Cavity wall insulation: Partial fill, Insulation Materials, Insulation thickness, Total fill, Thermal bridge.

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Partial fill.
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. If the cavity is then filled with some insulating material, no matter how impermeable to water the material is, there will inevitably be narrow capillary paths around wall ties and between edges of insulation boards or slabs across which water may penetrate. As a clear air space is considered necessary as a barrier to rain penetration there is good reason to fix insulation material inside a cavity so that it only partly fills the cavity and a cavity is maintained between the outer leaf and the insulating material. This construction, which is described as partial fill insulation of cavity, requires the use of some insulating material in the form of boards that are sufficiently rigid to be secured against the inner leaf of the cavity.

In theory a 25 mm wide air space between the outer leaf and the cavity insulation should be adequate to resist the penetration of rain providing the air space is clear of all mortar droppings and other building debris that might serve as a path for water. In practice, it is difficult to maintain a clear 25 mm wide air gap because of protrusion of mortar from joints in the outer leaf and the difficulty of keeping so narrow a space clear of mortar droppings. Good practice, therefore, is to use a 50 mm wide air space between the outer leaf and the partial fill insulation.

To meet insulation requirements and the use of a 100 mm cavity with partial fill insulation it may be economic to use a lightweight block inner leaf to augment the cavity insulation to bring the wall to the required U value.
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may be suspended in the cavity air space and raised to the level of the first row of wall ties and the batten is then withdrawn and cleared of droppings.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.

The suspension of a batten in the air space and its withdrawal and cleaning at each level of ties does considerably slow the process of brick and block laying.

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, as illustrated in Fig. 83. 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.

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. For small dwellings the Building Regulations do not limit the use of combustible materials as partial fill insulation in a cavity in a cavity wall.

To provide a clear air space of 50 mm inside the cavity as a barrier to rain penetration and to provide sufficient space to keep the cavity clear during building, an insulant with a low U value is of advantage if a nominal 75 mm wide cavity is formed between the outer and inner leaves. 

Fig. 83 Partial fill cavity insulation.

Insulation Materials.

The materials used as insulation for the fabric of buildings may be grouped as inorganic and organic insulants.
Inorganic 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.

Inorganic insulants are generally incombustible, do not support spread of flame, are rot and vermin proof and generally have a higher U value than organic insulants.

The inorganic 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.

Organic insulants are based on hydrdocarbon polymers in the form of therrnosetting or thermoplastic resins to form structures with a high void content, as for example polystyrene, polyurethane, isocyanurate and phenolic. Organic insulants generally have a lower U value than inorganic insulants, are combustible, support spread of flame more readily than inorganic insulants and have a comparatively low melting point.

The organic 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, isocyanurate and phenolic foams in the form of preformed boards or spray coatings.

The materials that are cheapest, most readily available and used for cavity insulation are glass fibre, rockwool and EPS (expanded poiystyrene), in the form of slabs or boards, in sizes to suit cavity tie spacing. With the recent increase in requirements for the insulation of walls it may well be advantageous to use one of the somewhat more expensive organic insulants such as XPS (extruded polystyrene), PIR (polyisocyanurate) or PUR (polyurethane) because of their lower U value, where a 50 mm clear air space is to be maintained in the cavity, without greatly increasing the overall width of the cavity.

Table 4 gives details of insulants made for use as partial fill to cavity walls.

Insulation thickness.

A rough guide to determine the required thickness of insulation for a wall to achieve a U value of 0.45 W/m2K is to assume the insulant  provides the whole or a major part of the insulation by using 30 mm  thickness with a U value of 0.02, 46 with 0.03, 61 with 0.04, 76 with  0.05 and 92 with 0.06 W/m2K.

Table 4 Insulating materials.

Assuming insulation with a U value of 0.03, then the thickness of insulation required = 0.66 x 0.03 x 1000 = 19.8 mm. This thickness is about one-third of that proposed by the first method that takes little account of the resistance of the rest of the wall construction.

Total fill.

The thermal insulation of external walls by totally filling the cavity has been in use for many years. There have been remarkably few reported incidents of penetration of water through the total fill of cavities to the inside face of walls and the system of total fill has become an accepted method of insulating cavity walls.

The method of totally filling cavities with an insulant was developed after the steep increase in the price of oil and other fuels in the mid-1960s, as being the most practical way to improve the thermal insulation of existing cavity walls. Small particles of glass or rock wool fibre or foaming organic materials were blown through holes drilled in the outer leaf of existing walls to completely fill the cavity.

This system of totally filling the cavity of existing walls has been very extensively and successfully used. The few reported failures due to penetration of rainwater to the inside face were due to poor workmanship in the construction of the walls. Water penetrated across wall ties sloping down into the inside face of the wall, across mortar droppings bridging the cavity or from mortar protruding into the cavity from the outer leaf.

From the few failures due to rain penetration it would seem likely that the cavity in existing walls that have been totally filled was of little, if any, critical importance in resisting rain penetration in the position of exposure in which the walls were situated. None the less it is wise to provide a clear air space in a cavity wherever practical, against the possibility of rain penetration.

Where insulation is used to fill totally a nominal 50 mm wide cavity there is no need to use insulation retaining wall ties.

With a brick outer and block inner leaf it is preferable to raise the outer brick leaf first so that mortar protrusions from the joints, sometimes called snots, can be cleaned off before the insulation is placed in position and the inner block leaf, with its more widely spaced joints is built, to minimise the number of mortar snots that may stick into the cavity. This sequence of operations will require scaffolding on both sides of the wall and so add to the cost.

Insulation that is built in as the cavity walls are raised, to fill the cavity totally, will to an extent be held in position by the wall ties and the two leaves of the cavity wall. Rolls or mats of loosely felted glass fibre or rockwool are often used. There is some likelihood that these materials may sink inside the cavity and gaps may open up in the insulation and so form cold bridges across the wall. To maintain a continuous, vertical layer of insulation inside the cavity one of the mineral fibre semi-rigid batts or slabs should be used. Fibre glass and rockwool semi-rigid batts or slabs in sizes suited to cavity tie spacing are made specifically for this purpose. 

As the materials are made in widths to suit vertical wall tie spacing there is no need to push them down into the cavity after the wall is built, as is often the procedure with loose fibre rolls and mats, and so displace freshly laid brick or blockwork. There is no advantage in using one of the more expensive organic insulants such as XPS, PIR or PUR that have a lower U value than mineral fibre materials for the total cavity fill, as the width of the cavity can be adjusted to suit the required thickness of insulation.

The most effective way of insulating an existing cavity wall is to fill the cavity with some insulating material that can be blown into the cavity through small holes drilled in the outer leaf of the wall. The injection of the cavity fill is a comparatively simple job. The complication arises in forming sleeves around air vents penetrating the wall and sealing gaps around openings.

When filling the cavity of existing walls became common practice, a foamed organic insulant, ureaformaldehyde, was extensively used. The advantage of this material was that it could be blown, under pressure, through small holes in the outer leaf and as the constituents mixed they foamed and filled the cavity with an effective insulant. This material was extensively used, often by operatives ill trained in the sensible use of the material. The consequence was that through careless mixing of the components of the insulant and careless workmanship, the material gave off irritant fumes when used and later, when it was in place, these entered buildings and caused considerable distress to the occupants. Approved Document D of the Building Regulations details provisions for the use of this material in relation to the construction of the wall and its suitability, the composition of the materials, and control of those carrying out the work. As a result of past failures this material is less used than it was.

Glass fibre, granulated rockwool of EPS beads arc used for the injection of insulation for existing cavity walls. These materials can also be used for blowing into the cavity of newly built walls.

Table 5 gives details of insulants for total cavity fill.

The required thickness of insulation can be taken from the two methods suggested for partial fill. In a calculation for total fill, the thermal resistance of the cavity is omitted.

Thermal bridge.
A thermal bridge, more commonly known as a cold bridge in cold climates, is caused by appreciably greater thermal conductivity through one part of a wall than the rest of the wall. Where the cavity in a wall is partially or totally filled with insulation and the cavity is bridged with solid filling at the head, jambs or cill of an opening, there will be considerably greater transfer of heat through the solid filling than through the rest of the wall. Because of the greater transfer of heat through the solid filling illustrated in Fig. 84, the inside face of the wall will be appreciably colder in winter than the rest of the wall and cause some loss of heat and encourage warm moist air to condense on the inside face of the wall on the inside of the cold bridge. This condensation water may cause unsightly stains around openings and encourage mould growth.

Thermal bridges around openings can be minimised by continuing cavity insulation to the head of windows and doors and to the sides and bottom of doors and windows.

Of late an inordinate fuss has been made about ‘cold bridges’ as though a cold bridge was some virulent disease or a heinous crime.

Solid filling of cavities around openings will allow greater transfer of heat than the surrounding insulated wall and so will window glass, both single and double, and window frames. To minimise heat transfer, cavity insulation should continue up to the back of window and door frames.

Where solid filling of cavities around openings is used the area of the solid filling should be included with that of the window and its frame for heat loss calculation. 

Table 5 Insulating materials.

Fig. 84 Thermal bridge.

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