Let the Sun Shine In
Updated: Mar 8, 2019
The tiny house faces south; the main roof overhang manages the sun that enters the clerestory windows, while the two canopies manage sun entering the lower glass. I chose windows with a higher solar heat gain for the south side only.
"Catch and Store Energy"
"Use and Value Renewable Resources"
"Apply Self-Regulation and Accept Feedback"
A few of the permaculture principles that inform the passive solar design of our tiny house. While the first two provided the original impetus to incorporate passive solar, the time has come to accept some feedback as to the success of the design. Feedback from the house itself.
First though, for those unfamiliar with the idea of passive solar, let me offer a very basic summary. Passive solar is a way of designing a building so that its heat regulation is largely done using the immense free and perpetual energy of the sun. No "active" systems (eg. photovoltaics) that require energy in manufacture and maintenance. Just smart design and choice of materials. At its most basic it consists of:
ORIENTATION - Line the building up so that the long side faces as close to true south as possible
GLAZING RATIOS - Install the correct amount of window glass on each side of the building
OVERHANGS - Calculate roof overhangs on the south side so hot summer sun is blocked, but winter sun can fully enter
THERMAL MASS - Incorporate sufficient high-thermal mass materials to absorb and slowly release solar heat
My rudimentary calculations for roof overhang placement on the tiny house based on how much sun I wanted to enter vs. block at various times of year, and where it might actually make contact with thermal mass once inside.
As you can imagine, there are more criteria, details, and nuances to consider depending on your latitude, climate, and the degree to which you want to calculate with precision. It's simple enough to ensure your roof overhangs block all sun from entering when its arc is high at summer solstice, and allow all of it in when its arc is low at winter solstice. But the trickier bit is deciding what amount of sunlight you'd like to allow in as you near the equinoxes (when the sun's arc is midway up in the southern sky). When you think about September 21st (roughly our fall equinox) and March 21st (roughly our spring equinox) the weather can vary wildly from year to year. Will it be quite warm and you'll wish for just a portion of the sun's heat to enter during the day, or will there be snow outside and you'll crave the maximum dose of warm sunshine coming through those windows? Given our northern climate, I decided to err on the side of more sunshine. My rationale was that even if the sun coming in those south-facing windows created too much heat in the tiny house for portions of the spring and fall, the temperatures outside are typically cool enough that opening a couple of windows should make things comfortable. And certainly the nights at those times of year are enough to cool things off quickly if need be.
Sun infiltration on September 9th (a week and a half before fall equinox). Top: The roof canopies are casting a shadow to about one third of the way down the flower box window and half way down the door window. Bottom: Sunshine hitting the floor and other surfaces inside the tiny house.
So, after observing the function of this passive solar design for nearly a year, how would I say it's performing? Overall really well, actually. The sun is coming in more or less as expected, hitting the thermal mass of the earthen floor and some brick detail, where its heat is retained within the highly-insulated tiny house envelope. With a bit of management in the form of opening and closing windows and blinds, the temperature can almost always be kept within a comfortable range (additional heating is still required in the winter). There are, however, a few things I acknowledge that could have been done differently to make this system optimal and hands-off:
More thermal mass. The amount of thermal mass you need can be calculated based on the size of your building, amount of southern glazing (windows), and the density of your mass materials. Too much thermal mass (an uncommon problem) and your home takes too long to heat up with the available sun; too little and your home overheats from the sun since there's not sufficient mass to absorb it. Ideally, we would have added more of it - a fact we were aware of, but that was sacrificed along the way as we made decisions about materials based on other criteria like functionality, efficient use of space, etc. Some brick on the interior of the north wall, or a tile/concrete/earthen countertop are a couple of places more thermal mass would have made sense.
Less west-facing glass. To avoid overheating, it's important to keep west-facing windows fairly minimal. This is the direction from which the hot afternoon sun enters from a point too low in the sky for overhangs to be effective. Again, we were aware of the fact that we were stretching this rule, but chose a little extra glass in order to provide a good view into the garden. Interior blinds help to shade that western heat when it's not wanted, but even more effective remedies would block some of that sun before it hits the window glass at all. For example, exterior shutters, a low retractable canopy, or a deciduous tree.
With the passive solar portion of the tiny house design I chose to stick to the broad principles, using a sort of try-and-see approach. We tried and now we're seeing what worked and what could be tweaked. The beauty of the virtually technology-free nature of passive solar is that it's merely a re-working of the placement, design, and materials as you construct a building - no need for any additional costs either up-front or ongoing. In fact, cost savings are essentially guaranteed as you reduce the need for "active" sources of heating and cooling. And unless you grossly miscalculate your design, small adjustments as you "apply self-regulation and accept feedback" are all that will be needed. One year in, we're certainly glad we laid the foundation for the sun to shine in and do some of the work for us!