Floor construction: what lurks beneath

Naj Haus subfloor – doesn’t look like much…

Now that I’m about to leave to build my roof, maybe it’s time I caught up on how I constructed my floor! The three big take-away thoughts I had on the floor were:

1. Floors look deceptively simple, but they encompass nearly every aspect of building the entire house.
2. Because of #1, they are a good training ground for learning most of the skills you will need later on.
3. Because of #2, the floor will probably take waaaaaay longer than you think.

But I should insert a caveat here – my floor was perhaps more complex than most due to a) the way I decided to integrate the house and the trailer, b) my choice of insulation, and c) my perhaps neurotic concerns about moisture and mold. The following walks through why I made the choices I did and then documents how I went about creating my floor.

(Note: this is all pretty technical and matter of fact; if you want to read about all my agonies and ecstasies during this time, see my Blood, Sweat and Tears… post!)

Integrating the trailer and the house: two different approaches

The majority of tiny houses on wheels are built on flat-bed utility trailers. If you buy a standard one off a lot, it needs to be customized before you can build a house on it. Typically, the cross ribs are welded at the top of the side rails to support a wood decking that will need to be all or partially removed. The side rails are usually on the inside of the wheel wells, which, without further customization, limits the width of your house. Often there are stake pockets and other additions welded on that need to be cut off. Many tiny house builders start with this type of trailer design and build the floor frame to sit on top of the trailer, bolting it to the cross ribs or the decking.

I chose a different approach, one in which the floor frame is dropped into the trailer frame so that the top of the floor is level with the top of the trailer. I ordered a trailer from Iron Eagle Trailers, outside of Portland, OR, that was custom-built for a tiny house. Iron Eagle has worked with PAD and other tiny house designers to drop the cross ribs to allow the floor frame to be set into the trailer. Holes are drilled in the side rails to bolt the floor frame securely to the trailer.

My trailer from Iron Eagle. Note how the cross ribs have been welded to the bottom of the side rails rather than level with the top, and one of the pre-drilled holes for bolting the floor frame to the trailer.

There are pros and cons to both the sit-on-top and the in-set methods and you’ll need to decide which makes sense for you. I suspect the sit-on-top method might be more simple and straightforward to construct. Depending on who’s doing the trailer customizations, this might also be the cheaper method, though from prices I’ve seen I’m not sure that is always true.

I went with the in-set method since I liked the idea that the floor was more firmly integrated with the trailer by being enclosed by the trailer frame and really bolted in, both horizontally and vertically. PAD’s excellent Go House Go construction manual explains how this works. I also knew there was a good track record with Iron Eagle building trailers for tiny houses. See my post on buying my trailer for how awesome they were to work with.

Additionally, two huge advantages of the in-set method are that it allows more interior house height and room for insulation. With a tiny house on wheels, you are locked into a maximum road legal height of 13.5′ or 14′ depending on which state you live in. The top of the trailer frame is typically about two feet off the ground. If you build your floor on top of the trailer, you have that much less height in your house (this usually means less room in the loft). Also with the sit-on-top approach, you will need to decide whether to sacrifice more interior height in order to use 2×6 floor framing for the extra insulation, or use 2x4s which limits how much insulation you can have. With the in-set method, you don’t have to make either sacrifice. Since you are dropping the floor down 5 to 6 inches below the top of the trailer, you get both a good amount of depth for insulation and that much more room up in your loft, which can make quite a difference.

The “side pocket” extension that adds an extra 6″ to each side. The angle iron allows you to bolt the walls down vertically to the trailer, in addition to the horizontal bolts through the side rails.

The other customization Iron Eagle did was weld on side pocket extensions along with angle iron on the outer edge. This way I could build out to the full 8.5′ width. I’d visited several tiny houses and having that extra foot of room on the interior is really noticeable. Because I want to have eaves off my gable roof, I will exceed the 8.5′ road legal limit but this just requires getting a temporary permit when you take it on the road (as long as you stay under 10′ wide). Since I don’t plan on moving it frequently, paying $30-40 and giving a day or two notice seemed like a good tradeoff for the extra room year round. (Note that this is true for the states on the west coast; it may differ in other locations so be sure to check it out if you’re considering going this route.)

Thoughts on insulation and moisture control

Before you start your floor, you need to make decisions about how it will be insulated. The floor doesn’t necessarily have to be insulated with the same materials you are using in your walls and roof. I agonized over this for quite awhile since I wanted something that was as energy efficient as possible but I also had concerns about the environmental and potential health impacts of many of the insulation types on the market. In the end, I decided to go with sheep wool from Oregon Shepherd since it’s natural, renewable, non-toxic, doesn’t off-gas, resists fire and pests, and handles moisture well, plus it felt good to support a local company. Wool has an R-value of about R-4 per inch which is in the middle range of the other options out there. It costs a little more but the benefits were worth it to me. The only real downside is that it is rather time- and labor-intensive to install, since it comes packed densely in boxes and you need to “fluff” it to allow air space around the wool fibers, which is what gives it its insulating properties (there are machines that can blow it in but I’ve heard that these don’t always work well, though I may look into this more for the walls.)

Last April, I helped Derin Williams of Shelter Wise pre-build the floor and one of the walls for the Casa Pequena tiny house build workshop. Derin is an expert in sustainable building and has been exploring a number of ways to create very energy efficient tiny houses. One of his methods is to create a thermal break (which slows heat loss) between the metal of the trailer and the frame of the house where possible. I liked his idea of inserting rigid foam board around the inside edges of the trailer to create a thermal break from the in-set floor frame. It also has the advantage of reducing the chance of condensation accumulating on the metal edge where it might rot the  wood. So, while I had opted not to have rigid foam board as the primary floor insulation, it did make sense to use it in these targeted areas. I also used it to fill the side pocket extensions.

Naj Haus floor frame – the pink is the rigid foam board. Note how there is a 3/4″ layer between the trailer and the floor frame.

So I had made my insulation decisions, but that led to a quandary on the proper use of vapor barriers and/or vapor retarders. This is a complicated and changing field of building design, one that I will only touch on. Basically, you want to prevent moisture from getting trapped in your walls and causing mold and rot. Moisture can get in either via outright leaks from poor rain protection or plumbing problems, or via air movement when it is in its vapor state. The source of the vapor may be on the outside finding its way in, or from cooking, showering or other activities inside the house that generate steam or water vapor that moves out through the walls. Because of the their compact size, tiny houses are more vulnerable to moisture-from-within problems than larger homes. HUD has produced a great publication on the problems that can arise from moisture in manufactured homes, which is applicable to tiny houses. (Be forewarned, it might give you nightmares! On a side note, it makes an excellent recommendation for putting down a sheet of plastic beneath gravel under your house to prevent water vapor from rising out of the ground and up into your house.)

The Green Building Advisor has a set of related articles on how the focus should be not so much on adding lots of vapor barriers, due to the issues with condensation if not used properly, but on making an air-tight building package that doesn’t allow the moisture-laden air to get in from the outside. It’s interesting reading, though you may end up feeling more confused than ever about where tiny houses fall into the equation. There’s a nice summation at the end of the article on air barriers that’s worth reading. In short, at minimum use house wrap and make sure every seam and potential leakage spot is well sealed.

Oregon Shepherd recommends having house wrap on the outside, but no vapor barrier on the inside, in order to allow the walls to breathe. As I understand it, house wrap is essentially a directional vapor barrier and a vapor retarder (actually I think it is technically a vapor retarder with different permeance levels depending on which side is facing out). It resists moisture from entering from the outside, but allows moisture generated on the interior to pass through.

For the floor, which can’t have house wrap on the outside since it would be shredded when on the road, I had to come up with some other solution. Many tiny house builders use metal flashing to line the underside of the floor frame. This has advantages in terms of protecting against flying rocks and debris on the road, preventing rot of the undercarriage, and discouraging insect and rodent incursions, but it also seemed like it might trap any moisture that might come from the interior. Since warm, moist air rises, this might not be a real problem unless you have a plumbing leak or are generating an excessive amount of moisture inside.

My solution: lining the interior of the floor frame with house wrap that is super tightly sealed from outside air. Note the purple rigid foam board that was later cut and placed between the floor frame and the trailer.

I decided to use marine-grade 1/4″ plywood for the undercarriage that was painted with a couple coats of oil-based paint. I then lined the floor frame with house wrap, made sure it was tightly sealed, added the wool, then attached the subfloor. My hope is that the plywood and house wrap will keep air and moisture from entering from outside but will allow any moisture from the inside to work its way out. There is a possibility that moisture  from the inside could collect between the house wrap and the undercarriage if the paint is really acting as a vapor barrier, but I decided that since I didn’t have any plumbing and am taking pains to reduce inside moisture from cooking and other activities, that this was probably a low risk.

Right before adding the subfloor plywood, hoping the wind doesn’t suddenly pick up!

The actual steps of how I built the floor are below in case they are helpful to others. However, since I am not an expert in construction and moisture control, you should do your own research and get professional advice on how to design your house shell. Your approach will likely vary given your particular climate; whether or not you have plumbing; your choices of heaters, air conditioners and other appliances that might affect interior moisture levels; and whether you have proper fans and vents  to handle water vapor.

Constructing the floor

Click on a photo to enlarge it. You can then click on the left and right arrows to scroll through the pictures in the viewer mode.

Ha! So now what do you see when you look at the finished floor? Not quite as simple as it first appeared, eh? After all that work it was extremely gratifying to have it feel really solid.