What you choose depends on what is available locally, what has the best R-value per thickness, cost, and weight. Aiming for 1 inch of insulation is adequate in a Pacific Northwest Climate, but if the boat spends a lot of time in very, very cold weather, you will probably want to insulate thicker than 2.54 cm (1 in) to get more R-value.

Types of insulation

Closed-cell spray foam. R-value per inch of R-7. A good way to insulate a hull is polyurethane closed-cell spray foam. Closed-cell foams tend to be water-resistant due to the closed nature of the cells that make up the foam. The more small and compact these cells are, the more resistant to water the material will be. This is how people insulate narrowboats. Closed cell foam is designed to expand to about 2.54 cm (1 in) of thickness when sprayed. The closed cell also acts as a vapor barrier, so water and moisture won't come through. While insulating, it also provides extra wall strength and buoyancy. The foam is sprayed directly onto the inside of the hull and adheres well to most surfaces. The foam can be trimmed, and sanded (don't breathe the dust). Any foam that is cut loose its waterproof properties, and ought to be sealed. Generally, the foam is covered up with panelling for a nicer, cleaner finish.

Polyisocyanurate foam board. Polyiso has a R-value per inch of R-6. Sold as rigid sheets, these are bonded to the bare hull using a strong adhesive, or spray foam (helps to close gaps). Insulation of this type must be sealed, and well-secured to the backing surface with no air gaps.

Extruded Polystyrene (XPS) Foam Board. A rigid foam board with a R-value of R-5. They are typically blue, or pink(Foamular). Like polyiso, it doesn't retain water but performs better in cold weather. XPS is more bendable than polyiso if you have really curvy walls. XPS manufacturers use HFCs, making this not the greenest option. Can lose its effectiveness over time due to off-gassing, and can retain moisture if exposed for a long time.

Expanded Polystyrene Foam Insulation Boards(EPS). A rigid foam board with a R-value of R-4. EPS boards are more permeable to air and moisture than XPS, but it doesn't retain moisture to the same extent because of it's breathability which lets it dry out. EPS is, in other words, composed of 2% polystyrene and 98% air. The manufacturing process results in a closed-cell structure, but not a closed-cell insulation board (due to voids that can occur between the beads).

What else is there?

What about Reflectix? Reflectix is a reflective insulation which functions best in specific situations. If you do not provide air space, Reflectix has less insulating value. It is useful to insulate areas that are impractical to insulate using other means, say due to lack of space, like around pipes or restricted areas. The product reflects heat back, rather than mass insulation such as the material examples stated above. On a boat, it might be useful to insulate a mast, or outside windows to reflect heat. **Don't place reflectix inside windows, as it can reflect heat back too well and can warp the windows.

To see how we insulated part of Pino, see installing insulation.