Practical Container Home Vapor Barrier Advice From The Sunshine State

Vapor Barriers in Container Homes? - UPDATE

3/22/22 Here is an update on this blog from Sterling Cox.

While I agree with his update, another solution we have found to be very effective is to simply use spray foam insulation which adheres to the container directly and gets in every nook and cranny, thus eliminating the need for a vapor barrier. However there is a cost implication to that choice, so Sterling offers up another take if spray foam isn’t the way you want to go.

When I built my container home back in 2014, I was figuring out pretty much all of it on the fly. There was one aspect of the project that was completely foreign to me: vapor barriers.

My neighbor Randy was a general contractor, and he said that all the shipping containers that he had used for storage would sweat on the inside. He suggested a vapor barrier as a defense against this condensation build up. It’s important to note that Randy only had experience with containers that were unmodified. No windows, no insulation, no people going in and out.

So I did some research and learned what I could. We bought some Tyvek house wrap and used spray adhesive to glue it to the interior walls of the container directly to the metal. I was convinced we had taken an important step towards making a well-built container home.

Since vapor barriers in container homes are a contentious subject, Julia asked if I would write an article about my experience. I love any opportunity to sound smart so I agreed.

Now, I’ve learned a lot since 2014. I can step inside my container home (it’s on our property in Georgia) and point out dozens of things I’d do differently today. It’s not exactly a Little Rascals clubhouse, but it’s amateur. But one thing that we really missed the mark on was the vapor barrier. I had it all wrong, because I didn’t understand the principle behind it. And I realized that many people, including professional builders, don’t either.

First off, Tyvek house wrap is not a vapor barrier. It always goes on the exterior, and it’s actually designed to let moisture escape. Secondly, it’s used on “stick-built”, wood frame houses with OSB (wood) sheathing which is totally different from the continuous and impermeable metal of a shipping container home. This means that the outside of the container home already is essentially an external vapor barrier.

A vapor barrier can go in only one of two locations in a standard home: in the interior of the home between the insulation and the finish wall (usually drywall), or on the exterior between the sheathing (usually OSB or plywood) and the cladding. Where it goes depends on what climate the house is in. Cold climates = interior. Warm climates = exterior.

A vapor barrier’s purpose is to block moisture, in the form of water vapor, from migrating into the wall cavities. There it can cause mold and other moisture-related damage. For this to occur, the air already has to have moisture in it. (So, hot arid climates arguably don’t really need a vapor barrier at all.)

Admittedly this has been an over-simplification of a complicated subject.

So, the question remains, do you need a vapor barrier in your container home? This is an excerpt from my book Making a Simple Container Home:

“In my opinion, it would only be effective to add a vapor barrier to your container home if it is going to be in a climate where it gets pretty cold and it is going to be occupied for long, continuous periods of time during the cold season. In this case, the vapor barrier would go over the interior side of the insulation, behind the interior wall paneling. Even in this case, it’s still debatable because you can’t have a vapor barrier on both sides of the wall.”

How do you prevent moisture from accumulating inside your container home?

Do an excellent job sealing around all the penetrations. This is very important.
Keep the windows open during nice weather.
While unoccupied, keep a window or two cracked to allow air flow and temperature equalization.
Avoid extreme differentials between inside and outside temperatures for extended periods of time.
Use air conditioning or a dehumidifier.

I hope this article is helpful to you.

Original Post

We are lucky to have another guest blogger today! Today’s post comes from a Super Cubes client in Florida, Sterling Cox, founder of Southworth LLC, who is converting a one-trip 20' container into a prototype living space, to be used as a production model. You can follow the pictures they are posting to Instagram at @swlivingspaces. Like so many other container projects, they are balancing their goal of using green building practices with being cost-conscious and also energy efficient. They recently added a vapor barrier to the container before insulating it.

Tonight's process:

We adhered a layer of Tyvek Home Wrap to the inside of the container for the vapor barrier using 3M all-purpose spray adhesive. This process was difficult with two people, and I imagine nearly impossible with one.

According to everything I read from what I determined to be credible sources, and my neighbor Randy who has been a contractor most of his life, a vapor barrier is essential. Containers are apparently notorious for condensation build-up on the inside or “sweating,” especially in humid climates. A vapor barrier blocks moisture from entering a home. It can be a foam or some other type of insulation, or more typically, a film or sheet. Conventionally the house wrap, Tyvek or another brand, is applied to the outside of a “normal” home being built, stretched taught and stapled in place before the siding goes on.

Reasons why we chose Tyvek:

It is the easiest to obtain through a retailer, and by far the cheapest option. The spray adhesive is necessary to have the film adhere to the wall as it gets rolled on.
To extoll the green benefits of our decision to go this route, I’d say the wrap itself is recycleable, a #2. This is a polyethylene, same as a milk jug, shampoo bottle or Smart Balance tub. The apparent R-value is non-existent, being that it is the thickness of construction paper.
The simple fact is that it was a necessary step in construction.

Why it is non-green:

Tyvek is manufactured by DuPont, a large chemical corporation, one that is undoubtedly manufacturing many products that, though useful, have difficulty diffusing into the ecosystem as it were. So be it. Sometimes awareness is all that you can offer.
The spray adhesive is manufactured by 3M, and simply by its odor and effectiveness, it is apparent that it is far from a green product. They advertise low VOC, though. Note: if you use this or any other spray adhesive in a container, cover your hair and wear long sleeves and pants. Wash your hands with mineral spirits afterwards.

If I had to go back:

I’d use Typar or HomeGuard house wrap. Both companies claim to have recycled content in their product, are part of the Green Building Council, and at least mention LEEDS in their respective websites. Another credit is that both companies actually address some sort of environmental awareness, whether or not it’s a marketing ploy. Note: both brands are more expensive than Tyvek.

As far as adhesives go, I’d order one that was from a smaller corporation, and had some sort of green credential. In my research, I find that at least attempting to sound green deserves a little credit. As I mentioned, being under a time constraint, we opted to use what we can get at the store as opposed to waiting for an order to be delivered.

Really if I had to go back:

I’d do some more research and find a spray-on vapor barrier. Naturally I’d look for something green, but even a petroleum-based product would be justifiable. The FoamIt Company manufactures a DIY spray foam that is apparently soy-based, but after reading the fine print it seems a bit dubious. No matter, any soy insulation is actually petroleum-based with soy content added. This kit offers 600 sq. ft. of 1” foam, which is very close to the amount needed to cover the two long walls, back wall, and ceiling of a 20’ container. It’s closed cell, so it’s an effective vapor barrier. The 1” of foam has a rating of R-7, which is a good start in conjunction with another type of insulation. The kit is $630, so cost is a factor. After you begin spraying, it’s apparently unusable after a month.

When all is said and done, I’ve spent under $150 to have a vapor barrier that is trusted by industry professionals and experts. I detested every minute of the application, the aesthetics of it severely irritate me and I honestly cannot wait until it is covered by insulation.

Postscript:

Certain types of readily available primers such as Zinsser’s BIN or Sherman William’s Moisture Vapor Barrier have acceptable perm ratings per most building codes, between 0.1 and 1 perm. Really really going back, I’d use something along those lines.

Another option I only briefly explored, but might end up being a very effective option would be to use some sort of spray on bedliner used to coat the beds of pickup trucks such as Rhino Liner.

You can email Southworth LLC at swlivingspaces@gmail.com.

Have you installed a vapor barrier on a container? What worked and didn’t work for you? What advice would you have for someone planning on applying one?

Below are some pictures from the project.