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 location. 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 systems are assembled from "materials" incorporated in
"assemblies" that are interconnected to create "enclosures". Each of these
three elements has measurable resistance to airflow. The minimum
resistance or air permeances for the three components are:
- Material 0.02 l/(s-m2)@75 Pa
- Assembly 0.20 l/(s-m2)@75 Pa
- Enclosure 2.00 l/(s-m2)@75 Pa
Materials and assemblies that meet these performance requirements are
said to be air barrier materials and air barrier assemblies. Air barrier
materials incorporated in air barrier assemblies that in turn are
interconnected to create enclosures are called air barrier systems.
Air Barrier System
An air barrier system is an assembly of materials that together are
designed, installed, or act to control the flow of air across the building
enclosure. The pressure boundary of the enclosure should, by definition,
be coincident with the plane of the air barrier system. The complete air
barrier system is comprised of materials and assemblies, each with their
own performance requirements.
Air-impermeable
A material or assembly having an air permeance equal to or less than
0.02 l/(s-m2) @ 75 Pa tested according to ASTM E 2178 or E 283.
Air Retarder
Materials and assemblies that do not meet the performance requirements
of air barrier materials and air barrier assemblies and air barrier
systems, but are nevertheless designed and constructed to control air flow
are said to be air retarders.
Building Enclosure
The building enclosure is the component (typically comprised of an
assembly of numerous materials and layers) of a building that acts as an
environmental separator between the indoor and outdoor environments. The
thermal, air pressure, and rainwater boundary lie within the confines of
the enclosure. A building enclosure controls heat flow, air flow, water
vapor flow, rain penetration, groundwater effects, the transfer of 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.
Note: The enclosure is a special type of environmental separator.
Environmental separators also exist within buildings as dividers between
spaces with different desired environmental conditions.
Conditioned Space
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 designed and constructed to
drain water. 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 flow is downward and outward.
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.
Habitable Space
Building space intended for continual human occupancy. Such space
generally includes areas used for living, sleeping, dining, and cooking,
but does not generally include bathrooms, toilets, hallways, storage
areas, closets, or utility rooms.*
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.
Occupiable Space
Any enclosed space inside the pressure boundary and intended for human
activities, including but not limited to, all habitable spaces, toilets,
closets, halls, storage and utility areas, and laundry areas.*
Outdoor Air
Air outside the building.*
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 The primary air enclosure boundary separating
conditioned air and unconditioned air. Typically defined by the air
barrier system.* 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.
Subflorescence/Sub-efflorescence
A potentially harmful accumulation of water soluble salts that
recrystalize just beneath the masonry surface as moisture in the wall
evaporates.
Thermal Control Layer
The thermal control is the layer (comprised of one or several
materials and formed into planes to create a three dimensional boundary)
that is designed, installed, or acts to form the thermal boundary in an
enclosure assembly. The layer may be partially penetrated by thermal
conductive elements.
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 material that has a permeance of 0.1 perm or
less. A vapor barrier is a material that is vapor impermeable. A vapor
barrier is a Class I vapor control layer. The test procedure for
classifying vapor barriers is ASTM E-96 Test Method A-desiccant or dry cup
method.
Vapor Control Layer (or Layers)
The element (or elements) that is (or are) designed and installed in
an assembly to control the movement of water by vapor diffusion. Language
use example: "A Class III vapor control layer is used to manage vapor flow
in this assembly."
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 Permeance
Classes Test procedure for determining vapor permeance class is ASTM
E-96 Test Method A-the desiccant or dry cup method.
- Class I: Materials that have a permeance of 0.1 perm or less
(Note: this is the definition of a "vapor barrier".)
- Class II: Materials that have a permeance of 1.0 perm or less
and greater than 0.1 perm (Note: This is the definition of a "vapor
retarder".)
- Class III: Materials that have a permeance of 10 perms or
less and greater than 1.0 perm
- Class IV: Materials that have a permeance greater than 10
perms
Vapor Retarder
A vapor retarder is a material that has a permeance of 1.0 perm or
less and greater than 0.1 perm. A vapor retarder is a material that is
vapor semi-impermeable. A vapor retarder is a Class II vapor control
layer. 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.
* From ASHRAE Standard 62.2 |
