Rooftop HVAC Systems: Pros and Cons
We’ve looked at the topic of ductwork on rooftops before here on our website and on our blog. Today, we will also take a look at some pros versus cons, and as we admit that there are some benefits to having ductwork installed above a rooftop, we want to candidly point out that we are overall against installation of ductwork on a rooftop when there are any other reasonable alternatives. More than anything else, ductwork on a rooftop causes more problems than it solves.
However, the conversation wouldn’t be complete if we didn’t talk about some of the reasons that people install ductwork on a rooftop to begin with. Firstly, it’s important to recognize that different types of specialty trade contractors have different interests. Roof contractors, if they care about the client’s long-term benefit and improvement of the roof portion of the property, are concerned primarily with maintaining a sound and effective roof system.
HVAC or mechanical or ductwork contractors by comparison should be concerned about the economics of their system and the long-term efficacy. It’s important though also consider though in the big picture that even for an HVAC contractor, while they might be just concerned with their work at the beginning of the life cycle of a HVAC system, they should think about the efficiency and economics of their mechanical system for the long-term lifespan of the system from an energy consumption perspective. Ductwork on a roof is contradictory to the majority of the principles of energy efficiency.
The picture below shows the HVAC Totalpak system which has both a condenser and evaporator and air handler all together in one unit on a rooftop. The benefit of this type of system is that it alleviates the need for taking up a mechanical closet space on the inside of the building or backyard space for a typical condenser unit.
That equipment sits on top of a pair of steel i-beams and from that elevated position, the ductwork is set on flimsy stands to run downwards towards the elevation of the low slope roof system below, where at a later point the ductwork goes through the roof system into the building.
(As a side note, we recently wrote an article, here on our website that you can read which explains several of the principles behind using steel I-beams as equipment stands on historic buildings here in Washington DC.) Even though the ductwork still goes and runs into and through the building, there’s much less horizontal runs of ductwork required, on the inside of the building, in this type of system since a large portion of the ductwork is running horizontally on the roof.
Nonetheless, some bulkheads will be required in most systems like this but the majority or a significant portion of those bulkheads can be avoided or omitted from the overall design if the HVAC unit is set on the rooftop. Bulkheads look bad, they essentially lower the ceiling height at the edges of a room and look like an odd or awkward obstruction in the ceiling.
In historic buildings like the majority of the ones here in Washington DC, where ornate crown moldings or millwork or installed on the ceilings or top of walls, the bulkheads require these elements to be reworked and or modified or removed altogether. That type of requirement can detract from the historic authenticity and aesthetic of a building’s interior.
Above, we discussed some of the primary benefits of having ductwork installed on the roof, it simply clears up interior floor space and even allows for more yard space and outdoor space like space in the yard really matters here in Washington DC because the yards are comparatively small.
The negatives or cons against installing ductwork exposed on a rooftop though are numerous and most likely outweigh the benefits, in almost all conditions are cases. Some of the negative trade-offs are that air heats up significantly in the summer and cools down significantly in the winter when it runs through this ductwork. HVAC or mechanical contractors may say that the amount of thermal transfer is minimal because the ductwork comes with a half inch lining on the interior of the duct, but it’s not true.
The amount of thermal energy loss in even a relatively short run of ductwork is massive, even when it has high quality insulation on the interior of the duct. Additionally or as an alternative, insulation can also be installed on the exterior of the duct, sometimes remedial attempts to help insufficient interior insulation, but it’s still not enough and the insulation generally installed on the exterior of duct work is highly susceptible to deterioration driven by exposure to ultraviolet rays.
One of the other very big problems with ductwork, exposed on top of roofs, is that it has a high tendency for leakage. Industrial flexible rubber and siliconized type coatings can be applied to help seal the ductwork better than the typical HVAC or mechanical contractors, but these materials are extremely expensive and the vast majority of contractors including most roofing contractors do not have access to or the experience needed to know how to install these materials properly. Our company can help with these type of issues, but it’s uncommon.
We provide this information here on our blog, and our website, to help our customers and future clients, and we recommend every building owner in DC who values the longevity of their roof (and their investments) and building use a contractor who values the simple and important principles of proper roof construction like Dupont Roofing DC.
Our company specializes in flat roofing here in Washington DC and we’re happy to help building owners of almost all types. Learn more about our company and the proper techniques of working with roofing on historic buildings in Washington DC here on our blog at DupontRoofingDC.com, and you can call us at (202) 840-8698 and email us at dupontroofingdc@gmail.com. We are happy to help and at least talk through options.
