Thermal Performance

 We have been gathering data on the passive solar performance of our house for a year and a half.  Although we plan to continue reporting on over time, there is enough data to suggest some trends.  The data comes from several thermometers within and below the house and one outside the house as follows:

Thermometer on first floor wall 
(the black speck to the right of the picture)

 1.  Living space thermometers at eye level in three rooms and one, read with binoculars, just under the highest point of the second floor vaulted ceiling.  These are labeled "Living Quarters" on the first graph.

2.  Thermal mass thermometers, one heavily insulated from room air at the junction of the vertical basement floor and the north concrete wall and at three depths below the concrete floor of the living room.   These are labeled "Thermal Mass" on the graph.

3.  Outdoor thermometer outside the back door labeled "Outdoor Temperatures" on the graph.

Temperatures have been recorded twice monthly -- on or near the first day and on or near the 15th of each month.  Analysis of the data for this writing revealed that using only one set of data per month was sufficient for painting an accurate picture so, with one exception, the first-of-the-month data were used.

Several piezometers were employed to monitor the behavior of the water table below the proposed house site, leading eventually to a system of French drains that lower the table.  Now we use one of the piezometers still protruding from the living room floor to take the temperature of the soil at three levels under the house down to 15' and to monitor the water table.

Overview

The data for the following graphs span 18 months, from January 2023 through June 2024.

The obvious take-away from the first graph is that, most of the time, the temperatures in the living spaces and in the thermal mass fluctuate in tandem with the thermal mass temperatures running slightly cooler than those in the living spaces.  What is apparent from the raw data in the charts below, more so than from the graph, is that the seasonal temperature changes of the thermal mass lag a month or two behind room temperatures, which is to be expected since air gains or loses heat faster than dry soil.

The second graph shows the benefits of a tight house (see the previous post regarding our blower door test results).  The temperature at eye level on the first floor, compared to 20 feet up at the intersection of the wall and the 

Thermometer at the peak of the
second floor vaulted ceiling

vaulted ceiling, diverged only slightly, marginally more in cold weather than in warm.  By contrast, convectional air movement in a typical -- leaky -- house results in temperatures that are distinctly higher at the ceiling than nearer the floor. 

The rationale for the insulation/watershed umbrella was to expand the size of the dry and insulated thermal mass beyond that directly under the house in order to protect more of the latter from outdoor temperatures.  Over time the mass would cool the house by absorbing heat during warm months and warm the house by re-radiating it into the house during cool months.  And, in so doing, the thermal mass would gradually warm from its lower Midwest legacy temperature of about 60 degrees with the effect being greater at shallow depths and less so deeper down.  But that is not quite what came to be.

The third graph shows temperatures in the soil beneath the living room floor at depths of 5', 10' and 15', taken with a thermometer on a string (pictured below) lowered into the piezometer from which some preliminary assumptions can be made.  First, the "legacy temperature" had already risen to 64 degrees, presumably due to the soil having been exposed to ambient temperatures during construction.  Second, a year-to-year comparison

The thermometer for measuring subfloor temperatures;
weight for measuring ground water levels

of the average temperature for the first 6 months of '23 versus the first six months of '24 differed by only one degree for each of the three depths.  It will be interesting to see if the close tracking holds for the rest of '24 and beyond.  If so, it will be good news because it suggests that the thermal mass may not be warming as fast as anticipated, if at all, in the face of global warming.

The last graph shows temperatures above the floor and at two shallow depths below the floor. The room temperatures were read on the wall thermometer shown in the first photo above.  The second temperature was recorded from water that had been standing for hours in one of the water lines running about a foot below floor level.  The third temperature was for 5 feet below floor level taken through the piezometer.  As might be expected, the room temperatures fluctuate more than those in the thermal mass and, as might also be expected, the water temperature fluctuates more than the soil does 5 feet down.  But as discussed below, all of the differences recorded for 2023 vs. 2024 between the three thermometers are largely insignificant, averaging only a few degrees.

We went to great lengths to control the water table with French drains at the time we excavated the house site.  The purpose of the drains was to syphon off ground water before it could saturate the soil and carry away the valuable heat stored 

Piezometer protruding through living room floor

in the thermal mass.  We monitored the water table by dropping through the piezometer to the 15' depth a spike nail attached to a cord (pictured above).  If water existed, the nail and the cord would be moistened in a way that could be measured. In 2023, water first appeared in mid-March, peaked at 32" in May and was gone by the first of July.  In 2024, it appeared in early May, peaked at 30" in early June and stood at 18" by mid-June, when this was written.  It would appear that the drains are holding the water to a level slightly deeper than 12' below the floor of the house and the highest levels exist for only 4 - 6 weeks.  These phenomena do not pose a serious risk to the thermal performance of that part of the thermal mass that influences indoor temperatures.

Discussion

While the graphs provide an overview, the data in table form flesh out the story.  However, it needs to be said that taking accurate temperatures with a thermometer small enough to fit inside of the piezometer was frustrating due to its small graduations. So the subfloor figures in the table are accurate to within +/- 2 degrees at best.  The bold figures on the table are the high and low temperatures over the 18 month period.

It is easy to see a couple of things with regard to high temperatures.  First, based on the one full year (2023) for which data is available, the highest temperatures in the living space, in the thermal mass and in the shallower depths in the thermal mass, were recorded in late summer and early fall instead of in the middle of the summer.  Second, the high readings for the 15 foot depth were so erratic that I was uncomfortable settling on one high figure.  Suffice it to say, though, that the numbers overall for the first couple of years do show a slight warming of the deeper thermal mass.

The low temperatures in the living space and thermal mass occurred during the late winter and early spring for both years, rather than in the dead of winter, but the ones for 2024 are higher than in 2023 by, in some cases, 4-6 degrees.  However, at the 15 foot level, readings year-over-year haven't changed much, maybe a degree or two.  

The outdoor temperatures are useful only in general terms because they vary so much and  cannot be captured with one or two readings a month.  A good example is the low of 35 in February 2023 and a balmy 56 in February of 2024.   However, they do show how the temperatures in our passive solar structure remain comfortably temperate without conventional HVAC, irrespective of the fluctuations of outside temperatures. 

SUMMARY

Seasonal temperatures in the living space and in the thermal mass move up and down slightly but do not deviate much from each other.  The thermal mass stays a little cooler than the room temperatures and changes slower.  There is only a weak correlation between temperatures inside and outside the house, which could be expected given the thickness of the wall and ceiling insulation and lack of air infiltration.  Temperatures within the house do not vary much between floor level and ceiling level, also due to lack of air infiltration.  Contrary to expectations, the temperatures in the thermal mass during the first half of 2024 appear to be almost identical to those in the first half of 2023.  The water table rises to within 12' of floor level but only for 4 - 6 weeks in the late spring or early summer and as such it does not threaten the efficacy of the passive solar design.

These are very preliminary observations based on 18 months data.  We will continue to monitor and report as more data become available.  

Our lower Midwest hot and humid climate is different than the high-plains / mountain west climate of the two pioneers whose designs informed ours (click on  "Featured Post" in the left sidebar to access a series of posts describing their passive solar designs).  So it is understandable that, with a warming globe, the summer temperatures in our house could reach the tipping point for future owners who will choose to add conventional air conditioning.  Our current photovoltaic array might even then continue to generate more electricity than consumed (meter running backwards) since its solar gain peaks during the air conditioning season.  If not, it could be minimally enlarged to handle the small amount of air conditioning that would be necessary for comfort.  A nice trade-off, though, is that, with global warming, inside winter temperatures might morph in the right direction.

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UPDATE:  December 31, 2024

The numbers are in for the latter half of 2024 with interesting implications for the entire data set.

The following table is different from the chart above (for the twelve months of 2023 and the first six months of 2024) in that the last six months of 2024 are included and it is reconfigured into cold vs. warm months.

(The bold figures in the chart are the high and low temperatures for each season.)

2023 versus 2024

The best way to look at the figures is to compare the cold months of 2023 to the cold months of 2024 and the same for the warm months.  As was pointed out in the original posting, the coldest temperatures did not occur during the middle of the winter.  They were recorded during March and April after the heat stored in the thermal mass was diminished to the extent that it warmed the house less efficiently.  Similarly, the warmest temperatures did not occur during the early months of summer but transpired during the late months of August and September when the uptake of heat by the thermal mass leveled off.

The figures for last six months of 2024 round out the picture so that additional observations are possible.  For example, I had predicted during the construction phase that it would take several years for the temperatures in the thermal mass under and behind the house and under the insulation/watershed umbrella to reach equilibrium, i.e., remain constant year-to-year.  As soon as we moved into the house in the late spring of 2022, I began taking the temperature of the house but in a ways that were not as encompassing as those that are reported here for 2023 and 2024.  But the less sophisticated figures did show temperatures that were colder in winter and hotter in summer than in the later years.  

So by 2023 I began to think that it might indeed take several years for temperatures to equalize and was surprised when the 2023-2024 temperatures seemed to indicate otherwise.  As a test, I averaged the figures for all nine columns for indoor temperatures and then combined them to give one temperature reading for cold months and the same for the warm months of 2023 as well as for the cold months and warm months of 2024.  The figures for the cold months differed by only one degree -- 69 or 2023 and 70 for 2024.  The figures for the warm months were identical at 74 degrees.  In a nutshell, it seems that the thermal mass temperatures have stabilized quicker than expected.

Interesting Correlation

The last graph in the original 18-month post labeled "Room and Shallow Thermal Mass" shows an interesting correlation between the room and the subfloor temperatures.  The room temperatures were taken from a thermometer on the wall in mid-house and the subfloor temperatures were recorded via water drawn from a water supply line running in the thermal mass about a foot below floor level.  The first take-away from the graph is that the temperatures at the subfloor level ("water temperature") and at five feet below floor level ran in parallel and separated from each other by only a few degrees, validating the use of the subfloor temperatures as representative of the thermal mass.  The second take-away is that the room and the subfloor (thermal mass) temperatures ran in parallel and close together except for a spike in room temperatures during the warm months.

This phenomena can be better appreciated in table form.

The striking thing is that air and thermal mass temperatures are clearly bunched by seasons --  "balanced" for cold months and "absorbing" for warm months.  This means that, during cold months, the temperature of the thermal mass and the living quarters were within at least three degrees of each other but not at the same level throughout the cold month season.  The heat in the thermal mass was indeed gradually depleted but not so fast that the room temperatures fell faster.  And the room temperatures never reached a level that could not be handled with a few mobile space heaters -- a situation that would be impossible in houses without HVAC that were not as tight, as well insulated and as earth-sheltered.

Unlike the matched cold month temperatures, there was a noticeable deviation, except for early (May) and late (October) months, between warm month temperatures in the living space and those in the thermal mass, especially as the warm season progressed to August. This means that, despite maximum uptake of heat by the thermal mass, it is unable to absorb heat as fast as the house accumulates it.

Typically, the conversation around passive solar is about staying warm in the winter, which is undoubtedly warranted in latitudes north of our St Louis area   However, the problem with passive solar this far south will continue to be staying cool in summer.  While global warming may improve our winter comfort, it will worsen our summer comfort to the extent that "passive solar" may include beefed up photovoltaic (solar) panels that support a minimum of air conditioning. 

Human Comfort and Mean Radiant Temperature

Another way of looking at the relationship between the air temperatures and the subfloor (thermal mass) temperatures is through the lens of mean radiant temperature.  The following passage is taken from the first blogpost listed under Understanding Our Project - Selected Posts at the top of the left column under my picture.

Mean radiant temperature (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 gain or loss than a 1 degree change in air temperature.  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.  For us this phenomenon held true in summer more than in winter.  Take for example  August 2024:  if the thermal mass had not stayed 12 degrees cooler than the room air, the comfort level would have been dictated by the 86 degree room temperature.  Instead, with the thermal mass at 74 degrees, the comfort level would have been around 78 degrees.  The fact that thermal mass and air temperatures were balanced during cold months meant that the heat stored in the thermal mass during the warm months was sufficient in quantity and availability to moderate air temperatures during cold months -- not enough to keep them from falling as low as the mid-60s eventually but enough, with the help of portable space heaters, to be comfortable.

Humidity

Another interesting phenomenon has been the amount of humidity in the house.  For the first year, humidity was unpleasantly high due to moisture trapped in the soil under the floor and under the insulation/waterproof umbrella, particularly the soil behind the concrete back wall, the only escape for which was through the floor and back wall into the living quarters.  By early 2023, the humidity stabilized to the 50 - 60% range for most of the year so that humidifiers are no longer necessary and portable dehumidifiers are utilized sparingly to get rid of humidity that originates within the house.  But the Energy Recovery Ventilator and exterior doors do admit moist air that has to be monitored and occasionally handled with dehumidifiers.