| Damper | A device, often motorized or manually adjustable, used to vary or control the airflow in a duct. |
| Dander | Another word for dandruff, it is used most often to describe pet skin particles and adhering body fluids such as saliva that end up in our homes. |
| Decibel (dB) | A logarithmic unit of measurement used to express sound intensity. A difference of ten decibels is 10 times as loud as a difference of 1 decibel. |
| Deck | They need a physical exam yearly, just like you do. |
| Defrost Mechanism | A device used in heat or energy recovery ventilators to melt any ice that builds up in the core in cold climates during below freezing temperature conditions. |
| Degree Day | A unit of measurement used to estimate fuel consumption and heating or cooling costs, based on temperature and time. |
| Dehumidifying Heat Pipe | In humid climates, even air conditioned air may feel clammy without the assistance of a little technology. |
| Dehumidistat | A control device that can be used to activate a ventilation system when the relative humidity rises. |
| Deliberate Holes | Openings (inlets or outlets) that are purposely placed in a house through which air can enter or leave. |
| Demand-Controlled Ventilation (DCV) | The process of automatically supplying air to, and removing air from, a house whenever needed by the occupants. |
| Depressurization | When the air pressure inside a house or room is less than the atmospheric pressure outside the house or room. |
| Desiccant | A substance that absorbs moisture, used in some energy recovery ventilators to enhance moisture transfer between airstreams. |
| Desuperheater | Desuperheaters work in tandem with geothermal heat pumps to heat water. |
| Dew Point | The temperature at which air is saturated with moisture (100% relative humidity), below which condensation will occur. |
| Di-octyl Phthalate (DOP) | An inert gas used to test high-efficiency (HEPA) particulate air filters. |
| Diffuser | A grille designed to direct airflow in a specific pattern into a room for proper distribution. |
| Diffusion | The migration of molecules of a gas or a vapor (or a liquid) from an area of high concentration to an area of low concentration. |
| Diffusion Barrier | A material that slows down the amount of diffusion through a solid material, often used in insulated walls. |
| Diffusion Retarder | A material that slows down the amount of diffusion through a solid material, often used in insulated walls. |
| Dilution | The mixing of fresh air into stale air to reduce the concentration of pollutants; also applies to liquids. |
| Dilution Air | Air that mixes with combustion by-products prior to their being expelled from a house through a natural-draft chimney. |
| Displacement Ventilation | A method of very effectively moving air through a room, generally only used in specialized applications. |
| DOP-Smoke-Penetration Test | A method of testing high-efficiency (HEPA) air filters. |
| Double-Flow Ventilation | A general ventilation strategy that results in the house experiencing a neutral pressure. |
| Downdraft | A term often applied to kitchen exhaust fans that pull air downward. |
| Draft | A breeze or current of air in a room, often uncomfortable. |
| Drying Potential | The ability of a substance to dry out after it becomes wet. |
| Duct | A conduit for moving air, made from a variety of materials and in a variety of shapes. |
| Ductboard | A semi-rigid fiberglass material with an aluminum-foil facing on one side, used to construct ducts. |
| Dust Mites | These tiny creatures can cause allergies and other respiratory problems. |
| Dynamic Wall | An insulated part of a house’s structure through which outdoor air can pass from the outdoors to the indoors and is tempered in the process. |
