Rebuilding Fire Resilient Passive Houses in Colorado

Homeowners in the U.S. can build and improve their homes to be more resilient to increasing climate extremes. We aim to educate homeowners about climate-related risks and opportunities to reduce potential damage to homes and save lives. 

 

In this series of Stories, we share examples of homes built to withstand local climate risks. This blog focuses on a project designed to be a safe home for wildfire victims. 

 

The RESTORE Passive House is a predesigned passive house aimed to replace homes destroyed in the Marshal Fire in Colorado in December 2022, which burned an estimated 1084 structures, and another 149 were damaged.

The team completed the design process, and the city of Boulder granted the permit for the first projects. The construction of two homes began on February 2023 in Boulder County, CO.  

Andrew Michler, a passive house designer, acts as the passive house consultant and champion for these projects and helps design and develop an awareness of the passive house concepts and benefits. Michler cooperated with Joubert Builders, and Harrison Architects, an architect firm experienced in passive house construction, and led the consultant for the first certified passive house in Seattle. 

 

Climate Risks

The RESTORE passive house Colorado was designed primarily to address the climate risk of wildfires, smoke, and extreme temperatures for prolonged periods.

In recent years Boulder is experiencing more extreme temperatures and weather events, including a significant increase in the number of high-temperature summer days. A temperature above 90 degrees Fahrenheit is not uncommon anymore, even in the fall season.

 

In addition, the air quality in the region has decreased. Cooling houses by opening windows is no longer optimal due to smoke from wildfires, and other airborne conditions, such as the ozone. The ozone in the Denver metropolitan area in Boulder was downgraded from “serious” to “severe.” The ozone levels in the area are higher than EPA’s air quality standard, which can cause breathing-related issues such as bronchitis, emphysema, asthma, and skin and eye damage.  

The Marshall Fire in Colorado was a collective wake-up call to wildfire risk within urban and suburban surroundings. While it occurred outside the Boulder City limits, it was the most destructive in Colorado history. The Marshall Fire happened mid-winter, which illustrates that there is no such thing as a fire “season” anymore. 

Wildfires also pose a risk of smoke damage even to distant homes. The Marshall Fire Colorado caused severe smoke damage to many houses and prevented occupants from returning to their homes though their houses were not destroyed. 

Climate Resilience Strategies
 

Passive House is well known for its extraordinary energy efficiency, which increases the home’s climate resilience. It is designed to maintain comfortable climate conditions for occupants without a significant need for heating or cooling. In power outages, a passive house can maintain livable temperatures for up to 36 hours, despite extreme temperatures outside. The main components that allow it are the airtightness of the house’s envelope and the thick and continuous insulation in the walls. 

Passive houses are naturally more resilient to wildfires due to several characteristics based on the basic principles of the Passive House. Note that the strategies are fire resilient only if put together in a proper assembly.

The following are specific climate-resilient strategies specified in the Passive RESTORE House Design:

• Densely packed cellulose insulation treated for fire. All the air is pushed out, preventing air from entering the wall system, which makes it much less flammable.
   
• Two inches mineral wool exterior insulation system.
   
• An airtight membrane named Intello helps seal the envelope while allowing moisture out of the wall cavity to avoid excess build-up. The membrane controls air while operating as a valve to help buildings dry. Other similar membranes required by code don’t allow vapor moisture through, which can lead to moisture build-ups and mold.
  
  
   
• Passive House Institute (PHI) -Certified Energy Recovery Ventilation (ERV) unit called Brink distributed by 475 High Performance Building Supply. One fan brings fresh air into the house and distributes it in the living space. Another fan takes the air out of the house, usually from more contaminant-risk areas, such as the bathroom or kitchen. The air flows through a membrane that can recover energy when the air is replaced. This way, the house gets a complete air change every two or three hours. A Passive House always incorporates a fresh air system in an airtight building that controls the amount of air infiltration to the building and can be enhanced by additional air filters.
   
• Simplified Form Factor: The structure’s design is simplified, originally, to preserve heat. A simple design reduces the area exposed to exterior temperature, which decreases heat loss. This strategy also protects from wildfires as the simple design eliminates nooks and crannies where embers usually accumulate, thus is less susceptible for firebrands to lodge into the home’s fabric and ignite. Michler thoroughly explains this concept in a video that walks us through his own certified passive house.
   
• Metal roofs are less susceptible to wildfires and don’t ignite as easily as timber roof systems. The roof is considered the most vulnerable element of the house to flames, so it’s crucial choosing a fire-resistant roof. Choosing fire-resistant building materials wherever possible in a high fire-risk location can save lives and reduce damage.
   
• Vents are eliminated within the house’s structure to prevent embers from entering the house. Although most codes require ventilating the roof to allow air penetration and avoid moisture and mold buildup, it is one of the primary ways for buildings to burn down during fire events. Even with fire-retardant vents, a significant amount of hot air can enter the roof system through them. Instead, this passive house has unvented closed-roof systems. The insulation continues to the roof deck, preventing airflow. The Intello membranes installed under the roof system help dry the roof without airflow.
   
• Fiber cement or cement board sidings that are fire-resistant.
   
• Triple pane windows by Alpen with tempered glass prevent heat from the fire to break the glass and prevent the embers from entering the house and burning the house from the inside. The tempered glass increases energy efficiency and makes the glass stronger and much more fire-resilient.