Discussions about moisture buildup and its effects are likely to be among the most enduring conversations in homebuilding and remodeling, particularly as more homeowners opt for better-insulated and more-airtight exterior walls.

In his Musings of an Energy Nerd column, GBA Advisor Martin Holladay recently sorted through key considerations for designing exterior walls that manage water condensation and perform well when exposed to cold weather. The variables in Martin’s analysis include the indoor relative humidity and the temperature of a wall component, such as the sheathing. If the sheathing gets cold enough and the indoor relative humidity is high enough, condensation can collect on the sheathing.

The condensation issue came to mind again this month when we spotted an Alaska Daily News story (sign-in required) about mold problems in houses in Fairbanks, where home energy conservation has been gaining importance since the 1970s, when fuel costs began to rise. The stimulus-funded expansion of the Weatherization Assistance Program and recent home-energy rebates have further advanced weatherization activity in the area.

The ventilation issue

But as more homeowners add insulation to attics and exterior walls, and air-seal the shell as best they can, complaints about mold have grown commensurately. A building inspector and retired energy auditor interviewed for the story noted that the design of the exterior-wall systems in the afflicted homes plays the most prominent role in mold growth, related health issues, and structural damage precipitated by long-term condensation problems.

Because these are existing buildings whose exterior walls are unlikely to be redesigned and rebuilt, however, homeowners are advised to examine a factor they can address more inexpensively: ventilation.

“People are still too often addressing one side of the energy equation … making walls thicker, increasing R-values and tightening homes. They are not addressing ventilation,” Steve Shuttleworth, a building official for the city of Fairbanks, told the paper.

As noted in another Musings of an Energy Nerd column, “Designing a Good Ventilation System,” builders nowadays typically choose from among three mechanical ventilation systems: an exhaust-only ventilation system based on one or more bath exhaust fans (this is the simplest system); a central-fan-integrated supply ventilation system (for better fresh air distribution); a heat-recovery ventilator or an energy-recovery ventilator connected to a dedicated duct system (the system that will operate at the lowest cost).

The choice depends on factors such as the size, layout of the house, and the budget of the homeowner. Martin’s column provides a thorough description of the options.