Thermal Lag – Calculating Heat Loss Into the Ground

Thermal Lag – Calculating Heat Loss Into the Ground

This page is about understanding thermal lag and thermal mass. These are helpful to understand for any in-ground constructions and, in our case, this includes the Duplicable City Center basement, Earthbag Village (Pod 1) living structures, Tropical Atrium, and Aquapini and Walipinis structures. In accordance with our open source philosophy and desire to help others interested in sustainable building too, we share here the results of our research into this topic.

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WHAT ARE THERMAL MASS & THERMAL LAG

thermal lag, thermal mass, soil temperature, heat capacity of soil, climate battery, in-ground building, in-ground construction, below ground construction, Highest Good housing, weather and soil, Earthbag Village, One Community, One Community GlobalThermal mass is a material’s resistance to change in temperature and the ability to absorb and store heat energy. Thermal lag is the name given to the delay in the stored heat being released from the material as the ambient temperature decreases. So thermal mass is a material that stores energy and how long that energy is able to be stored as surrounding temperatures change is called thermal lag.

For a material to provide appropriate levels of thermal mass, a combination of three basic properties is required:

  1. A high specific heat capacity; so the heat energy/kg is maximized
  2. A high density; the denser the material, the more heat it can store
  3. Moderate thermal conductivity – so the rate that heat flows in and out of the material is roughly in sync with the daily heating and cooling cycle of the building

Thermal lag will then depend on the effectiveness of the thermal mass and how extreme the ambient temperature variations are. The better the thermal mass is at holding heat from the hotter parts of the day, and the less extreme the ambient temperature fluctuations are, the slower the thermal lag and the easier it is to regulate temperatures within structures by understanding and using both thermal mass and thermal lag.

 

WHY ARE THERMAL MASS AND
THERMAL LAG IMPORTANT

thermal lag, thermal mass, soil temperature, heat capacity of soil, climate battery, in-ground building, in-ground construction, below ground construction, Highest Good housing, weather and soil, Earthbag Village, One Community, One Community GlobalThermal mass and thermal lag are particularly useful where there is a big difference between day and night outdoor temperatures. Thermal mass can be used to store heat when it is warm so that that energy can then be released/used when temperatures drop. Understanding how this works and how it relates to thermal lag is especially helpful in the case of in-ground structures and/or construction of Climate Batteries. In our case, the Duplicable City Center basement, Earthbag Village (Pod 1) living structures, Tropical Atrium, and Aquapini and Walipinis structures are all designed for in-ground construction. They also all include climate batteries, so we’ve invested a significant amount of time researching and understanding both thermal mass and thermal lag. We share here all we have learned in accordance with our open source philosophy and desire to help others interested in applying what we’ve learned too.

 

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CONSULTANTS ON THERMAL LAG

Vamsi Pulugurtha: Mechanical Engineer (Thermal lag calculations, designs, and renders)

 

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THERMAL MASS & THERMAL LAG DETAILS

Thermal mass and thermal lag details are discussed here with the following sections:

The graph below shows how temperature varies depending on the structure and materials used in the structure.

Thermal Lag Temperature over time

  • Appropriate use of thermal mass will not only provide comfort but also help in reducing heating and cooling bills.
  • Thermal mass is not a substitute for insulation. A high thermal mass material is not generally a good thermal insulator.
  • The basic function of thermal mass is to store and release heat energy. The three most important factors are emissivity, heat capacity and thermal conductivity.
  • In general terms, thermal mass comes into picture when the outside temperature is both above and below the desired indoor temperature during a normal 24-hour cycle. If the outside temperature stays warmer or stays colder than the indoor temperature, insulation would be ideal to use.
  • On the other hand the basic function of an insulation material is to obstruct or delay heat transfer which can happen via any combination of three basic modes of heat transfer, conduction, convection, and radiation. A properly insulated building stops heat flowing into or out of the building.

 

TEMPERATURES AROUND THE YEAR

  • If we take a look at the temperatures around the year for our location in the southwest, the temperatures fluctuate from 20.0º F to 90.0º F. The wide range of temperatures warrant a thermal mass integrated with sound design techniques, which means having appropriate areas of glazing facing appropriate directions with appropriate levels of shading, ventilation, insulation and thermal mass.
  • Thermal mass is most appropriate in climates with a large diurnal temperature range (day-night). For our specific location high mass construction with high insulation is desirable since the diurnal range is over 10° C. Since we have an underground/earth covered basement which protects the basement from solar radiation and provides additional thermal mass through earth coupling to stabilize internal air temperatures, ideal combination of thermal mass and insulation can be used to provide required thermal comfort very economically.
  • Based on the FEA analysis done the average temperature of the boiler room was estimated to be 85° F. The thermal properties of concrete, EPS insulation and other materials are provided in the table below.

Time lags for individual materials for specific thickness are listed in the table below.

There are two critical factors in determining heat storage capabilities of any material.

  1. The time it takes for heat wave to propagate from outer surface to the inner surface, named as time lag.
  2. The decreasing ratio of its amplitude, called decrement factor.

 

  • Time lag (hours) is the time delay of reaching the peak maximum outdoor temperature to peak maximum indoor temperature.
  • Decrement factor is the ratio of the indoor temperature’s amplitude to the outdoor temperature’s amplitude.
  • For our current structure, 10 inches of EPS insulation will give a time lag of 11 hours. 8 inches of concrete will give a time lag of 5.6 hours. Depending on the required combinations of materials and thicknesses, the time lag can be directly calculated from the table above.

 

SOIL THERMAL LAG

  • The mean annual surface temperature of any location depends on three important factors:
    1. The exact depth at which you are measuring the temperature.
    2. The proximity of geothermal heat sources.
    3. The proximity of ground water.
  • Air temperatures vary over 24-hour cycles. Soil temperatures go through similar cycles, but at greater depths the changes in ground temperature lag farther and farther behind air temperature and eventually the amount of temperature change is much less.
  • The major factor when we are talking about soil temperatures is the depth. At about five feet down, ground temperatures lag three months behind seasonal air temperatures. The lag keeps increasing as you keep going deeper and it reaches six months at 15 feet.
  • Going even further, 30 feet below the soil temperatures are constant, and are more or less equal to the average annual air temperature.
  • At around 32 feet (about 10 meters), the temperature of the soil takes on the average temperature of that location throughout the year.
  • Further down at more than 150 feet or more there is a steady increase of 2.6° C per 320 feet (100 meters).

 

BUILDING ABOVE GROUND VS UNDERGROUND

Different types of soil have different amounts of water and therefore different thermal conductivity levels. Thermal conductivity is measured in Watts per meter-Kelvin (W/m K). The thermal conductivity of soil is an important factor to consider when building underground.

Thermal Conductivity of different types of soil

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Below we can see the thermal differences between building above ground versus underground.

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  • Above pictures show a hot day with external atmospheric temperature being 95° F.
  • The contours on the left show the temperature distribution when the building is above the ground.
  • The contours on the right show building under the ground.
  • We can clearly observe a difference of 15° F in the average temperatures between the two scenarios and depending on the required temperature the area needs to be maintained at, additional things must be considered like insulating the external walls as well apart from insulating the boiler room or whatever may be the room with the heat source.

 

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Here is an analysis of the soil temperatures of the ground surrounding the basement.

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BOILER ROOM DETAILS

The thermal conductivity data for the boiler room is covered in the diagrams below.

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  • (Above image) Based on the previous analysis done on the boiler room, average temperature of the boiler room floor was found out to be 95° to 100° F.
  • In the current study the boiler room floor is assumed to be at 95° F to monitor the heat loss into the ground.
  • Three different materials are considered below the floor.

 

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  • We can clearly observe the changes in temperature as we go deeper into the ground.
  • The rate at which temperature drops depends directly on the temperature of the surrounding soil, which in turn depends on depth and the corresponding air temperature above the ground.

 

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  • When we look at the temperature contours on the floor going into the ground, the temperatures drop by about 2 degF (from 95 degF) by the time they reach the XPS insulation layer.
  • From XPS insulation layer just before reaching the aggregate base, the temperatures drop about 20 degF.
  • By the time we reach the aggregate base, the temperatures drop by 40 degF.

 

ENTIRE BASEMENT DETAILS

Thermal conductivity data for the entire basement, including the boiler room:

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CONCLUSIONS

We can clearly observe the difference in temperature dissipation for different types of soil with different thermal properties. For the soil types in the lower thermal conductivity range (0.22 W/m), we can observe a higher temperature transfer from the building into the soil. For the soil types in the higher thermal conductivity range (3.5 W/m), we can observe the temperature transfer is much lower compared to the lower thermal conductivity soil. (Ref: P17)

Depending on the soil type and external temperatures, we can get a very good understanding of how much heat is being dissipated into the ground from this data.

Under the ground constructions/coupling with the earth provides the occupants with resultant indoor temperatures which are far more stable. (Ref: P18 & P20)

Click to Enlarge

 

RESOURCES & REFERENCES

Related resources can be found below:

Thermal Conductivity – Wikipedia

 

References:

  1. Ahmed, T. (2012). Investigating The Impact Of Thermal Mass. Ryerson University.
  2. Asan, H. (2005). Numerical computation of time lags and decrement factors for different building
    materials. Department of Mechanical Engineering, Karadeniz Technical University, 61080
    Trabzon, Turkey.
  3. H. Asan, Y. S. (1997). Effects of Wall’s thermophysical properties on time lag and decrement
    factor. Department of Mechanical Engineeting, Karadeniz Technical University, 61080 Trabzon,
    Turkey.
  4. Hadi Ramin, P. H. (2015). Determination of optimum insulation thickness in different wall
    orientations and location in Iran. Advances in Building Energy Research.
  5. Mitalas, G. (1982). Basement Heat Loss Studies. DBR Paper No. 1045 of the Division of
    Building Research.
  6. Shea, S. M. (2013). Performance Evaluation of Modern Building Thermal Envelope Designs in
    the Semi-Arid Continental Climate of Tehran. http://www.mdpi.com/journal/buildings/.
  7. Srivastava, B. S. (2008). Influence of thermal insulation on conductive heat transfer through roof
    ceiling construction. Central Building Research Institute (CBRI).

Websites:

  1. Article: Passive Home Design Using Thermal Mass
  2. Article: Thermal Lag Heating
  3. Article: What’s the Difference Between Insulation & Thermal Mass?
  4. Article: Thermal Mass vs Insulation: Materials Choice
  5. Article: Cob, Straw Bale or Earthbag: Which Is the Best?
  6. Article: Autodesk.com – Thermal Properties of Materials
  7. Article: British Geological Survey (BGS.ac.uk) – Ground source heat pumps
  8. Article: Ground Temperatures as a Function of Location, Season, and Depth

 

 

SUMMARY

Having a temperature analysis of the surrounding air/soil inside and around the structures we are designing gives us a very clear picture of what is going on. With thermal mass and thermal lag info in hand, accurate and sound decisions can be made to create sustainable structures and designs. Using thermal mass to our advantage means our buildings will have a much higher average temperature in the winter, and a lower average temperature in the summer, which greatly reduces our heating and cooling costs throughout the year.

 

FAQ

Q: What is the difference between thermal lag and thermal mass?

Thermal mass is a material’s resistance to change in temperature and the ability to absorb and store heat energy. Thermal lag is the name given to the delay in the stored heat being released from the material as the ambient temperature decreases.

Q: What is the context of the basement and the boiler room within the larger One Community project?

 

 

(Full unedited copy from original word doc)

Thermal Mass & Thermal Lag

Thermal mass is a material’s resistance to change in temperature and the ability to absorb and store heat energy. Thermal lag is the name given to the delay in the stored heat being released from the material as the ambient temperature decreases.

For a material to provide appropriate levels of thermal mass, a combination of three basic properties is required:

  1. A high specific heat capacity; so the heat energy/kg is maximized.
  2. A high density; the denser the material, the more heat it can store.
  3. Moderate thermal conductivity – so the rate that heat flows in and out of the material is roughly in sync with the daily heating and cooling cycle of the building.

Thermal mass is particularly useful where there is a big difference between day and night outdoor temperatures.

The graph below shows how temperature varies depending on the structure and materials used in the structure.

Thermal Lag Temperature over time

  • Appropriate use of thermal mass will not only provide comfort but also help in reducing heating and cooling bills.
  • Thermal mass is not a substitute for insulation. A high thermal mass material is not generally a good thermal insulator.
  • The basic function of thermal mass is to store and release heat energy. The three most important factors are emissivity, heat capacity and thermal conductivity.
  • In general terms, thermal mass comes into picture when the outside temperature is both above and below the desired indoor temperature during a normal 24-hour cycle. If the outside temperature stays warmer or stays colder than the indoor temperature, insulation would be ideal to use.
  • On the other hand the basic function of an insulation material is to obstruct or delay heat transfer which can happen via any combination of three basic modes of heat transfer, conduction, convection, and radiation. A properly insulated building stops heat flowing into or out of the building.

 

Temperatures Around the Year

  • If we take a look at the temperatures around the year for our location in the southwest, the temperatures fluctuate from 20.0º F to 90.0º F. The wide range of temperatures warrant a thermal mass integrated with sound design techniques, which means having appropriate areas of glazing facing appropriate directions with appropriate levels of shading, ventilation, insulation and thermal mass.
  • Thermal mass is most appropriate in climates with a large diurnal temperature range (day-night). For our specific location high mass construction with high insulation is desirable since the diurnal range is over 10°C. Since we have an underground/earth covered basement which protects the basement from solar radiation and provides additional thermal mass through earth coupling to stabilize internal air temperatures, ideal combination of thermal mass and insulation can be used to provide required thermal comfort very economically.
  • Based on the FEA analysis done the average temperature of the boiler room was estimated to be 85F. The thermal properties of concrete, EPS insulation and other materials are provided in the table below.

Time lags for individual materials for specific thickness are listed in the table below.

There are two critical factors in determining heat storage capabilities of any material.

  1. The time it takes for heat wave to propagate from outer surface to the inner surface, named as time lag.
  2. The decreasing ratio of its amplitude, called decrement factor.
  • Time lag (hours) is the time delay of reaching the peak maximum outdoor temperature to peak maximum indoor temperature.
  • Decrement factor is the ratio of the indoor temperature’s amplitude to the outdoor temperature’s amplitude.
  • For our current structure, 10 inches of EPS insulation will give a time lag of 11 hours. 8 inches of concrete will give a time lag of 5.6 hours. Depending on the required combinations of materials and thicknesses, the time lag can be directly calculated from the table above.

 

Soil Thermal Lag

  • The mean annual surface temperature of any location depends on three important factors:
    1. The exact depth at which you are measuring the temperature.
    2. The proximity of geothermal heat sources.
    3. The proximity of ground water.

 

  • Air temperatures vary over 24-hour cycles. Soil temperatures go through similar cycles, but at greater depths the changes in ground temperature lag farther and farther behind air temperature and eventually the amount of temperature change is much less.
  • The major factor when we are talking about soil temperatures is the depth. At about five feet down, ground temperatures lag three months behind seasonal air temperatures. The lag keeps increasing as you keep going deeper and it reaches six months at 15 feet.
  • Going even further, 30 feet below the soil temperatures are constant, and are more or less equal to the average annual air temperature.
  • At around 32 feet (about 10 meters), the temperature of the soil takes on the average temperature of that location throughout the year.
  • Further down at more than 150 feet or more there is a steady increase of 2.6C per 320 feet (100metres).

 

References

  1. Ahmed, T. (2012). Investigating The Impact Of Thermal Mass. Ryerson University.
  2. Asan, H. (2005). Numerical computation of time lags and decrement factors for different building materials. Department of Mechanical Engineering, Karadeniz Technical University, 61080 Trabzon, Turkey.
  3. H. Asan, Y. S. (1997). Effects of Wall’s thermophysical properties on time lag and decrement factor. Department of Mechanical Engineeting, Karadeniz Technical University, 61080 Trabzon, Turkey.
  4. Hadi Ramin, P. H. (2015). Determination of optimum insulation thickness in different wall orientations and location in Iran. Advances in Building Energy Research.
  5. Mitalas, G. (1982). Basement Heat Loss Studies. DBR Paper No. 1045 of the Division of Building Research.
  6. Shea, S. M. (2013). Performance Evaluation of Modern Building Thermal Envelope Designs in the Semi-Arid Continental Climate of Tehran. http://www.mdpi.com/journal/buildings/.
  7. Srivastava, B. S. (2008). Influence of thermal insulation on conductive heat transfer through roof ceiling construction. Central Building Research Institute(CBRI).

Websites:

  1. http://www.yourhome.gov.au/passive-design/thermal-mass/
  2. http://www.hineslab.com/thermal-lag-heating/
  3. http://danieloverbey.blogspot.com/2013/05/whats-difference-between-insulation-and.html/
  4. https://treeyopermacultureedu.wordpress.com/natural-building/thermal-mass-vs-insulation-materials-choice/
  5. http://www.motherearthnews.com/green-homes/earthbag-cob-strawbale-zbcz1605/
  6. https://sustainabilityworkshop.autodesk.com/buildings/thermal-properties-materials/
  7. http://www.bgs.ac.uk/reference/gshp/gshp_report.html/
  8. http://www.builditsolar.com/Projects/Cooling/EarthTemperatures.htm/