FEMA P55 Coastal Construction Manual, Fourth Edition - Mad Cad

11 Designing the builDing envelope Volume II
Figure 11-27.
Enhanced
underlayment Option 1,
second variation: selfadhering
modifed
bitumen over the felt
is to avoid leakage into the residence if the felt blows of or is torn by wind-borne debris. (Taping the joints is
not included in the frst variation, shown in Figure 11-26, because with the self-adhering modifed bitumen
sheet applied directly to the sheathing, sheet blow-of is unlikely, as is water leakage caused by tearing of the
sheet by debris.)
Te second variation is recommended in moderate and cold climates because it facilitates drying the sheathing
because water vapor escaping from the sheathing can move laterally between the top of the sheathing and
the nailed felt. In the frst variation, because the self-adhering modifed bitumen sheet is adhered to the
sheathing, water vapor is prevented from lateral movement between the sheathing and the underlayment. In
hot climates where the predominate direction of water vapor fow is downward, the sheathing should not be
susceptible to decay unless the house has exceptionally high interior humidity. However, if the frst variation
is used in a moderate or cold climate or if the house has exceptionally high interior humidity, the sheathing
may gain enough moisture over time to facilitate wood decay. 12
Enhanced Underlayment Option 2. Option 2 is the same as the Option 1, second variation, except
that Option 2 does not include the self-adhering modifed bitumen sheet over the felt and uses two
layers of felt. Option 2 costs less than Option 1, but Option 2 is less conservative. Option 2 is illustrated
in Fact Sheet 7.2 in FEMA P-499.
12 Where self-adhering modifed bitumen is applied to the sheathing to provide water leakage protection from ice dams along the eave,
long-term experience in the roofng industry has shown little potential for development of sheathing decay. However, sheathing
decay has occurred when the self-adhering sheet is applied over all of the sheathing in cold climate areas.
11-28 COASTAL CONSTRUCTION MANUAL