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GBlist: Exterior thermal mass (Barkeater)




Commentary on Thermal Mass In Walls
   -- Bion Howard
---------------------------------------------
>> barkeater@winthrop.slic.com  Wrote:
   Subject: GBlist: effect of exterior mass
slipform stone house in upstate NY.// exterior wall section has 14-8" of
stone and concrete inside and out(mainly stone)... stress skin panels ?? thermal 
mass on the interior, but how much effect would the 8-14" of 
conc. and stone on the exterior have? 
     Tim McCarthy, Barkeater Design Build Co.
------------------------------------------------

The wall as described is a Paul Bunyun sandwich and will be dominated 
by the thick stress-skin panel' R-factor.  In a 30 inch thick wall with
this very thick insulation layer, the effects of BOTH mass layers will be
mitigated due to sheer quantities.  A rough calculation should show 
the wall you describe has a thermal time constant of several DAYS !

 FYI According to most thermal mass studies targeting "typical" walls:

  *  Exterior thermal mass has little influence over the annual
     energy consumption of residential buildings, but has been shown
     to help in some climates (most stuties have been on brick or
     block in standard 4 and 8 in thicknesses) through tempering the
     unwanted heat gains in summer. For example a light-colored brick
     exterior veneer wall (over R-15 insulated frame wall) will in
     most climates perform slightly better than the same "R" frame wall
     clad with siding over sheathing.

  *  Integral insulation ( a sandwich of  ||mass||insulation||mass||
     or truly integral insulation like a solid-wood (log) or strawbale
     wall section, or preinsulated block like Korfil HIR-R or SparFill,
     will perform on an annual basis as if it actually had a higher
     R-factor (lower conductance), but will not show as much of a
     proportional "thermal mass" performance benefit as...

  *  Exterior _insulation_ over mass walls.  Examples of this type of
     wall include: EIFS over CMU walls, SIP (structural insulated panel)
     with interior face brick veneer (or thin brick) where the thermal
     mass materials are deployed on the interior surfaces.  Such _walls_
     may display up to 20% to 30% lower overall annual energy consumption
     when compared to a similar frame wall.  Exterior insulation systems
     have been shown to be more effective in the South, and where passive
     solar direct gain heating is used. 

 Potential Savings = T-Energy * (%/100 * walls ) * (%/100 * mass benefit)

Thus, if the overall heating and cooling energy consumption was 80 million
Btu (lets not get into fuels here) then if the _potential_ mass savings is
estimated at 15% in the climate, and the walls create 30% of the building
annual heating and cooling consumption, then:

      PS =  80 MMBtu  * 0.15 * 0.30  =  3.6 MMBtu  (net 4.5%)

A generic estimate like this can also be backed out if you do two computer
estimates:  one for the building with frame walls, and one for the 
building with the mass walls (choose a computer tool that "does" mass )
with all other details held equal.  An example of a table of thermal
mass factors can be found in the Model Energy Code.
....................
Links to Web-sites with info on Model Energy Codes (and mass):

CABO -      http://cabo.org/download.htm
ORNL -      http://www.cad.ornl.gov/kch/mass.html
Consultant     http://diamond.gtii.com/riverbend/mec/
MEC-Check      http://www.energycodes.org/meccheck/mec.htm
MEC-Check      http://www.oikos.com/library/mec/

MIT codes site:
http://nimrod.mit.edu/depts/rotch/subjects/architecture/code.html

......................
A considerable literature on thermal mass in buildings exists, but the
reader must be careful of both:

   1.  inflated claims for thermal mass benefits; and

   2.  confusion between the use and benefits of thermal mass in
       passive solar buildings, versus the utilization of the 
       intrinsic heat capacity in structural walls of buildings

Passive Solar
-----------------
In the case of climate responsive design of residential and light commercial 
buildings the role of thermal mass is to provide a
diurnal heat storage capacity in building zones where sunlight is 
intentionally admitted through fenestration and where excessive temperature
fluctuations ( > 12 Deg F/day ) might occur reducing comfort due to the
absense of sufficient heat capacity due to the designed-in larger areas
of glass typical of such buildings.  In most production homes, the glass
are is about 13% to 18% of floor area, but in true passive solar homes
with the "direct gain" system, there may be a doubling of the glass area
facing the equator, as compared to a "typical" design.
      (see:  http://www.psic.org )

Envelope (building shell) Heat Capacity
------------------------------------------
In the case of the "intrinsic heat capacity" of the building envelope
the quantity and location (inside CS, integral, or exterior) of the mass,
as well as the quanity and location of insulating materials, combined with
other key factors:  climate (including solar radiation), building
occupancy, desired comfort, building type (high-mass residential walls
perform differently than high-mass walls in commercial buildings due
to differences in the fate of internal heat gains), ventilation rates,
heating and cooling system type and control strategy.  The mass is in
the walls and may not be specifically designed to absorb incoming solar
heat.  The role of mass appears to be reducing the amplitude and 
changing the time of occurence of peak heat flows through walls. This
effect has been documented to result in some annual energy savings in
temperate climate zones, where the outdoor temperature (an the Sol-Air
temperature of the wall surface) fluctuates above and below the average
indoor air temperature.

There is a large literature on thermal mass in both the passive solar
context, and pertaining to the thermal performance of the exterior
envelopes of buildings  (ASHRAE and DOE have an entire conference that
is devoted to these and other "building shell" studies every three years)

 ***  Contacts on Mass:

1.  Brick Institute of America,  Reston, VA 
2.  National Concrete Masonry Association, Herndon, VA
3.  Portland Cement Association, Skokie, IL
4.  National Log Homes Council, NAHB, Washington, DC
5.  Passive Solar Industries Council, Washington, DC
6.  Codes with Mass Provisions:

        1989 and later International (CABO) Model Energy Code (MEC)
            and hence in states that have adopted the MEC
        ASHRAE Standard 90.2-1993
        ASHRAE Standard 90.1-1989R
        California Energy Commission Title24 (certain provisions)
        HUD/FHA which recognizes the MEC
        NCS-BSC Model Code for Energy Conservation (MCEC)
        Special State Energy provisions in:
            Florida, Utah, Arizona, North Carolina (possibly others)

7.  Computer software that "does" mass (For more programs and info
       go to:   http://www.eren.doe.gov/buildings/tools_directory/  )

     Commerical Buildings (and res.)
        >  DOE2                 > BLAST/PC
        >  ENERGY-10
     Residential Buildings
        >  REM-Design           > WINGuide
        >  BuilderGUIDE         > SunCODE
        >  CALPAS/MicroPAS
 ###       

For more information on how to perform a thermal mass "study" in
an existing building, or of building plans, please contact me directly.

--------------- Contact Information------------------
        Mr. Bion D. Howard, Principal          
  Building Environmental Science & Technology   
  P. O. Box 1007, Upper Marlboro, Maryland 20773 USA
  bdhoward@ix.netcom.com <--> http://nrg-builder.com
     Voice#  410-867-8000  fax#  301-889-0889
-------------------------------------------------------          
 
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