[GSBN] Fwd: Modeling or measuring mass effect of interior plaster

[Derek Stearns Roff]

I think the effect is better documented than the cause.  There are a number of papers quantifying the positive effect of thermal mass inside the insulated building envelope.  Some of them have very sincere-looking numbers, especially the ORNL article.  Other articles say that in order to be effective, the thermal mass must receive direct insolation, and/or must have a diurnal temperature swing above and below the desired indoor temperature, leading to thermal lag and the flywheel effect.  I've never seen numerical documentation for those assertions.  The ORNL article, which is so widely quoted, does not contain the words "sun" or "solar", and makes no statement about the range and mid-point of daily temperature swings.

Some articles state that the thermal mass effect will save energy/money based on heating and cooling with fossil fuel or electricity.  This seems to be independent of the solar gain question, and leaves open the question of daily swingers like John.  Some articles have mentioned that furnaces and air conditioners work better when they run on longer cycles, but that seems unlikely to explain all of the dynamic thermal performance listed by ORNL, nor its variation between the chosen cities.  Bohdan's article also documents the dynamic thermal mass effect, without insisting on the cause.

I'd love to understand this better, and find an article with more convincing information on why and how the thermal mass effect functions.  So far, it seems like important details concerning the cause are not known.

Derek "just 'cause" Roff

derek at unm.edu<mailto:derek at unm.edu>

On May 12, 2013, at 9:21 PM, John Swearingen wrote:

Glad there is someone intelligent checking these things, Derek.  The GBA article, and I think the research, as well, are centered around looking at buildings with heavy mass in exterior walls, particularly in direct contact with the sun, so the expected performance of the mass is contingent upon the thermal lag of the wall system and the solar glazing.  Our plastered straw bale presents a different case.

I would consider the plaster on bales to be essentially interior walls, because they're backed by so much insulation. So then the question becomes, what is the effect of high interior mass, the flywheel effect, on energy usage. The flywheel is more effective in buildings that experience a greater diurnal temperature swing, whether because of direct solar gain or conduction through walls and windows.  Few climates don't have temperature swings, though, and so, if I understand correctly, that would account for the dynamic insulation effect of mass in a variety of climates.

John "Daily Swing" Swearingen





On Sun, May 12, 2013 at 6:58 PM, Derek Stearns Roff <derek at unm.edu<mailto:derek at unm.edu>> wrote:
Certainly worth reading, but the primary information source for the Green Building Advisor (GBA) article doesn't fully support the conclusions that the article presents.  GBA references an earlier paper written by researchers at Oak Ridge National Laboratory (ORNL), and most of the other references also use the ORNL data. http://www.ornl.gov/sci/roofs+walls/research/detailed_papers/dyn_perf/index.html  The ORNL data says that even the worst case location, Minneapolis, showed a dynamic insulation effect of almost 1.5 times, such that insulation of R-14 plus internal mass would function like insulation like R-21 in a building lacking significant internal thermal mass.  The best case was location was Phoenix, which, in one of the examples, attained a dynamic thermal performance of 2.58 times.

Both of these locations are the least likely, of the six analyzed locations, to have the diurnal temperature swing above and below the desired indoor temperature, for most of the year.  Denver, which I judge most likely to have those daily temperature swings, was only the third best, and closer in performance to Minneapolis than to Phoenix, with top ratings of 1.88.  In order from best to worst, the six cities analyzed were Phoenix, Atlanta, Denver, Miami, Washington, and Minneapolis.

Unfortunately, the ORNL article doesn't break anything down by season or daily temperature variations, so no data is available to say whether the advantages of interior mass are more pronounced in the summer, winter, or spring/fall.  Several of the articles referenced in this GBA article make statements similar to the one the John quoted for us, but none of them offer any data to support the idea.

Derek


On May 11, 2013, at 9:25 PM, John Swearingen wrote:

Martin Holladay, as if he were listening to our discussion, just published an excellent summary "All About Thermal Mass" <http://www.greenbuildingadvisor.com/blogs/dept/musings/all-about-thermal-mass?utm_source=email&utm_medium=eletter&utm_content=gba_eletter&utm_campaign=green-building-advisor-eletter>  at Green BuildingAdvisor.com<http://Advisor.com/>, with references to several studies that have been done.

Most of this has been covered in this discussion.  One thing he makes explicit is that mass is most effective in lowering energy usage when the diurnal temperature swing is above and below the indoor temperature (ie: warm days, cool nights).  He also points out that mass is most effective in reducing energy usage in cooling environments because the thermal lag will shift air conditioning usage to the cool night hours, when air conditioners are more efficient.

It's a good read.




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