[Greenbuilding] skepticism about mass wall values

[LarenCorie]

> From: Alan Abrams <alan at abramsdesignbuild.com>

> got into a debate recently about whether claims of high R-values
> for mass wall materials such as AAC really hold up in the real
> world, outside of Taos, NM, and a few other climates more
> pleasant than my own.

Hi Alan;

 All climates will have times, during the shoulder months,
when ambient temperatures cross into the comfort zone,
or temperatures when a house can heat itself with interior
generated heat and Solar, and exterior wall mass extending
heat, into the night can reduce heating loads. However, the
claims of general effect, and greater real world performance,
are really nothing more than a race to be the first to claim the
lowest hanging fruit (easiest heating job), as their own, when
other heat sources (such as Solar and interior generated heat)
may also be there, at the same time, making the effect of the
exterior wall mass, redundant. During the times when exterior
wall thermal mass will help heat a house, are the same times
when even minimal, poorly done, passive Solar heating (may-
-be not even intentional) could be doing just as well, or a
better job of it.   As for cooling, it can be done better in other
ways, too.    The idea of low Rvalue, massive exterior walls
(poor insulation, poor mass) exposed to both interior and
exterior conditions, is far from optimal, for energy performance.

> a claim was made that a 12" AAC wall has a functional
> R-value of over 20, much greater than its "steady state"
> value of something like R-12...

 It very well may. However, it will only be during times
when other options. such as insulation and simple Solar
strategies can be extremely effective, probably more so
than the exterior mass walls.  And, those other options
will continue to work during the heart of the heating
season, when the exterior mass wall strategy has no
effect, at all.

> ...I am not disputing the value of thermal mass,

  I think you should, in the context of it being in the exterior
walls (not that it has value, but how much).  When there are
exaggerated claims made, as to the true effectiveness of a
product, we should dispute it.

> my sense is that you should base at least peak load on
> ordinary R-values

Yes. U and R values are based on uniform values, that can
be used for comparison.  These companies try to hide their
unimpressive insulation values,  behind a deliberate smoke
screen of confusion, in order to fool customers.  It has one
intention, and that is to deceive  buyers.     There are many
names for that. Choose the one you like. They could, more
easily, be honest about the real Rvalue, and then say some-
-thing like: "This is why that may not matter" and give an
honest explanation of the way the mass effect works.

> With regard to its value during cooling season--in places
> where more energy is devoted to latent rather than sensible
> load--it seems like mass walls don't have huge value.

  And, unshaded mass exterior walls are likely to
cause a house to be much hotter through the night.
Mass can be a powerful tool, but there are many
factors that require considerable knowledge and
experience, in order to know how to use it.  For
instance, the elevation of thermal mass seems to
never be discussed, as to how it effects natural
cooling.

> like in "messy" climates such as the Mid Atlantic region.

Probably not a good place to use it, if any is.

   If the interior needs more thermal mass, then put it inside
the interior, rather than between the interior and the outdoors

 -Laren Corie-
 Natural Solar Building Design and
 Solar Heating/Natural Cooling/Energy
 Efficiency Consultation Since 1975
 www.ThermalAttic.com  (many new
 photos and pages, coming soon)

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