The honest comparison: a 30-pint dehumidifier removes roughly 480 ounces of water a day. A DampRid tub removes about one. That is not a small difference in degree, it is a difference in kind.
Every damp-house article recommends a bowl of rock salt or a bag of charcoal, and almost none of them say how much water it removes. That number is the whole story, so this page leads with it.
These products are not frauds. Calcium chloride genuinely pulls water out of the air, and it will keep a closet dry through a wet winter. The problem is the mismatch between what they remove and what a house produces: a single shower releases more moisture than a tub of DampRid will absorb in a month.
- Calcium chloride tub
- ~1 oz/day
- Silica gel, 1 lb
- ~6 oz total
- Rock salt in a bucket
- < 1 oz/day
- A 30-pint dehumidifier
- 480 oz/day
About 0.05 pints — refills last 30–60 days
Then it needs recharging in an oven
Slower than calcium chloride and messier
For scale — 400× a DampRid tub
Two different chemistries
Deliquescent salts — calcium chloride is the common one, sold as DampRid and similar — absorb so much water that they dissolve into their own brine, which collects in the lower chamber of the tub. They work at low humidity, they cannot be recharged, and the brine is corrosive to metal and harmful to plants and pets. They stop when the salt is gone.
Adsorbents — silica gel, zeolite, activated alumina — hold water on an enormous internal surface area without dissolving. They can be dried out in an oven or, in plug-in versions, by an internal heater, and used again indefinitely. Silica gel is what protects cameras, documents and firearms, and it is the right choice for anything you want kept dry for years.
Everything else on the folk list — baking soda, charcoal, coffee grounds, chalk, rice — absorbs a negligible amount of water. Charcoal and baking soda do adsorb odor molecules, which is why people believe they are working: the closet smells better while the humidity has not moved.
| Method | Water removed | Cost | Honest verdict |
|---|---|---|---|
| Calcium chloride (DampRid) | ~1 oz | $5–12 | Closet, safe, boat, stored RV |
| Silica gel, rechargeable | ~0.5 oz | $15–30 | Cameras, documents, firearms |
| Zeolite / plug-in desiccant | ~1 oz | $25–50 | Closet, small bathroom |
| Rock salt in a bucket | < 1 oz | $3 | Works, but slower and messier |
| Cat litter (silica type) | < 1 oz | $8 | Cheap silica gel, no more |
| Baking soda | ≈ 0 | $2 | Odor only |
| Charcoal briquettes | ≈ 0 | $5 | Odor only |
| Coffee grounds, rice, chalk | ≈ 0 | $2 | Myth |
| Houseplants | Negative | — | Transpiration adds moisture |
When it does not work
The limits matter more than the benefits — this is where most money gets wasted.
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The scale is wrong for a room
A family of four releases 2–3 gallons of water vapor a day through breathing, cooking, showering and laundry. Absorbers remove ounces. The arithmetic does not work at room scale, whatever the packaging implies.
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Houseplants raise humidity, they do not lower it
Peace lilies and Boston ferns appear on every "natural dehumidifier" list. Plants transpire: water goes from the pot into the air. NASA-era studies on plants and air quality measured pollutant removal, not moisture, and the effect was in sealed chambers. A room full of plants is a humidifier.
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Calcium chloride brine is corrosive
Keep the tubs off metal surfaces and away from pets and children. The liquid that collects is not just water.
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They mask, they do not diagnose
A closet that needs a fresh tub every month is telling you the wall behind it is cold or damp. That is worth investigating rather than subscribing to refills.
Doing it properly
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Confirm the space is small and enclosed
Closet, cabinet, safe, boat locker, stored vehicle. If the space is a room with a door people open, skip to dehumidifiers.
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Pick the chemistry for the job
Calcium chloride if you just want the space dry and will replace the tub. Silica gel if you are protecting valuables and want something you can recharge for years.
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Place it low and central
Cool damp air settles. Put the absorber at the bottom of the space, not on the top shelf, and leave air space around it.
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Give the space some airflow
A closet door left ajar an inch does more than the absorber does. Use both.
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Track how fast it fills
A tub that saturates in two weeks means the space is much damper than an absorber can handle — treat that as a diagnostic result, not a reason to buy more tubs.
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Recharge or replace on schedule
Silica gel goes into a 200 °F oven for a few hours. Calcium chloride gets replaced; there is no recharging it.
What it costs
Prices are US retail ranges in 2026 and running costs assume roughly 16 cents per kWh. Treat them as the shape of the decision rather than a quote.
| Absorber | Cost | Removes | Where it works |
|---|---|---|---|
| Calcium chloride tub, 1 lb | $5–$9 | ~1 pt per 3–6 weeks | Genuinely useful in a sealed closet, closet or boat cabin |
| Refill sachets | $2–$4 each | Same | Cheaper than replacing the tub; the container is reusable |
| Silica gel, 1 lb | $8–$15 | Under 0.5 pt, then needs drying | Rechargeable in an oven; best for sealed cases and safes |
| Charcoal or bamboo bag | $8–$20 | Negligible for humidity | Genuinely good at odor, close to useless for moisture |
| Rock salt in a bucket | $3 | A few ounces | The internet favorite; the arithmetic does not survive contact with a room |
How to tell it is working
The honest test for an absorber is the container it sits in. In a closet, a gun safe, a boat cabin over winter or a caravan in storage, a tub of calcium chloride visibly fills with brine and the enclosed air measurably dries. That is a real result in a genuinely small, genuinely sealed volume.
In an ordinary room the same tub collects a pint over several weeks while the household generates six to twelve pints a day. The absorber is not broken and the chemistry is not fake — the scale is simply wrong by a factor of a hundred or more. If someone reports that a tub fixed their damp room, what usually changed was something else at the same time: the weather warmed, the heating came on, or they started opening a window.
The one measurement worth taking is in the enclosure itself. Put a cheap hygrometer inside the closet with the absorber, shut the door, and read it a day later. If it holds below 60% while the room around it sits higher, the absorber is doing exactly what it should.
What to check before you commit
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Only use them in a sealed volume
A closet, a cabinet, a safe, a boat cabin, a stored caravan. Anywhere air exchanges freely with a whole house, the absorber will saturate against an unlimited supply and achieve nothing measurable.
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Keep the brine off everything
Calcium chloride solution is corrosive to metal and stains fabric and timber. Use a tub with a proper separated reservoir and stand it where a spill cannot reach clothes, tools or a floor you care about.
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Silica gel needs recharging, not replacing
A couple of hours in a low oven drives the water off and it is good again. Buying fresh sachets every time is the expensive way to use a material that is designed to be reused indefinitely.
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Do not buy one to fix a room
If the space is bigger than a closet, the money is better spent on venting a dryer, a timer for an exhaust fan, or the electricity to run a dehumidifier for a month.
Common mistakes
Buying tubs for a damp basement
You would need dozens, replaced constantly, to match one dehumidifier. Basements need a machine.
Believing the plant lists
Every "plants that dehumidify your home" article has the physics backwards.
Putting an open salt bucket on a wood surface
The brine escapes, and it stains and corrodes what it lands on.
Treating a fast-filling tub as success
It is a symptom. Something behind that wall is wet.
Common questions
Does DampRid really work?
Yes, within its scale. Calcium chloride removes roughly an ounce of water a day, which keeps a closet, safe or boat locker dry. It is about 400 times weaker than a 30-pint dehumidifier, so it cannot make a measurable difference to a room.
Does rock salt absorb moisture in a room?
It absorbs some, more slowly than calcium chloride and with more mess. In an enclosed closet it helps a little; in a room the effect is too small to register on a hygrometer.
Do houseplants reduce humidity?
No — they increase it. Plants transpire water from soil to air. Lists of "natural dehumidifier plants" have the direction of moisture flow backwards.
Does baking soda absorb moisture?
Barely. It adsorbs odor molecules well, which is why a fridge box seems to work, but its water capacity is negligible compared to silica gel or calcium chloride.
How do I know when a calcium chloride tub needs replacing?
When the crystals in the upper chamber have gone and the reservoir below is full of liquid, the absorbing material is spent. Some tubs go clear at that point; on others you simply see that the granules have collapsed into a shallow crust. Pour the brine away down an outside drain rather than a kitchen sink, and refill rather than throwing the container out.
Can I make a cheap absorber myself?
You can buy bulk calcium chloride — the same material sold as ice melt, provided it is the pure form without additives — and put it in a container over a mesh so the brine drains away from the crystals. It works and it is cheaper. What it does not do is change the scale of the problem: a homemade tub in an open room is still a tub in an open room.
Sources
- US EPA — Mold and moisture guidance — the 30–50% range and the 10 sq ft remediation threshold
- ASHRAE Standard 55 — Thermal environmental conditions for human occupancy — indoor comfort and humidity limits
- US DOE 10 CFR 430 — Dehumidifier test procedure — the 2019 move to 65 °F rating conditions