The One-Sentence Answer
If your garage is attached to the house and daylight shows around the door, it's fighting your furnace and your AC every single day. That's the rule of thumb. Now here are the exceptions that actually change the math.
When the leak matters less
- Your garage is detached — no shared wall, so the air exchange with living space is minimal.
- The door between the garage and the house is solid, weatherstripped, and kept closed — this acts as a second barrier even if the garage itself leaks.
- Your HVAC ductwork and water heater are not routed through the garage — nothing in there is directly tied to your comfort system.
If none of those apply — attached garage, shared wall, maybe the furnace or ducts are in there — the gap under the door is a bigger deal than it looks.
Why an Attached Garage Is a Thermal Weak Point
A garage door is the largest moving opening in most houses, and it's rarely insulated or sealed as well as a wall or a window. When it leaks, three things happen: hot or cold outside air gets into the garage, that air pushes against the shared wall and the interior door, and any ductwork or HVAC equipment sitting in the garage gets directly exposed to outdoor temperatures instead of conditioned air. Your thermostat is reading the house, but your system is working harder because the garage next to it is basically outdoors with a roof.
Why This Hits Harder Here Than in a Mild Climate
Inland summers push garage temperatures well above outdoor air temperature by mid-afternoon, since the space traps heat with no airflow. If that heat radiates through a shared wall or warms ductwork sitting in the garage, your AC runs longer to compensate. Winter brings the opposite problem — rain and cold snaps chill an uninsulated garage fast, and that cold transfers the same way. On top of the temperature swings, dry-season dust and wildfire smoke find their way in through the same gaps that let air through, which is a separate problem but the same root cause: a door that isn't sealing.
Where the Leaks Actually Are
Before you settle anything, check these three spots:
- Bottom seal (astragal): this rubber strip compresses and cracks over years of sun exposure. Look for flattened, torn, or hardened rubber, or a visible gap where the door doesn't quite meet the slab.
- Side and top weatherstripping: these strips wear out or fall off entirely, especially on older doors. Run a hand along the frame with the door closed — if you feel a draft, that's your leak.
- Panel gaps and warping: wood and older steel doors can warp from heat and humidity cycles, leaving gaps between panels that no amount of weatherstripping fixes.
A Quick Test You Can Do Tonight
- Close the garage door and turn off the garage lights.
- Have someone stand outside with a flashlight while you stand inside in the dark.
- Any light you see coming through around the edges or bottom is air coming through too.
- For the bottom seal specifically, try sliding a dollar bill under the door from outside — if it pulls through easily without resistance, the seal isn't doing its job.
Insulation Won't Fix a Bad Seal
This is the part worth settling clearly: an insulated door helps with radiant heat and noise, but it does nothing for air leakage if the seals around it are shot. A well-sealed single-layer door can outperform a poorly-sealed insulated one when it comes to drafts. If you're debating whether to spend money on a new insulated door versus just replacing weatherstripping, check the seals first — it's the cheaper fix and often solves most of the problem on its own.
One Local Wrinkle: Settling and Retrofits
Homes that have gone through foundation work or seismic retrofitting sometimes end up with a garage door frame that's shifted slightly out of square. Even new weatherstripping won't seal evenly against a frame that's no longer level, which is why a door that sealed fine for years can suddenly start leaking after other work was done on the house.
When to Call San Bernardino Garage Door
If the flashlight test shows daylight everywhere, or the door doesn't sit flush against the slab even after you've tried adjusting it, it's worth having it looked at rather than guessing. Our Garage Door Maintenance visits include checking seals, weatherstripping, and alignment, and we can tell you whether a seal swap will solve it or whether the door itself is sagging or warped. If the frame is out of square from settling or an old spring is letting the door hang unevenly, our Spring Replacement and adjustment work restores the balance that lets the door close flush in the first place — no amount of new weatherstripping fixes a door that isn't closing straight.
If your garage door has been letting outside air, dust, or smoke into the house, call San Bernardino Garage Door for a seal and alignment check. We serve San Bernardino, Highland, Redlands, Loma Linda, Colton, Rialto, Fontana, and Yucaipa, and we can tell you in one visit whether you need new weatherstripping or something bigger.