Insulation vs Thermal Mass: Less is More

Can you believe that using less insulation on the inside of your walls can actually result in increased interior comfort? A good way to think about this section is to consider every building as a combination of “Shelter” and “Comfort”. Most projects consider the exterior envelope as the primary means of providing shelter, while there are a number of ways to achieve internal comfort.

In the western U.S., we have become perhaps too comfortable with relying on insulation and mechanical systems as our primary method of achieving comfort inside our buildings. With the rising cost of energy, coupled with the advancement of holistic energy modeling software, we now know that thermal mass is perhaps the most powerful component to a successful energy management strategy.

Below is a graph of the basic principles of temperature, comfort, insulation and thermal mass.

Let's review some of the basics of achieving internal comfort.

INSULATION

“Insulation” is a familiar material and is measured in Rvalue. While most of us know what an R-11 or R-19 Insulation solution means, we might not fully understand what insulation does, and why or when it should be used. Essentially, insulation “lags” the time it takes for an outside climate to enter a building through the building envelope. Unfortunately, because insulation does not “damp” or lower the impact of the outside climate, we are left to “condition” the interior climate, most commonly with a mechanical heating and cooling system.

A surprising fact is that for commercial projects, little or no insulation is required for CMU walls in most climate zones. In California, (one of the toughest jurisdictions nationally for energy compliance), the Title 24 Energy Code requires zero additional insulation, per Section 120.7(b)4, Mandatory Insulation Requirement for Heavy Mass Walls. But why is that?

THERMAL MASS “Thermal Mass” is the ability of a material to absorb and store heat energy. High thermal mass materials, like concrete, or water, or a pile of rocks, require a lot of heat energy to change their internal temperature, while low mass materials, like timber and steel, change their temperature quickly when exposed to heat. The general effect of thermal mass is to regulate a more stable interior temperature. The building codes generally define an 8-inch CMU wall as a “Heavy Mass Wall”.

CMU has a direct impact on stabilizing interior temperatures due to a combination of both high thermal damping and even higher thermal lag. (Insulation provides a low degree of thermal lag, and has little ability to contribute to thermal damping).

Ideally, the right balance of both insulation and thermal mass is designed for the specific climate zone of each project. Due to the relative lack of common knowledge related to thermal mass in the Western U.S., CMU offers perhaps the largest opportunity for improving the energy efficiency of structures, both old and new, in nearly every climate zone. In extreme climates, insulation may be used to reduce the thermal conductivity of walls.

Also, the modular nature of CMU allows it to be used easily in the renovation of existing structures, by allowing assembly of the masonry walls in just the right amount, location and configuration.

For a more detailed view of these principles, I strongly recommend you watch the video version of BAS301V - "Concrete Masonry Fundamentals | Architect-to-Architect".

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