Insulation on a steel boat has 3 purposes:

  1. Reduce or eradicate condensation on the inside of the hull and deck
  2. Provide thermal insulation. That is, keep the inside of the boat warm in winter and cool in summer
  3. Provide sound insulation

On a steel boat we don’t want any condensation forming on the inside of the steel hull. Condensation forms when the steel is at a lower temperature than the air that’s against it. This temperature difference that occurs at the steel face causes the water in the air to change from vapour to to liquid, creating condensation. The amount of condensation can be quite excessive, and can eventually cause corrosion in the steel and rot in any adjacent woodwork.

    In choosing an insulation, we considered a number of aspects:

  • Cost of the insulating material, including cost of installation.
  • Installation effort. That is, how difficult is it to install, and how long will it take.
  • The thermal efficiency of the insulation.
  • The thickness of the insulation to achieve the required purposes. Not all insulation materials are equal! Different types of insulation need to be applied to varying depths to achieve the same insulation properties. We don’t want to sacrifice internal space because of the insulation we choose.
  • The permeability of the insulation. That is, how waterproof is the material. Porous materials must be vapour sealed to prevent moisture passing through the material and condensing on the hull.

What Insulation Materials Did We Consider?

There are many different materials available, however not all are suitable for insulating a boat. The following materials are generally used for steel boat insulation.

Spray Foam

Often called expanding foam, this is a spray-on polyurethane foam insulation with a closed cell structure, making it impermeable to moisture. It has the best insulating properties of all the common insulating materials. In the past this needed to be applied by a specialist contractor with expensive equipment, but technological innovations have produced DIY kits which are quite cost effective and easy to apply. This product does not need to be vapour sealed as it bonds directly to the steel and does not leave an air gap. It is capable of completely eradicating condensation in most situations. This is the most common method of insulation used in steel boats, and is almost universally used by boat builders.

Polystyrene or Polyurethane Boards

The boards need to be meticulously cut to size to ensure a good fit between the framing and vapour sealed to close of any gaps in the insulation. Installation requires a lot of cutting and usually results in a high amount of wastage. These boards have excellent insulating abilities.

Rock Wool (extruded silicone fibres)

Typically this is the cheapest option, and is available in rolls or batts (a compressed board like version). It has about half the insulating ability of spray foam, thus it needs to be approximately twice as thick to achieve the same insulation, thus sacrificing internal space. It also needs to be vapour sealed and cut it to size. It absorbes moisture and will eventually break down because of it.


This is a relatively new product made by the 3M company. It is the same material that is used in winter clothing (scarves, gloves, jackets, beanies, etc). It needs to be cut to size, glued into place and vapour sealed at the edges/seams to close any gaps. Thinsulate is an expensive option.

The following table compares the above insulation materials:

Material Cost Insulating Efficiency Ease of Installation
Spray Foam medium star star star star star star star star star star
Foam Boards medium star star star star star star
Thinsulate expensive star star star star
Rock Wool cheap star star star

We’ve chosen to insulate Yamoya using spray foam.