What is the the thermal mass heat storage capacity of 1/2 inch of sheet rock?

| Posted in PassivHaus on

I understand that the BTUs per degree of F per square foot of 4″ thick brick pavers is 9, of builder’s brick is 6.5 and of hard wood is 1.7 per (Johnston and Gibson, Towards a Zero Energy Home). What are the BTUs per degree of F per square foot of 4″ thick sheet rock?
I am building a 1550 sf one-story zero energy home in Bend Oregon. It will have .6 ACH@50p and will have 50 R walls and floor and 60R ceiling. The windows will have a .2 u-value and a SHGC of .6. The south facing living area is approximately 600 sf.

I want to optimize solar gain and diurnal temperature stability at the lowest cost possible. This is not a passive house, but a zero energy home built at the lowest cost possible. I have several questions related to this:

How can we determine the optimal south facing (100% clear) window area for optimizing passive solar gain?

Once we determine the optimal window area, how can we determine how much thermal mass we will need to keep the house from over heating (assuming we have appropriate window overhangs for this location)? We do not want to spend extra on unnecessary thermal mass, nor do we want to fall short of optimal.

Since we do not have a slab, but do have a wood floor, will double sheet rock (1/2″ x 2)) plus wood floors possibly suffice? Or will we need added thermal mass, like masonry wall (\$\$)?

I understand the the Passive House soft ware may answer some of these questions but I do not have access. What I am looking for are some straight forward formulas that any designer could use for answering these questions.

Lastly, I have read that if the south facing window area is less than 7%, the default thermal mass (wood floors, single sheet rock walls, furniture, counter tops etc) is sufficient. Under what circumstances might that be true?

Thanks, Joe .

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Replies

1. GBA Editor
| | #1

Joe,
1. I don't know the specific heat capacity of drywall; nor do I know whether it would be particularly useful to know that value. Perhaps a GBA reader can help with information on that topic.

2. Here are some rules of thumb for thermal mass in a passive solar home (from http://passivesolar.sustainablesources.com/):
● "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; ... 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."
● "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."

3. Here is another rule of thumb (from 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)."

4. Another rule of thumb (from 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."

5. Another rule of thumb (from 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."

"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."

7. Here is a link to a GBA article on passive solar design: Cost-Effective Passive Solar Design.

2. | | #2

he lists a value of .29btu/lb/degF as the specific heat capacity[the term you need to look for rather than thermal mass] of gypsum drywall.

I have no idea about what to do with that information, and pretty much most people seem to shrug

One point is that most of your walls will not receive direct sun for very long, compared to the floor. Direct sun can raise the temperature of the surface above room temperature, or for the purposes of this conversation, the sun can pound energy into the mass which could be retrieved later. Walls that do not 'see' the sun cannot do this, they can only absorb energy that is the result of them being cooler than the room, and will only give it up when they are warmer than the room. So not to say that the extra mass of 2 sheets in a north facing bedroom is useless, but it could be a waste of money.

Essentially a high mass floor exposed to south facing glass, would allow you to overglaze the south wall without overheating the room as badly. I don't think walls would work as well.

3. | | #3

Hi Keith, Thanks for the specific heat capacity per lb of gypsum dry wall. How does that compare to masonry or concrete?
Thanks, Joe

4. GBA Editor
| | #4

Joseph,
According to this source (http://www.engineeringtoolbox.com/specific-heat-solids-d_154.html), here are some specific heat capacity values:

Common brick: 0.22 btu/lb/degF
Concrete or stone: 0.18 btu/lb/degF

5. | | #5

Thanks Martin and Keith

6. | | #6

Occurred to me that a ceramic tile or stone floor would give you some floor mass for not a big upcharge from hardwood. just build a nice sturdy floor system for long life.

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