Whether you cruise down toward Baja or up in the PNW, installing insulation in your boat can make life much more bearable. It can reduce condensation in the cabin or turn a marginal heating or cooling system into a winner. In this article, we’ll look at various types of insulation and some methods of installing them.
The two main things in considering an insulation type are the “R” value and the suitability for the environment aboard a boat. The “R” value is simply a material’s resistance to heat flow. The higher the “R” value, the better an insulator. Some representative “R” values for common materials are:
Material “R” Value
- Wood – 0.91
- Reflectrix - 2.38
- Cork – 3.57
- Polystyrene Bead Board – 3.57
- Fiberglass (the insulation, not the hull) – 3.90
- Polystyrene – extruded – 5.00
- Polyisocyanurate – 5.56 Sprayed
- Polyurethane Foam – 6.88
- Vacuum Panels - 24.06
“R” values based on a one-inch thickness
Choosing an insulation material isn’t as simple as looking at an “R” value table and picking the best insulator. The method of installation, room available and durability of the material in the marine environment must also be considered. Let’s look at these materials in detail.
Wood: Low “R” value indicates why even wooden boats can benefit from additional insulation.
Reflectrix: Reflectrix is a combination of plastic bubble wrap and reflective aluminum foil. It works well in spaces that don’t have room for thicker insulation. It is easily cut with scissors and installation is simple.
Cork: Heavy, expensive and hard to find in thicker pieces.
Polystyrene Bead Board: This is the common white insulation board. The problem is that the beads flake off and the spaces remaining between the beads trap water, lowering the “R: value. Avoid it at all costs.
Fiberglass (the insulation, not the hull material): This is the standard fibrous home insulation material. It will pack down, absorb water and shed fibers aboard a boat. Avoid it.
Polystyrene – extruded: This is the common blue or pink insulation sheets found in home improvement stores. Being extruded, it is closed cell and won’t wick water. It is also inexpensive and easy to work with. This is my choice for general insulation use.
Polyisocyanurate: This is a foam panel covered with aluminum foil. While marginally better than extruded polystyrene, it is also more fragile and crumbles.
Sprayed Polyurethane Foam: It can be in a spray can or professionally applied by spraying on the surface being insulated. It is messy, requiring a lot of effort to clean up. An experienced applicator is required to get a uniform thickness and proper cure. It is often used to insulate steel boats, but can become waterlogged over time, reducing the "R" value.
Vacuum Panels: These panels are basically big thermos bottles. The vacuum provides the highest “R” value available. However, they are expensive and must be used in the standard sizes; they cannot be cut or trimmed to size. Any puncture will instantly destroy the insulating properties of the panel. These panels have a definite life, based on the slow permeation of the vacuum.
These examples are based on the work I did aboard my boat. They aren’t the only way to do the job but they worked for me.
I decided to add “ceiling” strips to the hull sides. On old time sailing ships, a ceiling was attached to the insides of the frames, allowing ventilation behind it and a path for the leaks to drain to the bilge without damaging cargo.
The hull was cleaned and prepped for epoxying. Vertical battens were epoxied to the hull. These consisted of three quarter inch plywood strips, faired and covered with a layer of six-inch wide fiberglass strips. These battens were sixteen=inches apart, allowing for a sixteen-inch wide piece of pink foam insulation to be placed in the pocket.
The ceiling strips were a combination of cypress and mahogany, 2-12” wide and a quarter of an inch thick. I cut these to length and screwed them to the vertical strips with #8 oval headed self-tapping screws and stainless finishing washers. After a section was installed, I removed the strips and varnished them on all sides.
After re-installing the first three or four strips, I pushed the insulation panels in place and finished re-installing all the ceiling strips. I ended up doing this in the vee-berth, the main cabin behind the settees and above the settees up to the hanging lockers and in the quarter berth.
I had installed a panelized overhead in my boat, consisting of a series of rectangular ring shaped panels epoxied to the overhead. Teak battens outlined the frames and the centers of the frames were covered with plywood panels covered in white nautical vinyl. The panels were held in place with Velcro. The Velcro holds the panels in place securely while allowing access to the underside of the deck.
The pockets in the center of the frames weren’t deep enough for three-quarter inch pink foam so I stuffed as many layers of Reflectrix as possible in the pockets. The “R” value, per inch, for Reflectrix wasn’t high, but there was no room for anything else.
Speaking of Reflectrix, there were several other areas aboard that didn’t have sufficient room to install any appreciable thickness of insulation. Behind the quarter-berth cabinets and behind the nav station were two such areas. In places like this, I installed as many layers of Reflectrix as I could.
The one remaining areas that allowed a lot of heat into the boat were the hanging lockers in the main cabin and the galley. You could feel the heat flowing in from the deck overhead and the hull side, which had a dark cove stripe at this point. The sides and top of these areas were covered with panels made up of cheap plywood covered with aged dirty gray vinyl.
I ripped out these panels, cleaned the hull and deck and painted these areas. My solution for refinishing these areas was to make up composite panels. I epoxied more pink foam to eighth-inch thick plywood. This plywood is commonly found as “door skin” material. Make sure you get the waterproof kind.
Once the epoxy kicked, I then contact cemented frosty white Formica to the plywood. I cut these panels to size and screwed them to the underside of the deck with a couple of stainless self tapping screws and finishing washers. I cut the panels going against the hull for a tight press fit, as I didn’t want to permanently glue them in place.
Each insulation project provided a noticeable improvement below deck. By the time my jobs were finished, my poor overworked 5,000 BTU air conditioner was performing like a champ. The insulation keeps it cooler in the summer, warmer in the winter and provides a significant reduction in noise below decks.
Tackle one area at a time and soon your boat will be well insulated.
Spring loaded closet rods holding the first layer of strips in place
First several strips screwed in place, ready to insert foam panels.
A cavity in the overhead ready for installing Reflectrix insulation.
Composite insulating panels installed in a hanging locker.
Overall view of installed ceiling strips and foam.
Another overall view of the ceiling strips and insulation in place.