The Super-Insulated Tiny House

Housing ideas come to me so fast and change with such regularity, while my writing and drawing lags behind hundreds of iterations.  No bother; I will offer my latest version of a tiny house for your consideration, and for the purpose of organizing my thoughts.  I get so far off topic every time I sit down to write something, attempting to explain every little facet of how I arrived at each thought that I get bogged down in details and grow tired of writing.  Try to follow along.

What I propose is a tiny house unlike anything I’ve ever seen before.  Not to say that it has not been done, it’s just that I have not found anything close to it.  I have condensed what I’ve learned over the years with regards to house construction, insulation, energy management, thermal mass, heating, cooling, ventilation, material toxicity, embodied energy, and conservation, into what I hope is a home which is sturdy, energy efficient, inexpensive, and pleasant to occupy.  There are many possible iterations of this particular design, many of which will lead to further designs down the road.  So let us begin with the latest:

The Container

The core structure comprising the home is a 20 foot long shipping container of the type seen in any port which carries goods across the sea, and at times, on rail and truck.  These containers come in several standard lengths including 30 foot and 40 foot, however I would prefer the 20 foot because it is lighter, cheaper to purchase, cheaper to build with, and I believe it offers just the right amount of space to occupy.  The internal dimensions are usually 19′ 4″ long, by 7′ 8″ wide, by 7′ 10″ high.  This provides a floor space of approximately 148 square feet.  In order to have space for two occupants to sleep, lounge, bath, cook, and utilize a composting toilet, I would prefer to locate a “High Cube” which has an internal height of 8′ 10″.  The other dimensions are equivalent to the numbers previously stated.

Interior Layout

The additional height allows for a split level configuration, not two full stories of course, but a cozier loft and cubby type configuration.  I do not have any diagrams depicting my design (they’re still stuck in my noodle). so I will do my best to describe it here.  Standing in the middle of the container facing one of the long sides, to the left would be loft area approximately 3.5 feet off the floor, extending from the short wall out to 8 feet, and from long wall to long wall, comprising an area of 61 square feet.  This loft area, unlike many tiny homes, would not be the bed but would rather be the lounge area with couch, cushions, computers, viewing screens, and the like.  The area beneath the loft would be the bed where one would find a shallow queens sized mattress.  These two levels could be switched, with a loft level being about 3.5 feet from the ceiling and the bed being up there, but I think I would prefer the lounge being higher in the structure so a better view can be appreciated from wide windows installed high on the side of the structure.  Access to the loft area could be accomplished by a ladder or preferably a large stepped storage structure built in somewhere.

Again, standing in the middle of the container, facing the long wall and looking the towards the other end, another split level configuration is employed for the kitchen and bath.  Unlike the lounge area, this platform is only 2.5 feet off the floor, and extends from the short wall out to about 8 feet, comprising another 61 square feet.  The exact orientation and placement of the kitchen and bathroom is not yet known, only that a bathroom sink will not be needed, and the placement of the shower and kitchen sink will be quite near to one another in order to minimize plumbing needed and to maximize water pressure and hot water temperature.  Toilet facilities are accomplished with separating composting system.  This could really be placed anywhere in the house since no fresh water or drainage is needed, but it most likely would remain a part of the shower in some way.

The space underneath the kitchen / bathroom platform would have the main purpose as an engineering / utility space.  This would comprise of an on-demand LP water heater, RV style fresh water tank with 12VDC pump and pressure tank, a grey water receiving tank for heat recovery, radiant floor heat equipment (if employed), battery compartment with outside ventilation, solar / wind charge controller, power inverter, breaker / fuse panel, and any other system that can be placed here for easy access.  The remainder of the space could be used for storage with access for everything being from a liftable hatch in the floor or similar assembly.

The entryway is placed in the middle of one of the long walls and is likely to be a full glass door to admit as much light as possible.  Above the doorway is a window for yet more light.  A wide window is placed in the lounge area on the end short wall, the long wall, or both.  A few more small windows might be placed in the kitchen area, and a well insulated translucent window in the shower.  Skylights and solar tubes would be nice, but likely will not be used due to concerns about possible water infiltration.

Insulation

The issue of insulation is a tricky matter.  Commonly, shipping container homes are insulated on the interior walls with spray foam, rigid board, or fiberglass between wooden studs.  I shy away from these methods because of these materials environmental impacts.  I could employ less toxic, lower embodied energy insulations on the interior walls, leaving the rugged steel shell exterior for a more industrial look, however I find the notion of insulating the inside to be a waste of floor space, and the level of thermal insulation I desire would make the inside of the container much smaller than it already is.  The other problem, much more serious, is that by leaving the steel shell exposed to the outdoor environment, indoor humidity generated from daily activities could condense against the inside wall of the steel on cold days, leading to mold and rot of wooden members or decay of insulation.  Another concern with an uninsulated steel shell has to do with the thermal mass of the steel box and the possible tendency for it absorb heat on a warm sunny day and not only cook the occupants inside while the sun shines, but also a lag effect might prevent the interior from cooling off at night sufficiently.  For these reasons and others, I’m opting to insulate the exterior.

Many super-insulated homes utilize a double exterior wall made of two 2×4 studded frames as much as one foot apart.  At a certain point in the construction, the cavity is filled with high density blow-in cellulose insulation.  By building two walls, the thermal bridging effect is avoided because the interior studs do not contact the exterior wall sheathing directly.  Thermal bridging still occurs at the top plate, bottom plate, and around windows and doors.

I’d like to draw on this idea of a very thick wall, but simplify it a bit.  Since the shipping container already represents the interior wall surface, I would like to essentially build a big wooden box around the smaller steel box.  The base of the structure is a series of 4×4 or 4×6 beams on which thick marine grade plywood would be screwed out to the perimeter of the finished platform.  How exactly the container would be supported, I’m not sure.  I need to gain more experience with framing.  One possibility is that a knee wall would be built with beams set across which the shipping container would be bolted to, leaving a foot of space underneath for insulation.  On the top plate of the knee wall is a studded wall with plywood sheathing on the exterior.  The foot or so between the wall and the container is filled with cellulose.  The wall would support a steel roof set to a slight pitch for rainwater catchment.  Enough space is left between the top of the container and the roof to allow for at least R-50 worth of insulation to be blown in, and still have sufficient air space for ventilation to keep everything dry.  The top of the container might be firmly anchored to either the roof structure or to the walls near the top.

I considered straw bale insulation for a time.  I have no experience building with the material, but I’ve researched it extensively.  The biggest issue I can see with straw bale walls is that typically they are plastered on both the interior and exterior with an inch or so of earthen or lime plaster.  This gives them a great deal of strength.  Butted up against the shipping container, I can’t see how I could really achieve this.  Also, I think that blow-in cellulose would just be much easier.  One idea that still nags at me, is making a bed of straw bale on the base wooden platform which the container would simply sit on.  I have little doubt that the bales could effectively support the box and also provide a great deal of insulation.  Issues with rot and rodent infestation concern me, but I’ll continue to consider it.

Facade

As far as the exterior finish goes, I haven’t really thought too much about it.  I’ve always liked rough lap boards.  Probably some kind of wood finish in the interior as well.  I’ll figure that out later.  Form follows function, right?

Thermal Mass

This last bit is one of my favorite topics when it comes to housing.  The steel of the shipping container, fully enclosed in an envelope of insulation, can provide the function of a thermal store of heat.  By absorbing and releasing heat, the steel serves to damper temperature swings inside.  I don’t think that 5,000 pounds of steel is small potatoes when living space is less than 160 square feet and the thing is super-insulated.  I really don’t know for sure, but I feel that the design as I’ve described it would be quite easy to heat and cool.  This is where it gets a little weird.  Water bottles.  Yes.  By building up the floor of the shipping container and lining the subfloor with a layer of used water bottles, full of water, I could reasonably add over a thousand pounds of water thermal mass to the floor.  If I chose to install radiant floor heat, I could run the plastic lines back and forth under these bottles, allowing the system to be charged up with heat which might even out the heat distribution in the floor and hold it over a longer period of time.  SImilarly, before installing the insulation, I could find a way to stack up water bottles against the exterior surface of the corrugated steel walls, thereby adding passive thermal mass to the walls.  Hell, I could even line the whole roof of the container in water bottles.

Miscellaneous

There are many other issues to discuss, but this article is getting really long, and I do not yet have any drawings to accompany my ramblings.  A few last things.  Because the house is designed with a shipping container, some means by which to provide fresh air are necessary.  Beyond openable windows and a ventilation fan, a heat recovery ventilator would be a good idea too.  I think I would want to install inspection ports in the the walls and floor to check on the moisture levels in the insulation and make sure things are staying dry.  I don’t think a vapor barrier is going to be necessary since the shipping container steel walls would keep most moisture from migrating through the insulated wall, other than around windows, doors, and vents.  Some sort of access to the attic space for inspection would be wise as well.

So far, the design seems like it would sit best on a well draining gravel bed, if not slightly raised on blocks.  It would be nice to have a structure which could be loaded onto a trailer and moved without it falling apart.  I’ll think about that one a bit more in the future.

I’ll keep doing my homework, and get to making some drawings.  Ta!

-M.C. Pletcher