{"id":111383,"date":"2020-04-30T13:57:11","date_gmt":"2020-04-30T17:57:11","guid":{"rendered":"https:\/\/www.constructionspecifier.com\/?p=111383"},"modified":"2024-09-24T16:44:10","modified_gmt":"2024-09-24T20:44:10","slug":"keeping-low-slope-roofs-dry-in-northern-climates","status":"publish","type":"post","link":"https:\/\/www.constructionspecifier.com\/keeping-low-slope-roofs-dry-in-northern-climates\/","title":{"rendered":"Keeping low-slope roofs dry in northern climates"},"content":{"rendered":"

by Dwight D. Benoy, PE, Pamela Jergenson, CCS, CCCA, BECxP, CxA+BE, and Gary C. Patrick, AIA, RRC, CSI<\/strong><\/p>\n

\"Photo<\/a>
Photo courtesy Inspec. Photo \u00a9 Matt Bryan<\/span><\/em><\/figcaption><\/figure>\n

When three separate wood-frame structures in the northern United States showed evidence of possible moisture infiltration in the roof areas, an independent architectural\/engineering consulting firm was called in to confirm the issue was present, determine the amount of damage, find the source, and design solutions. All three buildings had been built within the last 10 years, and shared similar design characteristics, with commercial space on the first floor and apartments on the upper levels. When all three assessments had uncovered premature failures to some structural components of the roof assemblies as a result of accumulated moisture in the truss space, a thorough investigation into the source had to be conducted.<\/p>\n

Background<\/strong><\/p>\n

All three buildings had similar non-ventilated, low-slope roof assemblies utilizing wood trusses with a polyethylene vapor retarder on the bottom of the trusses covered with a gypsum ceiling. Blown-in fiberglass or cellulose insulation filled the truss space from the ceiling to the bottom of the roof deck on all three buildings. Oriented strand board (OSB) of 13-mm (1<\/sup>\/2<\/sub>-in.) thickness was installed over the trusses as the roof deck. Rigid board insulation was installed over the OSB, followed by the roof membrane, which was a gravel-surfaced built-up roof in one case, a ballasted ethylene propylene diene terpolymer (EPDM) single-ply membrane on another, and a mechanically fastened thermoplastic polyolefin (TPO) single-ply membrane on the third.<\/p>\n

\"Figure<\/a>
Figure 1:<\/strong> The first repair option for the roof of the Coborn Plaza Apartments in Minnesota.<\/span>
Images courtesy Inspec<\/span><\/em><\/figcaption><\/figure>\n

Water intrusion was not evident in any of the buildings. A survey of the roof surfaces showed they were in good condition. Occupants in two of the buildings reported mold issues, which led to further investigation. Maintenance people walked on the roof of the third building and discovered soft spots, which turned out to be locations where the OSB roof deck had lost its structural integrity due to moisture degradation.<\/p>\n

It was apparent moisture-laden air had migrated into the truss space of all three buildings and condensed in the upper reaches of the roof assemblies. This resulted in excessive moisture buildup, mold, and rot of the OSB structural roof deck and structural trusses in a substantial portion of the roof area.<\/p>\n

\"Figure<\/a>
Figure 2:<\/strong> The second repair option.<\/span><\/figcaption><\/figure>\n

Ideally, the vapor retarder should act to minimize the amount of moisture vapor from the interior of the buildings getting into this space. In the northern climate where these projects were located, the vapor drive in the winter is mainly from the warm interior to the cold exterior. The warm interior air carries moisture vapor that will condense on surfaces below the dewpoint, the temperature at which condensation can occur.<\/p>\n

The investigations found many bypasses in the vapor retarder that would allow the warm, moist interior air to migrate into the truss space. The party walls between apartment units, which were a double-stud wall construction, interrupted the continuity of the vapor retarder.<\/p>\n

Similarly, all interior partition walls resulted in a discontinuity in the vapor retarder at the ceiling. This was exacerbated by the penetrations through the top plate of the wall by plumbing stacks and wiring. Penetrations through the ceiling, such as sprinkler heads and electrical boxes for light fixtures, were also found unsealed to the vapor retarder.<\/p>\n

Two of the buildings had ducts from bathroom and dryer vents running through the truss space. Some of these ducts were not well-sealed at the joints, thereby introducing moist air directly into a space with a potential for condensation.<\/p>\n

\"Figure<\/a>
Figure 3:<\/strong> Tall parapet at nonbearing roof truss.<\/span><\/figcaption><\/figure>\n

In the authors\u2019 opinion, the roof assembly would perform satisfactorily if the ceiling vapor retarder were perfectly constructed. However, in this type of construction, it was impossible to perfectly construct a vapor retarder at the ceiling level because of the discontinuities. Ventilation of the truss space is an ineffective option to manage moisture. Unlike a steep-slope attic space, there is very little, if any, room to create airflow over relatively long distances.<\/p>\n

Perhaps, the motivation to design an assembly as such would be to minimize the insulation costs related to the energy code requirements of recent years. While the amount of insulation installed exceeds the code requirement, the material and labor to install it were less than a code-compliant insulation installed above the roof deck. Filling the truss space with noncombustible insulation also eliminated the need for firestopping and draftstopping as noted in the 2000 edition of the International Building Code<\/em> (IBC<\/em>), in force at the time these buildings were constructed.<\/p>\n

\"Figure<\/a>
Figure 4:<\/strong> Typical invasive inspection opening above trusses (top) and below roof deck sheathing (bottom).<\/span>
Photos \u00a9 Dwight Benoy<\/span><\/em><\/figcaption><\/figure>\n

Installing the vapor retarder at the roof deck level affords a better opportunity to achieve a complete vapor retarder. This would be an easy way to provide continuity across party walls and to seal penetrations. The vapor retarder must also be continuous from the roof to the exterior walls. This might be accomplished by using sprayfoam insulation within the truss space at the exterior walls.<\/p>\n

On one building, the Coborn Plaza Apartments in central Minnesota, the problem was discovered approximately five years after the building was constructed when tenants of the top-floor observed mold on the gypsum ceiling and complained of musty odors. A mold remediation project was undertaken and included removal of the gypsum ceiling, vapor\/air barrier, and blown-in insulation. It was discovered the exhaust ducts for the bathroom and dryer vents were poorly installed in some of the units. These ducts ran through the structural trusses and exited the exterior walls through the rim area. This duct layout also bypassed the ceiling vapor\/air barrier, contributing excessive moisture to the truss space.<\/p>\n

\"Figure<\/a>
Figure 5:<\/strong> Approximate areas of roof deck and insulation replacement.<\/span>
Image courtesy Inspec<\/span><\/em><\/figcaption><\/figure>\n

The remediation work included the cleaning and sealing of the ducts, which was thought to be the only cause of the problem at that time. The moldy framing and structural roof deck were cleaned and painted with an anti-microbial paint. Some of the rotted deck was reinforced from below with additional OSB sheathing and framing.<\/p>\n

Inspection openings from the interior were then made to verify if the remediation was effective. Excessive moisture presence was discovered. It is important to know the moisture buildup had occurred in a matter of months following the remediation. Investigation began for another source of moisture. Hygrothermal modeling was conducted to confirm or deny the inadequacy of the vapor\/air barrier. Results indicated a propensity for moisture to accumulate.<\/p>\n

Designing the repairs<\/strong><\/p>\n

Due to the damages already experienced and the potential for more to develop, it was determined Coborn Plaza needed a full roof replacement. The primary challenge was to develop a complete vapor\/air barrier below the dewpoint temperature that also tied into the wall\u2019s vapor\/air barrier in order to envelope the building.<\/p>\n

Repair options were developed and hygrothermal modeling was conducted for all the ideas. The owner required all work to be conducted from above the ceiling to minimize disruption to the tenants.<\/p>\n

\"Figure<\/a>
Figure 6:<\/strong> Typical work at roof perimeter.<\/span>
Image courtesy Inspec.<\/span>
Photo \u00a9 Dwight Benoy<\/span><\/em><\/figcaption><\/figure>\n

First option<\/em><\/p>\n

This option was intended to create a complete vapor\/air barrier by installing sprayfoam over the polyethylene sheeting and bottom chord of the truss (Figure 1). This required the removal of the existing system down to the structural roof deck and also a significant portion of the deck to facilitate the vacuuming of the existing blown-in insulation out of the truss space and the installation of sprayfoam and new blown-in insulation. New tapered insulation and roof membrane above the structural roof deck were part of this solution.<\/p>\n

Second option<\/em><\/p>\n

The second option required removal of the existing system down to the structural roof deck and the replacement of any wet, rotted, and\/or moldy deck and blown-in insulation (Figure 2). A roof vapor\/air barrier would be applied on the structural roof deck.<\/p>\n

\"Figure<\/a>
Figure 7:<\/strong> Z-shaped vapor\/air barrier transition metal.<\/span>
Photos courtesy Inspec.<\/em> Photos \u00a9 Matt Bryan<\/span><\/figcaption><\/figure>\n

The application of sprayfoam insulation of at least 76-mm (3-in.) thick to the rim area was determined to be the most effective way in-situ to transition the polyethylene sheeting from the exterior walls to the roof vapor\/air barrier. The rim area is at the top of the exterior walls at the level of the 406-mm (16-in.) deep roof trusses.<\/p>\n

Sufficient insulation needed to be added above the structural roof deck to get the dewpoint temperature above the roof vapor\/air barrier. This insulation also needed to be tapered to provide roof slope to the existing, interior, primary and overflow roof drains. The hygrothermal analysis showed a minimum of 102 mm (4 in.) of polyisocyanurate (ISO) insulation was required in order to keep the dewpoint temperature above the roof vapor\/air barrier. This meant all roof drains would need to be raised to accommodate the increased insulation thickness.<\/p>\n

\"Figure<\/a>
Figure 8:<\/strong> Two-piece U-shaped vapor\/air barrier transition metal.<\/span><\/figcaption><\/figure>\n

Third option<\/em><\/p>\n

This option required removal of all the existing blown-in insulation in the truss space and installing a sprinkler system to satisfy the fire code. A new roof assembly above the structural roof deck included a roof vapor\/air barrier, tapered rigid board insulation, and membrane. This helped to let the dewpoint temperature occur above the roof vapor\/air barrier and minimize the amount of insulation. This option also required sprayfoam in the rim area, as described in the second option.<\/p>\n

The third option was quickly eliminated from further consideration because the owner decided against installing a fire sprinkler system above the top-floor ceiling due to the considerable disruption to occupants and the cost. Therefore, the blown-in insulation in the truss space needed to be maintained by selecting either the first or second option.<\/p>\n

The solution<\/em><\/p>\n

The second option was selected and developed into construction documents for bidding and construction (Figure 3). This was the best solution to achieve the goal of a complete vapor\/air barrier. It also exposed the existing assembly to allow for the removal and remediation of wet, deteriorated, and moldy roof components. This option also maximized the reuse of the structural roof deck and blown-in insulation that was still in acceptable condition.<\/p>\n

\"Figure<\/a>
Figure 9:<\/strong> Roof vapor\/air barrier at roof perimeter.<\/span><\/figcaption><\/figure>\n

Vapor\/air barrier continuity<\/em><\/p>\n

Vapor\/air barrier continuity from the wall to the roof is the key consideration and the toughest challenge for the repair design. Installing the roof vapor\/air barrier on top of the structural roof deck required transitioning the barrier through the deck to the rim area to complete the envelope. This was solved by designing a U-shaped sheet metal to wrap around the structural roof deck edge. This provides a surface on the bottom to receive the sprayfoam insulation applied to the rim area, and also a layer on top to which the self-adhering vapor\/air barrier could be bonded.<\/p>\n

Other considerations<\/em><\/p>\n

Besides selecting the second option, other design considerations included:<\/p>\n