10, 15 or 20 mm: which gap your berths need
All three variants hold the same monofilament structure. They differ only in how much air they keep between the mattress and whatever is underneath it. Which one is right follows from three things: how deep the berth base is, whether it touches the hull, and how long the boat stays shut.
Start with the berth, not with the mattress
The common mistake is to pick the thickness by mattress weight. Weight matters, but it is the second question. The first is what sits under the berth board: insulated joinery, a ventilated void, or bare hull laminate.
Where the base is built directly onto hull-contact structure, the cold side is close and the gap has to stay open under load. That is the case for 15 mm and above. Where the base sits over a ventilated void with air already moving, 10 mm is enough to break the flat contact.
10 mm: where space is the constraint
Crew cabins, guest berths and settee seating. The berth bases are shallow, the mattresses lighter, and every millimetre of headroom is spoken for. 10 mm breaks the flat contact without lifting the cushion out of its cut-out.
It is also the right choice for upholstery against cold coachroof panels, where the problem is contact rather than a deep moisture reservoir.
15 mm: the default for hull-contact berths
Owner's cabins, VIP staterooms and every berth that follows the hull. It gives enough gap against the cold side while still sitting flush under the cushion cut-out, which makes it the natural single specification for a refit or a fleet.
On a monohull the V-berth is the standard case: the cushion follows the laminate directly, two people raise humidity in a closed forepeak overnight, and the moisture collects underneath. 15 mm is what keeps that channel open.
20 mm: heavy systems and permanent occupancy
Master suites, uninsulated hull zones and berths in year-round use. Two situations call for it: a mattress heavy enough to close a thinner gap under sleeping load, and accommodation that never gets an unoccupied week to dry out.
Houseboats on permanent moorings and bunks in unheated compartments both fall into the second category, for opposite reasons: one is warm and humid all year, the other is cold and uninsulated.
When two thicknesses would both work
Take the thicker one if the boat sits closed for long stretches, and the thinner one if headroom is tight or the cushion has a fixed cut-out. Airflow is not a resource you can run out of, but a cushion that no longer fits its recess is a problem you notice every night.
Deep base or hull contact means 15 mm. Shallow base or limited headroom means 10 mm. Heavy mattress or year-round occupancy means 20 mm. If you are between two, order a sample of both. They are free for trade accounts.
Frequently asked
- Should I choose 10, 15 or 20 mm spacer fabric?
- 15 mm is the default for berths that sit against the hull, such as V-berths, hull double berths and owner's cabins. 10 mm suits shallow bases and tight headroom, such as crew cabins, settees and saloon seating. 20 mm is for heavy mattress systems and berths in year-round use.
- Does the mattress weight decide the thickness?
- Not first. The first question is what sits under the berth board: insulated joinery, a ventilated void, or bare hull laminate. Weight matters second, because a heavy mattress can compress a thinner layer under sleeping load.
- What if two thicknesses would both work?
- Take the thicker one if the boat sits closed for long stretches, and the thinner one if headroom is tight or the cushion has a fixed cut-out. A cushion that no longer fits its recess is a problem you notice every night.