BPI Envelope cheat sheet

Heat flows from higher to lower temperature by conduction, convection, and radiation; the three must be addressed together for effective envelope performance.

R-value is thermal resistance per unit area; U-factor is its reciprocal (U = 1/R). Lower U-factor means better insulation.

The stack effect drives air upward in buildings when indoor air is warmer than outdoor; pressure differences scale with building height and temperature difference.

The four control layers of a building enclosure are water, air, thermal, and vapor; their order and continuity determine durability and efficiency.

Pressure boundaries (air control layers) must be continuous and aligned with insulation to prevent convective heat loss and moisture-laden air movement into assemblies.

Thermal bridging occurs where framing, fasteners, or structural elements bypass insulation; it reduces effective R-value and can cause cold spots and condensation.

Dew point is the temperature at which air reaches 100% relative humidity; surfaces below dew point risk condensation.

Latent heat is energy absorbed or released during phase change (e.g., water vapor condensing); it significantly affects moisture-driven energy loads.

Air sealing should target the pressure boundary: top plates, rim joists, penetrations, attic hatches, and recessed lights; sealing reduces both energy loss and moisture transport.

Insulation R-value requirements vary by climate zone; effective R-value is reduced by gaps, compression, and thermal bridging.

The 'four-inch rule' for air sealing: seal all gaps and cracks larger than 1/4 inch to control infiltration.

Blower door testing quantifies air leakage; a common metric is air changes per hour at 50 Pa (ACH50) or CFM50 per square foot of enclosure area.

Insulation must contact the air barrier on at least one side; a gap between insulation and air barrier allows convective bypass and reduces performance.

Vapor-permeable insulation (e.g., fiberglass, cellulose) can allow drying; vapor-retarding insulation (e.g., closed-cell foam) can serve as a vapor retarder when installed properly.

Attic insulation should cover the top plates of exterior walls and maintain clearance around heat-producing fixtures (e.g., chimneys, recessed lights).

Rim joists are a major air leakage path; sealing and insulating them improves energy efficiency and reduces moisture problems.

Moisture moves by bulk transport (liquid water), capillary action, air movement (vapor-laden air), and vapor diffusion; air movement is often the largest contributor.

The vapor control layer should be installed on the warm side of the insulation in cold climates and on the exterior side in hot-humid climates.

Materials should be selected and layered to allow drying in at least one direction; avoid creating double vapor barriers that trap moisture.

Relative humidity above 70% indoors can support mold growth; maintain indoor RH between 30% and 50% where possible.

Rain screen and drainage plane design allow water to drain and dry; avoid reverse flashing and ensure weep holes are clear.

Condensation potential is assessed by comparing surface temperature to dew point; thermal bridges and uninsulated surfaces are high-risk areas.

Ground moisture barriers (e.g., 6-mil polyethylene) in crawl spaces reduce vapor drive into the building.

Vapor retarder class (I, II, III) indicates permeance; Class I (e.g., polyethylene) is most restrictive, Class III (e.g., latex paint) is most permeable.

ASHRAE 62.2 sets whole-building ventilation rates based on floor area and number of bedrooms; typical formula: Q = 0.03A + 7.5(Nbr + 1) CFM.

Exhaust-only ventilation can depressurize the building and draw in pollutants from garages or soil; balanced ventilation (HRV/ERV) avoids this.

Heat recovery ventilators (HRVs) transfer sensible heat; energy recovery ventilators (ERVs) transfer both sensible and latent heat.

Source control is the first line of defense for IAQ: seal combustion appliances, vent bathrooms and kitchens directly outdoors, and avoid storing chemicals indoors.

Carbon monoxide alarms should be installed near sleeping areas and on each level of the home.

Bathroom exhaust fans should be rated for at least 50 CFM intermittent or 20 CFM continuous; kitchen range hoods should be vented outdoors.

MERV ratings indicate filter efficiency; MERV 13 or higher captures fine particles but may require system design review for pressure drop.

Radon is a soil gas that can enter through foundation cracks; mitigation includes sub-slab depressurization and sealing entry points.

Window U-factor measures heat loss; lower U-factor means better thermal performance. Solar Heat Gain Coefficient (SHGC) measures solar heat admitted; lower SHGC reduces cooling loads.

In cold climates, select windows with low U-factor and moderate-to-high SHGC for passive solar gain; in hot climates, prioritize low SHGC.

The National Fenestration Rating Council (NFRC) label reports U-factor, SHGC, Visible Transmittance (VT), and Air Leakage (AL).

Air leakage (AL) rating for windows is measured in cfm/ft²; lower is better; a common threshold is ≤0.3 cfm/ft².

Door weatherstripping and sweeps should create a continuous seal; adjust thresholds and strike plates to ensure uniform contact.

Window flashing integrates with the water-resistive barrier (WRB) in shingle fashion (upper layers overlap lower layers) to shed water.

Double-pane windows with low-E coatings and argon fill improve thermal performance; warm-edge spacers reduce condensation at edges.

Installation defects (gaps, misalignment, missing flashing) are common causes of air and water leakage; follow manufacturer instructions and local codes.

Blower door testing measures building air leakage by depressurizing the building to 50 Pa; results are reported as CFM50 or ACH50.

ACH50 = (CFM50 × 60) / building volume in cubic feet.

The blower door can also be used for zonal pressure diagnostics to locate leakage paths between zones (e.g., attic, garage).

A manometer measures pressure differences in Pascals (Pa); typical indoor-outdoor pressure difference under natural conditions is 1–5 Pa.

Duct leakage testing uses a duct blaster to pressurize the duct system and measure leakage in CFM25 (cubic feet per minute at 25 Pa).

Infrared thermography can visualize temperature differences caused by air leakage, missing insulation, or thermal bridging; best performed with a steady indoor-outdoor temperature difference.

Pressure pan testing measures pressure differences across a register to infer duct leakage to outdoors; a reading above 3 Pa suggests significant leakage.

Commissioning of ventilation systems includes measuring exhaust airflow with a flow hood or balometer to verify design rates.