Air Leaks or Thermal Loss: What’s Worse?
Have we been brainwashed to consider only R-values?
Beefing up R-values and reducing air leaks are the twin rallying cries of builders focusing on energy efficiency. Regardless of the particulars of the house design, more insulation and fewer air leaks make houses more comfortable, more durable, and less expensive to heat and cool.
No one seems to argue that point. But Al Cobb wonders which is more significant.
"My real goal is to find the tipping point when a leaky building loses more energy via air changes then via the insulated envelope," he writes in GBA's Q&A forum. "I've had many answers where the losses from air leakage have been as low as 10% or as high as 50%."
Cobb believes home buyers have been "brainwashed" into thinking only about R-values, as energy codes give short shrift to the importance of airtightness. Energy modeling is especially frustrating, he says, because it asks for highly specific information on R-values but only broad generalizations when it comes to airtightness.
"Therefore, I'm looking for a study or analysis of homes (real or not) that have been modeled to the extent that heat loss from conductive and air infiltration losses are clearly defined," Cobb adds. "It only makes sense that as leakage rates increase, the decision to ignore air-sealing can be shown as a critical mistake."
Ain't no such animal
Good luck and God speed, suggests Robert Riversong: "Your question is similar to, 'What's the difference between an apple?'," he writes. "The answer could range from near zero to near 100%, and is entirely dependent on whole-house R-value and whole-house air exchange rate during normal operation (not under blower door testing). If you're asking about 'average' existing housing, there is some data on that. If you're asking about a particular new construction project, you have to do the heat loss analysis for that specific building including design or actual air exchange losses."
And as Riversong suggests, it might be useful to pin down exactly what we mean when we refer to "the code," as if there was a single, universal book of rules accepted by building inspectors everywhere.
"There is no 'The Code,' " he says. "There are international model codes which are becoming more comprehensive by the year and which each jurisdiction can choose to adopt or modify. And all building codes are minimum standards. You're free to exceed them as much as you'd like."
That said, there is always the HERS rating system, an approach to quantifying energy efficiency established by the Residential Energy Services Network. Its 0 to 100 scale compares the performance of a tested home to that of a house built to standards of the 2006 International Energy Conservation Code.
HERS inspections include both a blower-door testi to measure airtightness and a test to measure duct leakage.
Some truth to the argument
As to Cobb's basic premise that too much attention is focused on R-values, AJ Builder is right on board.
"R-value — My biggest pet peeve, especially when discussed here on this supposedly leading source of green building advice site!" he says. "You all love to quote rules and 'the codes,' yet two homes can be built with the same approved R-value insulation and have 100% different energy needs. The codes should spec insulation in more regard as to how continuous it is, in how it actually performs at all temperatures, when it is needed most and other aspects of the entire assembly. We all know that fiberglass batts in an attic that is at 0°F is not giving the same true R-value as, say, the R-value of other installs — but where [does] the beloved code address this fact?
"The code and discussions here would make much more sense if R-value [measured the R-value of the] actual whole assembly, rated at worst design temperaturei of stated climate," AJ suggests. "Otherwise the info is garbage."
That's exactly what I'm talking about, Cobb says. "The average consumer, building official, and architect is consumed with the perception of the importance of R-value," says Cobb. "That misperception is the basis of my question that started this thread. If modeling identifies the cost of ignoring good air-sealing, it can be used as a tool that educates all people about building better buildings by using products and systems that perform properly. The suggestion that the code has comprehensive standards is laughable."
Moreover, Cobb bristles at Riversong's tone: "I'm looking for data to help us build better homes and help the consumer make better choices," Cobb says. "What I'm not looking for is criticism of my professional capabilities."
Other solutions suggested
While Riversong suggests there is more of value in the model energy code on air sealing than Cobb or AJ acknowledge, J Chesnut has another idea, providing you're a math nut. You could, he says, play with heat loss formulas developed by ASHRAEi. J Chestnut points to some specifics in a 2005 publication which offer a way of calculating infiltration rates for load calculations.
Hunter Dendy adds this idea to the mix: "Maybe the simplest approach would be to model a house and then adjust those variables to see how it changes the performance/heating loadi of the model," Dendy writes. "You probably won't find an across-the-board answer to your question since there are too many variables involved in house design and site conditions. Better to look at it on a project-by-project basis, but after doing this for a while you will get a feel for it."
And in the end, that's where the discussion seems to put us. Thermal insulation and air sealing are both essential for high-performance houses, but there may be no cut-and-dried answer to Cobb's original question that fits all circumstances.
Here's how GBA Technical Director Peter Yost sees it:
Mathematically, you can constrain the two equations for convective and conductive heat loss and force them to be equal. And then in some part the relationship will be driven by the geometry of the building and the relationship between surface area and volume. But problems quickly arise:
1. Conduction is a field phenomenon and convection is a point effect. In the equations we just sum each effect and yet we know it is the sizes and locations of all the holes that are really important.
2. If we set the equations equal to each other, we are assuming that one does not affect the other. And yet we know that where and how the air leakage occurs can erode R-values.
3. Many building materials 'care' a lot more about convective than they do conductive heat loss because moisture moves with the air and can condense. So, we typically worry a lot more about convective than conductive heat loss in terms of durability.
And then there are the occupants. Their concern about the relationship between the two phenomena is complicated as much by the quality as it is the quantity of air exchange. While ASHRAE 62.2i gives us some guidance on how much fresh air — or more exactly, outside air — we should have in terms of occupant health and safety, how much outside air occupants need is a far from settled question and dependent on many more factors than the ones that architects and builders control.
There is no question that most folks, lay people and too many in our industry, are “R-centric” and give convective heat transfer short shrift. But a ton of great work has gone in to EPA’s Thermal Bypass Checklist to guide our priorities in cost effectively and practically tackling the big and typical holes. And both in terms of overall energy efficiency and building durability, the continuity of the air or convective barrier is just as important as and even codependent with the continuity of the thermal or conductive barrier.
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