After the patient labour of the stitched edge, this chapter is almost insulting in its speed.
Foam construction is what displaced traditional stuffing in factory work between 1955 and 1975, and it is what the great majority of modern domestic furniture uses today. A whole sofa-cushion set can be cut, bonded and wrapped in an afternoon. The trade-off, as Chapter 2’s cutaway diagram showed honestly, is lifetime: foam compresses where you sit and never recovers, and a foam-built seat is comfortable for ten years rather than a hundred.
We use foam for two kinds of work. First, on modern pieces being re-covered in their original modern style — everything since the late 1960s. Second, on loose seat cushions and back cushions where the customer wants the comfort of foam over a traditional sprung seat. Both are entirely valid uses; foam does what foam does well, as long as you set the customer’s expectations of its lifespan honestly.
The foam-density chart in the materials reference is the reference for everything in this chapter. If you haven’t internalised the difference between an 18 kg/m³ and a 36 kg/m³ foam, this chapter will read wrongly. Refer back if you need to.
What this chapter doesn’t cover is using foam in place of traditional stuffing on a heritage piece. Don’t. The shape will be wrong, the customer is being short-changed, and the chair was built for a different technology.
The two-foam principle
Almost every well-made foam cushion uses two grades of foam bonded together: a denser, firmer base layer that carries the structural load, and a lighter, softer top layer that provides the immediate comfort the sitter feels. The principle is exactly the same as the first-stuffing-and-second-stuffing distinction in traditional work; only the materials are different.
The base layer is high-density (HD) foam, typically 36 kg/m³ for a domestic seat cushion, 40–45 kg/m³ for a high-use one. The base takes most of the vertical load and is what determines the cushion’s lifespan — skimp on it and the cushion sags within five years. The top layer is lower-density (24–28 kg/m³) foam, providing the ‘give’ the sitter notices in the first 5 mm of compression. It carries no real structural load; it just feels soft. Without it, the cushion feels like a brick.
The two layers are spray-bonded together with a polyurethane contact adhesive (we use 3M Hi-Strength 90 or equivalent). Spray both surfaces, wait two minutes for the adhesive to flash off, press together. The bond is permanent. You can buy high-density foam (paid link) and softer top foam (paid link) cut to thickness from most suppliers.
A 50 mm HD base + 25 mm soft top is the standard ratio for a 75 mm seat cushion. Doubling the soft layer makes the cushion feel mushy and accelerates lifetime sag; halving it makes the cushion feel hard. Stick to roughly 2:1 base : top.
Cutting foam to size
Foam is sold in sheets, typically 2000 × 1000 mm in a range of thicknesses. The standard sheet thickness for seat-cushion HD foam is 50 mm; for back-cushion soft foam, 80 mm. You buy the sheets, lay out the cutting plan, and cut to size at the bench.
The cutting tool of choice is a serrated bread knife — a long, fine-toothed blade that cuts foam with a sawing action. A £6 kitchen-shop bread knife (paid link) outperforms every dedicated foam-cutting tool we’ve ever bought (see the ‘tools we wasted money on’ section of the toolkit). The trick is to keep the blade moving in long sawing strokes, not pressing down; press and the foam tears.
Mark before cutting. Use a thin felt-tip on the side face of the foam to mark cut lines. Cut just outside the line, not on it; foam compresses slightly under the knife and a cut on the line ends up undersized. Cut vertically, with the knife perpendicular to the foam’s top surface, working from one side to the other in a single pass.
Foam-cutting produces fine particulates. The FFP3 mask (paid link) is non-negotiable. We also keep the workshop vacuum running near the cutting bench; foam dust drifts and lands everywhere if not extracted promptly.
Bonding and assembly
After cutting, the two foam layers are bonded into a single cushion. Lay the HD base on the bench; spray a thin even coat of foam contact adhesive (paid link) across its top surface; lay the softer top layer on the bench beside it, also sprayed. Wait two minutes — the adhesive should feel tacky to a fingertip but not wet. Press the two surfaces together, working from one edge to the other to avoid trapping air bubbles. Press firmly with the heels of your hands; you cannot use a roller because the foam compresses unevenly.
Once bonded, leave the cushion under a light weight (a couple of magazines is plenty) for 30 minutes for the adhesive to fully cure. Then it can be handled normally.
Spray adhesives release VOCs. Open every window in the workshop, run the extractor fan, mask up. Don’t spray near a naked flame; the propellant is flammable.
The Dacron wrap
The bonded foam cushion is wrapped in Dacron polyester wadding (paid link) before the show fabric is fitted. The Dacron softens the foam’s hard edges, gives the cushion the puffy fullness customers expect, and stops the show fabric puckering against the foam’s cell structure (which happens visibly on cheap foam cushions where the Dacron step was skipped).
Cut a piece of Dacron about 50 mm larger than the cushion in every direction. Lay the cushion in the centre; fold the Dacron up over each side; secure with spray adhesive at the corners. The cover then goes over the wrapped cushion as a third layer of softness. Most domestic cushions get 25–40 mm of Dacron; premium cushions get more.
Dacron is sold by weight per square metre — typical grades are 100 g/m² (light), 200 g/m² (standard) and 400 g/m² (heavy). For foam cushions use 200 g/m². The light grade is for back cushions and chair backs; the heavy grade is for premium luxury work.
Tools & materials for this job
- High-density foam (36 kg/m³) for the base (paid link)
- Soft foam (24–28 kg/m³) for the top (paid link)
- Foam-rated spray contact adhesive (3M Hi-Strength 90) (paid link)
- Long serrated bread knife (paid link)
- Dacron polyester wadding (200 g/m²) (paid link)
- FFP3 dust mask (paid link)
A regular customer brought us a 1990s Heal’s three-seater sofa whose seat cushions had gone flat. The original cushions were 24 kg/m³ throughout — cheap factory foam that had compressed to about 60% of its original height. The customer wanted them to feel like new.
We made replacement cushions to the original dimensions: 50 mm HD core in 36 kg/m³, 25 mm soft top in 28 kg/m³, 200 g/m² Dacron wrap. Total foam cost about £90 per cushion; total labour about an hour each. The cushions feel firmer than the originals did even when new, because the foam grades are properly matched. The customer reckoned the sofa was “better than the day I bought it”. We didn’t mention that the original cushions were the cheapest the factory could fit.
The polyurethane foam used in modern upholstery was developed by Bayer in Leverkusen, Germany, in the late 1930s, but didn’t become commercially significant until the 1950s when manufacturing processes scaled up. Within a decade it had displaced horsehair from factory upholstery almost entirely, on the basis of price alone — foam was perhaps a fifth of the cost of horsehair, and the speed of foam construction was many times faster.
The lifetime trade-off — ten years of comfort versus a hundred — only became visible to consumers in the mid-1960s, when the first generation of foam furniture started to fail. By then the trade had reorganised around foam, the horsehair supply chain had largely collapsed, and the new norm was set.
- Single-grade foam. Cushions made of one grade throughout are either too hard (HD-only) or compress in months (soft-only). Two grades, bonded.
- Cutting too tight. Foam compresses under the knife. Cut just outside the marked line and the cushion settles to the right size; cut on the line and it will be undersized.
- Skipping the Dacron wrap. The cover puckers against the foam cell structure, the cushion looks cheap, the edges feel hard. Add the Dacron; it costs almost nothing.
- Wrong adhesive. Solvent-based adhesives can dissolve polyurethane at the bond line, producing a soft cavity that compresses faster than the surrounding foam. Use only foam-rated adhesives (3M Hi-Strength 90, Copydex Foam, or trade equivalents).
- Using foam on a heritage piece. See the chapter opening. Don’t.
Foam cut, bonded, wrapped. Whether traditional or modern, the seat now has its structural and shaping layers in place and is ready for the cover. The next chapter is the cover — calico under-cover, wadding over it, top fabric on top, and the corner-pleating technique that distinguishes good cover-work from indifferent.
New cushions made to your sofa’s original dimensions, foam grades properly matched, Dacron-wrapped and covered. Send a photo and the measurements for an estimate. Get a quote on your piece →