Mass Walls and Required Insulation R-Value – Energy Code

Note: This is based on the 2006 IECC and this section changes in the 2009 code, so its always important to verify which code is adopted.

Mass walls are walls that provide energy efficiency through mass rather than insulative value. The mass allows them to store energy during the day and release it through the night. In the right climate they can be a better choice than walls that are lightly framed and heavily insulated. The misconception is that mass walls do not have any required R-Value.

However, the International Energy Code specifies that mass walls must still meet a set of R-value requirements. It defines mass walls as walls that are made of concrete block, concrete, insulated concrete form, masonry cavity, brick, earth, adobe and solid timber or logs. The insulation must be at least 50% on the exterior or integral to the wall to count. Otherwise you’re back in the wood frame wall insulation requirements. In other words, brick veneer or log siding don’t count.

If you do have at least 50% of the insulative value on the outside or integral  then your mass wall R-Values are:

  • Climate Zone 1: 3
  • Climate Zone 2: 4
  • Climate Zone 3: 5
  • Climate Zone 4 (except Marine): 5
  • Climate Zone 5 and Marine 4: 13
  • Climate Zone 6: 15
  • Climate Zone 7 & 8: 19

As you get into the colder zones the mass wall R-Value more closely approximates the wood frame wall R-Value.

If you happen to be in Climate zones 1-3 there is an exception for insulation placement that doesn’t meet the criteria. You can up your R-Values slightly to:

  • Climate Zone 1: 4
  • Climate Zone 2: 6
  • Climate Zone 3: 8


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1 thought on “Mass Walls and Required Insulation R-Value – Energy Code”

  1. “…then your mass wall R-values are:

    Climate Zone 3: 5


    I understand the value of thermal mass as increasing the effective R-value. But are you saying that this number is 5 (for my zone 3), independent of mass type and thickness? Or do I misunderstand what these numbers mean? I have a 12″ solid brick wall which I’m planning to insulate with a system consisting of 1/2″ stucco, 1″ polyisocyanurate, 1/2″ airspace, vapor/radiant barrier foil, 1/2″ airspace. What would the masswall share of the R-value of the 12″ brick wall. I am using R2.4 for the standard R-value of the 12″ brick.

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