ASHRAE Journal - April 2023 - 18

COLUMN BUILDING SCIENCES
FIGURE 1 Vented Roofs. Vented roofs are energy inefficient compared to unvented
roofs. This is particularly the case where ductwork and mechanical systems are
located in vented attics. Ductwork leaks and leads to negative air pressures,
energy costs and part-load moisture problems.
Leaky Air-Handling Unit
And Supply Ducts
Air-Handling
Unit
Supply
Return
Supply
Depressurized Conditioned Space
Inducing Infiltration
Note: Colored shading depicts the building's thermal barrier and pressure boundary. The
thermal barrier and pressure boundary enclose the conditioned space.
FIGURE 2 Unvented Roof. An unvented conditioned roof solves the problem of
duct leakage and negative pressure.
Air-Handling
Unit
Supply
Return
Supply
Note: Colored shading depicts the building's thermal barrier and pressure boundary. The
thermal barrier and pressure boundary enclose the conditioned space.
windward side and negative wind pressures on the leeward
side. This includes sloping roof assemblies.
Ventilated attic spaces typically have soffit ventilation.
Soffit assemblies are not able to withstand high wind
events. When soffit assemblies collapse in high wind
events, wind can enter the attic space on the windward
side, pressurizing the attic space.
When this attic positive air pressure is coupled with
the negative pressure on the exterior surfaces of the
building's leeward side, the air pressure difference
across roof sheathing and roof cladding elements
18
ASHRAE JOURNAL ashrae.o rg
A P R I L 2023
such as shingles, tiles and metal roofing significantly
increases. This significant air pressure increase can lead
to loss of the roof structure. Unvented roof assemblies
resist high wind events more effectively than vented roof
assemblies because there is less risk of soffit collapse.
As noted earlier, vented roofs burn more than
unvented roofs. We were here before ( " Wildfire, "
ASHRAE Journal, August 2021). With respect to
wildfires, the issue is burning embers. Roofs are
particularly prone to burn ember exposure-from
the perspective of the roof covering and from the
perspective of roof venting.
Entry of burning embers is a concern in urban regions
with higher risk of wildfire occurrence. Homes built in
regions designated as Wildland Urban Interface Zones
are at a high risk for fire exposure. Wildfires burn with
speed and intensity that often lead to a high percentage
of homes lost in these communities. However, the ignition
of homes is typically not caused directly by flames
but rather by ignited embers becoming airborne and
entering vented roof assemblies. Obviously, unvented
roofs do not have this issue.
Also, as noted earlier, vented roofs are energy inefficient
compared to unvented roofs. This is particularly
the case where ductwork and mechanical systems are
located in vented attics. Ductwork leaks and leads to
negative air pressures, energy costs and part-load moisture
problems (Figure 1). An unvented conditioned roof
solves this problem (Figure 2).
For decades, one of the arguments for venting roof
assemblies is to " prevent shingles from being cooked. "
This has been discounted in many papers. A good piece
of work was done by Rudd1 who found that " the summertime
average daily temperature of roofing materials
is nearly unchanged whether vented or unvented, while
short-term peak temperature increases are not more
than 7 degrees F. "
It is also obvious that the color of the roofing layer
is more significant
regarding temperature
than venting or
not venting.
Now to the " except "
part. The only time
you should vent a
roof is to control icedamming
(Photo 6).
PHOTO 6 Classic Ice Dam. Can be prevented
and controlled with ventilation.
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ASHRAE Journal - April 2023

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