Closed Cell Foam vs Open Cell Foam: Which One Does Your Job Need?

Two foam sheets can look nearly identical on a shelf and then behave like completely different materials once water, weight or weather gets involved. The difference sits in the cell walls. Every bubble inside closed cell foam is sealed, so gas stays trapped and liquid has nowhere to travel. In open cell foam those bubbles are joined together, so air and water pass straight through the material.

That one structural detail decides whether a sheet insulates a pipe or soaks up a puddle. This guide covers what each type actually does, which one belongs in packaging, insulation, seating, outdoor builds and flotation, and how to check which kind you already have in the garage. It also clears up the two mix-ups that cause most of the wrong orders: assuming high density means closed cell, and assuming closed cell means fireproof.

Closed cell foam vs open cell foam: the short answer

Choose closed cell foam whenever moisture, weather, chemicals or repeated impact are part of the job, and choose open cell foam whenever comfort, breathability or sound absorption matter more. Closed cell sheets are firmer, they shed water instead of drinking it, and they hold their shape under load. Open cell sheets compress easily, breathe well and are far more pleasant to sit or lie on. Neither one is better in general terms, and cost is rarely the deciding factor.

The simplest way to hold the distinction in your head: sealed cells make a barrier, joined cells make a sponge. A barrier keeps things out and holds a shape. A sponge lets air and liquid through, and it collapses softly under weight. Every practical difference in the table below follows from those two behaviours.

What is closed cell foam, and what is it made from?

Closed cell foam is any foam where the gas bubbles formed during manufacture stay individually sealed rather than joining up. Blowing agent or nitrogen expands inside the molten polymer, the cell walls set around each bubble, and the finished sheet contains millions of separate pockets of gas. Because those pockets do not connect, the material resists water, resists vapour and carries load better than an open cell foam of similar weight. It also feels noticeably stiffer, which surprises people who expect all foam to be soft.

The term describes a structure rather than a single material, and that matters when you are comparing suppliers. Polyethylene is the workhorse for sheet products, either cross-linked for a finer and stiffer cell structure or non-cross-linked for a more economical sheet. Plastazote is Zotefoams' brand name for nitrogen-expanded cross-linked polyethylene, and it gets used loosely to mean closed cell foam in general, in the same way people say Hoover for any vacuum. Names like Evazote, Supazote, Stratocell and Plasticell sit in the same family, so the useful question is always which polymer sits behind the trade name. Spray-applied closed cell polyurethane insulation is a genuinely different product, installed by contractors into a cavity or roof rather than bought as a sheet, and it is worth knowing that if that is what your search actually meant.

What is open cell foam, and where does it belong?

Open cell foam has cell walls that broke or never fully formed during expansion, leaving one connected network of air passages through the block. Push a piece and you can feel the air leaving; release it and air floods back in. Almost all flexible polyurethane foam works this way, including standard upholstery grades, high resilience foam, memory foam and reticulated filter foam. Because air moves freely, the material breathes, cushions the human body properly and returns to shape after being squashed thousands of times.

That airflow is why open cell foam owns anything you sit, sleep or kneel on for long stretches, and why foam cut to size for cushions and upholstery is almost always an open cell polyurethane rather than a closed cell sheet. The same structure is a liability around water. A soaked open cell cushion holds a surprising amount of liquid deep in the matrix, gains real weight, dries slowly and gives mould something to live on if it never fully dries out. One more point specific to the UK: foam used in domestic upholstery has to meet the Furniture and Furnishings (Fire) (Safety) Regulations, so check the fire specification with your supplier before recovering a sofa rather than assuming any soft foam will do.

Is closed cell foam waterproof? Water, damp and outdoor use

If the foam will meet water, choose closed cell and stop weighing it up. Sealed cells mean liquid can only sit on the cut surface, so there is no wicking path into the sheet, no reservoir held in the middle and no slow gain in weight over a wet winter. Open cell foam does the opposite by design. Even the fast-drying grades stay damp for days once they are inside a cover, which is how outdoor bench cushions end up smelling before their second summer.

For sheet applications in wet or dirty environments, the ImpactSeal closed cell foam sheet is the type of product that gets specified. It is a rigid cross-linked polyethylene, waterproof and non-absorbent, resistant to chemicals, grease, mildew and bacteria, rated from -60°C to 90°C, and available with self-adhesive backing for gaskets and linings. Two honest limits go with it. It is not fire retardant, so it is the wrong choice anywhere a fire specification applies, including public seating and most building interiors. And closed cell foam does not breathe, so if you fix a sheet against timber or metal outdoors, give any trapped water a route out rather than sealing it in.

Packaging and protective inserts: which foam protects better?

For fragile, heavy or high-value items, closed cell polyethylene is the standard answer, and the reason is compression set rather than softness. A closed cell sheet springs back after each knock and keeps performing across repeated handling and multiple trips, while a soft open cell insert gradually flattens and stops absorbing anything. Closed cell foam also stays clean, sheds no dust into the packaging and can be die cut or routed into snug pockets that hold a component in one position. That said, open cell foam is genuinely the better buy for light void fill and one-way shipments where nothing is at risk, so do not pay for closed cell protection a lightweight parcel does not need.

Where the contents do need real protection, the Stratocell polyethylene foam sheet covers most cases. It is a closed cell polyethylene with strong impact absorption and resistance to moisture, dust and oils, and its non-abrasive surface will not scuff polished, coated or painted parts, which matters for anything cosmetic. Sheets come in 12mm, 25mm, 38mm and 50mm thicknesses, so you can layer a thin sheet as a surface wrap or use a thick one as a shock layer under a heavy item. As a rule of thumb, heavier and more fragile contents want more thickness and more foam between the item and the outer wall, and anything expensive enough to worry about deserves a drop test on a sample box before you commit to a full run.

Insulation, soundproofing and buoyancy: what each foam can and cannot do

Closed cell foam wins on insulation and flotation for the same physical reason. Trapped gas cannot circulate, which slows heat transfer through the sheet, and sealed cells refuse to fill with water, which is what keeps a buoyancy aid buoyant. Those two properties are why closed cell polyethylene turns up as pipe lagging, panel insulation, flotation inserts, van lining and kneeling pads used on cold wet ground. Open cell foam cannot do any of that job well, because the moment air or water moves through the material both the insulating value and the buoyancy disappear.

Sound is where the two get confused most often, and the answer runs the other way. Open cell foam absorbs airborne sound because the sound waves travel into the connected pores and lose energy as they go, which is why acoustic treatment panels and studio wedges are open cell. Closed cell foam reflects most of that energy instead, so it does nothing useful for echo inside a room. It earns its place in soundproofing build-ups as a dense decoupling layer that stops vibration travelling through a structure, which is a different task from absorption. If your goal is a less echoey room, closed cell sheet is the wrong material; if your goal is stopping noise passing through a partition, it belongs in the mix.

Two things buyers get wrong about closed cell foam

Most wrong orders trace back to two assumptions rather than to any lack of care. One is treating density and cell structure as the same measurement, which sends people looking for a high density closed cell foam when what they actually need is a firm open cell grade. The other is buying a replacement without checking what they already have, only to find the old sheet was a different material entirely. Both take a minute to sort out.

Density and cell structure are two different things

Density measures mass per unit volume, usually in kilograms per cubic metre, and it tells you nothing about whether the cells are sealed. Plenty of high density foams are open cell, including the firm upholstery grades used in seat bases, and plenty of closed cell polyethylene sheets are light in the hand while feeling far stiffer than a dense open cell block. Firmness is a third, separate property again. If your requirement is really support and longevity in a cushion, our guide to high density and average density foam is the more useful comparison to read.

How to tell if a foam sample is open cell or closed cell

Take an offcut, hold it under water and squeeze. Open cell foam releases a stream of bubbles, takes on water and stays heavy when you lift it out; closed cell foam gives up almost nothing and comes out close to dry. A second check works without a sink: press the sample to your lips and blow. Air passes through open cell foam easily and refuses to move through closed cell. Looking at a cut face helps too, since open cell foam shows visible open pores like a bath sponge, while closed cell foam presents a fine, matte, almost continuous surface.

Closed cell or open cell: which should you order?

Work backwards from the environment rather than from the product name. Anything facing water, weather, oils, chemicals, sharp temperature swings or repeated impact points to closed cell polyethylene, and that covers packaging inserts, gaskets, insulation, van and boat work, flotation and protective linings. Anything a person sits on, sleeps on or leans against points to open cell polyurethane, chosen by firmness and density rather than by cell structure. When both could technically work, ask which failure would annoy you more: a cushion that stays wet, or a sheet that feels hard.

If your job is protective packaging, our packaging foam range is the place to start, and full sheets can be cut down to the sizes your boxes actually need. Quick Foam has been cutting foam in Tyseley, Birmingham since 1971, so if you are unsure which material suits a specific build, describe the environment and the load and ask before ordering. Closed cell sheets generally sit at a higher price per square metre than standard open cell foam of the same size, which is worth knowing, though it is the wrong reason to pick a material that will not survive the conditions.

Frequently Asked Questions

Can I use open cell foam outdoors if it has a waterproof cover?

Open cell foam under a waterproof cover usually still gets wet, because rain finds seams and zips, and condensation forms inside the cover on its own. Once water is in, the cover slows drying rather than preventing damp. It works reasonably for a cushion stored indoors and used outside on dry days, but for anything left out permanently, closed cell foam or a quick-dry reticulated grade with drainage holes is the safer build.

Does closed cell foam work for a seat cushion or mattress topper?

Closed cell foam is a poor choice for seating and sleeping. It is too firm, it does not contour to the body, and it traps heat and moisture against the skin because no air moves through it. For seat cushions and sleeping surfaces you want an open cell polyurethane chosen by firmness and density, which is what the mattress toppers and comfort foam options are built from. Save closed cell sheet for the protective layer underneath.

How do I cut closed cell foam sheets neatly?

Use a long, very sharp blade and a straight edge, and take several passes rather than forcing one deep cut. A fresh snap-off knife blade or a long bread knife both work on polyethylene, and swapping to a fresh blade the moment it starts dragging makes more difference than technique. For thick sheets or curved shapes, a hot wire cutter gives the cleanest edge. Cut slightly oversize first, then trim.

Which adhesive sticks to closed cell polyethylene foam?

Polyethylene has a low surface energy, so many general adhesives grip poorly and peel away later. Look for a contact adhesive or spray adhesive that explicitly lists polyethylene or PE foam among its substrates, and always test on an offcut before committing to the real piece. For linings and gaskets, foam supplied with a self-adhesive backing avoids the problem entirely and gives a more even bond.

Is closed cell foam worth paying more for than open cell foam?

Closed cell foam is worth the extra only when the conditions demand it, meaning water, chemicals, temperature swings, load bearing or repeated impact. In those settings it lasts far longer and the cheaper option fails within a season, so it pays back quickly. For dry indoor comfort applications, paying more for closed cell buys you a worse result, so open cell is both cheaper and correct.

Will sunlight damage closed cell foam left outside?

Prolonged direct sunlight does degrade most polyethylene foams over time, typically leaving the exposed surface chalky, discoloured and brittle while the core stays sound. Shading the foam, painting it with a compatible coating or covering it with a UV-stable material all extend its life considerably. If a sheet will sit in full sun for years, ask your supplier whether a UV-stabilised grade is available before ordering.