Introduction
Every autumn in London, the same question starts landing on our phone lines: how do I heat my garage so I can actually use it through the winter? A workshop that is comfortable in October becomes unusable in January. A home gym goes cold. The car is fine, but your fingers are not. Heating a garage is one of the most satisfying home projects a southern Ontario homeowner can take on, because it turns dead space into a room you use for five extra months a year. It is also one of the easiest projects to get backwards.
The mistake almost everyone makes is buying the heater first. A garage is a large, leaky box with the single biggest uninsulated surface in the house sitting on one wall: the garage door. Pour heat into that box before you have sealed and insulated it, and you are paying to warm the driveway. This guide walks through the right order of operations for an Ontario garage, why the door itself is the decisive piece, how to choose a heater once the envelope is ready, and the two safety issues (condensation and carbon monoxide) that a heated garage forces you to take seriously. Every external link here has been checked against a live page on a recognized industry body or government source.
Why Ontario Homeowners Heat Their Garages
The reasons vary, but they cluster into a few groups. Some homeowners want a year-round workshop for woodworking, small-engine repair, or hobby projects. Some are building a home gym and do not want to give up a bedroom for it. Some want a warm place to get into a cold car on a January morning, or a spot to thaw and dry winter gear. And a growing number are turning the garage into a genuine bonus room, a project we cover in depth in our guide to converting a garage into living space.
Whatever the goal, the physics are the same. A detached single-car garage in London might need to be held at 10 to 15 degrees Celsius to be workable; a workshop where you handle tools bare-handed wants closer to 18. The colder it is outside and the warmer you want it inside, the more that temperature difference drives heat straight out through every uninsulated surface. In a typical Ontario garage, the largest of those surfaces, by a wide margin, is the door.
The Garage Door Is the Biggest Weak Point
Think about the surfaces of a garage. The walls back onto insulated house framing or are at least sheathed. The ceiling usually has an attic or living space above it. The floor is a concrete slab that barely moves temperature at all. Then there is the door: a single panel, often nine feet wide by seven feet tall on a single-car garage, or sixteen by seven on a double, made in many older homes of a thin single skin of steel with nothing behind it. That one surface can be a quarter to a third of the total exterior area of the room, and on an uninsulated door it is close to worthless thermally.
This is where R-value comes in. R-value measures resistance to heat flow: higher is better. A bare single-skin steel door has an effective R-value of roughly R-0 to R-2, essentially a cold sheet of metal. A quality insulated sandwich door (steel, foam core, steel) can reach R-12 to R-18 depending on thickness and core material. We break the numbers down in plain language in our garage door R-value guide, and it is worth reading before you spend a dollar on a heater. The short version: heating a garage behind an uninsulated door is like heating a room with one wall made of a cookie sheet.
The industry body that sets the measurement standards, the Door and Access Systems Manufacturers Association, is careful about how these numbers are reported. Their technical data sheet on garage door R-value and U-factor (TDS-163) explains the difference between a "calculated door section" number and a "tested installed door" number, and warns that the two are not directly comparable. When a salesperson quotes you an R-value, it is fair to ask which one they mean. The tested, whole-door figure is the honest one for comparing how a door will actually perform on your house.
Step One: Seal Before You Heat
Insulation slows heat moving through a surface. Sealing stops heat leaking through gaps around it. Both matter, and sealing is the cheaper of the two, so it comes first. On a garage door, four seals do the work.
The bottom seal is the flexible rubber or vinyl strip along the bottom edge of the door that presses against the floor. It is the single most important seal on the door and the one that wears out fastest, because it takes the abuse of every close and every winter freeze. If daylight shows under your closed door, or you can feel a draft at floor level, the bottom seal has shrunk, cracked, or hardened. Our bottom seal replacement guide walks through the fix.
The perimeter weatherstrip runs up both sides and across the top of the door, on the outside of the stop molding. It closes the gap between the door and the frame. The threshold seal, which sits on the floor rather than on the door, is the right tool when the slab is uneven or water tends to run in; we cover when it helps and when it does not in our threshold seal guide. And the seals between the panels matter on older doors, where the joints can whistle in a wind. Our broader garage door seal guide ties all of these together.
Sealing is inexpensive, mostly homeowner-friendly, and it is the highest-return step you can take before heating. There is no point running a heater against a half-inch gap under the door.
Step Two: Insulate the Door and the Rest of the Envelope
Once the door is sealed, the question is whether to insulate the door you have or replace it with an insulated one. The answer depends on the door.
Insulating an existing single-skin door
If your door is a flat single-skin steel or aluminum door in good mechanical shape, a retrofit insulation kit can be a reasonable middle path. These kits add rigid foam panels to the inside of each section. We give a candid assessment of what they do and do not deliver in our DIY insulation kits guide. The honest summary: a kit will noticeably cut radiant chill and dampen noise, but it will not turn a single-skin door into a true R-16 sandwich door, and it adds weight that can throw the door out of balance. If you add a kit, have the spring balance checked afterward.
Replacing with an insulated door
For most homeowners planning to heat a garage seriously, replacing an old single-skin door with a modern insulated one is the better long-term move. A polyurethane-core sandwich door seals better, runs quieter, resists dents, and holds heat far better than any retrofit. On the insulation core itself, there are two common materials, and they behave differently. Expanded polystyrene (the white bead-board foam) is cut to fit between the door's ribs; polyurethane is injected as a liquid that expands and bonds to both steel skins. Per DASMA's thermal data, polyurethane insulates better per inch than polystyrene, which is why the highest R-value doors almost always use a polyurethane core. Our why garage door insulation matters article covers the comfort, energy, and durability case in full.
One point that surprises people: insulating the door alone is not enough if the rest of the box leaks. The walls and ceiling of many Ontario garages are bare drywall or open studs. If you are committing to a heated space, insulate the ceiling and any exterior walls at the same time. The door is the biggest single weak point, but a heated garage is only as good as its weakest remaining surface.
Choosing a Heater for an Ontario Garage
With the box sealed and insulated, you can finally size and choose a heater, and it will be a far smaller, cheaper heater than you would have needed otherwise. There are four common choices for a residential Ontario garage.
Electric radiant and portable electric heaters
Electric heaters are the simplest option: no venting, no gas line, no combustion. A wall-mounted radiant panel or a hardwired 240-volt fan heater is a popular pick for a single-car garage or a workshop that is only heated when in use. Radiant units warm objects and people directly rather than heating all the air, which makes them well suited to a workshop where you want to feel warm at the bench without waiting for the whole room to come up to temperature. The trade-off is running cost: electric resistance heat is the most expensive fuel per unit of heat in Ontario, so electric heaters make the most sense in a well-insulated garage that is heated occasionally rather than held warm around the clock.
Gas-fired unit heaters
A ceiling-mounted natural gas or propane unit heater is the workhorse for larger or frequently used garages, because gas is cheaper per unit of heat than electricity in most of Ontario. The catch is that these are combustion appliances: they must be vented to the outside, they need a gas line, and installation is a licensed job, not a weekend project. Do not buy an unvented (vent-free) combustion heater for an attached garage. The combustion-safety and carbon-monoxide reasons are covered below and they are not negotiable.
Ductless mini-split heat pumps
The option that has changed the math in the last few years is the cold-climate ductless heat pump, or mini-split. A modern cold-climate unit produces usable heat well below minus 20 Celsius and delivers two to three units of heat for every unit of electricity, which makes it dramatically cheaper to run than resistance electric heat, and it provides air conditioning in summer as a bonus. The upfront cost is higher and it needs a licensed installer, but for a garage you plan to hold warm all winter it is often the lowest lifetime cost. Natural Resources Canada's overview of what makes an energy-efficient home is a good primer on how heat pumps and the building envelope work together, and Ontario homeowners should check for current federal or provincial rebates before buying.
A rough sizing rule
As a starting point for a sealed, insulated Ontario garage, plan on roughly 45 to 60 watts of electric heat, or about 150 to 200 BTU of gas output, per square foot of floor for a comfortably warm workshop. A single-car garage of about 250 square feet lands near a 4,000 to 5,000 watt electric heater or a 30,000 to 45,000 BTU gas unit. An uninsulated garage can need two to three times that, which is exactly why sealing and insulating first saves you money on the heater itself, not just on running costs. A heating contractor can do a proper load calculation, and it is worth having one for anything gas-fired.
The Condensation Problem a Heated Garage Creates
Here is the issue nobody warns you about. The moment you heat a garage, you introduce a temperature difference between warm, moist indoor air and cold surfaces, and warm moist air meeting a cold surface produces condensation. Drive a snow-covered car into a heated garage and the melting snow, plus the warm air, plus the still-cold steel of the door and any un-insulated surface, is a recipe for dripping water, rust, and even mold. We devote a whole article to this in our garage door condensation guide, and it becomes far more relevant once a garage is heated.
The defenses are the same ones that make a heated garage efficient in the first place: insulate the surfaces so they are not cold enough to hit the dew point, seal the door so humid outdoor air is not drawn in, and provide some ventilation and a way for meltwater to drain. A small amount of air exchange is not the enemy of a heated garage; trapped moisture is. If you are heating a garage and seeing water on the inside of the door, the fix is almost always more insulation and better drainage, not more heat.
Safety First: Attached Garages and Carbon Monoxide
This section is not optional reading. If your garage is attached to the house and you are adding heat, you are changing the airflow and combustion picture of a space that shares walls, and sometimes air, with where your family sleeps.
Two rules matter most. First, never use an unvented combustion heater, a barbecue, or a portable propane heater rated for outdoor use inside an attached garage. They produce carbon monoxide, an odorless, colorless gas that can be fatal, and a garage does not have the ventilation to make them safe. Any gas-fired garage heater must be a sealed or properly vented unit installed by a licensed technician. Second, install a carbon monoxide alarm in or near the garage and test it, especially if the garage is attached or has living space above it. Our guide to carbon monoxide risk in attached garages explains why a running car, a workshop, and a heater together raise the stakes, and what detection and ventilation you need.
A heated garage is a wonderful thing. A heated garage that is quietly filling with combustion gases is a tragedy. Spend the money on a proper vented appliance and a working alarm, and never cut that corner.
What It Costs to Heat a Garage in 2026
Costs fall into two buckets: getting set up, and running it. The setup cost is dominated by whether you replace the door and which heater you choose. A quality insulated replacement door is the single most valuable upgrade for a garage you intend to heat, and for a typical London home an insulated double-car door installed generally runs in the range of about $2,400 to $4,000 depending on size, thickness, and window and finish choices; you can model your own configuration on our door price estimator. Sealing work is far cheaper and often a same-day job.
Running cost depends on fuel, insulation, and how warm you keep it. This is where the earlier work pays off. Our insulated door energy savings guide shows how much an insulated door alone changes the numbers for a London homeowner. As a rule of thumb, holding a well-insulated single-car garage at a comfortable workshop temperature through an Ontario winter costs meaningfully less with a cold-climate heat pump or gas than with resistance electric heat, and costs far less in a sealed, insulated garage than in a leaky one regardless of fuel. The cheapest unit of heat is always the one you did not lose through the door.
Prices in this article are estimates for the London, Ontario area and exclude HST. Fuel rates and rebate programs change, so confirm current electricity and gas rates and any available incentives before you commit to a heating system.
A Realistic Plan: Seal, Insulate, Then Heat
If you take one thing from this guide, let it be the order of operations. Homeowners who heat a garage successfully almost always follow the same sequence:
- Seal the door. Replace a worn bottom seal, add or renew the perimeter weatherstrip, and address the threshold if water runs in. Cheap, fast, and high-return.
- Insulate the envelope. Insulate or replace the door, then insulate the ceiling and exterior walls. This is what shrinks the heater you need and the bill you pay to run it.
- Manage moisture. Plan for meltwater drainage and a little ventilation so a heated garage does not become a condensation trap.
- Choose and size the heater. Pick electric, gas, or a heat pump based on how often you will heat and how warm you want it, and size it to the now well-insulated space.
- Install safety. A vented appliance and a tested carbon monoxide alarm, non-negotiable on an attached garage.
Do it in that order and a heated garage is comfortable and affordable. Do it backwards, heater first, and you will spend the winter feeding an expensive heater against a cold steel door and wondering why the room never quite warms up.
Frequently Asked Questions
Do I really need to insulate the door, or can I just buy a bigger heater?
You can heat a leaky garage with a big enough heater, but you will pay for it every hour it runs, and a snow-covered car plus a cold door is a condensation problem waiting to happen. Insulating the door lowers the size of heater you need and the cost to run it, and it keeps the inside surface warm enough to reduce dripping. It is the higher-value spend almost every time.
What temperature should I keep a heated garage at?
It depends on use. A garage kept just above freezing, around 5 degrees Celsius, protects stored items and makes the car easier to start. A functional workshop is comfortable around 15 to 18 degrees. The warmer you hold it and the colder it is outside, the more the insulation and sealing matter.
Is an electric heater or a gas heater cheaper to run in Ontario?
Per unit of heat, natural gas is usually cheaper than resistance electric heat, and a cold-climate heat pump is often cheaper still because it moves heat rather than generating it. Electric resistance heaters are cheapest to buy and install but most expensive to run, which makes them best for a well-insulated garage heated only when you are using it. Confirm current rates before deciding.
Will heating my garage cause rust on the door and tools?
It can, if you do not manage moisture. The risk comes from warm humid air condensing on cold steel. Insulating surfaces, sealing the door, providing drainage for meltwater, and allowing a little ventilation all reduce it. See our condensation guide for the full set of defenses.
Can I use a portable propane or kerosene heater in my attached garage?
No. Unvented combustion heaters produce carbon monoxide and are not safe for an enclosed attached garage. Use a properly vented gas unit heater installed by a licensed technician, or an electric heater or heat pump, and install a carbon monoxide alarm. This is a safety line we do not bend on.
Does heating the garage help the attached house stay warmer?
A heated, insulated garage does reduce heat loss through the shared wall between the garage and the house, and it removes a large cold buffer next to your living space. It is a modest side benefit, not a primary reason to heat, but it is real, especially if there is a bedroom or bonus room above the garage.
My garage door is old. Should I fix the seals or just replace it before winter?
If the door is a single-skin door in rough shape and you are committed to heating the space, replacing it with an insulated door usually makes more sense than patching seals on a door that will still perform poorly. If the door is sound and reasonably modern, sealing and possibly a retrofit kit can be enough. Our cold weather garage door problems guide covers the winter issues an aging door tends to develop.
Conclusion
Heating a garage in Ontario is absolutely worth doing, and it is one of the few home projects that pays you back in comfort every single winter day. The secret is that it is not really a heating project; it is a sealing and insulation project with a heater at the end. Get the door sealed, get the envelope insulated with the door as your first priority, plan for moisture, and then pick a right-sized, safely installed heater. Do it in that order and the room warms up fast, stays warm cheaply, and stays dry.
If you are planning a heated workshop, gym, or bonus room this winter and you want an insulated door that actually holds the heat, our London team is happy to help. We can assess your current door and seals, quote an insulated replacement sized to your garage, and make sure the door is balanced and weathertight before the cold sets in. Call 519-619-7901, reach us through our contact page, or build and price an insulated door on our price estimator. For the routine that keeps any door sealing well year after year, DASMA's homeowner maintenance guide is a worthwhile annual read. AA Advantage Doors has kept London-area garages sealed, insulated, and running smoothly since 2009.
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