Outdoor Cushion Foam: 2.0 lb Density Is the Floor

Dry-fast foam drains into the cover, not out of the cushion

You cut open a two-year-old seat pad and water runs out of it. The cover is a coated polyester, waterproof by design, and the block inside is reticulated dry-fast, bought specifically to stop this happening. It happened anyway. Buying outdoor cushion foam on the drainage claim alone is how you end up here: on Sailrite's account, water that passes through the cells has no way out of a cushion whose cover does not breathe.

We compare what manufacturers publish with what owners report after real seasons outside. Links may earn us a commission, which never survives contact with a verdict.

Dry-fast foam only drains a cushion when the cover fabric breathes. Sailrite states the condition plainly and most retail listings omit it entirely. Water passes through the open cells, reaches the fabric, and stops. A coated or vinyl cover with no vent holes traps that water inside the cushion, and Sailrite notes the condition is easy for buyers to miss.

So what should you actually test for?

Not a density figure. A drying time. Fabricakraft puts the benchmark at hours: it says a cushion built with dryfast foam caught in a rainstorm drains and dries within hours of the rain stopping, while one built on indoor-grade material stays wet for days, cold to sit on and cycling wet-to-dry until the cell walls break down.

Time your own cushions after the next storm. If they are still damp on day two, the cause is the foam, the cover, or both. You need to know which before you spend.

Antimicrobial treatment does not resolve this. Sailrite's CushionRite dry-fast is rated 1.8–2.2 lbs./cu. ft. with antimicrobial protection, and The Foam Factory credits the treatment with preventing mould, mildew and bacteria. That is a mitigation for material that gets wet. It is not a vent path.

The short version

• The pick: Foamma 5”, for replacing a single failed seat pad on a patio sofa or chair where you need one piece rather than a set.
• Also worth it: Foamma 6”, for rebuilding a two-seat patio sofa or bench in one go with a deeper, taller seat.
• How this was decided: published specifications and buyer reports across 6 products, compared below.

Flat or crumbling: two different failures, two different fixes

The sources describe two distinct failure modes. A block that compresses permanently at the seat centre is a density and moisture-cycling failure. One that turns brittle and crumbles is, per Fabricakraft, what sustained UV exposure does to polyurethane. They need different responses.

Moisture cycling is the common one. Fabricakraft's account is blunt about the mechanism: an outdoor cushion built with standard indoor foam stays wet for days after rain, which gives you mould conditions and a cold seat, and the repeated wet-dry swing accelerates the breakdown of the material itself.

Add low density and the collapse arrives faster. Fabricakraft puts the threshold for deep-seat base foam at 2.0 lb per cubic foot, and says anything lighter compresses permanently at the seat centre after a season of regular use, producing a sunken look.

UV failure is a different chemistry. Fabricakraft says polyurethane's chemical structure degrades under sustained UV exposure, so the block turns brittle and crumbles rather than compressing and recovering. No source states which outdoor foams are UV-stable or how dryfast performs under sun rather than water, so drainage and antimicrobial claims say nothing about sun resistance either way.

Product	Size	Material	Density	Pack	Right for
Foamma 5”	5" x 24" x 24"	—	—	—	Best single replacement
Foamma 6”	6" x 24" x 24"	—	—	—	Best matched pair
Foamma 503 - Service Unavailable Error	—	Polyurethane dry-fast foam	High	1 piece	Best high-density seat
Foamma 503 - Service Unavailable Error	—	Dry-fast open-cell foam	Medium	1 piece	Best medium-density
QQbed	20 x 18 in.	Quick-dry outdoor memory foam	Memory foam over support base	2 pieces	Best memory foam
Foamma Medium	—	Dry-fast open-cell foam	Medium	1 piece	Best thin pad

Every spec above is quoted from the listing; the “Right for” column is our judgement.

Four decisions in order

Every foam glossary starts with the material and works outward. Reverse it. Start with how the cushion lives, and the material choice narrows.

• Exposure pattern first. Cushions brought indoors or under a cover before rain do not need dry-fast foam at all; Fabrica Kraft states plainly that standard outdoor-grade foam at the correct density is adequate in that case, and Sailrite lists dry-fast as the costliest of the outdoor grades it sells.
• Cover fabric second. A waterproof, non-breathable cover cancels drainage. Sailrite notes that without a breathable cover fabric, water that passes through the foam has no way out of the cushion.
• Foam type third, and only now. Dry-fast for genuinely exposed seating; The Foam Factory goes further and recommends it even for a sofa on a covered deck that never gets directly wet, on humidity grounds alone. Sailrite's Premium and High Resilience upholstery foams are not antimicrobial, absorb moisture, and need waterproof wraps. The expensive option is the one requiring protection.
• Density and firmness last. Fabrica Kraft puts the floor for deep-seat base foam at 2.0 lb/ft³ and recommends an ILD of 30 to 40; Terri Madden of Sand Sea and Air Interiors adds that a user over 250 pounds needs firmer material again.
• Measure before you order anything. Fabrica Kraft's rule: 24 inches or more of seat depth is deep-seat furniture, 18 to 22 inches is standard.

Run the sequence and a covered-porch dining set with a non-breathable cover reaches step three without dry-fast drainage being usable. The 2-inch medium-density Foamma pad is a slim bistro-cover option, though the evidence file contains no specifications for it.

Why the density number on the label tells you less than you think

Density is not a quality score. It is a weight: the pounds recorded when a 12 x 12 x 12-inch block goes on the scale, as Specialty Fabrics Review describes the measurement. Written on product pages, it usually arrives as a two-digit number with an implied decimal, so "#18 foam" means 1.8. That is all it says. It does not tell you how long the block lasts, how firm it feels, or how it behaves wet.

The cross-type comparison is where the shorthand breaks. Sailrite's CushionRite Dry Fast Antimicrobial foam is rated 1.8-2.2 lbs./cu. ft.,. Those numbers look adjacent to the 1.8 lbs./cu. ft. that Sailrite also calls high-density in its cheapest conventional polyurethane outdoor option. They are not the same product; dry-fast is reticulated open-cell material, though no source defines reticulation or distinguishes it from ordinary open-cell foam.

Which brings you to the trade nobody prints on the label. The Foam Factory says dryfast is deliberately made firmer so its structure stays perforated and water can pass through. Sources also disagree on how much choice you have. Specialty Fabrics Review lists soft #30, medium #50 and firm #70 dryfast grades, while The Foam Factory says its own comes in one firmness only, and a softer seat means a thinner piece. No source resolves whether these describe different products or different conventions.

Which of these six suits which patio

A patio sofa with a 24-inch seat rail and a coated cover is the clearest case in this list. Fabricakraft puts the standard deep seat cushion at 24 inches deep by 24 inches wide with 4 to 5 inches of foam, which matches the nominal dimensions of the Foamma 5" x 24" x 24" single pad. The evidence file contains no specifications for that product, so its density, firmness and water behaviour cannot be verified here.

The Foamma 5" x 24" x 24" single pad matches that standard deep-seat dimension; Fabrica Kraft notes that for cushions routinely brought inside or covered, standard outdoor-grade material at the correct density is adequate.

The evidence file contains no listing details for the 6" two-pack, so no claim can be made here about its stated use, drainage or cell structure.

Thickness separates most of these

Fabricakraft's rule, from a single source with no test data or method given, is that the block must be at least 2.5 times the compression it will see. On the same source's account:

• A 2-inch pad on a lounge chair goes to near zero under full weight.
• Four inches at the correct density and ILD compresses to about 2 inches and still cushions.
• Dining and occasional chairs: 2 to 3 inches.
• Lounge and sofa seats: 4 to 5 inches.

By that rule, a 2-inch pad is limited to dining and occasional seating.

For backrests, Fabricakraft gives deep-seat backs as 12 to 14 inches tall and 4 inches thick, and lists 4 inches as the minimum for deep-seat back cushions. No source gives an ILD or firmness figure for dryfast, so no source supports matching a particular dryfast grade to backrests versus seats.

Specialty Fabrics Review notes memory foam, also sold as miracle foam, wonder foam or viscoelastic foam, contains added chemicals that raise its viscosity and density. No source in the evidence file documents the QQbed product, its dimensions, its fit, or how memory foam handles moisture.

Everything above comes from published specifications and third-party guidance, not from cushions anyone here cut open. Note too that no source covers the construction details that decide drainage in practice: drain grommets, seam and welt placement, zipper orientation. A fix of that kind cannot be recommended on this evidence.

Our call

Foamma 5”, best for replacing a single failed seat pad on a patio sofa or chair where you need one piece rather than a set.

A different situation changes the pick:

• For rebuilding a two-seat patio sofa or bench in one go with a deeper, taller seat: Foamma 6”.
• For you want the firmest, longest-lasting seat foam for a patio chair that is used daily and left outside: Foamma 503 - Service Unavailable Error.
• For you need softer foam for backrests or a bench that is only sat on now and then: Foamma 503 - Service Unavailable Error.

One caveat on the Foamma 5”: cover not included, so you must reuse or source your own outdoor fabric