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.
Use of reflective glass is an option for
reducing unwanted solar gain.
How is the use of reflective coatings tuned for varying climates?
By the location of the coating on surface 2 or surface 3 of the IG panel.
What are the benefits and drawbacks of applying a reflective coating on surface #2 vs. surface #3 in an IG panel?
The SHGC is lower with a reflective coating on surface 2, but thermal stresses can be more common.
IG spacers are comprised of what 3 components?
Spacer, desiccant and sealant.
The IG spacer provides three functions, what are they?
Provides structural integrity
Carrier for the desiccant
supports the IG panel sealants
How is a ‘warm edge’ spacer differ from a traditional metal spacer?
How does this alter performance?
The typical metal is replaced with a material like PVC foam.
This reduces:
H - Heat flow around the edges of the IG unit and
C - the tendency for Condensation to form at the perimeter.
What is the function of inert gas fill in IG panels
Slow heat transfer
Reduce convective circulation
Low conductivity framing is
Sash and unit frame material with a low U value to complement low u-value glass.
Not really a question here - just remember the term.
Effective weatherstripping is used to seal between the ____ and _____ of a window.
Seal the contact areas between the fixed and movable section of the window.
Poor weatherstripping can cause either ____ or____ depending on whether there is too much, or too little.
Air leakage or unduly high operating force.
Energy performance is specified in what part of the building codes
IECC - International Energy Conservation Code.
The 2018 IECC offers two compliance paths. What are they?
Prescriptive path -
Total building performance approach (Modeling)
Describe the prescriptive method for IECC compliance
IECC prescribes one set of energy efficiency requirements for each component, regardless of the % of glazing in the walls, in different climate zones.
Describe the total building performance method for IECC compliance,
This method allows trade offs between the building envelope, mechanical system and lighting system.
How many climate zones are defined by the IECC
8
What criteria must be met for a product to be deemed as meeting IECC requirements?
U-factor and SHGC must meet specified requirements for the different climate zones.
In the IECC, skylight U-factors range from ___ in zone 1 to ____ in Zone 8.
Zone 1: 0.75
Zone 8: 0.50
Are skylight U-factors the same in the IECC and IRC
Yes
Are skylight SHGC requirements the same in the IECC and IRC?
No.
What maximum SHGC’s stipulated for Zones 4 - Zone 7-8 in the IRC?
None are stipulated.
What standard spells-out design trade-offs in the performance-based IECC compliance path for commercial buildings/
ASHRAE 90.1
Performance requirements for metal-framed fenestration products in commercial construction are higher or lower than for residential windows, doors and skylights?
Lower, especially in northern climates (Zone 4-8)
What is the maximum area for skylights allowed in the IECC prescriptive path?
3% of the roof area.
Give an example of how additional skylight area (above the prescriptive maximum) can be offset in the IECC performance compliance path?
Adding a system that dims interior lighting when adequate day-lighting is provided through the skylights and windows.
What performance parameters and fenestration types are rated by the National Fenestration Rating Council (NFRC)?
Hint: UVAS
U-factors
Visible transmittance
Air infiltration
SHGC values
for windows, doors and skylights,
How are NFRC ratings developed?
Combination of computer simulations and laboratory testing.
Are NFRC ratings accepted in model nationwide energy codes? How are ratings displayed on the fenestration?
Yes
Temporary labels.
What are the 3 criteria in NFRC simulation and testing standards, and what properties do they correspond to?
(This is not the 3 criteria for evaluation per NAFS)
NFRC 100 - U-factor
NFRC 200 - SHGC
NFRC 400 - Air Infiltration
2018 iRC requires compliance with which which NFRC testing standards?
NFRC 100 and 200
The definition of an energy star qualifying product is based on…
Fenestration Class & Rating Solar Heat-gain coefficient R-value Low-E coatings U-factor
U-factor
Solar Heat-gain coefficient
How do windows with low U-factors and SHGC impact building energy use?
What is the specific impact of low SHGCs?
They tend to lower energy use.
Low SHGC’s lower cooling energy use.
For Energy Star what procedures are cited for independent testing of:
U-factor
SHGC
IG Units must conform to:
NFRC 100
NFRC 200
ASTM E2190
