How to Insulate a Narrowboat: Spray Foam vs Rockwool

If you own a narrowboat, widebeam or canal boat, insulation is one of the most important upgrades you can make. The right insulation eliminates condensation, prevents hull corrosion, keeps you warm in winter and cool in summer, and dramatically reduces heating costs. But which insulation is best?

The Three Main Options

Why Rockwool Is Problematic on Boats

Rockwool (mineral wool) has been the traditional narrowboat insulation for decades, but it has serious drawbacks in a marine environment. It absorbs moisture — and in a narrowboat, moisture is everywhere. Once wet, rockwool loses its insulating properties, sags, and traps water against the steel hull, accelerating corrosion from the inside. Many narrowboat owners discover severe hull corrosion hidden behind rockwool insulation that has been silently holding moisture against the steel for years.

Why Spray Foam Is the Best Choice

Closed-cell spray foam solves every problem that rockwool and rigid board create on boats. It bonds directly to the steel hull with no gaps, provides its own vapour barrier so moisture never reaches the steel, conforms to every curve and rib without cutting or fitting, and adds minimal thickness — preserving the living space that is so precious on a narrowboat.

Read our full guide to boat and marine spray foam insulation for specifications, costs and how the installation works. Duratite provides narrowboat insulation at marinas and boatyards across the UK — call 01208 832602.