ASHRAE Journal - June 2020 - 78

COLUMN BUILDING SCIENCES

assemblies. Additionally, during high wind events,
vented soffit collapse leads to building pressurization
and window blowout and roof loss due to increased
uplift. Unvented roofs-principally due to the robustness
of their soffit construction-outperform vented roofs
during hurricanes: they are safer.
In coastal areas salt spray and corrosion are a major
concern with steel frames, metal roof trusses, and truss
plate connectors in vented attics. This is not an issue
with unvented attics.
Then why ever build a vented attic? Historically, there
were good reasons. The first and foremost reason was
to control moisture in attic spaces. However, we have
learned how to control moisture in unvented conditioned
attics. In hot, humid climates we have found that attic
venting actually causes moisture problems. Attic venting in hot, humid climates brings in outdoor hot, humid
air into attic spaces, making ductwork sweat and causing
mold growth on roof sheathing and roof framing.
The second reason was for comfort during the summer. Before the introduction of air conditioning and

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FIGURE 5 Controlling Condensation. The key to unvented attic and roof assemblies
is to control condensation or moisture accumulation on the underside of the roof
sheathing. One approach is to raise the temperature of the roof sheathing by insulating on the top of the roof sheathing.

attic insulation, venting an attic reduced the temperature inside of houses. With the advent of high levels of
attic insulation and the introduction of air conditioning,
venting attics no longer saves energy. Today, in modern
vented and insulated attics, the dominant form of heat
transfer is radiation from the underside of the roof deck
to the top of attic ceiling insulation. Ventilation does
not affect this heat exchange. The color of the roofing
and the type of roofing are more important in controlling radiant heat exchange in an attic than attic venting.
Radiant barriers are a common mechanism for directly
addressing radiation transfer.
The third reason was to control ice-damming. And yes,
we were here before ("Dam Ice Dam," ASHRAE Journal,
June 2010 and "De-Icing Ice Dams," ASHRAE Journal,
September 2018).

Controlling Moisture in Unvented Attics

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The key to unvented attic and roof assemblies is to
control condensation or moisture accumulation on the
underside of the roof sheathing. This can be done several ways. One way is to raise the temperature of the roof
sheathing by insulating on the top of the roof sheathing
(Figure 5).
If all of the insulation is on the top of the roof sheathing, it is clear that the temperature of the roof sheathing
is raised sufficiently to control condensation and moisture accumulation. Insulation added on the top of a roof
deck can be vapor impermeable or vapor permeable. All
of the rigid insulation board products work, including

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ASHRAE Journal - June 2020

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