Controlling condensation belowdecks

As any boat owner knows, natural condensation can be an annoying problem in a voyaging boat. Condensation forms at the dew point — the temperature at which water forms at the boundary layer. On a boat, that boundary layer is the interior surface of an uninsulated, or under-insulated, hull or deck. Condensation can be prevented by adding insulation and ensuring that the insulation is dimensioned so that its surface temperature will be above that of the dew point.

Hulls and decks are typically constructed of wood, or fiberglass, or bare metal, or a combination of materials, each of which can be characterized, in part, by its thermal conductivity — its ability to allow heat to pass through it. Often, this property is described by a heat-transfer coefficient — a relative number that expresses a measurement of change under certain fixed conditions.

By applying a sufficient layer, or thickness, of insulation to a hull, or to a deck with relatively high thermal conductivity, we are increasing its thermal resistance — or its R-value. We are then able to stay more comfortable, especially when we approach temperature extremes. We stay warm and dry belowdecks when the world outside is cold and wet; conversely, we stay cooler and drier inside when the outside world is oppressively hot. Boosting the R-value will decrease thermal losses and will reduce heating-fuel demands and costs, too.

Insulation is any material that impedes the transfer of heat. These materials are typically compared by their R-values. An insulating material that is one-inch thick, with a low thermal coefficient, will have a relatively high R-value.

There are a great variety of insulating materials on the market today, but three very effective insulating materials stand out for use in the marine environment: spray urethane foam; a radiant barrier insulation manufactured with an enclosed air space, such as Reflectix; and elastomeric, dry-cell foam, such as Armaflex. The primary differences between them are cost per square foot to install, application method and material properties.

Spray-on urethane foam
This type of insulation is common on both steel and fiberglass commercial fishing vessels. It is applied to a boat more easily before the interior skin is installed. After the interior is in place, it becomes somewhat more difficult to spray the insulation into remote areas. Also, the spray-on foam is usually best left to a professional with the proper equipment. Spraying foam can be messy and toxic — good ventilation is required.

Urethane spray foam is very effective as a thermal insulator and condensation inhibitor. The cost (per square foot) will be low to moderate, depending upon whether the installation is a do-it-yourself job or contracted out. One disadvantage to spray-on foam is that it generally needs to be covered with a liner because the surface is rough and unattractive and hard to clean. Also, spray polyurethane foam can be a fire hazard — especially on a metal boat that may require a welding repair to the hull.

The biggest advantage of sprayed polyurethane foam is that it has an R-value of approximately 6.0 per one-inch thickness, making it one of the highest R-value (per inch) insulators on the commercial market. Small do-it-yourself spray cans of expanding foam can be effective when used to seal small air gaps, cracks, irregularly shaped surfaces, and holes of less than three inches in diameter.

Reflectix insulation
Reflectix insulation ( has two layers of 96 percent reflective metalized aluminum separated by a 5/16-inch enclosed air space. Reflectix is effective as a radiant barrier; but, preventing heat loss is nullified if the material is applied directly to the interior surface of a hull, or deck, with high thermal conductivity. An air space facing one reflective side is required for this product to work as designed. R-values may be calculated when a reflective side of the product faces an enclosed air space — that is, an air space without free air flow.

Reflectix is flexible and can conform to almost any curved surface. It can have an enhanced R-value when it is installed properly. However, you need the extra space to take advantage of this type of layering. Simply stacking multiple layers of Reflectix, back-to-back, will only provide a marginal increase in efficiency, about R-1.1 for each stacked layer. On the other hand, with a sealed air gap of at least half an inch, between each layer of Reflectix, the R-value will increase significantly. Once again, air is a good insulator. For example, two layers of Reflectix, with a sealed air gap between each layer, can have an R-value of 6.0 — effective in controlling condensation. But to achieve this increase in R-value, the layers need to be installed according to manufacturer specifications. This may be difficult in some areas on a boat.

One significant advantage of Reflectix, is that it has the lowest cost-to-square-foot ratio of all three insulators. Also, it is easy to install. It comes in rolls of various widths and lengths. It is clean and non-toxic to work with. Generally, though, Reflectix, like spray foam, will need to be covered by a hull- or headliner, unless installed in an enclosed, hidden space, such as within a locker. Unless you do not mind being surrounded by shiny, aluminum foil in your exposed living and work areas, you will want a liner.

Made with Microban, AP Armaflex ( is antimicrobial. It is resistant to mold growth, fungi, and bacteria. It is safe (will not contribute significantly to fire), durable, toxin-free (manufactured without CFCs, HFCs or HCFCs) and fiber-free. Water absorption is only 0.2 percent by volume. The closed-cell structure of Armaflex prevents moisture from wicking and makes it an efficient insulation with an R-value of 4.2 per inch of thickness.

Armaflex SA (self-adhering sheet and roll insulation) is a flexible, elastomeric, thermal insulation, with exceptional condensation resistance, as well as sound-dampening characteristics. It is matte black in color, and may be applied as a stand-alone material, without the need for a hull- or headliner to hide or protect it. The adhesive backing is aggressive and adheres extremely well to curved and uneven surfaces, which are properly prepped (surfaces should be cleaned and degreased; isopropyl alcohol works well).

Armaflex is manufactured and supplied in both self-adhering sheets and rolls, and non self-adhering sheets and rolls. Flat sheets dimensions are three by four feet, with wall thicknesses ranging from a half inch to two inches. Rolls are typically supplied in four foot widths of continuous lengths, with thicknesses ranging from three-eighths of an inch to two inches.

The one disadvantage of Armaflex is that it is expensive; ten times as expensive as Reflectix, for instance, and about three times the installed cost of urethane spray foam. However, Armaflex is very easy to work with and apply, and it provides a finished look when used in exposed interiors not already covered and protected by a liner. With ease of installation, and exceptional material properties, Armaflex makes a first-rate insulation for thermal control, sound dampening, and most especially, condensation control.

Controlling condensation on Anna
We’ve found that by using a combination of the three types of insulation materials mentioned, we were able to make our living environment belowdecks on Anna, our Tayana 37 cutter, more comfortable by eliminating condensation and modestly decreasing thermal losses.

We wanted to be able to sail to a remote, moderately-cold locale, and still be relatively comfortable (dry and reasonably warm) within the cabin, and more specifically, without the use of a dehumidifier or forced-air, diesel-heating system that promised to consume our hard earned, alternate-energy amp hours. We wanted to be able to conserve power while at anchor, at a dock (without proper shore power) or under sail.

By using a gravity-feed, diesel day tank to supply our heating stove (an 18,000-BTU Sigmar 180, and insulating our more vulnerable surfaces like the uninsulated overhead areas, we are able to remain reasonably comfortable, with a minimal expenditure of diesel fuel (1.25 gallons per day) in cold-weather conditions. It takes zero amp hours of electrical energy to run the diesel heating system this way.

When the outside temperature dips down to the 20° F range, we will close off non-occupied compartments, so as to heat a smaller living space, until the outside temperature rises a few degrees. With our heating stove located amidships, the most difficult areas to reach, without a complex forced-air system or more than one heater, are the extremities of the boat in addition to any closed lockers.

Inside a small boat, relative humidity can vary widely. We have measured relative humidity at 78 percent in the V-berth (outside humidity was 98 percent at the time), while the main cabin registered 45 percent near the diesel stove. By circulating warm, dry air we can lower the inside dew-point temperature (and potential for condensation). To do this we use a stove-top fan to circulate warm, dry air to distant areas like the bow V-berth, some 17 feet away from the amidships stove.

Without an effective buffer zone of thermal insulation at the interface between colder outside air and warmer inside air, condensation is likely to occur. This is especially true when the relative humidity in the cabin is high (greater than, say, 80 percent) and the outside-to-inside temperature differential is significant. By maintaining at least the minimum surface air (dew point) temperature we will keep the inside of the boat comfortably dry.

Unless you can do that the atmosphere in the cabin will begin to resemble a rain forest. If the overhead in the V-berth, or sleeping area, is uninsulated in a cold climate, condensation will drip, steadily, and mold and mildew will prevail, embeding itself in fabrics and onto interior surfaces, often discoloring them and producing a strong musty odor. This is to say nothing of the potential health hazard.

If that is not enough to inspire you to insulate, the unpleasantness of being repeatedly awakened by drips of condensation falling on your face will probably do the trick.

On an uninsulated boat, when relative humidity is high and outside ambient temperature (or, seawater surface temperature) drops to below 50° F and inside ambient temperature is above 60° F, life down below is closing in on the dew-point threshold.

We have noticed that when there is at least a 10° temperature difference, between the outside world and the inside world a minimal amount of condensation can form on the vulnerable surfaces. When there is a 20° differential, condensation can be heavy. Of course, this varies from vessel to vessel, and is dependent upon the relative humidity, the materials used in the construction of the vessel and the amount of insulation on the vessel.

An insulation solution
Anna, has a stout, fiberglass hull, and a composite, three-quarter-inch thick deck (half-inch balsa core, enclosed within two layers of one-eighth-inch fiberglass). About 85 percent of the interior overhead space is minimally insulated, by a sealed, one-inch air gap — the space between the underside of the deck, and the thin liner panels, just below. Since we know that air has a low thermal coefficient (0.024), and that balsa (0.050) and fiberglass (0.040) are not too bad, either, we can speculate that we have at least enough insulation (an R-value of about 6.0 — based on the air space, plus the thickness of the deck core) in the overhead areas, where the enclosed air space exists, to prevent condensation. This is confirmed by the fact that during extreme temperature differentials, no condensation has formed.

The most critical area, overhead in the V-berth where we sleep, had no insulation, however. It needed an insulation material that would provide at least as much protection from thermal loss, as a one-inch, sealed air space.

The ideal material would need to meet our criteria: cover a surface area of about 50 square feet, with minimal thickness; be structurally stable enough so as not to require a headliner; be aesthetically acceptable; and completely eliminate the possibility of condensation forming wherever Anna might take us. After reviewing all the specs for the top three insulating materials, Armaflex SA appeared to be the best match.

To specify Armaflex, we needed to take into consideration the extremes: the maximum interior air temperature we expected (85° F); the minimum interior surface temperature (or dew-point threshold) we could live with (48° F); the minimum outside air temperature we might encounter (10° F); the minimum seawater surface temperature we anticipated anchoring in (40° F); and a maximum inside relative humidity of 85 percent. After plugging our numbers into the program that Armaflex makes available as a free download, we were able to determine the thickness of the Armaflex insulation we would require.

We used Armaflex SA sheets that were a half-inch thick, with an R-value rating of 3.1 (equivalent to about a one-inch enclosed air space). By adhering the Armaflex directly to a clean, bare, interior surface, we were able to achieve an effective, durable, attractive (at least in our opinion), and easy-to-apply insulation, which would stop condensation in its tracks.

After cleaning the bare surface with isopropyl alcohol and then drying the area, the next step is to make mock-up templates of the section we want to cover. We then transfer the template patterns onto the backsides of the Armaflex sheets. We cut out the patterns with a knife or scissors and simply pull off the adhesive protective backing, line up the sheet on the surface to be covered and press it on. Any small cracks between sections can be bridged with self-adhesive Armaflex sealing tape (same material, only one-eighth-inch thick). The tape both seals the small cracks and provides a finished look. Pretty simple when compared to some of the other insulating options and remarkably effective.

As a side benefit, we found that the inside ambient temperature (as measured in the more difficult to heat V-berth) averaged approximately 4° F warmer than it did before we added Armaflex. We tested the insulation during a 10-day cold snap, when outside temperatures hovered in the 20s F, both day and night. We were able to heat the main cabin to about 62° F during the coldest hours. The far ends of the boat were admittedly cooler in the early morning hours, about 6° to 8° F. This was acceptable to us, however, considering that the outside temperature was well below freezing and was not expected to last for more than a week or two.

Three additional steps we take to eliminate condensation and increase thermal comfort are: covering the overhead, Bomar hatches (a true heat sink) on the inside with a thin sheet of Plexiglas and weatherstripping around the perimeter. Doing this essentially creates a six-inch deep sealed air-space that completely stops condensation from forming and dripping from the metal framework of the hatch. We also circulate warm air from the diesel stove, with a Caframo Ecofan Air Plus (, which is powered solely by the heat of the stove-top surface (about 400° F on a Sigmar 180). Lastly, but not least, we keep the sleeping and cooking areas well ventilated with open dorades and louvered locker doors.

All these things combine to help keep us and the boat dry and reasonably comfortable when the weather inside tends to resemble the weather outside.

Rich and Cat Ian-Frese have been living aboard Anna, and cruising the waters of the North Pacific Ocean basin, since 2000.

By Ocean Navigator