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Last Updated on September 30, 2026 by Muhammad Toqeer
Your sump pit is an open pool of water inside a heated room, and a hole into the soil under your house. It evaporates around the clock and it lets ground air in, and a dehumidifier is paying to remove the result.
The fix is a lid — and it is not an upgrade. The residential code already requires a gas-tight cover on any sump pump that is installed. Most pits in older houses do not have one.
Two things an open pit does
It evaporates. A cool water surface under warmer basement air gives up moisture continuously, and the rate depends almost entirely on surface area. Your pit is a permanently wet dish sitting in a conditioned room.
And it breathes. An open pit is one of the largest deliberate holes in a basement floor. Warm air rising through the house has to be replaced from somewhere low down, and the air a sump draws comes from the drain tile and the saturated ground around your footings.
That second path is the one radon mitigation exists to close — which is why radon contractors seal sump lids, and why the lid sold for radon happens to solve a humidity problem at the same time.
How much water, actually
Contractor pages quote numbers for this with no method attached. Here is the arithmetic instead.
Evaporation from a still open surface runs at roughly E = exp(−(7.2 − 0.03236·T)) gallons per square foot per hour, with T in °F. An 18-inch basin is 1.77 square feet; a 24-inch one is 3.14.
| Basin | 55 °F | 60 °F | 65 °F |
|---|---|---|---|
| 18-inch | 1.5 | 1.8 | 2.1 |
| 24-inch | 2.7 | 3.1 | 3.7 |
Pints per day.
At 60 °F that is about 53 pints a month from an 18-inch pit, and 94 from a 24-inch one.
Be honest about the scale: this is not a flood. It is a couple of pints a day. What makes it worth closing is that it never stops — every hour, every day, all year, inside the space you are paying to condition. And it is the rare moisture source you can remove outright rather than manage with a machine.
If you run a dehumidifier in that basement, it is removing this water and you are paying for the electricity.
What the code actually says
This is the part that surprises people, and it is worth quoting directly:
“The 2009, 2012, and 2015 IRC require that any sump pumps that are installed have a gas-tight, removable cover.”
Gas-tight. Removable. Required. Not “recommended in damp climates”, not “a good idea for radon.”
On why:
“If the sump pit is open or has a loose-fitting lid, this water can evaporate into the air, raising the relative humidity inside the home, basement, and crawlspace.”
“Uncovered or improperly sealed sump pumps can also allow radon and other soil gases to enter the basements and crawlspaces and then mix with air inside the home.”
Both mechanisms, named in the same guidance.
A lid that sits on top is not a cover
This is where most pits fail, including ones that look finished. A compliant cover is three things at once:
Rigid. “Sump covers should be made of durable plastic or other rigid material.” A sheet of plywood is not it.
Gasketed. “…designed for air-tight sealing with a built-in gasket.” The seal is the point; the lid is just what carries it.
Mechanically fastened. “Mechanically fasten the cover in place with screws or other means that will maintain tight contact between the lid, the gasket, and the rim of the crock.”
ENERGY STAR asks for the same, requiring the lid to be mechanically attached and the sump fitted with a full gasket seal.
A loose lid meets none of the three, and a loose lid is what most basements have.
The complication nobody mentions
A sealed lid still has to let three things through: the discharge pipe, the pump’s power cord, and — where one is fitted — the float or alarm cable.
Those penetrations are where sealed lids leak. A cover with a ragged hole cut for the cord is not gas-tight, whatever it says on the box. Proper covers use grommets or sealed entries, and this is the detail to look at on a lid that has already been fitted.
Before you seal it, look in it
A pit that always has standing water is a bigger evaporating surface than one that empties.
If yours never seems to drop, that is worth understanding first — a pit that runs constantly when it has not rained usually has a source you can remove, and fixing that lowers the water level and the evaporation with it.
And check the pump itself is in reasonable shape before you screw a lid over it. If it is near replacement, doing both jobs at once saves opening it up twice.
What a lid does not fix
It is not a waterproofing measure. The pit is doing its job; sealing it changes what gets into the air, not what gets into the ground.
It is not a radon test. Sealing the pit is a standard part of radon mitigation, but whether you have a radon problem is a separate question with its own testing, and a lid is not an answer to it.
And it will not fix a basement that is damp for other reasons — a dirt crawlspace, an unsealed slab, uninsulated cold surfaces or a genuine leak. The pit is one source. It happens to be the easiest one to close outright.
When this needs a pro
Fitting or replacing the cover. It means working around live pipework, a pump cord and often a float cable, and the penetrations are what decide whether the seal is real.
Anything to do with radon. Testing, and any active mitigation system, is specialist work with its own standards.
A basement that stays humid with the pit sealed. At that point the pit was not the main source, and finding what is needs someone who can look at the whole envelope.
What you can do yourself: look at your lid. Is it rigid, is there a gasket, and is it screwed down? If the answer to any of those is no, you have found something the code already asked for — and it takes ten seconds to check.
FAQ
Can an open sump pit cause basement humidity?
Yes, in two ways. The standing water evaporates continuously — about 1.8 pints a day from an 18-inch pit at 60 °F — and the open pit lets air from the soil and drain tile be drawn into the basement.
Does a sump pit need a sealed lid by code?
Yes. The 2009, 2012 and 2015 IRC all require that any installed sump pump has a gas-tight, removable cover. Many existing pits have only a loose lid resting on the rim, which does not meet it.
What makes a sump cover gas-tight?
Three things together: rigid material, a built-in gasket, and mechanical fastening so the lid stays in tight contact with the rim. ENERGY STAR requires the same combination. A lid that simply sits on top meets none of them.
Will sealing the sump pit lower my basement humidity?
It removes one continuous source. How much difference you notice depends on how much else is contributing — a dirt crawlspace or an uninsulated slab may matter more — but the pit is usually the easiest source to eliminate outright.
Does sealing a sump pit help with radon?
Sealing the pit is a standard part of radon mitigation, because the pit is a direct opening into the soil. It is not a substitute for testing or for a designed mitigation system if you have a radon problem.
Can I just put a piece of plywood over the sump pit?
No. The guidance calls for durable plastic or other rigid material with a built-in gasket, fastened down. Plywood is neither gas-tight nor durable in a damp pit, and it leaves the two paths open.
