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

When looking at window air conditioners, one of the first things you may notice is that the installation kits are designed for vertical single or double-hung windows. While this is all well and good, it won’t work if you have a horizontal sliding window.

If you wonder how to install a window AC in a horizontal sliding window, we have you covered. This review will teach you everything you need to know to get a perfect fit in your horizontal sliding window, every time.

How To Install The Window AC In A Horizontal Sliding Window?

Installing an air conditioner in a horizontal sliding window isn’t any more complicated than a standard vertical window install. However, the main difference is filling the gap. Instead of a gap being on the sides of the AC, it will not be on top. We will cover a few methods to fill this gap.

Install Window AC in a Horizontal Sliding Window

Things Needed

Before you begin, you will need a few items. This may require a few trips to the hardware store, though, so make sure you keep track of everything you have and will need.

  • The window AC unit
  • Tape measure
  • ¼” foam or rubber insulation
  • Box of self-tapping sheet metal screws
  • Box of wood screws
  • Plexiglass pane*
  • Plexiglass cutter
  • Plywood*
  • Window screen frame
  • C-channel aluminum (3/8” and ½”)
  • Step-ladder

*-optional pieces. You will need one or the other but not both.

Step By Step Instructions

Once you have your materials, you are ready to begin the installation. However, you will need to prepare the AC itself as well as the framing and mounting. This instruction set assumes you have the saws and tools to cut your frames and pieces to size.

  1. Remove the bottom rail guide from the AC. You can bend and remove or bend and smash flush. Either way, the bottom rail guide is designed for vertical window catches. Horizontal windows do not have these, and this rail guide will get in the way.
  2. Measure the width and height of your AC. Next, cut C-channel pieces to fit on all four sides of the window unit. These C-channels will be the frame that rests in the window. Make sure your C-channel pieces fit to the edge on all sides and are snug to each other.
  3. Use self-tapping screws to secure the C-channel to the AC itself. You can also build a square frame to slide the unit into, but this extra step isn’t usually needed.
  4. Not that you have a window AC with a frame attached, you can open the window and put the AC in place.
  5. Using the hardware that is included with your window unit, you can use the L-shaped bracket mounts to secure the AC to the window sill. This should be done on the outside of the frame and window to prevent closure issues. Reminder: a 5-degree tilt to the back is required for proper draining.
  6. Close the window to the C-channel frame to ensure everything fits properly. Make any adjustments now. Once everything is snug and secure, use the self-tapping screws to secure the C-channel to the window frame (not the moving side) as well as the bottom of the window channel.
  7. Next, you need to decide if you are using plywood to cover the gap up top or plexiglass.

If Using Plywood:

Plywood can be painted or stained to make it more attractive. However, it will prevent light and visibility above the air conditioner. It is, though, the cheapest and easiest option.

  1. Measure the open space above the window unit that needs to be blocked off.
  2. Cut the plywood to size and secure it in place with wood screws.
  3. Use rubber or foam padding around the edges of the plywood, AC and window wherever there are small gaps or light shining through.

If Using Plexiglass:

Plexiglass is used to essentially make a new window pane that you can see through and allow more light to enter above the AC. It is more involved but offers a cleaner, sleek appearance to your project.

  1. Measure the open space above the window unit that needs to be blocked off.
  2. Use the screen frame materials to build a frame that will fit into the space above the air conditioner.
  3. Once the frame is assembled, cut the plexiglass with the plexiglass cutter to size, so it fits into the frame. Use silicone sealant to secure the plexiglass into place. Painters or masking tape will keep everything in place while the silicone cures.
  4. Once cured, place the plexiglass pane in place and secure it with more silicone, screws, or brackets.

Installing Horizontal Sliding Window AC Brackets

Most installations will utilize the mounting brackets that came with their ac units. However, it is important to keep in mind that these brackets are designed for vertical window closures. The height or mounting style may not work for your window frame.

You can use the existing brackets or use L-shaped brackets to make your own. However, it is important to note two major factors. First, mounting the bracket to the AC unit will keep it secure, in place, and from falling out of the window. Most AC bodies are made from sheet metal, so a standard self-tapping metal screw will work.

You should have plenty of room inside the machine for the screw head to not interfere with the internal parts of the system. Just make sure you don’t screw too close to the vents where the refrigerant lines run.

Second, there must be a downward angle to your AC unit from front to back. Five degrees down angle is recommended to allow proper condensation draining and prevent the unit from draining inside the home. As long as you stick to these two cautions and make sure to measure everything twice, you shouldn’t have any problems installing your brackets.

Horizontal Vs. Vertical Sliding Window For AC Unit Installation

When it comes to window air conditioners, the vast majority of models are designed for vertical window installation. These are the most common, most affordable, and ideal situations.

However, for those of us with rooms with horizontal sliding windows, it can be a problem to keep the room cool. There are very few models designed specifically for horizontal window installation, and those that are can be quite expensive.

Using the DIY install method above will cost you time and parts, which can range from about $250 to $500. Compared to the cost of a horizontal AC unit, though, it is a bargain.

Vertical wind AC installs are easier and faster, but with some time, dedication and know-how, a horizontal install can be more secure, more aesthetically pleasing, and much safer. Just ensure you utilize some sort of bar or window lock to prevent the window from being pushed open from the outside.

Can You Install Any Window AC In A Horizontal Sliding Window?

As long as the window unit you choose fits the opening of your horizontal window (with enough room to build and mount the frame you make), any air conditioner can install in any window.

The DIY process for converting a vertical install into a horizontal one will take a few hours, compared to the few minutes of a typical install. You can do the project yourself, too. However, it is typically ideal to have someone help you lift, mount and secure everything in place.

Related Article Best Sliding Window AC Units

Can Any Window AC Be Installed in a Horizontal Sliding Window

People Also Ask (FAQ)

Here are common horizontal sliding window questions most people ask.

How secure are window AC units in horizontal sliding windows?

When mounted correctly with a proper frame and brackets, a horizontal sliding window is entirely secure. Like a sliding glass door, though, horizontal windows only lock when closed fully. Therefore, when installing a window AC in a horizontal window, you will need a security bar or piece of wood in the window to prevent it from being pushed open from the outside.

Are window AC units allowed in a horizontal window in a neighborhood with an HOA?

This will depend on the rules of your Home Owner’s Association. Some will have guidelines as to which windows can house an air conditioner or if they are even allowed at all. Make sure you check the bylaws of the HOA agreement or speak with the HOA president to find out the rules and regulations for your neighborhood before you make a purchase.

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

If you wish to hire a professional to install the window unit for you, as well as cover the gap above, it can cost about $500 to $750 in parts plus their hourly labor fee. It should take a professional about 2 hours to install and secure the unit in place. For a DIY project, though, you only need to worry about the parts, which can range from $250 to about $500.

Can you put an air conditioner sideways in a window?

When boxed and not powered on, a window AC unit can be laid on its side. However, for operation, a window unit cannot safely operate on its side. It must be installed right side up and forward-facing for proper drainage, cooling, and fan operation.


Installing a window air conditioner isn’t a difficult task and usually takes less than an hour. However, if you have a horizontal sliding window, that installation becomes more difficult.

It is not impossible, of course. Creating a frame and gap closing panel is time-consuming, but when done right will look great and keep your home and AC unit secure. With the help of this guide, hopefully, you have a better understanding of what it takes on how to install a window AC unit in a horizontal sliding window.

Josh Mitchell

Josh Mitchell

My name is Josh and I am obsessed with DIY and improving my family home. HVAC topics can be tricky for homeowners so I decided to share my knowledge on the subject. When I am not working on DIY projects, you can find me at the beach or my local coffee shop.