[Greenbuilding] The dynamic R-value scam...

[Kat]

(second try. Why is it that my messages always come through blank???)

Thank you for pointing that out, Nick. Here's some simple language that 
backs you up (excerpted from the Summer 2005 Codelink, a publication of 
the State of Oregon Building Codes Division - entire article at 
http://www.cbs.state.or.us/external/bcd/pub/codelink/2005/summer_05.pdf):

"Thermal mass — not to be confused with insulation"
Oregon has seen a steady rise in the number of products and building 
systems used for constructing the walls of a home. The growing interest 
in energy efficiency and more sustainable building methods is behind 
much of this new market activity. Double stud or Larson Truss walls, 
structural insulated panels (SIPs), insulated concrete forms (ICFs) and 
a number of natural building materials such as straw bale or straw clay 
challenge traditional stud wall construction for market share.
They are challenging to consumers and building code officials trying to 
assess the energy performance of such assemblies. This is especially 
true of ICF systems, when a wall’s thermal mass may have some impact on 
the overall energy performance and comfort of the home. Manufacturer 
claims about product performance can confuse those interested in code 
compliance or a higher-performance building shell.

Code compliance issues

Masonry walls (and floors) are denser than their light-framed cousins. 
For this reason, care must be taken in characterizing the thermal 
performance of these elements of the building envelope. For the purposes 
of energy code compliance, the relevant thermal performance metric is 
conductive heat loss, represented by the whole-wall U-value. Its units 
are Btu/hr·ft2·°F and it is the inverse of whole-wall steady-state 
R-value, which is used as a shorthand way to describe the thermal 
performance of a number of building components or assemblies. 
Unfortunately, the use of “R-value” is not always consistent.
The energy code, by regulating U-value, indirectly regulates whole-wall 
steady-state R-value. This is not the same as dynamic or equivalent 
R-value, which usually takes into account the thermal mass or 
heat-storage effects of a building assembly. Neither is it the same as 
the R-value of the insulation that provides most of the thermal 
resistance of the assembly...

Climate

While it is often useful to account for the thermal mass, or thermal 
capacitance, of a building assembly, in Oregon’s climate, the heat loss 
for a building during the heating season is determined by the whole-wall 
U-value. In order for an assembly’s thermal capacitance to be of any 
benefit in slowing heat loss, diurnal (daily) outdoor temperatures must 
be above indoor temperatures for part of the day and below indoor 
temperatures for another part of the day (typically nighttime). During 
our heating season, these conditions are often found in the desert 
southwest; they are almost never found in Oregon.
During the cooling season, there are times when diurnal temperature 
patterns enable some benefit from good thermal capacitance. But because 
building cooling loads in Oregon are quite small compared to heating 
loads, code specifications are based on reducing heat loss in the winter 
months. Preliminary data from a mass wall assembly being monitored for 
performance suggest that this is appropriate.


Nick Pine wrote:
 > "Jeannie Babb Taylor" <jeannie at babb.com>
 >
 >
 >> ... the 1.2 per inch does not take thermal mass into account.
 >>
 >
 > Ignoring thermal mass is the honest way to do it.
 >
 > Nick