Article #1460 - REScheck Webcast Questions - Overhangs and Orientation

This article is based on questions asked during both the May 6, 2005 REScheck Training Webcast and the July 28, 2005 REScheck Training Rebroadcast as well as questions received through email following these events. The original language of the questions and answers has been modified to make them as generic as possible while retaining the applicable specific information.

Does a covered front porch count as an overhang if there is a second floor over porch?

Yes, any permanent extension counts as an overhang. In REScheck, you will need to separate out all of your walls and input the appropriate projection factor for each window or group of windows entered in the program. Keep in mind, this option is only available in REScheck for projects in an area of < 3500 heating degree days where the mandatory requirement for the solar heat gain coefficient of the windows is 0.40 and certain codes are selected in the Code menu.

How do you calculate the U-value for projection of overhangs?

Determining the overhang projection factor (PF).

The value for the overhang is not considered a U-value. It is called a "projection factor" which can help with compliance in locations < 3500 heating degree days where the code is 1998 IECC, 2000 IECC, or 2003 IECC or in Pima, Arizona < 4000 ft or Georgia. The option exists to use orientation and enter any overhangs that exist on the project.

The projection factor enables you to characterize the shading impact of horizontal overhangs or canopies that project outward from the plane of the window. The projection factor is the ratio of the distance the overhang projects from the window surface to its height above the sill of the window it shades. Overhang Projection Factor = a (distance to edge of overhang)/b (distance to bottom of window sill).

The following link offers additional information on this subject:

What is a Projection Factor?

I'm in Colorado, but I would like to enter Orientation and Projection Factor. How would I do this?

Orientation and Projection Factor are not available unless the project is in a location of < 3500 heating degree days and the code is 1998 IECC, 2000 IECC, or 2003 IECC or when Pima, Arizona < 4000 ft or Georgia. The reason these options are not available outside of the locations listed is that the program offers credit only towards a mandatory solar heat gain coefficient (SHGC) of .40.

How do you handle orientation of angled walls?

The orientation option is only available in REScheck for projects in an area of < 3500 heating degree days where the mandatory requirement for the solar heat gain coefficient of the windows is .40. If this option is applicable to your project, select Orientation from the Options menu to enable the Front Faces drop list for the building and create the Orientation column for walls, windows, doors, and basement walls. For walls, use the Orientation column to select the wall's orientation relative to the front of the building. To orient the right and left side of the building visualize yourself inside the building facing the front. Any windows or doors entered belonging to a wall with a set orientation will have the same orientation as the parent wall.

Front Faces represents the angle of the front face of the building relative to North. If the building front faces North, the building orientation will be 0 degrees; if the building front faces East, this will be 90 degrees and so on. Select one of the eight cardinal points in the drop list or choose Custom. Custom allows the user to specify an angle (in degrees) by rotating the building using a diagram.

Are south overhangs taken into account when calculating solar gain?

If you choose Orientation and Projection Overhangs from the Options menu, the software will take into account overhangs when calculating an adjustment towards the solar heat gain coefficient requirement. This option is only available if your project is in a location < 3500 heating degree days where the code is 1998 IECC, 2000 IECC, or 2003 IECC or in Pima, Arizona < 4000 ft or Georgia.

The adjustment to SHGC for overhang projections is based on work developed by the Technical Evaluation Committee for ASHRAE Special Project 53, under subcontract to PNNL in 1985-1988. The underlying data source was the ARES database. This work produced a set of multipliers specific to eight orientations along with a regression analysis based simplified formula. The relative orientation of the component with respect to "North" is first determined in order to select the correct set of coefficients to apply to the simplified multiplier formula. With the selected coefficients applied along with the glazing component projection factor, a multiplier results that can be applied to the component proposed SHGC. Note that projection factors do not apply to skylights. The multipliers and formula to be applied to the projection factor are:

multiplier = exp(A * atan(PF)) + M0 - 1

where the multipliers MO and A vary by orientation as follows:

The adjustment process will occur when a request for the building average adjusted SHGC is requested. The process will loop through all applicable glazing components and for each in turn, compute the projection factor multiplier then compute the adjusted area-weighted proposed SHGC and sum this into a running total that is then divided by the total fenestration area when all components have been processed.

When should I consider orientation for my project?

If your project is in a location < 3500 heating degree days where the code is 1998 IECC, 2000 IECC, or 2003 IECC or in Pima, Arizona < 4000 ft or Georgia,the option exists to use orientation and enter any overhangs that exist in the project. This option should be considered if your project is not meeting the mandatory requirement of 0.40 SHGC on any windows or glass doors.