Short answer: Yes, a little. Acrylic absorbs about 0.2% of its weight in water in 24 hours of immersion (ASTM D570) and roughly 1% at long-term equilibrium. As it absorbs moisture it swells: ACRYLITE measures dimensional changes of about 0.2% at 60% relative humidity and 0.6% at 100%. When one face is wet and the other dry, like a lid over an aquarium or vivarium, the wet side grows and the sheet bows toward the dry side.

Key facts

  • 24-hour immersion: 0.2% water absorbed (ACRYLITE cast and extruded, Plexiglas G); Plaskolite lists 0.4% for extruded OPTIX.
  • Long-term: about 0.8% at equilibrium (Plexiglas G, 28 days at 212 °F) and about 1.1% after 48 days of immersion (ACRYLITE cast).
  • Swelling from humidity: 0.2% at 60% RH, 0.3 to 0.4% at 80% RH, 0.6% at 100% RH (ACRYLITE).
  • Practical meaning: a 24 in. lid can grow about 0.14 in. on its wet face at 100% humidity, enough to bow it visibly.
  • It's reversible: dry the sheet evenly and it relaxes back toward flat.

How much water does acrylic absorb?

Very little by the standards of wood or nylon, and less than its reputation suggests. In the standard ASTM D570 test, a sample soaked for 24 hours at 73 °F gains about 0.2% of its weight in both cast and extruded ACRYLITE and Plexiglas G. Leave it in water for weeks and it keeps slowly taking up moisture: ACRYLITE's cast sheet reaches about 1.1% weight gain after 48 days, and Plexiglas G lists 0.8% as its long-term equilibrium. Acrylic doesn't rot, dissolve, or lose its clarity from water. It's used for aquariums and pool windows for good reason.

Does acrylic swell or expand with humidity?

Yes, and this is the part that matters for real parts. ACRYLITE gives dimensional changes of 0.6%, 0.4%, and 0.2% at 100%, 80%, and 60% relative humidity for cast sheet (0.6%, 0.3%, and 0.2% for extruded). For comparison, a 100 °F temperature swing changes the length by about 0.4%. So humidity can move acrylic about as much as a hot summer day does. Frames and channels have to allow for both; see acrylic thermal expansion.

Why does my acrylic lid warp over a fish tank or vivarium?

Because one side is wet and the other isn't. The underside of a lid over warm water sits at nearly 100% humidity and often a higher temperature. That face absorbs moisture and grows. The top face, in room air, stays the same size. A sheet with one face longer than the other has to curve: the wet side becomes convex and the dry side concave, so the lid bows up in the middle or its corners lift.

The same thing happens to vivarium doors with misting systems, humidor lids, and acrylic covers over hot tubs and steam sources. A heat lamp over one area adds a temperature difference on top of the moisture difference and makes it worse.

How do I keep an acrylic lid flat?

  1. Go thicker. Stiffness rises with the cube of thickness, so 3/8 in. resists bowing far better than 3/16 in. for the same span.
  2. Add stiffening ribs. Bond 1/2 to 1 in. tall strips on edge across the top face, like joists. A ribbed 1/4 in. lid stays flatter than a plain 3/8 in. one.
  3. Hold the edges. A frame or channel on all four sides resists cupping. Leave room for the sheet to grow in length.
  4. Vent it. Gaps, cutouts, or a mesh section let moisture escape and lower the humidity under the lid.
  5. Flip it. Turning a loose lid over every day or two lets the swollen face dry out while the other face takes its turn.
  6. Consider glass if the lid sits directly over hot, humid water and can't be ventilated or framed. Glass doesn't absorb water at all. Acrylic is still the better choice for drilled feeding holes, hinged doors, and anything that might be dropped.

We cut lids to size with holes, slots, and cutouts, and can bond on stiffening ribs; start in the instant quote or describe your project.

Does stored sheet absorb moisture too?

Yes. Plaskolite notes that sheet can develop a temporary wavy look as it absorbs moisture from the air in storage, which settles once the masking is off and the sheet acclimates. Store sheet flat or upright in a dry, ventilated space, and let it sit in the room where it will be installed before fitting it tightly into a frame. If you plan to heat it, keep it dry and follow the steps in how to bend acrylic.

Questions

Is acrylic waterproof?

Yes. Water doesn't pass through it or degrade it, which is why it's used for aquariums, pool windows, and boat glazing. It just absorbs a small amount into its surface, which causes slight swelling.

Will a warped acrylic lid go back to flat?

Moisture warping usually reverses when both faces dry to the same level, for example by taking the lid off for a day or flipping it. Warping from heat (a lamp too close) is permanent; see how to flatten warped acrylic.

Does polycarbonate absorb less water than acrylic?

Slightly. The LEXAN 9034 data sheet lists 0.15% in 24 hours for Lexan 9034 against about 0.2% for acrylic, but polycarbonate is also less stiff, so a polycarbonate lid isn't automatically flatter.

Sources