DIY Eco Building Glossary

Plain-language definitions of building science, insulation, ventilation, and Passive House terms — with formulas, examples, and links to free calculators.

• ACH50 — ACH50 is the airtightness of a building, measured as the number of complete air changes per hour when the home is depressurized to 50 Pascals during a blower-door test.
• Air Barrier — An air barrier is the continuous element of a building envelope that stops air leakage — and it has to be continuous on all six sides of the conditioned space (floor, walls, ceiling) for the air-sealing to work.
• Air Sealing — Air sealing is the construction practice of closing intentional and unintentional gaps in the building envelope so that air, moisture, and outdoor pollutants stop moving through the wall, ceiling, and floor assemblies.
• Blower Door Test — A blower door test is the standard measurement of building airtightness.
• Building Envelope — The building envelope is the complete physical separation between conditioned interior space and the outside environment — including walls, roof, floor (or slab), windows, and doors.
• Cavity Insulation — Cavity insulation is the insulation installed inside the framing cavities of a wall, ceiling, or floor — between the studs, joists, or rafters.
• Cellulose Insulation — Cellulose insulation is made from recycled newspaper or wood fiber treated with borate fire retardants.
• CFM50 — CFM50 is the airflow (in cubic feet per minute) needed to depressurize a building to 50 Pascals during a blower-door test.
• Climate Zone — A climate zone is a building-code classification of how cold or hot a location is, used to set prescriptive insulation, glazing, and ventilation requirements.
• Continuous Insulation — Continuous insulation is a layer of insulation installed on the outside of the structural framing, with no thermal bridges through the layer.
• Cooling Degree Day — A cooling degree day (CDD) is a measure of how hot a location is over a year, expressed as the cumulative degrees above a 65°F base temperature each day, summed over the year.
• COP — COP (Coefficient of Performance) measures the efficiency of a heat pump as the ratio of heat moved to electricity consumed.
• Design Temperature — Design temperature is the outdoor air temperature used for sizing heating or cooling equipment — chosen to be a "worst-case but not extreme" value that the equipment must handle while keeping the indoor space at the setpoint.
• Dew Point — The dew point is the temperature at which air becomes saturated with water vapor and condenses into liquid water.
• Effective R-Value — Effective R-value is the real-world insulating performance of a wall, roof, or floor assembly after accounting for thermal bridges through framing, fasteners, and structural members.
• Embodied Carbon — Embodied carbon is the total greenhouse-gas emissions associated with the materials and construction of a building, measured in kilograms of CO2-equivalent (kg CO2e).
• Energy Star — Energy Star is a US Environmental Protection Agency program that certifies products and homes that exceed federal minimum efficiency standards.
• ERV — An ERV (Energy Recovery Ventilator) is a balanced mechanical ventilation system that exhausts stale indoor air and brings in fresh outdoor air through an enthalpy core that recovers both sensible heat and moisture.
• F-Factor — F-factor is the heat-loss coefficient for a slab-on-grade foundation, expressed in BTU per linear foot of perimeter per °F of temperature difference.
• Frost-Protected Shallow Foundation — A frost-protected shallow foundation (FPSF) is a slab-on-grade or stem-wall foundation that uses horizontal "wing" insulation extending outward from the building edge to push the frost line down and outward instead of digging deep.
• Heat Gain — Heat gain is the rate at which a building takes in thermal energy from its environment in summer, expressed in BTU per hour at design conditions.
• Heat Loss — Heat loss is the rate at which a building loses thermal energy to its environment, expressed in BTU per hour at design conditions.
• Heating Degree Day — A heating degree day (HDD) is a measure of how cold a location is over a year, expressed as the cumulative degrees below a 65°F base temperature each day, summed over 365 days.
• HRV — An HRV (Heat Recovery Ventilator) is a balanced mechanical ventilation system that exhausts stale indoor air and brings in fresh outdoor air through a heat exchanger that recovers most of the sensible heat from the outgoing stream.
• ICF — ICF (Insulated Concrete Form) is a building system where hollow EPS foam blocks stack like Lego, are reinforced with rebar, and are filled with concrete.
• IECC — The IECC (International Energy Conservation Code) is the model energy code published by the International Code Council and adopted (often with state-level amendments) by most US jurisdictions.
• Larsen Truss — A Larsen truss is an exterior framing system that creates a thick insulation cavity outboard of the structural wall, named after John Larsen who developed it in the 1980s.
• Latent Heat — Latent heat is thermal energy associated with the phase change of water (vapor → liquid → ice) without changing temperature.
• Manual J — Manual J is the ACCA (Air Conditioning Contractors of America) standard for residential heating and cooling load calculations.
• Mineral Wool — Mineral wool (also called rock wool or stone wool, sold under the Rockwool brand) is an insulation made by spinning molten basalt or slag into fibers.
• Net Zero — A Net Zero home produces as much renewable energy onsite (typically from solar PV) as it consumes over a year.
• Net Zero Ready — A Net Zero Ready home is built to the same envelope and mechanical-system quality as a Net Zero home but does not yet have its onsite renewable-energy system installed.
• NFRC — The NFRC (National Fenestration Rating Council) is the US body that certifies window, door, and skylight performance ratings.
• Nominal R-Value — Nominal R-value is the rated insulating performance of an insulation product as printed on its label — the R-value of the cavity insulation by itself, before considering any framing, fasteners, or installation effects.
• Operational Carbon — Operational carbon is the greenhouse-gas emissions from running a building once it is built — heating, cooling, hot water, lighting, plug loads, and any natural gas, propane, or oil consumed on site.
• Owner-Builder — An owner-builder is a homeowner who acts as the general contractor on their own home — securing permits, hiring subcontractors directly, ordering materials, and doing some or most of the labor themselves.
• Passive House — A Passive House is a building that meets a specific high-performance standard for envelope quality, airtightness, and energy use.
• PHIUS — PHIUS (Passive House Institute US) is a North American Passive House certification body that adapts the PHI standard for US climate variations.
• Pretty Good House — Pretty Good House (PGH) is an informal performance standard developed by builders in the northeast US who wanted Passive House-quality results without the certification overhead.
• R-Value — R-value measures a material's resistance to heat flow per inch of thickness, expressed in ft²·°F·hr/Btu.
• Rigid Foam — Rigid foam refers to a family of board insulation products: EPS (expanded polystyrene, white "beadboard"), XPS (extruded polystyrene, often pink or blue), and polyiso (polyisocyanurate, foil-faced).
• Sensible Heat — Sensible heat is thermal energy that changes the temperature of air or another substance — the kind of heat you can feel directly with a thermometer.
• SHGC — SHGC (Solar Heat Gain Coefficient) measures the fraction of solar radiation that passes through a window and becomes heat inside the building.
• Slab on Grade — A slab on grade is a foundation type where the concrete floor slab sits directly on top of compacted soil and rigid sub-slab insulation, with no basement or crawl space underneath.
• Spray Foam — Spray foam is a two-part liquid plastic that expands and cures in place when applied.
• Thermal Bridge — A thermal bridge is a part of a building envelope where heat moves through the assembly faster than the rest of the wall, ceiling, or floor — most commonly at studs, headers, plates, fasteners, or balcony slabs that conduct heat much better than the insulation around them.
• U-Factor — U-factor measures how well a window, door, or assembly transmits heat — the inverse of R-value.
• Vapor Barrier — A vapor barrier is a layer of material designed to slow or stop the diffusion of water vapor through a building assembly.
• Vapor Retarder — A vapor retarder is a building material that slows water-vapor diffusion through a wall, ceiling, or floor assembly without stopping it completely.
• Visible Transmittance — Visible transmittance (VT) is the fraction of visible light that passes through a window — daylight, not heat.