In a recent post I made the case for using mean radiant temperature (MRT) the vehicle for understanding earth sheltered passive solar then followed that with a post outlining in bare bones fashion our green building project through the lens of MRT. This post adds flesh to the bones, describing in detail how our passive solar home will control mean radiant temperature to eliminate conventional heating and air conditioning.
But First..........
It seems appropriate from time to time to remind readers that I am not an expert in any of the fields involved with our project. Most of our design is based upon myriad sources that I have been sleuthing for many years now so I am standing on the shoulders of many experts and original thinkers, largely without giving them credit. But at the same time a lot of originality has gone into our project to make it more compatible with our climate, our building site, local building codes, using salvaged materials, working alone, spartan budgeting and, especially, designing and redesigning on the fly. By blogging, I would like to feel that I am paying forward for the insights borrowed from the experts and original thinkers while, at the same time, adding original content to the sustainability dialogue.
This posting is longer than usual but, for better understanding, I feel the complete story should be told without interruption. And, a reminder, click on the photos to enlarge them for better viewing.
But First..........
It seems appropriate from time to time to remind readers that I am not an expert in any of the fields involved with our project. Most of our design is based upon myriad sources that I have been sleuthing for many years now so I am standing on the shoulders of many experts and original thinkers, largely without giving them credit. But at the same time a lot of originality has gone into our project to make it more compatible with our climate, our building site, local building codes, using salvaged materials, working alone, spartan budgeting and, especially, designing and redesigning on the fly. By blogging, I would like to feel that I am paying forward for the insights borrowed from the experts and original thinkers while, at the same time, adding original content to the sustainability dialogue.
This posting is longer than usual but, for better understanding, I feel the complete story should be told without interruption. And, a reminder, click on the photos to enlarge them for better viewing.
The Challenge
The control of mean radiant temperature (MRT) for passive solar is a matter of having sufficient solar gain, then having the storage capacity to capitalize on the gain and finally having a building envelope that preserves the gain. The goal is to create an environment that radiates heat at the same rate as its occupants so that they are always comfortable.
SOLAR GAIN
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| An early sketch of our AGS system. The red line represents a conduit (one of nine) angling upward from the solar collector, located in front of the house, to daylight behind the house. It is 5' below the floor level at the front foundation and 3' below at the back foundation. Since it is inclined, heat from the collector rises naturally through it, warming the soil below the concrete floor and behind the concrete north wall (orange). The insulation/watershed umbrella is not shown but would lie just below the wavy black lines that represent the grade in front and back of the house. |
Winter Heating
During the three months of summer here in Collinsville, Illinois (near St Louis), the available sunshine is reduced by 31% due to cloud cover. Since days are long during June, July and August and the sun is more intense, the 69% of available sunshine nevertheless represents a lot more energy than can be harvested during the short days of winter. But shorter days are not the only problem -- the cloud cover during December, January and February is even worse than in summer, blocking 53% of the available sunshine. Fortunately the relative amount of sunshine between winter and summer is almost moot in our case because the surface area of glass in the solar collector will be a sixth of that of the 19 south-facing windows collecting winter sunshine. The collector will harvest energy more efficiently per surface area of glass than will the windows because of more available sun and a more favorable sun angle, but for the first couple of years, more useful warmth will come from the winter sun. Like the the hare-tortoise race, the solar collector, while slower, will eventually win because the two systems will heat the thermal mass from different directions. The heat from winter sun will warm the superficial layers of the thermal mass but not enough to eliminate the need for supplemental heat. The collector will gradually, over a year or two, warm the deeper layers from below where the heat will be trapped permanently and more able to stabilize the MRT year-round. In fact, Don Stephens, who, along with John Hiat, introduced the concept of AGS, speculated that designs like ours will eventually overheat and that part of the solar collector might have to be mothballed to control it. Until the temperature of the thermal mass stabilizes at a desirable temperature, we will resort to portable infrared heaters that, as opposed to permanently-installed infrared heaters, can be partially or fully retired when no longer necessary.
Update: Winter of 2022. Having said all of the the above, our expectations with the solar collector were misplaced (see post discussing second thoughts about the efficacy of the solar collector. In a nutshell, we had to bring the terminal ends of the conduits to a solar chimney and pull air through the system in order to overcome the backflow caused by the 60 degree ground temperature. Whether the revised system after a couple of summers will warm the earth enough to use the solar collector passively as originally planned remains to be seen. If not, maybe we will have to resort to calling a fan driven by our solar array a "passive solar" component.
Summer Cooling
Direct solar gain during summer is an issue that is less well resolved. One sees pictures of passive solar homes, earth sheltered and not, that have south facing glass unprotected by overhangs. Some even have glass that is tilted back at the top to be more perpendicular with the sun's rays in winter. I suspect that the latter could experience overheating during summer, particularly if shining on a dark floor. If the glass were perpendicular but unshaded, the amount of overheating might not be a huge problem if it was backed up by thermal mass of an appropriate color. This guess is based upon the fact that the high summer sun angle is so obtuse in our area that much more energy is reflected off of the glass than penetrates it.
In view of the above, we have elected to go with overhangs for all of our windows in order to eliminate entirely sun penetration from the latter part of May to the early part of September. The overhangs, in concert with windows that are inset 10" from the surface of the exterior wall, will however be somewhat counterproductive when they restrict solar penetration that would be welcome in late spring when the MRT has fallen to the lower 70s and allow some unwanted penetration in early fall when the MRT has risen to the upper 70s. (The unique design for the windows was covered in a recent post.)
Ventilation
Often lost in a discussion of passive solar is the extent to which heat generated by living in a house helps to heat the house. Cooking, water heating, showering, drying clothes, lighting and human bodies all warm the air and probably raise the MRT somewhat as well. But these factors also have a downside -- they compromise indoor air quality in an airtight structure and must be controlled by either an energy recovery ventilator or a heat recovery ventilator.
Summer Cooling
Direct solar gain during summer is an issue that is less well resolved. One sees pictures of passive solar homes, earth sheltered and not, that have south facing glass unprotected by overhangs. Some even have glass that is tilted back at the top to be more perpendicular with the sun's rays in winter. I suspect that the latter could experience overheating during summer, particularly if shining on a dark floor. If the glass were perpendicular but unshaded, the amount of overheating might not be a huge problem if it was backed up by thermal mass of an appropriate color. This guess is based upon the fact that the high summer sun angle is so obtuse in our area that much more energy is reflected off of the glass than penetrates it.
In view of the above, we have elected to go with overhangs for all of our windows in order to eliminate entirely sun penetration from the latter part of May to the early part of September. The overhangs, in concert with windows that are inset 10" from the surface of the exterior wall, will however be somewhat counterproductive when they restrict solar penetration that would be welcome in late spring when the MRT has fallen to the lower 70s and allow some unwanted penetration in early fall when the MRT has risen to the upper 70s. (The unique design for the windows was covered in a recent post.)
Ventilation
Often lost in a discussion of passive solar is the extent to which heat generated by living in a house helps to heat the house. Cooking, water heating, showering, drying clothes, lighting and human bodies all warm the air and probably raise the MRT somewhat as well. But these factors also have a downside -- they compromise indoor air quality in an airtight structure and must be controlled by either an energy recovery ventilator or a heat recovery ventilator.
STORAGE CAPACITY
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| Picture taken during troweling of the concrete floor; notice the extent of the floor and concrete north wall that will become part of the thermal mass; notice also the insulated concrete forms creating a "frost-protected shallow foundation" |
How the Thermal Mass Works
The primary thermal mass for our dwelling is (a) the soil beneath the concrete floor and (b) the soil under the insulation/watershed umbrella on all sides of the house, especially that behind the concrete north wall. Unlike classic earth shelters in which the concrete in the floor and walls is the thermal mass, the concrete in our house, while being that too, transports heat to and from the soil as its primary job. The amount of soil serving as thermal mass could be calculated I suppose. For example, the earth serving as dry and insulated mass under the umbrella behind
the north wall could be determined by multiplying the height of the uninsulated part of the wall (10') by the length of the wall (60') then by the width of the insulation/watershed umbrella (20') extending horizontally outward from the wall, etc. But the exact figure for this triangular quantity of soil is unimportant; what is important is that we are talking about a lot of thermal mass behind the wall alone to say nothing about that residing under the umbrella east, west and south of the house and under the floor of the house itself. It is safe to say that we will have plenty of storage capacity for any amount of solar gain even considering that some heat will bleed through the umbrella because it is only R-20 at its thickest. However, since it takes 6 months for a unit of heat to travel 20 ft through dry soil and the umbrella is 20' wide, most of the heat loss from under and behind the house will be inward through the concrete floor and north wall to warm the house in winter which is a good thing.
Air Movement - Winter
The thermal mass will cool the house during warm months just as it will warm the house during cool months. Since the stick-built walls and ceiling are so well insulated, most of the unwanted heat entering the dwelling in summer will be conducted through the windows despite the fact they are fully shaded as described in a recent post. Even though the windows are top-rated at R-4, they are like holes
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| The insulation/watershed umbrella under construction in the space between the solar collector on the left and the house on the right; it ranges from R-20 nearest the house to R-10 at its periphery; the umbrella rests on the shallow foundation footings, rendering them frost-free, and butting against the insulated concrete forms for a continuous vertical to horizontal thermal barrier |
Air Movement - Winter
The thermal mass will cool the house during warm months just as it will warm the house during cool months. Since the stick-built walls and ceiling are so well insulated, most of the unwanted heat entering the dwelling in summer will be conducted through the windows despite the fact they are fully shaded as described in a recent post. Even though the windows are top-rated at R-4, they are like holes
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| The red arrows point to the tall wall between the living quarters and the "vertical basement"; the magenta arrows point to the concrete north wall; the space circled by green is the airlock (see text) |
Another update: Winter 2022. Beginning January 1 of 2022, I monitored the temperature in the middle of the house in three places twice daily. It wasn't until we took occupancy in late February that we began using infrared heaters. Before doing so, the temperature at eyeball level on the first floor and 25' up at the ceiling of the second floor have remained within one degree of each other irrespective of outdoor temperatures and without assistance from fans of any sort. The temperature at the junction of the tall north concrete wall and the concrete floor started January in the mid-60's, meaning that the heat from the collector system had warmed the soil enough to be detected halfway through the winter. By mid-January however the temperature had dropped to 60 degrees and will likely stay there until the thermal mass is heated again during the warm months. The take-aways from these observations are that, with help from a fan, the collector system is capable of warming the thermal mass as planned and there is, unexpectedly, minimal stratification of the indoor air, at least in the winter -- good news but also a warning that it may take more warm weather seasons to reach a desirable heating-season temperature than originally imagined.
Air Movement - Summer
Summer will be a bit more challenging. The heat gain through the second floor windows will tend to collect under the second story ceiling and will have to be
pushed downwards along the slanted cathedral ceiling towards the concrete north wall, the top of which is about 6 feet below the tallest part of the second floor ceiling. Ceiling fans blowing upwards might be enough to push the air along but wall fans between the pairs of second story clerestory windows blowing towards the north wall may be needed instead of, or in conjunction with, the ceiling fans. Or maybe it turns out that the ceiling fans, blowing downwards, are forceful enough for the warm air to find the north wall and the concrete floor below. In order to hedge our bets, fan-supporting electrical boxes were installed in the ceiling of the vertical basement in case fans would help to push warm air down the cool north wall. And, not all of the heat from the first story windows will rise to the second floor ceiling. Because heat seeks cold, some will be immediately absorbed into the cooler concrete floor.
Tall Wall Design
In order for the air to reach the north wall in both seasons, it will have to pass through the tall wall (red arrows on the floor plan above) that separates the living space from the vertical basement . Accordingly, most of the wall has a continuous opening along its bottom just above the baseboard and another continuous opening at the top of the wall. This design for the tall wall,
by the way, is not original; John Hiat in his remarkable book, pointed out how important it was for the air in the living space to have plenty of access to the thermal mass of the north wall and the soil behind it.
Preliminary Observations
The metric "cooling degree days" (CDD) is used to analyze warm weather temperatures. At the time of this writing in the waning days of July, the temperature in the middle of the first floor of the house has not exceeded 82 degrees despite higher than normal CDD readings for both the month of July and for the cooling season as a whole. All that separates the interior from the 90-something outside weather are the windows and a layer of plywood sheathing covered with lumber wrap or 6 mil plastic. The second floor is another matter; temperatures there have tracked closely with outside temperatures. The difference between the two floors has mostly to do with thermal mass -- the lack
of it on the second floor and an abundance of it on the first floor. The mass was cooled sufficiently last winter to continue to absorb heat this late in the summer.
During the coldest part of last winter, the inside temperature stayed in the 40's and upper 30's despite the fact that there were no windows yet to collect winter sunshine. Only once for two days during a week of single digit outside temperatures did it drop below freezing, making it obvious that the indoor environment was being moderated by heat that had accumulated in the thermal mass during the warm season. The second floor temperatures were also moderated by summer heat emanating from the thermal mass although not as much as the first floor. Now that the windows are installed and winter sunshine will have access to the thermal mass, I would expect the inside temperatures next winter to stay above the mid-40's. Of course, once the house is insulated, the temperature of the thermal mass will no longer fluctuate with the seasons but these preliminary observations bode well for an efficient AGS system once the building is wrapped up.
[Update -- March 2020: The past winter was the first for harvesting solar heat through the windows. and the first since the shell was insulated. The result was that the temperatures stayed in the 40's and 50's except for one day when it dropped to 38 (compared to the previous winter when the temperatures were in the 30's and 40's and once fell below freezing).]
Air Movement - Summer
Summer will be a bit more challenging. The heat gain through the second floor windows will tend to collect under the second story ceiling and will have to be
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| John Hiat's indispensable book |
Tall Wall Design
In order for the air to reach the north wall in both seasons, it will have to pass through the tall wall (red arrows on the floor plan above) that separates the living space from the vertical basement . Accordingly, most of the wall has a continuous opening along its bottom just above the baseboard and another continuous opening at the top of the wall. This design for the tall wall,
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| A thermometer (circled) is located in the middle of the house |
Preliminary Observations
The metric "cooling degree days" (CDD) is used to analyze warm weather temperatures. At the time of this writing in the waning days of July, the temperature in the middle of the first floor of the house has not exceeded 82 degrees despite higher than normal CDD readings for both the month of July and for the cooling season as a whole. All that separates the interior from the 90-something outside weather are the windows and a layer of plywood sheathing covered with lumber wrap or 6 mil plastic. The second floor is another matter; temperatures there have tracked closely with outside temperatures. The difference between the two floors has mostly to do with thermal mass -- the lack
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| The temperature in early afternoon on July 13 when the outdoor temperature was 93 degrees; the highest reading of 82 degrees was reached later in the month during a week long heat wave |
During the coldest part of last winter, the inside temperature stayed in the 40's and upper 30's despite the fact that there were no windows yet to collect winter sunshine. Only once for two days during a week of single digit outside temperatures did it drop below freezing, making it obvious that the indoor environment was being moderated by heat that had accumulated in the thermal mass during the warm season. The second floor temperatures were also moderated by summer heat emanating from the thermal mass although not as much as the first floor. Now that the windows are installed and winter sunshine will have access to the thermal mass, I would expect the inside temperatures next winter to stay above the mid-40's. Of course, once the house is insulated, the temperature of the thermal mass will no longer fluctuate with the seasons but these preliminary observations bode well for an efficient AGS system once the building is wrapped up.
[Update -- March 2020: The past winter was the first for harvesting solar heat through the windows. and the first since the shell was insulated. The result was that the temperatures stayed in the 40's and 50's except for one day when it dropped to 38 (compared to the previous winter when the temperatures were in the 30's and 40's and once fell below freezing).]
Window Glass
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| Transparent glass overlooking the thermal mass (concrete floor and soil beneath it) on the first floor |
Interior Colors
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| Translucent glass for most of the second floor to compensate for the lack of approximating thermal mass |
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| Notice three features: (1) the maturing eastern red cedar shelter belt, (2) the unfinished upper termination of the 9 conduits that originate in the solar collector in front of the house and (3) the amount of earth sheltering for the 12' tall concrete north wall (the grade at its highest point is 8" below the top of the concrete portion of the wall with 5' of stick-built wall on top of it). The back of the house is covered with 6 mil plastic while waiting for steel siding |
PRESERVING THE GAIN
Solar energy is hard to come by so it makes sense to design a building that preserves it unequivocally. Many previous posts have described the green features of our building so I will only list some of them here (along with some linkages to informational posts), as a reminder of the effort that has gone into its design and execution.
-- 15” thick walls and 18” thick ceilings with
minimal thermal bridging filled with
rice hull insulation at R-3+ per
inch
-- Roof trusses
covered by two separated layers of sheathing that form a
ventilated air space (roof design)
--
Highly reflective steel roofing (light
gray) and steel siding (bright white)
--
Insulated/watershed umbrella extending below grade outward from the
house
in all directions (see “Featured Post” in
the left column above for a history of
the umbrella)
-- Frost protected shallow foundation
--
Plywood sheathing rather than OSB for better moisture control within the
wall
-- Vapor and air barriers for ceilings and exterior walls that favor drying over
vapor prevention
--
Fiberglass casement and awning windows (see recent post) for details on how
their design contributes to
preserving the gain)
--
Fastidious air sealing of the building envelope
--
Main entry shielded from north and west winter winds by house and garage
--
Sizable airlock inside main and garage entries to moderate incoming hot
or
cold air before it enters the living space (see drawing above).
--
Shelter belt of eastern red cedars shielding the house from north and
west
PHOTO-VOLTAIC PANELS
No discussion of passive solar would be complete without mentioning PV panels to generate electricity. A previous post covered the subject in detail so I will only summarize here. Our goal is to buy only enough PV to break even with the electric company through reverse metering.
With electricity taken care of, our only out-of-pocket energy costs will for the natural gas for cooking, water heating and clothes drying.
_ _ _ _ _
* For a information on AGS, click on "Featured Post" in the left column above or go to Don Stephens' definitive paper or, for an abbreviated explanation, go to Wikipedia.)
