One critical aspect of building performance is shading. With a properly designed building shell, the amount of solar radiation hitting the building’s opaque surfaces usually aren’t a problem. However, the windows are the weakest link in the shell. The presence or absence of shading can have a huge impact on cooling and heating loads, even with excellent quality windows.
You can control radiation intrusion to a certain extent by tuning the solar heat gain coefficient (SHGC), that glazing property that determines how much sunlight enters the building. However, additional radiation management may be needed to keep the building comfortable throughout the year. For instance, you may want more sunlight entering during the winter and less during the summer. With proper shading design and analysis, you can guarantee that a building stays comfortable all year while at the same time minimizing heating and cooling loads.
The Title 24 Approach
When determining if a building meets California’s residential energy code, the compliance software analyzes the glazing SHGC in each orientation to determine how much heat enters the building. This is a good first-level analysis, but it’s only so accurate—it ignores shading not attached to the building and some of the window nuances. A Title 24 energy model can include details on overhangs and side fins. In practice many modelers ignore those obstructions unless they need them to nudge a calculation from non-compliance to compliance.
The California modeling software makes basic assumptions about all windows. For instance, it assumes that all windows have a “bug screen”, which reduce radiation intrusion. It also assumes that each window has interior drapes that are closed at night and half-open in the daytime. There are also complex algorithms for estimating the effect of reflected exterior radiation. The algorithms assume that with the exception of user-entered overhangs and side fins, all windows are the same. Each one of these assumptions has a tendency to reduce the accuracy of the overall energy calculations.
The biggest omission in the Title 24 calculations, however, is the lack of any consideration of shading that is not attached to the building. There is no way to account for, for instance, a tall row of redwood trees on the south side of the building. Ignoring exterior shading guarantees that Title 24 calculations are not terribly accurate for any building that is not positioned in the middle of a broad flat field.
The Passive House Approach
The algorithms in the Passive House Planning Package (PHPP) allow much greater shading accuracy when used properly. Let’s start with windows. Figure 1, provided courtesy of Ed May of bldgtyp.com, shows the first set of variables that are analyzed:
dover and oover are the distances from the glazing edge to an overhang, and the extension from the glazing face to that overhang respectively. This is similar to the Title 24 definition of an overhang, with a few differences. Title 24 measures dover from the edge of the rough opening, not the glazing. Title 24 oover measures from the face of the building, not from the face of the window, which may be inset in the wall assembly. These are small, but potentially important differences.
dreveal and oreveal specify distances to a side fin or building projection. Like an overhang, the PHPP measurements are done slightly differently than those used for Title 24. dreveal is measured from the edge of the glazing to the base of the side fin. Title 24 measures from the edge of the rough opening. oreveal is measured from the face of the glazing, not the face of the building as in Title 24. Like the overhang calculations, these small differences can add up.
Note in Figure 1, there are two additional variables: dhori and hhori. These parameters can be used to account for objects, not attached to the building, that can block solar access. You can use these two variables to identify, for instance, adjacent buildings, rows of trees, or mountains. Unattached objects can have a huge impact on the solar radiance hitting a building.
But Wait, There’s More!
PHPP has additional capabilities for accurately analyzing shading. There is a window reduction factor for temporary shading. This is a percentage that shading can be increased, under occupant control, for blocking unwanted solar intrusion. It can be used to define window screens, interior shades, exterior shades, or other types of blockage such as variable-opacity “smart glass”.
There are also window shading reduction factors for winter and summer. These can be used in a variety of ways. For instance, you could have a shed adjacent to the west façade of a building. Behind that shed there could be a large deciduous tree that has no leaves in the winter. You can create winter and summer shading factors that consider those obstructions during both winter
and summer. Although these types of calculations aren’t necessarily simple, they are well- documented and doable. Certified Passive House projects have to take into consideration all relevant shading obstructions.
Fortunately, the Passive House Institute has created tools for radically simplifying shading analysis. There is a Plugin, called DesignPH, for the popular 3d modeling program Sketchup. You model the shell of the building, and include all the neighboring shading obstructions, including the location’s topology, if it’s relevant. DesignPH can then export the lion’s share of the project’s parameters into PHPP, including detailed shading analysis.
Figure 2, provided by Christian Kienapfel of PARAVANT Architects, shows a complex shading scenario for an Accessory Dwelling Unit (ADU). The combination of Sketchup, DesignPH, and PHPP provides an accurate assessment of heating and cooling loads, making it easy to size the mechanical equipment, and decide where to put external shading devices like overhangs.
Figure 3 show the same project with all the external shading obstructions removed. With no shading, and increased solar exposure, the winter heating load decreases by 37%, and the summer cooling load increases by 36%.
Complying with an energy code, whether it’s from California or another entity is one task. Most energy codes don’t require extreme accuracy, just improvement over a baseline, usually a prevailing building code. Although proper shading analysis is usually more complicated than code compliance, it needs to be considered to ensure that a building performs properly and that the occupants are comfortable. The Passive House Planning Package does a good job in this respect.
Author: Steve Mann
© 2021 by Home Energy Services