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[ Part 1 ] [ Part 2 ] [ Part 3 ] [ Conclusion ]
Case Study
Roof Ice Dam Remediation for Northeast Ski-Area
Condominiums
Part III
by Henri C. Fennell
President, H. C. Fennell, Inc. & FOAM-TECH
North Thetford, Vermont, USA.
Phase II Diagnostics and Repairs
Phase II non-destructive diagnostic information
was collected as in Phase I. The results showed more detailed information
about the locations and severity of the remaining heat losses as the overall
heat flow had been significantly reduced. Specific melt spots in the snow
cover were now visible and could be associated with interior construction
details. Due to the improvement from the first round of repairs, the flow of
melt water had been dramatically diminished and was concentrated below the
areas of remaining heat loss. As a result of monitoring the depth and
location of ice dam formations early in the season, the ice pattern was used
as an indicator of the volume of melt water that was coming from each
problem area above (This was useful in prioritizing the cost effectiveness
of Phase II repairs). Later in the season, the ice depth evened out along
the length of the eaves (except at the valleys, which still had the largest
ice accumulation), due to the melt water spreading horizontally as it was
held back behind the earlier ice formations.
Cathedral Roof Slope Warming – Phase II
Problems and repairs
Phase II testing was completed late in the winter
of 1997/98. The cathedral roof slopes appeared to be clear of problems
except for two areas - below the skylights and along the party walls. Based
on the infrared and melt pattern analysis, the following assessments were
made:
-
Wind washing in the lower slope was eliminated
except in minor spot locations at inaccessible soffit areas. No additional
work was performed in these areas.
-
Infiltration and heat loss at the
zero-clearance fireplace boxes was significantly reduced. The complexity
of this triangular ornamental box and the limited work access made the
work difficult at best. Some infiltration was still evident where the
front face of the box met the two outside walls; however, the amount of
warm air flow out of the top of the box into the roof slope vents was
virtually eliminated. No additional work was performed in these areas.
-
Infiltration of cold outside air along the
wall to roof intersection of the cathedral gable walls had been
eliminated. In the two middle units, these walls form the party walls with
the two end units. Even after air sealing procedures, the narrow strip of
roof over the party walls remained as one of the two most serious melt
areas, in part because they are directly above the valleys. With the wall
to roof intersection sealed, it is assumed that the heat source causing
this melt pattern was heat coming up from the wall cavities rather than
directly from inside the room itself. The air barrier in the cathedral
ceilings did cross over the party walls which have double 2x4 walls with a
space between them (See Detail / Photo Essay #11). As these bays are
inaccessible from the interior, and opening the roof was deemed to be too
expensive, the decision was made to block the flow of air from the walls
into the attic by sealing the roof bay at both ends. This did not preclude
the option of opening the roof at a later date, and it promised to limit
the heat reaching the roof and attic by cutting off the stack effect from
the double wall and creating a dead air space rather than a constant warm
flow. As the bottoms of these roof bays had been sealed when the eaves
were blocked and sealed, this only required sealing the readily accessible
top of the vent space from the attic.
-
The fan/light in the guest bathroom showed no
infiltration on the infrared scan and the melt area in the roof had been
significantly reduced considering that the duct was still routed through
the roof bay. While totally relocating this fan would have been the
optimal solution, the degree of improvement achieved without major
renovations was acceptable.
-
The repairs in the knee wall attics were
performing well. Both the infrared and melt patterns showed significantly
less warming at cold temperatures. The remaining warming, visible only at
near-freezing temperatures, was believed to be caused by the recessed
entry porch light.
-
Air infiltration around the skylights was
virtually eliminated; however, ice formation was noted immediately below
the skylights. As the infrared and melt pattern testing did not indicate
roof warming, and similar construction on both sides of the skylights had
not developed ice, it was assumed that the source of melt water in this
specific area was from the skylight itself. A Phase II test was developed
utilizing a piece of double-glazed tempered glass installed over one of
the existing inoperable skylights. If this test alleviates this problem,
better insulating glass or an outside storm panel will be proposed. This
unit and the unmodified skylight on the next unit were monitored to
determine the following:
-
Would additional glazing R-value prevent ice
formation below the skylights?
-
Was the melt caused by the glazing only, or
by the curb and framing losses around the skylight?
The result of this test was inconclusive.
Late-winter conditions did not produce enough snow to effectively compare
the volume of ice produced below the modified and unmodified units. Photos
of earlier snow cover indicated that the snow pack went clear up to the
skylight curbs. This indicated that the heat loss through the skylight curbs
was not a significant contributor to the ice build-up below them.
Attic Roof Warming – Phase II Problems and
Repairs
The main attic roofs had improved, but not as
much as was expected, given the magnitude of the initial improvements. Most
of the attic roof now held snow, the exceptions being the cricket attic
areas where the roofs of two units overlapped and above the middle unit
party wall. The small isolated attics (cricket attics) just over the party
walls between the middle and end units (See detail photo essay #8) were
especially problematic as this area was directly above the valleys which
concentrated the volume of melt water and ice in one spot. This was a good
example of the need to plan for two rounds of testing and repairs. The small
cricket area had not originally appeared as a unique problem from the rest
of the attic roofs when the Phase I testing (infrared and melt pattern
analysis) was performed. Only after correcting the main attic heat loss
problems could the crickets be identified as unique problem areas. While
these attics were relatively small, they still provided conditions conducive
to ice dam formation as well as roof leakage. As a result of the second
round of tests in the attics, the following "fine-tuning" details were
identified and addressed:
-
The flat cap areas were performing well over
the majority of the large ceiling surface area. The contractor had elected
to fill large gaps in the original insulation, add batts over fixtures and
bathroom fan ducts, and to use an air barrier material over the batts for
convection and ex-filtration control. This was successful in improving the
performance of the ceiling insulation except around the edges. At the
party walls and rafters the air barrier material stopped at the face of
the open framing. It was clear that the air barrier needed to be sealed to
the wall and roof sheathing to work as a secondary air barrier to stop
ex-filtration from ceiling penetrations. Sprayed polyurethane foam was used to
make these complex connections around the rafters, vent chutes, and double
party wall stud framing. Cut outs around plumbing and chimney penetrations
were also sealed (see Detail/Photo Essay #2).
-
Infrared testing showed that there was still
heat flowing into the attic from the top of the cathedral slopes. This was
coming from the vent space in the roof bays over the party walls, the
narrow spaces between the doubled skylight rafters, and at the boxed-down
I-beam at the slope to ceiling transition. While the beam had been air
sealed from below, there was little or no insulation over the steel.
Because the batt insulation had originally been installed from below, the
air barrier and insulation did not pass over the beam. To deal with the
reduced space between the top of the beam and the vent chute, the
available space was filled with polyurethane foam to provide an air barrier
and an adequate R-value. This foam seal was carried over to meet the air
barrier material that covered the attic ceiling batt insulation. The foam
was also used to seal the tops of the narrow spaces between the doubled
skylight rafters and the tops of the party wall bays (see item #4. below).
-
The infrared scans showed that the backs of
two dropped ceiling area walls were still cold. Further investigation
determined that the outside of these walls were unsheathed and open to a
vented soffit above the windows. Some of the batts had fallen out of these
bays. This area communicated with the attic above. The walls were
retrofitted with sheathing and sealed as in the other dropped-ceiling
areas.
-
The tops of the cathedral slope bays over the
party walls were sealed. These three bays had been the largest sources of
heat in the attics after the Phase I repairs. The bays had batts in them,
but no vent chutes or interior sheathing. They were open over their entire
length into the walls below. As a part of the foam contractor’s work,
these bays were filled as far down the slopes as possible. The two party
wall roof bays were the primary source of the warming in the small cricket
attics.
[ Part 1 ] [ Part 2 ] [ Part 3 ] [ Conclusion ]
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