Study Reveals Why Certain Homes Survived the Marshall Fire While Ones Around Them Did Not

One of the free lectures associated with the Oct. 1st tour of “green” homes was a fascinating presentation by Paul Kriescher of Bowman Consulting based on a study of the few homes which survived the Dec. 31st Marshall Fire while the houses around them burned to the ground. Click here for a PDF of Paul’s PowerPoint slides.

You’ll recall that it was the hurricane-force winds that were responsible for the fast spread of the Marshall Fire. Flying embers were what caused homes to catch fire in rapid succession. According to Paul, there’s a simple reason why those embers didn’t torch certain houses. It was because they didn’t get inside the homes or their attics.

The standard developer-built homes are “leaky” and built with ventilated attics. As I have explained previously, the standard procedure for finding and sealing the places where air can enter your home is to conduct a “blower-door test.”  (See graphic.) This involves installing a computerized fan in a doorway and sucking the air out of a house. The computer on that fan will tell you how leaky your house is — how many air changes per hour your home can expect during a certain wind speed. While that fan is operating, the technician can go through your house and determine all the places where air is coming into your home so that they can be caulked or otherwise sealed.

Many of those places are going to be around windows or on the rim joist — where your floor joists rest on the concrete foundation.

The goal is to get your home to a degree of air tightness at which you achieve two air changes per hour or less. Once you achieve that degree of air tightness, you then install an energy recovery ventilator (ERV) to bring filtered outside air into your home while expelling air from your home.

Making your living quarters more air tight can keep burning embers from entering your home. Combine that with having non-combustible exterior siding, decks and landscaping, and you go a long way toward preventing burning embers from being sucked into your home — and to keeping ash and smoke from making your home unlivable if it doesn’t burn down.

But the most critical area to seal is your attic. Your home probably has an attic which is vented. Blown-in insulation sits on your attic floor to keep your living quarters warm in the winter, while soffit vents combine with roof vents to draw outside air through your attic. This controls moisture buildup but is also ideal for drawing burning embers into your attic which can then light your entire house on fire.

Some builders have switched to building homes with “conditioned” attics, meaning that the underside of the roof is insulated and all vents eliminated. Thus, the attic itself is heated and cooled like the rest of the house. With no vents in your attic, those flying embers blow past your house instead of entering it.

There’s a subdivision in Arvada built by Meritage Homes called Richards Farm. It’s on the north side of 72nd Avenue, across from the Apex Center. Our agents were invited to tour it while it was under construction, and the builder showed us their conditioned attics. The reason the attics were conditioned had nothing to do with fire prevention. They were running heat ducts through the attic, and by insulating the attic, it made the ducts more efficient. But now we know the most important reason for conditioning an attic, and I suspect we’ll see building codes changed to require conditioned attics.

I learned another disadvantage of vented attics from participating in the 1994 Jimmy Carter Work Project, which built 30 Habitat for Humanity homes on the Cheyenne River Sioux Indian Reservation in Eagle Butte, South Dakota. Those one-story homes all had vented attics. Within months of completing those homes there was a blizzard which filled the attics of those homes with snow, which entered through the soffit vents. The snow then melted, causing the drywall ceilings to fall, causing immense damage. The homes had to be vacated and rebuilt on the inside the very next summer. The reservation had no building codes to follow, but if it did it would probably not have allowed vented attics for that reason.