Introduction
Recently, the design of residential
buildings has become an area of increased
research due to the potential to reduce
greenhouse gas emissions in this sector of
building construction. According to the
Energy Information Agency, in 2008
approximately 40% of the total U.S. energy
consumption was due to buildings. Of that
40%, 22% was caused by residential
consumption. * Through better design
environmental factors such as a buildings
use phase energy consumption can be
decreased. There has been extensive
amount of work done to determine what
some of these design improvements may be.
Smeds et al. notes that buildings with a
comfortable indoor climate and a low energy
use share the following characteristics
applied in unison: an area to volume ratio
resembling that of a globe, thermal
insulation, an extremely air tight building
envelope, use of passive solar gains through
windows while avoiding transmission
losses, and consideration of energy
conservation measures in the first step in the
design process.* On thermal insulation
Smeds et al. continues to note that insulation
thickness depends on building geometry and
internal loads and that building a wellinsulated envelope is essential because the
envelope is the most expensive building
component to retrofit. * This project is
primarily concerned with determining the
energy consumption of a residential building
of wood frame construction and of insulated
concrete forms (ICF)an alternative
construction.
The energy use during building
operation is responsible for far more energy
consumption than either the pre-use or postuse phase*. To accurately assess the use
phase of a building it is necessary to
consider the following in an energy model:
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Heating, Ventilation, and Air
Conditioning (HVAC) system
Lighting
Domestic Hot Water (DHW) use
Appliance and plug loads
However, operating phase energy use alone
is not the sole determinant in the
environmental impact of a building. To
determine this impact, a life cycle
assessment (LCA) will utilize the modeled
building performance to asses each
building’s: natural resource depletion,
greenhouse gas emissions, and waste
production. LCA comparison of wood
frame to ICF multifamily buildings is one
third of a research study being performed by
the Massachusetts Institute of Technology
(MIT) on behalf of Cemex a global supplier
and manufacturer of concrete products.
The concrete construction industry
has reached a low-point in residential
construction as the current housing market
remains stagnant. Wood frame
construction is the most common
construction type for both single family and
multifamily residential buildings* while ICF
construction is a less widely used method*
despite exhibiting considerable thermal mass
and inherent insulation. To evaluate
performance claims made by the industry to
support ICF construction each building
model will be simulated with climate data
for Phoenix, AZ and Chicago, IL.
Unfortunately, construction is not governed
by an overarching building code and varies
from state to state even to the county level.
The U.S. Department of Energy divides the
nation into eight climactic zones with further
subdivision by county. Phoenix lies in
climactic zone 2A while Chicago lies in 5B.
The American Society of Heating,
Refrigeration, Air conditioning Engineers
(ASHRAE) has developed an extensive
building code that accounts for the D.O.E.
climactic zones. ICF construction is the
same across zones while wood frame
construction has a large margin of building
practices. Each building is modeled to meet
the requirements of the 2007 ASHRAE
handbook for low rise buildings.
Methods
A single building type with a fixed
geometry is studied. The building type is a
low rise multifamily residential building.
The building has a total floor area of 1076
m2 and is composed of four stories each 3.05
m high. There are a total of 24 apartment
units of varying floor areas, six on each
floor, and two main circulation cores.
Glazing ratio is set to 18%. The occupancy,
plug loads, activity schedule, DHW use
schedule, HVAC use schedule, lighting
schedule, and respective set points are
modeled according to DOE Benchmark
Multifamily building*. Construction of
wood frame and mass buildings are made
according to ASHRAE 90.1-2007*where
applicable. Phoenix and Chicago are chosen
as reference locations for each building.
Site orientation is rotated 90° for each
construction type for each city. Weather
data is provided by D.O.E. for the reference
sites, which provides solar radiation and
outdoor temperature. Energy simulations
are done using Design Builder, a UK based
modeling software that uses the DOE’s
EnergyPlus engine to perform model
calculations. The climate data from D.O.E.
is used as an input file to Design Builder.
Simulation results from Design Builder
yield annual and monthly values for the
chiller system energy, heat generation
energy, total electricity, total gas, sensible
cooling, total cooling, and zone heating.
Hourly data is examined in Chicago during a
typical summer and typical winter week.
Wood Frame multifamily building
The exterior walls are a wooden
frame construction using platform framing.
Studs are a standard 2”x6” and set at 24 in.
on center*. Fiberglass board is the
insulating material. Windows are double
glazed, clear, with an air gap. The roof is
flat with fiberglass insulation entirely above
deck. The floor construction is an unheated
slab on ground without a basement or any
walls below grade. The U-values of the
components are shown in Table 1.
Insulated concrete forms multifamily
building
Exterior walls are made of insulated
concrete forms. The modeled forms used
2.5” of Extruded Polystyrene on both the
interior and exterior, steel reinforcement
both horizontally and vertically and eight
inches of cast in place concrete*. Windows
and frame are the same as for wood frame
construction. Again, the roof is flat with
insulation entirely above deck. There is no
cavity beneath the unheated slab on ground
floor construction. Component U-values are
shown in Table 1.
Heating, ventilation and air conditioning
Mechanical and natural ventilation
are modeled as off. A unitary fan coil
system is used with electric cooling and gas
heating. A four pipe fan coil is simulated
and does not include an economizer. In lieu
of mechanical ventilation, a total building
air infiltration of 0.7 ach and 0.56 ach is
used for wood frame and ICF construction
respectively. Performance coefficients set to
ASHRAE standard and
electricity use, gas use, chiller energy and
heat generation energy simulated by Design
Builder are shown in Fig 3-4. Annually the
ICF construction is seen to have an average
savings of 214% in the energy necessary for
heat generation as well as a 9.6% reduction
in the total energy from gas expended. The
ICF home however uses 4.77% more energy
through the chiller, but still saves .2% of the
total energy from electricity.
Chicago
Simulated space heating/cooling demand
Simulated total energy demand, chiller
energy & heat generation energy
Results
Phoenix
Simulated space-heating/cooling demand
A comparison of the wood frame
construction to ICF construction space
heating and cooling demand simulated by
Design Builder is shown in Fig. 1-2. Annual
comparison of the wood frame construction
to the ICF construction shows savings as
much as 254% in heating demand while
increasing cooling demand by as much as
5.16% for the building oriented at 90°. On
average the sensible cooling and total
cooling load were increased by 4.74% in the
ICF building while zone heating was
decreased 214%.
Simulated total energy demand, chiller
energy & heat generation energy
A comparison of the wood frame
construction to ICF construction total
Discussion
The performance modeling of buildings is a
necessary step in building design. Modeling
allows engineers and architects the ability to
decide which variables of design are
influencing the use-phase energy
consumption. As mentioned earlier, there
are a variety of building factors that must be
considered simultaneously to improve the
energy efficiency of a building. In this
project the main tested factors were building
insulation and air infiltration.
Thermal insulation of wood frame
homes is a variable that changes with the
building code followed. For the purposes of
this project, ASHRAE 90.1-2007 was used
and the prescribed minimum insulation
values could be seen to closely mimic the
insulation values for ICF construction.
There is difficulty in transferring the code
standard into an input file to be analyzed by
the simulation software. A layer by layer
replication of actual building practice is not
possible in the Design Builder software
because the framing and insulation in the
cavity section of a wall or roof cannot be
defined. To find the insulation of the cavity,
two possible methods are lifted from
ASHRAE 90.1-2007.
The first method uses the rated Rvalue of the insulation described for a given
climate zone to look up what the effective
insulation in a cavity will be based on
framing type. This method is simple to use
as the effective cavity insulation values
reside in A9.4C of the code. It is assumed
that ASHRAE has calculated the 3dimensional heat transfer necessary to create
such a table and therefore removes the need
to do such calculations. In Design Builder
the cavity is modeled as a single fiberglass
layer with an R-value equivalent to Table
A9.4C listing for the effective cavity Rvalue.
Another method is described in
ASHRAE and that is to calculate the cavity
R-value through parallel addition of Rvalues in the wall or roof assembly. The
percentages of cavity space that are 2”x6”
wood stud and insulation are multiplied by
their respective component R-values based
on a specific framing factor. In Design
Builder a new material class is created with
the specified R-value being the value
previously calculated. Layer by layer build
up is done around the modeled cavity layer
to produce the wall construction.
The
buildings in this project were modeled using
the first method for determining the cavity
insulation.
Ventilation and infiltration is also a
major determinant in how much energy the
modeled HVAC system will use.
Ventilation is scheduled air that is brought
in to the environment through the building
fans while infiltration is outside air that
enters the building directly through the
environment without pre-treating. Air
infiltration is due to imbalances in pressure
at various points in the building envelope
and the three sources of pressure are: wind
pressure, stack pressure and HVAC fan
pressure*. These simulations the air
infiltration was dominated by wind pressure
due to each building being low rise and the
absence of mechanical pressurization of the
residential building. Several national
laboratories and the National Institute of
Standards and Technologies have performed
investigations of building air tightness*.
Design Builder allows modeling of air
infiltration in terms of air changes per hour.
Currently the author is working on
calculating the air leakage of the chosen
building geometry. The infiltration values
used are based on an Oak Ridge National
Laboratory study that assumed 0.7 AC/h for
a wood frame single family home and
compared this to a ICF single family home,
which used a 20% reduction of uncontrolled
air infiltration*. Air infiltration testing of
single family ICF homes has been
undertaken by the Building Technology lab
to arrive at measured values of air
infiltration.
Acknowledgements
I would like to thank Ms. Monica Orta and
Dean Christopher Jones of the Office of the
Dean of Graduate Education for hosting
MSRP as well as the PA’s and staff. I also
would like to thank Dr. Ochsendorf and Dr.
Norford for mentoring me during the project
and my supervisors Ms. Andrea Love, Ms.
Marzena Fydrych, and Mr. Jason Tapia.
References