Glossary

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R

R-value
A measure of the resistance of a glazing material or fenestration assembly to heat flow. It is the inverse of the U-factor (R = 1/U) and is expressed in units of hr-sq ft-F/Btu. A high-R-value window has a greater resistance to heat flow and a higher insulating value than one with a low R-value.
Radiation
The transfer of heat in the form of electromagnetic waves from one separate surface to another. Energy from the sun reaches the earth by radiation, and a person's body can lose heat to a cold window or skylight surface in a similar way.
Rail
Horizontal member of a window sash.
Reflectance
The ratio of reflected radiant energy to incident radiant energy.
Reflective glass
Window glass coated to reflect radiation striking the surface of the glass.
Refraction
The deflection of a light ray from a straight path when it passes at an oblique angle from one medium (such as air) to another (such as glass).
Relative Heat Gain
The amount of heat gain through a glass product taking into consideration the effects of solar heat gain (shading coefficient) and conductive heat gain (U-value). The value is expressed in Btu/hr/ft2 (W/m2).
Relative humidity
The percentage of moisture in the air in relationship to the amount of moisture the air could hold at that given temperature. At 100 percent relative humidity, moisture condenses and falls as rain.
Retrofitting
Adding or replacing items on existing buildings. Typical retrofit products are replacement doors and windows, insulation, storm windows, caulking, weatherstripping, vents, landscaping.
RESFEN
or RESidential FENestration, A computer program used to calculate energy use based on window selection in residential buildings.
Roof window
A fixed or operable window similar to a skylight placed in the sloping surface of a roof.
Rough opening
The opening in a wall into which a door or window is to be installed.