Closed cell foam sheets are often chosen where low water uptake matters. For All Foam's current range, AFP30 is described in its technical data as waterproof and non-absorbent, while AFP45's current technical data gives maximum water absorption as 1% by volume. Those material properties are useful, but they do not automatically make a finished box, joint, seal or enclosure watertight.
Quick answer
Closed cell polyethylene foam can have very low water absorption because most of its cells are sealed. AFP30 is described as waterproof and non-absorbent, while AFP45 has a published maximum water absorption of 1% by volume. The finished assembly can still leak around edges, joints, fixings and other components.
"Waterproof", "water resistant" and "watertight" are not the same claim
These words answer different questions.
A foam supplier may describe the material itself as waterproof or give a measured water-absorption value.
A finished product may still need to stop water passing through:
- seams
- screw holes
- cable entries
- lid gaps
- joints
- interfaces with another material
That second question is about the whole assembly.
It is therefore possible for a low-water-absorption foam to sit inside a product that is not watertight.
What the AFP30 and AFP45 data says
AFP30 is the 30 kg/m3 nominal-density grade in All Foam's rigid cross-linked closed-cell polyethylene range. Its current technical data describes it as waterproof and non-absorbent.
AFP45 is the denser option in the same rigid cross-linked PE family, with a nominal density of 45 kg/m3. Its current technical data gives maximum water absorption of 1% by volume.
AFP45 also has higher published compression resistance than AFP30.
That mechanical difference should not be translated into an unsupported statement that AFP45 is "more waterproof".
Why closed cells limit water uptake
In open-cell foam, many cells communicate with neighbouring cells.
In closed-cell foam, the structure contains substantially sealed cells.
That makes bulk water less able to move through the foam body in the way it can through a sponge-like open-cell material.
The exact result still depends on the grade and test method, so use the published data for the actual product rather than a generic claim about all closed-cell foams.
What happens when the foam is cut?
Cutting exposes a fresh foam edge.
It does not turn the whole sheet into open-cell foam.
For a simple pad or insert, a clean cut mainly affects fit and finish.
For a sealing application, the important question is whether the cut piece fits the joint correctly and is compressed as intended. A poor fit can let water pass around the foam even when the material itself has low absorption.
Water can travel around the foam
This is one of the most important practical points.
A sheet can remain substantially non-absorbent while water travels:
- between the sheet and a wall
- under an unbonded edge
- through a fixing hole
- along a seam
- around an incorrectly compressed gasket
- behind an adhesive layer
If the application must be watertight, the joint design matters as much as the foam.
Splash, condensation and immersion are different exposures
A material may face very different conditions in service.
Examples include:
- occasional splashes
- repeated washing
- condensation
- standing water
- short-term immersion
- continuous outdoor wetting
Do not assume one successful condition proves suitability for all of the others.
The exact product documentation and complete application need to match the exposure.
Can water become trapped behind a closed-cell liner?
Yes.
Low water absorption by the foam does not mean water cannot sit behind it.
A liner attached over a damp surface can trap moisture at the interface if the design gives it nowhere to drain or dry.
Where that matters, consider:
- drainage
- ventilation
- edge detailing
- fixing method
- inspection access
- the moisture behaviour of the substrate
This is a system-design issue rather than a contradiction of the foam's water data.
Can closed cell polyethylene be used as a seal?
Closed-cell polyethylene can be considered for suitable gasket, spacer or seal applications where its mechanical properties and chemical compatibility fit the job.
A working seal still depends on:
- correct thickness
- enough compression
- even contact
- surface flatness
- joint geometry
- retained compression over time
- compatible liquids and chemicals
A closed-cell strip placed loosely in a gap is not automatically a watertight seal.
Does density decide water resistance?
Not on its own.
AFP45 is denser than AFP30, but density describes mass per unit volume.
Water behaviour should be taken from the water-related data for the exact grade.
Do not rank foam grades by waterproofing performance from density alone.
Outdoor use needs more than water data
A low-water-absorption material may still face other outdoor stresses.
Check:
- UV exposure
- operating temperature
- abrasion
- repeated compression
- wind or mechanical movement
- chemicals
- fixing durability
- fire or regulatory requirements
Water resistance is one part of outdoor suitability, not the complete specification.
Cleaning and drying
If a removable closed-cell insert becomes wet:
- remove surface dirt
- use a cleaning method compatible with the foam and any surrounding materials
- allow the insert and substrate to dry
- inspect for trapped grit
- check bonded edges before reuse
A non-absorbent sheet can still carry dirt or water on its surface.
AFP30 and WRAS approval
AFP30's current technical data sheet states WRAS approval for cold-water use.
That should be kept within its proper scope.
WRAS approval is not the same as:
- food-contact approval
- medical approval
- marine certification
- proof that a bonded assembly is watertight
The finished application may have its own separate requirements.
Common mistakes
- Treating low water absorption as proof that the whole product is waterproof.
- Assuming every closed-cell polyethylene grade has the same water data.
- Ranking water resistance from density alone.
- Ignoring cut fit, seams and fixing holes.
- Trapping moisture behind a liner.
- Copying a water-absorption figure from another supplier's PE grade.
- Treating WRAS cold-water approval as an unrelated certification.
- Assuming water resistance proves UV, chemical or fire suitability.
Quick answers
Does closed cell polyethylene foam absorb water? AFP30 is described as waterproof and non-absorbent. For AFP45, the current technical data gives maximum water absorption as 1% by volume. Other PE foam grades can have different published values.
Are cut edges still water resistant? Cutting does not turn the whole material into open-cell foam, but the cut quality and fit can matter where the piece forms part of a seal or joint.
Can water pass behind closed cell foam? Yes. Water can travel around edges, seams, fixings or interfaces even when the foam itself has low water uptake.
Does denser polyethylene foam mean more waterproof foam? Not automatically. Use the water data for the exact grade rather than density as a proxy.
Can AFP30 or AFP45 make an enclosure watertight? Not by themselves. Watertight performance depends on the complete enclosure, including joints, seals, fixings and interfaces.
