By Tony Marino — “Material moves. The only question is whether you plan for it.”
Every installer knows how to cut a hole.
Very few know how to cut a hole that won’t bite them later.
And that brings us to one of the most important — and most ignored — rules in PTAC installation:
If you don’t give the sleeve 1/8 inch of upward expansion room,
you’re building stress into the system before it ever turns on.
Most installers think cutting the PTAC opening “tight and flush” means a cleaner job.
Wrong.
A PTAC opening must have a controlled gap — specifically 1/8 inch of extra height — or the building, the sleeve, the wall, the insulation, the weather, and the unit itself will fight each other.
And when materials fight?
The sleeve twists.
The chassis binds.
The grille doesn’t seat.
The drainage pitch changes.
The blower vibrates.
The PTAC gets loud.
And the owner wonders why the “brand new unit” sounds like a cheap motel AC from 1989.
Amana Distinctions Model 12,000 BTU PTAC Unit with 3.5 kW Electric Heat
Let me show you why the pros cut high — and why everyone else ends up with callbacks.
📏 1. The 1/8-Inch Expansion Gap: What It Is and Why It Exists
Most installers never consider what happens after you shove a sleeve into a wall opening.
But here’s the truth:
Everything around that sleeve moves.
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The building moves
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The framing moves
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The sheathing moves
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The siding moves
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The sleeve expands
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The PTAC chassis heats and cools
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Moisture swells materials
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Weather compresses and releases the structure
A 1/8-inch expansion gap gives all of those forces room to happen without transferring stress to the sleeve.
Think of it like the expansion gap you leave around hardwood floors.
If you don’t leave room?
Things buckle, bind, shift, pop, or warp.
Same principle here — except instead of a floor buckling, your PTAC sleeve gets torqued into a parallelogram.
🧱 2. Why No One Teaches This (But Every Veteran Knows It)
Here’s the dirty secret:
PTAC manufacturers assume perfect framing.
Contractors assume rigid openings.
Instruction sheets assume square holes.
And apprentices assume construction is uniform.
Meanwhile, real-world installations include:
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Walls that sag
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Brick that expands
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Framing that shrinks
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Siding that warps
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Headers that bow
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Drywall that shifts
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Seasonal expansion and contraction
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Units that create internal thermal cycles
So why don’t people teach the 1/8-inch rule?
Because most installers don’t know how to diagnose the problems caused by not doing it.
They just think:
“PTACs are loud.”
“PTACs always vibrate.”
“PTACs always leak.”
No — a bad installation is loud, vibrates, and leaks.
A proper installation doesn’t.
⚠️ 3. What Happens When You DO NOT Leave the Expansion Gap
Let’s talk consequences.
Because the problems are predictable — and expensive.
Here’s what I see every year:
❌ Twisted Sleeves
If the sleeve is jammed tight against the top framing:
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Humidity swells the wood
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Pressure pushes downward
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The sleeve bows
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The chassis rails twist
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The PTAC vibrates
Instant noise.
❌ Chassis Binding
The PTAC chassis slides into the sleeve on steel rails.
If the sleeve compresses even slightly:
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The rails pinch
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The unit becomes hard to remove
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The blower rubs
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The condensate pan tilts
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The airflow path changes
This alone creates 80% of “unit sounds loud” complaints.
❌ Lost Drainage Pitch
Worst-case scenario:
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Sleeve gets pushed upward
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Drain pan’s angle reverses
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Condensate flows indoors
Now your PTAC becomes a carpet soaker.
❌ Exterior Grille Misalignment
If the sleeve warps, the grille no longer seats correctly.
This leads to:
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Whistling
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Turbulence
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Hot air recirculation
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Compressor overheating
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Higher static pressure
Sounds small; causes huge problems.
❌ Weatherseal Compression
If the unit pushes upward over time, the seal compresses into the air path.
Now you get:
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Back-pressure
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Noise
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Poor airflow
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Moisture infiltration
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Long-term mold
All from one missing gap.
🔧 4. Why 1/8 Inch? Why Not More or Less?
I get asked this all the time:
“Tony, how'd you settle on 1/8 inch?”
Simple — physics and field experience.
Let’s break it down.
🌡 A. Thermal Expansion of Materials
Metal expands and contracts daily as the PTAC cycles between:
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Cold coil temps
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Hot heat strip temps
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Outdoor temperatures
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Sunlight on the façade
Steel sleeves move.
Aluminum grilles move.
Plastic trim moves.
Brick moves.
1/8 inch is enough room for expansion without becoming a gap that causes drafts.
🧱 B. Structure Movement
Wood framing swells 1–2% seasonally.
On a 60-inch PTAC opening, that’s over 1/8 inch by itself.
💨 C. Air Pressure Dynamics
Internal pressure spikes from blower startup and unit cycling cause micro-flex in the sleeve.
1/8 inch prevents the sleeve from riding upward.
🧰 D. Real-World Tolerance
More than 1/8 inch becomes a drafting risk.
Less than 1/16 inch is useless.
1/8 inch is the perfect balance between movement tolerance and sealing integrity.
✏️ 5. How to Cut the Opening 1/8 Inch High — Tony’s Step-by-Step Method
Most installers think you cut perfect dimensions.
Wrong.
You cut for real-world conditions.
Here’s how I do it:
1️⃣ Mark the manufacturer’s sleeve height
Example: 16 inches tall.
2️⃣ Add 1/8 inch to the TOP ONLY
Not the bottom.
Not both sides.
Just the top.
3️⃣ Strike your horizontal cut line 1/8 inch above the spec
Use:
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Laser level
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6-foot straightedge
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Fine-tip marker
4️⃣ Score the line with a utility knife
This prevents tear-out.
5️⃣ Cut with a track-guided saw or multi-tool
No reciprocating saw for the top edge — it wanders.
The top edge must be clean and true.
6️⃣ Feather the top edge ONLY if necessary
If the wall is out-of-square (most are), feather the top, not the bottom.
7️⃣ Test with the sleeve
Push the sleeve in without shims.
You should see:
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Tight bottom
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1/8-inch reveal at top
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Even gap left to right
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No binding
Perfect.
🧱 6. The Gap Is NOT a Hole — Tony’s Sealing Instructions
A 1/8-inch gap is not left open.
It is:
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Created intentionally
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Preserved structurally
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Sealed correctly
Here’s how to handle it:
✔ Fill the rest of the rough opening with foam
But NOT the top gap itself.
✔ Use backer rod + caulk in the gap
This maintains flexibility.
✔ Add trim or flange cover
Looks professional and protects the seal.
❌ Do NOT foam the 1/8-inch gap
Foam expands, kills the gap, and destroys the purpose.
❌ Do NOT caulk rigidly
Rigid caulk prevents movement.
❌ Do NOT leave the gap empty
You’ll get drafts.
🧪 7. How to Know Your Gap Works — Tony’s Stress-Test Checklist
After installation:
✔ Sleeve slides freely
✔ Chassis inserts smoothly
✔ Chassis pulls out without effort
✔ Drain pan sits level and slopes outward
✔ No upward pressure on grille
✔ No wrinkle in the weatherseal
✔ No blower resonance
✔ No hum from side-loading
If ANY of these are wrong, your opening was cut too tight.
🔍 8. Why 1/8 Inch Makes the Unit Quieter, Too
This is the part everyone forgets:
Noise is a symptom of structural stress.
When the sleeve has upward pressure:
❌ The blower housing deforms
Creates a “buzz” sound.
❌ The condenser coil shifts
Causes compressor harmonics.
❌ The chassis rail twists
Causes blower-wheel scraping.
❌ The pressure chamber becomes asymmetric
Creates high-frequency whine.
Leave the gap → none of these happen.
🧯 9. How the 1/8-Inch Gap Prevents Condensate Problems
When the sleeve twists upward without a gap, the drain pitch changes.
That leads to:
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Backflow
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Drain pan overflow
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Indoor condensate drip
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Mold at baseboards
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Rotten carpet tack strips
Adding the expansion gap preserves the exact pitch you set during installation.
🧰 10. The Gap Helps With Grille Alignment (More Than You Think)
Exterior grilles require:
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Even pressure
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Square sleeve
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Correct seating
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Proper louver angle
If the sleeve shifts upward, the grille:
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Doesn’t latch
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Warps
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Vibrates
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Whistles
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Recirculates hot exhaust air
Suddenly the entire unit underperforms.
All because you didn’t cut 1/8 high.
🧠 11. The Physics Behind It — Why Materials Move Up More Than Down
This is important:
Gravity pulls down.
Thermal expansion pushes up.
Why?
Because:
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The bottom is anchored
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The top is free
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Heat rises
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Moisture diffuses upward
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Framing bows upward under tension
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Building sway loads transfer upward
So the top of the opening MUST be the relief point, not the bottom.
That’s why the gap goes on the top.
Always.
🔧 12. What Happens to a Tight Opening Over 2–5 Years
If you skip the expansion gap:
Year 1
Blower noise.
Vibration.
Chassis scraping.
Year 2
Drain pan tilt.
Sleeve twist.
Exterior grille popping.
Year 3
Indoor leaks.
Baseboard swelling.
Sleeve binding.
Year 4
Compressor overheating.
High ESP static pressure.
Whistling through grille.
Year 5
Unit failure blamed on “old PTAC.”
When really, it was the opening from day one.
📚 13. External Verified Resources Supporting This Practice
Here are reliable resources that align with best practices mentioned in this guide:
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Energy.gov – Air Sealing Guidelines
https://www.energy.gov/energysaver/air-sealing-your-home -
OSHA – Construction Saw Safety (for proper wall cuts)
https://www.osha.gov -
International Building Code (Wall Framing Requirements)
https://codes.iccsafe.org/ -
UL Guidelines for Electric Heat Components
https://ul.com/ -
ASHRAE Handbook – HVAC Fundamentals (Airflow & Pressure)
https://www.ashrae.org/technical-resources/ashrae-handbook
These are the same principles Tony follows in the field.
🏁 Final Word From Tony
Most installers worry about:
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Brand
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BTUs
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Heat strip size
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Voltage
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Grille design
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Sleeve color
But none of that matters if the opening is cut wrong.
A PTAC installation lives or dies by the gap you leave — or fail to leave.
“You can’t stop materials from moving.
You can only stop them from ruining your work.”
Cut it high.
Cut it clean.
Preserve the gap.
And your PTAC will run quieter, drain better, last longer, and serve the customer the way it's supposed to.
That’s why Tony does it.
And now you know why you should, too.
