Does R-Value Trump Thermal Mass?
Choosing between insulated concrete forms and wood-framed walls
Jesse Lizer’s plans for a new house in Climate Zone 6 call for a 60-foot long walkout basement wall on the north side. The three below-grade foundation walls will be built with insulated concrete forms (ICFs) with an R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. of roughly R-25.
Lizer’s question, posted in GBA’s Q&A forum, concerns the exposed north wall of the foundation. Should it be built from ICFs like the rest of the foundation, or would he be better off framing it as a double-stud wallConstruction system in which two layers of studs are used to provide a thicker-than-normal wall system so that a lot of insulation can be installed; the two walls are often separated by several inches to reduce thermal bridging through the studs and to provide additional space for insulation. with an R-value of 40, like the rest of the house?
With the wood-framed wall, he not only gets better thermal performance, but wood framing would also make it easier to put in windows and doors. “Plus,” he adds, “if a rough opening gets framed wrong or something changes, we are fixing wood and not concrete.”
“Would it still make a better/stronger basement system if all the walls were ICF?” he asks. “I realize the length is quite long, which is why I am curious if it would even help make any difference or not.”
The discussion is the topic for this week’s Q&A Spotlight.
Thermal mass vs. higher R-value
With a core of concrete, the ICF wall has more mass than a wood-framed wall, and Mike Collignon thinks this benefit is worth considering. “This often doesn’t show up in R-value discussions,” he writes, “but it [is] quantifiable and does make a difference in reducing air infiltration.”
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Added thermal mass, however, may not have much benefit in Lizer’s climate zone, Dick Russell says.
“Bear in mind that the ‘thermal mass’ effect of concrete is most useful in climates where there is a lot of outside temperature swing above and below the inside temperature,” Russell writes. “In prolonged cold weather, the mass really doesn’t help with temperature profile. True, it can provide a very airtight wall, but that can be achieved easily with a framed wall also.”
Philip Gross agrees, writing that the higher R-value of the framed wall has more advantages than the thermal mass of the concrete. One factor is that in an ICF wall, the mass of the concrete is wrapped in rigid foam insulation.
“With thermal mass you can store the extra heat and release it during the night,” Gross says. “Also, it prevents the structure from overheating during the summer as long as you have cooler nights. However, I am not so sure about the benefits of the thermal mass in ICF walls because of the interior insulation. I don`t think you can effectively store solar energy in an ICF wall and release it when needed.”
What about ICFs for the whole house?
Lizer is planning to use ICFs only for the foundation, not above grade. The double-stud exterior walls above grade will be insulated with blown-in fiberglass.
“Fiberglass? Ugh,” Collignon says. “In my opinion, you’re headed down the wrong road.”
Collignon e-mailed the thread to an old colleague, a professional engineer licensed in 20 states and one Canadian province, for a second opinion. According to Collignon, the engineer thinks Lizer would be better off building the whole house with ICFs. “Even with the double-stud configuration, unless he is using polystyrene foam in the wall, the old saying comes to mind, ‘fiberglass is a filter,’” Collignon quotes him as saying. “Air doesn’t blow through ICF walls.”
There are two additional points raised by Collignon’s engineer: first, if the concern is higher R-values, Lizer can use ICFs with 4 in. of foam on the outside face and 2 in. inside; and second, the windows and doors are the real energy-wasters. “Windows are R-1,” the engineer pronounces. “They are the ‘hole in the bottom of the pail of water’ ... all of the heat leaks out through the windows.”
Hogwash, writes Dick Russell. No one building a superinsulated house is going to use single-pane (R-1) windows, he says, adding, “Statements like that detract from whatever other useful thoughts he may have offered.”
In defense of fiberglass insulation
Fiberglass insulation is frequently criticized, at least in batt form. But John Zito thinks some of the blasts are undeserved. “Air doesn’t blow through fiberglass when all sides are air sealed,” he writes. “While I’m not a big fan of fiberglass for a few reasons (I would have chosen cellulose), it is effective when air sealed.”
On this count, Lizer seems to have done his homework. “Fiberglass, while not as ‘green’ (depending on your definition) as cellulose, it also has some advantages that make it the correct choice for me,” Lizer says. “When blown at pressures of 1.8-2+ psf, it has similar air convection current reductions as dense-packed cellulose, and at a slightly higher R per inch.”
As to the recommendation that the whole house be built with ICFs, Lizer isn’t buying in. For one thing, any wood-framed wall can be just as tight as, if not tighter than, an ICF wall. Further, benefits of thermal mass are marginal in a heating-dominated climate, and ICFs with thicker foam on the exterior “greatly increases the price over an already expensive product.
“I have priced out ICF all the way up to the trusses, and my materials cost nearly tripled for a wall with half the R-value,” he says. “Hitting similar R-values will make the price close to 5 times the cost of a double-stud shell.”
Our expert’s opinion
Peter Yost, GBA's technical director, added this:
There are a couple of key points to this issue:
- R-value and thermal mass in Zone 6. Yep, unless you have deep diurnal temperature swings, put your money and efforts towards insulation and air sealing in cold climates. And when I have been in well-designed passive solar homes with plenty of thermal mass in Santa Fe, New Mexico (a climate with nearly ideal diurnal swings) during the winter, I've concluded that it is best to expand your thermal comfort zone quite a bit in the early mornings until the sun catches up on that mass.
- Thermal mass and ICFs. Good catch on this one: All insulation, whether it be on one side of the concrete mass or the other, retards heat flow. ICF walls have interior rigid insulation that decouples the concrete mass of the ICF from the building’s interior, reducing whatever thermal mass effect it has.
- ICF and airtightness. While the airtightness of the field of an ICF wall is impressive, it’s not in the field of building assemblies (walls or roofs) where we get the bulk of our air leakage; that occurs at assembly transitions (for example, the transition between an ICF foundation and an above-grade wall, or the transition between an above-grade wall and the eave) and penetrations (windows, doors, plumbing stacks, chimneys). Really tight homes can be built of just about anything, but not without attention to these challenging spots, which are nearly as common in ICF assemblies as any other.
- Blown fiberglass and cellulose insulationThermal insulation made from recycled newspaper or other wastepaper; often treated with borates for fire and insect protection. and airtightness. Granted, the air permeability of cellulose is generally higher than spray fiberglass, but neither is airtight enough to qualify as an air barrierBuilding assembly components that work as a system to restrict air flow through the building envelope. Air barriers may or may not act as a vapor barrier. The air barrier can be on the exterior, the interior of the assembly, or both.; BOTH require six-sided containment and a dedicated, continuous, air barrier system.
Finally, I have never seen the open side of a walk-out basement with few enough openings and their configuration to really make any real structural advantage of an ICF over a framed wall. And boy, do they seem to get reconfigured over time as the use of the walk-out space changes. So, I really don’t see any real advantage, thermal or otherwise, to making the walk-out side out of ICF over any other assembly.
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