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Code Green

Mass Walls Increase Insulation Value

New provisions in the IRC and IECC are making houses greener

Thermal mass walls are often used in conjunction with passive solar design strategies to gather heat in the day and release it in the evening.
Image Credit: Rob Yagid, Fine Homebuilding 204

9 Steps to A Greener Code

New homes built using the 2009 International Residential Code (IRC) or International Energy Conservation Code (IECC) will be more energy efficient than ever. As a consequence, a builder’s world may become a bit more complex and, in some cases, a bit more expensive. Recent spikes in energy costs have increased the attention on regulatory measures that limit energy waste.


The code: Refined insulation standards have been applied to mass walls to increase their performance in both hot and cold climates.

What it means to you: Code allows mass walls to be built to two different thermal-resistant standards based on the configuration of insulation in the assembly. The IRC classifies a mass wall as an above-grade wall made of concrete block, concrete, insulated concrete forms (ICF), masonry cavity, brick, earth, adobe, compressed-earth block, rammed earth, and solid timber/logs. Insulation installed on a mass wall creates what’s called “thermal lag.” The insulation increases the time it takes for hot or cold temperatures to transfer from the mass into the living space, reducing the strain on mechanical systems. Insulating the interior of a mass wall is more expensive because code requires a greater thermal resistance, which means more insulation.

The 2009 building codes reflect practices that not only increase energy efficiency—air-sealing measures and increased insulation, for example—but also address sustainable building practices, such as moisture control.

Other segments of this series:

Part 1: Air Sealing

Part 2: Insulation

Part 3: Lighting

Part 4: Programmable Thermostats

Part 5: Insulating Mass Walls

Part 6: Efficient Windows

Part 7: Insulating Mechanical Pipes

Part 8: Exceeding the Energy Code

Part 9: Vapor Retarders


  1. Andrew Henry | | #1

    By how much?
    I understand the merits of mass walls and have read that they do increase insulation values, and used them in the addition on my house. Conceptually I can comprehend the benefits thermally, but are they actually increasing R-value or is it because their thermal and hygroscopic storage characteristics are a net benefit to indoor comfort?

    Anyone want to put numbers to this? I worry that the benefits are overstated.

  2. furniturefarmer | | #2

    mass walls are of limited value
    "Mass walls" are not good isulators and "thermal lag" will do very little to increase energy efficiency perfomance or comfort during the heating season in cold climate zones. The fact that the wall stores heat does not increase it's resistance to heat flow and if the heat is flowing in only one direction (inside to outside) for extended periods of time ,(say, November to March) the mass is virtually worthless. However, where exterior temperature swings close to the human comfort range (60-90 degrees F) happen on a daily basis storing heat in the walls can be very useful for cooling. For example, flushing a building at night with 65 degree air can remove heat from the mass so that as the building heats up during the following day, the mass is able to absorb heat and maintain comfort.

  3. GBA Editor
    Martin Holladay | | #3

    More on thermal mass
    Here's some information on thermal mass effects from the GBA Encyclopedia:

    "ORNL research has found that ICF houses use less energy than the typical wood-framed home. But the thermal mass benefits of ICF houses depend on location: Houses in Minneapolis and Chicago showed the least savings from the thermal mass effect, while those in Phoenix and in Bakersfield, Calif., had the most. In all cases, potential whole-house energy savings were 10% or less when the R-value of the wall was 25.

    "Canadian researchers who closely monitored the performance of a multiunit residential building with ICF walls reported, "The overall building is relatively airtight, due in large part to the continuity of the ICF wall assembly, as no extraordinary air leakage control measures were undertaken at the roof and foundation levels." One of the researchers, Duncan Hill, commented, "The concrete is a poured-in-place air barrier." However, the researchers concluded that an ICF wall offers no thermal mass benefits in Canada.

    "According to an article in Environmental Building News, high mass can enhance energy performance, but only when outdoor temperatures cycle above and below the indoor temperature in a 24-hour period. In parts of the country where outside temperatures remain well below the indoor set temperature for weeks at a time, the mass effect isn’t really a factor."

    GBA subscribers can read more here:

  4. furniturefarmer | | #4

    peak loads
    One of the only benefits of mass is that it tends to soften/shift the peak loads. This can potentially allow smaller heating and cooling systems and increases passive survivability. A super inulated air-tight house with lots of mass might ride out very cold temperatures during the coldest nights without needing as much heat. Likewise a properly shaded house with mass will shift the peak cooling demand to later in the evening when it is easier to deal with. The problem is that it tends to be expensive and generally not worth the cost.

  5. Kevin O'Meara | | #5

    temperature swings
    Bothe the ORNL and Canadian studies have been done examining the value of adding thermal mass. In one Canadian study, the project was a very energy efficient design but of low thermal mass, which represents most residential contruction. The addition of a 4 inch thick cement floor moderated the peaks and troughs of the daily temperature swings that the interior of the house experinced. The ORNL has also examined this as mentioned above, and found the best results with the thermal mass insulated on the exterior, with results showing anywhere from a 10% to 25% predicted energy savings. That same Canadian study also studied the use of drywall with a 30% content of microencapsulated paraffin (Mirasol by BASF). They found it even slightly better than the 4 inches of concrete to moderated daily temperature swings. The ORNL has done a similiar study and projected a payback in 3-5 years. Concrete stores about 30 btu/cubic foot X F. The paraffin at it's melting point, or phase change stores close to 2700 btu/cubic foot X F. Obviously the drywall would be easier, and cheaper to install in a retrofit situation. Drywall containing PCM is currently available in Europe, called "Smartboard", additionally National Gypsum here in the US recently had a press release stating they are testing a similiar product, called "Thermacore" in California.

  6. Robert Riversong | | #6

    Limited Value
    Thermal mass does not change the R-value of a wall system or thermal envelope, but it can effect what is called the "dynamic benefit for massive systems" (DBMS), which is a measure of the equivalent R-value benefit of thermal mass walls in variable climates and with various insulation configurations.

    What these studies have demonstrated is that mass-enhanced walls are of most benefit (up to 3 DMBS, or 3 times the effective R-value) in climates like Pheonix AZ in which there are wide diurnal temperature swings from well above to well below room temperature. It is in such climates that massive walls work best because their effect is to delay and dampen temperature swings. Massive walls also work well in Miami's climate because they are more effective at reducing AC loads than at reducing heating loads.

    But of the four insulation strategies that were tested: interior insulation, exterior/interior insulation (like ICFs), insulation sandwiched between mass, and exterior insulation - the most effective was a mass wall that is tightly-coupled to the interior environment and insulated from the exterior environment. ICFs were second worse after interior insulation and generally offer little mass effect (contrary to industry claims).

    It's not at all evident that the phase-change impregnated drywall offers much benefit and particularly much cost-benefit, as I suspect a high price increment for such materials. While the phase-change paraffin might itself have a very high latent heat, the small amount integrated into National Gypsum's wallboard offers only 22 BTU per square foot of storage capacity which isn't must different than what an 8" thick concrete wall will store for each degree F increase in temperature, and only four times what normal gypsum drywall can store for each 10 degrees of temperature rise.

  7. Andrew Henry | | #7

    Durisol mass wall
    I chose to use Durisol for my exterior walls because of two things. First, the trend for green material and recycled content led me to assign an unquantifiable value to Durisol and secondly the high mass thermal 'benefit' that this Durisol document suggested.

    The more I have learned about effective insulation strategies, the more I sense that a thermal mass structural wall material like Durisol does not stand up to received wisdom. That is to say their benefit to the thermal performance of a building is overstated. Nor is it cost effective, or easy to build, compared with some of the wall strategies I have seen here on GBA.

    That said, I noticed Durisol being used in a project that I assume John Straube may be involved with as it appears in a project (in Waterloo) that is in a couple of Building Science Insights (1, 2 see page 4).

    I'm curious as to what John Straube thinks about the thermal performance of a mass wall like Durisol, or any other mass wall that has a thermal barrier on the exterior side.

  8. DavidS | | #9

    What about comfort
    What I think might be missed here is the comfort of the occupants. Thermal mass closely coupled to the living space will provide a radiant source/sink for the occupants. In the heating season the thermostat can be lowered and the occupant will still be comfortable as compared to the same setting in a space with less mass. (compare the set point of the thermostat for comfort in a home with radiant heat compared to warm air) The opposite is true for cooling. So even though we might be splitting hairs on energy loss/gain in the lab, there can be energy gains by keeping the occupant comfortable with less energy.

  9. Cameron Ware | | #10

    Mass walls benefit understated
    Martin, I appreciate your bringing up this out this ORNL report:

    "ORNL research has found that ICF houses use less energy than the typical wood-framed home. But the thermal mass benefits of ICF houses depend on location: Houses in Minneapolis and Chicago showed the least savings from the thermal mass effect, while those in Phoenix and in Bakersfield, Calif., had the most. In all cases, potential whole-house energy savings were 10% or less when the R-value of the wall was 25."

    This is very interesting: This is an ORNL thermal mass study. Folks use this report to say that there's only a small improvement between ICF and conventional construction. This report shows the mass superiority basely solely on thermal mass. .

    I find your "typical wood framed-home" to be awkward in the context of the study. Why?

    Your typical R-value for ICF about 23. Your typical home is substantially lower. Even with 2x8's and R-23 foam you will not have R-23 due to the fact that heat will prefer the path of least resistance (the stud). Your average builder will say he has R-23 since that's what he has in the cavity. A builder trying to correct for the studs might calculate an average (e.g. 20% studs at R-8 and 80% foam at 23) -- about R-20. The average (although well intentioned means nothing) the actual R-value based on 1/sum(u values) = 1/(.2 x u value wood + .8 x u value foam) is just under 17. This is assuming use of foam and no air infiltration.

    Why bother making this point? Because if you read the small print at ORNL -- this report is a thermal mass study -- specifically it establishes exactly what benefit thermal mass provides - if any --- therefore all wall systems utilized have the same R-value. Obviously, one can build a true R-23 in a wood framed wall system. But it's not that easy.

    Not to get into too much detail but similar fine print exists for the ICF comparision to other mass systems (again AS IT APPLIES TO SOLEY TO THERMAL MASS) -- e.g. CIC (Concrete Insulation Concrete) and Interior and Exterior Mass.

    Martin is 100% correct in that two of these systems CIC and Interior Mass perform even better than ICF. However, these systems are also subject to the same small print -- each system has and identical computer assigned R-value - how else would one correctly extract the benefit of thermal mass alone. Not holding R-value constant would have made this report confusing at best.

    A 2x8" foam wall system, or a CIC requiring 5 plus inches of internal foam to get to the typical ICF R-value --- or an Interior Mass system that utilizing the same exterior foam content and the same thermal mass of 6 inches of concrete --- each of these (better than ICF) wall systems are very rate indeed.

    You can meat or exceed ICF (on R-value) with staggered stud wall (where the studs are not in contact) or a 2x10 with foam.

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