Thermal Performance - First 18 months

 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 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.

An  earlier post described using several piezometers to monitor the behavior of the water table below the proposed house site that eventually lead 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, 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.  By contrast, convectional air movement in a typical -- leaky -- house results in temperatures that are 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.  And, with global warming, a thermal mass temperature that is not rising would be a plus for summers and a non-issue for warmer winters.

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 stood in one of the water lines running less than 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 did not appear until 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 chart 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 chart are accurate to within +/- 2 degrees at best.  The bold figures on the chart 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.  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 will morph in the right direction.