Designing with ICF BLOCKS – The Pros and The Cons

Designing of an ICF block structure has some major pros and a few cons. In this page we will be looking at the positive advantages and some of the negatives for the designer.

ICF Block – Design Pros

With any ICF block system the designer can rest easy knowing that that the thermal performance will perform to the design expectations. This not always the case with traditional construction or timber frame. Both these build types can suffer from the bad or indifferent builder taking liberties with the insulation installation. Traditional brick construction rarely meets its designed performance. Bricklayers are paid to lay bricks. The insulation is often viewed as being an nuisance! Too often is the cavity insulation not properly placed, and air gaps between the inner skin and the insulation are much too common. These air gaps are from sloppy attachment and the mortar snots on the cavity face of the inner skin. Here is an academic article on the performance gap in domestic buildings

The ICF structure is invariably going to be airtight, due to the solidity of the dense monolithic concrete core. Traditional construction suffers from the tiny air gaps in the perpendicular joints on many brick joints.

The monolithic concrete core of ICF blocks also creates a structure of high structural strength and integrity. Perfect for disaster resilience, and protecting the inhabitants agianst the unexpected.

It will be much quieter for the residents than any lightweight structure. People enjoy the sense of protection from an ICF structure.   There is less room for builder error than one may see in both traditional or lightweight construction. 

ICF Blocks – Cons

Polystyrene ICF Blocks have some limitations due to the modular nature of the product.  The need to vertically line up the internal ties and the male/female connections can cause some design issues. This often caused by window widths and heights not keeping to the module. Some systems have issues with certain types of floors.

Ideally wall lengths should be designed to multiples of the modules.  When using one of the North American systems a further nuisance layer is added by the fact that everything is manufactured in imperial sizing.

Imperial Sizing Issues

The imperial sizing can cause issues with follow on events like fastening plasterboard. The internal fastening points are set for 4 ft plasterboard which measures in at 1220mm.  With UK plasterboard being 1200mm things can quickly go out of skew.

The way round this is to dry line with battens, which will reduce the living space. Dot and dab fixing of the plasterboard is not an option as this compromises the fire rating.

Material Issues

ICF blocks are always made of EPS. It is the only insulation that can be moulded to shape. This material’s main advantage is that it is cheap to make and buy.   High grade  EPS with tight and well-fused beads is an excellent product with a good moisture resistance. It can be difficult to assess quality without examining the interior of the board as the surface usually fuses better than the interior. 

Moisture will enter much quicker when the surface is cut. Wet insulation is no longer an insulant but a conductor of heat. The ingress of moisture will cause the material to degrade much quicker.

There can be a fair amount of wastage with ICF blocks, but they are quick to assemble, though in truth, that is a small part of the assembly time.  They build best when coming off a very level surface.  Unlike traditional brick construction there is no levelling compound of the mortar so inconsistency in the set out can come back to bite you later. 

Here at Polarwall, we also suggest building from a flat and level surface, but in reality we can build off anything as the sloping swimming pool picture to the side shows.  Also the Polarwall formwork can be adjusted to level on each and every course.

The Tie Material Within the block

There are three types of material used to make the crosstie within an ICF Block.

The first is a polystyrene bridge. The very earliest ICF systems used this. The low cost of production is the main advantage. However, the size and width of the bridge can inhibit proper concrete consolidation. The width of the bridge can also cause uplift of the block as the flowing concrete will try to float lighter materials.

The second type of tie is a steel tie. Being steel this is extremely strong in itself. Designers need to be aware that the steel ties are thermal bridges and will reduce the thermal performance of the ICF block by around 30% according the the BRE U-Value Calculator. Some claim the steel ties adds to thermal mass and provides extra reinforcement. Neither of these claims are true. The steel is going in the wrong direction and thermal mass inside an ICF is mostly nonsense. The steel ties become a series of heat exchangers built into the wall.

The third type of tie is a plastic tie. This is the best option with any ICF system. Plastic has a better Lambda value than concrete so there is no thermal bridging unlike a steel tie. A plastic tie allows a better flow of concrete inside the structure for good consolidation. However ties with flat horizontal surfaces that can add to uplift.

Polarwall – Design Pros and Cons  

Polarwall used to call itself an ICF but now prefers to use the name of Permanent Insulated Formwork (PIF).  The reason for the change of name (and acronym) is because we are so unlike the  EPS moulded ICF block.   Polarwall is assembled using flat planks of insulation and consequently any insulation could be used.  We do not offer an EPS option as the inconsistency of the material is not ideal.  Extruded polystyrene (XPS) is the best ICF material.  This is much stronger than EPS (both in compression and tensile strengths),  has a far higher moisture resistance and a better thermal performance.  It also has a high consistency in terms of quality.  

We would love to use some high performing insulation boards such as phenolic or PIR boards but these do not have the inherent strength to resist the concrete pour or the moisture of the fluid concrete. 

The boards we use have a special textured surface designed to bond to cementitious materials,  Consequently when the concrete cures against the board it creates a “complete composite material”.   

Only Non-Modular System

Being a “plank” system means that there is no modularity to concern the designer.  Therefore,  any height or shape, straight , curved or angled are easily and simply formed.  In fact, curved walls are not any more expensive than straight walls either in terms of materials or assembly.   Whatever the designer designs and to whatever length, height, or spec then the builder will build.  

Our small section ties allow for perfect compaction of the concrete and also allow us to use welded wire mesh in retaining walls. Engineers love specifying welded wire mesh as it reduces potential builder error in the steel fixing. 

Polarwall is usually site assembled. It can also be be pre-assembled (or site assembled) in large panels for extremely fast build speeds. 

Polarwall give free training to designers and builders.