Little Room for Error
Stricter Code Requirements And Modern Materials Have Raised The Stakes For Today's Builders
Today's homes are engineered for comfort and healthier living conditions. Their walls are sealed to eliminate drafts, and they include thick thermal insulation as well as high-performance heating, cooling and fresh-air ventilation systems. They're a far cry from older homes.
They're about to become even more so. Pending requirements set by the 2021 version of the International Energy Conservation Code will push the average new home close to Net Zero Ready, which means it will use so little power that a modest solar array will satisfy its annual needs. Subsequent code updates promise to be even stricter.
Not surprisingly, many homeowners worry about the unintended consequences of this trend, especially the code's air sealing requirements. Common questions we hear include "Don't walls need to breathe?" and "Won't a tight wall trap stale air in the home and make us sick?"
These are legitimate concerns. Air sealing and insulation do create risks. Fortunately, those risks can be fully mitigated by someone with knowledge and skill—the most knowledgeable of today's builders and designers will approach the house as an interdependent system in which walls, roofs, windows, insulation, air sealing and mechanical equipment all play key roles.
Success at this approach requires a firm grasp of building science—a discipline that describes how heat, moisture and air flow through a structure and how they interact with one another and with the home's mechanical equipment. Building science also includes best practices for keeping rain and ground water out of the structure (no small matter considering that 80% of building failures are water-related).
Training in building science also helps builders understand the limitations imposed by today's materials. Take the example of framing lumber. Thirty years ago, framing lumber came from mature trees and was able to absorb lots of moisture without problems. That lumber was placed in drafty walls where it could easily dry out. The drying happened automatically and didn't require special consideration.
But old-growth lumber is no longer available. In today's homes, engineered and new-growth lumber—neither of which stores much moisture—are put into nearly airtight walls. The builder has to be very deliberate about making sure the wall will be able to dry.
The bottom line is that ever-more-strict code requirements implemented by a well-intentioned but uneducated builder can lead to moisture problems and stale indoor air. On the other hand, the new code requirements give the educated pro a helpful roadmap for creating a home that's comfortable, quiet and healthy.
Which brings us back to the "walls need to breathe" question. The answer is that walls don't need to breathe but people do, which is why fresh-air ventilation takes on more importance in today's homes. What walls do need is the ability to dry out when they get wet, even when tightly sealed and framed with new-growth lumber. It's like your clothing: you want it to retain body heat, but to release sweat and other vapors that you don't want.
The construction details needed to ensure that result differ by whether you live in a hot, cold or mixed climate and whether it's dry or humid outside most of the year. That's why you need an educated local builder who knows what does and doesn't work in your particular climate.
Q: What does housewrap do?
A: Housewrap (known in the trades as weather-resistant barrier, or WRB) helps keep drafts out of the house and also provides a secondary drainage plane for any water that gets behind the siding. But while it sheds liquid water, it’s also permeable enough to let water vapor from inside the structure escape to the outside, which helps keeps moisture from accumulating in living spaces and wall cavities. Some sheathing products have a protective overlay that takes the place of housewrap.