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Insulation Formed Concrete Walls

Star The building science associated with Carnation Walling. 


Best ICF system

Having gained experience with ICF (Insulated Concrete Form) construction and having studied building science, this describes what I believe to be the ideal "ICF" system.  As we shall see, it is probably better to call it an "IFC" (Insulation Formed Concrete) system.  I refer to it as Carnation Walling.  In accordance with building science best practices it puts all the EPS insulation on the outside of the concrete wall to couple the concrete thermal mass with the house inside temperature.  On the inside of the wall, Carnation Walling uses regular low cost construction grade plywood that is stripped off after the concrete hardens and is reused.  As well as being better from a building science perspective, Carnation Walling is about half the cost compared with traditional ICF.

Carnation Walling is a *method* of building walls rather than a proprietary product.  It requires a particular set of steps and these steps are described in detail here (but it's best to read the current page first).


Building Science

Everything needs to start with Building Science, ie start by figuring out the perfect wall design (that meets building code requirements).

Insulation requirements

Insulation is measured in R-value.  Type 1 EPS gives you about R-4.17 per inch thickness (in cold climates).  Type 2 EPS gives about R-4.5 per inch (in cold climates).  For walls, 2012 energy code in cold climates in America (climate zones 6, 7, and 8 which includes Wyoming, Montana, North Dakota, South Dakota, Minnesota, Wisconsin, Vermont, New Hampshire, and Maine) requires a minimum R-value of 25.  That equates to a Type 1 EPS thickness of 6 inches.  That is the minimum code requirement, but many people in cold climates will want to make a super energy efficient house and that means they will want to go up to about R-40 for the walls (10" of Type 1 EPS or 9" of Type 2 EPS), but a more common choice will be about R-36 which is about 8.6" of Type 1 EPS or 8" of Type 2 EPS.

What is needed is a walling system that meets 2012 Energy Code in cold climates at least cost and yet is easy to optionally increase to a greater R-value for those who want a super insulated house.

Insulation on outside

The next question is where to put the insulation.  Traditional ICF systems (in addition to only providing about R-23) have half the insulation on the inside of the concrete and half on the outside.  By considering building science and the perfect wall design we see that the insulation should actually all be on the outside of the structure, ie on the outside of the concrete.  You want the structure (the concrete) on the inside of the building envelope so it stays warm and dry.  For maximum occupant comfort you want to get the concrete as close to the inside of the wall as possible in order to thermally couple the huge thermal mass that concrete provides with the warm inside house temperature.  You want the thermal mass to be at the same temperature as the inside living space, ie about 70 degrees F.  The water liquid control layer wants to be on the outside of the wall and EPS (plus stucco) with its low water permeability provides this nicely.


Other Benefits

These benefits also apply to traditional ICF as well as Carnation Walling (IFC).  The term ICF/IFC is used to refer to either.

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 and IFC 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/IFC house.  Firstly it is very well insulated thanks to the polystyrene.  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/IFC exterior walls require less energy to heat and 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/IFC 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/IFC 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/IFC 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/IFC provides a safe shelter from hurricanes and other violent storms.  ICF construction is becoming very popular in areas prone to twisters.  An ICF/IFC 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/IFC 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/IFC 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/IFC 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/IFC 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 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/IFC.

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/IFC 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 and IFC 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 OSB (Orientated Strand Board) and some plywood 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/IFC is completely inert in addition to the EPS, so ICF/IFC avoids health problems.

Avoids allergy issues

Because ICF/IFC 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 HRV (or ERV) 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.


Carnation Walling design details

R-36 insulation

Carnation Walling provides an 8" thickness of EPS on the outside of the concrete and deliberately no insulation on the inside of the concrete.

The EPS used is Type 2 (50% higher density than regular Type 1 EPS).  The polystyrene density is 1.5 lb/ft3 (0.024 g/cm3).   The compressive spec is 15 psi which makes it stand up well to abuse on the construction site.  Just applying 3/4" stucco is sufficient to avoid dents from kids on bikes.  The use of higher density EPS also increases the R value per inch to about R-4.5 at 40 degrees F, so the house is better insulated.  Carnation Walling (as described) provides R-36 insulated walls.  You could modify the Carnation Walling implementation to increase or decrease the amount of insulation, but R-36 is about right for the well insulated houses that we should be building.

Note that this is a continuous R-36 thermal break rather than the very inferior batt insulation used in stud wall construction.


Forming concrete

The main principle used to provide a concrete form strong enough to withstand the pressures associated with wet concrete is a vertical 2x4 baton on both sides of the wall and a tie that goes between the batons through the wall concrete.  The ties actually go through the concrete inside 8" (or 12") cut lengths of low cost half inch electrical conduit so they can easily be removed after the concrete has set.  The plastic pipe also holds the forming the right distance apart to make the required concrete cavity width.  Concrete cavity widths of 12", 8", and 6" are easily accommodated by just using different lengths of plastic conduit pipe.

Forms need to be fairly watertight.  Loose forms permit either loss of cement (in thin slurry form), resulting in honeycomb, or loss of water causing sand to streak.


Forming concrete on the outside

The use of two glued together layers of EPS with staggered overlapping joints makes a continuous solid wall of EPS that is both strong to withstand wet concrete pressures as well as being waterproof to stop concrete slurry leaking out.

Regular commodity EPS sheets are used.  They are not a special design from a particular manufacturer, so the cost of the EPS is a lot less than buying it in proprietary ICF block form.  Type 2 EPS sheets are used because they can withstand being knocked about and Type 2 provides a slightly higher R value per inch.

In my implementation I use 6" thick EPS for the first layer and 2" thick EPS for the second layer.  The 2" thick second layer is thin enough to follow any distortion of the 6" thick first layer and ensures a good adhesive joint.  A thinner second layer also allows for the option of using higher density EPS if you need to make the wall highly resistant to kicking and denting.  In most cases using 15psi EPS for both layers is perfectly fine.


Forming concrete on the inside

The reason of course that half the EPS is on the inside in a traditional ICF system is that it is used to constrain the wet concrete to form the wall.  When using traditional ICF, I strip away the interior ICF EPS once the concrete has hardened.  This is a lot of work and is also of course very wasteful of EPS.  It leaves the wall with only about 2.5" of EPS insulation, so lots of EPS sheets need to be attached to the outside of the wall to get it up to the required insulation value.

With Carnation Walling, on the inside of the wall, two sheets of plywood (one thin and one thick) with staggered overlapping joints (together with 2x4 bracing) withstands the wet concrete pressure and stops concrete slurry leaking out.

Regular construction grade half inch plywood is used on the inside.  This is typically known as CDX plywood and in practice 15/32" is often sold as 1/2" plywood.  Once the wall concrete has solidified, the wall ties are removed and then the construction grade plywood is removed .  The plywood sheets are typically used in their full uncut 8 foot x 4 foot form so can be reused for other concrete wall pours or for building other things.  The only need to cut the 8'x4' sheets is to fit around windows and any obstacles or lengths associated with the house you are building.  By keeping the plywood as 8'x4' sheets as much as possible it reduces cutting costs and allows it to be better reused after it is stripped from the solidified concrete.  Also bigger sheets means less joints for the concrete to try to seep through.

There is also a thin sheet of smooth surface plywood between the concrete and the thick construction grade plywood.  Only the thick construction grade plywood is removed after the pour because the thin plywood is designed to stay in place to form a nice wall surface that can be painted.

The plywood (both the thin and the thick) is totally standard and brought from your local supplier of choice, which significantly reduces costs.  It is best to choose formaldehyde-free plywood, particularly for the thin plywood as it stays in place inside the house.  The use of two layers of plywood with staggered overlapping joints and 2x4 lumber over the joints between 8'x4' sheets means that the plywood does not need to be tongue and groove.

As well as being great from a building science perspective, not having EPS on the inside makes it easy to implement things like elevator shafts and swimming pools without having to strip away the EPS like with ICF systems.


Wall ties

The 8'x4' plywood sheets (with the batons) on the inside and the 8'x4' EPS sheets (with the batons) on the outside attach using concrete wall ties of my own design.  An important driver here is keeping everything to a minimal cost.  In strategically important places, the ties through the wall are made from 3/8" threaded steel rod plus nuts and washers (total of about $1.50), but the rods are removed after the concrete pour and can be reused.  Where the tie is not strategically important, it can be done using regular 14 gauge galvanized wire (which is even lower cost), though I tend to prefer to use the reusable threaded ties everywhere.


Keeping the wall square and true

Angled adjustable bracing made from 2x4s is used to ensure the walls are square and perpendicular.  The 2x4s are used with the flat face against the plywood as that imposes the least requirement for straightness on the 2x4 sticks.

In addition to the wall bracing, the fact that the concrete ceiling (done with Carnation Flooring) is concrete poured at the same time as the Carnation Walling is very useful for holding the walls vertical and for holding the building square. Without this it would be necessary to use a lot more bracing.


Coping with weather during construction

The outer EPS walls and the EPS Carnation Flooring above are built well before the rebar is added and certainly a long time before the plywood that constrains the concrete on the inside.  This is great logistically when building a house because most of the work can be done inside a warm dry enclosure. This is way better for the workers and avoids the plywood and other wood being damaged by rain.  Also working inside is much safer than working on ladders up the outside of the building.


Holding the rebar in place

The vertical and horizontal rebar with lots of low cost cable ties is used to hold the wall tie in the right position (both horizontally and vertically).  This is necessary for getting the plywood on and of course the rebar needs to be securely held in the right locations during the concrete pour.

Because the rebar is added from the inside after the polystyrene wall is built means that there is no problem having large sheets of EPS.  In contrast, ICF blocks have to be only 1 foot tall if the horizontal rebar needs to be every 1 foot (which it does in the case of my house in an earthquake zone).

Structural engineering typically calls for the center of rebar above window and door openings to be 2.25" from the edge of the concrete.  In the case of 1/2" rebar that means the support needs to be 2" above the concrete edge.  Typically the cutting of the opening in the EPS above a window is placed on an integer X number of feet from the slab, but the cavity part of the bucking will extend 1.5" up from that, so the height of the edge of the concrete will be at X' 1.5".  That means the rebar support above openings needs to be at X' 3.5".  The diameter of the plastic conduit tube is just over 27/32" which can be approximated to 1", so the support height above the center of the wall tie is about 1/2".  This means the centers of the wall ties should be at X'3".

Below window openings, structural engineering provides more flexibility in the placement of the rebar.  Under window openings it is typical to cut the EPS at a height of X'9".  The cavity bucking will mean the concrete edge is at X' 7.5".  It's good to have a concrete covering over the rebar of 3.5" below windows.  For 1/2" rebar and a support height of 1/2" above the tie center that means the rebar support is best at X' 7.5" - 0.5" - 0.5" - 3.5" = X' 3"

Carnation Walling allows rebar to be placed at X'9"  or X'3" above slab height, where X is an integer number of feet up the wall.  In most cases it is a wall tie at X'9" that is used to support the rebar.  When needed, eg above and below window openings or at the bottom of the wall, a wall tie can be added at an X'3" position to support the rebar.

Carnation Walling actually provides full flexibility in the placement of rebar because the wall ties can be put and used wherever you want, whereas ICF blocks have a fixed plastic web that determines where the rebar can be placed.  In the case of an ICF block it is in practice hard to even use the lower web position because the rebar has to be threaded through from the wall end.  With Carnation Walling it is easy to use the lower position as the rebar is added from inside the building.  If you really wanted to with Carnation Walling you could even put wall ties at whatever height you want, but it is unlikely that you would ever want to deviate from the X'9" and X'3" positions.


Making corners and T pieces

Corners are made on the job site by just gluing the edge of one EPS sheet to the face of the other EPS sheet.  It is strengthened but gluing on the second layer of EPS sheet that does the over-lap and under-lap the opposite way round.  Even though in practice this provides enough strength, it is typical to also use an L shaped wooden 2-by corner baton to give extra peace of mind.

In the row of 8x4 EPS sheets above it is typical at the corner to reverse the over-lap under-lap pattern as it all adds to the strength and wall squareness.

Concrete seepage at the corner is prevented using foam adhesive and/or foam gap filler.  Because the outer EPS is thick it provides a large gluing area.  Just do a square cut with a hotwire cutter or serrated calving knife or saw.  Put foam adhesive and/or foam gap filler on the cut surface and butt it up to an uncut inside edge of the EPS block on the other wall.  You can easily make T pieces as well as make any corner angle you want.


Finishing the wall inside

Even though the thick construction grade plywood is removed after the concrete pour, the thin plywood stays in place.  Unlike the thick plywood, the thin plywood used has a nice smooth finish that is suitable for painting.

The thin plywood will have 1 foot spaced 7/16" holes in it.  These are filled with finger dabs  of PL-Premium adhesive (which is a great filler compound) and when dry this is sanded to make the whole wall a nice smooth painting surface.  The PL-Premium also glues the plywood to the inside of the tie rod plastic conduit tube to further ensure the thin ply remains firmly attached to the concrete.


Finishing the wall on the outside

Typically the outside EPS is stuccoed.  There is typically a 5/16" scratch coat, then a 5/16" brown coat, then an 1/8" top coat.  That's a total stucco thickness of 3/4".

It is best to apply the stucco within 6 months of putting up the EPS walls so that UV sunlight does not make the EPS surface powdery.  The stucco sticks best to a non-powdery EPS surface so you want to avoid the job of having to use a stiff brush or sanding block on the EPS surface to remove the powder.


Fire resistance

Building code requires that any EPS (even fire treated EPS) on the inside of the house envelope is covered with a naturally fire resistant product, eg drywall.  There is no requirement for an internal covering when using Carnation Walling because there is no internal EPS used as part of Carnation Walling.  On the inside is just concrete with a thin layer of plywood.

Because the only EPS is on the outside of the wall, there is no real need (other than building code) to add fireproofing chemical to the EPS mix even though untreated EPS is flammable.  Having said that, it is still some slight benefit to add fire retardant to the EPS on the outside of the wall in case your house is subjected to a forest fire and you want to reduce the amount of exterior damage.

Carnation Walling uses regular commodity EPS sheets and all EPS sheets are treated by the EPS manufacturer with a brominated flame retardant called Hexa-bromo-cyclo-dodecane (HBCD).  The HBCD percentage in the EPS is about 0.7%.  It is claimed by the EPS industry that emissions in a fire are no more toxic than burning wood, but some people do worry about its long term health effects and it is possible that it may get banned (starting first in Europe).  A ban would have a huge negative effect on the traditional ICF industry, but would not effect Carnation Walling.  Long term health worries are irrelevant in the case of Carnation Walling given that none of the fire retardant with its associated potential emissions will be inside the house envelope.  If HBCD does get banned then it will either just be left out of the Carnation Walling EPS sheets or the EPS sheets will move to another flame retardant such as Polymeric FR as and when it becomes available.


Cost comparison

It is worth comparing the cost of Carnation Walling with traditional ICF.  To do this it is best to find how much it costs to form a wall section that is 4' wide and 1' tall (the size of a typical ICF block).  I call this a "block unit".  I have found GreenBlock to be a good ICF block and the least expensive, so this is used in the comparison.  The comparison includes getting to an R36 wall and the cost of the bracing.  The itemizing for Carnation Walling was also useful for driving out cost.  A large house like mine needs about 3000 of these block units.

Traditional ICF (GreenBlock)

8" cavity ICF block (with delivery and tax)                                         $20.00
EPS (5") for the outside to get to R36 (with delivery)                         $4.00
Long screws from bracing to web (2 off) (reused 2.1 times)             $0.20
2x4 lumber (bracing, inc angle bracing) (reused 3 times)
Long screws 6in for attaching external EPS (1 off)                             $0.40
Large washers to attach external EPS (1 off)                                        $0.07
Foam adhesive
Total per block unit                                                                                $28.00


Carnation Walling

EPS (6") 4'x1' (with delivery)                                                                 $4.80
EPS (2") 4'x1' (with delivery)                                                                 $1.60
1/2" electrical conduit (4 x 8" lengths = 32")                                        $0.55
3/4" electrical conduit (10 x 3/8" lengths = 6")                                    $0.11
3/8" threaded steel rod (2 x 2' lengths = 8',  reused 3 times)             $0.88
3/8" steel zinc plated nuts (4 off, reused 3 times)                              $0.14
3/8" large steel zinc plated washers (8 off, reused 3 times)             $0.29
1.5" nylon washers with 3/8" hole (4 off)                                             $0.72
8'x4' plywood 15/32" thick (Reused 2.4 times)                                   $1.00  
8'x4' plywood 3/16" thick (Not reused)                                               $1.56
Galvanized wire 14 gauge (2 lengths of 2.5')                                      $0.32
1-1/2" small head nails
3/4" large head nails
2x4 lumber (bracing, inc angle bracing) (reused 3 times)
Foam adhesive
Total per block unit                                                                               $15.00