Group 4.B - Building Codes, Part II: Energy Efficiency and Thermal Performance Flashcards
Windows typically comprise what percentage of a homes exterior wall area and what percentage of the heating and cooling needs
10-25% of area
25 - 50% of heating cooling
The basic measurements of thermal performance are:
Thermal conductivity and Solar heating.
What is the measure of thermal conductivity?
U-factor
How is total heat transfer through the fenestration calculated?
U-factor multiplied by the temperature difference from interior to exterior.
U factor is defined as:
The heat transfer per time unit, per area, per degree of temperature difference.
The technical expression for U-factor is
Btu/hs.f.Deg.F
Incident solar radiation affecting fenestration is broken down into three types based on frequency. What are they?
Ultraviolet (UV)
Visible light
Infrared.
What is Solar heat gain coefficient?
The fraction of (primarily) infrared solar radiation transmitted through the glass.
How is Solar Heat Gain Coefficient expressed and what does this value mean.
As a decimal number between 0 and 1.0
A lower SHGC value means less solar energy transmitted and more shading ability.
Where is a Low SHGC desirable?
Where is a high SHGC sought?
Low:
Warm climates
commercial buildings in most climates.
High:
Cold climates where heat gain is beneficial.
What does LSG (Light to Solar Gain) index refer to?
How is it calculated?
A gauge of the efficiency of a glass product in transmitting daylight, while blocking solar gain.
Calculated as the ratio between Visible Light (VT) and SHGC: (LSG = VT/SHGC)
What energy transmission strategy is best for a window optimized for cold climate solar heat gain?
High LSG, High SHGC.
Allow maximum daylight and solar gain while reducing transmission of low-energy radiation, the type emitted by interior sources.
What component of a window is the primary tool in reducing U-factor
Insulating glass (IG) units.
Components included in IG units to create energy efficient buildings include:
Varying glass type and thickness Coatings Tints Gas fills Sealants Spacer materials.
Emissivity is a measure of what material property:
How is it expressed
The ability of a material to re-radiate infrared radiation absorbed by that material.
Expressed as a decimal number between 0 and 1,
Low E coated glass with an emissivity of 0.04 reflects what percent of radiant heat?
96% of long-wave radiation is reflected.
The numerical value of emissivity represents…..
…the percentage of radiation absorbed and retained by the material - the inverse of the percentage reflected.
Surfaces in an IG panel are reference by numbers counting from
a. The outermost glass surface, then in
b. The innermost surface, out
A. numbering starts on the exterior face of the external sheet.
A low-E coating on surface 3 (the air-space side of the interior sheet) provides what function?
It reflects heat back into the room to reduce heat loss through the window, keeping the space warm.
(It also reflects heat into the air-space, where conductive heating transfers some of that heat to the interior, but don’t tell anyone)
How are Low E coating are made:
From a microscopically thin layer of metal or metal oxide (silver or tin oxide) .
A low E coating on surface 2 (the interior face of the outer sheet) provides what function:
Reflects exterior heat back to the outside, keeping the interior cool.
The DOE defines spectrally selective” glass as that with a LSG of:
1.25 or higher.
i.e. VT = .6 and SHGC = .48
.6/.48= 1.25
What is the benefit of spectrally selective glass?
Allows day-lighting while minimizing heat-loads - minimizing artificial cooling AND lighting requirements.
Reflective glass coatings comprise….
metallic layers applied to float glass, that reflect solar energy.
