Don't Try This on Home: Wall-over-wheels
Well, I guess that brilliant wall-over-wheel idea I had wasn’t so brilliant after all.
When I built my first little house on wheels, I framed the walls over and around the wheel wells so they’d be integral to the walls themselves. I thought that visually demonstrating how this wasn’t just some shed or playhouse sitting on a flatbed trailer, but was, by all appearances, something entirely new would be important to helping the U.S. get its collective head around some relatively “new” (very old, actually) ideas about the merits of smaller homes. How much that integral wheel design had to do with success on this front, I’ll never know. What I do know is that, with the goal of opening people’s eyes to a new concept already well under way, we can now make use of much better means of building a home on wheels.
After renovating a travel trailer’s interior for my own habitation, my primary education in residential architecture and construction was gleaned exclusively from designing and building my first little home on wheels from scratch. Designing it, with virtually no experience in the field of contemporary architecture, took three years. Building it, without much in the way of official construction know-how, took another two years. In the 14 years since that five year period, I’ve come to understand a lot about what I got right and what I got wrong.
For this early model, I used my wall-over-wheel design. It’s pretty, even if I do say so myself... pretty risky in relative terms of leaks and thermal bridging.
It’s not that my original design for integral wheels is inherently flawed in any serious way. If you do just as I prescribe in The Small House Book, early videos and old plans, with weep holes, water dams and adequate framing, it’s highly unlikely that you’ll have any leaks or structural problems in one of these structures. Some folks, like Dee Williams and Dan Louche, have developed their own, much improved, ways of mitigating the chance of damage, but, with any potential for such problems compounded by undue expenses in terms of material costs, labor and complexity, I just can’t understand why anyone is still building this way.
By now, I see no sense in incorporating a horizontal surface that wants to catch rainwater and carry it into the wall; I see no reason to use an unrated piece of relatively thin metal wheel well to support a 5’ span, above which rests a sizable piece of someone’s home, and, it’s hard for me to see any justification for filing a 9” x 3” x 66” section of bulwark with wheels when that same cavity could, far more easily, be filled with insulation. The amount of space left over for some sort of thermal barrier in the typical wall-over-wheel composition is just ½”.
For this more recent one, I scaled the house to fit between the wheel wells. The snug fit makes it look just as integral to the wheels, but the waterproof membrane remains unbroken, its construction is easier and more affordable, it has far more structural integrity and there’s more than insulation to guard against the condensation and water damage associated with thermal bridging.
Architect Kelly Davis at SALA designed this beautiful structure for Dan Dobrowolski. If you look closely at the bottom-left side of the structure, you can see that the house is built over its wheels all together. This is another tried and true way to ensure against the potential problems associated with my original wall-over-wheel design. I use this method for my Marmara and Marie Colvin designs and all of my current plans for forthcoming works.
Consider the whole envelope
When this wall-over-wheel method is coupled with the use of a trailer’s metal framing as an assembly of floor joists, the thermal bridging at these points, alone, can easily add up to a thermal sinkhole 1/8 the size of a home’s total square footage. Any house company’s claims of superior insulation in its floor, walls and/or roof should be considered in this context. Just because a manufacturer claims that their structures contain 15-Rs of insulation or more doesn’t mean that that same R-value exists throughout their product’s entire building envelope. Some of the most popular, portable houses out there are sold under the pretext that they contain as much as 20-Rs of insulation. What these companies won’t tell you is that as much as 13 square feet of the floor, walls and roof contain less than 3-Rs of insulation on average and that there’s less than 1-R in nearly half of that. In a cold climate, you’d have to have, at least, three sporting friends with decent upper-body strength and ice scrapers on hand to keep the resulting frost on your home’s interior at bay.
If you intend to build a house on wheels, don’t cut some big hole in your wall’s waterproof membrane to stick a big ‘ole rain-catching, thermal-bridging wheel well in it. Be sure to leave enough room in your walls for no less than 3-Rs of insulation and preferably something more like 10 - 40. This is most easily accomplished by building your structure above the wheels, on a deck-over trailer, or by making it narrow enough that it’ll fit between the wells of a standard car-hauler/flatbed trailer.
Do not try this with your home if its framing is designed to encompass the wheels. The floor and wall framing is broken in a house that’s framed to fit around the wheels and it can’t take much lateral stress. If, for example, you wanted to lift your abode off its chassis and onto the ground, or visa-versa, your house would likely to break in half. The little structure in this photo is the Four Lights Tiny House Company Anderjack being jacked up at three corners while the fourth hovers without incident or worry — a testament to the structural integrity of a house designed to fit between the wheel wells.