Editing Thermal mass

{{short description|Use of thermal energy storage in building design}}

[[File:Heavy_vs_light_weight_school_classroom.JPG|thumb|Graph showing the effect of Heavy-weight and Light-weight constructions on the internal temperature of a naturally ventilated school classroom]]
In building design, '''thermal mass''' is a property of the matter of a building that requires a flow of heat in order for it to change temperature. 

Not all writers agree on what physical property of matter "thermal mass" describes. Most writers use it as a synonym for [[heat capacity]], the ability of a body to store [[thermal energy]]. It is typically referred to by the symbol ''C''<sub>th</sub>, and its SI unit is J/K or J/°C (which are equivalent). 

However: 
* Christoph Reinhart at MIT describes thermal mass as its volume times its [[volumetric heat capacity]]. <ref>{{Cite web|url=https://ocw.mit.edu/courses/4-401-environmental-technologies-in-buildings-fall-2018/c03cdb9ea591216d81a3f2febd616a3c_MIT4_401F18_lec12.pdf|title=4.401/4.464 Environmental Technologies in Buildings}}</ref>

* Randa Ghattas, Franz-Joseph Ulm and Alison Ledwith, also at MIT, write that "It [thermal mass] is dependent on the relationship between the specific heat capacity, density, thickness and conductivity of a material" <ref>{{Cite web|url=https://cshub.mit.edu/sites/default/files/documents/ThermalMassBenefit_v10_13_0920.pdf|title=Mapping Thermal Mass Benefit }}</ref> although they don't provide a unit, describing materials only as "low" or "high" thermal mass.
* Chris Reardon equates thermal mass with volumetric heat capacity . <ref>{{Cite web|url=http://www.yourhome.gov.au/passive-design/thermal-mass|title=Thermal mass &#124; YourHome}}</ref> 

The lack of a consistent definition of what property of matter thermal mass describes has led some writers to dismiss its use in building design as [[pseudoscience]].<ref>{{Cite web |date=2022-07-21 |title=Thermal Mass and the Warming Climate |url=https://www.greenbuildingadvisor.com/question/can-high-thermal-mass-systems-backfire-in-a-warming-climate |access-date=2024-12-05 |website=GreenBuildingAdvisor |language=en-US}}</ref><ref>{{Cite web |date=2022-07-17 |title=Sizing HVAC System for ICF House |url=https://www.greenbuildingadvisor.com/question/unique-icf-home-that-im-having-trouble-sizing-hvac |access-date=2024-12-05 |website=GreenBuildingAdvisor |language=en-US}}</ref><ref>{{Cite web |date=2024-11-25 |title=Concrete, Thermal Mass, and Stable Ground Temps |url=https://www.greenbuildingadvisor.com/question/concrete-thermal-mass-and-stable-ground-temps |access-date=2024-12-05 |website=GreenBuildingAdvisor |language=en-US}}</ref>

==Background==
{{main|Heat capacity}}
The equation relating thermal energy to thermal mass is:
:<math>Q = C_\mathrm{th} \Delta T\,</math>
where ''Q'' is the thermal energy transferred, ''C''<sub>th</sub> is the thermal mass of the body, and Δ''T'' is the change in temperature.

For example, if 250&nbsp;J of heat energy is added to a copper gear with a thermal mass of 38.46&nbsp;J/°C, its temperature will rise by 6.50&nbsp;°C.
If the body consists of a homogeneous material with sufficiently known physical properties, the thermal mass is simply the mass of material present times the specific heat capacity of that material. For bodies made of many materials, the sum of heat capacities for their pure components may be used in the calculation, or in some cases (as for a whole animal, for example) the number may simply be measured for the entire body in question, directly.

As an [[extensive property]], heat capacity is characteristic of an object; its corresponding [[intensive property]] is specific heat capacity, expressed in terms of a measure of the amount of material such as mass or number of moles, which must be multiplied by similar units to give the heat capacity of the entire body of material. Thus the heat capacity can be equivalently calculated as the product of the [[mass]] ''m'' of the body and the specific heat capacity ''c'' for the material, or the product of the number of [[mole (unit)|moles]] of molecules present ''n'' and the molar specific heat capacity <math>\bar c</math>. For discussion of ''why'' the thermal energy storage abilities of pure substances vary, see [[Heat capacity#Factors that affect specific heat capacity|factors that affect specific heat capacity]]{{Broken anchor|date=2025-01-14|bot=User:Cewbot/log/20201008/configuration|target_link=Heat capacity#Factors that affect specific heat capacity|reason= The anchor (Factors that affect specific heat capacity) [[Special:Diff/896999528|has been deleted]].}}.

For a body of uniform composition, <math>C_\mathrm{th}</math> can be approximated by
:<math>C_\mathrm{th} = m c_\mathrm{p}</math>
where <math>m</math> is the mass of the body and <math>c_\mathrm{p}</math> is the isobaric [[specific heat capacity]] of the material averaged over temperature range in question. For bodies composed of numerous different materials, the thermal masses for the different components can just be added together.

==Heat capacity in buildings==
Christoph Reinhard describes the impact of heat capacity this way:<ref>{{Cite web |last=Reinhard |first=Christoph |date=July 12, 2021 |title=4.401/4.464 Environmental Technologies in Buildings |url=https://ocw.mit.edu/courses/4-401-environmental-technologies-in-buildings-fall-2018/c03cdb9ea591216d81a3f2febd616a3c_MIT4_401F18_lec12.pdf |access-date=December 10, 2024 |website=MIT.edu}}</ref> 

{{quote|
* If the outside diurnal temperature swing frequently oscillates around a desired (balance point) temperature, adding thermal mass may increase the hours of comfort in a given time interval.
* Thermal mass may act as a liability to keep a space comfortable e.g. when it is only used intermittently.
* Thermal mass has really no effect if the direction of heat flow through the building envelope stays constant for extended periods of time.{{verify quotation|date=January 2025}}}}

Heat capacity is not normally calculated in the engineering of buildings. In the United States and Canada, national building codes and most state and local jurisdictions require that heating and cooling equipment be sized in accordance with Manual J<ref>{{Cite book |title=Manual J Residential Load Calculation |series=ACCA Technical Manuals |url=https://www.acca.org/standards/technical-manuals/manual-j |access-date=2024-12-10 |publisher=[[Air Conditioning Contractors of America]] |language=en |archive-url=https://web.archive.org/web/20240907142225/https://www.acca.org/standards/technical-manuals/manual-j |archive-date=2024-09-07}}</ref> of the [[Air Conditioning Contractors of America]]. The Manual J process uses detailed measurements of a building's dimensions, construction, insulation, air-tightness, features and occupant loads, but it does not take into effect the heat capacity. Some heat capacity is presumed in the Manual J process, equipment sized according to Manual J is sized to maintain comfort at the first percentile of temperature for heating and the 99th percentile of temperature for cooling. The process presumes that the building has sufficient heat capacity to maintain comfort during brief excursions outside of those extremes.

===Construction examples===
* [[Earthship]]
* [[Rammed earth|Rammed earth wall]]
* [[Trombe wall]]

==See also==
* [[Specific heat capacity]]
* [[Thermal energy storage]]
* [[Thermal inertia]]

==References==
{{reflist}}

{{HVAC|state=collapsed}}

{{DEFAULTSORT:Thermal Mass}}
[[Category:Heating, ventilation, and air conditioning]]
[[Category:Heat transfer]]
[[Category:Mass]]
[[Category:Thermodynamics]]