Frequently Asked Questions
We are very happy to answer any questions that you may have. If you don't find the answers here, please telephone or email us and we'll do our best to help.
01 Are there any design limitations?
No. The ICF system we specialise in offers excellent design flexibility. Unlike other ICF systems, there are no modular repeats. Instead, as a panel system, boards are simply cut to size for wall lengths and heights. With 4 basic components (extruded polystyrene boards, ties, corner pieces and rails), we can build pretty much any structure, from curves to stepped walls and corbels. In fact, reinforced concrete provides architects and engineers with increased design options through the use of retaining structures and large cantilevers. Added to this is the ability to maximise adverse and difficult to develop sites because of the lightweight but ultimately robust nature of ICF structures.
02 Is there any flexibility to make changes as you build?
Yes. One of the major benefits of our chosen ICF system is the ability to build independent walls. Sometimes, as a result of design or engineering changes perhaps, it is not possible to finish a section of wall. With other ICF systems work has to stop because a course has to be completed at a time. Our preferred system allows one to stop along a section of wall, continue building upwards and then join the wall with the H-rail at a later point. This has proved to be beneficial on several occasions. It also means that large panels can be built independently on a jig, lifted into place and joined with connector strips. In fact, on some projects this method is the preferred construction choice, resulting in excellent build speeds.
03 How do you build the walls?
A course of U-rails (sometimes H-rails) joined together by ties are secured to the concrete footings or slab and then 300mm wide extruded polystyrene panels are placed into the rails. More rails are put on top of the panels and then another course is constructed. Corner braces go in after the first course and the alignment props after 3 courses. Plumb corners result in true formwork and the fact that we are able to pour up to 3.3 meters at a time leaves plenty of time to align the wet concrete for completely upright and straight walls.
04 How do you get the concrete into the walls?
Concrete placement is usually achieved with a boom pump through a hose reduced down to a diameter of 75mm. This gives us excellent control of the flow of concrete and minimises the pressure on the formwork. On occasion we use line pumps, concrete hoppers and have on occasion used a tele-handler with a bucket on the end.
05 Does the concrete need vibrating?
On placement, a vibrator is used to ensure that there are no voids are left in the concrete. Excellent compaction of the concrete is achieved with minimal vibration which tales place immediately the concrete is poured.
06 How many storeys can be built?
Ultimately a building's height is determined by the structural engineer, but in our experience 5 to 6 storeys seems to be the optimum height.
07 Does the bracing hold the walls up?
The formwork is totally self-supporting and the props are used solely to keep the walls straight. With the combination of the H-rails and alignment props, we pride ourselves on being able to build the straightest walls in the ICF industry.
08 How high can you pour concrete in one go?
We usually pour a full storey height at a time, up to approximately 3300mm.
09 How soon after pouring can the braces be removed?
As a general rule, we allow a minimum of 24 hours before removing the props. Where we have cantilever overhangs which are propped, we leave it to the discretion of the structural engineer.
10 What stops the panels lifting during the concrete pour?
This ICF system is unique in that the H and U rails clamp the boards within the channels and mechanically stop the boards lifting during the pour. The cross ties stop the outside panels from separating, giving a very robust formwork into which concrete can be poured.
11 Can I get a mortgage on an ICF house?
Yes. There are no issues with mortgages on ICF houses as ICF is an accepted Modern Method of Construction in the UK. Qualifying ICF systems have full BBA certification and providers like the NHBC, LABC and Premier Guarantee all provide structural warranties.
12 How warm are the finished walls?
U-values to passive house level can be achieved, but in our experience the standard 80mm extruded polystyrene either side of a 155mm core (0.18W/m2K) is more than enough to build a house which is extremely easy to keep warm.
13 Is the formwork weathertight?
The extruded polystyrene provides an excellent weather proof finish. On one occasion we had a project where the developer had financial difficulties which meant that the project remained unfinished for 5 years. The ground floor polystyrene was left exposed to the elements all this time and beside a small amount of discolouration, the building remained intact and perfectly dry.
14 Is a DPC required?
Because of the inevitable steel dowels or starter bars from a slab as a key for the concrete core, a conventional DPC is rarely an option. A simple and effective solution is a liquid hand or spray applied DPC of which there are a number on the market with BBA certification.
15 What about floor attachment?
One of the many reasons we favour ICF is because of the simplicity with which intermediate floors are attached. Our preferred construction method are pozi-joist and timber i-beam type floors because of the ease with which they are tied into the superstructure. A course of polystyrene is omitted internally and timber used instead. The timber is cast into the concrete using bolts and the joist hangers attached to the timber - a very simple and cost effective solution. There are also proprietary ICF joist hangers which are placed in the formwork prior to concrete placement and become cast in once the concrete has cured. For beam and block or concrete hollow core floors, the walls are poured and left to cure. Once cured, the flooring is laid onto the concrete wall and then the formwork built up over and the floors cast in. Again, a very simple process. A further option is cast-in-situ concrete floors which can be designed to accommodate significant spans.
16 What about internal walls?
We regularly use the ICF formwork to construct internal walls, particularly when building apartments. The system is flexible in that the polystyrene can be stripped back to expose the concrete core, or left on to increase thermal performance. The ability to build 206mm and 251mm concrete core walls is particularly useful for party walls. All proprietary internal wall systems from timber and metal stud to concrete block, are simple to tie into the concrete by exposing the core and using conventional mechanical fixings.
17 How thick are the walls?
Walls vary in thickness depending on the U-value required and the structural requirements designated by the engineer. We install a 315mm thick wall most regularly - 80mm of polystyrene either side of a 155mm thick concrete core, with a U-value of 0.18 W/m2K above ground. Retaining walls and basements vary in wall thickness, but usually have a concrete core thickness of 206mm, 251mm and 302mm.
18 Can the system be used for basements?
ICF is arguably the perfect construction method for basements and our chosen ICF system in particular. Modular block ICF systems are limited in terms of steel placement, whereas we are able to facilitate the use of steel mesh in construction - significantly speeding up build times. The lightweight nature of the system lends itself very much to building below ground and surprisingly substantial structures can be achieved quickly with a relatively small workforce.
19 What foundations are required?
This ICF system can be built off any foundation type and is in fact often used to create the foundations themselves. Strip footings can be cast using the ICF formwork, as well as used to create a raft type foundation.
20 Why extruded polystyrene?
There are two types of polystyrene used in ICF construction - Expanded Polystyrene (EPS) and Extruded Polystyrene (XPS). EPS is basically lots of plastic balls heat welded together by compression. XPS is plastic pushed through an extrusion mould and impregnated with air using a blowing agent. We have used both types and found the XPS to be significantly more robust when pouring concrete. The compressive strength of XPS is greater than EPS and is also more waterproof. In short in our opinion, XPS wins any head to head with EPS when using it in ICF formwork.
21 How is cladding attached to the walls?
One of the benefits the ICF system we use is the fact that the plastic rails are cast into the concrete at 300mm centres. These rails provide a substantial mechanical fixing point for internal finishing sheet materials like plasterboard and cement board. Conventional external cladding systems like timber, slate, tiles, stone and brick slips are fixed back to the concrete core, as are more substantial finishes like brick, stone and granite quoins. Render is usually a proprietary render designed for ICF with a plastic mesh in the base coat covered by a finishing coat. Conventional sand, cement and lime render can be used but must be put onto a render carrier.
22 How are services chased into the walls?
Prior to plaster-boarding, electrical services are easily chased into the wall by removing 20-25mm of material from the face of the internal polystyrene panel with the use of a hot wire cutter or multi-tool. Water pipes and 32mm and 40mm waste pipes are chased into the walls in the same manner if desired and even 110mm can be buried up to 80mm deep into the walls to keep boxing sizes to a minimum.