Mass Walls Increase Insulation Value

By how much?
by Andrew Henry

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.

mass walls are of limited value
by Dave Brach

"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.

More on thermal mass
by Martin Holladay, GBA Advisor

Andrew,
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:
http://www.greenbuildingadvisor.com/green-basics/insulated-concrete-forms

peak loads
by Dave Brach

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.

temperature swings
by Kevin O'Meara

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.

Limited Value
by Robert Riversong

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.

Durisol mass wall
by Andrew Henry

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.

Here are the links
by Andrew Henry

Here are the links that I didn't anchor properly.

http://www.durisolbuild.com/Webdocs/Durisolthermalperformance.pdf

http://www.buildingscience.com/documents/insights/bsi-014-deciding-on-en...