Posted By Jetgraphics on 05/19/2009 11:59 PM
http://en.wikipedia.org/wiki/Specific_heat_capacity
Specific heat capacity, also known simply as specific heat, is the measure of the heat energy required to increase the temperature of a unit quantity of a substance by a certain temperature interval.

In an insulated volume, the specific heat of the air and materials determines how much heat energy is needed to change the temperature. Adding thermal mass affects the rate at which the temperature changes. However, that lag is only useful when the climate has large swings between day and night. In a hot / humid climate, adding thermal mass doesn't save energy.

[b]The big question: why did the American building gurus decide on R13 and R19 as the "suggested" wall insulation? Was it a coincidence that it matched fiberglass batts within a 3.5" and 5.5" wall cavity?[/b]

And instead of blowing $$$ on "super" windows that barely can reach R4, why not specify insulated shutters and use inexpensive windows?

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There's a history there, and it's entirely related the cost of energy vs. the installed cost of insulation, the cost of money and where the crossover where building for higher R values goes present-value-negative in an intermediate-term financial analysis that assumes constant or low-inflation energy pricing.  The original 3.5" batt insulation was R11 (and they're still available), not R13, but  R15 high-density batts are now available for standard 2x4 construction.  I haven't seen recently, but R7 or R8 "econobatts" used to be pretty common too (and work a lot better than empty wall-cavity balloon frame construction that was common in the cheap-energy 1940s.)  Similarly, R22 high density fiberglass batts are available for 5.5" cavities (and blown high density fiberglass hits R23 in that depth.)

DOE R-value recommendations have been bumped a few times over the past few decades, but they don't change with the same volatility as actual energy pricing.  (What would have been cost-effective in a 10 year NPV analysis at last summer's $4.50-5.00/gallon heating oil might seem pretty ridiculous at this summer's $2/gallon heating oil.)  Changes to building codes tend to be slow, and generally have to have a clear economic or safety argument to make it into law.  Forcing people to build to standards that are un-economic is rightly seen as an unfair burden, so there's a healthy margin built in- the true economics vary, and code minimums are truly the no-brainer minimums. Anybody who cares to do the math can figure out what makes sense for THEM.  But even after last year's energy spikes the general housing marketplace is more willing to spend money on more space, or more luxury features than energy efficiency features.  It's easier to sell the cush-factor of radiant floor heating than the high-efficiency systems that can take full advantage of the low temperature operation for double-digit percentage efficiency gains.  If it's not NPV+ in 3 years (a ridiculous standard, but measureable in the marketplace), the general public just isn't investing there.

Green is hard to sell (commercial or residential), even when demonstrably cost-effective, which is why minimum standards have to be incorporated into building codes to make any headway in the big picture, or when making broad policy shift based on values surrounding global warming or national security considerations, beyond the simple economics.  California Title 24 is a good start, as are many local code upgrades like that in Austin TX.  But those too are broadly economic,  but calculated with a sharper pencil & longer time horizions.  It's a lot easier & cheaper to address it in new construction- retrofit is where the big bucks are going to be spent.  Something like 80% of the buildings that will be standing in 2050 are already built.  Prescriptive practices & methodologies (whether building codes, LEEDS, or EnergyStar) will have to eventually move ever more toward measured building efficiency if broader goals are to be met.

OK enough thread-drift- back to the thermal-mass discussion...

In most of the lower 48 thermal mass inside a well insulated envelope will save energy and improve comfort, but the amount it saves will vary widely by climate zone & season.  In hot humid climates with high latent AC loads it won't buy you too much, but still some- mass affects only the sensible AC loads.  By lengthening the cycles and reducing the numbers of cycles, the efficiency of heating/cooling systems goes up and with high thermal mass setting up the controls for efficiently-long minimum cycle lengths won't result in overheating/overcooling.  With 2-3 heating/cooling cycles per DAY (instead of per hour), the startup wear & tear on the equipment from is much lower too.  And when the daily average temperatures are moderate (even if the daily temperature swing is not) the building can "coast", with minimal or no AC/heating inputs.  In some situations the annual percentage energy savings will be into double-digits, but in others it'll be hard to measure low single-digits or "in the noise".