The first post on the insulation/watershed umbrella delt with its configuration as described by John Hiat in his self-published book, "Passive Annual Heat Storage; Improving the Design of Earth Shelters". This post is an attempt to explain why the umbrella is so essential for the function of the AGS system. The "Annualized GeoSolar" terminology, by the way, is taken from a paper by Don Stephens that complements and advances Hiat's original design; the "insulation/watershed umbrella" terminology comes from Hiat. A thorough discussion of the AGS system is found in three earlier posts: first post, second post, third post. The umbrella is covered in the third post.
The Function of the Umbrella As Part of the AGS System
The insulation/watershed umbrella increases the size of the thermal mass that is heated passively by the summer sun via the solar collector and the nine conduits running beneath the floor. Without the umbrella, the thermal mass would consist of only the concrete floor, the rock sub-base and that part of the underlying soil that remained dry. The concrete earth contact walls would also contribute but not the earth behind them because they would have to be insulated. The thermal mass would still be considerable because our ranch house is spread out so much. But, even so, why take a chance on the solar collector not being able to keep up with the heat loss from the mass during the cold months? Better to risk an overheating problem than an under-performing system.
Heat will be lost from the house mostly in four ways: (a) the envelope of the house will be the main source of heat loss irrespective of how tight or insulated it is, (b) some heat will be lost directly from the thermal mass outward through the insulated foundation above the umbrella and (c) some heat will be lost through the energy recovery ventilator that uses fresh outside air, even in winter, for indoor air quality. The heat losses through the envelope and ERV will be made up by heat moving out of the thermal mass into the house. The heat losses through and under the foundation are simply lost until recharged by the solar collector during next warm weather season,
Late Winter Blues
However, there will come a time at the end of winter when the heat that is readily available from the top layers of the mass has been used up and heat from lower down may not migrate fast enough to the surface to keep up with the heat loss through the envelope, foundation and ERV. In which case, the ambient indoor temperature may drop a few degrees until Spring when the average outside temperature rises above that of the house at a time when the solar collector has already begun to reheat the thermal mass. So it's all about getting enough heat into the thermal mass before the cold season then miserly keeping it there to compensate for heat losses from the house as long as possible in order to minimize a chillier indoor temperature in late winter and wait out the recharging of the solar collector in the spring.
The rate at which heat leaves the mass during the winter will be slowed somewhat by heat gain through the south-facing windows as well as heat generated by living in the house -- cooking, lighting, human bodies and hot water use. Both Stephens and Hiat have already warned that the ambient temperature will be lower for the first couple of years until the input from the solar collector is capable of hitting our target of 74 degrees year-round. At that point, it remains to be seen just how much the temperature drops in late winter/early spring, if any.
How Does It Work?
Heat seeks cold so the exterior winter environment is a prime target for the heat in the house and in the thermal mass. Heat loss from the house is covered in other posts while heat loss from the mass is our focus here. It exits the periphery of the mass in the winter as high up as possible. In our case, the insulated concrete foundation and umbrella-insulated footing force most of the escaping heat to pass under the footing. With our insulated concrete wall and without an umbrella, the top of the footing would be the departure point for the heat. With the umbrella, the departure point moves laterally to the outer edge of the umbrella.
According to Hiat, it takes 6 months for a unit of heat to travel through 20 feet of dry earth. Once the outside average temperature is lower than that of the thermal mass, heat loss begins. The first sketch shows what happens with an umbrella in place. The dashed and dotted lines represent the interface between warm and cold earth. Dashed line "A" depicts
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| Click on image to enlarge for viewing. |
the maximum of heat storage possible at the end of the warm weather season. If the umbrella is 20' wide as Hiat suggests, the dashed line "a" represents the maximum of amount of heat that could be lost in 6 months. Notice that the umbrella virtually eliminates any heat loss from under the house, assuming that the insulation in the umbrella stops all heat loss. In reality, its R-factor is sufficient to stop enough loss to keep that part of the floor nearest the wall much warmer than it would be without it.
At one point, in order to save a few dollars and a little effort, I toyed with the idea of making the umbrella 16' wide and it wasn't until I sketched out the two options and realized how much heat would be lost from under the house did I forget about the narrower umbrella. Even if the ranch style footprint would still provide enough thermal mass to compensate for the additional heat loss, the floor next to the exterior walls would probably be uncomfortably chilly towards the end of the heating season.
The second sketch shows the maximum amount of heat loss without the umbrella ("C" would be the maximum of heat available and "c" would be the maximum of heat lost in 6 months). The +/-20 foot floor expanse between the two dotted lines would undoubtedly be uncomfortable and more so the closer you come to the outside wall.
Role of the French Drains
The French drains are 10' below the floor and essentially limit the vertical dimension of the thermal mass to 10" which should be more than adequate for AGS system. Heat migrating into the living space from any deeper would not be very helpful because it would barely make it through the floor before the cold weather season was over. If the water table were to rise to the level we know it can (by monitoring piezometers for several years before beginning construction), it would bleed heat away from thermal mass. The drains make sure that it stays at the harmless level of 10" below the floor. For more on the French drains, go to first post, second post and third post detailing the design, fabrication and installation of the drains.
2 - 3 Year Time Delay
The umbrella largely solves the problem of conserving as much heat as possible within the thermal mass. The remaining question is whether our design for the solar collector/conduits will pump enough heat into the thermal mass to fill it out to the maximum. According to both Hiat and Stephens, one should expect it to take 2 - 3 years to max out. Afterwards, Stephen says, the problem could be too much heat such that some of the system would need to be mothballed. In order to speed things up, we may wait a couple of years to install the sun shades over the south-facing second story windows so that the summer solar gain will go into the thermal mass and augment the solar collector heat. And, if push comes to shove, we may install infrared heaters as supplemental heating for the first couple of winters, particularly if the code requires some sort of thermostatically-controlled heat.
Earth Contact North Wall
The foregoing discussion does not take into account the north wall. Without the umbrella next to it, it would have to be insulated on both sides for all or nearly all of it height. By moving most of the vertical insulation horizontally into the umbrella, only the top three feet or so need to be insulated vertically, viz., that part exposed above the grade, that part next to the topsoil covering the umbrella then a foot or so below the umbrella. And, by reorienting the insulation, most of the two-story wall and the earth behind it creates so much thermal mass as to make an earth sheltered roof discretionary or unnecessary.
Incidental Heat from Other Sources
Heat during the cold months will be lost principally through the envelope of the house, especially through the windows, and through the energy recovery ventilator as it exchanges +/-30% of the indoor air every hour. Most of the replacement heat will migrate out of the thermal mass but there will be incidental heat from other sources as well. Most will come from sunshine through the south-facing windows. But human body heat, heat from lighting (which we hope to minimize with LED), cooking heat and heat lost from hot water during bathing will also contribute. And, as mentioned above, we may invite the summer sun in through the second story windows for a year or two.
Cooling Season
The function of the AGS system during the cooling season is pretty simple. Our goal is to maintain a year-round floating temperature in the low 70s, expecting it to rise above the average floating temperature a couple of degrees by the end of the cooling season and fall a couple of degrees by the end of the heating season. Heat seeks cold so, at these temperatures, any summer heat penetrating the envelope will go immediately into the thermal mass without altering the ambient temperature, or at least not until the last weeks of the cooling season when the cumulative amount of heat uptake is more than the top layers of the thermal mass can absorb and distribute at the 20' per half-year rate. Heat gain through the envelope will not come through the windows. The windows will be shaded by overhangs that block the summer sun but admit the winter sun. The exception could be the delay of overhangs for the second story windows as mentioned above.
The Rest of the Story
At the time of this writing in early winter, I had excavated for the umbrella without knowing for sure when actual installation could begin due to the weather. The excavation was a matter of removing enough soil to allow the bottom of the umbrella to rest on the footings and slope downhill -- about two feet of soil had to be removed.
By going ahead with the excavation when the likelihood of installing the umbrella yet this
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| Insulation protecting the footings under the foundation walls |
year was pretty remote, the footings were a little more exposed. So I purchased 2" thick expanded polystyrene sheets, sawed them in half lengthwise, laid them over the footings and weighted them down with stones. Eventually, the insulation will be recycled as vertical insulation on both sides of the concrete garage walls.
Hang on, additional posts on the umbrella will follow as it is installed after the spring rainy season.
