It’s the classic Catch-22 of building envelope design: By keeping water vapor out of a home through insulation and air sealing, you also keep it in. This conundrum was explored in depth at an educational session last week at the 2012 Greenbuild International Conference and Expo in San Francisco.

Passive House expert Prudence Ferreira urged attendees not to overlook moisture mitigation in new and remodeled high-performance homes. “It’s great if you’re building an energy-efficient home with beautiful fixtures and finishes,” she said. “But if it starts growing mold in 10 years, you’ve kind of shot yourself in the foot in terms of sustainability.”

To avoid moist interiors that can lead to unhealthy mold growth, it’s important to prevent both liquid and vapor moisture from intruding on a home. “If you’re not controlling your moisture flow, it can lead to building failure,” Ferreira said. “Moisture gets into a home quickly and moves out much more slowly through vapor diffusion.”

Vapor control is especially important for tightly sealed, well-insulated homes, said Ferreira, principal of Integral Impact. “Just going for thermal control and not thinking about how vapor is or is not going to move through the house is a bad idea.”

Ferreira and fellow presenters Achilles Karagiosis, global director of building science for Owens Corning, and Florian Antretter of Hygrothermal Building Analysis offered the following tips to help building pros win the battle over moisture infiltration:

--Rainscreens, which accelerate the removal of liquid water and use air flow to enhance drying, are highly recommended. (Click here for a Building Science article on rainscreens.

--Ferreria likes Class 3 vapor barriers for their permeability. “If you use Class 1 or 2, you start playing with fire a little bit,” she said. “The more vapor open you leave things, the better we’re finding it is for the house, especially in super tight Passive House construction.”

--Permeable air barriers include plywood, plaster, and  latex or enamel paint over gypsum board. Avoid oil paint, which is a Class 1 vapor barrier.

--Adequate insulation is important not only for energy efficiency, but to keep out moisture as well. Dewpoints occur because cold air can’t hold as much moisture as warm, and it condenses inside a home that is not thoroughly insulated. “Ninety percent of moisture in a building assembly comes from the air, not from water itself,” Ferreira pointed out.

--Control humidity through continuous, balanced ventilation. “Remember that if there is greater humidity on the outside of the home, the water will want to come inside and vice versa," she said.

--Think about the temperature setting inside the home. Vapor load increases exponentially when indoor air is much colder than outdoor, Karagiosis said, increasing the potential for condensation.

--Learn about the WUFI modeling program, available for free download here. It encompasses climate, construction, and material data and shows energy and moisture fluxes into and out of a home. “It allows users to really assess the overall performance of the whole building,” said Antretter.