Cargo trailer ventilation?

[Texas-Mark]

I have a 12x6 cargo trailer conversion. It has powered roof vent and a side window. During the day when I am not using it, the powered roof vent runs when the internal temp reaches 95 degrees. Obviously it will never be cooler inside than the outside ambient temperature, but it is always 10-20 degrees hotter inside. With the window closed, it will be 20 degrees hotter even with the fan running since there is no fresh air intake. With the window open, it will still be 10 degrees hotter, but since the window is right next to the the ceiling vent, there is not really that much overall air flow other than in the window, and right out the vent,

I have a 5K BTU AC that i can remotely turn on, but the hotter it it is inside, the harder it is is to cool it down, and I have to fire it up a lot earlier if I plan to go out to it. Note: We have been having 100+ days for weeks now. I have panels on the roof that offer some shading and a sun screen shade on the West side that helps a little.

So my question is, has anyone installed a fresh air intake somewhere like in the floor, and did it help? I don't want to be making holes for just a degree or two difference.

 

[df00z]

I am in the process of doing this now. Was 90 out, 95 in trailer. I actually am doing insulation, cellulose in walls as they are finished, foamboard on top. It seems to be working. I ran a 300 watt AC when it was 85 out and it got down to 70. It wasn't doing anything before. I have a chinese ERV on order. I think insulation is key, especially the roof where the sun hits and it turns into a solar oven.

A vent will only do a little, the radiant heat off the roof is nuts.

I thought about sucking in air from below, but i don't think it'd be enough.

I have an air monitor as well, co2 and vocs are pretty bad with no ventilation.

If you have insulation already, especially in the ceiling, crossflow can't hurt!

 

[Texas-Mark]

I only have insulation for the roof (foamboard).

cellulose in walls as they are finished,

Are you saying the walls were already finished and you added insulation, or you are adding it as you finish it? I have nothing in the walls now (they already had wood paneling). If you added it after the fact, how did you do it?

 

[df00z]

I unfortunately am stupid and finished them, then realized I wanted to insulate. It was like hundreds of dollars of paint. I didnt want to dismount the walls, plus they used these like self drilling screws that like to damage threads on the way out.

I am adding cellulose. I was able to stuff it in from the top and bang on the wall for it to settle. The walls don't seem to have any airflow, so I am just throwing some greenfiber in for moisture management. It is super messy. Each wall partition was taking about 1.5 5 gallon buckets. The studs are 16 on center. 1 inch wall depth.


If there is a large temp difference, water can condense on the back of the plywood or on the metal wall. Greenfiber absorbs moisture and averages it.

Dismounting the walls and foam would be best. Blowing in foam on finished walls is not a diy thing from what I can see, you can leave air gaps and it can cause mold. Or the walls can bulge.

 

[df00z]

Lemmie rephrase the process. I calculated the volume of a bucket at 1300 inches cubed. The walls are around 1100. I broke up the cellulose with a metal drywall mixer wand in a drill. I used a multitool to expand the opening on top of the walls which will be hidden by moulding so that it was wide. I hand stuffed, tapping on the inner and outer walls and occasionally ramming it down with some plastic tubing.

The walls took 1.5-2 buckets. So I can estimate the density. I went slow. I expect it to average out and settle some in the walls.


Blowing it in did NOT work.

I expect a moderate change, as the walls didn't seem to be putting off a lot of heat uninsulated. Little air flow. Little convection.

Ceiling has a lot of airflow and no radiant barrier without insulation. Walls had plywood at least.

I only did it for moisture management because I am putting flat moulding around the top which will further cut ventilation in the walls.

The ERV and AC should manage moisture OK. Might need to actually run a dehumidifier in the winter if I sleep in there, still.

Each wall section took me about 30 minutes to fill. Was very annoying.

At 1.5-2 buckets...I'd have to weigh the buckets but it's probably a density around 2 lbs per cubic foot which is loose fill not dense packed. 1 inch of insulation isn't gonna do much, just manage condensation better than the back of a wood panel will.

I can find no data on cellulose use in an RV. Greenfiber says it's OK for a poolhouse and an RV will be less extreme than that.

 

[wme]

Very temporary hack. Get some foil faced rigid foam and build a u shaped channel from the window to near the floor. You will get better air circulation, but you are still dealing with outside air temp.

 

[Texas-Mark]

FWIW, I have had the AC in there for about 5 years now. Used to run it with a generator before solar. Earlier this year is when I put the window in. When I cut the hole for that, I saw no signs of mold or anything on the inside of the paneling. So I don't think condensation is a huge issue or concern.

Thanks for the details on how you did the insulation on the walls. For me, I think it is more trouble than it's worth since I already have all sorts of stuff inside.

Very temporary hack. Get some foil faced rigid foam and build a u shaped channel from the window to near the floor. You will get better air circulation, but you are still dealing with outside air temp.

That is not a bad idea or something similar. Maybe I can temporarily use a section of dryer hose to go from the window to the front floor area to see if that makes any difference. Perhaps even use an intake fan. As I said earlier, I realize it will never be cooler than outside ambient, but I would like to get it down to about a 5 degree or less difference if possible.

 

[pollenface]

Having an air gap above the roof also helps, like solar panels elevated a couple of inches to allow air to pass underneath (shade)

 

[Texas-Mark]

Having an air gap above the roof also helps, like solar panels elevated a couple of inches to allow air to pass underneath (shade)

My panels are elevated, but only about an inch. Nothing else I can do about that at this point.

 

[wme]

That is not a bad idea or something similar. Maybe I can temporarily use a section of dryer hose to go from the window to the front floor area to see if that makes any difference. Perhaps even use an intake fan. As I said earlier, I realize it will never be cooler than outside ambient, but I would like to get it down to about a 5 degree or less difference if possible.

Make a window insert like a portable AC and run 2 hoses.

 

[iamrich]

My old trailer was much smaller (5x8), but I did foam insulation in the walls and roof and it made a huge difference. I was able to cool the interior with a less than ideal window AC setup (don't laugh, I froze my butt off one night with this setup as there was no thermostat) and a 2kw generator.

 

[Texas-Mark]

So as a test, I propped the side door open and blocked off the top of it letting air come in just from the bottom. It made no difference. Looks like I have to live with 10 degrees over ambient on 100+ degree days. Which BTW is getting really old, with no relief in sight.

I suppose another option is to just run the AC for a several hours each day since I already have solar available.

 

[0truck0]

You have to insulate it. If the plywood walls are already up and nothing behind them then take the plywood off and insulate it and put plywood back up - is the only way...
Insulate the roof even more, double what the walls are.
Beware that if you have metal framing and put the outside and inside walls against the metal studs then that metal will very efficiently transfer heat across helping to heat the interior wall. Best practice is to isolate the metal framing if you can. The inside plywood will go a long way to do this.
I have two inches of foam board in the ceiling (isolated) and one in the walls. The top inch has the metal framing, but the bottom inch has wood framing to isolate heat transfer.
I notice that where the solar panels are the ceiling is much cooler than where the sun hits the roof, so even corrugated plastic covering the roof will help, just hold it on with a brick or something when parked at home.
My back door of cargo trailer has aluminum framing, aluminum outer skin and aluminum inner skin, so this was transferring huge heat across. Insulating helped, but the frame still conducted heat to the inner aluminum skin, so I put more insulation over that to reduce heat. THis worked.
The trailer is black, but it does stay cool.
Having the roof mostly covered with panels and the AC unit keeps the sun's heat at bay too.
I do have two small windows and two roof vents that I keep cracked, but when parked and no fan it will get warm eventually, hotter than outside of course, but with fan on full speed the difference is a lot closer and with a 9000 btu AC going it will cool down in just minutes even on 95 deg days.
I do not run the fan during the day, just keep one vent opened (it has a cover) and the windows cracked (they are teh frameless flip-out ones so can be opened during rain storms). It never gets more than 10 degs hotter inside, hotter but not crazy different.

 

[OffGridInTheCity]

I did a 7 x 14 cargo trailer -> camper. Used 1" silver board (foam board) all around - floor, ceiling, walls, corners, every-nook-cranny including the tailgate. Put in a max fan roof/center (which we rarely use - was a waste of big $) but also a ~9" shower/bathroom fan in the vertical wall at head height that we just leave 'open'. Occasionally turn it on for kitchen fumes or a big one. This has worked well for overall ventilation.

We use a 9,000BTU mini-split and it cools/heats quite nicely. Have camped as low as 17F at night and as high as 100F and perfectly comfortable.

 

[JRH]

I have a 12x6 cargo trailer conversion. It has powered roof vent and a side window. During the day when I am not using it, the powered roof vent runs when the internal temp reaches 95 degrees. Obviously it will never be cooler inside than the outside ambient temperature, but it is always 10-20 degrees hotter inside. With the window closed, it will be 20 degrees hotter even with the fan running since there is no fresh air intake. With the window open, it will still be 10 degrees hotter, but since the window is right next to the the ceiling vent, there is not really that much overall air flow other than in the window, and right out the vent,

I have a 5K BTU AC that i can remotely turn on, but the hotter it it is inside, the harder it is is to cool it down, and I have to fire it up a lot earlier if I plan to go out to it. Note: We have been having 100+ days for weeks now. I have panels on the roof that offer some shading and a sun screen shade on the West side that helps a little.

So my question is, has anyone installed a fresh air intake somewhere like in the floor, and did it help? I don't want to be making holes for just a degree or two difference.

How much does the 5000 btu ac unit reduce the temp inside from the outside ambient.? ..And how much from what the temp was inside before you turned it on…? the reason I ask is Im doing a project now along these lines..
in those temps vents will help but not much……I know as I have several fan powered ones… sucking from the bottom and venting from the top… in dead summer it’s minimal reduction over ambient…maybe 5-10 degrees.
so I put a 5000 btu ac unit in this past Monday…and a dedicated inverter just for it..and turned off the fans..for about 200 watt hours or less once it’s cooled down x 6 hours per day …it’s a huge ….. huge difference …the multiplus fell by 35 degrees ..it’s now about 75 ish when running hard , the charge controller is 20 degres cooler at 70% load and now its chilly inside the trailer…..about 68…the compressor runs about 15 min per hour during the heat of the day And 8 min per hour when in the morn or late afternoon.
I turn it off about dark and turn it on about 12…
your mileage may vary…….
im not done… still experimenting…. but this is a amazing how better the gear runs and ( how much better it is for the batts) stayin* cool all day…….everything is about 70 F inside..
good luck to ya…
J.

 

[Bluedog225]

Agree with foil insulation. With air gap. 2” on ceiling. 3/4 on walls. Worked for mine.

painting roof white is huge.

 

[Texas-Mark]

Put in a max fan roof/center (which we rarely use - was a waste of big $)

Just curious, why was it a waste? Because you rarely use it or it did not perform? With my fan, unless I have the window open a crack, it's not really going to do anything since the trailer is sealed up pretty well. But I had nearly a 10 degree difference between running the fan and just leaving the vent cap open.

As for more insulation, that is no longer an option. I do have the ceiling done, but I am not ripping the walls down. Too much already in place to start over. I do have a large sun screen on one side (sort of a lean-to) that shades the west side in the afternoon.

 

[JRH]

Agree with foil insulation. With air gap. 2” on ceiling. 3/4 on walls. Worked for mine.

painting roof white is huge.

I insulated mine heavily behind the inside plywood about 2 inches plus wood.on all 6 sides….. im painting Cool Top on it next week…. I do have one 4 inch inline fan running inside 24 -7 ( 5 watt hr) to keep all the airstirred up and more equal top to bottom …
…But the AC…oh lord …wonderful……power wise ,it’s using up about 2 100 watt panels to run it during the heat of the day…and perhaps a one panel after about 6 o’clock…I have tested it on a Kilowatt meter and the Victron app for two days and that what’s its showing.…great results…won’t need it after sept or before June….
if one considers the loss of efficiency you Dont suffer when the gear is cool , the power used in operating it is much less than it appears.

J.

 

[Texas-Mark]

How much does the 5000 btu ac unit reduce the temp inside from the outside ambient.? ..And how much from what the temp was inside before you turned it on…? the reason I ask is Im doing a project now along these lines..

If I fire up the AC before it gets above about 85 inside, it will cool it down to 75 and keep it there provided I am not going in and out a lot. But if it is already 110 inside, it takes a couple of hours to get it comfortable. Hence the problem. I randomly use the trailer, so if it's already blazingly hot, I have to suck it up for a while. I can turn the AC on remotely from my phone, so that helps a bit if I know I will be going out there later.

Prior to putting the solar panels up, I was using a generator for the AC so it's not like I could run it when I was not using it. But now I have 800W of panels on the roof, a Rover 40 and 300Ah of 12V Lifepo4. The AC draws 45 amps (@12V). So I don't have enough solar to run it all day if the compressor needs to run non-stop. But I can put it on a timer to run for a few hours every day if I want to try and keep it from getting too hot.

BTW, I had put up these "freezer strips" on the inside of the door, which help a lot with the opening and closing of the door.

 

[Texas-Mark]

But the AC…oh lord …wonderful……power wise ,it’s using up about 2 100 watt panels to run it during the heat of the day

What type of AC?