House Cement

Insulated Concrete Forms--A Concrete Revolution
by John Straube
Insulated Concrete Forms (ICFs) are an increasingly popular type of lostform
system used to build concrete walls. According to the Portland Cement
Association, nearly 20,000 homes--from affordable 1,000 square foot starter
homes to 30,000 square foot mansions--were constructed using ICFs
last year. Although no separate statistics are kept, the use of ICFs in the ICI
sector is also growing quickly.
The forms, made of some insulating material, are either provided as
complete interlocking blocks or as separate panels connected with ties. The
two insulation layers used as formwork not only provide continuous
insulation, but a space for services to be placed, and a backing for
finishes on the inside and outside.
The North American use of ICFs can be traced back to Buckminster
Fuller's Stockade Building System, an ICF made of magnesium-oxy-chloride
bonded wood fibre. Although Fuller's company failed, ICFs made of bonded
wood fibre were successfully used in Europe as early as the 1930s. These
became important in housing and multi-family buildings after World War II,
when energy, materials, and skilled labour became scarce. The leading
European ICF product was introduced to North America in 1953. Unlike the
foam plastic used by most of the more recently launched products, this type
of ICF was--and still is--made of cement-bonded recycled wood chips. The
material has been used in over 15 million square feet of mostly commercial
and institutional buildings since then. Foam-based ICF blocks have since
been introduced, but significant market penetration only seriously began in
the 1990s, and now numerous companies produce ICFs with varying details.
The shape of the concrete in the voids is one of the features that
distinguishes one ICF from another. Flat slab systems produce a continuous
wall thickness just like other formwork and hence can be engineered using
standard approaches. The concrete wall produced by grid systems has a
waffle-like pattern which results in varying concrete thickness and reduced
concrete use. Post and beam systems, comprised of individual closely-spaced
horizontal and vertical columns of concrete, use the least volume of
concrete.
Most ICFs provide 50-63 mm (two to 2.5 inches) of insulation on both
faces of the concrete core, which is between 100 to 200 mm (four and eight
inches) thick. The result is a solid wall assembly from 200 to 300 mm thick
with good thermal resistance.
The insulation material is usually expanded polystyrene (EPS), but
can be cement-bonded polystyrene, or cement-bonded wood fibre. In many
systems, ties connect the two layers of insulation together. These ties are
often made of plastic, but can be made of light-gauge steel. These ties are
also used as locations on the face of the ICF to which one can fasten
finishes, or to keep rebar within the core in place during pouring.
In the last five years new ICF manufacturers have proliferated.
There are now more than 50 companies in North America, many offering
slightly different wall shapes, means of attaching finishes, or wall
thicknesses. While each system has its advantages, most are very similar in
performance and methods of construction.
ICFs have many advantages as a wall system. They typically provide
sufficient strength to meet most needs (the 25-storey Windsor Hilton was
built with the traditional cement-bonded recycled wood chip form of ICF),
and the strength of reinforced concrete is often seen as reassuring for
owners in hurricane and earthquake regions. The materials are typically
moisture tolerant, and so mould, rot and corrosion are rarely a problem.
Almost all ICFs blanket a building with insulation--from R8 to over
R20--with few thermal bridges such as wood or steel studs. This means that
the interior surface temperature is comfortably high and uniform. The
thermal mass of the concrete helps to moderate temperature swings, which is
especially useful for reducing air conditioning energy consumption. Systems
that place the concrete mass closer to the interior will have a greater
potential for energy savings. Air leakage through and air movement within
ICFs is far lower than frame construction because ICFs use dense insulation
and concrete.
The lack of thermal bridging, thermal mass, and air tightness are
real, measurable and significant benefits of ICFs. The space-conditioning
energy consumption of a building built of ICFs will be significantly less
than for standard construction. However, some salespeople have promoted
their ICF product's R-value as being 40 or 50--these claims are misleading
at best and fraudulent at worst. While "effective R-values" of close to this
level are possible in unique situations, such claims should be ignored
without an engineer's detailed dynamic analysis of the specific project.
The performance--structural, thermal, moisture, acoustic--of an ICF
wall is in most cases superior to that of walls framed with light-gauge wood
or steel. However, the installed cost is also significantly higher,
especially in house construction. The popularity of ICFs is evidence that
the industry considers the cost increase for a relatively small part of the
house cost (the walls) to be worth the significant benefits.
Many of ICF's benefits are shared by masonry cavity walls with at
least 100 mm (four inches) of cavity insulation. The installed cost of
masonry is, however, often higher, and the skilled trades required to build
masonry are increasingly difficult to find. Unlike normal reinforced
concrete and masonry walls, ICFs can be constructed in cold weather. Their
insulating nature retains the heat of the concrete even in very cold
weather, allowing curing without tenting or heating.
Fire performance is one attribute that can become an obstacle in
commercial construction. Although most ICFs can provide a high level of
fire
separation by virtue of the continuous concrete core, the flame spread and
smoke produced values for foam plastics require the use of a thermal
barrier. In many cases gypsum wallboard can be used to protect the foam
(cement-bonded (delete wood - cement-bonded foam too) products require
no fire protection). The increasing
number of commercial projects being built with ICFs is a testament to their
growing acceptance by building officials.
Almost any finish can be applied to the inside and outside of an ICF
wall. Some products may require slightly different methods of application
however. Masonry, stucco, EIFS, and siding can all be applied directly over
ICFs. Most systems have fastening surfaces to attach siding such as wood,
vinyl, and aluminum. Interior finishes are typically drywall or direct
applied plaster.
ICFs are likely here to stay both in all types of construction. Their
virtues of energy-efficiency, durability, and lower labour input are
becoming increasingly sought after while the size of the cost premium is
steadily dropping.
John Straube teaches in the Department of Civil Engineering and the School
of Architecture at the University of Waterloo.