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Building With ICFs in a Fire-Prone Region

Lesley_Farm_Girl | Posted in General Questions on

So, we are building a house and have started an ICF basement. We are considering continuing the main floor walls in ICF and possibly the roof since the cost of lumber has increased so dramaticly. We live in Central Montana. I like the idea of having an air tight house that would be fire and natural disaster proof for the most part. Anyway, our original house plan is not for ICF but for 2×6 wood wall construction. We are using Amvic blocks. I noticed the site has design software. Do we need an engineer to plan our changes and how expensive might that be? I’m interested in the roof system, but my husband is nervous about having concrete on the roof and the weight.  He was interested in having a steel beam roof. I also mentioned that some people use SIPS for their roofs. Do you have any advice? Also, what are your thoughts on the R value of ICFs and would you need additional insulation?

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  1. Expert Member
    RICHARD EVANS | | #1


    I think it is great that you are doing your research to build the best home possible. I also think it is great that you've come to GBA for advice on next steps. Welcome!

    Personally, I would never use ICFs in an area prone to wildfire. Sure, your walls might still be standing after the blaze, but then what? The EPS foam would have melted away along with your siding. Interior EPS foam would have also at least partially melted if not completely, leaving protruding plastic ribs everywhere. The interior and exterior walls would almost certainly be uneven as the EPS would have melted irregularly.

    Restoring such a melted mess back to proper living conditions would be expensive and nasty work. Tearing it down would be expensive too as you would need to bust-up concrete and re-bar.

    ICF forms typically have an R-value of around R22. This likely doesn't even meet code in your climate but builders use them anyway. Changes to ICF walls (such as moving a window/door) can be tricky and may require an engineer. SIPS roofs carry numerous risks if not executed perfectly.

    You can build the way you proposed, but your house would not be very energy efficient. It would not offer any advantages in the event of a wildfire. A home built this way (with ICFs, Steel Beams/ SIPS/ concrete roof) would also have a dramatic impact on the environment compared to conventional lumber. You could likely build a house with lumber and cellulose and then burn hundreds of barrels of diesel fuel in your yard and still not achieve the embodied energy of the home your are proposing.

    I realize lumber costs have become ridiculous. If I were in your shoes, I would be exploring alternatives as well. I would still stick with stick-framing though. Do some research on Advanced Framing. Peter Yost's article on GBA this morning is a great starting place, given your project.

    Best of luck!

  2. GBA Editor
    Kiley Jacques | | #2

    I second Dana’s recommendation to go with stick framing, despite the spike in costs. In addition to the article he referenced,
    Materials Matters, you might want to read this piece: Three Easy and Essential Advanced Framing Techniques. And, given your justified concerns around building in a fire-prone region, consider reading
    Designing a High-Performance Wall in Wildfire Country. Between the three posts, you are sure to glean information to help you with your decision making. And, no doubt, they will lead to more questions for the GBA community. Welcome aboard!

    1. Lesley_Farm_Girl | | #10


      I was looking at advanced framing earlier... 2x6 walls with cellulose and two layers of mineral wool on the exterior and being clad in corregated metal. Would we have to use ferrowing strips to attach the metal siding to the exterior siding?

      There are several reasons I am leaning towards ICF... The lack of people in my area willing to try new building practices... With multiple layers, the availability and cost of lumber at this time, the simplicity of an all-in-one system (ICF), the record of positive outcomes after natural disasters for ICF, the air tightness and thermal bridging factor, etc...

      I am definitely a person who would like to build environmentally consciously. But, I also wonder if this house will be standing for numberous years to come if it would drastically cut the impact over time.

  3. Jon_R | | #3

    Stick framing plus exterior gypsum (eg DensGlass, DensDeck) makes sense to me (for fire).

  4. jackofalltrades777 | | #4

    Looking at data from wildfires and looking at what happened to ICF homes AFTER a wildfire hit that area, is the best way to answer your question.

    The Camp Fire in 2018 in Paradise, CA was devastating. One home that survived direct nearby flames was an ICF home. Many other recent fires have factual evidence that ICF/concrete homes not only survived fires but had little to no damage and were deemed inhabitable without having to do any work on them. Most ICF homes are finished off in synthetic stucco and not "siding" as noted.

    During fire wall testing, ICF walls (6" concrete core) are subjected to continuous gas flames and temperatures of up to 2000°F for as long as 4 hours. None of the ICF walls failed while the wood frame walls collapsed within 30-45 minutes. The concrete core also prevented the fire by slowing the conduction of heat from one side of the wall to the other by not allowing enough heat through to start a fire on the cool side of the wall for 3 hours.

    The International Building Code gives ICF walls (6" core) a 4 hour fire rating while wood frame walls get 20 minutes.

    1. JC72 | | #6

      @Peter L Has there been inspection of the concrete core? Concrete when subjected to intense heat will crack and flake off and if hot enough fire will weaken the re-bar.

      1. jackofalltrades777 | | #7


        To my understanding the ICF houses shown and the ones I read about that survived wildfires, were structurally sound and intact.

  5. Expert Member
    Peter Engle | | #5

    The 4 hour fire rating is helpful, but it is more oriented towards the structural safety of the building and its occupants than the cost of restoration after a fire. The concrete cores will survive the fire, preventing collapse of the structure, but it might still be an expensive repair. I have seen several cases where the radiant heat from a gas grill was enough to melt the EPS foam behind a synthetic stucco skin. The wall looked normal, but there was nothing but air behind the skins, as the EPS had melted into a blob. It takes surprisingly little heat to do this. I have no doubt that the radiant heat from a wildfire would do significantly more damage than that. Still, tearing off the skin, adding a new layer of EPS and synthetic stucco would be cheap compared to replacing the entire house and contents.
    FEMA has some good information and does endorse the use of ICF construction. They do recommend cement stucco or masonry skins over ICF so the cores don't melt. If you go with ICF, you should consider using cement stucco rather than synthetic. A full 1" of stucco on metal lath would transmit far less heat than the synthetic stucco skins. Another choice would be 5/8" Dens-Glas sheathing with direct-applied synthetic stucco over that.

    1. jackofalltrades777 | | #8


      Nice name ;)

      As mentioned, even if the EPS foam melts on the outside of ICF homes, which happened in one wildfire that was studied. They simply installed new foam utilizing concrete screws and washers. They then added the stucco coat and all was back to normal. The concrete wall remains structurally sound which means the roof doesn't collapse and destroy the entire home. The house is able to be inhabited.

      Nothing is 100% fireproof. As given enough time, heat, fuel and circumstances, anything and everything can burn. The World Trade Towers would still be standing today, if only the planes were not carrying so much fuel. The Boeing 767 loaded with 20,000 gallons of jet fuel is what eventually led to the failure. The jet fuel kept burning which weakened the steel and concrete and led to the collapse. If not for the fuel source, the impact of the planes alone, would not have brought the towers down.

      The point is that concrete and steel do very well in fires. Not talking about the "environmental carbon footprint" of building with concrete & steel but simply how they fare in a wildfire. A stick wood frame home vs an ICF home, when it comes to fires, is an apple to oranges comparison. Heavy timber is another topic. Glulam/LVL beams do quite well in fires as the outside gets charred and provides a protection coat for the inner part of the beam, keeping it structurally sound.

      So it's important to keep the outside "fuels" around the home, cleared and far away. That means no trees/vegetation within 30-40 feet of the structure. When you watch video or pics of the recent fires. The homes had trees sitting within just a few feet of the home. With such a fuel source nearby, the house doesn't stand a chance.

  6. Lesley_Farm_Girl | | #9

    Thanks for everyone's feedback. The laters comments on wildfires are consistent with the research I have done. There are R30 blocks available for upstairs. I know a PGH would have R40 insulation on the main level walls... But, I have also heard you need to factor in that these walls are absolutely air tight making the real R Value greater? Is this true...? No thermal bridging.

    Do you think using corregated metal siding and roofing would work well. That is what we were planning to use.

    Also, do you think a steel roof would not be as fire proof as using the new ICF roof system? Also, how would you insulate a roof properly using R30 ICF blocks and the ICF roof system and/steel roof framing?

    I want to make sure I have the correct amount of insulation in the right places so we don't have problems in the future.

    1. jackofalltrades777 | | #11


      If you want, just leave your email as it might be easier to communicate that way instead of posting everything here. I researched and built a home in a wildfire area. So I do have considerable knowledge in this area.

      Concrete/ICF roofs are VERY expensive and labor intensive. I would lean towards conventional truss roof with a steel roof on top with metal soffits and fascia.

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