Of course, most buildings have some sort of mechanical ventilation system to create a desired flow of conditioned air along a desired path. These systems work best when infiltration from the building enclosure is minimized or at least controlled. If the ventilation system is attempting to pressurize a space, that pressure will be compromised if air is being pushed out through unintended openings in the enclosure. Conversely, a room with intended negative air pressure will be prone to pull outdoor air and associated contaminants through any leaks in the building enclosure, causing increased stress on the mechanical system. The impacts of this leakage on the energy cost and operation of the equipment are direct and obvious. Once again, the key to achieving the desired performance is finding the right balance between desirable air flows and unwanted air infiltration.

Moisture control. Common everyday routines of the inhabitants of a building can produce airborne water vapor which must be addressed and controlled to prevent potential problems. Some of the sources of this water vapor or moisture can include basic human functions like respiration and perspiration, particularly prevalent where large groups of people occupy the building. Operating fundamental building elements such as appliances, equipment, or using showers are a clear source of adding water vapor to the air. And of course there are unintended additions such as pipe leaks or other malfunctions that need to be addressed as they happen. Toward all of these ends, it can be important to understand the intended uses and activities inside a building from the beginning of design to be sure that there is an appropriate understanding of the potential moisture level in a building. Otherwise, it is possible that any ventilation system is simply circulating humid air throughout the whole building.

With the indoor moisture sources are accounted for, our focus turns to controlling its movement, particularly in terms of the building enclosure. There are essentially two ways that moisture moves from inside a building into an enclosure assembly. The first is capillary action where a material or product is porous enough to allow water to permeate through it or even wick it from one location to another in the material. In this case water vapor diffuses through a material (like gypsum board) from a high pressure area to a low pressure area. Typically, the amount of water transferred in this manner is fairly small unless there are unusually humid or high pressure conditions. The second common means of movement is the bulk transfer of moisture. Here moisture flows with air through an opening or gap in the assembly in part due to air pressurization differences that can move and carry substantial amounts of water vapor with it. This is the usual and largest cause of unwanted moisture movement into an enclosure assembly. In either case, that moisture can end up condensing inside an assembly, cause water droplets to form, and lead to deterioration, mold or both.

Turning to the exterior of the building, leaks in the exterior layers can allow rain, snow, or ice to infiltrate a building enclosure system. From there it can run down into the building from above, it can be pulled sideways when the wind blows or even be wicked up from below if the source is soil or other grade level (or below) water sources. Wind can worsen this situation since high pressure forces moisture to areas of lower pressure, i.e. outside to inside.

Design Considerations: Connecting the Dots

In the normal context of designing and renovating buildings, how does all of the above come together? Overall, the key is found in maintaining the right balance between all of these performance issues with the rest of the building design criteria. Of course, if we try to create a building that is perfectly formed, functions perfectly and is totally cost contained, we may find ourselves never building that building since it likely doesn’t exist. Economy must be balanced with the aesthetics and function including controlling sources of problems in assemblies that can’t be accessed after construction. Nonetheless, there are several key points to look at holistically in the process of creating successful building designs.

Accounting for Climate
As noted, the local climate plays a big part in how the enclosure performs and how the assemblies in the envelope are designed. In cold climates the concerns can include thermal bridging in winter which not only leaks heat it can create cold building surfaces that produce frozen condensation. This then becomes a dormant issue that is delayed for weeks or months until temperatures rise and the condensation melts. The resultant water becomes a problem that could have been avoided if the thermal bridge was recognized and treated in the design and construction process using truly continuous insulation. In a hot and humid climate, the thermal bridging issue becomes an issue in the summer as heat is absorbed and wicked into the building, increasing the air conditioning load.

Vapor and moisture penetration are affected by climate as well. Since the intent of a vapor barrier is to avoid condensation occurring on components inside an enclosure assembly the conventional approach is to place that barrier on the side of the enclosure with the most moisture, which is typically the warmer side. Therefore, the location of the vapor barrier is tied directly to the location of the building. So, for a northern climate like Minnesota, conventional practice would locate a vapor retarder to the warmer, more humid side which would be the heated interior side in this case. Conversely, in a warm climate like Florida placing the vapor retarder on the warm side means it goes toward the exterior of the assembly heated by the sun and climate, not on the typically air conditioned interior side. But what is the right location in a mixed climate of hot and cold temperatures? In those cases, there may be reasons to keep options open to allow for a predominant or variable vapor direction. In a coastal climate like the northwestern US that receives lots of rain and humidity, it will be important to allow for drainage and drying in enclosure assemblies. In the end, the whole point is to keep moisture out of the enclosure while maintaining comfortable interior temperature and humidity levels.