If your window is too narrow (under 23"), too short (under 13" opening), or the wrong type (casement, awning, hopper) for a standard window AC, you have two main alternatives that deliver real cooling: a through-the-wall AC unit or a portable air conditioner. A through-the-wall unit is the better long-term solution for homeowners; a portable AC is the only option for renters who can't modify the structure.
Before jumping to alternatives, let's first verify whether your window truly can't accommodate any window AC model — the market has expanded significantly, and some compact models fit openings as narrow as 18".
Is Your Window Actually Too Small?
Standard window ACs require a minimum opening of 23" wide and 13" tall. But several compact and specialty models fit smaller openings:
Minimum Window Dimensions by AC Type
| AC Type | Min Width | Min Height | Min Sill Depth | BTU Range |
|---|---|---|---|---|
| Standard Window AC | 23" | 13" | 4" | 5,000–25,000 |
| Compact Window AC | 18"–21" | 12" | 3.5" | 5,000–8,000 |
| Casement/Vertical AC | 15" (width) | 20" (height) | 4" | 8,000–12,000 |
| U-Shaped (Midea U) | 22" | 14" | 4" | 8,000–14,000 |
| Low-Profile (GE ClearView) | 24" | 13.5" | 4" | 6,000–12,000 |
Measure Again — Correctly
The most common reason people think their window is too small is incorrect measurement:
- Width: Measure the opening between the inside edges of the window frame with the sash fully raised, not the exterior frame dimensions.
- Height: Measure from the top of the sill to the bottom of the raised sash. If your sash doesn't raise high enough, check for a sash lock or stopper that limits travel.
- Sill depth: Measure from the interior edge of the sill to where the window sash closes. Include any interior lip or ledge.
Check for sash stops. Many windows have removable plastic or metal stops that limit how far the sash opens. Removing these (they usually pop or unscrew) can add 2–4 inches to your opening height, potentially making a standard AC fit.
Truly Incompatible Windows
If after measuring correctly your window still can't fit any AC model, it's likely one of these types:
| Window Type | Why ACs Don't Fit | Best Alternative |
|---|---|---|
| Casement (crank-out) | Opens outward, no horizontal sill | Casement AC or portable |
| Awning (hinged at top) | Opens outward/upward | Portable AC |
| Hopper (hinged at bottom) | Opens inward, blocks interior | Portable AC |
| Fixed/picture window | Doesn't open at all | Through-the-wall or portable (vent elsewhere) |
| Very narrow (under 18") | Too narrow for any AC | Through-the-wall or portable |
| Very short (under 12") | Sash doesn't raise enough | Through-the-wall or portable |
| Basement egress | Code may prohibit blocking | Portable AC (vents to window) |
Alternative 1: Through-the-Wall AC (Best for Homeowners)
A through-the-wall (TTW) AC installs in a permanent sleeve cut into an exterior wall. It works identically to a window AC but doesn't require a window at all.
How It Works
- A metal sleeve (chassis) is permanently installed in a rectangular hole cut through the exterior wall
- The AC unit slides into the sleeve, similar to inserting a drawer
- Indoor air circulates through the evaporator; outdoor heat exhausts through the condenser
- To replace the unit, slide it out and insert a new compatible unit
Through-the-Wall Specifications
| Spec | Typical Range |
|---|---|
| BTU Range | 8,000–14,000 |
| CEER | 10.0–13.0 |
| Noise | 42–54 dB |
| Sleeve Size | 26" W × 16" H × 16" D (standard) |
| Wall Thickness | 4"–16" (adjustable sleeve) |
| Weight | 55–100 lbs |
| Price (unit only) | $400–$800 |
| Price (sleeve) | $50–$120 |
| Installation Cost | $200–$600 (professional) |
| Total Cost | $650–$1,520 |
Installation Requirements
Cutting a hole in an exterior wall requires careful planning:
- Structural assessment: The opening cannot cut through load-bearing headers, studs that can't be modified, or structural beams. A contractor can identify safe locations.
- Wall material: Wood-frame walls are easiest. Brick, concrete, and masonry require specialized cutting tools.
- Insulation: The sleeve must be insulated around its perimeter to prevent thermal bridging.
- Electrical: A dedicated outlet must be positioned near the installation location.
- Exterior weatherproofing: The sleeve needs flashing and caulking to prevent water infiltration.
- Building permits: Many municipalities require a permit for exterior wall modifications.
Pros of Through-the-Wall
- Doesn't use any window — your windows are free
- Permanent, clean installation
- Easy future replacement (slide out old unit, slide in new)
- Better noise isolation than window units (sealed wall vs. single-pane glass)
- No seasonal installation/removal needed
Cons of Through-the-Wall
- Requires cutting a hole in your wall (not for renters)
- Professional installation recommended ($200–$600)
- Limited BTU range compared to window ACs
- Hole remains even if you remove the unit later
- Building permits and HOA approval may be needed
Example: Brownstone in Brooklyn The Nguyens have a pre-war Brooklyn brownstone with non-standard windows (20" wide, too narrow for any window AC). They hired a contractor to install a Friedrich through-the-wall sleeve in their living room for $1,350 total (sleeve, unit, and labor). The 12,000 BTU unit cools their 500 sq ft living area. Three years later, when the unit needed replacement, they simply slid out the old one and slid in a new compatible unit for $550 — no contractor needed.
Alternative 2: Portable AC (Best for Renters)
A portable air conditioner sits on the floor and exhausts heat through a flexible hose connected to a window adapter kit. It's the only no-modification option for spaces with incompatible windows.
How It Works
- The self-contained unit sits inside your room
- A flexible exhaust hose (5"–6" diameter, 5–7 ft long) connects the unit to a window adapter panel
- The adapter panel sits in a partially-opened window (only needs 4–6 inches of opening)
- Hot air exhausts through the hose; cooled air blows into the room
- Condensation is either auto-evaporated, drained through a hose, or collected in a tank
Why Portable ACs Work with Small Windows
The window adapter for a portable AC requires far less window space than a window-mounted unit:
| Measurement | Window AC Requirement | Portable AC Requirement |
|---|---|---|
| Window Width | 23"–36" | Any width (adapter is adjustable, typically 20"–48") |
| Window Height (opening) | 13"–20" | 4"–6" of opening (just enough for the adapter panel) |
| Window Type | Double-hung, sliding (horizontal) | Any openable window (double-hung, sliding, casement, awning) |
| Sill Depth | 4"+ | Not applicable |
Portable AC Specifications
| Spec | Range |
|---|---|
| BTU Range (DOE SACC) | 5,500–12,000 |
| CEER | 8.0–11.0 |
| Noise | 48–58 dB |
| Floor Space | 15" × 17" (typical footprint) |
| Weight | 48–80 lbs (on casters) |
| Hose Length | 5–7 ft (limits placement) |
| Price | $250–$700 |
Single-Hose vs. Dual-Hose
| Feature | Single-Hose | Dual-Hose |
|---|---|---|
| How It Works | Pulls room air through condenser, exhausts outside | Pulls outdoor air through condenser, exhausts outside |
| Negative Pressure | Yes (pulls hot air in through gaps) | No (uses outdoor air for condenser) |
| Efficiency | Lower (10–20% less than dual-hose) | Higher |
| Price | $250–$450 | $400–$700 |
| Availability | Common | Limited selection |
| Best For | Occasional use, budget buyers | Regular use, efficiency matters |
Single-hose portable ACs create negative pressure. By exhausting room air outside, they create a slight vacuum that pulls hot air in through gaps around doors, windows, and walls. This is why they're 20–30% less efficient than window ACs. Dual-hose models avoid this by pulling outdoor air for condenser cooling through a separate hose. If buying a portable AC, choose dual-hose whenever possible.
Portable AC Window Adapter Solutions
For truly difficult window situations, aftermarket adapter kits extend compatibility:
| Window Type | Adapter Solution | Approximate Cost |
|---|---|---|
| Casement (crank-out) | Plexiglass panel cut to window shape | $20–$40 DIY |
| Sliding (vertical) | Vertical slider kit | $20–$30 |
| Sliding (horizontal) | Horizontal slider kit | $15–$25 |
| Awning (hinges at top) | Custom panel with hose port | $25–$50 DIY |
| Very small windows | Reduced-size adapter panel | $15–$30 |
| No window | Vent through dryer vent, ceiling, or wall port | $50–$200 |
Example: Studio Apartment with Casement Windows Aisha rents a studio in San Francisco with casement windows that can't accept a window AC. She bought a Whynter ARC-14SH dual-hose portable AC ($480, 10,000 BTU SACC) and a casement window adapter kit ($30). The adapter uses a plexiglass panel that fits in the casement opening with the hoses running through sealed ports. It's not as efficient as a window unit, but it keeps her 350 sq ft studio at 74°F during summer. Her monthly electricity increase: about $45.
Example: Basement Room with Hopper Window Jake's basement bedroom has a small hopper window that opens inward and is only 16" wide. He uses a Black+Decker portable AC ($280, 6,000 BTU SACC) with a custom-cut foam board adapter for the hopper window. The adapter holds the exhaust hose while the window tilts open just 4 inches. It cools his 120 sq ft room adequately, though at 52 dB it's noticeably louder than a comparable window AC would be.
Decision Matrix: Which Alternative Is Right for You?
| Your Situation | Best Alternative | Why |
|---|---|---|
| Homeowner, no suitable window | Through-the-wall | Permanent solution, better efficiency, quieter |
| Renter, can't modify structure | Portable AC | No structural modifications needed |
| Homeowner on a budget | Portable AC (short-term) | Lower upfront cost, consider TTW later |
| Building with HOA restrictions | Through-the-wall (with approval) | May be permitted where window ACs aren't |
| Room with no window at all | Through-the-wall | Only option that doesn't need any window |
| Casement or awning windows | Portable AC with adapter | Simplest solution for these window types |
| Need cooling in multiple rooms | Portable AC (moveable) | One unit can serve different rooms |
Key Takeaways
- Measure your window again carefully — many "too small" windows actually fit compact or casement-specific AC models.
- Through-the-wall ACs are the best permanent alternative — they're as efficient and quiet as window ACs, don't require a window, and last 10–15 years.
- Portable ACs are the only option for renters — they require no structural modification, just a partially-open window for the exhaust hose.
- Portable ACs are significantly less efficient than window or through-the-wall units (20–30% more electricity for the same cooling).
- Dual-hose portable ACs are worth the upgrade over single-hose models to avoid negative pressure and improved efficiency.
- Through-the-wall installation costs $650–$1,520 total but is a one-time investment that pays back through efficiency and convenience.
- For casement windows specifically, purpose-built casement/vertical ACs ($350–$550) are a better option than portables.