Window AC Installation — Complete Guide for Every Window Type

"I just bought a 10,000 BTU window AC and the installation guide is 2 pages of tiny diagrams. How do I actually install this thing properly without it falling out the window?"

We get it. You've got a heavy AC unit, a confusing manual, and a window that may or may not cooperate. The good news: you can install a window AC yourself in 30–45 minutes with basic tools and zero HVAC experience.

This is the only window AC installation guide you'll need. We cover every window type (double-hung, sliding, casement), every common problem (window too small, window too large, sideways installation), plus the stuff most guides skip — proper tilt angle, electrical requirements, support brackets, sealing, and winterization.

Before we start, you need two things figured out:

1. The right BTU size for your room. If you haven't sized your AC yet, use our BTU calculator here before buying. An oversized unit short-cycles and leaves you clammy; an undersized one runs nonstop and never catches up. (Also worth understanding: the difference between SACC and ASHRAE BTU ratings — they can change which unit you pick.)
2. Your window type and dimensions. Measure the width and height of your window opening (not the glass). We'll tell you exactly what fits below.

Let's get this thing installed.

Tools and Materials You Need Before Installing a Window AC

Gather everything before you start. Nothing worse than having a 60-pound AC unit balanced on a sill while you hunt for a screwdriver.

Tools:

1. Phillips-head screwdriver (or drill with Phillips bit)
2. Tape measure
3. Torpedo level (small bubble level)
4. Pencil for marking screw holes
5. Utility knife or scissors (for cutting foam seal)

Materials (most included with your AC unit):

1. Foam weather seal strips (usually included)
2. Accordion-style side panels (usually attached to unit)
3. L-bracket(s) for window sill mounting (usually included — 1 or 2)
4. Window lock bracket (usually included)
5. Mounting screws (Type A and Type B — included)
6. Support bracket (sold separately — required for 2nd floor and above)

Optional but recommended:

1. Extra foam insulation tape (the included stuff is often thin)
2. Plexiglass or rigid foam board (for extra side panel insulation)
3. A helper (seriously — these units are heavy and awkward)

How to Install a Window AC Unit (Step-by-Step for Double-Hung Windows)

Most window air conditioners are designed for standard double-hung windows — the kind with two sashes that slide up and down. This is the default installation method that works for about 80% of homes.

Step-by-Step Installation for a Standard Double-Hung Window

Step 1: Prepare the window.

Open the lower sash as far as it goes and remove the window screen. Clean the sill of any debris, dirt, or old caulk. Check that the window frame is in good condition — a rotted sill cannot safely hold an AC unit.

Step 2: Attach the top mounting rail to the AC unit.

Most units come with a metal mounting rail that screws onto the top of the AC chassis. Attach it using the Type A screws included. This rail hooks over the window sill and prevents the unit from tipping outward.

Step 3: Extend the side curtain panels.

Slide the accordion-style side panels into their tracks on each side of the unit. Don't fully extend them yet — just make sure they move freely.

Step 4: Place the AC unit on the window sill.

This is the heavy part. Lift with your legs, not your back. Center the unit in the window opening so equal amounts of the side panels will extend on each side. The unit should sit with its front (indoor) edge against the back edge of the window sill.

⚠️ Safety warning: Never lean out the window while holding the AC. Always have a helper steady the unit from inside while you position it.

Step 5: Lower the upper sash onto the mounting rail.

Bring the window sash down so it rests on the top mounting rail. The sash should press firmly against the rail. This is your primary anchor — the window's weight holds the AC in place.

Step 6: Secure the side panels.

Extend the accordion panels until they reach the inside edges of the window frame on both sides. Mark the screw holes with a pencil. Drill pilot holes with a 1/8" bit, then screw the panels to the window frame using Type B screws.

Step 7: Install the L-brackets.

Most units include one or two L-brackets that attach the bottom of the AC to the window sill. Loosen the screw on the side of the unit near the bottom, slide the bracket over it, tighten, then screw the bracket into the sill. Repeat on the other side if a second bracket is included.

Step 8: Install the window lock bracket.

Screw the lock bracket into the top of the lower sash frame. This prevents the window from being opened from outside and keeps the sash from accidentally sliding up.

Step 9: Check the tilt and seal the gaps.

Use your torpedo level to verify the unit tilts slightly toward the outside (we cover exact specs below). Stuff the included foam seal between the upper and lower sash where they meet. Check for any remaining gaps around the unit and fill with foam.

Step 10: Plug in and test.

Plug the unit directly into a grounded 3-prong outlet within 6 feet of the window. Do not use an extension cord — GE, Frigidaire, and most manufacturers explicitly prohibit this, and it can void your warranty or create a fire hazard. Turn it on and enjoy.

How to Properly Tilt Your Window AC Unit

Should a window AC be tilted? This is one of the most common installation questions. Here's the deal: most window ACs need a slight rear tilt of 1/4" to 1/2" so condensation drains outside instead of leaking into your home.

The tilt doesn't need to be dramatic. We're talking about the thickness of a few coins.

Tilt Angle Specifications by Manufacturer

Modern exception: Many newer units (especially Midea U-shape and recent GE models) have internally sloped drain pans that channel water to the rear automatically. These units can be installed perfectly level. Always check your manual first.

How to Measure and Adjust the Tilt

1. Place a torpedo level on top of the AC unit, running front-to-back.
2. The bubble should be slightly off-center toward the indoor side (indicating the outdoor side is lower).
3. If the unit is level or tilting inward, place wood shims or plastic shims under the front edge of the unit on the window sill until you achieve a 1/4" to 1/2" rear tilt.
4. Secure the shims so they can't shift from vibration.

⚠️ If your window AC is tilted inward (toward the room), water will pool and leak inside. This is the #1 cause of window AC water leaks. Fix it immediately.

⚠️ Don't over-tilt. Anything beyond 1/2" creates instability and can cause internal components to operate at an improper angle, reducing efficiency and potentially causing freezing issues.

How to Seal and Insulate Around a Window AC

Gaps around your window AC are the enemy. They let hot, humid air infiltrate your room, force the AC to work harder, waste energy, and invite insects. A well-sealed installation can cut your running costs noticeably.

Sealing Methods (Best to Good)

Here's the approach we recommend:

1. Foam the sash gap first. Stuff the included foam strip (or buy thicker weatherstrip tape) into the gap where the upper sash meets the lower sash. This is usually the biggest air leak.
2. Extend and screw the accordion panels. Make sure they're snug against the window frame on both sides. Use every screw hole provided.
3. Add rigid foam behind the panels. Cut XPS foam board to size and press it behind the accordion panels. This adds real insulation value (the accordion panels alone have almost none).
4. Seal the bottom. Check where the unit meets the sill. Any gaps here let warm air in from below. Fill with foam tape or rope caulk.

A properly sealed window AC keeps your room cooler and reduces the humidity load — which matters more than most people realize. (More on how ACs remove humidity here.)

Window AC Support Brackets: Do You Need One?

The short answer: if your AC is on the 2nd floor or above, yes — install a support bracket. It's cheap insurance against the unit falling and hurting someone.

When a Support Bracket Is Required

NYC residents, take note: Under Local Law 11/98 (the Façade Inspection & Safety Program), window ACs in buildings six stories or taller must be secured with supporting brackets. The brackets must be structurally sufficient to support the unit's full weight. Penalties start at $1,000 plus $250 for every month of non-compliance.

How to Install a Window AC Support Bracket

Most retail support brackets (available at Home Depot, Lowe's, Amazon for $20–$50) follow this installation process:

1. Position the bracket below the window on the exterior wall.
2. Center the bracket with the window opening.
3. Extend the adjustable arms until they touch the exterior wall.
4. Check that the bracket is tilted slightly downward toward the outside (matching the AC tilt).
5. Secure the bracket to the building exterior with the provided lag screws or masonry anchors.
6. Place the AC unit on the bracket. The unit should rest on the bracket AND the window sill — the bracket provides backup support, not sole support.

⚠️ Never use improvised supports. Bricks, wooden blocks, phone books, or stacked objects under a window AC are not safe and can shift from vibration or wind.

Installing a Window AC in a Sliding Window

Sliding windows (the horizontal kind that move left and right) are the second most common window type for AC installation, and they come with 12,100 monthly searches for a reason — people struggle with them.

Here's the deal: you can absolutely install a window AC in a sliding window, but you'll need either a purpose-built sliding/casement AC unit (which sits vertically) or an adapter kit with a custom filler panel.

How to Install a Window AC in a Horizontal Sliding Window

1. Open the sliding panel fully to one side.
2. If using a vertical-style AC (designed for sliding windows), position it in the opening with the exhaust facing outside.
3. The unit sits on the window track or sill. Most sliding window ACs include a track-mounting bracket.
4. Fill the remaining open space above the unit with the included filler panel or a cut piece of plexiglass.
5. Secure the filler panel with screws into the window frame.
6. Seal all gaps with foam weatherstrip tape.
7. Use a security bar or window lock to prevent the sliding panel from being pushed open.

If you're using a standard (horizontal) window AC in a sliding window, you'll need to build or buy a support shelf that sits in the window track, since the sill on sliding windows is often too narrow. This is a more advanced DIY project — if you're not comfortable with it, a portable AC may be a better option. (Here's our guide on venting a portable AC without a window.)

Can You Install a Window AC Sideways? (Vertical Window Installation)

Can you turn a window AC on its side? No — you should never install a standard window AC unit sideways. The compressor, fan motor, and drainage system are all designed to operate in the horizontal position. Running the unit sideways can:

• Damage the compressor (oil pooling on the wrong side)
• Prevent proper condensation drainage
• Cause excessive vibration and noise problems
• Void your warranty

The solution for vertical windows: Buy a vertical window AC unit specifically designed for this application. These units (9,900 monthly searches — people need them) are taller and narrower than standard models, and their internals are oriented for vertical mounting.

Popular vertical window AC brands include Frigidaire, Soleus Air, and Koldfront. They fit windows as narrow as 15.5 inches wide and are compatible with both sliding windows and narrow casement-style openings.

Window AC for Casement Windows (Why It Usually Doesn't Work)

Casement windows — the kind that crank outward with a handle — are one of the most frustrating window types for AC installation. With 14,800 monthly searches, this is clearly a pain point for a lot of homeowners.

Why Standard Window ACs Don't Fit Casement Windows

Standard window ACs are designed to sit on a sill with a sash closing down on top of them. Casement windows don't have this setup — they swing outward on hinges, and the crank handle protrudes inward. There's no sill to rest the AC on and no sash to hold it in place.

Forcing a standard AC into a casement window is unsafe and not recommended.

Casement Window AC Alternatives

You have three real options:

1. Buy a casement window air conditioner. These are purpose-built narrow units (about 15.5"–16" wide) that slide into the window opening vertically. Brands like Frigidaire (FFRS1022RE), Haier, and Perfect Aire make casement-specific models in 8,000–10,000 BTU sizes.
2. Use a portable AC. A single-hose portable AC exhausts through a small panel in the window and doesn't require any window sill support. This is often the easiest solution for casement windows. (Here's how to drain a portable AC.)
3. Install a mini-split system. For a permanent solution with better efficiency, a ductless mini-split mounts on the wall and doesn't need a window at all. Higher upfront cost, but superior cooling and efficiency.

Window Too Small for Your AC Unit (Solutions)

Your window is 20 inches wide and the AC needs a minimum of 23 inches. Now what?

Minimum Window Dimensions by AC Size

Solutions for narrow windows:

1. Remove the side panels. GE and other manufacturers note that "the air conditioner can be installed without the side curtain panels if needed to fit in a narrow window." You'll need to seal the side gaps with custom-cut foam board or plexiglass instead.
2. Buy a smaller BTU unit and supplement. Downsize the AC and pair it with a ceiling fan to distribute cold air more effectively. A fan lets you set the AC 4°F higher and still feel just as cool.
3. Consider a casement or vertical unit. These start at just 15.5" wide and come in 8,000–10,000 BTU sizes.
4. Go portable. A portable AC only needs a 5"–8" opening for its exhaust hose panel.

Window Too Large for Your AC Unit (Solutions)

The opposite problem: your window is 44 inches wide and the accordion panels only extend to 38 inches total. You've got a 6-inch gap on each side that's letting hot air pour in.

How to Fill Large Gaps Around a Window AC

1. Cut plexiglass or rigid foam board to size. Measure the gap between the fully extended accordion panel and the window frame. Cut a piece of 1/4" plexiglass or 1" XPS foam board to fill it exactly.
2. Screw or tape the filler into the window frame. Use small screws for plexiglass, or heavy-duty weatherproof tape for foam board.
3. Seal all edges with foam weatherstrip tape. Even small gaps around the filler let air through.
4. Consider a larger AC unit. If the gaps are extreme (10+ inches on each side), a larger-capacity unit with wider panels may be a better fit both physically and for cooling capacity.

Plexiglass is the better choice for security — foam board can be pushed in from outside. If your window is on the ground floor or accessible from outside, always use plexiglass and secure it with screws.

Does a Window AC Pull Air From Outside?

This is one of the most common misconceptions about window air conditioners, and it comes up constantly (especially during wildfire season).

The answer: No. Window air conditioners recirculate indoor air. They pull warm air from your room through the front grille, cool it by passing it over the evaporator coils, and blow the cooled air back into the room. Heat is expelled outside through the rear of the unit, but no outside air enters your home through the AC's normal cooling cycle.

The unit is essentially a sealed system. The indoor air side and the outdoor air side are separated.

One exception: Some window ACs have a small "Vent" or "Fresh Air" lever on the control panel. When open, this allows a small amount of outside air to mix with the indoor air. During wildfires or high pollution days, the EPA recommends closing this vent and setting the system to recirculate mode.

Another exception: If your AC is poorly sealed with gaps around the side panels or between the sashes, warm outside air will infiltrate around the unit — but that's an installation problem, not an AC design problem. Seal those gaps (see our sealing section above).

So no, you don't need to worry about your window AC pumping smoke, pollen, or pollution into your home during normal operation. The only air it's cooling is the air already inside.

Electrical Requirements for Window AC Units

Getting the electrical setup right isn't optional — it's a safety issue. Here's everything you need to know in one table.

BTU to Amps to Circuit Size Table

Quick rule of thumb: For 115V units, figure roughly 0.8 amps per 1,000 BTU. For 230V units, about 0.44 amps per 1,000 BTU.

Can You Use an Extension Cord With a Window AC?

GE, Frigidaire, and most major manufacturers say absolutely not. The GE installation manual literally states: "DO NOT use an extension cord with this Window Air Conditioner. DO NOT use surge protectors or multi-outlet adapters."

Here's why: most window ACs have a 6-foot power cord, and extension cords add resistance that causes voltage drop. When the compressor kicks on, it draws 2–3x its running amps for a brief surge — combined with the voltage drop, this can overheat the cord, trip breakers, or start a fire. If your outlet isn't within 6 feet of the window, hire an electrician to install a dedicated outlet ($150–$300).

Winterizing Your Window AC (Cover vs. Remove)

Once cooling season ends, you have two choices: pull the unit out and store it, or leave it in place and cover it. Both approaches work — the right one depends on your situation.

Cover vs. Remove: Pros and Cons

If You Remove the Unit

1. Unplug the AC and wait 24 hours for it to dry internally.
2. Remove with a helper — lift with your legs, not your back.
3. Drain any remaining water from the base pan.
4. Clean or replace the filter.
5. Store upright in a dry location. Never store a window AC on its side — the compressor oil can migrate into the refrigerant lines and cause damage on startup.
6. Close and lock the window. Install a temporary insulation panel if the window doesn't seal well.

Best Window AC Covers for Winter

If you're leaving the unit in place, you want both an indoor and outdoor cover:

• Indoor cover: Insulated fabric cover that fits over the room-facing side of the unit. Blocks cold air transfer into your room. $10–$20 at any hardware store.
• Outdoor cover: Breathable (not fully waterproof) cover that shields the exterior from debris and ice. Must be breathable — a fully sealed cover traps moisture and causes mold and rust. $10–$25.

In addition to covers, seal the gaps around the unit with extra foam weatherstrip tape for winter. Pay special attention to the sash gap and the side panels — these are the biggest sources of cold air drafts.

Common Window AC Installation Mistakes (And How to Avoid Them)

We've covered most of these throughout the guide, but here's a quick reference of the biggest installation errors we see:

Wrong Tilt Angle (Causes Water Leaks Inside)

If your unit is level or tilted inward, condensation pools in the drain pan and eventually overflows into your room. This is the single most common cause of window AC water leaks. Fix it with 1/4"–1/2" rear tilt using shims.

Poor Sealing (Causes Humidity Problems and Higher Bills)

Gaps around the unit let warm, humid outside air infiltrate continuously. Your AC has to work overtime to cool AND dehumidify this extra air. Seal every gap with foam, weatherstrip, or plexiglass.

No Support Bracket Above Ground Floor (Fall Risk)

A window AC unit weighs 40–80 lbs and most of that weight hangs outside the window. Without a support bracket, one strong gust or a loosened sash and you've got a deadly projectile. Install a bracket for any unit above the 1st floor.

Using an Extension Cord (Fire Hazard)

We can't stress this enough: no extension cords, no power strips, no multi-outlet adapters. Plug directly into a grounded wall outlet. If you don't have one close enough, get an electrician.

Not Securing the Window Sash (Unit Instability)

If the upper sash isn't locked in place with a window lock bracket, it can slide up over time from vibration. This destabilizes the unit and creates gaps. Use the lock bracket included with your AC (or buy one for $5).

Installing in an Incompatible Window (Unsafe Mounting)

Forcing a standard AC into a casement, awning, or hopper window creates an unsafe installation that cannot be properly sealed or supported. Use a casement AC unit or a portable AC alternative instead.

FAQ

How long does it take to install a window AC?

30–45 minutes for a standard double-hung window installation. Add 15–20 minutes if you're also installing a support bracket. First-time installers should budget an hour.

Can one person install a window AC?

Technically yes for small units (5,000–8,000 BTU, under 45 lbs). For anything larger, we strongly recommend a helper. The unit is heavy and awkward, and the trickiest part — placing it on the sill — is much safer with two people.

Do all window ACs need to be tilted?

Not anymore. Many modern units have internally sloped drain pans and can be installed level — always check your specific model's installation guide. If the manual doesn't mention tilt, try installing level first and add a 1/4" rear tilt only if water leaks inside.

How do I know if my window can support a window AC?

Check three things: (1) the window sill is solid wood, vinyl, or metal — not rotting or cracked; (2) the window opens wide enough (see our dimension table above); and (3) the window locks securely. If any of these fail, either repair the window first or use a portable AC.

Can I run my window AC 24/7?

Yes. Window ACs are designed for continuous operation. The compressor cycles on and off automatically based on the thermostat setting. For the most efficient cooling, set it to the ideal AC temperature (78°F when home, higher when away) and use the energy-saver mode if your unit has one.

How often should I clean my window AC filter?

Every 30 days during cooling season. Remove the front grille, pull out the filter, wash it with warm soapy water, let it dry completely, and reinstall. A clogged filter reduces airflow, increases energy consumption, and can cause the unit to freeze up. If your unit still isn't cooling well after cleaning the filter, it may need a refrigerant recharge.