Building Science
Glossary
Here are the
ABC's of building science terminology, according
to Building Science Corporation. There are a lot
of construction glossaries available, on and off
the web. This one is unique in that it is centered
on building science and uses the Builder's Guides
as a primary reference and source of terms.
Whenever possible, a link to the most relevant BSC
web site document is provided for more detailed
information on the term or concept. Stay tuned as
these initial ABC's grow into a full-blown
glossary of over 100 terms unique to our industry.
Air
Barrier: Air barriers
are systems of materials designed and constructed
to control airflow between a conditioned space and
an unconditioned space. The air barrier system is
the primary air enclosure boundary that separates
indoor (conditioned) air and outdoor
(unconditioned) air. In
multi-unit/townhouse/apartment construction the
air barrier system also separates the conditioned
air from any given unit and adjacent units. Air
barrier systems also typically define the location
of the pressure boundary of the building
enclosure. In multi-unit/townhouse/apartment
construction the air barrier system is also the
fire barrier and smoke barrier in inter-unit
separations. In such assemblies the air barrier
system must also meet the specific fire resistance
rating requirement for the given separation.
Air
Barrier; Performance Requirements:
Air barrier systems typically are assembled from
materials incorporated in assemblies that are
interconnected to create enclosures. Each of these
three elements has measurable resistance to
airflow. The recommended minimum resistances or
air permeances for the three components are listed
as follows:
- Material
0.02 l/(s-m2)@75 Pa
- Assembly
0.20 l/(s-m2)@75 Pa
- Enclosure
2.00 l/(s-m2)@75 Pa
Air
Retarder: Materials and
assemblies that do not meet the performance
requirements of air barrier materials and air
barrier assemblies, but are nevertheless designed
and constructed to control airflow are said to be
air retarders.
Building
Enclosure: A building
enclosure is an environmental separator. It
separates the interior environment from the
exterior environment. A building enclosure
controls heat flow, air flow, water vapor flow,
rain, groundwater, light and solar radiation,
noise and vibrations, contaminants, environmental
hazards and odors, insects, rodents and vermin,
and fire. A building enclosure provides strength
and rigidity and must be durable, aesthetically
pleasing and economical.
Conditioned Space: A
conditioned space is the part of the building that
is designed to be thermally conditioned for the
comfort of occupants or for other occupancies or
for other reasons.
Diffusion: The movement
of individual molecules through a material. The
movement occurs because of concentration gradients
and thermal gradients, independent of airflow.
Drainage
Plane: Drainage planes
are water repellent materials (building paper,
housewrap, foam insulation, etc.) which are
typically located behind the cladding and are
designed and constructed to drain water that
passes through the cladding. They are
interconnected with flashings, window and door
openings, and other penetrations of the building
enclosure to provide drainage of water to the
exterior of the building. The materials that form
the drainage plane overlap each other shingle
fashion or are sealed so that water drains down
and out of the assembly. The drainage plane is
also referred to as the "water resistant barrier"
or WRB.
Equivalent Leakage Area of a building (EqLA or ELA):
Quantitative expression of the airtightness of a
building enclosure. EqLA is the method set by the
Canadian General Standards Board in which a blower
door depressurizes the building enclosure to 10
Pascals and the leakiness of the enclosure is
expressed as a summary hole in square inches. ELA
is set by the ASTM equivalent procedure at a
pressure differential of 4 Pascals.
Foundation, Water-managed:
Systems for at or below-grade enclosure assemblies
where gravity (drainage) is used to move liquid
water away from the structure, relieving
hydrostatic water forces.
Grade
Beam: A foundation wall
that is cast at or just below the grade of the
earth, most often associated with the deepened
perimeter concrete section in slab-on-grade
foundations.
Housewrap: Any of the
numerous spun-fiber polyolefin rolled sheet goods,
or perforated plastic films designed to function
as drainage planes.
Indoor
Air: Air in a
conditioned space.
Insulating Sheathing:
Non-structural insulating board products with
varying R-values and a wide variation in vapor
permeability and drainage characteristics.
Materials include expanded polystyrene (EPS),
extruded polystyrene (XPS), polyisocyanurate (most
often foil-faced), rigid fiberglass, and mineral
wool.
Jump
Duct: A flexible, short,
U-shaped duct (typically 10-inch diameter) that
connects a room to a common space as a pressure
balancing mechanism. Jump ducts serve the same
function as transfer grilles. Used when return
ducts are not located in every room.
Kiln-dried Lumber: Any
lumber placed in a heated chamber or "shed" to
reduce its moisture content to a specified range
or average under controlled conditions. For
softwood framing lumber, the moisture content of
KD lumber is somewhat based on regional
conventions but is most often an average of 12% by
weight. In comparison, the moisture content of
thoroughly air-dried softwood framing lumber is
15% to 20%.
Low-E:
Most often used in reference to a coating for
high-performance windows, the "e" stands for
emissivity or re-radiated heat flow. The thin
metallic oxide coating increases the U-value of
the window by reducing heat flow from a warm(er)
air space to a cold(er) glazing surface. The best
location for the coating is based on whether the
primary heat flow you want to control is from the
inside out (heating climates) or the outside in
(cooling climates).
Mechanical Ventilation:
Controlled, purposeful introduction of outdoor air
to the conditioned space.
Outdoor
Air: Air outside the
building. It can enter the conditioned space via
the ventilation system, or by infiltration through
holes in the pressure boundary or designed
ventilation openings.
Ozone:
03 instead of 02. This 3-atom molecule is an even
more active oxidizing agent than its more common
2-atom relative. At ground level, ozone is a
pollutant and in the upper atmosphere it is a
solar shield (location, location, location).
Touted for its ability to "clean" air in room or
household ozone generators, this application
actually does more harm than good-ozone's highly
reactive nature tends to accelerate the breakdown
of synthetic materials in homes such as paints,
plastics, and ever-available volatile organic
compounds, often with less-than-desirable results.
All told, we look to protect ozone in the heavens
and shun it here at home, inside and out.
Permeance: The physical
property that defines the ease at which water
molecules diffuse through a material. It is to
vapor diffusion what conductance is to heat
transfer. The unit of measurement is typically the
"perm."
Pressure
Boundary: Air barriers
define the location of the pressure boundary of
the building enclosure. See also Air Barrier.
R-value:
Quantitative measure of
resistance to heat flow or conductivity, the
reciprocal of U-factor. The units for R-value are
ft2 °F hr/Btu (English) or m2 °K hr/W (SI or
metric). While many in the building community
consider R-value to be the primary or paramount
indicator of energy efficiency, it only deals
conduction, one of three modes of heat flow, (the
other two being convection and radiation). As an
example of the context in to which R-value should
be placed, 25% to 40% of a typical home's energy
use can be attributed to air infiltration.
Thermal
Boundary: The layer in a
building enclosure that controls the transfer of
energy (heat) between the interior and the
exterior. It is a component of the building
enclosure and it may, but does not have to align
with the pressure boundary.
U-factor: Quantitative
measure of heat flow or conductivity, the
reciprocal of R-value. While building scientists
will use R-values for measures of the resistance
to heat flow for individual building materials,
U-factor is always used as a summary measure for
the conductive energy measure of building
enclosures.
Vapor
Barrier: A vapor barrier
is a Class I vapor retarder. Vapor barriers are
materials that are vapor impermeable.
Vapor
Impermeable: Materials
with a permeance of 0.1 perm or less (rubber
membranes, polyethylene film, glass, aluminum
foil)
Vapor
Permeable: Materials
with a permeance of greater than 10 perms (housewraps,
building papers)
Vapor
Retarder: A vapor
retarder is the element that is designed and
installed in an assembly to retard the movement of
water by vapor diffusion. There are several
classes of vapor retarders:
- Class I vapor
retarder - 0.1 perm or less
- Class II vapor
retarder - 1.0 perm or less and greater than 0.1
perm
- Class III
vapor retarder - 10 perms or less and greater
than1.0 perm
The test
procedure for classifying vapor retarders is ASTM
E-96 Test Method A - the desiccant or dry cup
method.
Vapor
Semi-Impermeable:
Materials with a permeance of 1.0 perm or less and
greater than 0.1 perm (oil-based paints, most
vinyl coverings)
Vapor
Semi-Permeable:
Materials with a permeance of 10 perms or less and
greater than 1.0 perm (plywood, OSB, most
latex-based paints)
Water
Resistant Barrier: A
water resistant barrier (WRB) is also referred to
as a drainage plane.
Wind-Washing: The
phenomenon of air movement that occurs due to wind
entering building enclosures typically at the
outside corners and roof eaves of buildings.
Wind-washing can have significant impact on
thermal and moisture movement and hence thermal
and moisture performance of exterior wall
assemblies.
Xeriscaping:
Climate-tuned landscaping that minimizes outdoor
water use while maintaining soil integrity and
building aesthetics. Typically includes emphasis
on native plantings, mulching, and no or limited
drip/subsurface irrigation.
Zero
Energy House: Any house
that averages out to net zero energy consumption.
A zero energy home can supply more than its needs
during peak demand, typically using one or more
solar energy strategies, energy storage and/or net
metering. In a zero energy home, efficiencies in
the building enclosure and HVAC are great enough
that plug loads tend to dominate and so these
homes must have the added focus of high efficiency
appliances and lighting.
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