Knowledge guide

Everything You Need to Know About Insulating a Cargo Trailer

Everything You Need to Know About Insulating a Cargo Trailer

Insulation is a commonly overlooked aspect of a DIY cargo camper conversion, and that's somewhat understandable.

You get your trailer, and you're anxious to get to building the fun stuff like flooring and cabinets.

However, neglecting this critical process can make the difference between enjoying your trailer in anything less than the most ideal weather conditions.

Check out our video below for details on our own trailer insulation.

Should You Insulate Your Cargo Trailer?

To insulate, or not to insulate? THAT is the question!

Answer: Yes. You should definitely insulate.

Think about it. Would you ever consider buying a home or renting an apartment with nothing but a thin layer of metal between you and the elements? No!

There is no single RV or camper on the market that isn't insulated, and there's a reason for that. Insulation allows temperature regulation. Without it, the interior of the shell is susceptible to all different kinds of heat transfer.

Even the smallest amount of insulation makes a noticeable difference in regulating the temperature inside a trailer.

To understand why insulation is essential, you must first have a basic understanding of how heat transfer works.

How Insulation Works

Science Class: Radiation, Conduction, Convection

You may remember from grade school that there are three primary forms of heat transfer: radiation, conduction, and convection. All of these types of heat transfer affect the inside temperature of the camper.

Here is a fantastic cartoon that outlines in practical words what these science words mean:


In technical terms, radiation is heat transfer in the form of electromagnetic waves. The obvious example that matters to us is heating from the sun. The trailer sits in the sun; it gets warm. Simple as that.

Conduction is also pretty simple. Heat is transferred from one material to another. The example from grade school is a boiling pot whose handle gets hot. In the context of a camper, the outside of the trailer is hot from the sun (radiation), and that transfers through the layers in the wall to make the inside layers hot.

Convection is a little more tricky but still pretty simple. Heat is transferred through the air to heat up other air.

In a camper, once the walls are warm from the sun (radiation) and that heat is transferred into the inside through the walls (convection), the adjacent air molecules are heated up and will then heat up other air molecules. Thus, it gets hot inside the camper.


In these examples, we are talking about heat transfer into the trailer.

Imagine a hot day in full sun. In this context, we care about keeping the inside cool, and we accomplish that by limiting the amount of heat that makes it into the trailer that will fight with our cooling solutions such as AC or fans.

However, this also matters in the context of keeping the trailer warm. When it's cold outside, we want to keep the heat inside the trailer.

Convection happens when the warm air inside the camper mixes with the colder air and cools it off. Conduction occurs when the warm inside walls heat up the cold outside walls, and heat escapes that way.

Radiation is a little more challenging to explain in the context of cooling but knows that all materials give off radiant heat when they are warm.

So why does all this matter when it comes to insulation? Well...

Insulation Slows Down Heat Transfer

Simple as that.

Think about it. The thicker material is, the further heat has to travel. The type of material also dramatically affects heat transfer.

That's why there are so many different types of insulation. Some materials are of various densities and have air pockets and try to maximize insulation by attacking all three types of heat transfer. Read about how closed-cell foam insulation works if you really want to see how complex and interesting insulation is.

The good news is someone along the way figured out that we mere morons can't understand all that and created a simple method to measure how good an insulating material is: R-value.


R-value is a simple measure of how resistant a material is to heat transfer.

Higher R-value, better insulation. Easy as that.

So next time you are at Lowes or Home Depot, take a look at the insulation section and note how different types of insulation have different R-values.

Thermal Bridging

One more important concept to understand is thermal bridging.

Heat generally follows the path of least resistance. Meaning, if your walls are insulated but there are exposed beams that contact the outside walls, heat will make it inside through those beams.


You reduce thermal bridging by covering up every exposed surface you can with some material to facilitate heat transfer.

In a cargo trailer, that means putting insulation between the steel studs and on top of them. This adds a little complexity to the build because you have to cover your studs with something.

Types of Insulation for Cargo Trailers

Ok, so now that we understand how heat transfer works and how insulation quality is measured using R-value, let's get to the good stuff.

What is the best type of insulation for a cargo trailer? Honestly, it is a matter of opinion. A few types of insulation are worth considering in a cargo trailer, and a few that are not even worth thinking about.

Rigid Board: XPS or Polyisocyonate

Rigid board is by far the most commonly used type of insulation in a cargo trailer. It's cheap, it's square, and it's available at virtually all big box stores.

There are two kinds of rigid board: XPS and Polyisocyonate (generally referred to as "polyiso").


XPS is the pink board with a pink panther on it at Home Depot. At Lowes, it's the lime green board.


XPS has an R-value of 5.0 per inch at 75F and 6.0 per inch at 15F. Meaning, it works a little better in cold weather as compared to hot weather.


Polyiso looks pretty much the same everywhere. Polyiso is sort of shiny and reflective on one side and generally white on the other. Put the shiny side out!


Polyiso has an R-value of 5.6 per inch at 75F and 5.0 per inch at 15F. It's the opposite of XPS in that it's better for warm weather.

Pros of Rigid Board

  • Cheap
  • Impermeable to water vapor
  • Readily available
  • Nice and square, so they fit neatly between the wall studs.

Cons of Rigid Board

  • Creates air gaps on uneven surfaces, which can be an ideal place for water to get trapped and form mildew
  • Decent R-value, but other solutions have better insulating properties.
  • Stiff material that can't be stuffed into cracks and crevices
  • Need additional layer of something over the steel studs to eliminate thermal bridging
  • Will definitely blow off the top of your 4Runner if not adequately secured. Don't ask me how I know this.

Bottom Line & Recommendations

XPS and Polyiso are excellent choices for cargo trailers. If you go this route, consider these tips:

1) The biggest concern is that you can end up with leaks and mold if you do not adequately seal up the walls and fill the cracks and crevices. If you go with a rigid board to insulate your cargo trailer, go above and beyond to seal up all the gaps. Consider going over all visible interior seams with window-grade silicon or caulk before placing your rigid board.

Photo credit:
Photo credit:

2) Use canned closed cell foam such as Great Stuff to fill all the cracks and crevices around the board's edges. Leave no gaps exposed!


3) Reduce thermal bridging by covering your studs with additional insulation. Good options are fan-fold insulation, like an R-value of 1, an extra layer of rigid board (thicker the insulation, the more interior space you lose), or other thin materials like Thinsulate or Relectix. When it comes to limiting thermal bridging, R-value is less of a concern than simply breaking up the connection between materials.

I used spray foam in my trailer build (read more below), but I would definitely consider rigid board a viable option, and I highly considered it. If you go this route, use the above advice for a perfectly insulated trailer!

Spray Foam Insulation

There are a LOT of opinions about spray foam insulation. When I began researching insulation, I kept coming across folks who either loved or hated this solution.

Much of my research was from the van life community, which is incredibly opinionated about spray foam.

I came across some professional van installers who swore that spray foam distorted the panels of vans every time they had used it. And other folks say that mechanics refuse to work on vans that have spray foam installed.

Regardless of these opinions and personal experiences, there are a few facts that I simply couldn't ignore and ultimately led to us choosing spray foam as our insulation method.

Let's look at spray foam as a potential method of insulation.


Closed-Cell vs. Open Cell Spray Foam Insulation

The first important distinction about spray foam insulation is that there are two major types: closed cell and open cell.

In open-cell foam insulation, the tiny cells of the foam are not entirely closed. During the expansion process, air fills the open space inside the material. This makes the foam softer and squishier feeling than closed-cell foam. I am not even sure if it would be possible to use open-cell foam in a trailer, given how much it expands.

In closed-cell foam insulation, all of its tiny foam cells are closed and packed together. They are filled with a gas that helps the foam rise and expand and become a better insulator. The final dried project is tough and feels dense and hard.


Long story short: closed cell is the only product to consider. No exceptions. It's space-saving, more bang for your buck, and functionally superior to open-cell foam.

When thinking about spray foam insulation for a cargo trailer, you effectively think about closed-cell foam exclusively.

For the remainder of this guide, I mean closed cell spray foam insulation when I say spray foam insulation.

Advantages of Spray Foam Insulation in a Cargo Trailer

Spray foam has an R-value of 6.5 per inch, one of the best if not the best R-values per inch of any material on the market.

In addition to being effective insulation, there two more considerable advantages in the context of a cargo trailer: structure and water sealing.

I had read about how much foam would change the structure of a cargo trailer, but I was still shaken by just how hard the walls became. The previously flimsy metal walls became solid, rock-like, dense walls that virtually do not bend or flex at all. It legitimately made the aluminum walls feel like concrete panels rather than thin aluminum.

In addition, because spray foam is sprayed as a liquid and rapidly becomes a solid within seconds of making contact with the wall, it fills every crack and crevice. Literally no area is left exposed when properly applied. This is huge for a cargo trailer because no surface is genuinely level or square from the factory.

The water-sealing effect is a massive improvement over rigid board as it's virtually impossible to get mold in your walls behind spray foam insulation.

We also took advantage of spray foam to stick to the underside, giving you an insulated floor.


Disadvantages of Spray Foam Insulation

This material is not without its drawbacks. There are reasons not to consider using it.

Firstly, we can't ignore that many people have had negative experiences with it, having distorted and bent the metal panels and ruined the shape and look of the exterior. Given, most of these folks were applying spray foam to a van rather than a boxy trailer.

Closed-cell foam works by combining two different chemicals that cause the expansion and hardening of each other. The reaction is thermogenic, meaning it creates heat. Heat and aluminum are not suitable dance partners, and it's easy to see how aluminum under tension getting warm could lead to warping.

My response to this in the context of a cargo trailer is: who cares?

Trailers are giant metal boxes. They are already warped. It doesn't matter if it becomes more warped. And in my personal, n=1 experiment, I didn't even have this problem. But my foam was applied in an indoor setting by professionals. Something to consider.

Secondly, something I learned that I totally wasn't expecting when I had my foam sprayed professionally: there's a technique to applying this stuff.


You don't just spray it in there like a can of spray paint. The professionals had an eye for it and knew when to stop to let it rise, how fast and thick to spray it in certain areas, and just how thick it would be once dry. I definitely would have goofed this up. Watch our video to see just how they did it.

Lastly, spray foam is a hugely messy job. It literally gets everywhere. It takes a ton of taping things off because once it's there, it's there. It will scrape off if necessary, but once it's dry, it's pretty hard to cleanly remove.

If I ever have to rewire brake lights or replace any functional trailer wiring (not my camper wiring but like the running lights and stuff), I will just have to chop the wires at either end and run new ones because those factory wires are buried.

DIY Spray Foam Kits

If you're considering spray foam insulation in your cargo trailer, you have inevitably come across kits to do it yourself. Common ones include:

  1. Tiger Foam - Bought directly from the manufacturer
  2. Froth Pak - Generally available at Lowes
  3. Touch 'n Foam - Generally available at Lowes or Home Depot
  4. Other Brands available on Amazon
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My advice: don't do spray foam insulation yourself. This feels like blasphemy.

After all, you are building your own camper, for crying out loud. It's not like you can't handle it. Well, I disagree. And after I've seen it done professionally, this is a prime example of the Dunning Kruger Effect in action.

First off, you have to calculate precisely how much you need. There are calculators online, such as the one on the Tiger Foam website here.

The problem is, the kits only come in pretty large increments. Meaning, if you calculate you need 800 sqft worth of material, and then you don't get the thickness precisely right, you can run out of foam with one corner left and be forced to buy a whole other kit just to do a small space.

Secondly, as I mentioned above, the technique the professionals use was impressive. I'm not saying that you wouldn't pick it up after a few mins of doing it, but I guarantee you won't spray it in as neatly as someone who does it all day, every day.

Next, these kits come with two big cans of chemicals. Watch some videos on how to use a DIY kit. You have to shake them both up, keep them at set temperatures, and then test to make sure they are mixed equally.

If by chance, in the darkness of your spray job, you don't realize that they are not mixing equally, you can end up spraying a ton of one can but not the other. Resulting in a goopy mess of chemical foam that will not harden.

That's what happened to Ryan when he sprayed his Cargo Trailer.

Lastly, we have to talk about the cost because that's really the whole reason we are building our own camper, right?

The fact is, you may not save any money using a DIY kit. Professional insulation installers buy huge drums of chemicals and use trucks dedicated to operating spray equipment. When you calculate how much spray foam you need, you may find that a professional installation is comparable to the DIY kit. That was certainly the case for me.


I had spray foam insulation installed in my 7x14' cargo trailer by Mullins Insulation in Kingsport, TN, for a flat $1000. I calculated $800 of DIY kits and materials to do it myself. By paying $200 more, I didn't have to deal with the mess, got a super clean job done, and quite frankly had no stress about the whole job.

Have spray foam insulation professionally installed.

Pros of Spray Foam Insulation

  • Excellent R-value per inch
  • Rigid and solid. Will give additional strength to trailer walls
  • Water sealing and conforms to surfaces
  • Fills crooks and crannies
  • Easily covers metal studs, reducing thermal bridges
  • Can be sprayed on the underside, insulating the floor

Cons of Spray Foam Insulation

  • Messy. It will literally get everywhere.
  • More expensive than other forms of insulation
  • Could possibly distort metal walls
  • May not be a great DIY job. Consider professional installation.

Bottom Line & Recommendations

Overall, we are incredibly pleased with our decision to use spray foam insulation. Here is our advice regarding the decision to use it:

  1. Have it professionally installed. We've covered this in-depth above.
  2. Install furring strips first. I cover this in detail in our two videos about installing spray foam and installing furring strips, but here's the gist: It's hard to attach furring strips over the top of spray foam because you can no longer see the metal studs once they are covered in foam. I used a screwdriver to find them, and it was a massive pain in the ass. If I had to do it again, I would install my furring strips first and ask the professionals to do their best to leave them uncovered in foam. There will inevitably be overspray, but it can be easily scraped over with a razor or belt sander. If you choose to install furring strips first, be sure to fill the hollow studs with some canned spray foam like great stuff, and consider using a thin piece of Reflectix or other insulating material between the furring strips and steel studs.
  3. Have the underside sprayed to insulate the floor. They sprayed 3-4 inches under our trailer, and it made a noticeable difference. If you have the underside sprayed, consider installing your gray water tanks first. We didn't do this and will have to scrape out space for them when we get to that point. If they were already installed, they could have been covered in foam, giving the water tanks a little protection from the elements. Maybe that's not a good idea, though. I don't know.
  4. Tape everything off really well first. We were lucky that the pros did an excellent tape job before starting, but maybe don't trust that to them. Be prepared to cover the backside of the wheels as to not get any foam on the brakes.
  5. Do any job that requires cutting holes in the trailer first. Windows and fans, obviously, but also consider the smaller jobs like the shore power plug and the water inlet. I didn't do this and, as such, don't have a great way to build a little wooden frame to attach the plug and inlet to. A small problem I will have to solve now…
  6. Can you spray foam the doors? I am not sure. Maybe. Though after having insulated those myself using XPS board, I could see that being a problem. If doing it again, I'd probably stick to XPS or polyiso for the doors just for simplicity. They were a big enough pain to disassemble and reassemble as it is. Although I don't believe doors are built the same on all trailers, it's something to consider.


Rockwool is a material that I have only recently become familiar with, but it's a favorite among the van life community.


Rockwool comes in sheets similar to traditional fiberglass insulation and is an excellent option for insulating a cargo trailer. Also known as mineral wool, it's a rock-based mineral fiber insulation comprised of basalt rock and recycled Slag. Basalt is a volcanic rock abundant in the earth, and Slag is a by-product of the steel and copper industry. The minerals are melted and spun into fibers.

It's resistant to water, rot, mold, mildew, and bacterial growth and completely fills wall cavities to eliminate gaps and voids. It also has a pretty good R-value of 3.0 per inch.

Rockwool is also an excellent sound-dampening material. With its naturally dense, non-directional fiber structure, insulating with this material will trap soundwaves and reduce vibration to provide enhanced noise reduction. Talk about a good night's sleep!

Wear protective clothing and a face mask whenever you work with Rockwool, as the installation can be quite an itchy experience.

Another hazard is that inhaled Rockwool can get trapped in your lungs. There have been some indications that this can lead to serious health problems. Not nearly as much as fiberglass insulation. But still not something to ignore.

Pros of Rockwool

  • Water-resistant
  • Fits into nooks and crannies
  • Environmentally friendly
  • Sound dampening

Cons of Rockwool

  • Expensive
  • Not a great R-value for the space it takes up and cost.
  • Pretty thick stuff that will have to be squished
  • Itchy process

Insulation Types You Definitely Do Not Want to Use

Fiberglass Insulation


Classically used in homes, this is probably the first type that comes to mind when you hear the word "insulation."

Although it has an R-Value of 3.2 per inch, fIberglass is not a good choice in cargo trailers.

Fiberglass insulation absorbs water, grows mold when it gets wet, puts particles in the air that are known health hazards (the jostling of movement causes this), and it's itchy and painful to install.

I cannot for the life of me figure out why this material is standard in campers.

Wait, yes I can: it's cheap.

Blegh! Don't use this.



Why does this material always come up?

Reflectix is a terrible insulation option. It's basically bubble wrap coated in plastic tinfoil.

It has an R-value of 1 per inch. ONE.

Use it as a window cover and as a minimal thermal bridge.

"Blown-In" Insulation

Usually, people are referring to cellulose material when talking about blown-in insulation.

I cannot imagine that this would work in a constantly moving trailer.

This material absorbs water, is a reservoir for mold, and won't stick to the walls nicely. Use other materials.

The Best Insulation for a Cargo Trailer

If you've made it this far, I hope you will see that the best insulation is somewhat of a matter of opinion. There are definite pros and cons of lots of different types of insulation.

In our opinion, some combination of these types of insulation is the way to go. After having insulated our own 7x14' cargo trailer, these are solutions I would consider:

Walls and Ceiling

Option 1: Seal all seams with caulk. Use XPS or polyiso rigid board between all-steel studs. Use an additional layer of rigid board over top of studs to reduce thermal bridging or use a thin material such as Reflectix or foldable thin XPS. Fill all the cracks with canned spray foam.

Option 2: Spray foam the whole thing with closed-cell foam. Have it professionally done. Install your furring strips on top of your steel studs first and have some thin material behind the furring strips to act as a thermal bridge.


Rigid board the interior. Fill all seams with canned spray foam and seal every crack possible with caulk.


Option 1: Cut out small rigid board pieces for the slots and glue or tack them to the underside of the floor. Use canned spray foam around the edges. This is a labor-intensive job for sure.

Option 2: Have the underside professionally sprayed. This is the option we went with.

Final Thoughts on Insulating A Cargo Trailer

We are thrilled with our spray foam-ed cargo trailer. If we had to do it again, I definitely think we would choose to go the spray foam route.

We would do a few things differently to make the next steps easier (outlined above), but overall we are incredibly satisfied.

I hope this guide was helpful for you in your own cargo trailer adventure.

Let us know in the comments below how you decided to insulate and if you agree with our recommendations above.

Hit us up if you have questions, and good luck!

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