PARTIAL FILL CAVITY INSULATION

Generally speaking partial fill cavity insulation leaves a 50mm gap between the insulation layer and the outer leaf of the wall, which acts as a barrier to prevent external moisture reaching the internal leaf and can be used on buildings of any height. The purpose of the air space in a cavity wall is as a barrier to the penetration of rainwater to the inside face of the wall. If the clear air space is to be effective as a barrier to rain penetration it should not be bridged by anything other than cavity ties. It is important to leave clear air space of all mortar droppings and other building debris that might serve as a path for water. Good practice, therefore, is to use a 50-60mm wide air space between the outer leaf and the partial fill insulation. To meet insulation requirements and the use of a 100mm cavity with partial fill insulation it may be economical to use a lightweight block inner leaf to augment the cavity insulation to bring the wall to the required U-value. Partial-fill is the standard choice for most new developments. The 50mm gap has the advantage of reducing the chance of any moisture that does penetrate the outer leaf bridging through to the internal structure. For that reason, the system is required by many local authority building control departments especially in the UK’s many ‘exposed’ locations. 

PARTIAL FILL MATERIALS

The materials used for partial fill insulation should be of boards, slabs or batts that are sufficiently rigid for ease of handling and to be retained in a vertical position against the inner leaf inside the cavity without sagging or losing shape, so that the edges of the boards remain close butted throughout the useful life of the building. They are supplied at 455mm wide to allow easy installation between standard vertical wall tie spacings and minimum on-site cutting and waste.

Those partial fill materials can be grouped as:

• Inorganic insulants
• Organic insulants

Organic insulants are made from naturally occurring materials that are formed into fibre, powder or cellular structures that have a high void content, as for example, glass fibre, mineral fibre (rockwool), cellular glass beads, vermiculite, calcium silicate and magnesia or as compressed cork. Organic insulants are generally non-combustible, do not support spread of flame, are rot and vermin proof and generally have a higher U-value than inorganic insulants. The organic insulants most used in the fabric of buildings are glass fibre and rockwool in the form of loose fibres, mats and rolls of felted fibres and semi-rigid and rigid boards, batts and slabs of compressed fibres, cellular glass beads fused together as rigid boards, compressed cork boards and vermiculite grains.

Inorganic insulants are based on hydrocarbon polymers in the form of thermnosetting or thermoplastic resins to form structures with a high void content, as for example polystyrene, polyurethane and phenolic. Inorganic insulants generally have a lower U-value than organic insulants, are combustible, support spread of flame more readily than organic insulants and have a comparatively low melting point. The inorganic insulants most used for the fabric of buildings are expanded polystyrene in the form of beads or boards, extruded polystyrene in the form of boards and polyurethane and phenolic foams in the form of preformed boards or spray coatings. With the recent increase in requirements for the insulation of walls it may well be advantageous to use one of the somewhat more expensive inorganic insulants such as XPS (extruded polystyrene), PIR (polyisocyanurate) or PUR (polyurethane) because of their lower U-value, where a 50mm clear air space is to be maintained in the cavity, without greatly increasing the overall width of the cavity.

Partial fill materials used as insulation for the buildings may be also grouped as:

• Expanded Polystyrene
• Phenolic Boards
• PIR Boards
• Mineral Wool Batts.

EXPANDED POLYSTYRENE

Partial Fill Polystyrene Cavity Wall Insulation Boards are a far more cost-effective alternative to PIR that offers a 20% reduction in thickness. This insulation will reduce your carbon footprint and help to decrease heating bills by limiting heat loss. Polystyrene Cavity Wall Insulation Board (EPS 70) (1200 x 450mm - coverage 0.54m²) is manufactured from low lambda polystyrene and is a square edged panel that offers excellent thermal performance when used within masonry wall constructions to satisfy the relevant U-value requirements. 

Products:

SPI Stylite Partial Fill EPS 

Jablite Partial-Fill Cavity Wall insulation  EPS 70 (white) and EPS HP 70 (grey) 

PHENOLIC BOARDS

Partial fill cavity board is a premium performance insulation with a fibre-free rigid thermoset phenolic core. It provides a level of performance that is unrivalled in the world of rigid thermoset insulation materials with a thermal conductivity of just 0.018 W/mK across all thicknesses. However, phenolic products are more expensive up front and installation can require more attention to detail on site.

Products:

SR/CW Cavity Wall Insulation Board Xtratherm Safe-R  

Kingspan Kooltherm K108 Phenolic Cavity Insulation Board 

PIR BOARDS

PIR partial fill cavity boards are a high performance insulation with a fibre-free rigid thermoset polyisocyanurate (PIR) core. They are used to insulate partial fill cavity walls, and have usually have a thermal conductivity of 0.022 W/mK. Rigid products offer better thermal performance allowing you to specify a thinner wall or achieve a better U-value at the same thickness. 

Products:

Kingspan® Thermawall TW50 

Celotex® CW4000 Cavity PIR Insulation Board

Xtratherm® XT/CW Thin-R Partial Fill Cavity Insulation

Xtratherm® Xtroliner XO/CW

MINERAL WOOL BATTS

Mineral wool partial fill batt is designed to provide thermal insulation in partial-fill external masonry cavity walls, with no height restriction (additional requirements apply for buildings above 25 metres in height for partial fill). It can be used regardless of height and in cavities in areas of 'very severe' exposure without rendering the outer leaf. Semi-rigid batts of glass mineral wool or rockwool are the traditional choice. Their flexibility allows them to fit snugly against the masonry as well as knit together at abutments.

Products:

Rockwool® HP Partial Fill Cavity Slab

Knauf Rocksilk^® RainScreen Slab

URSA 32 Cavity Wall Insulation Batts

Superglass® Superwall 36 

URSA® 32 Cavity Wall Insulation Batts

PARTIAL FILL CAVITY INSTALLATION

Usual practice is to build the inner leaf of the cavity wall first, up to the first horizontal row of wall ties, then place the insulation boards in position against the inner leaf. Then as the outer leaf is built, a batten may be suspended in the cavity air space and raised to the level of the first row of wall ties and the batten is then withdrawn and cleared of droppings. Insulation retaining wall ties arc then bedded across the cavity to tie the leaves and retain the insulation in position and the sequence of operations is repeated at each level of wall ties. Insulation retaining ties are usually standard galvanised steel or stainless steel wall ties to which a plastic disc is clipped to retain the edges of the insulation. The ties may be set in line one over the other at the edges of boards, so that the retaining clips retain the corners of four insulation boards.

Related articles:

All types of partial fill cavity insulation

How to choose the right Kingspan insulation board for construction?

How to work out how many square metres of insulation I would need?

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Best 6 passive fire protection cavity barriers.

4 types of full fill cavity insulation explained

Cheaper alternative/equivalent to kingspan insulation

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