BPI Envelope key terms

The continuous layer of insulation in a building assembly that separates conditioned interior space from unconditioned or outdoor space, and which must align with the air barrier for the enclosure to perform as designed.

A continuous, sealed material system that resists uncontrolled airflow between conditioned and unconditioned spaces. It must be airtight, durable, and joined to itself and to other building components at every penetration and transition.

Unintended movement of air through gaps, cracks, and penetrations in the building enclosure. It carries both heat and moisture, so it affects energy use, comfort, and condensation risk at the same time.

Outdoor air entering a building through unintentional openings driven by wind, stack effect, or mechanical pressure differences. It brings in pollutants and moisture while increasing heating and cooling loads.

Indoor air escaping through unintentional openings in the enclosure. In cold weather, exfiltration deposits warm moist air inside cold assemblies, where it can condense and cause hidden decay.

Pressure-driven airflow caused by buoyancy differences between indoor and outdoor air. Warm air rises and escapes high in the building while outdoor air enters low, creating a vertical pressure gradient that intensifies with height and temperature difference.

The horizontal level in a building where indoor and outdoor pressures are equal, so neither infiltration nor exfiltration occurs there. Above it air tends to escape; below it air tends to enter.

The defined envelope surface that separates the test volume from outside during diagnostic testing. All openings in this boundary must be controlled so measured pressure differences reflect the intended zone.

A diagnostic procedure that mounts a calibrated fan in an exterior doorway to depressurize or pressurize the house, then measures airflow needed to hold a set pressure difference, revealing envelope tightness.

A normalized leakage metric equal to the blower-door airflow at a 50-pascal pressure difference divided by house volume. It allows tightness comparisons among homes of different sizes and shapes.

The estimated total area of a single sharp-edged orifice that would leak the same airflow as the whole enclosure at a specified reference pressure. It converts measured leakage into an intuitive size.

A testing method that uses pressure measurements between connected zones to estimate how leaky an interior boundary is relative to the exterior envelope, often expressed as a percentage of total leakage.

A shallow pan with a pressure tap temporarily sealed over a duct register or grille. It measures the pressure difference across that opening during a blower-door test to indicate duct leakage to outside.

Air escaping from or entering ductwork through seams, joints, and disconnected sections. Leaks in ducts located outside conditioned space waste conditioned air and can draw pollutants or moisture into the system.

A path of high-conductivity material, such as wood framing or steel studs, that bypasses insulation and conducts heat through an assembly. It lowers effective R-value and can create cold spots prone to condensation.

The practice of placing insulation in full contact with the air barrier and thermal boundary, without gaps, compression, or wind washing, so the rated performance is actually achieved in the field.

Air movement through or across fibrous insulation that strips heat from the material and degrades its effective R-value. It is prevented by tight air barriers and dense, well-fitted installation.

A circulating airflow pattern within an insulation cavity or assembly, driven by temperature differences, that moves heat from warm surfaces to cold ones and reduces overall thermal performance.

A measure of a material's resistance to conductive heat flow per unit thickness. Higher values mean better resistance; assembly R-values must account for framing, gaps, and other thermal bridges.

The overall rate of heat transfer through an assembly, equal to the reciprocal of its total R-value. It is used to rate windows, doors, and whole wall sections, where lower numbers indicate better performance.

The fraction of incident solar radiation admitted through a window and released as heat indoors. Lower values reduce cooling loads and glare; higher values help with passive heating in cold climates.

A microscopically thin metallic or metal-oxide film applied to glass that reflects long-wave infrared radiation. It reduces radiative heat transfer and can be tuned to control solar gain.

The percentage of visible light passing through glazing. It trades off against solar control: coatings that block heat often also darken the glass and reduce daylight.

A rating of how well a window or door resists forming condensation on interior surfaces under standardized conditions. Higher values indicate warmer interior surface temperatures and less moisture buildup.

A low-conductivity component placed between conductive framing parts, such as aluminum window frames, to interrupt heat flow and raise interior surface temperatures so condensation is less likely.

The temperature to which air must cool, at constant moisture content, for water vapor to begin condensing into liquid. Surfaces colder than the dew point collect moisture.

The amount of water vapor in air expressed as a percentage of the maximum the air could hold at that temperature. It rises as air cools even when absolute moisture stays the same.

A material that limits diffusion of water vapor through an assembly. Its correct placement depends on climate, and installing it on the wrong side can trap moisture inside the wall.

Movement of water vapor through a material driven by vapor pressure differences rather than bulk airflow. It is generally slower than air-transported moisture but still matters in cold assemblies.

A measure of how readily water vapor passes through a given thickness of material, usually expressed in perms. Low-permeance materials act as vapor retarders; high-permeance ones allow drying.

The water-shedding layer in a wall or roof assembly, positioned behind cladding, that directs liquid water downward and outward to flashing and weep openings rather than into the structure.

A sheet or coating behind cladding that keeps liquid water from reaching sheathing and framing while allowing the assembly to dry. It must integrate with flashing at penetrations and transitions.

Waterproof material installed at joints, penetrations, and edges to redirect water away from the enclosure. Missing or reversed flashing is a leading cause of concealed water damage.

A cladding strategy with a drained and ventilated air gap between siding and the water-resistive barrier. The gap equalizes pressure, limits wind-driven rain entry, and promotes drying.

Movement of liquid water through narrow pores and gaps against gravity, driven by surface tension. It can pull moisture upward into framing and masonry through tiny cracks.

The total water vapor introduced into a space by occupants, cooking, bathing, laundry, plants, and infiltration. It must be removed by ventilation or controlled to keep indoor humidity in a safe range.

The volume of outdoor air supplied to or removed from a space per unit time. It is set by code or design standards to dilute pollutants and control humidity without excessive energy penalty.

A balanced mechanical ventilation device that transfers sensible heat from exhaust air to incoming fresh air through a core, reducing heating energy while supplying filtered outdoor air.

A balanced ventilator that transfers both heat and moisture between exhaust and supply airstreams. It is useful in humid climates where excess outdoor moisture would otherwise raise indoor humidity.

A mechanical device that removes air from a specific space, such as a bathroom or kitchen, creating negative pressure that draws replacement air in through openings and leaks.

Outdoor air supplied to replace air removed by exhaust devices so that building pressure stays near neutral. Without it, large exhaust flows can depressurize the house and cause backdrafting.

Reversal of flue gas flow in a naturally drafted combustion appliance, allowing combustion byproducts to spill into the living space. It is caused by depressurization or blocked flues and is a health hazard.

The space containing fuel-burning equipment, tested for pressure, spillage, and carbon monoxide under worst-case conditions to confirm that combustion gases vent safely outdoors.

A designed route that lets air travel from rooms back to the air handler. Blocked or undersized pathways depressurize rooms, increase duct leakage, and reduce system airflow.

The systematic closing of unintended openings in the enclosure with sealants, gaskets, foam, and rigid materials. It reduces heat loss, moisture transport, and pollutant entry, and improves pressure balance.