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Q&A Spotlight

Designing a High-Performance Wall in Wildfire Country

This owner-builder wants walls with a high R-value that address the region’s high risk of fire

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Wildfire testing: A test wall undergoes a destructive test at the Insurance Institute for Business & Home Safety Research Center. The organization is a good source of information for builders working in areas prone to wildfires.
Image Credit: IBHS
Wildfire testing: A test wall undergoes a destructive test at the Insurance Institute for Business & Home Safety Research Center. The organization is a good source of information for builders working in areas prone to wildfires.
Image Credit: IBHS
Windblown embers are a serious risk to structures during wildfires. Building details that minimize possible entry points help reduce the risk of fire. This test took place at the IBHS Research Center.

Will Welch has chosen to build his high-performance house in Nederland, Colorado. The site is at the border of Climate Zones 6 and 7, and it poses some challenges: it’s at an elevation of 8,600 feet; the area gets a generous amount of snow and wind; and the number of heating degree days tops 8,800 a year.

But Welch has one more concern: the threat of wildfires.

“My priorities are a tight building envelope with high-R walls and more environmentally friendly materials that also resist fire, but I’m also trying to get the most bang for the buck on those materials,” Welch writes in a post at the Q&A forum.

His plans currently call for a 2×6 framed wall rather than a double-stud wall, mainly as a way of preserving all the interior space he can. Wall cavities would be insulated with dense-packed cellulose or a mixture of closed-cell spray foam and cellulose, with 5/8-inch drywall on the inside and 5/8-inch plywood sheathing on the outside. On the exterior, Welch would use 4 inches of mineral wool insulation, covered with a mix of corrugated metal and fiber-cement siding.

“I’m leaning away from rigid foams for the exterior insulation because a number of them either don’t perform well in fires or in the cold, have thermal drift over 5-10 years, or are rough on the environment,” Welch says. “That said, exterior mineral wool doesn’t look nearly as cost-effective as something like recycled rigid foam.”

Welch has three questions:

  • Is his choice of mineral wool over rigid foam reasonable?
  • Given the climate, what is the best location for the water-resistive barrier (WRB)?
  • Should he add 2 inches of closed-cell spray foam to the inside face of the wall sheathing to increase the wall’s R-value?

That’s the topic for this Q&A Spotlight.

The closed-cell foam option

GBA editor Martin Holladay thinks that selecting exterior mineral wool rather than rigid foam makes sense if fire safety is high on Welch’s list or priorities.

But the case for installing 2 inches of closed-cell spray polyurethane foam into wall cavities isn’t very strong because it would add only about R-5 to the total wall assembly compared to just using cellulose between the studs.

Maybe so, Welch replies, but the foam also would have value as an air barrier — more so than the 2 inches of cellulose it would replace.

As for locating the WRB, Holladay says it may be installed either between the plywood sheathing and the exterior mineral wool, or between the exterior mineral wool and the furring strips. He suggested that Welch consult a GBA article on this issue, “Where Does the Housewrap Go?”

“The main factor that drives the decision on WRB location is the the integration of the WRB with your window flashing,” he says, “and that depends on whether your windows are innies or outies.”

What about a double-stud wall?

Would Welch’s choice of a 2×6 frame with exterior insulation really provide much more interior floor space than a double-stud wall? Some readers don’t think so.

“Other considerations aside, the difference in depth of your two wall options are pretty close,” writes Malcolm Taylor. “[You’re comparing] a double-stud wall at around 12 inches and a 2×6 wall with 4 inches of exterior insulation at around 10 inches. You aren’t getting much more floor space with the mineral wool.”

If losing too much interior space is a concern with a double-stud wall, Robert Opaluch adds, just increase the footprint of the foundation accordingly. Dimensions should be in multiples of 2 feet or 4 feet so the sheathing would line up with studs with less cutting.

“I’ve been having the 2×6 vs. double-stud debate for months myself (it seems like an enduring one on this site and others),” Welch replies. “If I were to build a roughly 25′ x 80′ rectangular structure, a 12-inch double-stud would increase my foundation by 5%, and with the additional lumber, I think it would be more costly than a 2×6 wall with exterior insulation because the exterior insulation doesn’t require foundation underneath it, correct?

“Some or all of the double-stud lumber cost would be offset by the cost of the exterior insulation, and the labor might be a wash if I can find a contractor with exterior insulation experience, but the double-stud foundation would cost me more,” he continues. “Is it just preference at this point (2×6 with exterior insulation vs. double-stud) or would one of these designs actually be more cost-effective with my cold climate and wildfire concerns?”

Lowering fire risks

Tim R. has another concern about adding a layer of exterior insulation: building details that could increase the risk of fire.

“I would be concerned about the exterior insulation with the wood furring strip and possible gaps where embers could get in and ignite the plywood strips,” Tim explains. “The metal siding also needs its gaps filled to prevent ember intrusion.”

Building codes for the Wildland-Urban Interface in San Diego County, California, might be useful, Tim says. And for extra protection, Welch could consider adding gypsum sheathing on the exterior of the walls to get a fire-rated wall assembly.

Opaluch suggests that any wall or attic venting should include screens to prevent airborne embers from getting inside in the event of fire. Other tips: avoid using roof trusses because they are more vulnerable to fire; chose fiberglass window frames over vinyl window frames; consider mineral-wool insulated window shutters; and pay careful attention to landscaping practices that minimize fire risk.

Omitting exterior insulation

Continuous exterior insulation is an effective way of reducing thermal bridging, but so is insulation on the interior, says Jon R., leaving just gypsum sheathing and siding on the exterior.

“Exterior gypsum sheathing (no plywood) combined with double studs, continuous rigid foam on the interior side, or strips of polyiso rigid foam on the interior side studs addresses thermal bridging without flammable exterior furring,” Jon R. writes. “Strips are hard to beat on material cost and R-value per inch. If you do go with exterior insulation, the exterior furring strips could probably be treated to be more fire-resistant.”

He refers Welch to an article by a builder named Stephen Bonfiglioli detailing his technique of adding strips of rigid foam to the inside edges of 2-by framing.

If he were to choose that route, adds Taylor, Welch should plan on doing the work himself. “The labor component is too high to get a contractor involved,” he says.

Some thoughts on advanced framing

Some builders frame houses so that wall studs fall on 24-inch rather than 16-inch centers — one element of a framing method called “advanced framing.” This reduces the amount of thermal bridging, making for higher overall R-values by increasing the amount of wall cavity that can be devoted to insulation.

But, suggests Roger Berry, proceed cautiously.

“I would be very concerned with wind loading if not in a very protected spot,” he says. “My house grumbles in some of the 50+ mph gusts we get and my framing is rather tank-like. The idea of 24-inch framing may become more theory than reality once you have windows and doors placed. A plan with lots of free wall area might yield some gains against thermal bridging, but with exterior insulation I suspect the difference might pencil out to be quite trivial.”

Finding a builder familiar with advanced framing techniques could prove to be a challenge, he adds, and the price difference between 5/8-inch sheathing and the more typical 1/2-inch sheathing for 16-inch on-center framing is probably worth considering. Further, 24-inch on-center framing may not meet requirements for the siding Welch uses.

Berry also makes these suggestions:

  • Check on fire insurance options now rather than later, and make certain that building details such as a sprinkler system or roof assembly will pass muster with insurers.
  • A simple roof plan will minimize the number of “ember traps” regardless of roofing materials.
  • Avoid 16-foot wide garage doors and use multiple smaller doors instead to reduce the risk that high winds will buckle the doors and allow burning embers inside the house.
  • Buy the best windows you can afford, and if Welch must choose vinyl frames it would be best to avoid dark colors.
  • A foam break to reduce thermal bridging will be more effective on the exterior than it would on the interior, and adding foam strips to the interior seems “very labor-intensive and fussy.”

Our expert’s opinion

Peter Yost, GBA’s technical director, added these thoughts:

When I was at Building Science Corp., Steve Quarles of the University of California Berkeley Center for Fire Research and Outreach worked with us to integrate wildfire and moisture management strategies in buildings in areas prone to wildfire. It was really cool stuff. Quarles is now Chief Scientist for Wildfire and Durability with the Insurance Institute for Business & Home Safety (IBHS). I gave him a call and asked for his recommendations for Will Welch’s home and any home in an area prone to wildfire. Here are the points he made:

  • Get the Wildfire Partners designation available in Boulder County, Colorado. For new homes in Boulder County, there is really no difference between code compliance and the Wildfire Partners designation, but the designation can be helpful in obtaining fire insurance. Wildfire Partners, a program strongly recommended by Steve, works extensively with existing homes and businesses in wildfire-prone areas.
  • Address structure and site equally. Wildfires often jump to structures as wind-blown brands or embers (see Image #2 below). You need both maximum defensible space around your home and structural defense, including simple designs, fire-resistant and heat-resistant materials, and fire-friendly construction details.
  • Get the roof right; it is the most vulnerable part of the structure. Chose a simple design that creates the fewest pockets where debris can accumulate. The roof should have Class A cladding or be rated Class A by assembly. Class A claddings are materials like clay tiles. Standing-seam metal roofs can be Class A by assembly if rolled roofing is used under the standing-seam metal cladding. Finally, include closed, horizontal soffits. The roof framing is not the problem; it’s the places where embers and brands get purchase, such as open-framed eaves. Closed soffits provide less purchase, and closed horizontal soffits provide less purchase than sloped closed soffits.
  • If you vent assemblies, use 16-gauge wire with a 1/8-inch grid and consider intumescent paint on vents. The more robust wire and smaller weave better protects against sparks and embers; the intumescent paint can be heat-activated to close off the vents.
  • Detail decks correctly. Gap deck boards by 1/4 inch instead of 1/8 inch. Increase joist spacing to 24 inches on center. Face the top of each joist with self-adhering foil-faced tape. And use a deck board that qualifies for California Building Code Chapter 7A. For more information, see this fact sheet from the National Fire Protection Association.
  • Invest in tempered glass in your windows. Tempered glass is three to four times more heat-resistance than standard annealed glass.
  • Go with rigid mineral wool to match the fire-resistance of your wall claddings. Steve chose the rigid mineral wool over the Kooltherm, but he emphasized that a true clear space of at least 6 inches between finished grade and the bottom of the cladding is critical. Yes, 6 to 8 inches clear space is in every building code, but this is a pretty common deficiency in new homes.
  • Use the information resources that IBHS has created. They are the most extensive and up-to-date resources and references for wildfire management available.


  1. Peter L | | #1

    Why not ICF?
    Just curious as to why ICF with a 6" concrete core was not used as an option?

    The concrete wall structure is fire rated for 4 hours instead of 20-30 minutes as most wood frame walls are. You can then add synthetic stucco to the exterior to fireproof the EPS. There are NO weep screeds, exposed cavities, drainage planes, etc. in the wall assembly to have embers travel into. Synthetic stucco leaves no gaps for embers or bugs to get into.

    Plus in areas of high winds, ICF is rated for 200mph winds. No wind racking in an ICF home. My ICF house is dead silent in 50mph winds. I live in wildfire country and received discounts from my home owners insurance due to the concrete wall. Plus I have a Class A fire rated standing seam metal roof.

  2. User avater GBA Editor
    Martin Holladay | | #2

    Response to Peter L
    This article summarizes a Q&A thread. In the original thread, a GBA reader named Will Welch announced that he was planning to build a house with 2x6 walls. He asked a few specific questions, and the GBA community did our best to answer his questions.

    Other readers, at other times, have announced their plan to build a house with ICF walls. It's certainly one way to build. It just wasn't Will Welch's way.

  3. Christopher Peck | | #3

    Is it really possible to ignite furring strips?
    John Straube was here (Sonoma County, CA) a couple weeks ago presenting on continuous insulation and someone asked him if furring strips could catch fire. He said "no, a 3/4" furring strip just doesn't have room to catch." He went on to say maybe if the gap was a couple inches, maybe. It was an important answer to me since I have remodeled my entire house with continuous foil-faced poly-iso, 3/4" thick furring strips, and HardiPlank siding. The thought that I had inadvertently introduced an ignition source was disconcerting, so I was glad to have John confirm my thought. But it would be nice to hear another expert poke a hole, or possibly confirm, this fear.

  4. User avater GBA Editor
    Martin Holladay | | #5

    Relevant quote from Building Science Corp. document
    In the document that John Clark provided a link to, Joe Lstiburek wrote:

    "The fire implications of too large of an air gap behind a cladding are huge. Air flowing behind a cladding can feed a fire in the cladding or feed a fire in the insulation behind the cladding. Both happened in spades in the Grenfell Tower. The buoyancy of fire heated air flowing upwards in the air gap behind the cladding acted like a blow torch or a bellows feeding the fire. When the air gap is small…less than ¾ inch…the friction from both surfaces bounding the air gap limits the air flow. The boundary layer on both surfaces is an effective fire stop."

  5. Christopher Peck | | #6

    Thank you John and Martin for the confirmation
    I do recall Joe's comments about Grenfell, so that's some consolation my memory isn't totally failing.

  6. John Burk | | #7

    Steel siding
    My build is 2x6 0n 16" with 1/2" ply, 4" reclaimed foam, 2" sprayed closed cell (dense pack unavailable in this part of Ontario), unventilated cath with 6" foam exterior and 2" sprayed, Vic West roof. Nice to hear the comments on strapping.
    Looking into Tru Log steel siding out of Colorado, anybody used it? Original plan was Hardie.

    Very happy with my build, quiet and stable interior temps.

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