The Comprehensive Window Glossary: 50 Fenestration Terms Defined for Homeowners

## The Homeowner's A-to-Z Fenestration Dictionary - **Air Infiltration (AI):** The amount of air leaking through a window frame under 25 mph wind pressure (measured in cfm/sq ft). - **Apron:** A decorative horizontal wood trim piece installed directly beneath the interior window stool. - **Argon Gas:** A non-toxic, odorless inert gas sealed between dual panes to reduce conductive heat loss. - **Balance:** A mechanical spring or weight assembly inside window jambs that holds double-hung sashes open. - **Casement:** A window sash hinged on the side that cranks outward. - **Desiccant:** A porous molecular drying agent packed inside spacer bars to absorb internal moisture. - **Egress:** An opening large enough for emergency exit and firefighter rescue (minimum 5.7 sq ft). - **Fenestration:** Any opening in a building envelope, including windows, doors, and skylights. - **IGU (Insulated Glass Unit):** Two or three glass sheets bonded to a spacer bar to form a sealed thermal unit. - **Low-E (Low-Emissivity):** Microscopic silver coatings that reflect radiant heat while transmitting visible light. - **Mullion:** The structural vertical or horizontal post connecting two adjacent window units. - **NFRC:** National Fenestration Rating Council; independent non-profit rating thermal performance. - **R-Value:** The resistance to thermal heat flow (the mathematical reciprocal of U-Factor: $R = 1/U$). - **Sash:** The inner movable or fixed frame that holds the glass panes. - **SHGC:** Solar Heat Gain Coefficient; fraction of solar heat admitted through the window. - **STC (Sound Transmission Class):** Rating scale quantifying sound attenuation performance. - **U-Factor:** The rate of heat loss through the entire window assembly. - **Warm-Edge Spacer:** Non-metallic composite spacer that reduces perimeter glass condensation. - **Weep Hole:** Small drainage slots at the bottom exterior frame sill allowing rainwater to drain out. Speak with confidence when consulting ServiceFind Windows Pro for a **Fair Quote**.