How did we insulate our bus conversion for four-season efficiency?
The difference between insulating a bus conversion well and skipping that step altogether is equal to the difference between livable and not so livable space.
Insulating our bus conversion was one of the first major interior projects after the demolition process. It's without a doubt one of the most important parts of the entire bus build.
If you really want to adventure, I’m talking scorching deserts to white peaks, you need to be prepared for any and all weather. It all starts with meticulously insulating your space.
See What’s Inside
After removing all our old panels we got to see how much of a crap job was done to insulate the bus during the manufacturing process.
Squares of 2” fiberglass insulation sliding around with major gaps at the edges, yuck.
I didn’t even know they made fiberglass batt this thin! Clearly we were going to have to start from scratch.
There’s more than one way to go about insulating a bus conversion or tiny space of course. With a bus you’re working with thin walls and a whole lot of conductive steel.
I want to preface here by saying this was my first major project during the build and the only thing I knew about insulation is that it was itchy (the fiberglass batt kind). To the internet and YouTube I went!
Radiant barriers, XPS, ISO, EPS, closed cell SFI, open cell SFI, fiberglass, you get the point. There was a lot to choose from and each one claimed they were better than the last. At the very least I knew it was important to find the highest r-values per inch for our thin walls and ceiling. While that was true, I came to realize I could do better if I also incorporated a radiant barrier.
Let’s rewind for a moment and talk about the different factors involved in insulating. Before we get into this you’ll need to pause here and get a formal six year education in classical and statistical thermodynamics…(Jeopardy theme song)...Great, your back! Okay, let’s start with the different forms of heat transfer: Convection, conduction, and radiation.
Convection: Heat is transferred through the movement of matter in the form of a fluid. Example: Water swirling in a pan before boiling and air movement in the atmosphere creating wind.
Conduction: Heat is transferred via direct contact of two materials. Example: A hot handle of a wood stove burns your hand, OUCH!
Radiation: Heat is transferred through electromagnetic waves. Example: Sunlight through windows heats the inside of a car or bus.
In our universe all heat is trying to reach equilibrium and will move from a hotter temperature to a lower temperature. In other words, if it’s 95 degrees outside the universe wants it to be 95 degrees inside. Since we can’t argue with the universe we can never really stop this from happening, but we can slow it down in the form of, you guessed it, insulation.
Most insulating materials are designed to slow the conduction of heat, and is described in r-values. The higher the r-value the slower heat will conduct through it. However, they are not so good at blocking heat in the form of radiation. This is why you can have a well insulated attic with high r-value materials but it will still be hotter than the outside ambient temperature on a sunny day. That’s where the radiant barrier comes in.
Reflective surface insulation (essentially plastic bubble wrap with an aluminum polyester coating) can be used to block 90%+ of radiant heat. I’ve seen these types of insulations used in so many bus and van applications but one of the most forgotten aspects of this installation is the air gap.
There NEEDS to be a ¾” or greater air space between at least one side of a radiant barrier for it to work. Although aluminum works awesome at reflecting and not emitting radiation, it IS, however, a great conductor of heat. If it’s touching two surfaces without an airspace it will simply conduct all the heat you’re trying to block right through it.
What about Convection?
You might have noticed I’m not really talking at all about convection. Well, that’s simply because I didn’t really do anything during the insulation process that does or does not affect convection.
Not to say someone couldn’t play around with this concept and use some form of convection to their benefit. I just can’t think of a good application other than heat shields around a wood stove, but that doesn’t really add a heat benefit, just safety. If you have any creative ideas please share them below.
Our Bus Conversion Insulation Process
Okay, so we know we need to insulate to slow conduction AND we want to create a radiant barrier to reflect radiant heat. But what products are we going to use and how are we going to apply them?
Here’s what we did: I’m going to focus on the ceiling right now since that’s where you’ll have the most heat loss and gain. First thing to do was remove all the interior panels. After this we could see there was about 2” of space to work with before you go beyond the frame ribs.
We chose to start with 2” XPS rigid insulation here. It boasts an r-value of 10 which is pretty high for just 2”. However, the key word here is rigid, which meant it came in a flat, unbendable sheet THAT WAS BY NO MEANS BUS CEILING SHAPED!
Cutting for the Curve
I had to cut over 200 individual pieces to follow the curve of the bus roof. Each piece was placed and glued ¼” apart from each other then Great Stuff Spray Foam was used to fill in the ¼” gap. Yeah, I didn’t really think this first project through very well. It took three weekends to complete and was extremely messy. By the time it was finished it looked like the inside of some xenomorph alien cocoon.
IT WORKS SO WELL THOUGH. So I guess that's a positive. 🙂
No matter which way YOU choose to insulate your bus, Great Stuff Gaps and Cracks is going to be involved. I wish I would have counted how many cans I bought because it was a lot more than I thought I was going to need.
I probably made five plus trips to the hardware store and each time thinking I had purchased enough. PLUS, if you pause for too long the nozzle clogs and the can is wasted. It would have made a lot more sense to buy the professional level cans in bulk and use their reusable spray nozzle.
Next step to insulate our bus was the radiant barrier. We used Reflectix double sided insulation here. First a layer was glued and stapled to the xps covering the entire ceiling. It’s important to use a foam safe glue when adhering something to XPS, other adhesives can actually melt the foam.
Make sure you tape the seams with reflective heat tape, not insulation tape. You’ll otherwise lose the radiant reflection in those areas.
Next I took 1” thick wood blocks and tech screwed them to the frame ribs. These would serve as the spacers to achieve the air gap needed for proper radiant reflection.
Lastly I used more of the Reflectix and glued them to the backs of the interior panels, then tech screwed them back to the ceiling. At the time the bus was still painted its original prison black and it was one hot summer. If you touched the outside of the bus it would burn your skin. After the insulation went up it was completely cool to the touch on the inside even when the sun was beaming down.
We're getting somewhere!
I built out the walls to about 3” and simply went with 3 ½” thick fiberglass insulation with an r-value of 13. We didn’t go to crazy with insulating the walls since all our windows would pretty much counter any extra efforts put into making them better.
Insulating the Floor
I did choose to insulate the floor on our bus conversion, something only short people have the benefit of doing unless you do a roof raise. Luckily Meag and I are both 5’6”; well I’m 5’6 ¼” and Meag is 5’6”. Yes, it matters.
I used 1 ½” XPS glued to the floor then ½” subfloor ply glued to the XPS and tied in with L brackets at the walls.
Our flooring is ¾” reclaimed maple and with the ceiling dropped an extra inch we only had 6’ clearance at the center of the bus. No one’s doing gymnastics in our space. I don’t regret the height loss at all, though.
Our floors are still pretty chilly in the winter but I can only imagine how cold it would be if we didn’t have any insulation. Expert tip: Wear cozy slippers.
What about the Windows?
At this point the biggest areas of heat loss and gain are the windows.
We could have chosen to lose some and replace them with paneling but we wanted to keep as much light in our tiny space as possible. Besides, it’s wonderful being able to open all the windows and doors on a summer day.
We don’t have any air conditioning, but with a little breeze and the windows open the bus stays relatively comfortable even on hot days. No, we can’t be in Arizona when it’s 100 degrees, but I don’t know anyone who’d want to do that anyway.
What I’d do Differently
Don’t get me wrong, our insulation works great, but if I could go back in time there’s a lot I would do differently.
First off, I would spring for closed cell spray foam insulation walls to ceiling and all the ribs. The reason I initially shied away from it was because it’s very expensive. Tanks, spray guns, and a professional application can run into the thousands. But after spending way too much time cutting and installing the XPS I think it’s worth it. You also get a little more r-value per inch, around 6.5. And it coats the ribs of the bus which is one area that conducts a lot of heat.
I’ve seen videos of some people spraying foam inside holes they drilled in the ribs. The idea is to fill the air space with foam and insulate the ribs. Don’t waste your time with this. Heat simply conducts through the steel around anything sprayed inside.
I still highly recommend the radiant barrier, it makes a huge difference. However, I think the second layer might have been a little overboard. Yes, it added R-1 to everything but it isn’t enough to justify the cost. Only one layer with air space on one side is necessary. Plus it would have been time saved.
I think a radiant barrier would have been beneficial on the walls as well. This is especially true at sunrise and sundown when the sun is hitting the side of the bus the most.
Lastly, and I’m still thinking about what to do here, I want to do something about the windows. I don’t want to cover any of them permanently but perhaps some removable insulated panels at night or really hot sunny days. The problem is the hassle of putting up and removing them daily and then of course storing them out of sight. Curtains have helped but are no match for Northeast winters. If anyone has any creative ideas out there, share it below.
There you have it my friends, the space between the walls. Good luck out there and stay frosty. Wait, no, stay toasty. Wait, no, stay both frosty and toasty depending on your meteorological situation.
Check out our post about our tiny wood stove keeping us cozy year-round!