Considerations for historic buildings:
POAH and POAH Communites Window Limiter Policy: Windows should be limited to opening 4” maximum.
For each unique window type or wall assembly provide details for the head, sill and jamb conditions.
The details must clearly demonstrate the water control for the window opening (flashing and drainage).
The details must also clearly demonstrate how the air barrier of surrounding assemblies will be transferred to the window.
Window openings shall be fully flashed with flashing at the sill and jambs of the window opening
The window opening shall have provision to drain any incidental water on the flashing in the window opening. The sill flashing shall drain over the drainage plane of the wall or over the wall cladding.
The window shall be fully air sealed to the window opening at the entire interior perimeter of the window frame
For mulled window units, the junction of windows shall include:
Provision for drainage from the joint
Continuous air barrier across the interior side of the joint
Windows must comply with local energy code.
To verify if specific window energy properties comply with the local code requirements, look for the NFRC label.
The National Fenestration Rating Council (NFRC) label is needed for verification of energy code compliance. The NFRC label displays whole-window energy properties and appears on all fenestration products which are part of the ENERGY STAR program (www.nfrc.org).
Windows must be ENERGY STAR certified. Often energy star certified windows meet or exceed energy code requirements.
Proper window installation is necessary for optimal performance, to avoid air and water leakage. Always follow manufacturers’ installation guidelines and use trained professionals for window and skylight installation.
Window performance is based on location and climate. See below for climate zone requirements. To find your climate zone,
Windows must meet the following U-Factor and Solar Heat Gain Coefficient (SHGC) requirements.
Physical Sizing: all AC units must be sized to fit the thru-wall sleeve.
Thu-wall Sleeves: insulate and seal the perimeter of the wall sleeve to avoid drafts. Make sure thru-wall sleeves drain condensate to the exterior.
Insulated AC Covers for Winter Use: all thru-wall sleeves should be covered during winter months with an interior, hard-plastic, insulated cover. Use covers that are deep enough to go over both the sleeve and the AC unit so AC units do not need to be removed during winter months. See the AC Cover section of the Basis of Design for more information.
Use energy efficient AC unit: See the AC Unit section of the Basis of Design for more information.
POAH’s preferred strategy for in-window AC units is to install a new insulated sash panel with integral AC sleeve. The details shown below may need to be modified for each specific window condition.
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The rate of heat loss is indicated in terms of the U-factor (U-value). This rate of non-solar heat loss or gain through a whole window assembly is measured in Btu/hr-sf-°F. The lower the U-factor, the greater a window’s resistance to heat flow and the better its insulating value.
Solar Heat Gain Coefficient (SHGC)
The SHGC is the fraction of incident solar radiation admitted through a window. SHGC is expressed as a number between 0 and 1. The lower a window’s solar heat gain coefficient, the less solar heat it transmits. Whether a higher or lower SHGC is desirable depends on the climate, orientation, shading conditions, and other factors.
Visible Transmittance (VT)
The VT is an optical property that indicates the amount of visible light transmitted. VT is a whole window rating and includes the impact of the frame which does not transmit any visible light. While VT theoretically varies between 0 and 1, most values are between 0.3 and 0.7. The higher the VT, the more light is transmitted.
Air Leakage (AL)
AL is expressed in cubic feet of air passing through a square foot of window area (cfm/sf). The lower the AL, the less air will pass through cracks in the assembly. AL is very important, but not as important as U-factor and SHGC.
Condensation Resistance (CR)
CR measures how well a window resists the formation of condensation on the inside surface. CR is expressed as a number between 1 and 100. The higher the number, the better a product is able to resist condensation. CR is meant to compare products and their potential for condensation formation. CR is an optional rating on the NFRC label.