So I have put an 8,000 BTU window unit AC in one of the E exit windows of this bus. Ratings say the AC should cool 250 sq feet and the bus cabin is 180 sq. feet. This AC doesn't even touch it. I've put reflectix in the windows, and even hung double layer silver tarps over the walls and windows and still not cooling. So I tarped off the back 1/3 of the bus so now we're talking about 80 sq feet. It'll cool it down after the sun goes down. Leaving the AC on all night, by about Noon, you can feel the heat radiating off the ceiling and the tarps hung on the walls with reflectix over the windows and it can't keep it. Temp slowly rises to 80 then 85 and won't cool back to 72 until well after dark again.
I realize this is all factory insulation and nothing else has been added. I did end up putting a silver tarp over the top of the bus in that back 1/3 section and it seems to help just a little. I'm just wondering does anyone else know for a fact what works as a thermal barrier so hot sun isn't transferred in this tin can? I have a feeling it would work much better if I could park in the shade, but where I'm at, there is no shade to park under.
I don't want to spend a fortune on things like painting the roof with products like Silver Seal or TropiCool, and take down the metal on the ceiling and use some type of insulation if it's not going to do any good.
Anyone have any input on what they've done that worked to where they can park in the hot sunlight and a reasonable size AC unit can keep up?
So earlier I talked about the R ratings that the window AC unit is designed for. Now let’s talk heat transfer.
Heat transfer happens in three ways: radiation , convection , and conduction.
Radiation requires line of sight, and only heats surfaces. For your bus, radiation is hitting your roof, walls, and windows. It heats the surfaces of all three, but also passes through your windows and heats the surfaces in your bus. To combat radiation, you have options: 1. Create a reflective surface that bounces the light energy off your bus’s surfaces. This would be things like a cool roof (TropiCool or equivalent high reflectance/emittance paint), reflective coatings on the exterior of your windows (on the interior, the radiation will be absorbed by the glass on the way in and on the way out after it bounces off the reflective surface on the interior), or an offset shade structure that doesn’t touch the bus (an array of solar panels works well if they’re space on rails off the roof).
Once the surfaces are heated by radiation, the heat then moves through the assembly (skin, support structure, insulation, and interior skin) via conduction. That is how the interior of your walls and roof become hot. Those interior surfaces then transfer heat to the interior space via radiation and convection.
Convection is the process of transferring heat via a moving fluid, such as water or air. There are natural convective currents that happen as the interior surface of your bus get hot. The air against those surfaces warms up and begins to rise (air expands as it heats and becomes less dense, therefore it rises, like oil does within water). Those convective currents are why the areas up high in a house are hotter than the areas down low (anyone with a loft space has experienced this).
To prevent conduction through the walls and roof, one must slow down the transfer of heat. This can be done via insulation and by creating what is called a thermal break. If you took an infrared camera and took a picture of your bus’s ceiling or walls when it’s hot, you’d see that the support structure bridges the insulation and creates a rapid path for heat to go from the outer skin to the inner skin. The absolute best way to combat this conduction is to remove the inner skin, mount a set of wooden “studs” transverse to the bus’s structure (they would run lengthwise down the bus), and spray foam it with rigid, closed-cell foam. This will do a few things: minimize thermal bridging, create a much higher R-value assembly (R-5 per inch thickness), and create a vapor barrier because it is closed cell. That last one is really important to prevent condensation on the inside of your walls and ceiling. To see this process, check out Chuck Cassady’s YouTube page (
Chuck Cassady). That page is AWESOME for learning these concepts.
To prevent conduction through your windows, you need to change your windows. Remove the old bus windows, fill in the void with sheet metal, then install dual pane RV windows. Create the structure in the wall as I talked about above (see Chuck’s videos - they’re incredible), and spray foam around those RV windows.
Chuck also puts in 2-4” of rigid insulation above the floor pan and below the subfloor to create a thermal break there. I can’t recommend seeing his videos enough. His builds typically use 18-24k BTU/hr mini-splits for full-length buses using these methods.
If you have any questions about HVAC technologies, I’m more than happy to help. As I said in my last post, I’ve been an HVAC professional for over two decades, in design, construction, and forensic analysis of failed systems.
Obviously, if you’re already living and/or weekending in your bus, not all of these are viable options. More than anything, I wanted you to understand the thermodynamic principles of heat transfer in your bus so you don’t throw good money after bad. One thing I can tell you for certain that does not work - a radiant barrier does no good whatsoever if it doesn’t face the sun. All radiant heat transfer requires line of sight. Don’t pay for any material for radiant protection that doesn’t face the sun.
Hope this helps.
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