EPS Cavity Wall Insulation | Thermaboards


Expanded Polystyrene (EPS) insulation is effective in reducing the thermal transmittance (U-Value) of the walls of new and existing buildings. Recommended density for wall insulation is both standard or high density. High density is recommended for cold humid areas.

EPS cavity wall insulation is made from standard EPS boards in thicknesses and edge profiles to suit the applications. EPS boards are for use as a complete or partial fill to reduce the thermal transmittance of cavity walls with masonry inner and outer leaves. The installation of EPS insulation during the construction of walls must be carried out by competent contractor.

Design Data

EPS boards are not to be considered as contributing to the structural strength of the walls. The width of the EPS boards and any additional gaps are considered as the cavity for structural purposes. Cavity walls must comply with the rules of the National Building Regulations or be designed in accordance with SANS 10164-1 by a competent person. Adequate wall tie densities must be achieved in both vertical and horizontal planes.

Properties In Relation To Fire

It is highly unlikely that the EPS board will ignite in the cavity. The use of the EPS boards will not introduce any additional hazard in respect of behaviour in fire when compared with traditional walls. However, in terms of SANS 10177-5, EPS boards on their own are considered combustible. Cavities are to be bricked closed horizontally and/or vertically around the perimeters of defined fire rated compartments, and around any openings e.g. windows, doors, to prevent movement of toxic fumes or hot gases. EPS boards must not be stored near flammable liquids, waste etc. and not exposed to heat or open flames.

Thermal Insulation

For the purpose of U-Value calculations to determine if the requirements of building regulations or other regulations are met, the thermal conductivity (k-value) of the insulation may be taken as 0.037 W/(m.k.) This value allows for an increase in thermal conductivity due to ageing and water absorption.

Where the insulation has not been continued into window or door reveals due to a lack of clearance, there will be a risk of cold bridging at these points. Where door and window frames are to be replaced, it is recommended that their size be adjusted to permit the reveals to be insulated.

Depending on constructional details, cold bridging can also occur at the eaves and at ground-floor level, and care should be taken to minimize this, e.g. roof or loft insulation should continue over the wall head, ensuring that ventilation openings are not obstructed.

Technical Specification

a) The walls are constructed leading with either the inner or outer tier. If a residual cavity is specified, the inner tier must then be constructed ahead of the outer tier and the insulation fixed to the cavity face of the inner tier. Fixing EPS boards in this manner enhances the thermal performance.

b) EPS boards are fixed to the cavity face of the leading tier with wire ties which are additional to the cavity ties specified. These additional ties may be standard cavity ties or may be made from 2 mm diameter L-shaped galvanized wire. They must be installed at the same height and mid-way between the specified cavity ties. The EPS boards must be held firmly in place while these additional ties are bent in position. Ties are bent up and down alternately to secure the lower and upper edges of the EPS boards.

c) The installation procedure is as follows:

  • A section of the leading tier is built with wall ties in position, with consideration being given to the required height of the first fit of insulation. The first row of ties is placed in position.
  • Excess mortar is removed from the cavity face of the leading tier and the EPS board is attached firmly with the wire ties. The edges of the EPS boards are carefully notched to accommodate the ties.
  • Construction of the leading tier continues ensuring that the exposed edges of the EPS boards below remain free of mortar.
  • After a few rows of EPS boards have been positioned the building of the second-tier progresses insuring that if a residual cavity is required the cavity ties and cavity itself remain clear of mortar droppings.
  • The leading tier is continuously built, EPS boards installed and the second tier built, as described above until the full wall is completed.

d) The thickness of the wall tiers and the cavity widths define the horizontal spacing of the cavity ties. Ties are installed at vertical spacings of 600 mm and coincide with the horizontal joints of the EPS boards. Where window and door openings are provided, and at control joints the vertical spacing of the cavity ties must be reduced to 300 mm. EPS boards must be cut to accommodate these ties. During installation of the EPS boards care must be taken not to loosen the cavity ties embedded in mortar joints.

e) The use of cavity boards and battens helps to keep cavities and the exposed top edges of the EPS boards free of mortar droppings.

f) EPS boards may be cut using a sharp knife or a fine tooth saw to fit round window or door openings. No gaps must be left in the insulation. Cut pieces of EPS boards must be glued in position to completely fill open spaces.

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