A case study in condensation

Evaluating and preventing attic condensation in a residential building

  • Photo 1: Brown stains on the underside of a ceiling in one townhouse
Photo courtesy of Wiss, Janey, Elstner Associations Inc., Minneapolis.
  • Photo 2: This photo shows frost and melting ice on the underside of the roof deck. Note the brown staining on the loose-fill insulation.Photo courtesy of Wiss, Janey, Elstner Associations Inc., Minneapolis.
  • Photo 3: These were the conditions at an inspection opening in the roof eave. At this location, new aluminum panels were installed over the original plywood soffit panels. The vents in the aluminum panels were spaced at 3 feet (1 m) and did not coincide with the vents in the underlying plywood soffit (spaced at 8 feet [2.4 m]).
Photo courtesy of Wiss, Janey, Elstner Associations Inc., Minneapolis.
  • Photo 4: A separated furnace exhaust vent was discharging directly into the attic.
Photo courtesy of Wiss, Janey, Elstner Associations Inc., Minneapolis.
  • Photo 5: This photo shows conditions at the top end of a vent pipe. The pipe terminated at the bottom of the roof deck, and no clamping device was provided to secure the pipe in position. The top circular edge of the vent was leaning against the sides of the opening cut in the roof deck. The pipe was venting into the attic space rather than to the exterior.
Photo courtesy of Wiss, Janey, Elstner Associations Inc., Minneapolis.

Condensation on the undersides of roof decks in attics can be a common problem for different residential roof systems. Condensation in attics can occur more frequently in the colder climates of the upper Midwest and Northeast and typically on roofs that have inadequate ventilation and insulation, as well as attic bypasses. When dryer vents, bathroom fans or furnace exhaust ducts vent directly into an attic space, the problem is intensified.

Although building codes recommend and require adequate attic ventilation above roof insulation to remove heated air and water vapor that has entered an attic from the surrounding air or a structure's occupied portion, the codes are not strictly adhered to during the design or construction phases. In addition, attic bypasses and openings in dryer vents and furnace ducts must be sealed to minimize moisture infiltration into an attic. This would prevent troublesome condensation within the attic space and on the roof deck's underside. Unfortunately, joints in vents and furnace ducts sometimes are not installed in an airtight manner.

The primary signs of severe condensation in an attic space (as viewed from the living spaces) are water stains on ceilings, windows and walls. Such stains generally are the result of melting frost and/or icicles on a roof deck's underside. When staining because of melting frost and/or icicles occurs to a building's interior and damages interior finishes, prompt action is required to limit water damage. Water also can saturate insulation, reducing its effectiveness, and contribute to conditions that would be favorable for the development of stains that appear to be mold, as well as long-term deterioration of sheathing and framing.

A case study

The types of problems described were evident in the case study that follows. Roof leakage problems were alleged by residents of a townhouse building located in St. Paul, Minn., in 2003. The Minneapolis branch office of my company, Wiss, Janney, Elstner Associates Inc., was retained to investigate the reported roof leakage problems and provide an opinion of the likely cause(s) of the alleged water leakage.

The building was constructed between 1973 and 1974 and is a one-story, split-level wood-framed structure. There are 10 units in the building, and each unit consists of two levels. The exterior walls are clad with vinyl siding. The typical roof structure consists of asphalt shingles, felt paper underlayment, wood sheathing on wood trusses and loose-fill/unfaced-batt fiberglass insulation over gypsum wallboard attached to the bottom chord members of the trusses. Party walls are wood stud framing sheathed with fire-rated gypsum wallboard. Numerous vents are present on the roof, and preformed perforated metal panels are integrated with the soffits. In each unit, the space conditioning systems consist of forced-air, natural gas units. These units are located in utility rooms on the lower level. No fresh air exchange system was installed in the units. A laundry room with a washer and dryer is located on the upper level of each unit.

Wiss, Janey, Elstner Associates attended a group meeting and performed a site inspection in February 2003. During our initial inspection, snow was present on the roof and the outside temperature was below freezing. Following are some of our findings.

Background information

During our inspection, we were informed by the maintenance representative of the townhouse association and residents the original construction used vertical beveled wood siding. Between 2000-01, vinyl siding was installed directly over the original wood siding. The asphalt shingles on the building were removed and replaced in summer 2002 because of hail damage. In October 2002, stains were observed on the ceiling of the upper level. Curtains on rear elevation windows were removed and replaced because of brown stains. The stains appeared to be caused by moisture that migrated from the top of the rear elevation windows.

No repair work had been performed on the washers and dryers or interior of the units preceding our site visit. A turbine attic vent was installed on the roof in one unit, and a portable dehumidifier was in the attic of another unit. These measures were taken by residents in an attempt to reduce attic humidity levels.

Interior inspection

We observed water stains on the ceilings, walls and millwork around windows. Several brown stains were observed on the upper-level ceiling, as shown in Photo 1. In one unit, portions of the ceiling had been painted in an attempt to conceal the brown stains. However, the stains redeveloped after the painting. In another unit, brown stains were observed on window frames, curtains and gypsum wallboard below the sills of north-facing windows. Most stains originated from the intersection of the roof deck and wall and were traced to melting frost and icicles that had formed on the deck's underside. Gaps and spaces were present between the perimeter of wall vents and gypsum wallboard in the upper-level ceiling.

Attic space

We also saw frost and melting icicles on the underside of the plywood roof deck, several nails that protruded through the decking and the roof vents' bottom surfaces. (See Photo 2.) The amount of frost diminished toward the eave. Water from melting ice and frost was dripping onto the truss framing, insulation and ceiling at many locations. The icicles were located directly above stains on the ceiling's underside. Brown stains were observed on the loose-fill insulation, which suggested previous freeze/thaw conditions in the attic.

No visible daylight was observed between the top chords of the roof trusses along the eaves. This suggested the eave vents were plugged or covered. This observation prompted removal of the aluminum soffit panels at an isolated location for further inspection of the venting at the eave. At this opening, we observed the vents in the aluminum panels were spaced 3 feet (0.9 m) on center. Above the aluminum panels, there was an original plywood soffit with vents spaced 8 feet (2.4 m) on center. The vents in the aluminum panels were not positioned over the vents in the original wood soffit (see Photo 3). The vent in the original soffit partially was blocked with loose-fill insulation. Significant air movement into the attic space was detected when the aluminum panel was removed.

An exhaust stack for the furnace was disconnected and vented directly into the attic in one unit, as shown in Photo 4. In addition, the exhaust duct for the dryer was lying on the bottom chord of the trusses and was not directed to the exterior. In this unit, the dryer was in operation during our inspection and significant moist air was discharging into the attic. This unit appeared to be the major contributing source of moisture accumulation in the attic. Also, the exhaust duct for a bathroom vent was disconnected at the housing.

The top ends of rigid exhaust vent pipes (from the kitchens and bathrooms) terminated flush with the roof deck's underside. No clamping device was used to secure the pipes to the roof deck. The edges of these pipes were leaning against the sides of the openings cut in the roof deck. These pipes appeared to be venting into the attic space as shown in Photo 5.

Loose-fill insulation was applied over the ceiling to a depth of about 6 inches (152 mm) in the original construction. The insulation felt wet to the touch and was water-stained at many locations. In one unit, the loose-fill insulation was overlaid with about 6 inches (152 mm) of unfaced-batt insulation, and a portable dehumidifier was in operation during our inspection.

Water stains were observed on the surface of a cast-iron vent stack. The stains appeared to be the result of water penetration through separations at the top of the pipe.

Roof system inspection

We observed the roof surface to document the overall condition of the asphalt shingles and exposed flashings and identify probable sources of water entry. We saw vent pipes and relief vents were installed, and the insect/bird screens installed at the ends of the dryer vents were plugged with lint about 1/4 of an inch (6.4 mm) thick. No visible water entry points were observed in the flashing at the vents or pipes. Vent stacks projected about 12 inches (305 mm) above the roof. Separations were observed at the tops of some vent stacks that appeared vulnerable to water penetration.


The investigative work we performed identified several likely moisture sources that caused or contributed to the ceiling stains.

At the eave, the new aluminum soffit panels that were installed during the remodeling were placed directly over the original plywood soffit. When this was done, the necessary precautions were not taken to ensure the vents in these two soffits coincided. This condition prevented adequate air movement through the eave, prevented proper ventilation of the space between the insulation and roof deck, and contributed to the formation of troublesome condensation occurring in the winter months. It also is likely a lack of ventilation contributed to warmer roof deck temperatures and melting of frost and icicles.

In addition, the dryer vent duct and furnace exhaust vent were disconnected and discharging moist, warm air directly into the attic space. The furnace pipe that was disconnected and exhausting directly into the attic represented a safety hazard and required immediate correction. This resulted in significant condensation within the attic.

Other conditions that contributed moisture into the attic space were loose connections in exhaust pipes, vent penetrations in the ceiling (creating air bypasses) and locations where vent pipes improperly terminated in the plane of the roof deck. Lint that had plugged the insect/bird screens at the outlets of dryer vents inhibited moisture discharge to the exterior. This likely contributed to elevated humidity levels within the townhomes. These conditions, combined with inadequate ventilation, caused condensation.

We also found water stains on the ceilings located directly below locations where separations were present at the tops of vent stack flashing boots.


Following is a list of recommendations for correcting condensation-related problems similar to those Wiss, Janey, Elstner Associates found in the building in St. Paul:

  • Install a fresh air exchange system to prevent elevated indoor humidity levels
  • Install continuous soffit and ridge vents
  • Seal all attic bypasses
  • Vent dryer and furnace flue pipes directly to the exterior
  • Properly seal all ductwork for airtightness
  • Install rain caps over roof vents to prevent water penetration

By taking these relatively simple precautions, moisture accumulation in attics can be avoided or, at least, significantly reduced.

Collins O.Y. Ofori-Amanfo is a senior enginneer for Wiss, Janney, Elstner Associates Inc., Minneapolis.


Be the first to comment. Please log in to leave a comment.