The biggest air leak in your home is right under your nose.

Or, underfoot, more accurately.

It’s the hole that a plumber cuts in the subfloor to make room for the drain assembly at the bottom of a bathtub. Because the drain assembly is big, plumbers make way for it by cutting a generous-size hole that gives them some “wiggle room” when connecting plumbing pipes. This big hole often is left open to the space below.

If you have a first-floor bathroom over a crawl space or unheated basement, you likely have one of these giant energy wasters in your home.

“Having that big hole for the bathtub drain can waste about as much energy as leaving one of your windows open a few inches, all day, every day,” says Allison Bailes III, president of energy consultant firm Energy Vanguard. “The main difference is that you can easily close the window, but most people don’t even know about this other open window.”

And BTW, that hole also is big enough for good-size critters to crawl inside your home!

Plumbers may plug this hole with a wad of loose insulation, but that’s an imperfect solution — fiberglass insulation can sag over time, especially if it gets damp. The preferred method is to insert foam board that’s cut to fit around the pipes, then seal the board in place with spray foam insulation.

You won’t know if you have a problem unless you crawl under your bathroom and take a look at the tub drain assembly from underneath. Patch the hole with pieces of foam board screwed in place, then seal any gaps with spray foam insulation ($8/can).

A plumber will do the job, too, but that’ll set you back $100-$150. Probably worth it when you think about energy savings, comfort, and keeping mice and camel crickets out of your house!