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ICF (Insulated Concrete Forms)

Star An innovative poured concrete wall forming system that also provides house insulation.


Giant Lego set

Building with ICF is just like playing with a giant Lego set.  You use the blocks to build yourself a Lego house and then you fill up the cavity in the Lego walls with reinforced concrete.  The Lego blocks are made of polystyrene and their first function is to form a mold for the concrete.  The polystyrene blocks stay in place in the finished concrete house and perform their second function which is to provide thermal insulation.  You end up with a house that's extremely strong thanks to the reinforced concrete and extremely good from an energy perspective thanks to the polystyrene insulation and the thermal mass that the concrete provides.

ICF assorted blocks

Using my experience with ICF, I have now designed my own walling system system called Carnation Walling which is an Insulation Formed Concrete system (IFC).


Now starting to take off

The benefits are being realized

The ICF idea has been around for about 40 years, but the building trade is pretty conservative and moves at a glacial pace, so it is only fairly recently that ICF construction has started to take off.  The various benefits are detailed below.  There is also a negative in that it can cost about 5% more to build an ICF house.  This extra cost is however a small price to pay for a dramatically reduced heating bill and the fact that the house will last at least 5 times longer than a wooden house.  It also is nice to be in a concrete house if you get hit by an earthquake or a tornado.


Benefits of ICF

Low heating bill (and saving the planet)

It's nice when saving you money happens to coincide with the side effect of also helping save the planet.  That's the case with ICF construction because you won't need to burn much fuel in your house to keep it in the nice 72 degree comfort zone inside.  There are two reasons why it does not take much to heat an ICF house.  Firstly it is very well insulated thanks to the polystyrene that that the ICF bricks are made of.  Good insulation means there is only a very small heat loss from inside your house to the cold outside.  Secondly the thermal mass stores heat from the sun during the day and releases it in the evenings and night.  The thermal mass also avoids needing air condition because the cool from the night keeps the house cool during the hot daytime.

Lower cost HVAC system

Homes built with ICF exterior walls require an estimated 44% less energy to heat and 32% less energy to cool than comparable wood frame houses.  Consequently, your HVAC systems can be downsized and are therefore less expensive.  Unless you live in a particularly hot part of the world you may well decide that thanks to the thermal mass of the concrete and the polystyrene insulation you can do without installing air conditioning.  You may have been considering a geothermal heating system because of its great efficiency, but the reality is that with ICF walls the payback period on the geothermal capital investment will be really long and probably will not be a good use of your money.  Even an inherently inefficient heating system such as electric radiators will perform well in an ICF house because they will not need to be on much.

Lasts at least 5 times longer

Not only does it psychologically feel good to be building a house that is designed to last for 500 years or more, but it's also a very green thing to do.  It takes a bunch of energy to create the materials necessary to build a house.  In the lifespan of a house that lasts 500 years you would need to build 5 houses that only last 100 years.  If you were to be negative about a concrete house and say that it takes twice the energy to build a concrete house compared with a wooden house, that would mean that a concrete house is still two and a half times more efficient in terms of world energy usage compared with a wooden house.  Also of course a house with a dramatically longer life will maintain its resale value way longer.

Won't rot like wood

ICF walls are made of polystyrene and concrete, and neither of these will rot.  Wood on the other hand rots when it gets damp.  In wooden houses the walls will get damp inside.  It is unavoidable due to condensation that happens inside the wall.  The rot of course seriously reduces the life of a wooden house.

Withstands earthquakes

There are two different strategies for designing an earthquake proof house.  You can make it of something approaching rubber or you can make it really strong.  The problem with a rubbery house is that everything inside gets very badly shaken up during the quake.  People inside get hit by flying objects and furniture.  A better strategy for a building that is not too big (such as a residential house) is to just make it really strong so it does not break when the earth shakes.  If the earth under your house liquefies and sinks unevenly then even a very strong ICF house may need to be jacked up on one side and underpinned after the quake, but it will provide good protection for you and your family.

Withstands hurricanes and other storms

ICF provides a safe shelter from hurricanes and other violent storms.  ICF construction is becoming very popular in areas prone to twisters.  An ICF house is also the right type of house to shelter in if a tsunami comes your way.  Obviously it's good to go to an upper floor, but your can be pretty sure the house will not get washed away because it has great strength and mass due to the reinforced concrete.  Having an ICF house in a storm prone area will likely reduce your cost of house insurance considerably and may make the difference in your ability to even get insurance.

Draft proof

ICF walls have no gaps unless you specifically design in a hole.  In contrast wooden walls require tremendous vigilance during construction to avoid gaps, and that construction site vigilance is often lacking.  In a wooden house those drafts, as well as reducing comfort, also carry moist air to cold surfaces, creating condensation that then leads to mold.  Also air movement through walls is the most significant source of building heat loss.

Mold proof

Neither polystyrene or concrete are a food source for mold so mold will not grow as there is nothing to sustain it.  Wood in contrast is a food that mold loves.  Mold comes in lots of varieties and most of the varieties make you sick.  Typically the effects are not enough to kill you but they can seriously impact your quality of life.  It's an unfortunate fact that most wooden houses have mold growing in the walls.  This is because water vapor gets into the wall cavity and wood is a great food source for mold.  ICF walls are solid without an air cavity.

No termites or carpenter ants

Wood is a good food source for termites, and carpenter ants love to make their home in damp wood.  Neither concrete or polystyrene are food sources, so you avoid all these problems.  You also avoid the bad health effects of spraying chemical to deal with the infestations you get in wooden houses.

Sound proof

Even in a family setting it makes for more harmonious living to not hear what everyone else is doing all the time.  Teenagers play loud music and even worse like to learn instruments such as electric guitars.  Sometimes it's nice to go to another room to quietly read a book without being distracted by someone else watching TV.  It's also nice to not hear other people's bathroom and bedroom sounds.  ICF internal walls (and polystyrene formed concrete floors) provide very effective sound proofing because of the combination of solid concrete to take out the low frequencies and the more fluffy polystyrene to take out the higher frequencies.  Of course if you live near a major road, rail tracks, an airport, or have a neighbor with a noisy lawn mower then you will really appreciate the fact that the outer walls of the house are made of ICF.

Supports a decent roof

If you just have wooden walls on a house then you have to be really careful to have a light weight roof because the walls cannot stand much weight.  That means you cannot use thick lumber in the roof trusses and you cannot use things such as clay tiles.  You even have to be careful not to use too many layers of felt tiles.  ICF walls have no such problems.  You can use good thick lumber to frame the roof and you can use whatever tiles you want with no worries.  You can even easily support the weight of lots of water storage tanks in the attic that gravity feed your house and yard.

No nasty chemicals

The polystyrene used to make almost all brands of ICF bricks is EPS (Expanded Polystyrene).  The Polystyrene raw material is completely inert and non-toxic, and it's expanded using just steam to make EPS.  XPS (eXtruded PolyStyrene) in contrast uses noxious gas to expand it and in my opinion is best avoided because the gas leeks out over a period of about 10 years after manufacture.  Wood such as plywood or OSB (Orientated Strand Board) is also in my opinion best avoided because it's manufactured using urea formaldehyde which it gives off as a carcinogenic gas.  The concrete that fills the ICF is completely inert in addition to the EPS, so ICF avoids health problems.

Avoids allergy issues

Because ICF walls are totally inert, dust free, and air tight, there is nothing to give you allergy issues.  If you are allergic to things in the outside air (such as pollen), it is good to make the house air tight and then filter in one place all the air coming into the house.  It's also good to use an EVR or HRV air heat exchanger system in the same location so that the exhaust air is used to heat the incoming filtered air.

Design flexibility

Personally I'm sticking with straight walls and right angle corners, but ICF can fairly easily make whatever corner angles you want and can even do curved walls.  If you want to do a circular lookout tower then you can.

Withstand an explosion

Hopefully this is not a requirement.  In my case I live a mere 65 miles from Mount Rainier (a big volcano).  When it next erupts (which may well be within the 500 year design life of the house), I don't want the pressure wave from the blast to knock the house over.  In fact I want the house to be a safe place to protect the inhabitants from the blast.


Some negatives

Changing walls after the build

Concrete walls are really strong so it's an expensive proposition to change your mind about where you want the walls after the walls have been built.  It's worth also being fairly sure about where the doors and windows should be.  At a push you can use a jack hammer to make extra openings in a concrete wall but it's best avoided as it won't be as cheap as doing it with a wooden wall.

Thermal response time

Because you have a very large thermal mass, if you have been away on vacation for a week, it will take a few hours for the house to get back up to temperature after you turn the heating back on.  Luckily Home Automation (such as Insteon) make it easy to access your house thermostat via your cell phone or PC, so you can tell the house to turn the heating on when you are a couple of hours away from returning.  You can also if you want design the heating system so that it gives some immediate heat as well as gradually in parallel bringing the concrete up to temperature.

Slightly more expensive

Estimates of how much more expensive it is to build an ICF house typically give a figure of 5% more per square foot.  Personally I tend to distrust all estimates that talk about cost per square foot as there are just too many variables.  Anyhow, it is probably right to say that ICF construction costs a bit more than wooden construction.  An ICF house will however give you a far superior structure in which to implement your interior fixtures and fittings.  If your fittings and fixtures are fairly up market then you will probably not even notice the extra cost of the house structure.  If you are just doing very basic fixtures and fittings then the structure cost will be a higher percentage of the overall house cost, but you will have the assurance of a really strong house that will last at least 5 times longer than a wooden house and will give you dramatically reduced monthly bills.