Have we been brainwashed to consider only R-values?
POSTED ON DEC 20 BY SCOTT GIBSON – GREEN BUILDING ADVISOR
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
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 airsealing
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.”