The New Ice Age - A Practitioner`s Perspective on Ice Damming

Transcription

The New Ice Age - A Practitioner's Perspective on
Ice Damming Across Canada
W. Allen Lyte, B.Tech., A.Sc.T., RRO
Chris Love, B.Tech., RRC
ABSTRACT
There is significant property damage to buildings in Canada as a result of ice damming. Ice dams
form through the repetitive melting and freezing of roof top snow that can cause ice to build up at
the eaves. The accumulation of ice can prevent the passage of melt water drainage and cause it to
back-up below the roofing materials. The resulting leakage may only be evident in the form of a
water stain; however, the deterioration can affect concealed components ranging from the roof
deck to wall assemblies. Safety can become a concern if ice builds up beyond the building’s
structural capacity, or if ice starts to fall.
Proper design can help avoid ice dams and/or limit their affect. The primary design features
required to avoid ice dams include a sufficient thermal barrier and air barrier to prevent heat loss
from the building reaching the underside of the roof deck. Adequate ventilation is required to
help replace any heated air under the roof deck with exterior air to maintain as cool an
environment as possible to minimize snow melt. Finally, if ice damming still occurs a waterproof
barrier, called “eave protection” or “ice dam protection”, is installed under the primary roofing
material to direct water back out of the roof assembly.
Even when the above are included in the design, ice damming can still occur for two general
reasons. One, poor design/construction techniques resulting in heat loss, and/or insufficient
ventilation to remove the heated air from under the roof deck. Two, while the design/construction
may meet local code requirements, it does not address local climatic factors that tend to promote
ice damming. Case studies are used to review the affect of the climate with buildings built to
code and also using similar design/construction techniques. Readily available test procedures to
identify ice damming causes are discussed. Case studies referenced illustrate common causes of
ice dams and options to manage these specific cases.
1. INTRODUCTION
This paper reviews strategies used to deal with ice damming for specific Canadian regions with
the use of case studies. The affect of code minimums, appropriate design/construction and
weather variances are the key factors that will affect these strategies. The affects of weather with
ice dams will be Ice damming is defined by Canada Mortgage and Housing Corporation’s
document Attic Venting, Attic Moisture and Ice Dams as, “Ice dams are the large mass of ice
that collects on the lower edge of the roof or in the gutters. As more melting snow (or rain) runs
down the roof, it meets this mass of ice and backs up, sometimes under the shingles and into the
attic or the house.” [CMHC].
The attached bibliography provides sources that summarize ice damming. [Baker 1967] is one of
the earlier recognized documents explaining the basics of ice damming. [Straube 2006] updates
the various causes of ice damming and appropriate actions to reduce the ice damming.
11th Canadian Conference on Building Science and Technology
Banff, Alberta, 2007
Attempts to rectify ice damming issues often focus on issues with the building construction and
not the specific weather the building experiences. Weather data should be reviewed to help
identify regions which may be more susceptible to the formation of ice dams. Areas with more
snow that has the potential to melt will likely form ice dams.
Better understanding the principles of snow
and ice allows for a better understanding on
how snow forms, collects and changes its
structure from fluffy white snow to hard dark
ice. The bibliography includes several
sources for further information on snow
including [Armstrong, Terence, Roberts,
Brian, and Swithinbank, Charles 1973] and
[Gray and Male 1981]. These sources go
into depth on the types of snow, including
their crystalline structures, and how this in
combination with fluctuations with weather
will affect snows accumulation and densities.
Key principles discussed include, friction
and threshold shear velocity, which helps
explain how and when snow will slide from
the rooftops, and the melt rate of snow,
Sketch 1 - Typical ice damming formation [CMHC]
which is the key component of ice dams.
The characteristics of snow in its natural state can be correlated to better understand ice damming
problems. The avalanche characteristics of snow are discussed as they relate to roofs in [Taylor
1983]. Snow and ice trajectory is critical to avoid damages similar to Case Study No. 1.
Code minimums for insulation and ventilation were set to address other issues other than ice dams
and they do not address specific climatic regions. The minimum amount of eave protection
coverage specifically meant limit damages from ice dams treats broad climatic regions the same.
The eave protection minimums also don’t account for the roof configurations can collect snow
and form larger ice dams. “Cookie cutter” designs have a greater chance of ice dams developing
when they are designed for one region successfully and then built in different climatic regions.
2. ASSESSING THE POTENTIAL FOR ICE DAMMING
2.1 Construction Checks to Prevent Ice Damming
Prior to commissioning a building, a series of simple tests can be completed to check for
deficiencies that may contribute to ice damming. As a starting point this will determine if the
building was built as designed and to code. First a visual review should be completed of the air
barrier. Ideally this is completed prior to the installation of the interior finishes and attic
insulation, to allow access for repairs, if required. Smoke pencil testing can be utilized to confirm
defects in the air barrier. Once the attic space is insulated, attic temperatures should be checked
to make sure it isn't too warm. The attic ventilation provided should be confirmed. There is no
definitive temperature that the attic should be; however, the closer to the exterior air temperature
the better. As a rule of thumb the attic should be similar in temperature to an unheated garage.
Due to the difficulty accessing attic spaces, it is often not feasible to review the entire attic.
Infrared imaging can identify small and large heat sources that could be missed otherwise. Often
defects can be easily sealed or insulated once identified. If possible, the defects should be
corrected as they are identified and the adequacy of the repair confirmed with the infrared
camera. The lower costs and availability of infrared cameras can allow the installer to check their
own work.
2.2 Attic Temperature Diagnosis
If the attic is determined to be too warm it may be difficult to identify the heat source. Tracking
the temperature swings over time relative to interior and exterior temperature will help identify
the problem. The following Case Study No. 2 illustrates how this data can be used with weather
data to help identify the reasons for heat gain into the attic causing the ice dams.
Attic temperatures that rise and fall with interior temperatures are indicative of heat loss into the
attic and/or inadequate ventilation. An attic temperature that approaches or fluctuates with, the
interior temperature would indicate a severe heat loss problem. If the attic temperature does not
drop when the exterior temperature lowers, then this would indicate insufficient ventilation, as
was the situation with Case Study No. 2 after snow covered the roof vents and reduced the
ventilation.
Snow fall can affect attic temperatures by insulating the roof deck, preventing heat loss through
conduction. The effective insulation value of the snow will vary depending on its depth and
density. Regions with excessive snow with low densities will have higher insulating values.
More importantly a heavy blanket of snow can reduce the attic ventilation if the standard ridge or
mushroom vents are covered. This problem can be identified if the exterior and attic air
temperatures stop swinging together after a heavy snowfall and the attic temperature starts to
warm up. Snow loads can be identified in the National Building Code by city. The mean snow
depth is graphically represented with maps of the region (see Figure 1 illustrates the mean snow
depth for the month of February). The first component required to form an ice dam is snow. A
quick glance of the following map will identify areas of snow accumulation. These areas include
mountainous locations, costal regions and inland areas subject to lake affect snow for example.
Figure 1 - February mean snow depth (cm) [Brown et al 2003]
2.3 External Attic Heat Source
Solar heat gain is generally a factor when attic temperatures rise higher than interior or the
exterior temperatures on sunny days. The building orientation to the sun, prevailing winds
distributing roof top snow, reflectivity of the roofing material and regional location can increase
the affect of the solar heat gain of the attic space. The greater the mean solar radiation for the
specific region, the greater the chance of increased attic heating. It is important to note that solar
radiation generally increases over the winter as the days get longer. Late winter is generally when
there is the most snow accumulation available to melt and to form into ice dams as well. The rate
of snow melt can be estimated using the daily solar radiation.
2.4 Snow Metamorphosis
Another important property of snow is its potential to create melt water which is the main source
of leakage once the ice damming cycle starts. Areas with higher snow loads producing high snow
water equivalents (see Figure 2 indicating the snow water equivalent for February) have the
potential to create ice dams that can surpass the typical code requirements for eave protection
provided if held back. The melt water is the second component of an ice dam. The areas with
high snow water equivalents are generally the same areas that experience the greatest snow
accumulation.
Figure 2 - February snow water equivalent (mm) [Woo 1997]
3 CASE STUDIES
The following two case studies illustrate how the above procedures were used to identify ice
damming problems, their causes and the recommended repair strategies. Only the techniques
discussed above that were applicable were used for each case study. The above recommendations
are not an exclusive list, but rather common approaches that were applicable for the case studies.
As with most work of this nature the level of review and corrective action is limited by time and
budget constraints.
3.1 Case Study 1 – Ontario Ski Resort Compared to Similar Located in British Columbia
and Quebec
3.1.1 Background
a) Building Description: This case study reviews the design of a three to four storey ski resort
built on the south shore of Georgian Bay at the base of the Blue Mountains in Ontario. The steep
sloping roofs were covered with a mixture of asphalt shingles and architectural metal (see Photo
No. 1). The roofs were drained by external gutters to downpipes. The roof lines were divided by
fire walls extending through the roof. Dormers were regularly cut into the roof. Construction of
the building followed standard residential procedures, including wood framed walls clad with
stucco or wood cladding.
Ice Damming History: During the first winter of operation large icicles developed over entrances
to the building and ice crashed through fabric canopies at cafés below (see Photo No. 2). The
falling ice was from large ice dams breaking away from the roof. The roofing material and eave
protection were sufficient to prevent leakage. Full ice dam protection, from eave to ridge, was
supplied at most locations. However, a means of protecting pedestrians and property from the
falling ice needed to be developed.
Photo 1: Georgian Bay ski resort roof variations.
Photo 2: Ice from the roof falling to grade.
3.1.2 Problem Analysis
a) Design/Construction to Code: The construction team began by looking at the attic construction
for answers as to why the ice formations were occurring with such severity. The attic was
insulated to RSI 5.6 (R-32), which met the local code minimums. The ceiling air barrier included
a polyethylene sheet (also acting as the vapour retarder) and painted gypsum board. Continuous
soffit vents, ridge vents and mushroom roof vents were installed on the roof to meet the code
minimum 1/300 free ventilation area for the attic.
Spot temperatures of the attic space confirmed it to be warm enough to promote melting of the
snow. Deficiencies were detected in the insulation and air barrier. The insulation was locally
deficient around attic mechanical equipment penetrations such as the elevator shaft. The rigid
insulation was poorly fitted and secured over the elevator shaft. The polyethylene air/vapour
barrier was installed continuously; however, multiple penetrations were made by conduits and
piping after it was installed. The mechanical trade did not restore the seal around the conduits and
pipes they carried into the attic. These construction deficiencies were corrected once identified;
however, the ice problems continued.
b) Weather Affects: The construction team believed the ice problems were due to construction
deficiencies. Buildings of a similar design constructed in Whistler, British Columbia and MountTremblant, Quebec had not reported ice damming to the same degree as the Ontario site. The
problem facing the design team was that a building built to local building codes was nonetheless
experiencing ice damming problems. Weather analysis is the key differentiating factor between
the Ontario built ski resort and the British Columbia and Quebec based ski resorts. Weather data
has been summarized in Table 1.1 for the three sites.
Table 1.1 – Data Summary
LOCATION
* MEAN SNOW
DEPTH (cm)
** SNOW WATER
EQUIVALENT (mm)
*** MEAN DAILY
RADIATION
(MJ/m2)
14
Southern Georgian Bay,
20
40
Ontario
Whistler, British Columbia
150
500
Mount Tremblant, Quebec
50
150
* For the month of February [Brown et al 2003]
** For the month of February [Woo 1997]
*** Average for the months of January, February and March [CANSIA]
13.3
15.3
All three sites have significant accumulations of snow, although the Ontario ski resort has the
lowest levels of the three. The Ontario ski resort is on average much warmer with higher day
time highs that promote melting of the snow. We have reviewed the temperature and snow
accumulation for the months of January, February and March in 2006 for the three ski resort
locations to compare the cyclic temperature range (see Figures 3, 4 & 5). The maximum and
minimum temperatures are indicated with the yellow and pink lines respectively. The change in
temperature is indicated with the dark blue line. The cyan coloured line represents the snow on
the ground. The graphs show a much greater swing in temperature for the Ontario resort that
rises frequently above the freezing point. The graphs for the British Columbia and Quebec ski
resorts show much more consistent temperatures that rarely rise above freezing. This resulted in
melting snow during day time highs in Ontario and freezing of the snow melt once temperatures
dropped again. The cyclic action quickly built the ice dams at the Ontario ski resort causing more
icicles and falling ice hazards (see Photo No. 2).
Weather in Southern Georgian Bay, ON - Jan-Mar 2006
35
Weather in Whistler, BC - Jan-Mar 2006
300
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25
250
300
25
15
250
Max
0
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-5
Min
Change in T
Snow on Ground
Degrees Celcius
Degrees Celcius
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60
70
80
90
200
100
-5
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-15
-15
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-25
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-35
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Days after start of year
Figure 3 - Southern Georgian Bay Weather Data
-35
100
Days after start of year
Figure 4 - Whistler Weather Data
cm of Snow on Ground
15
200
Max temp
Min temp
Change
Snow on ground
Weather in Mont-Tremblant, ON - Jan-Mar 2006
300
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25
250
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5
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50
60
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-5
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-15
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-25
-35
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Days After Start of Year
Figure 5 - Mount Tremblant Weather Data
Degrees Celcius
200
Max
Min
Change in T
Snow on Ground
In addition, the amount of solar radiation or
amount of sunlight available is slightly
higher for the Ontario ski resort compared
to the Whistler resort. The exact amount of
impact is unclear, but nonetheless a factor to
be considered. The increased heat from the
solar gain directly increases the rate of snow
melt from the exterior. The solar gain also
heats the exposed areas of roofing raising
the attic air temperature quicker than the
attic ventilation can remove the warmed air.
This causes the snow on the roof to melt
from the underside. Snow covered roof
areas will insulate the attic from the top side
aiding to this melting affect. Snow has an
insulating value of about RSI 0.046/cm (R
0.66/inch) depending on the snow’s density.
The snow melt will then freeze when
contacting the colder eaves.
3.1.3 Solution Approach
a) Code Minimums and Design/Construction: Obvious deficiencies were corrected in the building
envelope in lieu of additional investigation to determine the exact causes of the elevated attic
temperatures. Internal attic heat sources were reduced to meet good design/construction
standards. The attic ventilation was upgraded beyond codes to help control the ice damming.
Unfortunately the complicated building design at the Ontario site and warmer, fluctuating climate
made it difficult to completely stop all the ice dams. The ice dams were not causing interior
leakage due to the full ice dam protection membrane coverage, from the eave to the ridge, which
exceeded code. Still a building built to meet or exceed the code developed ice dams that became
falling hazards.
b) Ideally the design of the roof slopes would allow snow and ice to fall freely to unoccupied
space at grade, such as landscaped areas. The trajectory of snow and ice falling from a roof can
be estimated using the building eave height, length and slope of the roof, coefficient of friction of
roofing material and amount of snow [Taylor 1983]. In lieu of a total building redesign, the
Owners chose to implement a snow and ice removal management program to address problem
locations before it could start to develop ice dams.
Snow guards were installed to prevent uncontrolled falling snow and ice prior to removals. It is
important to note that snow guards need to be designed to resist the ice/snow loads down to the
building structure or they will fail. The force of the snow on the snow guard can also be
calculated [Taylor 1983]. The first attempt at snow guards were only secured into the roof
cladding and were torn off by the force of sliding ice from an ice dam. At the worst accumulation
areas stronger snow/ice guards were installed. Heat tracing was utilized in conjunction of the
snow/ice guards to melt snow and ice to the roofs drainage system. Monitoring of the ice
damming is ongoing to help control the ice forming and limit related damages.
3.2 Case Study 2 - Ontario Long Term Care Facilities, Muskoka site Compared to
Peterborough and London
3.2.1 BACKGROUND
a) Building Description: The long term care building reviewed is an expansion of the original
facility. The building consists of three wings of residential rooms joining to a centre core
occupying common areas and offices (see Photo No. 3). There are a total of 160 residential units
in this part of the facility built in the Muskokas of Ontario. The asphalt shingled roofs have a
moderate 4/12 slope. Faux dormers broke up the roof slope.
b) Ventilation: The light framed construction included raised trusses that allowed full depth
insulation to meet the exterior wall. The asphalt shingle roof sheds water to external gutters,
drained by leaders to grade. Ventilation was supplied by continuous soffit vents at the 300mm
overhang and ridge vents at the peak, including the dormers. The total ventilation area that has
been provided is about twice the minimum amount required by local codes.
Photo 3: Concentrated snow accumulation between dormers.
Photo 4: Ice damming at Muskoka Long Term Care Facility
c) Insulation: The attic ceiling construction consists of (from the inside out) painted gypsum
board interior finish secured to the bottom cord of the wood truss, polyethylene air/vapour barrier
and blown fibreglass insulation applied to a depth to achieve about RSI 5.6 (R-32). The ceiling
level is raised over the corridors for the mechanical ducts and pipes. The mechanical services are
concealed by an acoustical drop ceiling.
d) Mechanical Heating System: The building is heated by two forced air heating systems and a
hot water radiator system. The main forced air system heated resident areas and the secondary
system heated the common and corridor areas. Radiators are mounted to the ceilings along the
exterior walls. The hot water for the radiators is heated to 60°C. Building operations staff
thought the ceiling mounted radiators may be contributing to the attic heat gain and reduced the
circulation to the radiators. The radiators output was set at 20°C and the forced air thermostat to
24°C to limit the operation of the radiators. The heating controls in the stairwells are limited and
they are maintained at approximately 27°C.
e) Ice Damming History: During the first winter of service over a foot of ice built up along the
eaves. The ice damming progressed past the eave protection causing interior leakage. The
following summer it was discovered that the soffit vents were blocked by tarps that were installed
by the insulation contractor. The tarps were removed to increase the ventilation by allowing the
required circulation. However, the following winter the ice damming remained the same (see
Photo No. 4). This was perplexing to the designers since they have identical buildings located in
London and Peterborough, Ontario, that did not experience the same degree of ice damming.
The need to review the climate was obvious, which has been summarized in Table 2.1 for all
three sites.
Table 2.1 – Data Summary
LOCATION
* MEAN SNOW
DEPTH (cm)
50
10
** SNOW WATER
EQUIVALENT (mm)
150
40
Muskoka, Ontario
Peterborough,
Ontario
London, Ontario
10
20
* For the month of February [Brown et al 2003]
** For the month of February [Woo 1997]
*** Average for the months of January, February and March [CANSIA]
*** MEAN DAILY
RADIATION (MJ/m2)
16.6
15.3
13.3
3.2.2 PROBLEM ANALYSIS
a) Design/Construction to Code: The melting snow flows to the eaves where it freezes, creating
icicles and then ice dams over 300mm thick. The resulting leakage typically occurs at the valleys
where the snow accumulation is the greatest and creating larger ice dams. At some units water
leaked past the eave protection down onto the ceiling over the polyethylene vapour retarder. It
would collect and flow into the wall and not become visible until it exited at a window head or
along the floor at the base of the wall. The multitude of factors contributing to the ice damming
in Muskoka required analysis to determine what the primary causes were.
b) Winter Maintenance: To help control the ice dams maintenance staff removes snow away
from the ridge vents so that the attic ventilation is not impeded. The need for the snow removal at
the vents is confirmed with the increase in attic temperatures after a snow fall in the month of
February and the vents were not cleared (compare Figures 6 and 10 from about February 15th to
the end of the month). To protect the building and pedestrians maintenance crews remove icicles
before they are allowed to fall. At some locations plywood is placed over the windows at grade
to protect them from falling ice. Thick ice at locations of leakage is removed with hot
pressurized water. Maintenance staff removed sections of external gutter at the northwest wing,
along the parking lot, to determine if it would allow snow melt to drain off the roof instead of
backing up to form the ice dams. At one of the large dormer located over the northwest wing, an
electric exhaust fan has been installed to help remove warm air from the attic. Neither
modification reduced the amount of ice damming. The attic temperatures in the northwest wing
were similar to the east wing attic that did not have any ventilation upgrades. The one electric fan
was likely not sufficient for the size of the northwest wing.
c) Weather Affects: The temperature and snow accumulation for the months of January, February
and March in 2006 for the locations for all three long term care facilities were compared (see
Figures 6, 7 & 8). All three locations have fluctuations in the temperature. However, the
increased snow depths in Muskoka, about 5 times more, made the ice damming that occurred
more severe. The Muskoka weather is conducive to create ice dams; however, neighbouring
buildings in Muskoka did not experience the same degree of ice damming. The likely
explanation for this is that the much larger long term care facility was maintained at a much
warmer temperature than typical commercial or residential buildings.
Weather in London, ON - Jan-Mar 2006
Weather in Muskoka, ON - Jan-Mar 2006
300
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300
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250
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-5
Max
Min
Change in T
Snow on Ground
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-5
100
100
-15
-15
50
-25
-35
-35
0
Figure 7 - London Weather Data
Weather in Peterborough, ON - Jan-Mar 2006
35
300
25
250
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80
90
150
100
-5
100
-15
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-25
-35
Degrees Celcius
200
5
20
0
Figure 6 - Muskoka Weather Data
10
50
-25
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150
100
Max
Min
Change
Snow on ground
d) Prior to our initial review the radiators
were set to 27°C in specific units to see if
they increased heat loss into the attic. The
exterior ambient air temperature was about
4°C with 40% relative humidity during our
initial review. Spot temperatures were
taken using digital and non-contact
thermometers to see if there was a
corresponding increase in the temperature
of the attic air and underside of the roof
deck. There was not a significant increase
in attic temperatures over the units with the
radiators turned on.
0
Figure 8 - Peterborough Weather Data
Smoke pencils confirmed air leakage into the attic around ceiling penetrations (e.g., light fixtures,
nursing call wires, pipes, etc.). The number of penetrations quickly added up since they repeated
for each of the residential units.
There were also areas of disturbed blown insulation. However, the areas of the greatest concern
were related to the insulation over the raised corridor and the construction of the fire walls. The
insulation at the sides of the raised corridor was only laterally restrained with stripes of plastic
stapled across the wood framing and had fallen away, leaving large areas of exposed interior wall
construction that lead to direct heat conduction into the attic space. The masonry fire walls were
not well sealed at the ceiling junctions and had numerous voids in their construction. Warm
interior air could be felt flowing from these defects, which were also a fire safety issue at the fire
wall. The deficiencies found during the preliminary review suggest enough heat loss sources to
cause the reported ice damming.
e) Attic Temperature Monitoring: Data loggers were installed during a subsequent visit to monitor
the temperature variations in the attic. These temperatures were compared with the interior and
exterior, starting midway through January to the end of March of 2006 (see Figures 9, 10 & 11).
The interior sensor was placed in a typical unit on an interior wall, about a meter below the
ceiling. The interior temperature is represented by the purple line in Figures 9, 10 & 11. In
general, the interior temperatures are maintained at a higher level than would be typical in
0
150
100
Degrees Celcius
5
200
Degrees Celcius
200
Max Temp
Min Temp
Change
Snow on ground
residential occupancy for the comfort of the long term care tenant’s comfort. The east and
northwest wings had sensors installed in the attics. The sensors in the attics were placed on the
top chord of the roof truss below the roof sheeting approximately at the mid-point of the roof
slope. The temperatures recorded in both attics were close to the same, represented by the yellow
and cyan coloured lines in Figures 9, 10 & 11. In the same figures the dark blue line represents
the exterior temperature measured on site.
Muskoka Long Term Care Facility - Ice Damming
30
20
TEMP. (C)
10
0
25-Jan-06
26-Jan-06
27-Jan-06
28-Jan-06
29-Jan-06
30-Jan-06
31-Jan-06
-10
-20
-30
TIME (Jan 2006)
Exterior
Interior
East Attic
NW Attic
Figure 9 - January Data Logger Temperatures
40
30
20
TEMP (C)
10
0
01-Feb-06
08-Feb-06
15-Feb-06
22-Feb-06
-10
-20
-30
-40
TIME (Feb 2006)
Exterior
Interior
East Attic
NW Attic
Figure 10 - February Data Logger Temperatures
40
30
TEMP (C)
20
10
0
01-Mar-06
08-Mar-06
15-Mar-06
22-Mar-06
29-Mar-06
-10
-20
-30
TIME (March 2006)
Exterior
Interior
East Attic
NW Attic
Figure 11 - March Data Logger Temperatures
The attic temperatures should be approaching the exterior temperature. The attics are cooling
with the exterior temperature, but for much of the time they do not cool quickly enough. There is
about 12 hour lag for the attic temperatures to drop with the exterior temperatures. Before the
attic can cool down to the maximum low temperature, the exterior temperature begins to warm
again with each new day. The data shows us that the ventilation is inadequate or there is
excessive heat gain in the attic surpassing the venting capacity. The difference between the attic
and exterior temperatures ranges from a few degrees to over 20°C.
The greatest difference occurs at times of snow accumulation. The snow blocks the ridge vents,
reducing the attic circulation from the soffit to the ridge. The cyan coloured line in Figure 6
represents the snow on the ground for Muskoka. The increased snow depth appears to reduce the
amount of attic ventilation. As the snow starts to melt in March, the attic and exterior
temperature start to converge (see Figures 6 & 11). From the data collected it would appear
maintaining clear ventilation would help keep the attics a cooler temperature.
a)
Infrared Imaging: Spot temperatures using the infrared non-contact thermometer used
during the preliminary review could not cover the whole attic efficiently and accurately. The
infrared scan of the attic was done to confirm and identify sources of heat loss due to thermal
bridging, inadequate insulation and air leakage. The day of the infrared scan the exterior
temperature was -2°C with 89% relative humidity. From our initial visual review the infrared
scan confirmed and quantified the heat loss at poorly insulated areas and areas of air leakage.
Additional locations identified by the infrared imaging that was not identified from our initial
visual review included:
i)
plumbing vent pipes from the warm moist air being vented;
ii)
exhaust stacks from the buildings mechanical systems;
iii)
small sections of attic run ducts not adequately insulated; and
iv)
thermal bridging at truss web locations.
During the infrared scan we also identified the warmest temperature at these new locations. The
majority of heat loss areas had temperature readings above 20°C. The surface temperature of the
underside of the plywood roof sheathing was also checked at several locations averaging around 3
to 4°C.
3.2.3 SOLUTION APPROACH
a) Exceed Code Minimums and Address Design/Construction defects: The base repair
recommendations included the following:
i)
Air Sealing: As identified by a visual review and an infrared scan the areas of air leakage
required sealing to prevent warm air leaking to areas beneath the roof deck. The construction
deficiencies need to be air sealed include all pipe, conduit and fire wall penetrations. To achieve
adequate bond to dirty surfaces inside the attic spray-in-place urethane foam insulation is
generally recommended.
ii)
Ventilation: Improved ventilation during snow accumulation can help remove warmed
attic air, as indicated by the weather and data logger information. The current ridge vent design
has reduced efficiency when covered with snow. Additional raised vents can be added to maintain
ventilation when the ridge vent is covered in snow. Caution should be taken to avoid providing
new ventilation in instances where the air barrier is defective. In some instances the roof vents
may create a negative pressure in the attic space. Instead of balance of intake and exhaust of
exterior air the negative pressure can draw warmed interior air through the deficiencies in the air
barrier.
iii)
Insulate Interior Attic Heat Sources: Clearly seen with the infrared scan the attic ducts,
boiler vents and some plumbing vent pipes are heating the attic space. Some of these items are
insulated, but only to reduce performance loss of the specific duct or pipe. This amount of
insulation is not sufficient to prevent the air in the attic from being warmed. These heat sources
need upgraded insulation.
iv)
Attic Insulation Upgrading: Poorly placed insulation from original construction
installation need to be corrected. Foot traffic from the above repairs in the attic will compact and
reduce the thermal resistance of the existing blown insulation. New insulation will be required
above code requirements, and eliminating thermal bridging can result in improvements. Care
must be taken to avoid blocking eave ventilation.
b) Optional repairs recommended included the following:
i)
Roof Improvements: Extend eave protection and full roof underlayment to prevent backed
up water leaking into the building. Roof underlayment (not installed during construction) might
have prevented the leaks. Increased eave protection is critical at areas where the snow collects,
such as valleys around dormers. A cost effective time for this would be during initial design or
when the roofing material is replaced with an existing building.
ii)
Heating Cables: Heating cables can provide an uninterrupted flow of water through the
frozen portions of the roof, which are typically the eaves. Heat tracing gutters and leaders will
also help remove water from the roof eave. This should be a last resort considering
energy/environmental concerns.
iii)
Snow Removal: Snow provides the thermal balance which can act to maintain the attic
space at an elevated temperature. Removal acts to cool the space sufficiently to reduce ice
damming (and removes the source of water). This is most critical at the valleys of this facility.
To date we have been informed that the air barrier deficiencies have been repaired, additional
insulation added and the buildings mechanical system has been re-balanced. To date there have
been no ventilation upgrades completed. Winter 2007 is the first season since our review and
there have been no reports of ice dam damages. It is important to note that there was limited
snow until the middle of January 2007. Ice damming conditions are being monitored on an
ongoing basis.
4. CONCLUSIONS/RECOMMENDATIONS
What have we learned from the case studies? Code minimums for insulation, ventilation and ice
dam protection can be insufficient, especially for climates prone to ice damming. The solutions
for ice damming in one climatic region may not work for a building in a different climatic region.
The proportion of snow accumulation, temperature variance and solar radiation will affect the
formation of ice dams. Buildings with attic deficiencies located in climates prone to ice dams
become more difficult to diagnose.
The ski resort case study located on southern Georgian Bay in Ontario had some heat loss into the
attic from insulation and air barrier deficiencies. The correction of these defects reduced energy
loss, but did not resolve the ice dams. The regions cyclic temperatures would melt and form ice
from the snow. The resulting build up of ice became a falling hazard and life safety issue. Ice
dam leaks were prevented due to the use of eave to ridge ice dam protection, which is a good
recommendation in high risk ice dam locations. When being relied upon the sealing and
performance of the ice dam protection membrane is also important (see Graham 2006). Falling
ice could have been managed in the design phase if the roof slopes could direct the snow and ice
away from occupied areas below. However, the roof design, similar to that of the ski resorts
located in British Columbia and Quebec, included changing roof slopes, stepping levels and
dormers that shed ice over entrances or sitting areas. These complex roof shapes allowed snow to
collect and add more volume to the ice dams when they occurred.
In the second case study there were numerous factors that may be contributing to the ice dams at
the long term care facility in Muskoka. The phased repair approach taken by the Owners can
determine if the simple fixes alone reduces the ice damming. The long term care facility in
Muskoka had heat loss into the attic. The numerous conduits and pipes at each unit of the long
term care facility should have been sealed. This would have stopped much of the heat loss
occurring through convection. The attic insulation over the ceiling can be a problem, especially if
the interiors are heated as high as this long term care facility. The poorly supported area of
vertical insulation over the raised corridors created voids in the insulation and did not prevent
thermal bridging at wood framing members. Possibly the greatest insulation deficiency is not
even part of the building envelope at the long term care facility. This included ventilation ducts,
plumbing vent pipes and hot exhaust stacks. The insulation provided for such equipment is only
designed for the performance of that specific device. The mechanical/plumbing heat sources in
the attic should be insulated beyond the manufacturer recommendations to a minimum of RSI 3.5
“R-20”. One of the last easy fixes to reduce ice dams is to increase the attic ventilation, or
maintain what has been provided. Clearing the blocked soffit vents improved the ventilation at
the Muskoka case study. However, the higher snow loads at this location blocked the rooftop
vents, reducing the attic ventilation after heavy snow falls. Raising the height of the rooftop vents
would help maintain the attic ventilation requirements in this case.
From the two case studies we have learned climatic regions with snow that promote the formation
of ice dams generally have temperature swings above freezing. The greater the snow depths and
swings in temperature above freezing, the greater the potential for ice dams. Weather played a
role with the ice damming formation for both case studies; however, attic deficiencies also needed
to be identified. The heat loss into the attic and the attic ventilation capacity was checked using a
range of common tools, ranging from smoke pencils and thermometers to infrared cameras and
data loggers. These tools helped identify the design/construction deficiencies since there are
usually several of them contributing to the ice damming. Fixing all deficiencies is expensive and
still may not guarantee the ice damming problem solved. Systematic repairs were phased over
time to determine what reduces the ice damming effectively to avoid unnecessary repair
expenses.
REFERENCES
Brown, Ross D., Gridded North American Monthly Snow Depth and Snow, Water Equivalent for
GCM Evaluation, by Ross D. Brown1* , Bruce Brasnett2 and David Robinson3, [Original
manuscript received 11 March 2002; in revised form 16 August 2002], [Brown, 2000]
1 Meteorological Service of Canada, Climate Research Branch, Dorval QC H9P 1J3,
2 Meteorological Service of Canada, Canadian Meteorological Centre, Dorval,
3 Dept. of Geography, Rutgers University, Piscataway, NJ U.S.A
Canadian Solar Industries Association, CANSIA, “Peak Solar Maps”,
http://www.cansia.ca/solarmap.asp CMHC, Canada Mortgage and Housing Corporation’s, Attic
Venting, Attic Moisture and Ice Dams
MSC, Metadata for Snow CD, Snow CD (A Compilation of Canadian daily snow depth
observations (cm) and bi-weekly snow course data (depth - cm, SWE - mm) up to the late1990s.), Ross Brown, 31-DEC-1999; and Metadata for Daily Snow Depth CD update, Daily
Snow Depth CD update (A Compilation of Canadian daily snow depth observations (cm) and biweekly snow course data (depth - cm, SWE - mm) up to the late-1990s), Ross Brown, 31-DEC2003 [MSC, 2000]
Woo2 , Ming-ko, Ph.D., PH (A.I.H.), A GUIDE FOR GROUND-BASED MEASUREMENT OF
THE ARCTIC SNOW COVER1 , December 1997, [Woo, 1997]
1 This report was prepared for the Climate Research Branch, Atmospheric Environment Service
2 Geography Department, Hamilton College, McMaster University, Hamilton, Ontario L8S 4K1
BIBLIOGRAPHY
Armstrong, Terence, Roberts, Brian, and Swithinbank, Charles, Illustrated Glossary of Snow and
Ice Second Edition. Yorkshire: The Scolar Press Limited, 1973.
Baker, M.C., Canadian Building Digest, Ice on Roofs. CBD#89, Ottawa: National Research
Council of Canada, 1967.
Graham, Mark S., Analyzing Self-Adhering Underlayments, Professional Roofing, National
Roofing Contractors Association, November 2006, p. 30.
Gray, D.M. and Male, D.H., Handbook of Snow Principles, Processes, Management and Use.
Toronto: Pergamon Press, 1981.
Straube, J.F., Building Science Press, Ice Dams, Building Science Digest 135, October 30, 2006.
Taylor, D.A., Canadian Building Digest, Sliding Snow on Sloping Roofs. CBD#228, Ottawa:
National Research Council of Canada, November 1983.

The New Ice Age - A Practitioner`s Perspective on Ice Damming

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