How to Install a Casement Window Air Conditioner

Casement Awning Window opening uPVC aluminium frame with space for text
coffeekai / Getty Images
Project Overview
  • Working Time: 1 - 2 hrs
  • Total Time: 1 - 2 hrs
  • Skill Level: Intermediate
  • Estimated Cost: $400-600

When you are hot, lacking central air, and the proud owner of slider or casement windows, you have an interesting trifecta of cooling issues.

You probably already know that most window-unit A/Cs were made for windows that open up and down, such as double-hung windows. Only a few A/C units are vertically structured to accommodate sliders and casements, and these models are all expensive.

To make matters even more challenging, after you spend the money on this expensive machine, you have to figure out how to install it.

These instructions pertain to the popular Frigidaire Casement/Slider Room Air Conditioner, ranging from 8,000 to 12,000 BTU. But these instructions are broad enough that they can apply to most other casement or slider style A/C units. The only difference would be the horizontal measurements and how they correlate to the window frame.

Casement Window A/C Installation Basics

Though these instructions might look complex, this is actually a simple assembly and installation. The Frigidaire A/C instructions, in particular, may look daunting. But installation can be done with less trouble than it appears. Here are some basics to begin with before moving on to installation.

Install the Unit From Inside the House

You will install the unit itself completely from inside your house. However, you do need to drive one screw into the side of your house, approximately 1 foot below the level of your windowsill.

You will need to do this from the outside. Leaning over the window, while inside, is not only dangerous, but you will not be able to place the hole precisely. Do this on a safely positioned ladder on the outside.

Unit Is Supported by a Platform Assembly and the Window

The casement window A/C employs a redundant support system. The back (outside) is held up by a metal brace called a platform assembly that you install on the house siding. The front is held up by a wood support base that you create with your own materials and place on the windowsill. The unit should then clear the lower edge of your window, but only barely. The greater the clearance, the more space you need to fill in or accept as an open-air gap.

Fill the Top Gap With Plastic or Particleboard

Once the A/C unit has been installed, there will be a sizeable gap at the top of the unit. This is where the casement window A/Cs different from slider window A/Cs. The slider window A/Cs usually have an accordion-style plastic gap cover that extends from the side; the casement window A/C has no such feature.

Some units may come with a thin plastic board to fill in that area. If not, inexpensive hard plastic sheeting or thin particleboard are available at your local home improvement store.

Safety Considerations

High BTU casement window A/Cs are extremely heavy at close to 80 pounds. Plus they are unwieldy and difficult to grasp. It is necessary to have an assistant help you with lifting the unit and holding it in place while it is being secured. Follow all installation instructions precisely to avoid having the unit fall out of the window.

What You'll Need

Equipment / Tools

  • Screwdriver
  • Square or drywall T-square
  • Pencil
  • Measuring tape
  • Ladder
  • Plastic cutting tool (if needed)


  • Casement window A/C unit
  • Wood boards of appropriate length and depth (for support base)
  • Hard plastic sheeting or thin particle board (if needed)
  • Exterior grade plywood piece (if needed)


These instructions assume that the unit will be installed on the right side of the window.

  1. Install the Support Base on the Windowsill

    You will begin by installing a wooden support base of your own making on the windowsill, and then the metal manufacturer-supplied platform assembly on top of that support base.

    The reason for this user-created wooden support is so that the window unit is not resting all of its weight on the thin edge of the window frame. The window frame is not structurally supportive.


    To create a platform, knowing the support base width and height is crucial. Cut wood boards to the width of the unit, plus another 2 inches to account for the window frame on the right side. For the Frigidaire unit, this will be 17 inches. Functionally, it does not matter if you cut a little too long. From an aesthetic standpoint, though, you want it to be roughly the width of your window unit A/C.

    The board height should be as tall as the height of your window's bottom track. The idea is to avoid having the window track support the unit.  

  2. Determine the Center Point

    In terms of placement, determining the center point of the eventual window opening is the single most important part of this installation.

    Start by finding the leading edge on the right side of the window. The official instructions might call it the window jamb, but it isn't clear what window jamb means. Thinking of it as the leading edge makes more sense.

    In other words, you need to find the left-most point of the window frame. Your frame may have several edges, but it will have only one edge that will butt up against the side of the A/C unit.

    It helps to hold a square or drywall T-square against that leading edge so that you can see where the line falls across your support base.

    From this line, measure leftward 8 11/16 inches and mark that point.

  3. Begin Installing the Platform Assembly

    Hook the platform assembly over the front edge of your support base, with the long end protruding outside. Drive a screw into the center hole of the platform assembly, lining it up at the point you just marked, sinking it firmly into your support base.

  4. Finish Installing the Platform Assembly

    Go outside and mount a ladder against the side of the house.

    Put the platform assembly together per manufacturer instructions. Position the platform properly and drive a screw into your siding, provided you have a solid base. If you do not, you will have to first secure a support board with exterior grade plywood or a similar material.

  5. Position the A/C Unit in the Window

    Go back into the house. Open up the window to the width of the A/C, plus another 4 or 5 inches.

    With a partner, lift the A/C and slowly push it through the window opening and onto the platform assembly. Be very careful, as the A/C is prone to slipping off at this point.

    Close the window sash against the left side of the A/C. Make sure the sash is behind the metal bracket that is attached to the side of the unit. It is this metal bracket pressing against the sash that holds the A/C in place.


    It is strongly recommended to use a helper for this step. But if you decide to pick up the unit by yourself, hold the unit on opposite corners. (Looking at the unit from the front, your left hand would be on the upper-left back corner and your right hand on the lower front-right corner.)

    Set up a landing pad that is the same height as the window. An old table that can be scratched is perfect for this. You will lift the A/C unit onto the landing pad (instead of directly into the window). Then, when you are ready, you only need to slowly slide the unit toward the window.

  6. Make Sure the A/C Is in Proper Alignment

    All air conditioners, including your casement/slider unit, extract moisture from the air, process it in the machine, then expel it. When you have a full-sized, permanent, central unit, water discharge happens without you even knowing about it.

    With window units, water is collected and discharged. But with these units, the water drains out of the back.

    You want the unit to tip slightly downward toward the back (outside). Many manufacturers recommend a 1/4-inch drop from front to back. Keep in mind, too, that after you have that 1/4-inch drop set up with your platform assembly, the drop will increase when you place the heavy unit on top of the assembly.


    Some units are designed with an integrated slope and should be installed level. Check the product manual to be sure. 

  7. Fill the Top Gap

    Before removing the panel frame, measure for height.

    Pull the panel frame up and off completely. This panel frame needs to be filled in. Otherwise, your house will be open to the outside.

    One solution is to purchase a clear acrylic or polycarbonate sheet, which you can find at most home centers. If you don't want clear plastic, any type of lightweight particleboard will work. Some boards even come pre-laminated with melamine.

    When measuring for Frigidaire casement A/C units, use the following:

    For width, using a plastic cutting tool (usually found in the Plexiglas and acrylic section), cut a window that is 15 inches wide. You only have a 1/4-inch to 1/2-inch tolerance, so be exact about this.

    For height, measure the height of the opening. Next, add 1/4-inch plus another 1/4-inch to account for the panel frame grooves in which the acrylic will rest. So, if your window opening is 16 inches tall, you need to add 1/4-inch and another 1/4-inch for a total of 16 1/2 inches.

Casement Slider Window AC
Frigidaire / Electrolux