Special Maintenance Considerations - Effective Building Maintenance: Protection of Capital Assets - page 127

months, an internal pressure inallareasofthebuilding of at least 0.009 psi
(0.25” wg) would be required.
Because of the vagaries of construction, it is impossible to compute
the exact amount of outdoor air that must be introduced to maintain ex-
actly 0.25” wg positive pressure in the building. Thus, themostcommon
approachusedbydesignersistodeterminetheexhaustrequirementineach
areaandthenintroduceenoughoutdoorairinthatareatooffsettheexhaust,
plusanadditional10-20%asa“safetyfactor.” At all times, when an exte-
rior door is opened, air should exfiltrate (exit) from the building .
For indoor air in the summer to be at an acceptable 50%RH or less,
the HVAC systems must supply their air at a 55°F or lower dewpoint
temperature.
Finally, most buildings are not occupied around the clock. There-
fore, the building owner must address unoccupied periods, ranging
from overnight to weekends to summer vacations, to maintain low hu-
midity level and help control mold growth. Care must be taken to en-
sure that all building exhaust fans are shut down and all outdoor air
dampers are closed during unoccupied periods.
The recommended approach to preventing high humidity from oc-
curring during these periods is to establish both high limit temperature
and high limit humidity space setpoints, monitored by space tempera-
ture and humidity sensors, that will cause the HVAC controls to cycle
the HVAC systems on to maintain these setpoints. Ideally, this would be
done for each air-handler. But, because central chiller and boiler plants
may have a minimum load requirement, care must be taken by the de-
signer to ensure that enough systems are cycled on to provide this mini-
mum load—or there is no benefit to the unoccupied period control.
Since there are no people in the building during these unoccupied
periods, ventilation air is not required and the outdoor air dampers
should be closed (which eliminates the majority of the moisture load
imposed on the building).
In summary, Table 4-12 represents a “checklist” that can be used to
define the maintenance requirements to control mold in facilities.
DEALING WITH FAILING MASONRY WALLS
Brick,concrete,andstonemasonrycavitywallsallowgreaterwaterintru-
sionandhavegreaterresultingmoldproblemsthanothertypesofexteriorinish.