How To Install A Window AC Unit In A Horizontal Sliding Window

So, you’re planning to install a new window air conditioner to keep your home even more comfortable in the summer. You’ve already set aside the budget and even have the ideal date on your calendar. All you need to do now is get the best air conditioner for horizontally sliding windows and install it appropriately.

This guide will take you through the process of selecting the right horizontal sliding window air conditioner, assembling the installation tools and materials, and correctly installing the air conditioner into your horizontally sliding window.

How to Install a Window AC Unit in a Horizontal Sliding Window 

Once you’ve purchased the right horizontal sliding window air conditioner, you can begin installing it.

Here’s how to proceed;

Tools and Materials 

  • Horizontal window AC kit
  • A box of screws
  • Electric drill
  • Flat-head screwdriver
  • Measuring tape
  • Scissors
  • Wrench
  • Weatherstripping
  • Ladder (maybe)

Step by step instructions 

You’ll also need a second pair of hands during the installation. Once you have that and the tools and materials, begin the installation.

Step 1: Choose the Perfect Window

Which of your windows do you want to install the air conditioner? The choice is yours. However, keep a few points in mind.

First, you need plenty of clearance around the air conditioner. Generally, you need at least 20 feet of clearance from potential obstacles on all sides of the AC, inside and outside the house.

Secondly, the window for a horizontal air conditioner must be within reach of a three-pronged electrical outlet.

how to install a window ac unit in a horizontal sliding window

Step 2: Assess the Strength of the Windowsill

Horizontal window air conditioners are heavy, with most units weighing around 100 pounds. So, you must ensure that your windowsill is strong enough to bear significant weight.

If the sill on the selected window shows signs of weakness or age, replace it or select a different window.

Step 3: Position Window Stripping on the Window 

To protect the bottom part from weather elements, you need to create a weatherproof base on which the window air conditioner will rest once installed. Weatherstripping does this excellently.

So, cut a piece of the weatherstripping, remove the adhesive at the back, and position it on the windowsill, pressing firmly to help it stick to the surface.

Step 4: Install the Support Bracket into the Windowsill

Most brackets arrive pre-assembled. Alternatively, the instructions for assembling the bracket are usually included in the guide. Make sure the top of the bracket tilts back slightly by ¼ to ½ an inch.

This enables the air conditioner to tilt slightly when installed to prevent rainwater pooling. Also, remember that most brackets are installed in the middle of the sill.

Step 5: Attach the Bracket to the Siding

Attach the support bracket to the exterior siding using an electric drill and a screwdriver. The exterior portion of the bracket is attached directly beneath the windowsill or further down the siding, depending on the size and weight of the unit.

how to install air conditioner in sliding window

Step 6: Assemble the Air Conditioner 

We usually recommend getting a pre-assembled unit. Otherwise, you need to assemble it personally. Generally, the assembly process requires attaching guides along the sides and top of the AC unit using screws.

You need a hex wrench, typically supplied in the AC kit. The installed guide frames the AC and offers a place for a plastic curtain component.

Step 7: Attach Weatherstripping to the Side Guides 

Cut enough weatherstripping for the side guides and remove the adhesive component at the back. Then stick the weatherstripping to the back of the side guides.

This setup provides an airtight seal when you finally install the air conditioner.

Step 8: Get the Plastic Curtain Ready 

The plastic curtain goes in the gap between the top of the AC and window frames. So, subtract the height of the air conditioner (including the side guides) from the height of the sliding window to determine the amount of plastic curtain needed. Then cut enough plastic curtain (often supplied with the AC kit).

For instance, if the AC is 40 inches tall and the window frame 50 inches high, you need 50-40 = 10 inches of window curtain.

Step 9: Install the Window Curtain and Frame onto the AC

Slide the plastic curtain into the frame (also included in most window AC kits) and attach the frame to the air conditioner. The frame legs go into the side guides you installed earlier. Some frames easily slide into place. Alternatively, use screws and a screwdriver to attach it to the AC.

Step 10: Insert the Air Conditioner

You’ll need an extra pair of hands for this step. Together, lift the AC and slide it into the window opening until it rests against the support bracket and sits flush with the side window jamb

Step 11: Screw the AC to the Bracket

First, ensure that the air conditioner is evenly positioned within the window opening and supported by the bracket. Then, screw the bracket to the air conditioner from outside the house. You’ll need several screws.

Step 12: Screw the Guide and Curtain to the Window Frame

Use the electric drill to drill holes through the bottom guide into the window track. Then use the screwdriver and screws to attach the two parts.

After that, drill holes through the curtain frame into the window jamb and use screws to attach the two pieces.

Step 13: Install the L Bracket

You’ll notice that your window AC kit comes with an L bracket. This bracket goes into the lower corner and meets the jamb to prevent anyone from opening the window from outside.

First, position it. Then drill holes through it and into the window frame. Drive screws into the holes to hold it firmly in place when you are done.

That’s all! You can now plug in your horizontal window air conditioner and test it.

How Do Horizontal Sliding Window ACs Work? 

how sliding window air conditioners work

Also known as casement air conditioners, sliding window air conditioners are slightly different from the traditional window AC unit. Unlike standard window ACs, which are vertically oriented, casement units are longer and taller.

Most of them are 20 inches or taller. They are also characterized by precut fasteners and inserts that hold them o the window frame.

These unique features enable them to be firmly installed into windows that slide from left to right with extra plexiglass or foam insulation.

Casement window AC units work from left to right. First, warm, stale indoor air enters the air conditioner on the left, where strategically positioned filters trap airborne dirt.

Then the air passes over cold refrigerant coils, becomes cool, and returns to your home as cool air, gradually lowering indoor temperatures. The rest of the cooling process is similar to any modern air conditioner.

Horizontal vs. Vertical Window ACs

The two main differences between horizontal and vertical window air conditioners are orientation and size.

  • Orientation: Horizontal air conditioners are horizontally oriented, i.e., they work from side to side, typically left to right. They are designed for windows that slide from left to right (or right to left). On the other hand, standard/vertical window ACs work up-down and are designed for windows that open up and down.
  • Size: Horizontal window AC units are slim and tall, whereas a standard/vertical AC unit is wide and short.

Pros and Cons of Sliding Window ACs

  • Pros: Horizontal window air conditioners are affordable, easy to install, and highly customizable. They also drain water more easily than vertical window air conditioners and offer more support.
  • Cons: Unfortunately, horizontal window air conditioners are rarely attractive, thus watering down outdoor décor efforts. They also tend to be very heavy. The weight can make them difficult to install yourself.

Factors to Consider When Choosing a Horizontal Window AC 

Once you’re ready to go shopping, you need to note the most important considerations when selecting a horizontal sliding window air conditioner. Four factors are especially critical;

Cooling Capacity (in BTUs)

The cooling capacity in British Thermal Units (BTUs) is critical because it directly determines the quality of air conditioning in the allocated room/space. An undersized unit won’t cool the room down fast enough or may fail to cool it to the desired temperature anyway.

Meanwhile, an oversized AC often results in dangerous short cycling and high energy bills. So, make sure to size the AC right.  

Size/Dimensions (in Inches)

The air conditioner’s dimensions are just as important as determining whether the appliance will fit into the selected window. You may have to take it back if it doesn’t fit. Don’t forget to consider the weight too.


The best air conditioners, no matter the style, are highly efficient. So, compare Seasonal Energy Efficiency Ratings (SEER) before settling on an air conditioner. For example, the best window air conditioners are rated 16 SEER or higher.

Ease of Installation 

Finally, make sure you’re getting an easy-to-install air conditioner. For instance, check the installation kit to ensure it contains most of, if not all, the materials needed for installation. The installation bracket is especially critical.


  • How much does it cost to install a horizontal sliding window AC? 

The average cost to install a window air conditioner in 2022 is $546. However, this is the cost to install an average 8,000 BTU AC to serve 340 square feet. Depending on the AC size, efficiency rating, and many other factors, the actual costs may be higher or lower.

  • How do I secure an AC in a horizontal sliding window?

Support brackets offer the most effective way to secure an air conditioner in a horizontal sliding window. Secure the brackets to the window frame and the side of the house using wood screws for wood or vinyl windows or sheet metal screws for metal frames.

  • Is it safe to install a window air conditioner in a horizontal sliding window?

Yes, installing a window air conditioner in a horizontal sliding window is perfectly safe. However, if you’re concerned, consider a sliding window lock. You can make one manually or purchase a customized unit from the store.

  • Can you install a window AC in a horizontal sliding window in an HOA neighborhood?

Unfortunately, there’s no straightforward answer here. Generally, you can do so in single-family homes but not in a condo or apartment. Spare some time to check with your homeowners’ association (HOA).

  • Can you install a portable air conditioner in a sliding window? 

Yes, you can install a portable air conditioner in a sliding window. The most important thing is to ensure the AC is held firmly in place. Importantly, ensure a weathertight seal around it to prevent air loss/inefficiency.


There you have it. Now you know how window ACs work, the pros and cons of a window AC, and, most importantly, how to install a window air conditioner in a sliding window. Feel free to comment and share the article, and if you have any questions, we’d love to hear them.