A while back, we took a look at this roof in the pictures below. It’s a TPO, thermopolyolithin roof system with an elaborate and extensive ductwork system on top of the roof. We aren’t certain of the exact history, but it’s very interesting to look at from a distance and take it all in. The long system of metal ductwork spans out like a spider, maybe a daddy long legs, on top of the roof.
In general we should start by saying that we generally advise against exposed ductwork on rooftops because it can cause a lot of problems both for the HVAC system and for the roof if not done with great care to coordinate between the roofing and the HVAC system. It’s rare to find a contractor who specializes in both. For example, here at Dupont roofing, we are a roofing contractor that specializes in flat roofing. We don’t do HVAC work, but we often have to fix problems caused by HVAC systems. As well HVAC systems rarely do both HVAC and roofing. These are naturally separate trades because they require separate fields of expertise that only have limited specific overlap.
As a result of this separation and distinction between trades, HVAC system installers often don’t recognize the level of required protection as much as they should in respect to caring for the roof and coordinating to make sure their penetrations and respective sealing work is properly done. If the flashing at the base of the ductwork penetration is not done properly and properly sealed to the sides of the penetrating ductwork, leaks can happen at that location. As well, if the ducts are not properly sealed for an exterior installation, leaks can also happen at the internal parts of the ductwork and then run into the interior of the house. Even in cases where leaks are minimal, the lined ductwork insulation become become saturated, like a sponge retaining and holding water, and cause damage to the duct work or the insulation and cause poor air quality for extended periods of time before the damage becomes evident and known.
This particular system is set high above the roof membrane which may have been intentionally installed in this way to allow space for the roof installation at a later time or date. That type of spacing is somewhat rare but nonetheless helpful. In this case, the duct is wrapped with a radiant and insulator layer to both improve our value and minimize heat or cooling transfer, essentially minimizing energy loss. Nonetheless, even with an insulative type wrap on the exterior of the duct, it’s almost always required to also have an insulated lining on the interior of the duct as well because the wrap on the exterior, alone, is insufficient to maintain proper thermal insulation values. The difference between the exterior air temperatures in the interior tempered air on the inside of the duct can be so significant that it can cause serious problems with condensation and thermal energy loss.
This issue occurs both in the summer and in the winter, particularly in our climate zone here in Washington, DC. The outside air temperatures in the winter can be so cold that condensation can occur on the inside of the duct work if not properly insulated and conversely the temperatures on the inside of the duct can be comparatively so cold that it can cause condensation on the outside of the duct work in the summer. If this condensation accumulates the point that it ponds and or runs down the ductwork, it can cause both rust and deterioration of the duct work or leakage into the inside of the building.
When you look at the ductwork system like this it’s hard to imagine that there’s not a intense amount of thermal transfer happening between the tempered air and the exterior outside air at both of the main hot and cold seasons of the year. However, people often install ductwork like this on the rooftop because space in Washington DC buildings is simply limited and naturally people want to have as much of the interior space on encumbered by ductwork in bulkheads as possible.
If you look closely at the picture below, you can see that the metal angle used as supports to hold the ductwork up in the air is set upon rolled pieces of scrap TPO or similar membrane material. The building code and related installation requirements mandate that auxiliary or separate elements must be only set upon the roof system with a slip sheet or similar type of separate sacrificial material installed between a support system like this and the main roof membrane. Here, these scrap pieces of material are better than nothing, but there are limitations with the way this material is being used. In this configuration and context, the material is not secured to the legs of the support system, by more than gravity alone. That’s not a sufficient method of connection to ensure the sacrificial slip sheet type elements stay in place through iterations of heavy weather conditions with high winds, for example.
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. 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.
