Tiny Passive Solar House

I'm tying to design a house that uses passive solar technology in a tiny form factor - 16x24ft and I'm not sure how much of the strategies I should apply.

It's going to be a mostly open floor plan, and it's in a southern climate - Tennessee, so I'm more concerned about cooling than heating.

Obviously, I need some thermal mass, and I'd prefer to have it decoupled from the soil - mainly because there is an existing foundation that I'd like to be able to use and if I can't use that, I'd prefer to use piers than pour a new one.

I've bought a couple books on passive solar design, but they seem to be geared more towards larger houses, and I wasn't sure how well this scales down.

The southern side deciduous and northern side evergreens will of course come into play, but I'm not sure how much glazing I need and how much thermal mass - do I need a clerestory window, or are a few normal sized windows adequate?

The smaller the space, the easier it will be to overheat with solar gain. Conversely, the more internal mass you have, the more stable the temperature will be. I don't know if you have a structural system in mind yet, but I would consider cob for its mass and tempering ability. If as you say cooling is a greater concern than heating, cob will moderate day/night swings and possibly help with humidity if that is a particular concern in Tennessee. I would go with a moderate amount of south windows, well shielded by overhangs to prevent warm season direct sunshine from entering. Clerestory windows, especially operable ones to allow thorough ventilation in hot weather, would be good mainly for bringing light to the north side. 16' is not very deep, and you probably don't need them for most layouts.

Right, due to the low cost of PV designers are comparing cost of performance & risk ditching mass designs moving the cost of glazing to it & high levels of insulation, air sealing, and ventilation systems. The risk of overheat is high, some seeing it even in cold climates since the design parameters are complex needing an energy model like free BEOPT which uses manual J and other DOE passive solar software, one zone very easy. The problem with BEOPT is it does not have a lot of mass material properties (density, specific heat, atteburg limits, etc) in it but, there is limited capability to create your own if you know them. With those knowns one can generate BTU’s/hr based on diurnal swings, and estimate peak load time lags. If you’re tied to the grid with surcharges at peak load this can estimate peak cooling and heating loads shifts at lag time frames (eg: 6 hr) that cost less like at night, saving’s compared to more PV usually loses. That results in lower glazing on the south and west fecades, low as possible SHGC/u-values. BEOPT or SAM will also estimate electrical loads to size solar arrays and estimate the cost and pay back of PV vs mass over 5-50 years.

Pier and beam foundations are good on these in case you ever want to move them, I think the cash invested in insulation or PV is better served than a trailer unless one plans on moving often, in that case metal will take the vibration and fatigue much better. 1-2” inches of internal plaster like lime or earth will crack if moved and would humidity and heat buff. The only way it will reduce HVAC loads is if there is phase change. Again, you can define that in WUFI models, ~ 970 BTU/lb of water vapor in mass/ reductions. If hygromass is not designed right it reacts slow so cooling and dehumidification can be worse.

If you place value on comfort, even humidity and lack of hot-cold spots you may be able to prove the mass na’ers wrong, depends on the cost of windows and how well you design.  

If you go bed loft gambrel/ dormers make sense to battle stack affect in shoulder seasons. Wall fold up I’d go with leave the loft for something else. Good rule of thumb for roof mount PV 7/12 although if you run BEOPT it will optimize the angle to track the sun per site location.

Tiny spatial designs are not easy, done right it’s architecturally genius very challenging. Some find it is not what they thought after they live there, from bad designs. If you find any good design ideas let me know. I’m working on some models.

It's so funny, I'm always looking at the other end of the spectrum - how I can keep a gigantic living space warm in the frigid north up here!

Along the lines of finding something that can be done more to scale though for your purposes, there's actually a Kickstarter that comes to mind. The reason being is that it's going to be a sustainable home design library, but everything is modular, so you can pick and choose the size of your home, what parts you want, etc, without altering the entire design. I kind of feel like it's like building with legos.

All of the designs (once this project is funded in about a week) are going to be building code ready and interchangeable too, should be a handy system!

Here's a link to the thread that explains the project a little more.