Replies

1. GBA Editor
   Martin Holladay | Apr 24, 2013 06:43am | #1

   Brian,
   All thermal mass inside your building envelope (on the interior side of your insulation) helps somewhat when it comes to passive solar design. But some types of thermal mass are more effective that others. Ideally, any concrete in your home intended to act as thermal mass for passive solar design will be: (a) 4 inches in thickness or less, and (b) will be located where the sun hits the concrete or thermal mass.

   A big potted plant with a big ball of soil in a dark-colored pot qualifies, as long as the pot sits somewhere where the sun hits the pot.

   The classic solution is a dark-colored 4-inch-thick concrete slab on the south side of your house. But you could also build concrete furniture -- perhaps a concrete bench -- and put it near a south window.

2. 
   Bowler222 | Apr 24, 2013 07:11am | #2

   Thanks for the response Martin. I guess what I was looking for was rules of thumb to determine how much mass I need when not using traditional thermal masses such as concrete slabs and walls. For example a single small potted house plant isn't going to do much for me, however several large potted trees might. I'm trying to determine that line, if there is one.

3. GBA Editor
   Martin Holladay | Apr 24, 2013 09:08am | #3

   Brian,
   Here are some rules of thumb -- some of which are contradictory:

   http://www.eere.energy.gov/erec/factsheets/passive_solar.html
   "The ideal ratio of thermal mass to glazing varies by climate."

   Green Building Guidelines from SBIC: "The rule of thumb is that the thermal mass should be about six times the area of the direct-gain, south-facing glass. ... For most thermal mass materials, their energy effectiveness increases up to a thickness of about 4 inches. Mass thicker than 4 inches typically does not absorb and release heat quickly enough to be effective and worth the additional investment."

   The Green Studio Handbook by Alison Kwok and Walter Grondzik: "A general rule is to provide a concrete mass of 4-6 inches thickness that is about 3 times the area of the solar glazing. This assumes the mass is directly irradiated by solar radiation. A ratio of 6:1 is generally recommended for mass that receives only reflected radiation."

   http://solarhowto.us/ThermalMassForPassiveSolarHeatStorage.html
   "A rule of thumb for designing a comfortable Direct Gain system, which lets sun shine directly into rooms, is to distribute the thermal mass materials over a large area of walls and floor, about nine or ten times the area of south glass and about 4-6 inches thick (for earth materials)."

   http://passivesolar.sustainablesources.com/
   Direct gain system rules of thumb (Austin):
   Do not exceed 6 inches of thickness in thermal mass materials.
   Do not cover thermal mass floors with wall to wall carpeting; keep as bare as functionally and aesthetically possible.
   Use a medium dark color for masonry floors; use light colors for other lightweight walls; thermal mass walls can be any color.
   For every square foot of south glass, use 150 pounds of masonry or 4 gallons of water for thermal mass.
   Fill the cavities of any concrete block used as thermal storage with concrete or other high mass substance.
   Use thermal mass at less thickness throughout the living space rather than a concentrated area of thicker mass.
   The surface area of mass exposed to direct sunlight should be 9 times the area of the glazing.
   Sun tempering is the use of direct gain without added thermal mass. For most homes, multiply the house square footage by 0.08 to determine the amount of south facing glass for sun tempering.

   http://arch.ced.berkeley.edu/vitalsigns/res/downloads/rp/thermal_mass/mass-sml.pdf
   In order to be effective as a thermal mass, a material must have a high heat capacity, a moderate conductance, a moderate density, and a high emissivity. It is also important that the material serve a functional (structural or decorative) purpose in the building. .... Concrete and other masonry products are ideal, having a high capacity for heat storage, moderate conductance that allows heat to be transferred deep into the material for storage, high emissivity to allow absorption of more radiation than that which is reflected. When sized properly, concrete is effective in managing diurnal energy flow.

   * * * *

   Here's my advice, Brian: if you are trying to substitute some other material for concrete in your calculations, aim for roughly the same weight or volume (assuming that you are using materials of a similar density to concrete) as is recommended in the above rules of thumb. The result won't be mathematically perfect, but your answer will put you in the right ballpark -- and after all, these are simply rules of thumb.

4. 
   Bowler222 | Apr 24, 2013 09:14am | #4

   Thanks Martin!

5. 
   Tristan Roberts | Apr 24, 2013 10:03am | #5

   In my experience, a lot of people put large potted plants outside during the summer where the plants can be healthier and be repotted, etc. If these are part of your plan be sure that they will be there when they are most needed.

6. 
   user-1119462 | Apr 24, 2013 03:09pm | #6

   Don't forget that large plants transpire large amounts of moisture.

   Regards, Tony.