RV Condensation

I'm working on the second major iteration of the Minibago, based on a Chevy Sprint; https://www.youtube.com/watch?v=EKoq3B8PRIg  I want to insulate it this time, for use in a cold climate.  The only time I car-camped below freezing, I got a thick layer of  frost on the windows.  So, I'm sure that if warm air gets onto the steel body behind the insulation, it will cause many problems.  Experiments show that if I use Lepage Tiite Foam aerosol as a mortar for chunks of styrofoam it provides an adequate vapour barrier.  However, the shape is complex, and the foam is hard to control, so I'm sure there will be flaws in the workmanship.  I've added rustproofing at the low crevices, but now I'm wondering if I should also install a cotton rope to wick out any moisture.  The theory is that the rope will not admit much air, but will remove minor accumulations of water, and indicate the extent of any problem.  However, as water condenses, it reduces the volume of air, which would draw more in, and that might not stabilize.  Other options would be to drill drain holes, or install a tube to blow out with compressed air.  Any thoughts appreciated.  

With all of the schoolie conversion sites there are many tips on creating that thermal break required to prevent condensation in vans and busses. Propane gives off a lot of water so I would look at one of the Chinese diesel heaters or one of the small wood stoves for heat. I'd take a gander at some of those sites and their solutions to condensation.

Robert Ray wrote:With all of the schoolie conversion sites there are many tips on creating that thermal break required to prevent condensation in vans and busses. Propane gives off a lot of water so I would look at one of the Chinese diesel heaters or one of the small wood stoves for heat. I'd take a gander at some of those sites and their solutions to condensation.

Thanks.  I certainly have stumbled on a big topic.  I don't see anything about wicking moisture out of enclosed spaces, and I do see a lot about heating and ventilation, but nothing about air-to-air heat exchangers.  With one of those running, heat loss would be minimal, and it should be possible to maintain the absolute humidity close to that outside, so there would not be condensation even on the windows and other cold spots.  Have you run across either of those solutions?

While I don't know if this is relevant to your topic there is a product called Damp Rid that might offer a solution.

https://permies.com/t/118103/Permie-dehumidifier-idea

I am definitely not a HVAC  expert, my understanding of a heat exchanger is that you would need a heated fluid running through the system for it to operate. What will be your source of the heated fluid? You are working with limited space. For whatever reason I was thinking Sprint van not a micro Sprint car. Whatever you select creative use of space and considerations of power draw will definitely guide your build.  I am a fan of the Chinese diesel heaters. We use them in our greenhouses. went from propane to a diesel heater in my studio. "Hcalory" are relatively new and are self-contained as far as fuel tank. Something that you could carry in the car and place outside saving space. They make three different styles.
https://www.bing.com/videos/riverview/relatedvideo?&q=hcalory+chinese+diesel+heater&&mid=435C39672BE3161CD197435C39672BE3161CD197&&FORM=VRDGAR

Anne Miller wrote:While I don't know if this is relevant to your topic there is a product called Damp Rid that might offer a solution.

https://permies.com/t/118103/Permie-dehumidifier-idea

Dessicants are suitable for use in a closed container to reduce humidity for better storage.  They don't have the capacity needed for moving air with a manageable volume, and would need to be re-dried frequently.  

Robert Ray wrote:I am definitely not a HVAC  expert, my understanding of a heat exchanger is that you would need a heated fluid running through the system for it to operate. What will be your source of the heated fluid? You are working with limited space. For whatever reason I was thinking Sprint van not a micro Sprint car. Whatever you select creative use of space and considerations of power draw will definitely guide your build.  I am a fan of the Chinese diesel heaters. We use then in our greenhouses. went from propane to a diesel heater in my studio. "Hcalory" are relatively new and are self-contained as far as fuel tank. Something that you could carry in the car and place outside saving space. They make three different styles.
https://www.bing.com/videos/riverview/relatedvideo?&q=hcalory+chinese+diesel+heater&&mid=435C39672BE3161CD197435C39672BE3161CD197&&FORM=VRDGAR

There are fluiid-to-fluid heat exchangers, and air to fluid or fluid to air, such as a car "radiator" but I specified an air-to-air heat exchanger.  A very basic one just has two concentric tubes with the flow in them going in opposite directions.  My neighbour's condensing furnace has the "smoke" cooled to 100 C in the condenser, and then it is blown out the wall.  However, on the way, the intake air for the furnace comes in through a larger, concentric tube, pre-warming the combustion air and further cooling the exhaust in its PVC tube.  There are also cubes with layers of exchange surfaces.  Commercial ones are pricy, but less-rugged home made versions are cheap.
The first version of my camper was featured in Rolling Homes by Shelter Publications, and is the smallest one I know of that is highway-legal.  It is also fully self-contained for storm or stealth.  It gets almost 50 MPG US, and is intended for more travel than camping.  If I had an inter-city courier business, I'd try to store all the waste engine heat for use at night with phase-changing wax.  I could travel 500 miles a day on the carbon footprint of my house in winter.  
The basic setup should work for almost any car that has to be used for a living space.  The driver's seat is not used in camp.  The passenger side is a bed.  Sitting sideways on the bed puts me in the kitchen, with my legs between the driver's seat and a custom cabinet.  The sink is in a drawer that comes out just over the bed, and the stove is over my legs when in use. Details in the video or book.
I'm thinking I might try a small rocket stove for heating.  I could shut it down to extract charcoal to make it carbon-negative with the charcoal being scattered as a soil amendment, or I could burn the charcoal to avoid a visible condensation plume for stealth camping.  

Whether fluid or air exchanger, during the night what medium and where is the heat being stored for the temp exchange? Is your design going to use the heated wax for the exchange? If you will be using the wax, what do you see as the volume of wax and anticipated temperature of the wax in its storage vessel. What length of time do you anticipate the heat reservoir will be able to keep the exchange up to inhibit condensation? Interesting concept but the available cubic inches will make it a tricky solution. Looking forward to see version 2.0.

Robert Ray wrote:Whether fluid or air exchanger, during the night what medium and where is the heat being stored for the temp exchange? Is your design going to use the heated wax for the exchange? If you will be using the wax, what do you see as the volume of wax and anticipated temperature of the wax in its storage vessel. What length of time do you anticipate the heat reservoir will be able to keep the exchange up to inhibit condensation? Interesting concept but the available cubic inches will make it a tricky solution. Looking forward to see version 2.0.

Rough estimates are encouraging with a 20 gallon tank, but exact numbers depend on the level of insulation achieved, and the local weather.  The heat exchanger would conserve most of the heat usually lost to a vent.
A gasoline engine puts about 25% of the potential energy in the fuel out the shaft, 25% into the coolant, and 50% out the exhaust pipe.  So, the general plan would be to use a water muffler to recoup exhaust heat, and circulate that, and the coolant through a heat storage tank.  The tank would be mostly filled with closed tubes of paraffin.  It has a heat of fusion close to that of glauber's salt, without the problems.  It does shrink a lot when freezing, so that has to be allowed for.  That temperature can be adjusted up with stearic acid to improve heat release and increase capacity slightly.  I expect that just moving a blanket would suffice for regulating car temperature, but it might need a fan, etc.
However, I don't have an inter-city courier business or much motivation to travel, and if I decide to look for a new home again, I can probably do it in the summer.  So, perfecting this rig is probably more trouble than it would be worth.  Now, I'm thinking of just adding foam where it is easy to get a perfect seal to the metal, and making sure that any condensation returns to the air the next day.  

After thinking about sail boats, which have more problems with water than just my condensation issue, I realized just how wonderful a bilge is.  A dry bilge is pretty rare, but a wet one is not a problem until there's enough water to make it worth pumping.  
In the car, I can just use the gutters on each side that extend from the box sections under the doors as collectors for any running condensation from farther up, with small drain holes.  
I'll insulate much of the bodywork above that, Where there's good enough access that I'm confident about getting a good seal from the foam to the metal to create a vapour barrier.  Places where gaps are needed for various reasons can just drip into the gutters, with a bit of planning.  I plan to install chunks of styrofoam that fit loosely together by injecting foam through a hole through the foam, so it will displace any air behind the chunk, and then expand into the gaps like mortar.  Unless someone here can think of a snag, I'll probably get to work tomorrow.  

Regarding condensation on the door skin side, vehicles are already vented for that. They are designed to allow rain water to fall through if it gets past a seal up top. When that doesn’t happen, you get rust-through. You can prepare for that by pouring water all over it before you work on the interior and make body panel drain modifications to let the water out. Convertible cars have the most engineering focused on that. It’s the same concept as a vapor barrier on a typical house.

The biggest problem is the interior!

I’m a former full-time RVer of over 5 years, 20 years in the industry, and now owner of an award winning RV resort. I’m a FIRM believer that all RVs should have a small dehumidifier. In warmer climates, the air conditioner works great for that, but in cooler, humid environments the AC will not run, humidity builds up fast, comfort is lost and mold sets in.

You must to condense the moisture from the air.

The simplest and cheapest solution is a basic dehumidifier. Run the drain hose outside, and let it run on auto ALL THE TIME. Yes, that’s a drain on electricity especially when off grid, but there are no easy shortcuts. You must seal the inside from the outside, and condense what’s on the inside.

Something else to consider is fresh air exchange. If you seal it up too tight, you won’t get a quality air supply. There are energy return ventilation systems that address this problem so you get air exchange with minimal loss in energy and environmental control. Those must be sized according to the interior space.

RVs do not need ERVs because they leak air like a tent. Schoolies and 4 season RVs on the other hand CAN be tight as a drum.

My suggestion is to tighten up with insulation followed by dehumidification and ventilation.

I also live off grid, and can run a ~3500 SF house on 5k solar, and a 3K backup generator using this approach in south Texas.

Forgive me if I misunderstood your original post, but this is a potentially complex problem. I tried to keep my response simple by saying, “insulate, dehydrate and ventilate”.

Hi Durrell,
It is great to have your experience for this.  My recollection of Texas is both hot and humid.  For my northern climate, what would you think of using an air-to-air heat exchanger instead of a dehumidifier?  If there's a significant difference in temperature it puts out fairly warm, dry air similarly to a dehumidifier, for less energy drain.  It could keep the absolute humidity inside almost the same as outside, making condensation very unlikely.  The commercial ones are pricy, but a custom-fit DIY version could be cheap.  

Bob Stuart wrote:My recollection of Texas is both hot and humid.  For my northern climate, what would you think of using an air-to-air heat exchanger instead of a dehumidifier?

Heat exchangers (vs heat pumps) differ in terms of dehumidification in several ways.

A heat exchanger transfers heat between two or more fluids, or between a solid surface and a fluid, without allowing them to mix. In the context of dehumidification, a heat exchanger can be used to remove moisture from air by cooling it below its dew point, causing the water vapor to condense and be removed. However, a heat exchanger itself does not actively dehumidify the air; it merely facilitates the transfer of heat.

On the other hand, a heat pump is a device that transfers heat from a cooler space to a warmer space, using mechanical work. In the context of dehumidification, a heat pump can be used to actively remove moisture from the air by cooling it below its dew point, causing the water vapor to condense and be removed. Unlike a heat exchanger, a heat pump actively dehumidifies the air by transferring heat.

I hope this helps. Heat exchangers are probably not what you’re looking for.

That's an interesting point about the possibility of the heat exchanger itself icing up, and one I'd have to keep in mind.  However, I should have probably stated that the purpose of the heat exchanger is to just expel considerable quantities of moist indoor air while recovering the heat for a corresponding intake of fresh, dry air.  

Spent 3 winters in an rv here in Eastern Canada.  Nights are pretty  standard at -10 to -20.  Use a small wood stove for your heat, with a propane furnace backup because your fire will go out while you sleep.  A programmable thermostat can be used even off grid.  Maximize your use of "Reflectix" insulation, primarily as window covers, and additional ceiling insulation (your heat all rises).  Floors must be spray foam insulated (a horrible job but an absolute must do) to prevent frozen feet.  The best case for condensation is using wood heat and above all else ventilation.  Yes that means Crack a window open in the winter.  Moisture in your walls comes from the interior so if possible use a vinyl coated wall board or a Reflective vinyl vapor barrier behind your wall board.  Other outside wall moisture comes from leaks. R5 foam is no roaring hell in the winter so Reflective insulation which will return up to 90% of the heat loss is a much better deal.  The most important thing to consider is ventilation that allows your heat to dry the air and is essential for a wood stove.   Do not rely on any open burning propane or electric,  you will freeze.
If possible skirt the vehicle, even if it is only with tarp material on a frame.  Do Not use straw bales. THESE ARE MOUSE HEAVEN. They usually come with the mice already housed, and they will come looking for you. Find a way to double glaze your windows (think old style storm windows).  Small computer fans can be ceiling mounted to even up the temperatures.  Best of luck.  As a side note, make sure all your plumbing is inside the heated space and not under the floor.

With such a tiny house storing 20 gallons of heated wax is a pretty big volumetric bite out of available space. My problem with propane heat is the volume of condensate that comes with their use. I'm with Johnathan on the small wood stove, that could be used as your cooking appliance as well. Venting the humid inside air to the outside will probably be key to whatever design you are looking into. Just exchanging heat while retaining interior moisture from sweating, breathing and propane usage might not address the condensation issue. Ventilation sounds like an important piece of the puzzle.

Robert Ray wrote:With such a tiny house storing 20 gallons of heated wax is a pretty big volumetric bite out of available space. My problem with propane heat is the volume of condensate that comes with their use. I'm with Johnathan on the small wood stove, that could be used as your cooking appliance as well. Venting the humid inside air to the outside will probably be key to whatever design you are looking into. Just exchanging heat while retaining interior moisture from sweating, breathing and propane usage might not address the condensation issue. Ventilation sounds like an important piece of the puzzle.

The whole point of the heat exchanger is to have high rates of ventilation without wasting much heat.  Moist air goes out and dry air comes in, but pre-warmed for free.  Agreed that a wax tank would need significant volume, but so does a wood burner plus fuel.  

The first version of my camper was featured in Rolling Homes by Shelter Publications, and is the smallest one I know of that is highway-legal.  It is also fully self-contained for storm or stealth.  It gets almost 50 MPG US, and is intended for more travel than camping.  If I had an inter-city courier business, I'd try to store all the waste engine heat for use at night with phase-changing wax.  I could travel 500 miles a day on the carbon footprint of my house in winter.  
The basic setup should work for almost any car that has to be used for a living space.  The driver's seat is not used in camp.  The passenger side is a bed.  Sitting sideways on the bed puts me in the kitchen, with my legs between the driver's seat and a custom cabinet.  The sink is in a drawer that comes out just over the bed, and the stove is over my legs when in use. Details in the video or book.
I'm thinking I might try a small rocket stove for heating.  I could shut it down to extract charcoal to make it carbon-negative with the charcoal being scattered as a soil amendment, or I could burn the charcoal to avoid a visible condensation plume for stealth camping.

Sounds ingenious!
Any link or other information? Would love a "tour"

Bob Stuart wrote:That's an interesting point about the possibility of the heat exchanger itself icing up, and one I'd have to keep in mind.  However, I should have probably stated that the purpose of the heat exchanger is to just expel considerable quantities of moist indoor air while recovering the heat for a corresponding intake of fresh, dry air.  

Look into Energy Return Ventilation (ERV) systems. They are compact fresh air return systems that may do what you need. Last time I researched them for a tiny house, Panasonic had the best results.

Thanks for the terminology, Terrell.  What I'm finding is too big, too expensive, and unsuitable.  I don't want any humidity recovery, and maximum heat recovery.  For now, I'm  doing some insulation because it has to go in first, but I'll just finish the unit for occasional, above-freezing use for now.  If I do need it for continued and/or colder use, I'll add a wood stove first, and see if I need enough ventilation to make the heat recovery job worth it.  One nice thing about wood heat is that I can extract charcoal for biochar to be a good citizen, not just a fuel waster.  If I expect to be driving 20% of the time, I'll go to the wax tank to store engine heat instead.  

Thanks, David.  I have been using roughly-carved styrofoam blocks set in a "mortar" of spray foam, trying to avoid all air gaps behind and between, but I know the job can't be perfect.  I've tried to arrange for drainage, but again, it can't be perfect.  If I were doing it again, I'd use all spray foam.  Considering all the fuss I've had, doing a perfect vapor barrier with fiberglass might have been easier.  

Bob,

We are going into our 4th year in an RV. We have been all over the US during a variety of times of the year. We have tried all manner of ways to beat the condensation. One way would be to be using a diesel or gas heater inside the RV to help get rid of it. That does not help much if you are in the shoulder seasons or in a small RV where your body and breath actually help keep it warm. You risk a lot of issues including electrical issues if the humidity is not mitigated, trust me.

Ultimately, we just made sure we had the solar capacity for a dehumidifier. We have 168 SQFT of space, 7 bodies and a 45 pint dehumidifier. It draws around 4 amps, running an average of 20-30 mins an hour, sometimes less and not more. We open the rig up all day and only close it after dinner and showers. That means we only need about 20 AH capacity (or less) to use it in a 24 HR period and we have easily met it, even during extended boondocking trips.

Food for thought. I hope you get it sorted out.