Installing Cat Flaps in Double Glazing

Why You Cannot Just Cut a Hole

This is the part that surprises most cat owners. You have a perfectly good double-glazed patio door. You have a cat that yells at you seventeen times a day to open it. You think: I will just get someone to cut a circle in the glass and pop a cat flap in there.

That is not how toughened glass works.

Every pane of glass in a Canadian residential door or window is toughened safety glass. The Ontario Building Code requires it. During manufacturing, the glass is heated to about 620°C and then rapidly cooled, which creates internal tension that makes it four to five times stronger than regular glass. That tension is also why it cannot be modified afterward.

Cut into toughened glass and the entire pane disintegrates. Not cracks. Not a clean break. It shatters into thousands of small granular pieces, instantly. This is actually a safety feature — those small pieces are far less dangerous than jagged shards. But it means your double-glazed unit is a finished product. No modifications. No drill bits. No angle grinders. Nothing.

Quotable: Toughened glass has exactly two states: intact and in a thousand pieces on your floor. There is no middle ground where you can "just cut a small hole."

The Actual Process: New Glass, Pre-Cut Hole

Here is what really happens when you install a cat flap in double glazing. It is straightforward, but it requires precision.

Step 1: Buy the Cat Flap First

This sounds backward, but it matters. Every cat flap model requires a different hole diameter. The Cat Mate 210 series needs a 215mm circular hole (or 260mm with the glass-mounting adapter). The SureFlap Microchip flap needs a 212mm hole. The PetSafe models vary from 216mm to 292mm depending on whether it is a cat door or a small dog door.

Buy the flap. Open the box. Find the glass-cutting template or the specification sheet. That template is what your glazier needs.

Step 2: Measure Everything

A glazier visits your home and measures your existing double-glazed unit. They need:

• Width and height of the glass pane (not the frame — the glass)
• Cavity depth — the space between the two panes, typically 12mm, 16mm, or 20mm
• Glass thickness — usually 4mm per pane in residential units
• Hole position — centre of the circle, measured from the bottom edge and each side

The hole position is where most DIY measurements go wrong. The flap needs to be low enough for the cat to walk through comfortably but high enough to clear the frame rebate at the bottom. For an average-sized cat, the centre of the hole sits roughly 250-300mm from the bottom of the glass. For a Maine Coon or other large breed, bump that up to 300-350mm.

Quotable: Measure your cat's shoulder height. Add 20mm of clearance. That is where the bottom of the flap opening should sit. Get this wrong and you have a $600 window with a hole your cat refuses to use.

Step 3: Manufacturing

The glazier sends the order to a glass manufacturer. Here is what happens at the factory:

1. Two panes of annealed (un-toughened) glass are cut to your exact dimensions.
2. A circular hole is cut in each pane at the specified position using a CNC glass cutter or specialized core drill. This only works because the glass is still annealed at this stage.
3. Both panes go into a tempering furnace at 620°C. The rapid cooling process toughens them, and now the holes are permanently part of the glass structure.
4. The two toughened panes are assembled into a sealed double-glazed unit with a spacer bar, desiccant, and either air or argon gas fill in the cavity.

The result is a standard insulated glass unit (IGU) that happens to have a perfectly round hole through both panes. The seal around the hole is maintained by the cat flap's mounting hardware, which typically uses compression gaskets on both the interior and exterior faces.

This manufacturing process takes 10-15 business days. Rush orders exist, but they cost more.

Step 4: Installation

The glazier returns, removes your existing sealed unit from the frame, and installs the new one. The frame does not change. The beading or rubber gaskets get reused or replaced. Then the cat flap hardware goes into the pre-cut hole.

For most patio doors, the entire swap takes 1-2 hours. The longest part is carefully removing the old unit without damaging the frame.

Choosing the Right Cat Flap for Glass

Not every cat flap works in double glazing. You need a model specifically designed for glass installation, with a tunnel depth that spans the full thickness of the sealed unit — typically 24-28mm for a standard residential double-glazed unit (two 4mm panes plus a 16-20mm cavity).

The Four Types

Type	Cost (CAD)	How It Works	Best For
Basic 4-way manual	$40-$80	Slide lock to set in/out/both/locked	Indoor cats with occasional outdoor access
Magnetic	$60-$120	Cat wears collar magnet; flap unlocks when magnet is near	Keeping most strays out
Infrared	$80-$150	Cat wears collar key; infrared sensor unlocks flap	More selective than magnetic
Microchip	$120-$250	Reads cat's implanted ISO microchip; no collar needed	Toronto homes. Full stop.

Why Microchip Wins in Toronto

Canada had roughly 8.9 million pet cats in 2024, and Toronto's raccoon population is one of the densest in North America. Those raccoons are not shy. They have figured out magnetic cat flaps. They have figured out lightweight manual flaps. They will enter your home at 2 AM, open your kitchen cabinets, and leave muddy handprints on your countertop.

A microchip cat flap reads the 15-digit ISO standard microchip that your vet already implanted in your cat. It only unlocks for programmed animals. Raccoons, skunks, opossums, and the neighbourhood's feral cat colony stay outside where they belong. Models like the SureFlap and PetSafe Microchip door store 20-40 pet profiles and run on four AA batteries that last 6-12 months.

Quotable: A raccoon that has learned to use a basic cat flap is a raccoon that now considers your kitchen an extension of its territory. Spend the extra $100 on microchip.

The 4-way locking feature on most models is also genuinely useful. Set it to "in only" at night, and your cat can come home from a late patrol but cannot leave again until morning. Toronto's coyote population has expanded in recent years, and keeping cats indoors overnight is now standard veterinary advice in the GTA.

Positioning: Where the Hole Goes

The location of the cat flap in the glass panel matters more than people expect.

Bottom edge clearance. The hole cannot go right to the bottom of the glass. The frame rebate — the channel the glass sits in — covers the bottom 15-20mm. Your hole centre needs to clear that rebate plus leave enough solid glass below the hole for structural integrity. Minimum 60mm from the bottom edge of the glass to the bottom of the hole.

Side positioning. Centre the hole horizontally in the glass unless you have a specific reason not to. Off-centre holes look odd and can create weak points near the edges.

Height for the cat. Measure your cat from floor to the top of its shoulders while standing. That measurement, minus the frame rebate height, minus the radius of the hole, gives you the approximate centre point. For most domestic cats, the shoulder height is 230-260mm. Large breeds hit 300mm or more.

Multiple cats, different sizes. If you have a 4kg tabby and an 8kg Maine Coon, size the flap for the larger cat. The small cat will manage. The large cat will not squeeze through a small-cat flap and will instead resume yelling at you seventeen times a day.

What It Costs in the GTA in 2026

Here is a realistic breakdown for a cat flap installation in double glazing in the Toronto area.

Item	Cost Range (CAD)
New double-glazed unit with pre-cut hole (standard patio door panel)	$450-$900
Cat flap hardware (microchip model)	$120-$250
Cat flap hardware (basic 4-way manual)	$40-$80
Professional measurement and installation	$150-$300
Total (with microchip flap)	$720-$1,450
Total (with basic flap)	$640-$1,280

Smaller glass panels cost less. A cat flap in a half-glass back door with a 600mm x 400mm glass area will be at the low end. A full-height patio door panel at 2000mm x 800mm will be at the high end, because double glazing is priced per square metre.

If your existing unit has argon gas fill or low-E coating, the replacement unit should match. Downgrading the glass specification to save $50 is not a good trade when you factor in long-term energy costs.

Alternatives If You Do Not Want New Glass

Maybe you rent. Maybe the unit is brand new. There are workarounds.

Panel insert for sliding doors. A plexiglass or aluminum panel fits in the track of your sliding patio door with a cat flap built in. Cost: $200-$400. You lose some door opening width, and the seal is never as tight as integrated glass.

Through-wall installation. A cat flap can go through an exterior wall instead. This requires cutting through siding, sheathing, insulation, and drywall, then installing a tunnel with proper flashing. Cost: $300-$600. Works well in older Toronto homes with brick or frame walls.

Solid door panel. If your back door has a solid lower panel below the glass, a cat flap goes directly into that section. Cheapest option: $80-$200 including the flap, and most handy homeowners can do it with a jigsaw.

Common Mistakes

Ordering the glass before buying the flap. The hole size depends entirely on the flap model. Buy the flap first.

Measuring the frame instead of the glass. The new sealed unit replaces the glass, not the frame. Measure the daylight area — the visible glass — and the full unit dimensions including the edges that tuck into the rebate.

Forgetting the cavity depth. A cat flap tunnel that is too short for your sealed unit will not seal properly. Too long and it sticks out awkwardly. The cavity depth of your existing unit tells the manufacturer exactly how thick the new unit needs to be.

Installing too high. Cats are not dogs. They do not jump up to enter a flap. The opening needs to align with their standing shoulder height or lower. A flap installed 400mm off the ground is a flap that most cats will ignore.

Skipping the draft excluder. Even with the cat flap's built-in gaskets, the junction between flap and glass can be a cold-air entry point. A brush-style or magnetic draft excluder integrated into the flap design makes a measurable difference in GTA winters, where exterior temperatures regularly hit -15°C or colder.

Quotable: The most common cat flap failure in Toronto is not the glass. It is the owner who measured the frame, ordered the wrong size unit, and now has a $500 sealed unit sitting in the garage.

When to Call a Glazier

This is not a DIY project. The measurement and installation require someone who works with sealed glass units daily. One wrong dimension and you are ordering a second custom unit at full price.

At Installix, we handle the entire process: measurement, manufacturing coordination, and installation for cat flaps in double-glazed doors and windows across the GTA. If your patio door glass is older and showing signs of seal failure anyway, a cat flap installation is a good excuse to upgrade the sealed unit at the same time — new low-E coating, argon fill, and a happy cat in one visit.

Already have a broken or fogged sealed unit? Check our guide on foggy window repair — if your glass needs replacing regardless, adding a cat flap hole to the new unit is minimal additional cost.

If you are also considering window screen repairs while the glazier is on-site, it is a good time to bundle the work.

For a quick overview of our residential glass work, see our residential window services page.

FAQ

Can I cut a hole in my existing double-glazed window for a cat flap? No. Double-glazed units use toughened safety glass, which is legally required in Canadian residential windows. Toughened glass cannot be drilled or cut after manufacturing — it will shatter into thousands of small pieces immediately. The only option is to order a brand new sealed double-glazed unit with the circular hole pre-cut into both panes before they go through the tempering furnace. Your existing frame stays. Just the glass unit gets swapped.

How much does it cost to install a cat flap in double glazing in Toronto? In the GTA as of 2026, expect to pay $450-$900 CAD for the new custom double-glazed unit with the pre-cut hole, depending on the size of the glass panel. The cat flap itself costs $40-$250 depending on whether you choose a basic manual flap or a microchip-reading model. Professional installation labour runs $150-$300. Total all-in cost is typically $650-$1,400 CAD.

What size hole is needed for a cat flap in glass? Most glass-fitting cat flaps require a circular hole between 210mm and 260mm in diameter. The exact size depends on the flap model and whether you are using an adapter ring. For example, the Cat Mate 210 series needs a 215mm hole without adapter or 260mm with the glass-mounting adapter. Always buy your cat flap first and check the manufacturer's template before ordering the glass.

How long does it take to get a cat flap installed in double glazing? Allow 2-3 weeks total. The process is: measurement visit (30 minutes), glass manufacturing (10-15 business days), and installation visit (1-2 hours). The manufacturing wait is unavoidable because each sealed unit is custom-built to your exact dimensions with the hole in the correct position.

Should I get a microchip cat flap or a standard one? In Toronto, a microchip cat flap is worth the extra $80-$170. Raccoons in the GTA are bold, dexterous, and absolutely will push through a basic flap if they smell food inside. A microchip flap reads your cat's existing 15-digit ISO microchip and only unlocks for programmed pets. Most models store 20-40 pet profiles. No collar magnets to lose, no raccoon dinner guests.

Can I install a cat flap in a sliding patio door? Yes, but the approach differs. For a sliding glass patio door, you have two options: replace one of the double-glazed panels with a new unit containing the pre-cut hole, or install a separate pet door panel insert that sits in the sliding track. The panel insert is cheaper ($200-$400) and does not require custom glass, but it reduces your door opening width and is less secure. A proper glass replacement is the better long-term solution.