The impact of thermal mass vs insulation | Monday BS

A common misconception, especially in the world of sustainable or “green” building, is the impact of thermal mass vs thermal insulation. A lot of this confusion comes from well meaning design movements like “Passive Solar” and earthships. Often, in single-family design, there is an outsized expectation that the effect of thermal mass will be much greater than it is on the performance of the house. In other words, in residential buildings the impact of thermal mass is secondary compared to other elements of the building envelope (e.g. insulation, air sealing, windows, shading). On the other hand, in larger buildings with highly variable occupancy, thermal mass can start to be part of a strategy to optimize mechanical systems.

Research sources (some in German language):

What is this series? We’re trying a thing. Join Emu Co-Founder, Mariana Pickering, for a super casual and quick 5 -10 min video every Monday covering some commonly asked B.S. (Building Science) questions we get as Passive House educators. Requests for future topics highly encouraged in the comments!


  1. 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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