Design - Passive Solar and Mean Radiant Temperature

The number of visitors to our building site has been steadily increasing as it begins to look more like a "real" house.  And now, while its bones are still exposed, is a good time for the uninitiated to see it and get a feel for how it will work.  (Note: This post and the next two were written approximately two years before we moved into the house.)  Some of the visitors have been groups who do self-guided tours of the grounds and then come inside for a look around followed by a sit-down session on passive solar in general and earth sheltered passive solar in particular.  We compare three types of construction:  (1) typical stick-built houses, (2) classic 1970-80-era earth sheltered houses, as ably articulated by Rob Roy in his book, and (3) what has come to be known as Annualized GeoSolar houses like ours as described first by Hiat and then Stephens.  (For details on AGS, click on "Featured Post" in the column to the left; it will take you to three posts that follow the evolution of AGS.)


I have always had difficulty describing the many nuances of passive solar and earth sheltering.  A couple of months ago, I read again for the umpteen-time Edward Mazria's book, where, on page 64, he discusses the relationship of mean radiant temperature and human comfort.  This time it hit me that the concept of mean radiant temperature would be the perfect vehicle for making passive solar more understandable.

Mean Radiant Temperature

Understanding Mean Radiant Temperature

A cold evening campfire is one of my favorite things but, being skinny, I need to sit at exactly the right distance from the fire to stay comfortable.  If I sit too close to the fire or it blazes up, I quickly get too hot; if I sit back too far or the fire dies down, I begin to chill.  When I visit Missouri's underground caverns, I need a warm wrap.  Otherwise, the low temperature of the enveloping rock soon raises goosebumps.  The reason for these phenomena is that my comfort level depends upon a balanced thermal environment whereby the wave energy radiating from the fire or the walls of a cave that my skin absorbs is more or less equal to the wave energy that I am emitting -- equal actually to a 100w incandescent light bulb.  The mechanism at play here is called mean radiant temperature (MRT) and here’s how it works.

The feeling of comfort for us humans is best realized by maintaining a thermal environment in which the human body can lose heat at a rate that is equal to its production – no shivering, no sweating.  The need to lose heat stems from the fact that the body is essentially a heat engine with a thermal efficiency of only 20% (Mazria).  The waste-heat (80%) is dissipated in three ways:  perspiration, convection and by radiation to surrounding objects (walls, floors, furniture, etc. and, in the case of earth sheltering, thermal mass).  Of the three mechanisms, radiation accounts for about half of the heat loss with perspiration and convection (heat carried away by air) accounting for the rest. 

MRT is simply the average temperature of solid matter in the surrounding environment and it is more important for comfort than the air temperature in the same environment.  In fact, a 1 degree change in MRT has a 40% greater effect on body heat loss than a 1 degree change in air temperature (Mazria).  Therefore, when designing living space, it is far more efficient to control MRT than it is to control ambient air temperature.  And the higher the MRT, the lower the air temperature can be.  For example, if we can maintain the MRT at say, 76 degrees, the ambient air temperature could be as low as 62 degrees but our comfort level would be the same as if the air temperature were 70 degrees. Although MRT applies to matter such as wall studs, drywall, wood floors and furniture, it takes something much more massive to provide comfortable environments.

In a nutshell, the temperature of the thermal mass of a structure is more important for human comfort than the interior air temperature.

Mean Radiant Temperature in Stick-Built Homes

In stick-built homes, it is impossible to maintain a reasonably comfortable thermal environment without HVAC systems even with plenty of south-facing windows, because there is no thermal mass for storage and insulated 2 x 4 walls rated (optimistically) at R-13 or 2 x 6 walls rated at R-19 (optimistically) hemorrhage heat in winter and absorb heat in summer.  The temperature of the entire structure is at the mercy of outdoor temperatures that can be up to 30 degrees too hot or 70 degrees too cold.   Consequently, it takes a robust HVAC system to keep up with the heat gain or loss through the building envelope.  And, in lieu of thermal mass in which to store heat, the HVAC system cycles on and off repeatedly in order to keep the air warm or cool enough.  Meanwhile, the effect of MRT on human comfort actually becomes a negative -- in winter, the human body radiates heat faster than the cold walls and, in summer, it radiates heat slower than the warm walls.  And, heaven forbid, the occupants' furnace fails while they are on a winter vacation; the water damage from freezing is not a pretty sight!

Mean Radiant Temperature in Classic Earth Sheltered Passive Solar Homes

The thermal mass in the classic earth sheltered passive solar home is limited to the concrete in the floor, exterior walls and sometimes ceilings at the exclusion of the soil below, behind and above the concrete.  This peculiar situation occurs because the soil is kept from

A nearby earth sheltered passive solar home built just
after the oil embargo in the late 70s - early 80s; the living
quarters are one room deep and the roof is fully earth
sheltered.

being part of the thermal mass by insulation applied to the outside of the concrete shell. However, this arrangement does protect a large portion of the building envelope from extreme summer and winter temperatures, a significant improvement over stick-built homes.  The problem is that the amount of solar gain through south-facing windows and the limited storage capacity of the concrete shell are not able to keep up with the loss of heat through the insulation behind the shell and under the floor, to say nothing about heat loss through the south-facing stick-built wall.  Consequently, the mean radiant temperature remains so cool in winter that supplemental heat is the norm although the amount of supplemental heating is much less than stick-built structures because it has only to raise the temperature, say, 10 degrees – the difference between the soil temperature beyond the insulation and a 70 degree temperature in the living space.  As in a stick-built house, though, most of the supplemental heat goes towards keeping the air temperature comfortable.  But at least the modest amount of thermal mass in the form of concrete is enough to store some excess solar or supplemental heat as well as any waste heat from cooking, water heating, showering, drying clothes, illumination and radiating from human bodies.  Any heat that does make its way into the mass and is held there rather than bleeding through the insulation and into the cold soil would indeed improve the MRT of the living space, something that could never happen with a stick-built home.  A major advantage of the classic earth sheltered passive solar house is that the coolness of the MRT in summer means that conventional air conditioning is sometimes unnecessary (gleaned from conversations with owners of classic earth sheltered homes in our area).  And the probability of frozen pipes is negligible.

To be sure, there are non-classic earth sheltered passive solar designs that are more MRT-centric than just described but they are not as common.  Typically they utilize the most efficient thermal mass possible -- water -- in containers (like darkly-painted metal "oil" drums or polymer vessels of various shapes and sizes) staged to collect winter sunlight through south-facing windows during the day and release heat at night and on cloudy days.  Less commonly, roof ponds comprising water in waterbed-like bags strategically situated on the roof are used to heat in winter and cool in summer.

Mean Radiant Temperature in Annualized GeoSolar Homes

What sets our Annualized GeoSolar earth sheltered passive solar home apart from stick-built construction and the classic earth sheltered home is the total absence of supplemental heat or conventional HVAC.  This is possible by controlling profoundly the mean radiant temperature in three ways:   (1) increasing solar gain by harvesting the summer sun to supplement and vastly exceed the heat-gain from the winter sun, (2) significantly increasing the storage capacity of the thermal mass, and (3) retaining heat (winter) or rejecting heat (summer) with a R-60 to R-73 building envelope.  Based on the Hiat/Stephens design, we expect eventually a year-round comfort level in the mid-70s with a fall-off  to the upper 60s by the end of winter and an uptick to the mid-80s by the end of summer. 

It may take a few years for the temperature of the thermal mass to stabilize during which we will use infrared space heaters for supplemental heat and, with numerous ceiling fans and the use of windows for nighttime ventilation, may or may not have to worry about air conditioning.  However, the kicker is that our build is patterned after designs by authors who live in climates -- Montana and Washington state --  that are dryer and cooler than ours near St Louis, MO where summertime temperatures and humidity levels are more like the southern states.  With global warming, our passive heating will improve but passive cooling might become even more challenging.  We have to remain open to adding a minimal amount of air conditioning and trust that our photovoltaic array will handle the additional load or, at least, with net-metering, that our electricity cost on a yearly basis still zeroes out.

(Thanks to Jason Graklanoff, my engineer friend, for his thoughtful input to this post.)

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Two additional posts continue the story of how our house works.  The first one is an outline, the second delves into the details.