Attic Ventilation Guidelines to Minimize Icings at Eaves

Attic Ventilation Guidelines to Minimize Icings at Eaves

By Wayne Tobiasson, James Buska and Alan Greatorex
ABSTRACT
determine how their attic temperature influenced icing. We
observed that problematic icings developed very slowly, il
at all, when the outside temperature was above 22T Such
icings can be avoided by sizing natural, and if necessary,
mechanical attic ventilation systems to maintain an attic
temperature of 30°F when the outside temperature is 22T.
N COLD REGIONS, ICICLES AND ICE DAMS
may develop on roofs that slope to cold eaves.
Ventilating the space below the snow-covered roof
_ with outdoor air to create a “cold” ventilated roof is
often an effective way to avoid such problems. Several
buildings in northern New York were instrumented to
Introduction
Icicles and ice dams form at the eaves of some roofs in
cold regions. Water that ponds behind ice dams may leak
into the building since most steep roofs are configured to
shed water, not hold back standing water.
Figure t shows two roofs located near Watertown, NY. The
two photos of identically constructed buildings were taken
within minutes of each other. One roof contains large ice
dams and icicles, but the other is ice free. Why? The snow
on top of the chimney of one roof is the clue to the differ¬
ence in behavior. That building was not being heated, while
the other building was at room temperature.
Figure 1 Two identically-constructed roofs photographed at the same
time The building on the right, with no icings, was unheated
This example is used to illustrate that building heat, not
the sun, is the primary cause of ice dams and icicles on roofs.
When the sun melts snow on roofs, it also warms the eaves,
and this tends to minimize the growth of icicles Certainly,
some icicles can form on unheated buildings and from solar
heating, but they are usually small, infrequent, and do not
cause chronic problems.
Pioneering work on ice dams 1, done in 1976, concluded
that a combination of insulation, ventilation and correct
house design is needed to reduce ice dam formation More
recent studies 2’4 also promote use of cold ventilated roofing
systems to reduce icings at eaves. These studies also indicate
that icings can be reduced by increasing the slope of the
roof, by making the surface slippery so that snow slides off,
by not installing gutters, and by reducing the overhang at
the eaves However, on roofs without gutters, too small an
overhang can cause wetting of the walls below or formation
of icings on them A 12-in overhang is often a good com¬
promise in cold regions Also, allowing snow to slide off
roofs can create hazards 2 Snow guards may be needed to
hold snow on slippery roofs 5
Problems in Upstate New York
A few years ago, many buildings were built at Fort Drum
near Watertown, NY All these buildings have standing seam
metal roofing systems above ventilated attics. Standing seam
metal roofing systems have both strengths and weaknesses
when used in cold regions6. Some of these roofs have
remained clear of icicles and ice dams (Figure 2 ) Several have
experienced some problematic icicles and ice dams (Figure 3)
January 1998 Interface • 17
and others have experienced severe icicles and ice damming
(Figure 4). The range in performance is related to the ability
of each building’s attic ventilation system to remove heat
that enters the attic from the warm building below and heat
produced by HVAC equipment located in the attic.
Figure 2. Some creep of snotv and cornicing is evident, but no icings
occurred on this roof.
Figure 3. Minor icings occurred along the eaves of this roof and problem¬
atic icings developed at the base of valleys.
Figure 1. Severe, problematic icings developed all along the eaves of this
dining facility.
We have developed recommendations for solving these
specific problems and have attempted to better understand
how and when icicles and ice dams form. During our first
winter of study (1990-’91), four buildings were monitored to
study a range of icing problems from “some” to severe. By
“some,” we mean that minor icings have occurred along the
eaves and large, problematic icings have developed at some
locations such as the base of valleys, as shown in Figure 3. A
nearby building not experiencing icing problems was also
monitored as a control. This paper describes those findings
and uses them to develop ventilation guidelines to minimize
icings at roof eaves. We also monitored these buildings after
they received attic ventilation improvements. Those readings
verified that our guidelines are effective.
Initial Measurements
Outside air temperature was measured in a small weather
shelter in the vicinity of these buildings. Attic air tempera¬
ture was measured near the middle of each attic. All temper¬
atures were measured with thermistors. Temperature mea¬
surements were taken once an hour from Nov. 1, 1990 to
April 10, 1991. Battery-operated data collection systems
stored the data between our periodic visits to Fort Drum. An
engineer at Fort Drum periodically photographed these
buildings for us.
Findings
Observations of these buildings and others indicated that
problematic icings seldom grew when the outside temperature
was above 22°F.
Plots of attic air temperature vs. outside air temperature are
presented in Figures 5, 6, and 7 for buildings experiencing no
icing problems, “some” icing problems and severe icing prob¬
lems, respectively. Figure 6 is representative of two other
buildings that also experienced “some” icing problems. The
least-squares equation of best fit and its correlation coeffi¬
cient (r2) are presented on each figure. Horizontal and verti¬
cal lines, representing, respectively, an attic air temperature
of 30°F and an outside air temperature of 22°F are also pre¬
sented on each figure. The portion of each graph to the
right of the vertical 22T line is warmer than conditions
observed to create icings. The portion of each graph below
the 30°F horizontal line is also not within the “icing enve¬
lope” because the attic is then so cold that snow on the roof
is not melted by building heat.
We chose 30°F for the horizontal line instead of 32“F since
we expect that there were places in the attics that were
somewhat warmer than the places where our thermistors
were located.
Of the four quadrants created in Figures 5-7 by the 22°F ver¬
tical line and the 30”F horizontal line, the upper left quad¬
rant defines the problem area (i.e., the “icing envelope”).
For the roof with no icing problems (Figure 5), very few data
points fall within the icing envelope, as expected. For the roof
with “some” icing problems (Figure 6), about 6% of the obser¬
vations fall within the “icing envelope.” For the roof with
18 • Interface January 1998
Figure 5: Attic temperature vs. outside air temperature for a roof experi¬
encing no icing problems.
Figure 6 Attic temperature vs outside air temperature for a roof experi¬
encing “some” icing problems
Figure 7.- Attic temperature vs. outside air temperature for a roof experi¬
encing severe icing problems.
Figure 8. Lines of best fit for the three roofs shown in Figures 5-7, along
with similar lines (dashed) for two other roofs also having “some” icing
problems.
severe icing problems (Figure 7), 23% of the data (i.e., 23% of
the time during the winter) falls within the “icing envelope.’’
The separate line of points in Figure 7 that runs down
toward the lower left corner of the graph represents a 5-day
period when the heating system of that building was off due
to mechanical problems. Those points provide further evi¬
dence that building heat is the primary source of icing prob¬
lems since, once cool, that building performed out of the
“icing envelope.”
The lines of best fit for these three roofs are shown
together as solid lines on Figure 8 along with similar lines,
(shown dashed), for the other buildings we monitored that
also had “some” icing problems. This information suggests
that icings can be avoided by sizing attic ventilation systems
to maintain an attic temperature of 30’F when the outside
temperature is 22T.
Calculations
With knowledge of the thermal resistance of the ceiling
and the indoor and attic temperatures, the conductive heat
losses from a heated building into its attic can be deter¬
mined To this can be added any heat introduced to the attic
by HVAC equipment and any ducting located there. If the
assumption is made that during the design condition, the
roof is covered with an insulating blanket of snow that
reduces conductive heat losses from the attic to near zero,
then all the heat in the attic must be removed by ventilating air
The following equation applies?:
Q = 51 4H/(ta-to)
where Q = airflow rate required to remove heat (cfm), H =
heat to be removed (BTU/min), ta = attic temperature (’F)
January 1998 Interface • 21
and to = outside temperature (°F). This equation uses a spe¬
cific heat of 0.24 BTU/lb °F and a density of 0.081 lb/ft 8 for
30°F air.
When attic air and outside air temperatures of 30°F and
22°F respectively are used, the above equation reduces to:
Q = 6.43H
If this airflow is to be provided by natural stack effect with
cold air entering the attic all along its eaves and exhausting
all along its ridge, the flow rate created when the attic has
nearly equal intake and exhaust openings7 is as follows:
Q = 221.3A[Ab(ta-to)/(ta+460)]0-5
where Q = stack-induced flow (cfm), A = free area of inlet
openings (ft 2), Ab = height difference between inlet and
exhaust openings (ft), ta = attic temperature (°F) and t0 =
outside temperature (°F). This equation uses a discharge
coefficient for the openings of 0.65, and a gravitational con¬
stant of 32.2 ft/s 2. If the inlet and outlet areas are not about
equal, a correction must be applied. 7
When attic air and outside air temperatures of 30°F and
22°F respectively are used, the last equation reduces to:
Q = 28. 3A Ab 0 5
To determine the free area of inlets needed to cool an attic
enough by natural, stack-induced ventilation, the second and
fourth equations are equated. Then,
A = 0.227H/Ab0-5
The coefficients in the above equations changed slightly
from those in previous versions of this paper. 8 The free area
of inlet openings (A) is about 8 percent less since this report
uses a density of 0.081 lb/ft 8 for 30°F air instead of the stan¬
dard air density of 0.075 lb/ft 8 that we used in our previous
reports. Note that the free area of inlet openings (A) is in
square feet Multiply by 144 to get it in square inches. If the
required inlet and outlet areas can be provided so as to ven¬
tilate the entire attic, natural ventilation will suffice to keep
the attic cool enough to prevent icings. If the required inlet
and outlet areas cannot be provided, mechanical ventilation
will also be needed.
A design firm used our calculations to develop recommen¬
dations for attic ventilation improvements for several build¬
ings at Fort Drum. Those improvements were made in 1993
on four of the buildings we had been studying. We contin¬
ued to monitor these buildings to determine the effect of the
modifications.
The attic described by Figure 7 had experienced severe
icing problems. However, it needed help in the form of
improved natural ventilation or mechanical ventilation for
less than 23% of the winter. We were not able to provide
enough inlet area to completely solve this attic’s icing prob¬
lems using only natural ventilation. Thus, several large fans
were installed near the ridge as shown in Figure 9. The fans
were not dampered. This allows the fan openings to serve as
outlets for natural ventilation, thereby reducing the amount
of time that mechanical ventilation is needed.
The fans are thermostatically controlled, since they are
needed infrequently. They operate only when the attic tem¬
perature is above 30°F and the outside temperature is below
22’F. We installed instrumentation to monitor when the fans
are used. During the winter of 1995-’96, these fans were used
only 20% of the time.
Figure to compares the modified building to a similar
unmodified building for the period Nov. 15, 1993 to Feb.
23, 1994. Both buildings were having similar severe icing
problems before one was modified. The portion of each data
set to the right of the 22°F outside air temperature line in
Figure to relates to natural ventilation since the fans cannot
operate when it is warmer than 22°F outside. The dramatic
difference in that portion of the two data sets indicates that
natural ventilation has been improved significantly. We
expect that much of this improvement would not have been
achieved if the fans contained louvers that were opened only
when the fans were on.
The “hunk” taken out of the data set for the building with
improved attic ventilation reflects the contribution of the
mechanical ventilation system. The mechanical system has
been able to keep that attic out of the “icing envelope” most
of the time. Without mechanical ventilation, it appears that
the attic would have operated within the icing envelope for
a significant amount of time with problematic icings expect¬
ed. This verified our feeling that natural ventilation alone
would not solve the icing problems being experienced by
some of these buildings.
Figure t1 shows the two buildings just discussed on the
same date (Jan. 12, 1994). The unmodified building is sub¬
jected to severe icings all along its eaves. There are only a
few small icicles at the base of the valleys of the building
with improved attic ventilation. All other irregularities along
the eaves of that roof are snow cornices, not icicles.
Seven large fans were installed to mechanically ventilate
this attic. Each one consumes about one kilowatt of power.
Using the calculations discussed in this paper, four such fans
would be enough to do the job but the designers used seven
fans. To determine if only four fans would suffice, on Feb.
23, 1994, we had three of the fans turned off and blocked
with sheet metal to preclude both mechanical and natural
ventilation through them.
Figure 9: Attic ventilation fan installed near ridge of modified building.
22 ■ Interface January 1998
The natural ventilation portion of the data for openings
provided by four fans did not change noticeably from that
when openings were provided by seven fans. When mechan¬
ical ventilation was needed, the four fans kept the attic out
of the “icing envelope” almost as well as the seven fans did.
No large icings formed on the modified building with only
four of the seven fans working. These findings convinced us
that the design approach presented in this paper can be used
to size natural and mechanical ventilation systems for solv¬
ing icing problems. We ultimately recommended using five
fans on this facility in order to have some redundancy in
case of operational problems with any of the fans.
Using the test results from the four buildings modified in
1993, we worked with the Fort Drum Directorate of
Engineering and Housing to design attic ventilation improve¬
ments for the remaining 53 buildings. Attic ventilation modi¬
fications were completed on all those buildings during the
summer of 1995. The winter of 1995-96 provided plenty of
snow and cold weather to adequately test the modifications.
There were no reports of problematic ice dams or icings
on any of the modified buildings. By changing these roofs
from hot, poorly-ventilated systems to cold, well-ventilated
systems, the meltwater that formed problematic icings at the
eaves was not generated in cold weather.
Summary
We determined appropriate design temperatures of attic
ventilation systems to minimize icing problems by monitor¬
ing several buildings in northern New York, all but one of
which were experiencing icing problems.
Problematic icings appear to develop very slowly, if at all,
when the outside temperature is above 22°F. We feel that,
owing to variations in temperature within an attic, design
should be based on an attic temperature of 30°F.
Thus we recommend that, to eliminate icing problems,
attic ventilation systems be sized to maintain an attic tem¬
perature of 30°F when the outside temperature is 22°F.
Fifty-seven buildings experiencing icings were modified
using these guidelines to improve attic ventilation. Severe
icings did not form on them after they were modified.
Instrumentation installed to monitor their performance has
Figure tO: Attic temperature vs. outside air temperature for a building
with severe icing problems and a similar building with improved attic ven¬
tilation experiencing no icing problems.
validated our design approach.
Properly designed attic ventilation systems that create cold
ventilated roofs avoid the many problems associated with ice
dams and icicles along roof eaves.
Our research was conducted on relatively large buildings.
Other work we have done using the same design approach
indicates that icing problems on most smaller residential
buildings can be solved by providing or improving on natur¬
al ventilation (mechanical ventilation is usually not needed).
In homes it is important to ensure that the natural ventila¬
tion provided to cool the roof is not somehow blocked.
Also, heating and ventilating ducts that pass through the
attic should be well sealed and insulated and the heat they
add to the attic should be considered when sizing the venti¬
lation system. Finally, good insulation and continuous air
barriers between the living space and the attic are essential
so as to minimize the passage of heat and warm air into the
attic. In cold regions, vapor retarders are often necessary to
reduce moisture migration. The ventilation provided to mini¬
mize icings also serves as a second line of defense against
accumulation of moisture in attics.
Figure 11: Both buildings from Figure to photographed at the same time. The unmodified building shown at the left was experiencing severe icings. The
other building with improved attic ventilation bad no icing problems.
January 1998 Interface • 23
Acknowledgements
This work was funded under DA Project 4A762784AT42,
Installation Management in Cold Regions, Task BS, Work
Unit 019, Deterioration Resistant Building Technology for
Cold Regions. The authors thank Gary Dahl, Chief of
Buildings and Structures, Directorate of Engineering and
Housing, Fort Drum, NY, for his support and assistance on
this project.
This is an updated version of the paper, “Ventilating Attics to
Minimize Icings at Eaves,” that appeared in Issue 21 of Energy and
Buildings, published in 1994 by Elsevier Science S.A.
s
1 Grange, H. L. and Hendricks, L. T., “Roof-snow Behavior
and Ice Dam Prevention in Residential Housing,” Bulletin
399, Agricultural Extension Service, University of
Minnesota, St. Paul, MN, 1976.
2. Tobiasson, W., “Roof Design in Cold Regions,” Proceedings
of the First International Conference on Snow Engineering, CRREL,
Hanover, NH, 1989, Special Report 89-6, pp. 462-472.
3 Mackinlay, I., “Architectural Design in Regions of Snow
and Cold,” Proceedings of the First International Conference on
Snotv Engineering, CRREL, Hanover, NH, 1989, Special
Report. 89-6, pp. 441-455.
4. De’Marne, H., “Field Experience in Control and
Prevention of Leaking from Ice Dams in Northern New
England,” Proceedings of the First International Conference on Snotv
Engineering, CRREL, Hanover, NH, 1989, Special Report.
89-6, pp. 473-482.
5. Tobiasson, W., Buska, J., and Greatorex, A., “Snow
Guards for Metal Roofs,” Interface, January 1997, pp. 12-
19.
6. Tobiasson, W., and Buska, J., “Standing Seam Metal
Roofing Systems in Cold Regions,” Proceedings loth
Conference on Roofing Technology, Rosemont, IL, 1993,
National Roofing Contractors Assoc., pp. 34-44. Also
available as CRREL Mise. Paper 3233.
7. ASHRAE Handbook: Fundamentals, IP Edition, American
Society of Heating, Refrigerating and Air Conditioning
Engineers, Inc., Atlanta, GA, 1989, p. 23.8.
8. Tobiasson, W., Buska, J., and Greatorex, A., “Ventilating
Attics to Minimize Icings at Eaves,” Proceedings of the Cold
Climate HVAC ’94 Conference, Rovaniemi, Finland, March
1994. This report was updated and published in 1994 by
Elsevier Science, S.A. in Issue 21 of Energy and Buildings
and a condensed version of that paper in I-P units was
also published in the March/April 1995 issue of Home
Energy magazine.
Wayne Tobiasson James Buska Alan Greatorex
About The Authors
the Roofing Industry Educational Institute (RIEI), and is an
honorary member of RCI.
James Buska is a Research Civil Engineer with CRREL.
He has been conducting applied research on cold regions roofing
technology for the last 13 of his 21 years tvith CRREL. Buska
has B.S. and M.S. Degrees in Civil Engineering from Montana
State University. He is a member of ASCE.
Alan Greatorex is a Civil Engineering Technician with CRREL.
He has tvorked on building technology for the past 24 years, with
much of that focusing on moisture in roofs and snotv load design crite¬
ria. Greatorex has an associate degree in Architecture and Building
Technology from Vermont Technical College.
Wayne Tobiasson, now retired, was a Research Civil Engineer
with the U.S Army Corps of Engineers’ Cold Regions Research and
Engineering Laboratory (CRREL) in Hanover, NH. He has a B S. in
Civil Engineering from Northeastern University and a Masters of
Engineering from Dartmouth College. He is a member of the American
Society of Civil Engineers (ASCE), has been a member of the faculty of
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