Insulated Concrete Forms (ICF) evolved rapidly since the 1960’s; as a method of constructing buildings in Europe; since then the technology has become commonly used in Europe and North America. ICF was first introduced into Australia in the 1980’s. Benefits of the building material include:
1. Light weight and easy to construct
ICF can be managed easily by an owner builder or used in a larger development given the weight and ease by which the blocks are assembled and where necessary cut to size. There is significant weight in the metal reinforcing inserted into the ICF blocks however since these are layed individually after each row of blocks is put in place, reinforcing is easy to manage and spacings in the ICF provide pre-defined supports to ensure that reinforcing is installed correctly.
Bracing of the ICF is essential to ensure walls remain perpendicular and do not distort (“blow out) at the base once concrete is added to the structure. The fixing of bracing is possibly the most critical aspect to ICF construction and to make this easy some ICF manufactures supply custom designed supports with each order (note however this can significantly increase the cost). Supports often provide a safe platform at wall height so that concrete can easily be added to the ICF structure once each level is complete. Concrete is generally pumped into walls and agitated to ensure that there are no gaps in the concrete, maximising the strength of the structure once cured.
2. Thermal Insulation and Thermal Mass
ICF by it’s nature provides good thermal insulation with Thermal R-Value’s typically around 4.0; R-Value is a measure of thermal resistance so as a comparison cavity (double) brick typically has an R-Value around 1.1. This means that Cavity brick conducts heat 4 times more quickly than an ICF construction. R-Value and Insulation is of course only one factor to consider; the other is thermal mass. Thermal mass is the ability of a material to absorb and store heat energy. A lot of heat energy is required to change the temperature of high density materials like concrete, bricks and tiles. They are therefore said to have high thermal mass. Lightweight materials such as timber have low thermal mass. So in the case of Cavity brick, even though the bricks are poorly insulated, it takes some time for bricks to heat-up and then cool due to the energy stored in them. As an example, if you have lived in cavity brick home you may notice that on a hot day the bricks heat up slowly and then stay warm and continue to transmit heat into the house well into the evening after the temperature has cooled outside. The opposite can be observed on a cold winters day.
So what about timber or steel frame constructions which are clad with insulation that has a high R-factor? Like ICF; heat is slow to leave and enter the the stud wall due to the insulation; so on one hand this is better than cavity brick; there is however a disadvantage which is the low thermal mass of the walls. As soon as heat does pass through the insulation, it has a more immediate impact on the temperature in the house.
Insulation efficiency of walls is only one consideration and losses due to windows and movement of air (lack of air tightness) can often be a much bigger factor impacting the energy efficiency of the entire house; despite this use of ICF can be the difference between needing single or double glazing in Sydney; or if using double glazing it can result in a significant reduction in heating and cooling costs in summer and winter.
3. Acoustic Insulation
According to the Australian Government (see Your Home website) “Noise can interfere with sleep, rest and conversation and cause fatigue, irritability, headaches and stress. Surveys show that noise is an important environmental concern for most Australians. We all need to contain and reduce noise and protect ourselves from sources of noise in order to enjoy a healthy life. Thoughtful design and practice can reduce the impact of noise on our lives and improve the quality of our living environment.”
ICF is a good sound insulator mostly due to the concrete contained in the structure as closed-cell materials, such as expanded polystyrene, are not good at absorbing sound. As an example Eco Block claim a STC (Sound Transmission Class) rating of 54 for their 280 Series ICF which means loud speech cannot be heard at all through the ICF wall. Drywall attached to both sides of a wall provides very little difference to STC and in-fact adding drywall to both sides of the wall with an unfilled furring space can reduce STC due to mass-air-mass resonance similar to the action of a drum. If greater levels of acoustic insulation are required fibrous insulation can be added in the furring space between the Wall and Drywall.
It is important to consider that walls as one element of design that provides noise insulation, as with thermal insulation windows can be the weakest link when it comes to noise insulation; so if noise could be an issue when designing your home, you need to make sure all the structural elements will provide adequate noise insulation.
4. Fire resistance
Polystyrene and polyurethane foam do not burn and support flame or fuel fire and ICF products are generally treated with added flame retardant, making it virtually self-extinguishing. With the addition of external renders and re-enforced concrete; buildings constructed using ICF have walls rated at around 4 hours which means in the case of a bush fire, the fire itself will often pass over the structure without the house catching on fire.
As with the other insulation properties, weakness can be introduced in other elements of the structure (for example a roof with eaves); however if other components of the structure are well designed there has been evidence of whole suburbs of houses being burnt out in bush fires and leaving concrete and ICF structures only scorched by the flames.
5. Durability, Strength and moisture resistance
The single house left standing after Hurricane Katrina was of an ICF construction, with many people thinking that the image was a fake. One of the reasons ICF constructions have been seen to stand up to natural and man made disasters is the strength of re-enforced concrete which is at the heart of all ICF construction. The inorganic nature of the materials used in ICF mean that they are impervious to moisture and they do not offer any food value to rodents or insects. It is this combination of strength from concrete and durability and moisture resistance from the forming that make ICF construction superior to many traditional construction systems and materials.
Are there any disadvantages to ICF
One of the possible disadvantages of ICF is wall thickness with external/load bearing wall widths ranging from 150-350mm plus the thickness of external and interior cladding. In comparison Cavity Brick construction has a width of approx. 260mm. Selecting the optimum width ICF for your needs is important to ensure that interior space in the building is not compromised; this can also reduce the amount of concrete required which in turn reduces the cost of construction.
For interior walls the main considerations are load bearing capacity (in particular for multi-story dwellings) and physical separation of internal rooms. Noise transmission is the most likely difference that will be observed between different internal wall construction materials. Minimum thickness ICF of around 150mm is clearly an option however this comes at a price when you consider the loss of livable space in a room. An acceptable alternative that will provide better noise insulation and structural strength are some of the concrete forming solutions that do not include polystyrene insulation. One that we have used for interior walls of ICF constructions is a 110mm product from Dincel which results in 100mm thickness of concrete vs 80mm which would be achieved from a 150mm ICF interior wall.
ICF Construction
Construction of an ICF building typically starts with a concrete foundation with steel reinforcing protruding from the slab at a required distance such that the slab can be tied to the structure of the walls that will be formed later.
Walls are formed in a process of stacking the lightweight blocks together in a similar manor to children’s building bricks. Blocks vary depending on the system and also the usage (single story residential vs commercial construction and fire rating is also a factor). Some examples include:
Zego
a) Domestic Forms that are a single block with cross re-enforcing made of the same polystyrene material;
b) For larger residential and commercial buildings products like Zego Fire Form are recommended, which come as two vertical sheets of insulation with a number of joiners that hold the sheets together.
Eco-Block
Have a range of ICF products with different widths, they are similar in design to the Zego Fire Form but cost effective from small residential through to commercial applications. Eco-Block have available corner sections specifically designed to finish the Corner of the buildings; this requires a little more planning upfront however it can save time in construction.
As walls are constructed, steel reinforcing is inserted into the walls according to the specifications of the engineer, the amount of reinforcing will depend on the structure of the building, the location of the wall, and calculation of loads. All the ICF systems that I have reviewed include spacers for reinforcing so that it is easy to ensure that each layer of steel is located correctly.
Space needs to be left for windows and doors as the walls are constructed and once walls are complete leveling of the walls is essential, if necessary the wall can be leveled with foam under the bottom course. After leveling the walls they are tied to vertical wall bracing which is fixed to the concrete slab and ground on both sides of the wall to prevent movement and avoid “blow-outs” (widening or even worse failure of the ICF structure at the bottom of the walls) as concrete is poured.
Bracing used to secure the walls can also be used to provide a safe walking structure at the wall height, when the walls are ready for concrete to be poured. Most of the ICF manufacturers provide bracing systems, some (ie. Zego) mandate the use of their proprietary bracing with each construction, others offer the builder flexibility to use their own bracing if this is preferred.
Polyurethane foam is applied to any gaps and potential spaces between the bottom of the wall and slab; this is to ensure that there is no leakage of concrete once the walls are poured and to secure the bottom course to the slab.
One of the advantages of ICF construction is there is minimal requirement to move heavy materials during the construction, most of the weight in the walls is a result of concrete which is poured once the ICF structure and form-work is in place.
Some forward planning is often necessary when it comes to booking supply of concrete and a concrete pumping truck and builders need to ensure they have adequate labor on site Concrete is pored into walls progressively followed by agitation to ensure that there are no air gaps in the concrete; more concrete is then poured until the entire wall cavity has been filled with concrete.
Internally ICF walls can be lined by gluing plasterboard directly to the surface of the ICF. Externally multiple coasts of an acrylic render are typically applied starting with a base coat and fiberglass mesh for strength; this is followed by a texture coat and finally an armor coat which ensures that the render is water tight.
References:
Australian Government Your Home Website
Sound Transmission Class Ratings for Concrete Masonry Walls
Australian ICF manufacturers:
Hi Brendan
I like your article on ICF
Just wondering if you would do an article on tilt up ICF? pros and cons
The web site below have this product.
http://www.formcraft.com.au/products/formtilt/
Thanks for the feedback Mal, happy to take a look, FormTilt certainly looks interesting.