Humidity and Living Aboard

[TheLake]

Hi all,

There are lots of discussions around humidity, but it seems to be a bit confusing (differing words of wisdom, sometime contradictory).

The basics:
- living aboard a 42' CHB Tricabin in Sidney, BC.
- during the winter, boat heated primarily with a diesel stove
- during the winter, typically put internal clear film over the windows and doors to block out breezes and to provide a bit more insulation
- during the summer, like to have the windows open in bathroom and stateroom for fresh air/breezes
- have a whole-boat dehumitifier in the engine room with vent lines bringing in air from the engine room and from the aft cabin floor vent
- usually have one or two pellet-type dehumidifiers around the boat
- during the winter, often have the round plug in dehumidifiers around the boat

I find that during the winter with the diesel stove on the boat is very dry

When the diesel stove is not on (fall, spring, summer), the boat is fairly wet (humidity between 70 and 80). Running the dehumidifier can lower the humidity to about 60, but doesn't seem to get lower than that.

Showers and cooking obviously don't help - even with the head window open during the shower and a solar powered head vent fan (but truthfully, this only seems to occasionally run - the batteries never seem to last on these things).

I was doing some reading, and there was discussion about closing up the boat. I didn't think much of it until I looked at the external humidity in the area (was about 86%) - and realized that open windows would be bringing in this humid air - and may be keeping my internal humidity up (despite running the dehumidifier).

I also have been overly confused about the seasons and humidity. I thought that humidity would only be a problem in the winter when the temps are colder and the air can't hold as much moisture, and thus lots of room for the water to condense inside the boat. Thus, I only thought about putting any sort of dehumidifiers on the boat during the winter.

Now I realize that the humidity is quite high in the summer outside, and maybe that is part of the "wet" boat feeling.

Bilges are generally quite dry all year long.

So, I guess I am looking for some help on trying to understand the right approach. Including:
1. Is there a concern about having windows and doors open during the summer months
2. Should I be running the dehumidifier during the summer months (even with the windows and doors open)
3. Should I be continuously running the dehumidifier? Or just during the nighttime, or just during the daytime?
4. Should I look to be putting weatherstriping, etc on the doors to try and seal the air leaks on the boat as much as possible? (Should the boat be as air tight as possible? And if so, how does one deal with diesel stoves that require a regular source of fresh air)
5. Should I install a "blower" in the engine room to keep the air moving in there?


Other general advise on how to best manage the humidity levels on a live aboard in the Pacific NW.

Thanks,

Chris

 

[Moonfish]

Great and well thought out questions. You're right, humidity is a year-round concern, although it is much more of an issue in the winter with the interior and exterior temperature differences. We've been living aboard 12 years not too far south of you and pretty much do many of the same things (including plastic heat-shrink film on the main cabin windows). The primary difference, however, is we have a Wallas forced air diesel heater as opposed to a diesel stove. That heater draws the bulk of it's fresh air from an outside vent and thus allows us to keep the boat "closed up" during the winter. Here's what we have found works great for us (and haas kept this boat essentially mold and mildew free):

1. Wallas Spartan heater (runs 24/7 October-May)
2. Household propane fireplace in main cabin (with double intake/exhaust flue)
3. 2x household AC dehumidifiers belowdecks (8 hrs/day in winter, every few days in summer)
4. Heat shrink plastic (October-May)
5. H2Out Space Dryers (18 different sizes throughout the boat)
6. Kanberra Gel (also placed throughout the boat)
7. Engine room blower for bathroom fan

The Wallas is in the engine room, so even if the temps drop below freezing for a few days the ER stays around 40F degrees. In addition to the household sized dehumidifiers, we also have a small counter top AC unit in the pilothouse that runs 24/7. During the winter I end up emptying the main dehus about every two days. Summer once a week or so. If we're cooking something like pasta we definitely run the galley fan. But the engine room blower is important as all three of us shower on the boat. We typically keep the door to the head closed and the blower running until the mirror is clear. Then the large dehu does a good job with the rest.

The H2Out Space Dryers are great as they are rechargeable and you don't end up with any caustic water byproduct. The Kanberra Gel is tea tree oil that evaporates in the air and kills mold and mildew (and many viruses). My wife often says she would not be a liveaboard without those products and diesel heat. We sell both products in our store, so message me if you're interested in learning more or purchasing them.

 

[ksanders]

I lived aboard in Seward Alaska for several years.

In the summer you open the windows and deal with it, because at least it's warm outside.

Fal, and spring, and winter I ran two portable dehumidifiers, plug in type. This improved comfort greatly. Yes, in the dead of winter they were less used, but I kept them on and the humidity set at 45%

Less and I started having nasal problems, and more and the boat felt "wet"

 

[TJM]

Well, you are going about it the right way. I am also a live aboard in southern New England (USA). As you have said, warm air can hold more moisture. When warm air comes against colder surfaces the moisture comes out of suspension and condenses onto the cooler surface. Our problem is in the winter time when the boat is closed up and we cook and shower aboard. I have sealed up my boat and engine room less every year. Heat loss by convection, or air flow is acceptable. It brings in dry air. I have insulated against conduction or heat loss thru surfaces such as walls and windows. I have learned I would rather loose heat thru open doors and windows than have a moldy boat. The first year I panicked because my engine room sides had about 4" inches of water on the sides. I thought I had a leak somewhere, but it was just condensation on the walls. Now I keep my engine room vented and heated to 40° F.

 

[SeaDogAK]

One thing I would mention is that your dehumidifiers have to be the kind that actually remove moisture - water - from the air, and the water has to go somewhere where it will not re-evaporate into the air. The small round dehumidifiers are just a heater and a fan, so they reduce the relative humidity by warming the moist air. But they don’t actually remove any water from the boat.

I run a dehumidifier in the galley sink with the water outlet in the sink so that the water it puts out goes down the drain. And then fans to circulate the air so that the moist air below decks can be dehumidified by that unit.

 

[dhays]

- have a whole-boat dehumitifier in the engine room with vent lines bringing in air from the engine room and from the aft cabin floor vent

This confuses me a bit. If you have a dehumidifier that is bringing in air from the ER, you are really bringing in outside air from the vents into the ER designed to keep your engine breathing. If you are running a dehumidifier, you want the boat closed up. The whole point is to make the inside, dryer than the outside. Think of how the AC in a car works. It recirculates the cool air instead of trying to continually cool the hot outside air.

- usually have one or two pellet-type dehumidifiers around the boat
- during the winter, often have the round plug in dehumidifiers around the boat

The pellet desiccants I don't think are terribly effective, but they do help if you have lockers that don't get much air circulation. The round things you are talking about simply are slow speed fans that very slightly warm the air and keep it moving around. I think they can be helpful to use in addition to a real dehumidifier.

I find that during the winter with the diesel stove on the boat is very dry

That is a nice thing about diesel or wood heaters, they tend to keep things drier. That combined with something to move the air around helps a lot.

I don't live aboard but I keep a dehumidifier running 24/7 on the boat most of the year when we aren't using it. The most effective are the compressor type of dehumidifiers. They do a great job but are noisy and draw a lot of power. Less effective, but much quieter and less of a power hog are the Peltier type of dehumidifier. I've used both. I would run a compressor type of dehumidifier in the salon and then a small mini Peltier in the head and master cabin.

As SeaDog does, I run the drain of the big dehumidifier into the galley sink and have drilled holes in the mini Peltier than I keep in the head so it drains into the head sink.

 

[Moonfish]

I have sealed up my boat and engine room less every year. Heat loss by convection, or air flow is acceptable. It brings in dry air.

Good point that I hadn't fully realized. It is somewhat counterintuitive (for me, anyway) that winter air is "dryer" than summer air. Probably because around here it rains a lot more in the winter and that just seems wetter...

 

[Carriage Guy]

We spend a lot of time in all 4 seasons cruising on our little boat so are particularly sensitive to humidity. We use a Wallas forced air heater, occasionally a rooftop (air-to-air) AC and have insulated the cabin walls of our v-berth with foam that is designed to insulate AC ducts in large buildings. We've been very happy with our boat and stay mold free.

I received a degree in mechanical engineering years ago and so am supposed to know "all things of thermodynamics" but that was years ago and I didn't specialize in that field in my career.

With that said, our humidity control strategy was based on the Psychrometric Chart that among other things shows the relationship between the water content of air, temperature and relative humidity which makes it easier to understand how and why relative humidity can vary within a boat and from season to season. The charts can get really busy, I found a less busy one at Psychrometric Chart Use. Attached is a hand sketch of that chart with some example points added.

On the chart, the air temperature as measured by a thermometer is along the bottom axis, the amount of water in the air (humidity ratio) is along the right axis, and the relative humidity are the sweeping arc lines.. The upper-left most relative humidity line is saturation or 100% humidity-- anything above or to the left of that line represents condensed liquid.

Starting at any point on the chart, moving left or right reflects changing just the temperature of the air. That could be from a change in seasons or the difference in location of a poorly air-circulated boat (closer to the cold cabin wall). Moving up and down reflects changing the amount of water in the air. That could be from exhaling, sweating, cooking, bathing, or de-humidifying.

The key constant to consider is the moisture content of the air. The temperature of the air can easily vary considerably from the middle of the cabin to the cabin wall. The relative humidity in the middle of the cabin may be 80% but the relative humidity along the cabin wall may be 100% because the same air is now at a lower temperature-- nothing changed the pounds of moisture per pound of dry air but the relative humidity increased because the local temperature decreased.

(Don't know why the following is bolded) So why do diesel forced air heaters work so well? They take cold, humid air (100%) in from the outside with a constant amount of water (point "A"), heat it to lower relative humidity (40%, point "B"), displacing other internal air that has higher humidity from breathing, showering, etc.​

So why does breathing or showering increase the humidity? Adding water to the air at a constant temperature increases it's relative humidity (80F, 50% Point "C" to 80F, 100%point "D"). If too much moisture is added beyond the air becoming saturated, the excess will condense out as water drops.

So why do water drops condense on my cabin wall or on the outside of an ice tea glass? From dropping the temperature of the air beyond it's saturation point. Comfortable air at 80F, 50% humidity (point "C" again) dropped to 50F will reach saturation and start to condense (100% humidity, point "E"). The only way to keep that from happening is to keep the temperature up or the amount of moisture in the air down.

So why does a car or boat airconditioner create cooler, dryer air? They create a cold radiator surface to cool the air which is sized to also cause the air to reach saturation (the "C" to "E" transition mentioned above). Some moisture in the air is condensed out (why your AC dribbles) and that cold air at 100% relative humidity is then released back into the cabin as it warms and lowers in relative humidity to mix with other warmer air to create a cooler, lower relative humidity mix.
Humans breath out so many pounds of moisture per day and cooking and/or showers add more moisture to the cabin air. If that air is not de-humidified to remove that moisture or if that air is not exchanged with dryer air from the outside, it will eventually start to condense and the relative humidity reaches saturation. In the summer exchanging even 80% relative humidity outside air for 100% relative humidity air in a wet shower is a good move.

Things we do: mix the air with a fan to keep the temperature more constant at the cabin wall, remove excess water before it can condense, minimize the addition of water to the air, insulate the cabin wall to decrease the temperature drop at the wall in cold weather.

--Mike

 

[Moonfish]

Awesome post, Mike!

You also reminded me that we have added insulation as well. While we had the overhead down (for access) in our daughter's forward cabin we cut half-inch foam boards and installed between the overhead and subdeck. Made a difference. We also used spray adhesive and affixed thinner mylar-like insulation wherever we could behind cabinets and drawers. That made a difference in our aft master cabin, even though we've yet to insulate the overhead.

I think for us liveaboards, the moral of the story is every little bit helps!

 

[TheLake]

Thank you everyone for your comments and thoughts. This is exactly what I was looking for you have given me lots to think about.

My Dickerson Diesel stove has a coil in it and heater hose with antifreeze that goes to two radiant heater heads (one aft and one in the v-berth). I find that they don't blow very hot air from the heater heads...but am thinking that I need to flush out and refill the antifreeze.

Chris

 

[Carriage Guy]

Awesome post, Mike!

You also reminded me that we have added insulation as well. While we had the overhead down (for access) in our daughter's forward cabin we cut half-inch foam boards and installed between the overhead and subdeck. Made a difference. We also used spray adhesive and affixed thinner mylar-like insulation wherever we could behind cabinets and drawers. That made a difference in our aft master cabin, even though we've yet to insulate the overhead.

I think for us liveaboards, the moral of the story is every little bit helps!

At the risk of beating a dead horse, I'll add one more observation about installing insulation in our boat.
Say the middle of our cabin is 70F and the outside of the hull is 30F. Everywhere between those two points, the temperature is slightly less from the inside toward the outside-- that temperature difference is what is driving (or results from) the heat loss. A small large amount per inch for good insulator materials (air, foam), a much greater smaller drop per inch for poor insulator materials (glass, metals).
Somewhere between the inside and outside of the boat the 100% saturation temperature will be reached for the air that happens to be in the boat at that point. The trick is to make sure there is no or little moist air at that point and points further outboard. Doing that keeps condensation from forming at that point. By the way, that's why houses are built with a plastic "vapor barrier" on the living side of an outside wall -- to keep the moist interior air away from the temperature point of the insulation batt where condensation could otherwise occur.
We made sure our insulation was flush against our hull and thick enough so the inner surface was kept warm enough and our cabin air dry enough not to reach saturation--therefore no sweating.
For example, insulating the cabin door of a hanging locker would probably cause the air inside the locker to chill below it's saturation point and sweat out onto the hull. Better would be to insulate the hull wall inside the locker enough to keep the air temperature near the hull high enough to stay above the air's saturation point. Keeping the door open and circulating the air with a fan would also help keep the temperature of the air against the hull from reaching saturation.
Any dead air space between the insulation layer and the hull is prone to sweating and mold, although if sealed from air exchanges (eg. tight plastic film on the inside of a bost window) the amount can be minimal. Worse still would be a dead air space near the hull that allows air to circulate to some degree (replenish moisture as it is condensed out) as it is chilled.That would be the same process as an AC but the "dribble" of water would be deposited against your hull instead of removed from your boat by a drain. An example of that would be a poorly sealed (floppy) inner plastic film on a boat window that continuously let air exchange and sweat against the outer glass window resulting in a trickle of the AC "dribble".
Hopefully this deeper dive makes some of the other helpful comments make sense.
--Mike

 

[Comodave]

And put a couple of vents in the door so that you get air circulation in the locker. Or keep the door open a bit.