Estimate R-value, thickness, and cost for your marine insulation project.
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Download NowInsulation turns a fiberglass box that sweats and bakes in the sun into a livable cabin, and it is the difference between a boat you can cruise in comfort and one you avoid at the extremes. The right thickness depends on the climate you sail and the insulating value of the material you choose.
Different materials resist heat flow at very different rates per inch. A closed-cell foam packs a lot of R-value into a thin layer, while weaker insulations need thicker build-ups that eat interior space, and only closed-cell types tolerate the damp a boat lives in.
Enter the hull, deck and cabin areas, choose a material and climate zone, and the calculator returns the thickness each surface needs, the average R-value, the total volume to order and a cost estimate for the whole job.
Thickness = target R-value ÷ R-per-inch; Volume (ft³) = Σ(Area × thickness) ÷ 12| Material | R per inch | Marine Suitability |
|---|---|---|
| Closed-cell foam | 6 - 7 | Best; resists water |
| Polyiso (faced) | 6 | Good with vapour barrier |
| Rigid EPS | 3.5 - 4 | Fair; keep dry |
| Open-cell foam | 3.5 | Poor; holds moisture |
Approximate thermal resistance and moisture behaviour by material.
Formula verified against standard naval-architecture references · Updated October 2026
It depends on climate. Tropical boats need modest values focused on the deck, temperate cruisers benefit from moderate R across hull, deck and cabin, and cold or arctic sailing wants substantially higher values and thicker build-ups. This tool sets a per-surface target for each zone and back-solves the thickness from the material's R-per-inch, giving you a defensible starting point for your waters.
Closed-cell foam is the marine default because it combines a high R-value per inch with a refusal to absorb water, so it stays effective and does not hold damp against the hull. Faced polyiso works well with a good vapour barrier. Avoid open-cell foams and fibreglass batts in wet locations, since they soak moisture, lose their value when damp and invite rot and mould in a boat that cannot dry out.
Usually yes, mainly to stop condensation rather than to keep warmth in. A bare fiberglass hull is a cold bridge: warm, moist cabin air turns to water on it and drips. Insulating brings the inner surface closer to room temperature so the damp stops condensing, and it also noticeably improves thermal comfort. The deck matters most, but sides and bulkheads round out a dry, livable interior.
Thickness is the target R-value divided by the material's R-per-inch, so a high-performance foam needs less than a low one for the same result. In a temperate zone with closed-cell foam, hull linings often land around two inches and decks a little more. Space is the constraint, so the calculation balances comfort against how much interior you can afford to lose.
Yes, it is the great enemy of boat insulation. Water held against steel corrodes it and against wood rots it, and a wet insulator loses most of its value and grows mould. That is why the marine rules are water-resistant material, a vapour barrier on the warm side, and a way for any incidental damp to drain or dry rather than sit. Never seal absorbent insulation flat against a cold, wet laminate.
Only with care. Building codes assume a dry, ventilated wall; a boat hull sweats, takes water and rarely dries fully. Fibreglass batts and open-cell foams that work in a house will hold damp and fail in a hull. Closed-cell spray or rigid marine foams, faced boards with proper barriers and good drainage are what make insulation durable at sea, so choose for the wet reality rather than the house norm.