The short answer to what foam to use in a boat is closed cell foam for anything that gets wet. Closed cell PE suits buoyancy blocks, closed cell EVA suits deck pads and seats, and open cell foam belongs only inside a cushion built to drain.
iFoam cuts closed cell PE and EVA foam to size at its Cape Town and Durban branches, for boats on the coast and on inland dams.
Why Closed Cell Foam Comes First on a Boat
Closed cell foam is made of sealed bubbles, so water has no path through it. Open cell foam is a connected network that fills with water like a sponge.
Myth: Any foam floats.
Reality: Open cell foam loses most of its buoyancy as it soaks up water, and then it holds that water against the hull.
Our guide to open cell vs closed cell foam covers the structure in detail. On a boat, the choice splits into three zones: buoyancy, the deck and seating.
Foam for Boat Buoyancy
Buoyancy foam keeps a holed or swamped boat afloat, so deciding what foam to use in a boat starts here. The foam has to stay dry inside after hours underwater. Size blocks for the voids under floors and seats against your own vessel’s flotation requirement, not by eye.
Closed Cell Polyethylene Keeps Its Buoyancy
Closed cell PE is light, does not rot and does not take on water, which is why it suits buoyancy blocks. Our article on what pool noodles are made of shows the same principle at work in a much smaller piece of foam.
| Grade | Density | Shore C hardness | Character |
|---|---|---|---|
| PE33 | 33 kg/m³ | Around 17 | Light closed cell polyethylene |
| PE45 | 45 kg/m³ | Around 26 | Firmer, with good impact resistance |
| T5 | 80 kg/m³ | Around 30 | PE and EVA blend that recovers well after compression |
All three sit within our closed cell foam sheets range. A higher Shore C figure means a firmer foam.
EVA Foam Boat Flooring and Deck Pads
EVA (ethylene vinyl acetate) is the usual material for boat deck pads. It is soft underfoot and does not soak up spray, and our EVA foam sheets page covers the range.
Choose a softer grade for long days on the dam and a firmer one for a casting deck. Our guide to PE vs EVA foam density explains the trade-off.
Key principle: Our EVA45 grade is 45 kg/m³ at around 17 Shore C and recovers well after compression, which helps a deck pad hold its shape under foot traffic.
Deck pads are cut to the outline of the deck and are usually fixed with an adhesive backing. Send us the deck measurements or a drawing and we will quote on the cut pads.
Foam for Boat Cushions and Seats
A boat seat takes rain, spray and sun, and there are two sound ways to build one.
- Keep water out: Our EVA33 grade, at 33 kg/m³, is listed for boat seats and life jackets, and it wipes dry.
- Let water through: A quick-draining open cell foam inside a breathable cover dries out instead of staying soggy.
Standard upholstery foam is the wrong choice on deck. It holds water, and trapped water is how mould starts.
How Salt Water, Sun and Fuel Affect Boat Foam
Salt water worries boat owners most but does the least harm. Once the cell structure is right, exposure is the second test of what foam to use in a boat.
| Exposure | Closed cell PE | Closed cell EVA | Open cell foam |
|---|---|---|---|
| Salt water | Little harm | Little harm | Holds it unless built to drain |
| Direct sun | Ages it over time | Holds up well, still best kept covered | Ages it over time |
| Trapped water | Does not absorb | Does not absorb | Leads to mould |
Wipe fuel and oil spills off foam pads promptly. For hatch seals, EPDM rubber stands up well to UV, ozone and rain, but it swells in contact with oil and fuel. Check the fuel rating of any seal that sits near the engine.
Ordering Marine Foam Cut to Size
We cut foam to size at our Cape Town and Durban branches and deliver anywhere in South Africa. Our guide to foam cut to size explains how the process works. Send us three things:
- Zone: Tell us the foam is for buoyancy, the deck or seating.
- Space: Send measurements or a drawing of the void, deck area or seat base.
- Exposure: Say if the boat runs on the sea or a dam, if the area is covered, and how close it sits to the engine.
Resources
- iFoam, closed cell foam sheets for waterproofing and marine jobs.
- iFoam, open cell vs closed cell foam.
- iFoam, EVA foam sheets.
- iFoam, PE vs EVA foam.
- iFoam, foam cut to size.
Frequently Asked Questions
What is the best foam for boat buoyancy?
Closed cell foam, usually polyethylene. Its sealed cells cannot fill with water, so it holds its lift even if the hull is holed and the foam ends up submerged. Open cell foam soaks up water and should never be used for flotation.
Is marine foam different from regular foam?
Yes. Marine foam is chosen to cope with water, sun and salt. For flotation and decking that means closed cell foam, while marine seat cushions often pair a fast-draining open cell foam with a breathable cover so the seat never stays wet.
Can EVA foam be used for boat flooring?
Yes. EVA is closed cell and soft underfoot, which makes it a common deck pad material. Pads are cut to the shape of the deck, and the grade and thickness are picked for comfort and wear.
Does salt water damage foam?
Very little in closed cell PE or EVA. Sunlight and trapped water cause far more damage over a boat’s life. A fresh water rinse after each trip and covered storage both help the foam last longer.
Final Thoughts
Knowing what foam to use in a boat comes down to two tests: the cell structure and the exposure. Closed cell PE suits buoyancy, closed cell EVA suits decks and seats, and open cell foam belongs only in a cushion built to drain. Send us the space rather than a product code, and request a quote or technical advice for your boat.