What's it really like living in a concrete bunker? This is part one of a two part post on our ICF (insulated concrete form) home. I'll discuss what ICF construction is, how it differs from conventional construction, why we chose Insulated Concrete Forms, how much energy is saved (for heating and cooling) by using ICF construction, costs, whether is suitable for the do-it-yourselfer, and how an ICF home performs during emergencies. Let's get started!
Living in a Concrete Bunker – Our Story
When the SHTF, the old cliché is to hide out in a concrete bunker. Since there weren’t any readily available in our area, we decided to build our own. No, this isn’t a complete underground structure that would be tough to get permits for and likely get you labeled as “one of those people” by the local authorities. The beauty of this bunker is that outside and in, it looks mostly like a “normal” home. Only those in the know realize that we’re protected from everything from tornadoes to someone driving a truck into our wall.
What do you mean by “concrete bunker”?
Our home is constructed using Insulated Concrete Forms (ICFs). These forms consist of two layers of insulating foam (approximately 2 inches thick each), inside and out, which sandwich an 8 inch thick layer of concrete. They measure roughly 16 inches tall by four feet long by 12 inches wide. The concrete core is eight inches thick. Plastic framework within the forms stabilizes the form and provides strips that can be drilled into to mount drywall, cabinets, wood trim, etc.
These forms are assembled, Lego-style, laced with rebar, and then filled with concrete. Because the concrete and rebar provides the strength of the wall – not the form – a wall is framed out with the ICF forms, V-buck openings, and bracing, and then poured in “lifts”, i.e. they fill a wall about three forms high with concrete, let it set, and then fill another three forms in height, and so on. Our home uses these forms from the foundation to the roofline. Other options are basement/foundation only, or for building/retrofitting a safe room within the shell of a home.
ICFs can be built completely underground, but in our case we decided to build a walk out ranch. This takes advantage of earth sheltering for the lower level/basement, but also allowed us to incorporate ample windows on the south side for daylighting and passive solar gain. The home is built to be handicap accessible, so a ranch design was better than a two story in our situation. If need be, we could close off the top level and retreat entirely to the bottom level with only minor modifications, or we could house additional family members in their own semi-private space, which was the original design intention.
Windows are placed to maximize cross ventilation when weather permits. The house is exceptionally airtight, so we also installed a Fantech HRV (Heat Recovery Ventilator). An HRV brings outdoor air inside and preheats it with stale indoor air before it enters the home – fresh air in, stale air out. The HRV could also be fitted with specialized filters to address toxic air problems outside the home. During extremely humid weather, we do have some minor condensation/mildew in the basement if I don’t keep the air moving. In floor radiant heating eliminates this in cooler weather.
Because they are heavily insulated, I have not seen condensation on the walls. Drywall is screwed into plastic strips in the ICF forms, allowing the home to be plastered and finished like a more conventional home – no ugly, bare concrete walls. The only indication you have from inside that the house is not standard construction are the deep window wells and doorways – and the noise reduction and overall sturdiness of the home. ICF construction is also much more pest resistant – nothing eats or can chew its way in through concrete. (Termites can use the foam for nest material, so in termite prone areas, pre-treated forms should be used.)
In winter, when I cover the windows with insulating cellular shades, it creates a cool microclimate in the window well which can lead to condensation if the blinds are kept closed too long and the HRV isn’t run. This is less of a problem in the south windows, more of one on the north windows. I’ve taken to applying a layer of insulating window plastic on the inside of the north windows at the beginning of the heating season. (In retrospect, we should have probably upgraded to the triple pane windows with argon fill on the north side of the house, instead of a good quality double pane.) Our window frames are vinyl to reduce heat loss from the home and prevent rot.
Warm in Winter, Cool in Summer
One of the primary reasons we chose Insulated Concrete Form construction was for its energy efficiency. Because the walls are uniformly insulated with a solid sheet of foam inside and out, there are no channels for air to seep through. You will not find hot and cold spots on an ICF wall. In stick built construction, your effective r-value is greatly diminished by air infiltration and heat transfer along wooden studs and around outlets and other wall penetrations where the fiberglass insulation does not fill the area completely (spray foam insulation is better, but you still have heat transfer along the studs).
Thermal images courtesy of Reward Wall Systems.
The amount of concrete used in the structure gives the home high thermal mass. This means that is holds temperature very well. Heat it up, and it stays warm – you don’t lose heat to the surroundings. Fill the house with cool night air in the summer and close it up during the day, and it stays cool. Windows at the east and west ends of the home allow us to take advantage of winds off Lake Michigan to flush warm air out of the home.
In addition to the concrete walls, we also have a TempCast wood burning masonry stove in the basement. Our masonry stove has primary and secondary combustion chambers and a serpentine flu surrounded by brickwork that captures the heat from the fire and slowly radiates it into the home. Unlike convention wood stoves, the masonry stove is fired with a short, hot fire once or twice a day, and then the flu is closed, trapping the heat within the masonry instead of sending it up the chimney. The masonry stove contributes additional thermal mass.
Update 2017: We would suggest a simpler fireplace, or possibly even a rocket stove with an external feed. The tempcast is nice but it is not burning properly and we have been unable to get it resolved after years of effort.
We do have a forced air system for air conditioning, primarily for humidity. How much we use it depends on the year. Could we get along without it? Possibly, but because of the tightness of the home I believe mold would become a significant problem. We have had no problems with cracked or shifting walls or leaks.
Update 2017: We have a crack in the basement floor mainly because the people pouring the floor didn't understand they needed to add an expansion joint. We still have no visible settling or cracking on interior or exterior walls.
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