General question about thermal mass vs. insulation

Hello all, AFAIK, high thermal mass just means the interior of the structure will be generally the average of the high + low temperature / 2. Meaning if the daily high is 90 degrees, and the low 75 (SC Low Country in the summer), and we use a building technique with high thermal mass like Cob, Earthbag, or Rammed Earth, we can figure the inside temperature will be 82.5 degrees. Is this an accurate way to estimate what a structure will be like on the inside?

I'm going to say no not very accurate determining interior surface temps from outside air temps in low country with high summer heat and humidly levels using those materials that have a high moisture storage content, not without considering enthalpy or latent heat of evaporation. As a result, there should be significant temp differences between inside and outside air especially when solar gains and losses are considered.

Thermal Mass is defined more by an individual material's ability to absorb and store and then slowly release heat. Water stores more heat than stone for instance. And stone stores more heat than rammed earth. However a "wall" made of black painted milk jugs filled with water, will not be able to store as much heat as a stone wall that is three feet thick. The stone wall may take much longer to absorb the heat necessary to be able to be more useful in terms of heating the space. There are many variables based on what is desired.

Insulation (or isolation) on the other hand is the ability of a substance to resist the transference of heat through it. It is generally measured as the R value. The R is the resistance that the material has to the transfer of heat for every inch of material. Styrofoam is better than straw bale by R value, for instance. However, a straw bale wall that is many inches thick has a higher total R value than a two inch barrier of Styrofoam. It takes thicker Styrofoam (than 2 inches) to be better overall than straw bale.

Whatever materials you use for insulating or retaining heat (and their thickness), will determine how warm the interior of a space can be, given equal heat. What heat is hitting the surfaces and for how long, also play a roll in the warmth of the space.

The two concepts, though both useful in a structure, and can be combined, are up to individual circumstances to determine on a case by case basis as to when to utilize and to what degree. The idea of the build itself can be chosen to include one or the other or both to one degree or another and so it comes down to being partly personal choice as to what is the desired outcome considering of course the resources that a person wants to put into a project.

I would definitely say that the question as you posted it in this thread can be answered in the "no" category.

I hope that's helpful.

Pretty sure the question had to do with heat transfer coefficient not specific heat or conduction. If your a student or pro trying to determine building surface temps based on convective air flows and moisture the best software out there is WUFI, although there are some issues with ASHRAE 160 the air loops have been simplified, boundary conditions and climate files are more accurate for US building's now.

ORNL has a limited free version with earth mass on their website. Climate files get at Energyplus.

Hi everyone, thanks for your posts. I can see it is much more complicated than I had imagined. Right now I live in England and my garage is an old stone barn. It is roughly the average of high and low temps, every day, in there. Always warmer than the low temp and lower than the high. I figured that this was a general principle. I am really interested in using Rammed Earth to build my dream home when I return home, but I was told by a local architect (I'm really lucky to have a licensed natural building architect in my area back home) that cob and RE are poor choices due to their low insulative value... but I only see cob and RE used traditionally in hot climates so I'm not sure what she is talking about or why low insulation would be a deal breaker.

Matthew, your Architect is incorrect mass has insulation value even denoted in IRC building code as high or higher than some insulation's when wet, but it is dynamic not static like r-value. In cold climates an insulating thermal brake core may be required many have used effectively all over the internet. Many have used R-5-10 EPS in cold climates I prefer mineral wool board, or, run small electric PV hydronic radiation in the walls and floor, cool with a mini-split. I'd be surprised if you need a thermal break in SC at 12"-18" thick and depending on how conductive your soil is or whether it is low or high (k) soil. A pro RE designer knows how to design low-K soil Rammed Earth. Have her run a simulation model like WUFI at different thicknesses, or u-k values and/or heat transfer calcs..... It has many natural materials like earth types in the software. Your bigger concern in low country due to all the rain and humidity is making sure it is surface sealed and designed correctly. I use a silioxane or silane sealer on mine and it is holding up well. Good luck!