where the floor and deck are both on the same plane.  This is a major challenge especially for stopping water infiltration.  Troughs below the floor surface can carry water that enters to safely pass to the outside, but not foolproof.


Screens are the bane of existence for window and door designers. Insect Screens obstruct the entry of light and reduce the clarity of vision.  The can be fixed, sliding or swinging in function and some even roll up into canisters.  Except for the roll-up or pleated varieties they never go away.  For full opening movable glass walls a decision needs to be made whether to keep the bugs out or just go natural and open up everything to free movement between the inside and outside. 

Are Window Walls Expensive?

They can be, but it is all relative.  Here are a few general truth’s:

  • glass walls are more expensive than frame walls
  • operable windows are more expensive than fixed ones
  • doors are more expensive than windows

That being said, there are a few hidden gems that defy the generalities.  We’ll show you more about that in the full guide.

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Window Wall require skill to satisfy all of your needs.  To do it correctly you need to juggle lots of variables all at the same time:

    • Vision or Movement?
    • Metal or Wood?
    • Ventilation or Fixed?
    • What function works best for my purpose?

This guide will help sort out the options to make the decisions organized and simple, fill out the form today

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