Sealing the Envelope
- By Michael Fickes
- December 1st, 2009
The epidemic of mold that has struck K-12 schools in recent years has led architects, building contractors and maintenance technicians to pay more attention to school building envelopes — to keep out the bad and keep in the good.
The Collaboration for High Performance Schools
(CHPS) has developed a series of manuals covering best practices in the design, construction and maintenance of high performance schools. Volume II: Design and Volume IV: Maintenance and Operations cover building envelopes.
Overall, the design manual focuses on traditional issues such as the envelope’s color, insulation, thermal mass and acoustical properties and how those characteristics affect the comfort of occupants.
The manual also discusses newer, high performance concepts such as cool roofs, a term that describes how a roofing surface can limit heat transfer into a structure.
While the overall performance of the envelope contributes to good indoor air quality (IAQ), major chapters of the CHPS design guide detail techniques used to control the infiltration of outside air and moisture through the envelope and into the building. These are the key techniques in the fight to produce good IAQ and prevent mold.
Preventing Air Infiltration
Air infiltration destroys the efficiency of heating, ventilating and air conditioning (HVAC) systems, increases wear and tear on the HVAC system and boosts the cost of utilities.
Anti-infiltration techniques include the design of the connections between the building envelope, windows and doors.
Another technique is to install air barriers. The manual describes three barriers: combined air and vapor barriers, which include asphalt membranes and spray-on urethane foams; vapor permeable air barriers that allow water vapor to pass through; and multi-component systems, which combine air barrier materials.
Preventing Moisture Infiltration
If the infiltrating air carries moisture, it can spawn mold growth. Other causes of mold include leaks that allow rain and snow to get through the envelope and condensation of water vapor on interior surfaces. Moisture control techniques include vapor barriers and vapor retarders. The first blocks moisture out, and the second simply slows the vapor down without preventing its migration.
Where would you use a vapor retarder? According to the CHPS design manual, schools built in mild California climates use retarders to limit the migration of water vapor to cooler parts of a structure where the moisture will condense out of the air onto building surfaces and perhaps initiate the growth of mold.
Even the best-designed and built envelope cannot resist air and moisture infiltration without regular inspections and maintenance for systems before they fail.
Exterior and interior walls require regular inspection, especially the areas around the windows, foundation and roofing system. Inspectors should aim to ferret out air and moisture leaks and arrange for immediate maintenance.
Windows and doorframes will need to be re-caulked on a regular schedule. Exterior wood will need re-painted and re-stained to preserve sealing capacity.
In the end, of course, just as the best-designed envelope cannot do its job without maintenance, the best maintenance program cannot repair a poorly designed and built system. As always, it takes good design, good construction and good maintenance to do the job right.