The impact of thermal mass vs insulation | Monday BS

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1. Marianna
   I do agree that thermal mass isn’t the be all end all of of temperature management in a home. However Passiv Haus gives it pretty short shrift in modeling and therefore its not used for what it is, as a tool.

   It does however effect the physics of the materials in a home. You didn’t actually explain what it is, just that you feel it isn’t important. This isn’t common sense however and without a basic understanding of physics or specific heat capacity of material we can all understand how it feels in Colorado to be in a basement in the summertime. This is a mass dominated environment, the outdoors are blazing hot and dry, the underground indoor space is cool or even cold. An upstairs living room in front of a bay of windows, even in a passiv haus with a poor overhang strategy, will be roasting even in light of the great insulation and superior windows and awesome ventilation.

   This space is not dominated by the mass but the solar gain.

   Another example of thermal mass effecting comfort is standing on a cool tile floor in a bathroom. Let’s say someone just hopped out of a 102 F shower only to stand on floor tile that’s 68 F. The mean radiant temp causes the floor to draw heat out of your body, and for some people this triggers a sense of pain. Now how could this be if the room is well heated with a forced air heating system? This makes no sense as we have great under floor insulation so it must be comfortable. We have a tight air seal and an ERV actively managing the humidity in our bathroom environment. My feet must be wrong… right?

   No, this is an example of thermal mass and how it affects the built environment. The specific heat capacity of the mass of the tile floor and substrate cause the floor to take on heat from the lower specific heat capacity air in the room which, amazingly, also has mass and can carry heat-thermal mass. In a home, if you use one of many strategies to heat the mass instead of the air, the effect can be that the floor may be closer to body temperature and therefore more comfortable on your feet while not overheating the space around your head or core that have a greater effect from your metabolism.

   The real crux of considering thermal mass as a tool to affect the comfort of your home or its real world effect on energy usage is how it exploits the source of power and even more difficult how to get the mass to give up the heat at the right time and space required.

   The examples I have provided above, heating mass with external power sources, is of course net neutral vs heating the air space. Because, as you guys would do, you measure the amount of energy it takes added to your heated space vs heat loss through the envelope thus the power demand required to maintain the homes pleasant environment. In that case, it does actually matter and you have minor effects for the time cycle e.g. a slab floor can take hours to give up its heat compared to plush carpet that can radiate heat in moments because of the increased surface area at 100 to 200 times the rate of the slab. You can observe this in a sunny room with large windows. With a dark slab floor with low emissivity, you will find some of the infrared reflected and bouncing around the walls in the room. You can observe this with a thermal camera seeing where the light hits the wall then increasing the surface temperature. Some of the air and particles in the air will warm and of course the floor will absorb much of the heat. If you change the emissivity of the floor by increasing a white color in the floor you will reflect more of the infrared away increasing the amount of infrared hitting the walls and ceiling. Now, where the thermal mass comes into play is the way in which the floor can absorb as much heat as the specific heat capacity of the floor, its mass and its density and some random material characteristics. Once the floor begins to exceed the temperature of the room it will begin to not only reflect the infrared but emit its own infrared. This can be good if the room is cold (compared to human comfort) or bad if the space is already warm and begins to overheat, driving cooling loads. This is what you were referring to when you observed how passive solar homes had problems with overheating. This is of course a problem in passive houses as well in the spring and fall. Now if you remove the slab example and we move to an example in the same room but with plush carpet, the carpet reflects infrared in the same way with different amounts of reflection based on color and reflectance. The difference is extremely low thermal mass as well as insulation from the mass below it, but this is compounded by the high surface area of the fibers increasing the amount of heat given up to the air through convection. This room overheats much faster than either slab example, but also loses heat through those dreaded fenestrations at the same rate but has little stored heat. Unfortunately the room will have to be be heated at night when renewable energy has less options. Good thing we have net zero to green wash our energy source for us.

   The real way to observe thermal mass and specific heat capacity is as a tool to store heat from the sun. If the heat comes from a non renewable source then your posit is correct, it has no value and should be ignored because now it’s just the energy flywheel but your net energy requirements are the same. If on the other hand you recognize that capturing solar energy in as many creative ways as you can and removing it so you don’t overheat and storing the energy to release at a time you need it, to avoid burning fossil fuels, you are on the right path to helping solve our seemingly insurmountable climate crisis. This ultimately is the real problem we face as a society using the sun when it isn’t present. The current engineering hurdles society faces is battery tech. Many crude technologies are superior to the fancy LiFePo4 batteries everyone covets. One of those is gravity and another is specific heat capacity of material or thermal mass.

   This is physics of course not Passivhaus. Passivhaus is about marketing a high quality energy modeling system, which is great but don’t turn Passivhaus into greenwashing ignoring actual science. Encourage creativity to solve problems of energy usage, beauty and comfort.

   I’ll close this with a final quick observation for the people who didn’t stop reading. The problem with getting heat out of the thermal mass is the crux. Concrete and steel (high mass objects) give up their heat slowly and take heat out of convecting air slowly. They conduct heat well and store it well but when your house is cold the surface area of say a slab (due to it’s smooth texture) reduces the area able to give up heat, it’s a bad radiator. That’s why hydronic heat needs to be so warm. So you need to increase the surface area for it to give heat up fast enough to matter.

   Additionally if your not convinced, just say you don’t exploit the lower delta T for insulation below grade due to the several months thermal cycle of subgrade soil. I can already feel the eyes glaze over of the one nerdy guy like me who is still reading, but I literally could go on for the next few hours describing the awesome current technologies used currently like molten salt solar farms to strategies like evacuated solar collectors and their problems to overcome and the potential solutions they can offer to controlling heat and comfort.

   Unfortunately, thermal mass is hard to model and effective execution of a strategy is the reason it’s passed over by Passivhaus practitioners. I personally love it because it offers strategies for homes without optimal solar orientation or multifamily solutions. I’m over the conspicuous consumption of the wealthy building effigies to their success so they can say, “look I saved the environment by building a house 10 times bigger than I need! I flew across the planet to talk about it and I must have saved the planet ten times over!” We need people who will pluck the creativity out of their very souls to bring these strategies to cost effective everyday use.

2. Let’s say your post is “short but sweet.” I guess the explanation is simple but informative. A residential building would benefit more from insulation, while thermal mass benefits larger buildings. Thank you for eliminating the confusion or misconception of many property owners. But I’m curious whether or not thermal mass and insulation could be combined. Will this provide better results? Any ideas would be appreciated.

   1. Of course one can combine thermal mass and insulation. However, the effect of thermal mass impacts performance in a close to negligible manner in residential construction. Our approach is: if there is thermal mass anyway, cool (eg a slab on grade). If not, it’s not that beneficial to add it to a house for the sole purpose of improving energy performance

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