Q: Should the exit plumbing be pressure-tested for leaks in today’s super-tight houses?

A: Steve Demetrick, a residential builder and remodeling contractor in Wakefield, R.I., responds: The integrity of the waste plumbing is definitely something to be thinking about when you’re building any kind of house, including a super-tight one. We recently completed a new Certified Passive House in Rhode Island, and we made sure that the exit plumbing was pressure-tested before attaching it to the sewer line (the attachment point to the sewer line was outside of the air barrier of the house).

Along the same lines, it is also wise to pay attention to any water line traps in a tight house, especially the smaller lines from minisplits that are hard piped into the drain lines. These traps are typically made from smaller-diameter pipes that only drain water during the summer. A long, dry winter can easily dry out one of these traps and allow sewer gases into the house under the right air pressure conditions.

I’m aware of this phenomenon from working on large summer homes in my area. A couple of times, clients who live elsewhere for nine months of the year wondered why their house smelled like sewer gas after I had remodeled one of the bathrooms over the winter. In every case, a trap in a different bathroom—which hadn’t been used for all those months—had dried up and was letting sewer gas into the home. Summer homes with smart caretakers don’t usually have that problem.

But for the gases to be drawn into the house, there has to be negative pressure inside. In a properly built super-tight house, the ventilation system should be balanced. A balance between the amount of intake air and exhaust air will minimize areas of negative pressure in the house, minimizing the chance of sewer gases entering the house.

For super-tight houses, it may be worth mentioning that if a trap does dry out, the gas won’t necessarily flow into the house. In order for air to flow into the house, you need two openings—in this case, one would be the sewer line, and the other would be a hole somewhere else in the house—along with a pressure difference between the inside of the house and the outside. If this situation existed, it would create suction on the open sewer pipe and draw gases inside.

But that other hole and the pressure difference are less likely to exist in a super-tight house. To give you some perspective, when you open a window in a Passive House in the winter in New England, the cold air doesn’t rush into the house, as it would in a normal leaky house. These houses are so tight that the air doesn’t have anywhere to go.