Southrise: Challenges and Progress

In the July Hillside newsletter, you heard about the 12-unit Southrise building exceeding Passive House standards for airtightness. The fact that Southrise is a multifamily hempcrete structure makes this particularly exciting news: Very few hempcrete structures have ever hit these super low values. 

This newsletter will cover building challenges and progress made by the crew on Southrise. Jason Eaton, a foreman in the crew, was enthusiastic that they have surpassed their airtightness goals above and beyond what has been done before.

The Southrise building is making progress!

The number of leaks in the units is close to zero. “We’re not talking pencil-hole, we’re talking pinhole,” Jason described. And even though the scale is so small, the crew has still been able to locate these pinholes of air leakage and seal them up. 

The units are tested by placing a large fan in the front doorway to blow air out of the unit, creating negative pressure inside. During testing, the equipment supplied by Mike Browne of Advanced Building Analysis was initially unable to generate reliable air leakage readings because of insufficient airflow through its sensors.

Airtightness is measured in cubic feet per minute of air leakage, or CFM, and the square footage of the unit is taken into account. The CFM of air leakage of an average home is around 0.4, but Southrise had to have a score of less than 0.06 CFM/ft2 to be certified as a Passive House building. Our last test results revealed Southrise’s measurement to be just 0.01 cfm/ft2 of air leakage!

Blower door testing at Cottage Court; methods remain consistent at Southrise.

Overcoming obstacles

The construction crew had to go through lots of trial and error to get to this point. They learned a great deal from attempting certain methods and having to adjust course as they proceeded through the air sealing stages. 

A hempcrete wall before being smoothed and sprayed with plaster

Insulation and plaster are very important aspects of a good air seal, and temperature and weather changes can affect the application of these materials. Hempcrete turned out to be more cold-weather tolerant than anticipated, but the crew ran into trouble with natural plasters.

The first strategy was to use the exterior natural plaster as the primary air seal layer. The wall panels for Southrise were assembled on site, so the first coat of plaster was applied when the panels were horizontal. Hempcrete and plaster did not cooperate as temperature declined in the fall and winter of 2023 and too thick a layer of plaster had been applied.

The slow curing speed of the hempcrete when covered with excess plaster led to freezing damage. Even panels that had been plastered in August earlier that year were damaged, months before temperatures dipped below freezing. Curing rates had decelerated enough for frozen moisture to compromise bonding between the hemp fibers and the plaster.  

Jason, along with the crew, has now managed to repair the freezing damage to the plaster and outer layer of hempcrete in the last month. “We might be the crew with the most concentrated knowledge of these building techniques, just from the amount of trial and error we’ve gone through.” 

Changing course

Because of the obstacles that occurred with exterior plaster, air sealing would have to be accomplished in a different way. The solution was Zip R6, or insulation panels an inch and a half thick, on all interior walls. 

Zip insulation inside a Southrise unit

Now, in October, the crew is putting the final coat of plaster on the outside of Southrise, and it’s looking good! Their goal is to meet Passive House air seal certification on all of the Southrise units. Jason reported that the unit with the weakest air seal beat the Passive House standard by a factor of two, and the strongest unit beat it by a factor of 40. “We were told we should change careers and build submarines instead!”

The final step: Outer wall plaster

What are the benefits of an airtight home?

The air quality of a Southrise unit will be unmatched. When completed, each unit will receive a Zehnder Energy Recovery System that is constantly pulling fresh, filtered, outdoor air into the unit and discharging air from bathrooms and kitchens at 92% energy transfer efficiency. This ensures fresh air is brought into the units, without compromising the temperature inside.

A Zehnder unit in Cottage Court

Indoor air is constantly being filtered through MERV13 class filters that reduce airborne allergens, and particulates generated from wildfires and vehicle traffic. Conservation of temperature-regulated fresh air is another benefit. Hempcrete is quality outer wall insulation, but the air seal brings it to a new level of effectiveness when keeping heat inside. With solar energy providing electricity to the units, and a tight air seal, we’re giving residents complete control over the air that they breathe at zero cost in heating and electricity bills.

By David Hall and Sadie Woodward