Knowing how to figure out the plenum space dimension is an essential part of successfully completing the ARE Building Section Vignette. This vignette is fairly straightforward and really only has 1 correct solution. Where candidates typically get into trouble is when it comes to figuring out the minimum plenum space between the ceiling and the floor slab above. If you’re not able to properly size your plenum space, it will usually result in a failed ARE exam score so it’s essential that candidates understand this issue.
What Is The Plenum Space?
The plenum space, as it applies to the building section vignette, refers to the space used for mechanical and structural systems above the ceiling but below the floor above. Enough space must be given in the plenum so that ducts can properly run throughout a building without conflicting with structural members or even other mechanical components.
Since the CDS vignette requires you to draw the most efficient building section possible given the program requirements, the extra space in the plenum must be kept to a minimum. What causes a lot of people to fail this ARE exam is the fact that they draw the plenum dimension at the section cut line. While you should always draw what is being cut through in that specific section, you still must leave proper space to accommodate the worst case scenario on each floor.
To be safe, many candidates often leave too much space for the plenum and this will certainly count against them too. NCARB wants candidates to provide the most efficient solution, meaning that it will count against you if you leave too much space where it’s not needed.
In order to provide the correct plenum dimension on the building section vignette, you must figure out the worst case scenario on each given floor. You will need to take into account both joist depth as well as the depth of the duct. This is commonly referred to on the ARE Forum as the “Joist/Duct Combo”.
Your goal when trying to solve this ARE vignette is to figure out the maximum joist/duct combo for each area and leave enough space to satisfy this dimension. What you’ll want to do is carefully examine the background floor plans and determine what the maximum joist duct combo is by adding the depths of the 2 elements together.
For example, say that you have an area with 24″ deep joists and 12″ deep ducts on the first floor but closer to the mechanical chase you have 18″ deep joists with 24″ deep ducts. Since the second joist/duct combination is larger (42″ is greater than 36″), you will need to provide that much space above your ceiling, regardless of where the section is cut.
The part that confuses many ARE candidates is the fact that you are providing a dimension that is usually not shown where the section is cut. Since your goal is to draw a building section, make sure that you draw the proper size of both the joists and the ducts where the section cut line is. The key to the whole plenum space issue is to leave the appropriate space even if what you’re cutting through is not as big as the plenum.
In addition to the plenum space, you will also need to accommodate the size of the light fixtures. This dimension is given to you in the program so make sure that you add on this additional dimension the maximum joist/duct combo in each situation. Remember, you will probably have to calculate the joist/duct combo 3 times (first floor, second floor, double-height space) depending on the floor plans. Fortunately, you will not be asked to draw the light fixtures in your section but you are responsible for leaving proper space for them.
The issue of the plenum dimension is highly debated on the ARE Forum because it’s such a critical portion of the ARE construction documents vignette. The plenum refers to the area between the ceiling and the bottom of the floor or roof above. Included in this space needs to be room for lights, ducts, and joists. By figuring out the worst case scenario, you can properly leave the minimum amount of space to satisfy all of the requirements.
For more information about the CDS vignette and the proper steps it takes to pass it, please see the ARE Advisor ebook series.