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    Page 1 of 1

    One last sticking point

    New postPosted: Wed Jan 28, 2009 12:18 am
    by design4U
    Codeman,

    Since you are so helpful we have one last question on energy. As in the last request, the 2006 codes apply.

    The client wants a non-permanent overhang that can be detached from the building during the winter months on a commercial building (office in climate zone 4). When it is attached, the overhang will have a projection factor of 0.40.
    What is the maximum SHGC for the vertical fenestration that is shaded by the overhang. A reduction will help with costs.

    Re: One last sticking point

    New postPosted: Wed Jan 28, 2009 11:06 am
    by Jerry Peck - Codeman
    Yes, you certainly have a sticking point, but probably not the sticking point you are thinking of.

    design4U wrote:The client wants a non-permanent overhang ... When it is attached, the overhang will have a projection factor of 0.40.


    Actually, "when it is attached" it will not have a projection factor at all.

    This is because the 2006 IECC states:

    (underlining and bold are mine)
    - 502.3.2 Maximum U-factor and SHGC. For vertical fenestration, the maximum U-factor and solar heat gain coefficient (SHGC) shall be as specified in Table 502.3, based on the window projection factor. For skylights, the maximum U-factor and solar heat gain coefficient (SHGC) shall be as specified in Table 502.3.
    - - The window projection factor shall be determined in accordance with Equation 5-1.
    - - - PF = A/B (Equation 5-1)
    - - - - where:
    - - - - - PF = Projection factor (decimal).
    - - - - - A = Distance measured horizontally from the furthest continuous extremity of any overhang, eave, or permanently attached shading device to the vertical surface of the glazing.
    - - - - - B = Distance measured vertically from the bottom of the glazing to the underside of the overhang, eave, or permanently attached shading device.
    - - Where different windows or glass doors have different PF values, they shall each be evaluated separately, or an area-weighted PF value shall be calculated and used for all windows and glass doors.

    That shading device can only be included in the PF calculation if "permanently attached", and the overhang you are proposing is not "permanently attached".

    Now, though, if we were to set that aside (not that it can be set aside for construction purposes, but for the purpose of answering the other part of the question):

    You stated the projection factor was to be 0.40. That is equivalent to, for example, a 4 foot horizontal overhang shading glazing which has the bottom of the glazing 10 feet vertically below the bottom of the overhang.

    The SHGC for Climate Zone 4 (except Maine) with a PF of 0.25 or greater is NR (No Requirement). As shown in Table 502.3

    However, as stated above, that was just for calculation purposes and to answer your question. As the overhang you are proposing is not permanently installed, it cannot be counted. This is because the PF could be used in the energy calculations to reduce costs, then the shading device never installed for use - totally defeating the requirements and reason for the code.

    Codeman