Oct 14, 2026Bike Accessory Buying Guides

Closed-Cell vs Open-Cell Foam for Bike Cushions

Closed-cell vs open-cell foam for bike cushions: water absorption under 1%, compression-set data and the spec numbers that decide comfort and lifespan.

Quick answer
The difference between closed-cell and open-cell foam is whether the bubbles are sealed or connected. Sealed cells (EVA) keep water out — under 1% absorption in datasheet testing — and hold their shape far better. Connected cells (PU, HR foam) are softer and cheaper, but they soak and stay wet. For rain-exposed passenger cushions, closed-cell wins on water and hygiene. For plush indoor comfort, open-cell still has the edge. Send us your cushion's exposure and load and we will spec the cell type with numbers.
Key takeaways
  • Closed-cell foam has sealed, unconnected cells; open-cell foam is over 50% connected — the difference between a float and a sponge.
  • Closed-cell EVA (Zotefoams EV50) absorbs under 1% water (ISO 2896); 22 hours at 25% compression leaves only 3% set after 24-hour recovery.
  • Typical bike-cushion HR foam runs 40-60 kg/m3 with 59% rebound and 8.1% compression set — comfortable, but it absorbs and retains water.
  • Heat matters too: microcellular PU (PORON 4701-30) is rated at most 10% compression set at 70°C (ASTM D3574) — the temperature class of a sun-heated parked cargo box.
  • Whatever the cell type, load-bearing cushions need a support factor (65%/25% IFD) of at least 2.0.
How we verified
This is a data review of published sources: Zotefoams EVA datasheet (ISO 2896 water absorption), Rogers PORON 4701-30 (ASTM D3574), Foam Factory's density/ILD catalogue, PFA guidance on support factor, and bike-saddle foam figures from patents US9670307 and EP1818347. Cell-structure definitions come from general foam-industry references. We did not run our own lab tests — every number below is traceable to a published source.
Two cell types, one test
Foam is a solid full of gas bubbles. In closed-cell foam the bubbles are sealed shut; in open-cell foam over half the bubbles are connected to their neighbours, so air and water move through the material like a sponge.
For bike cushions this is not trivia. A passenger cushion on a cargo bike lives outdoors: rain, puddles, hose-downs and damp garages. That environment sorts the two materials quickly.
What the data says about water
The closed-cell case is measurable. Zotefoams' EV50 EVA datasheet reports water absorption below 1% (ISO 2896). Open-cell PU foam does the opposite — it absorbs and retains water, dries slowly, and in damp storage can support mould and mildew, which is exactly the failure mode riders complain about on cheap cushions.
One nuance before you dismiss open-cell outright: some manufacturers produce engineered dry-fast open-cell foams with large cells designed to drain rather than retain. These can work outdoors when the cushion design includes drainage and airflow. The rule is not closed-cell only — it is absorb versus drain. A standard open-cell cushion with no drainage path will stay wet; a dry-fast grade with drainage can perform.
What the data says about shape
How a foam holds its shape under load and heat is captured by compression set — how much thickness it fails to recover after being squashed. The spread across common materials:
Closed-cell EVA (EV50): 25% compression held for 22 hours leaves just 3% set after 24 hours of recovery.
  • HR foam, the 40-60 kg/m3 class typical in bike cushions: 8.1% compression set, with 59% rebound.
  • Microcellular PU (PORON 4701-30): at most 10% compression set at 70°C per ASTM D3574 — the high-temperature class that matters for a cushion sitting in a sun-heated cargo box.
Figure 1: Compression-set data across foam types (pending upload).
Figure 2: Closed-cell versus open-cell comparison (pending upload).
The comfort trade, honestly
Closed-cell EVA is firmer and less conforming than soft open-cell foam — that is physics, not marketing. If your product brief says plush, closed-cell demands a lower-density, softer EVA grade (and a check that it still passes the compression-set and support-factor numbers), or a dry-fast open-cell with drainage. If the brief says weatherproof and hygienic, closed-cell is the default.
Two more numbers keep the spec honest. First, density and hardness are independent: Foam Factory's catalogue alone lists 29 kg/m3 at ILD 35, 45 kg/m3 at ILD 50, and 29 kg/m3 at ILD 100 — same density, three times the firmness. Second, for any load-bearing cushion, PFA guidance requires a support factor (65% / 25% IFD) of at least 2.0; below that, the cushion bottoms out. Bike-saddle patent data puts typical 25% IFD at 50-80 lbf — a useful anchor for passenger cushions as well.
What it means for importers
Choose the cell type by exposure, not by habit. Rain-exposed or washable products: closed-cell (or a documented dry-fast open-cell with drainage). Indoor plush goods: open-cell can win.
  1. Write five numbers into the spec: material grade, density (kg/m3), ILD or IFD at 25%, support factor (65%/25%), and compression set at a defined temperature.
  1. Ask for water absorption with the test standard — waterproof foam without ISO 2896, or an equivalent, is an adjective, not a spec.
  1. For sun-exposed cargo products, require the compression set at the 70°C class (ASTM D3574); ambient-temperature data understates the ageing.
  1. Sample-test the worst case: soak the sample, load it, dry it, cycle it. Ten minutes at the tap beats a datasheet argument.
  1. See how we spec cushions and QC for our own range at /seat-cushions, the foam-numbers guide in /posts/foam-density-ild-comfort, or send your cushion brief to /contact-us.
Limitations
All figures come from published datasheets and standards, not our own lab testing.
  • Open-cell absorption is stated qualitatively — suppliers rarely publish figures; verify per material.
  • Dry-fast open-cell foams are a real option but datasheets vary widely; test the drain path, not just the foam.
  • Support-factor and IFD ranges come from PFA guidance and patent data; confirm against your own load case.
FAQ
What is the difference between closed-cell and open-cell foam?
Cell structure. Closed-cell foam has sealed bubbles that water and air cannot enter; open-cell foam has more than half its cells connected, so it behaves like a sponge — soft and breathable, but it absorbs and retains water.
Which foam is better for bike cushions that get rained on?
Closed-cell. EVA in the closed-cell family absorbs under 1% water in ISO 2896 testing and dries instantly, while standard open-cell foam soaks and can mildew in damp storage. For weather-exposed cushions, that settles the choice unless you have a documented drainage design.
Is closed-cell EVA comfortable enough for a passenger cushion?
It can be. EVA is firmer than soft PU, so specify a lower-density, softer grade and back it with numbers: support factor at least 2.0 and compression set appropriate to the temperature class. If plush is the priority and the product stores dry, open-cell remains the comfort leader.
How do I stop a bike cushion from soaking up water?
Either keep water out (closed-cell foam, sealed cover, taped seams) or get it out fast (dry-fast open-cell foam plus drainage paths, airflow and quick-drying cover material). A standard open-cell core in a leaky cover is the worst of both.
What numbers should I ask a foam supplier for?
Five: material grade, density in kg/m3, 25% ILD or IFD, support factor (65%/25%), and compression set at a stated temperature and test standard (for example ASTM D3574 at 70°C). Add ISO 2896 water absorption if the cushion sees rain.
Buying or specifying passenger cushions for a bike range? Send your load, exposure and comfort brief to our team at /contact-us — we spec the cell type, grade and support-factor numbers, then sample in 7-12 days.
References
Zotefoams EV50 EVA datasheet — water absorption (ISO 2896) and compression recovery — https://www.zotefoams.com/
  1. Rogers PORON 4701-30 — compression set at 70°C (ASTM D3574) — https://www.rogerscorp.com/
  1. Foam Factory — density and ILD catalogue (HD36 / Lux HQ / Super Lux) — https://www.thefoamfactory.com/
  1. Polyurethane Foam Association (PFA) — support factor guidance (65%/25% IFD) — https://www.pfa.org/
  1. US9670307 — bicycle saddle foam figures (HR foam, IFD ranges) — https://patents.google.com/patent/US9670307
  1. EP1818347 — bicycle saddle foam data — https://patents.google.com/patent/EP1818347
  1. General foam reference — closed-cell and open-cell structure definitions — https://en.wikipedia.org/wiki/Foam
  1. ISO 2896 — water absorption test method reference — https://www.iso.org/standard/60012.html