Thermal Mass in the Sunspace:
Hypothesis, Methods, Data, and Analysis

Hypothesis

As stated previously on the hypothesis page, we believe the sunspace floor will act as thermal mass to help even out temperatures there.

Methods

In order to gauge whether the sunspace slab influenced the ambient temperature of the sunspace, we decided to look for the "lag" that is characteristic of thermal mass in action. This lag is caused by the ambient temperature of a space reaching its high or low consistently before the thermal mass associated with that space reaches its peak high or low temperature.

To check for this lag, we measured and compared the temperature of the slab and the ambient temperature of the sunspace. The proper way to measure the temperature of the slab for this analysis would be to calculate an average temperature over different depths in the slab. However, we were only able to measure the surface temperature of the slab because we could not drill in the Siegel's floor, so in this analysis, we use surface temperature as a proxy for the average temperature of the whole slab.

A section diagram of the sunspace
showing the location of the temperature sensors used
to test thermal mass performance in the sunspace

To evaluate this in our case, we used the data from two of the temperature sensors we placed in the sunspace. One measured the ambient temperature of the space. The thermistor of this sensor was located about 6 feet off the ground, and covered with a radiation shield so it would not receive any direct sunlight. The other data set we used is from a sensor measuring the temperature of the slab. It was located underneath a table in the sunspace, out of the direct sun, but in the center of the room. The thermistor of this sensor was placed against the tile floor and held in conductive contact with the floor by masking tape.

Data and Analysis

This graph shows the ambient and slab temperatures in the sunspace, and indicates where lag is occuring (14 k gif)

The graph above shows the ambient and the slab temperatures plotted for the second week in our investigation. We have identified the peak ambient temperatures and placed a vertical dashed lines on the graph at those points. The peak-lines associated with high temperatures are in red, and the lines for low temperatures are in blue. We have then placed circles on the graph to help identify the "lag" between the peak ambient temperatures (indicated by the vertical lines) and the peak slab temperatures. As you can see, in the circles highlighted yellow, a small lag is noticeable. In the rest of the circles, and especially the ones highlighted gray, there is little or no lag indicated. We take this as showing there is some effect from the thermal mass of the slab, but not a very pronounced one. And, as indicated by the weather icons at the bottom of the chart, the lag effect seems most pronounced on sunny days when the slab can really charge.

Conclusions

Our measurement techniques may have taken away from our ability to get an accurate picture of the action of the thermal mass sunspace floor. In the temperature measurements we could not drill down into the Siegels' floor to get temperatures at different levels in the slab, so we have to use a top temperature as proxy. But, we feel that we can still draw some conclusions from this data. Our graph shows either no action or only small action of the thermal mass, and so we believe it is not a strong force in affecting the temperature inside the sunspace.

Comments to author: vitalsigns@
ced.berkeley.edu

All contents copyright (C) 1998. Vital Signs Project. All rights reserved.

Created: 04/23/96
Revised: 09/09/02

http://www-archfp.ced.berkeley.edu/vitalsigns/workup/siegel_house/sgl_smas.html