Concrete forms have taken a new shape-and purpose. Insulating concrete forms (ICFs) are rigid plastic foam forms that hold concrete in place during curing and remain in place afterwards to serve as thermal insulation for concrete walls. The foam sections are lightweight and result in energy-efficient, durable construction.
ICFs consist of insulating foam, commonly expanded polystyrene (EPS) or extruded polystyrene (XPS). The three basic form types are hollow foam blocks, foam planks held together with plastic ties, and 4 x 8 panels with integral foam or plastic ties. ICFs can be used to form various structural configurations, such as a standard wall, post and beam, or grid. They provide backing for interior and exterior finishes.
Insulation values of ICF walls vary depending on the material and its thickness. Typical insulation values range from R-17 to R-26, compared to between R-13 and R-19 for most wood-framed walls. The strength of ICF structures relative to lumber depends on configuration, thickness, and reinforcement.However, ICF walls are designed as reinforced concrete, having high wind and seismic resistance.
There are many ICF wall types. Products are differentiated
based on the type of form and the shape of the concrete sections. Products
are further differentiated by how forms attach to each other,
how finishes are attached to the wall, insulating values, foam
types and other features. Introductory information on the most
basic product types follows. The book, Insulating
Concrete Forms for Residential Design and Construction, available from ICFA, includes
an in-depth discussion of design principles, details, types of ICFs, field assembly, and performance and cost data.
ICF walls provide higher R-values (between R-17 and R-26) and lower air infiltration rates than typical wood frame construction (typically R-12 to R-20).Thermographic testing by the NAHB Research Center of an ICF home showed that a solid ICF wall (clear wall with no windows or penetrations) had fewer cold spots than a similar wood-framed wall. However, selection and installation of many other elements of a house, such as windows, ceiling insulation, air sealing, and HVAC equipment, all have an impact on the overall energy efficiency of the house. Houses constructed with ICF walls will have up to a 50% decrease in capacity of HVAC equipment over framed wall construction.
Foundation walls built with ICFs are easier and faster to construct than either concrete masonry unit (CMU) or cast-in-place (CIP) concrete foundations depending on total area and house plan. Insulating forms protect the concrete from freezing and rapid drying. Concrete can be poured in ICFs when ambient temperature is as low as 10°F, requiring only the top of the form to be protected with insulating blankets. In extremely hot weather, in which evaporation is a concern, the top of the form need only be covered with plastic sheeting. The walls of a properly-constructed ICF home are resistant to loads imposed by high winds, and can be designed for all seismic zones. With regard to durability, foam and concrete hold the potential for improved building durability over wood construction because they are more resistant to moisture and less attractive to termites and other pests. ICF walls are more rot-resistant and durable than wood-framed walls.
Wind Resistance and Seismic Resistance. See Benefits/Costs area for details.
ICF forms are lightweight and easy to erect. Bracing and alignment systems are provided by most manufacturers. Normally, concrete can be placed in a house foundation wall in one day. Appropriate concrete placement equipment (such as a pump truck) may not be available in some locations. Use of ICFs does change the construction sequence, and initially may require a greater amount of coordination.
Attachment of siding, veneers, and interior drywall are relatively easy; details are provided by each manufacturer. Methods for attaching interfacing materials is different from traditional building materials. For example, electrical wiring is routed behind the wall surface by cutting grooves in the foam.
There are many manufacturers of ICFs; some distribute directly to concrete contractors or builders, while others distribute through authorized building products distributors.
ICF material cost ranges from about $1.75 per square foot to about $3.50 per square foot. Cost of installation labor, reinforcement, bracing, and concrete placement will be additional.
On average, ICF homes cost about two to five percent more than wood-framed construction. However, contractors installing ICFs for the first time, because of training required and a learning curve, may find that total installed cost is from five to seven percent.
There are no operational costs. Energy costs for houses with ICF full height walls will be significantly lower than for framed wall construction.
ICFs must meet standard prescriptive structural design requirements for cast-in-place concrete walls in the building codes. The plastic foam insulation on the interior surface requires special attention to meet fire resistance provisions. The International Residential Codes (IRC) contain prescriptive methods for building below- and above-ground walls. In February 2003, the International Code Council Evaluation Service (ICC-ES) was formed and issues ICC Evaluation Reports. The ICC also maintains “Legacy Reports” issued by the former four building product evaluation services.
Code Adoption Status and the Prescriptive Method
In May of 1998, the NAHB Research Center completed work on the Prescriptive Method For Insulating Concrete Forms In Residential Construction (Prescriptive Method) which was funded by the Department of Housing and Urban Development (HUD), the Portland Cement Association (PCA), and the National Association of Home Builders (NAHB). The first edition of the Prescriptive Method served as the source document for building code provisions in the International Residential Code (IRC).
The second edition of this Prescriptive Method was published in January 2002, and expanded on the first edition by adding provisions for Seismic Design Categories C and D (Seismic Zones 3 and 4). Wall construction requirements, using Grade 60 reinforcing steel and concrete mixes with selected compressive strengths, were included. In addition, tables throughout the document were simplified.
The Prescriptive Method includes definitions, limitations of applicability, below-grade and above-grade wall design tables, lintel tables, construction details, various construction and thermal guidelines, and other related information for home builders, building code officials, and design professionals. A prescriptive approach to ICF design eliminates the need for engineering in most applications. The provisions of this document were developed using accepted engineering practices and practical construction techniques. However, users of the document should verify compliance with local code requirements. The Prescriptive Method includes provisions for majority of ICF systems including flat panel and plank systems and grid systems (waffle and screen).
The first edition of the Prescriptive Method was accepted for inclusion in the 2000 International Residential Code (IRC). The IRC includes provisions for the use of ICFs in both above- and below-grade applications. Bear in mind that the IRC is a model code. By regulation or legislation, States or localities will adopt provisions of the IRC and IBC. However States or localities adopt, they have the option to add or remove requirements as they see fit.
Structural Design of ICFs Covered by Prescriptive Method
Where the IRC and Prescriptive Methodare not yet accepted, when certain ICF types are not covered by the Prescriptive Method, or when buildings do not meet the applicability limits of the Prescriptive Method, engineered designs (usually with sealed sets of plans) may be necessary in order to obtain building permits. For systems and applications that are not covered by the requirements in the Prescriptive Method, the NAHB Research Center, under sponsorship of the Portland Cement Association (PCA), completed the publication entitled Structural Design of Insulating Concrete Form Walls in Residential Construction. This publication, available from PCA, is a guideline for the design of single- and multi-unit residential structures using ICF wall systems. It includes step-by-step design procedures for ICF, a comprehensive design example, and many design aids, such as graphs, charts, and tables, to assist design professionals.
Most ICF manufacturers have taken steps of their own to have their proprietary systems approved by various model code organizations. Evaluation Reports produced by code bodies are available from those manufacturers. Most ICF
manufacturers will also provide design services if necessary.
Table 1. Applicability Limits for the ICF Prescriptive Method
||Number of Stories
||2 stories above grade plus a basement
|Design Wind Speed
||150 mph (241 km/hr) 3-second gust (130 mph (209 km/hr) fastest mile)
|Ground Snow Load
||70 psf (3.4 kPa)
|Seismic Design Category
||A, B, C, D1, and D2 (Seismic Zones 0, 1, 2, 3, and 4)
||Unbalanced Backfill Height
||9 feet (2.7 m)
|Equivalent Fluid Density of Soil
||60 pcf (960 kg/m3)
|Presumptive Soil Bearing Value
||2,000 psf (96 kPa)
||Wall Height (unsupported)
||10 feet (3 m)
||Floor Dead Load
||15 psf (0.72 kPa)
|First-Floor Live Load
||40 psf (1.9 kPa)
|Second-Floor Live Load (sleeping rooms)
||30 psf (1.4 kPa)
|Floor Clear Span (unsupported)
||32 feet (9.8 m)
||Maximum Roof Slope
|Roof and Ceiling Dead Load
||15 psf (0.72 kPa)
|Roof Live Load (ground snow load)
||70 psf (3.4 kPa)
|Attic Live Load
||20 psf (0.96 kPa)
|Roof Clear Span (unsupported)
||40 feet (12 m)
Local Code Issues/Barriers
Potential issues or barriers for the use of ICFs may be encountered, and include the following items:
- General unfamiliarity of code officials and inspectors with the product
- Fire issues due to the use of foam
- Termites and the use of foam below-grade
- Structural concerns, especially for high loads due to backfilling, wind, earthquake; special constructions; attachment/integration of walls, floors, roofs; and proper filling of forms with concrete
- Moisture protection
- Attachment of finishes
Builders should consult with the ICF manufacturers and local code officials to resolve any code issues.
Bruce Davis Construction: Washington Square, La Plata, Maryland
Hopke Buildings & Grounds: MADE to Last Home, Sturgeon, Missouri
Hughes Construction: Lexington, North Carolina
Lancaster County Career & Technology Center (LCCTC): Mt. Joy, PA
ICFs are commonly installed on standard spread footings or on-grade concrete slabs. Layout lines are snapped and the ICFs stacked or set in place, typically in an interlocking fashion. Walls are braced and aligned. Steel rebar is placed where required in the hollow cores. Concrete is placed, typically with a concrete pump in 4-foot heights, and consolidated with care so not to create a "blowout," or to rupture the form. Curing takes about seven days.
After curing, standard construction materials are used to complete the roof, floors, and interior walls.
Basic Construction Steps
Below is a typical construction sequence for building walls
with ICFs. For the most part, these steps apply to both above
and below grade. Note that many details are not included and
that exact procedures or sequence may vary according to type,
manufacturer, code requirements, and/or preference. Check with
the manufacturer to determine specific construction details.
Basic Construction Steps: Walls
- Place dowels (rebar) in footings, foundation wall, or slab as required.
- Place temporary braces along first course to align the ICF forms and to prevent movement.
- Set blocks on concrete footings. Concrete can be recently placed and uncured.
- Place termite shield if required by code authority.
- Complete one course all the way around.
- Set horizontal and vertical rebar as required.
- Subsequent courses should typically be staggered so that vertical joints do not line up from one course to the next. Make sure vertical and horizontal cavities line up.
- Cut for openings as required (or cut out after entire wall is built)
- Install bucks/opening blockouts. A buck may be one of three types: recessed, protruding, or "channel." A pressure-treated 2x buck is frequently installed to provide an attachment surface for windows and doors. Alternatively, a
water-resistant membrane may be used between wood and ICFs. Some prefabricated plastic and vinyl bucks are now available and becoming widely used. Sizing a buck is key to efficient installation of windows and doors. Whether the windows have "masonry style" window frames or frames with nailing flanges, the rough opening should be sized appropriately to accommodate the actual windows size.
- Brace forms as required. Strong, temporary bracing of all walls and openings in ICF walls is important to
keep them plumb and square during the concrete pour and to support the weight of the concrete until it achieves the desired strength. Bracing is needed at corners, window, and door openings, periodically along the length of walls, and at the top of the forms. Top braces square the forms and provide a surface to check wall height and cut uneven blocks.
- Place anchor bolts and ledgers as required. Floor system attachment options include ledger, pocket, embedded
joist hangers, or direct bearing. Ledgers may either be pressure treated wood or may include a water-resistant membrane. Bolts and ledger are placed before pour, with foam cutouts around bolts to allow concrete to back up ledger (ledger face must not "bear" only on foam). Embedded joists require cutting out the foam and inserting wood spacers before the pour to create a pocket in which to seat the joist. Some code authorities also require the embedded joist to be fire-cut. See details provided by manufacturer.
- Sleeve penetrations.
- Foam seal joints as required (possibly per course). Foam sealant can be used along joints to secure blocks until
concrete is poured.
- Pour concrete in 2 to 4 foot lifts using chute or pump per manufacturer's instructions. A "high flow"
concrete mix that will move well through a pump is typically used. A free-flowing mix is paramount to allow concrete to flow into all interior spaces of the form. Failure to follow manufacturer's instructions for bracing and lift can result in a blow-out. If a blow-out occurs, it can be quickly repaired with lumber or plywood and some form of attachment/bracing.
Changes in Current Practice Likely with the Use of ICFs
The use of ICFs may require the following changes in current practices throughout the construction process:
- Finishes, trim, cabinets, and interior partition walls may require special attachment methods.
- Moisture protection: Products used for moisture protection of foam below-grade must be non-petroleum based.
- Utility penetrations: Through-the-wall penetrations require pre-planning and sleeving, or later drilling through concrete. The use of wireless lighting switches, such as that provided by Lightning Switch (www.lightningswitch.com), can reduce labor and add flexibility for users of ICF walls.
- Installing wiring and plumbing in walls - Foam must be grooved
out for pipes and wires.
- HVAC equipment may be sized smaller than for conventional
Warranties vary by manufacturer, but typically cover any damage to the structure due to faulty manufacture for a period of 25 to 30 years.
ICFs allow trade contractors to construct concrete walls without a significant investment in reusable wood and metal forms. Because they use non-biodegradable materials, they are not subject to rot. They can increase the temperature range for pouring concrete to below freezing by insulating the concrete until fully cured.
ICFs may be used for either above- or below-grade walls. The comparative benefits and limitations of ICFs must be considered in light of the systems which it replaces. In the United States, foundation walls are typically cast-in-place (CIP) concrete or concrete masonry unit (CMU) block walls, whether for basements, crawlspaces, or stem walls.
Benefits: Foundation Walls
ICFs can be used for full basements, crawlspaces, or stem walls for slabs. Possible benefits of ICFs when compared to CMU or CIP concrete foundations include:
- Protection of concrete from temperature extremes -- Insulating forms make it easier to protect the concrete from freezing and rapid drying. Concrete can be poured in ICFs at ambient temperature as low as 10°F, requiring only the top of the form to be protected with insulating blankets. In extremely hot weather, in which evaporation is a concern, the top of the form need only be covered with plastic sheeting.
- Foundation walls built with ICFs may be easier and faster to construct than either CMU or CIP foundations depending on the area and house configuration.
- With ICFs, forms do not need to be removed as with normal CIP concrete using wood or metal forms, eliminating another visit by installation crews to revisit the site to remove forms.
- Especially where finished basements are desired, the cost differential may be quite small.
- ICF walls are ready for interior finishing, although some products may requiring furring out first.
- Carpentry crews can be trained to build with ICFs quite easily. Studies have shown that the learning is overcome during the first three hours of building with ICFs.
- Total labor plus material costs may be less than CMU foundations.
- When used as a stem wall for slabs, ICFs provide built-in slab edge insulation for enhanced energy efficiency because the interior slab is poured completely inside the exterior ICF wall. ICFs provide an easier method for placing edge insulation than conventional methods.
- Scheduling of trades can be simplified because specialty foundation construction-related trades may not be needed.
Benefits: Above-Grade Walls
ICFs can be used in place of wood framing for most above-grade situations, placed on slabs or basement or crawlspace walls. Possible benefits of ICFs over wood framing include:
- Strength, namely resistance to high winds and with wind borne debris. Can be designed to resist seismic forces and abnormal loadings.
- Energy efficiency / Comfort
- Thermal Mass
- Noise abatement
- Reduced number of subcontractors and construction steps
- Extension of the building season
Costs: Above-Grade Walls
Above-grade ICF walls cost more to build than typical wood framed walls. As wood-framed walls approach the thermal insulation value of ICFs, cost differential decreases. In most cases, material costs (concrete and forms) are primarily responsible for increased costs, while labor costs are often similar to wood framing. Cost premium depends on relative material prices, labor efficiency for each system, necessity for engineering, and effect on other practices or trades, among other factors. The cost premium for ICF houses is smaller in areas such as high-wind regions that
require additional labor, time, and materials for special construction of wood-framed houses.
Cost differential can be expressed several ways, either per square foot of wall area, per square foot of floor area, or as a percentage of the total cost of the house to the builder or buyer. According to an NAHB Research Center study, costs are estimated to increase by 1 to 8 percent of total house cost 4 over a wood-framed house.
Results from the NAHB Research Center's Demonstration Homes Project showed that total costs for construction of ICF foundation walls can be less than that for block walls. One ICF system had total costs of $1.25 per square foot of house floor area compared to $1.27 per square foot of house floor area for the block wall based on the construction of a short (~ two-foot) "stem wall."
For foundations, ICFs cost about the same or less than CMU or CIP wall systems. ICFs cost about the same
as for block construction with furring and insulation, but can be erected in one-third of the time.
Costs: Potential Added Costs for ICF Construction
- Engineering: One builder reported a slight cost increase for engineering services. The builder is a structural engineer and normally does his own engineering, but he had an outside structural engineer involved with the design.
- Because of the thick walls that ICFs produce, finishing doors and windows may add expense. Doors and windows will require extension jambs, hand trimming, or drywall returns. The use of drywall returns may be more cost effective than hand trimming.
- Trade contractors (siding, drywall, plumbing, electric, carpentry) may charge extra because of unusual tasks such as routing in foam for wires or pipes, installing furring strips for siding installation, screwing vinyl siding in place, or using adhesives and screws for hanging drywall. On one demonstration house, most of the trades did not charge more for working on an ICF home. The framer charged more, but the increase was offset by not requiring other trades. The electrician charged 5% to 6% more.
Costs: Offsets to Additional Costs
Because ICF construction is inherently energy efficient and airtight, heating and cooling equipment and ductwork can be downsized, resulting in a lower cost for equipment.
Siding costs can be reduced when Exterior Insulation and Finish Systems (EIFS) are used for the exterior finish, since the foam used for these finish systems is already in place.
One builder from the NAHB Research Center Demonstration Homes Project reported a decrease in customer service costs due to fewer callbacks.
When basements are intended to be finished, there would be some cost reduction by eliminating insulation
and interior framing along exterior walls.
Costs for Homeowners
Because ICFs are well-insulated and airtight, homeowners will have lower operational costs (utility bills) than a typical wood-framed house.
ICF homeowners' insurance may be discounted because of the disaster-resistant concrete construction. One builder reports that a ten percent reduction was typical.
Costs: Learning Curve Impact
Builders should expect to see higher costs than expected for the first few houses. As with any new product or technique, there is a learning curve in order to reach typical efficiency and cost. For ICFs, three or four jobs appears to be a sufficient number to overcome the learning curve. Once the process is understood, ICF wall construction typically takes about the same time as wood-framed wall construction. The learning curve can apply to others involved
in construction, from engineers and architects to builders, trade contractors, and code officials. Additional time is needed for background research as builders decide upon system type, manufacturer, best methods of construction such as methods for providing attachment for trim, and choose compatible components such as waterproofing
and finish materials.
1. PCA Wind Tunnel Tests
2. VanderWerf, Peiter, Insulating Concrete Forms
3. VanderWerf, Pieter
4. NAHB Research Center, Inc., Demonstration Homes Project