It seemed like a straightforward project, but in the words of Confucius, “Life is really simple, but we insist on making it complicated.” It was a 25-year-old, pre-engineered building which had originally been constructed as a temporary warehouse on our military base—simple gable roof, batt insulation installed under the roof supported by the purlins, and gable end vents with louvered openings. Later, it was converted to office use. A dropped ceiling was added, batt insulation was rolled out over it, and ducted supply and returns were added to provide space heating and cooling.
Over the years, condensation and leaks caused some of the under-roof batts to droop down, leaving the underside of the metal roof exposed. We were called in to help but regarding an air quality issue, not a building envelope issue. The air handler was installed backwards, so the mechanical room with the computer server was acting as a return plenum; the louvered opening in the wall was closed because the hot, humid air being dumped out was screwing up the servers. The heat source was a strange ductwork experiment which ran a portion of the supply air through the old warehouse’s gas-fired unit heater. But I digress.
There are basically two strategies of design for an attic space: vented and unvented. Vented attics are the traditional solution – the horizontal ceiling surface serves as the thermal, air, and diffusion barrier. The attic space is well-ventilated, and the sloped roofing material becomes the bulk moisture barrier. The four enclosure barriers are liquid moisture, thermal, air, and vapor diffusion retarder. With an unvented attic, all four barriers happen at the sloped roof plane.
In principle, I prefer unvented attics, and modern building science also supports this. Every recessed ceiling light, duct connection, and plumbing penetration is a potential breach in the ceiling thermal and air barrier for a vented attic. Attic vents add hot, humid air into the enclosure in the summer and cold, dry air in the winter. Providing air distribution in the attic is somewhat nonsensical.
In the summer, nice cool air is taken from the air conditioner, dumped into metal boxes (ducts) with lots of joints and minimal insulation, and then sent through the most miserably hot portion of the entire building. In the winter, it reverses to heated air being sent into those same metal boxes through the coldest portion of the building.
An unvented attic is a simpler, cleaner system. Put all the lights, ducts, and other stuff nice and cozy within the sealed building enclosure, and life is good. In his air barrier testing, Phil Emory noticed that many of the connections between wall-to-roof perform better than those between wall-to-ceiling.
For this 25-year-old, pre-engineered building, I recommended removing the ceiling insulation, closing the gable vents, and adding a spray foam air/thermal barrier to the underside of the sloped roof. Exposed foam can be a fire protection issue, so this required coordination.
Then, when we got bad news about the budget, the priority was to correct the egregious issues with air quality. Keeping people from getting sick was (rightfully) determined to be a more important priority than our building science experiment. With no money to do the correct “textbook solution” to the building enclosure, it was time to recommend Band-Aid Plan B.
But that came with so many questions: Do we remove the sagging under-roof insulation and remove the batt insulation above the dropped ceiling, which by this point had gaps and missing pieces, or leave them both in place? What about the open gable vents?
The under-roof batt insulation had been compromised thermally, no robust air barrier was originally designed, and the nature of this system presented thermal bridging at the structural purlins. The batt insulation above the ceiling was no longer continuous after years of maintenance activities, and pieces were missing. If you believe this system could provide a continuous air barrier, maybe consider purchasing swamp land in Florida. Over years of cold winters, squirrels had nested in the batt insulation using the gable vents. Fortunately, in the mixed climate zone of Washington, DC, we did not have to lose sleep over a vapor diffusion retarder.
Our solution was as follows:
- Leave the under-roof insulation in place. Push the sagging pieces back up into place and affix them within the limits of working above an existing suspended ceiling. Considering the metal roof’s underside as a prime condensation spot, the insulation helps it stay above the dew point and act as a reservoir for moisture – holding it until it can evaporate on a sunny day and prevent dripping on the suspended ceiling. Even in its compromised state, this helps mitigate the temperature differential (delta T) between the conditioned interior space and the outside environment.
- Leave most of the batt insulation above the ceiling. Remove those sections fouled by squirrel feces. Add additional insulation in kind and attempt to make it continuous without being obsessive. While an imperfect air barrier, it still performs as a thermal barrier, addressing the delta T between conditioned interior spaces and semi-conditioned attic space.
- Close the gable vents. In the winter, they introduce cold, dry air into the building enclosure; in the summer, they introduce hot, humid air into the building enclosure. Make the closure reversible so that if a future moisture issue develops, the building can be brought back to its existing state. Currently, we are making up for the building enclosure sins by wasting energy in order to avoid moisture problems. Making the solution reversible lets us hedge our bets.
- Visually monitor the situation four times per year, once for each season. Because we are making modifications to the building’s mechanical system, changes to pressure, temperature, relative humidity, air changes, and other wild-card factors will occur. Changing the mechanical system has the potential to change the rules for the building enclosure system. Increased condensation at the metal roof deck’s underside and subsequent water drips could become an issue if moisture-laden attic air falls below the dew point. This may require allowing more heat to escape from the conditioned space below in the winter, or in other words, wasting energy but preventing water damage. Conversely, if there are ice damming issues, the attic space may be too warm and require partially opening the vent seasonally.
Buildings are complex systems. Combine demands for durability, energy efficiency, air quality, and comfort for all occupants with a limited budget to accomplish everything, and your blood pressure will soon reach unsustainable levels. As professionals in the building industry, we are called to help building occupants and owners with our knowledge and skills. This requires us to keep the fundamentals in mind, make logical decisions based on sound building science, and swallow our pride if we are wrong.
