Pushing the Envelope
- By Michael Fickes
- March 1st, 2010
Not many years ago, an epidemic of mold rampaged through K-12 school buildings across the country, rocketing mold prevention to the top of the priority list for the design of school building envelopes. Today, however, mold concerns have receded, and the primary building-envelope design preoccupation since the mid-1970s has re-emerged — energy efficiency.
“We haven’t conquered mold, but we have learned how to minimize opportunities for it to grow and thrive,” says Rodney D. Wiford, AIA, REFP, LEED-AP, a project manager with Fanning Howey, a Celina, Ohio-based architectural firm whose specialties include K-12 school building design. “It’s still out there as a concern, but it is becoming less of a problem, especially in masonry buildings — a direction most new schools are taking.
“Without minimizing the importance of mold prevention, the contemporary concern is energy efficiency.”
Given the problems of rising energy costs and global warming, energy efficiency as both a cost control measure and sustainable building technique carries particular import in the built environment today.
Statistics show that buildings that house schools, offices, retail stores, restaurants, factories and homes use 70 percent of the electricity generated in the U.S. Within the commercial and institutional real estate sectors, office buildings use the most electricity and school buildings rank second.
All told, the cost of all of the energy used in the nation’s 124,000 public and private school buildings totals a whopping $12 billion per year, according to an estimate in the Advanced Energy Design Guide (AEDG) for K-12 School Buildings issued late last year by the Department of Energy (DOE), the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) and other national groups interested in the built environment.
The Design Guide, which can be downloaded at www.ashrae.org/publications/page/1604, contains plans, tailored to geographical regions across the U.S., that the authors believe will cut energy use in new and renovated K-12 schools by 30 percent. If that estimate holds true for implementations of the Design Guide’s plans, 30 percent of a school building’s energy costs would flow back into programs and facilities.
The Design Guide characterizes the building envelope as a key design element that can produce significant energy savings if properly designed and constructed. The envelope includes an outside skin, layers of insulation, vapor barriers and interior walls.
While the Design Guide covers all building systems, it looks at the building envelope in great detail. It recommends cool roofs, insulation values for floor slabs, walls and roofs, kinds of doors and glass area (no more than 35 percent of the total wall area). It also recommends ways to minimize thermal breaks in the envelope as well as the effects of thermal breaks.
All in all, effective envelope design aims to keep hot, cold or wet weather out and conditioned hot or cool air in. “You want to minimize the transfer of the wrong energy in or out, depending on the season,” continues Wiford.
Insulating Foam to Seal the Envelope
The outer skin of a K-12 school, today, commonly includes masonry walls with doors, windows and vents and a roof composed of a membrane penetrated by vents, fans and heating, ventilating and air-conditioning (HVAC) equipment.
In addition, the skin uses materials — like caulk — to seal the spaces around doors, windows, vents, HVAC equipment and other building components that penetrate the building envelope. Finally, the skin includes intersections or seams that knit together structural components such as the roof and walls.
“Seams and skin penetrations can allow outside cold, heat and moisture to leak in, causing mold and wasting energy, ” Wiford says. “Caulking and other sealants that go on the outside skin don’t stop all leaks that might get through the envelope. Today, however, there are new foam insulation products that can help.”
Wiford recommends applying foam insulation to fill voids that develop underneath the caulking typically used to seal seams. “The problem is under the caulk,” he says. “Foam works there and prevents energy leakage.
“Traditionally, we have relied on insulation stuffed into voids, but insulation doesn’t form a continuous barrier. Foam fills the voids completely.”
The foam is a spray on product that fills voids, expands a bit and then hardens. It doesn’t get rock hard, but it does form an insulating barrier more solid and substantial than loose insulation stuffed into the walls.
The walls are easy to insulate, says Wiford, who also cautions against overdoing it. Insulation has an optimum thickness depending on geographical location. Exceeding the optimum thickness raises cost but not insulating effectiveness.
Wiford also notes that the roof-wall intersections, doors, windows, vents and exhaust fans that penetrate the envelope from outside to inside cause most energy-loss problems in buildings. The foam has proven effective in sealing and insulating these openings.
Most of the components of walls and roofs, including insulation and foam, aim to prevent costly leaks of cooling and heating energy. Some components must also work to prevent mold-producing moisture from getting through the outer skin into the inner wall or building interior.
In masonry buildings, exterior brick materials absorb moisture and actually pass it through to the cavity between the exterior and interior walls. “We design the exterior wall to give this moisture a way to flow back outside of the building,” Wiford says. “It’s a weeping system that condenses the incoming moisture, allows it to run down the brick to the bottom of the wall, where it flows back outside.”
That takes care of a lot of moisture, but some amount of moisture still gets through masonry walls — as well as exterior walls made of other materials — and can’t get back out. Moisture barriers prevent that moisture from getting onto surfaces where it can lead to the growth of mold or cause other problems inside the wall.
Traditionally, vinyl sheeting has formed moisture and vapor barriers.
Recently, however, a new product came to market. It’s a black spray-on material that resembles paint after it dries.
Proving the Envelope, Pushing On
After a building has been constructed, the building team can run tests to prove that it is energy and moisture tight. “We run thermal imaging scans of the building, find air leaks and repair them,” says Wiford. “This is a new capability. In the past, we relied on doing the work correctly. Today, it is possible to go back and test for problems right at the start.”
Some energy auditors favor the infrared thermal imaging cameras alone. Others use the imaging technology in conjunction with a blower that pushes air into a closed building. Air leaks show up as black streaks on the image.
Air leaks usually indicate some problem with the insulation, which today can be repaired with foam.
On the principle that wet insulation conducts heat faster than dry insulation, the same technology can identify roof leaks that need repair and even more subtle moisture leaks that might eventually cause mold.
In the end, measures taken to design a tightly sealed building envelope will cut heating and cooling costs substantially while effectively managing the threat posed by mold.
Once that job is done, the Department of Energy’s Advanced Energy Design Guide for K-12 schools can help to squeeze more energy waste and more savings out of other key building systems, including lighting, HVAC and the service water heating system.
Remember, taken together, the design guide recommendations promise to wring 30 percent out of the energy costs paid by a typical K-12 school building.