How an Overheated Garage Is Secretly Taxing Your AC
If your house has an attached garage, there’s a good chance it turns into a small oven every summer afternoon in the Treasure Valley. Temperatures inside a closed, sun-soaked garage can climb well past the outdoor air temperature, and that heat doesn’t stay contained. It pushes through the shared wall, the door into your house, and sometimes straight through your ductwork, making your AC work harder than it should.
This is one of those summer comfort issues that’s easy to overlook because the garage feels like a separate space. It isn’t.
Why Garages Get So Hot
A few things stack up against an attached garage in the summer:
- Minimal insulation. Garages are often built to a lower insulation standard than the rest of the house, especially the garage door itself.
- Dark surfaces and direct sun. Asphalt driveways and dark garage doors absorb heat and radiate it inward.
- No active cooling. Unlike the rest of your home, the garage isn’t on your thermostat, so heat just builds with nowhere to go.
- Trapped exhaust heat. Cars that have just been driven add residual engine heat to an already warm space.
By late afternoon, it’s common for a closed garage to sit 20-30 degrees hotter than the outside air.
How That Heat Gets Into Your Home
The wall (or door) separating your garage from your living space isn’t a perfect barrier. Heat moves toward cooler space, so:
- Shared walls conduct heat into adjacent rooms, especially bedrooms or laundry rooms built over or next to the garage.
- The door between the garage and house is a common weak point if it isn’t properly weatherstripped or insulated.
- Ductwork routed through the garage is the biggest hidden offender. Many homes run supply or return ducts through the garage ceiling, and if that ductwork isn’t well-sealed and insulated, it’s essentially absorbing garage-level heat before that air ever reaches your rooms.
- A garage water heater or furnace closet, common in a lot of Boise homes, adds even more ambient heat to a space that’s already overheating.
The result: rooms near the garage run warmer than the rest of the house, your thermostat compensates by running the AC longer, and your system works overtime to fight heat that’s leaking in from a space it was never designed to cool.
Signs Your Garage Heat Is Affecting Your AC
- One or two rooms near the garage are noticeably warmer than the rest of the house
- Your AC seems to run longer in the afternoon than its rated capacity should require
- You can feel warm air coming through vents in rooms adjacent to the garage
- The wall or door shared with the garage feels warm to the touch
What Actually Helps
The fix usually isn’t about cooling the garage itself. It’s about stopping the heat transfer:
- Insulate and seal ductwork routed through the garage. This is often the single biggest improvement. Sealed, insulated ducting keeps your conditioned air conditioned, instead of picking up heat on its way to the vent.
- Weatherstrip the door between the garage and house. A simple, inexpensive fix that closes one of the most direct paths for heat transfer.
- Insulate the garage door and shared walls, if they aren’t already. This slows down how much heat the garage absorbs and radiates inward in the first place.
- Keep the garage door closed during peak heat hours when possible, and consider a vent fan to keep hot air from sitting and building all afternoon.
- Have your ductwork inspected. If you’ve never had someone physically check duct routing and insulation, especially in older homes, it’s worth knowing exactly what’s running through that hot space.
When It’s Worth a Closer Look
If you’ve noticed uneven temperatures in rooms near your garage every summer, it’s not something you have to just live with. A lot of the time, the real culprit isn’t your AC unit at all. It’s ductwork or insulation quietly losing the battle against a 130-degree garage.
At Gem Heating & Air Conditioning, we inspect duct routing as a standard part of our HVAC evaluations, including the parts most companies never check, like ductwork running through garages and crawlspaces. If certain rooms in your home never seem to cool down the way the rest of the house does, we’re happy to take a look and find out why.
If your house has an attached garage, there’s a good chance it turns into a small oven every summer afternoon in the Treasure Valley. Temperatures inside a closed, sun-soaked garage can climb well past the outdoor air temperature, and that heat doesn’t stay contained. It pushes through the shared wall, the door into your house, and sometimes straight through your ductwork, making your AC work harder than it should.
This is one of those summer comfort issues that’s easy to overlook because the garage feels like a separate space. It isn’t.
Why Garages Get So Hot
A few things stack up against an attached garage in the summer:
- Minimal insulation. Garages are often built to a lower insulation standard than the rest of the house, especially the garage door itself.
- Dark surfaces and direct sun. Asphalt driveways and dark garage doors absorb heat and radiate it inward.
- No active cooling. Unlike the rest of your home, the garage isn’t on your thermostat, so heat just builds with nowhere to go.
- Trapped exhaust heat. Cars that have just been driven add residual engine heat to an already warm space.
By late afternoon, it’s common for a closed garage to sit 20-30 degrees hotter than the outside air.
How That Heat Gets Into Your Home
The wall (or door) separating your garage from your living space isn’t a perfect barrier. Heat moves toward cooler space, so:
- Shared walls conduct heat into adjacent rooms, especially bedrooms or laundry rooms built over or next to the garage.
- The door between the garage and house is a common weak point if it isn’t properly weatherstripped or insulated.
- Ductwork routed through the garage is the biggest hidden offender. Many homes run supply or return ducts through the garage ceiling, and if that ductwork isn’t well-sealed and insulated, it’s essentially absorbing garage-level heat before that air ever reaches your rooms.
- A garage water heater or furnace closet, common in a lot of Boise homes, adds even more ambient heat to a space that’s already overheating.
The result: rooms near the garage run warmer than the rest of the house, your thermostat compensates by running the AC longer, and your system works overtime to fight heat that’s leaking in from a space it was never designed to cool.
Signs Your Garage Heat Is Affecting Your AC
- One or two rooms near the garage are noticeably warmer than the rest of the house
- Your AC seems to run longer in the afternoon than its rated capacity should require
- You can feel warm air coming through vents in rooms adjacent to the garage
- The wall or door shared with the garage feels warm to the touch
What Actually Helps
The fix usually isn’t about cooling the garage itself. It’s about stopping the heat transfer:
- Insulate and seal ductwork routed through the garage. This is often the single biggest improvement. Sealed, insulated ducting keeps your conditioned air conditioned, instead of picking up heat on its way to the vent.
- Weatherstrip the door between the garage and house. A simple, inexpensive fix that closes one of the most direct paths for heat transfer.
- Insulate the garage door and shared walls, if they aren’t already. This slows down how much heat the garage absorbs and radiates inward in the first place.
- Keep the garage door closed during peak heat hours when possible, and consider a vent fan to keep hot air from sitting and building all afternoon.
- Have your ductwork inspected. If you’ve never had someone physically check duct routing and insulation, especially in older homes, it’s worth knowing exactly what’s running through that hot space.
When It’s Worth a Closer Look
If you’ve noticed uneven temperatures in rooms near your garage every summer, it’s not something you have to just live with. A lot of the time, the real culprit isn’t your AC unit at all. It’s ductwork or insulation quietly losing the battle against a 130-degree garage.
At Gem Heating & Air Conditioning, we inspect duct routing as a standard part of our HVAC evaluations, including the parts most companies never check, like ductwork running through garages and crawlspaces. If certain rooms in your home never seem to cool down the way the rest of the house does, we’re happy to take a look and find out why.
