This invention relates to a thermally insulating building element, such as a brick, block or the like.

It is not unusual for building bricks or blocks to contain perforations in order to improve their thermal performance, to reduce their mass or to provide a key for cementing together. However, it is not generally appreciated that the shape, orientation and distribution of the perforations is critical in determining the resulting thermal performance.

It is an object of the present invention to provide a building element with improved thermal insulating properties.

According to a first aspect of the present invention, there is provided a building element having two opposed facing surfaces which define a heat transfer area for the flow of heat along a path from one surface to the other and which are separated by a distance B millimetres, the element having voids therein which constitute between 10% and 90% of the volume of the element, each void having a notional plane of maximum cross-sectional area and a maximum dimension d in a direction perpendicular to that plane, wherein: a) the aspect ratio of the voids (as defined herein) is high; b) the value of d is 6 millimetres or less; c) the perpendiculars to said notional planes of greatest

cross-sectional area lie within 30° of the perpendiculars to said opposed surfaces; d) at least 40% of the heat transfer area has a minimum number N of voids in series in the path of heat flow from one of said opposed surfaces to the other, where N equals B/16 rounded up to the nearest integer; and e) at most 30% of the heat transfer area has no voids in said path of heat flow.

The term "aspect ratio" as used herein means//A/d, where A is the area in square millimetres of the notional plane of maximum cross-sectional area of a void, and d is as defined above (in millimetres). The values of A and/or d can be the same for all the voids, or can vary from void to void. Typically, the. aspect ratio of the ^" voids will be in the range 4:2 to 1000:1.

The term "facing surface" as used herein has the same meaning as is conventionally used in the building industry, and refers to those surfaces of the building element which are intended to form part of the inner or outer face of e.g. a wall constructed from such elements.

According to a second aspect of the present invention, there is provided a wall structure composed of a plurality of building elements as defined herein, wherein the said opposed facing surfaces are arranged along opposite sides of the wall structure. In this way, the notional planes of greatest cross-sectional area of the voids are within 30° of the planes of the sides of the

wall structure.

The present invention is based upon the discovery that an improved thermally insulating effect can be obtained by providing a relatively large number of relatively thin voids in series in the path of heat flow, as compared with providing a relatively small number of relatively thick voids in series. The various conditions laid down by the invention control various parameters of the voids to achieve an optimum effect: thus, condition (b) provides for a maximum value for the thickness of the voids, while condition (d) provides for a minimum number of voids in series in the path of heat flow over a defined minimum percentage of the heat transfer area.

Considering condition (b) in detail, ideally d is made as small as possible so that the largest possible number of voids can be provided in series in the path of heat flow. However, this must be tempered with manufacturing considerations which may place a lower limit on the practical value of d, depending upon the material and production techniques employed. It has been found that a considerable improvement in the thermal insulating properties is obtained with a volume of d no greater then 6 m (tied in with the other conditions laid down by the invention) , but even better results are obtained if d is made less than 5 mm. The situation is improved still further as the value of d is reduced to a maximum of 4 mm, and beyond that to 3.5 mm, 3 mm and even 2 mm. For practical purposes, a value of d of substantially 4 mm has been found to be advantageous.

Turning now to condition (d), this provides on the one hand for there being a minimum numberN of voids in series in the path of heat flow over a certain percentage of the heat transfer area, and on the other hand for this latter percentage to be no lower than a minimum figure. The value of N is given by B/16 where B is the distance in millimetres between the said opposed facing surfaces of the building element: if this is not integral, then the value of N is obtained by rounding up the actual value of B/iδ to the nearest integer. For example, a typical building brick has a dimension B equal to 102.5 mm, which gives a value for N of 7. Thus, over the defined minimum percentage of the heat transfer area, there must be at least seven voids in series in the path of heat flow. This condition, coupled w;Lth the maximum value for d prescribed by condition (b), ensures optimum thermal insulation performance.

An even better performance can be obtained by increasing this minimum number of voids in series to B/13, again rounded up to the nearest integer if not already integral. For the typical brick described above, this gives a value of 8 for the minimum number of voids in series.

Condition (d) also specifies that the required minimum number of voids in series extends over at least 40% of the heat transfer area. This is to ensure that thermal bridging (i.e. the flow of heat through the areas between the voids) is minimised, and it has been found that 40% is the minimum figure needed to achieve

a significant improvement. Ideally, this figure should be as high as practicable whilst ensuring that the integrity and strength of the building element are not adversely affected to a material degree. Indeed, if possible it would be preferred that 100% of the heat transfer area has the required minimum number of voids in series. For practical purposes, however, it has been found that acceptable results are obtained if the percentage is a minimum of 45%. Even better thermal insulation properties are obtained by increasing this minimum percentage to 50%, 60% or even 70%.

Condition (e) provides for a certain maximum percentage of the heat transfer area having no voids in the path of heat flow, again to avoid thermal bridging effects which would otherwise destroy the benefits obtained from conditions (b) and (d) . In an ideal situation, this percentage should be as small as possible (even as low as 0%, i.e. where no part of the heat transfer area has no voids in the path of heat flow) . However, this must be balanced by the practicalities of manufacture, for example.. It has been found that 30% is the maximum value at which a significant improvement is obtained, although better results are achieved by reducing with percentage to a maximum of 20%. As a practical compromise, a maximum of 10% has been found to produce very acceptable results.

In general terms, a plurality of "thin" voids in series is required because the thermal conductivity of a void reduces with decreasing value of d, although the exact value of this reduction

depends largely on the properties of the surfaces enclosing the void. The implication of this effect is that several "thin" voids in series are significantly more resistant to heat flow than a single void with a "d" value equal to the total of the "thinner" voids. For example, a void with d=8 mm and a surface emissivity of 0.93 has a thermal resistance of 0.132 m ² °C/ , whereas two 4 mm air gaps in series have a combined thermal resistance of 0.186 m ² °C/ and four 2 mm air gaps in series have a combined thermal resistance of 0.236 m ² °C/W, an improvement of 79% over a single 8 mm air gap. The effect of the reduction in thermal conductivity of a building element containing thin air gaps in series becomes increasingly noticeable as d (the thickness of the voids) decreases below 6 mm and N (the number of voids in series) increases beyond B/16 for larger proportions of the heat transfer area than 40% and larger volume fractions of slots. All these factors together are necessary for high thermal performance results to be achieved, as the later examples will show. Although the factors are inter-related, they may compensate for each other to some extent. For instance, where the proportion of heat transfer area having the maximum number of slots in series is only 40%, improvement in performance may be achieved by having a greater N and a lower d or higher volume fraction.

With reference to condition (c), the orientation of the voids is such that the perpendiculars to their notional planes of greatest cross-section lie within a certain angle of the direction of expected heat flow. This orientation maximises the resistance

to heat flow, by maximising the number of voids through which the heat must flow through any section of the block. Ideally this would be a maximum when the perpendicular to the notional plane of greatest cross-section of the voids is parallel to the direction of expected heat flow through the block, i.e. parallel to the perpendiculars to the opposed facing surfaces. However satisfactory results can be obtained when this angle is increased to 5° or even 15°. It has been found that a maximum value of 30° can be tolerated without significantly reducing the thermal insulating properties of the building element.

Although it is preferable, the notional planes of largest cross- section of the voids need not be parallel to each other.

The voids may for example take the form of open-ended slots extending between opposed faces of the brick or block. These slots can be rectilinear and can have a thickness which is substantially constant along the length thereof. Preferably the voids are arranged so that there are no cold bridges along the direction of anticipate flow, i.e. there is no line which can be drawn uninterrupted by voids between those faces of the block between which the expected heat flow occurs.

In a preferred embodiment, the voids take the form of rectilinear slots extending in.a series of rows across the building element. Conveniently, each said row contains a plurality of slots with the slots in each row being separated by bridging portions, at

least some of the bridging portions in the various rows being arranged in a generally Y-shaped configuration.

Examples of shapes of voids which can fall within the recited mathematical definition are planar or dished discs, buttons, platelets or lamellae; oblate spheroids or any generally planar geometric shapes, e.g flat squares, or triangles, or rectangles.

The building element of the invention may, for example, be made of fired clay or any other suitable material, such as Portland or magnesia cements, alumina cement, concrete, plaster or synthetic resin. Additional fillers (thermally insulating or otherwise) such as grogs or shales may be incorporated in the matrix material. The matrix material may also contain other conventional additives, such as special clays or process aids. The matrix material may also be porous, i.e. contain air pockets or spherical voids either naturally occurring or introduced for the purpose of reducing the density of the matrix material.

In the case of building elements produced from clay, the voids may be produced in the clay before firing by a variety of techniques. For example, slots may conveniently be stamped into or cut out of the element by mechanical means. They may also be directly introduced by a suitable moulding or extrusion process. It is also possible to introduce combustible materials of appropriate shapes such that after firing the necessary voids remain.

The building element of the invention can advantageously be employed to improve the thermal insulation of, for example, the inner or outer leaves of a cavity wall.

The invention will be further described by way of example only with reference to the accompanying drawings, in which

Figure 1 is a generalised perspective view of a building element according to the present invention, and defines a co¬ ordinate axis system therefor;

Figure 2 is a perspective view of a first embodiment of a building element in accordance with the invention, in the form of a brick;

Figure 3 is a plan view of the brick shown in Figure 2;

Figure 4 illustrates various high aspect ratio voids which may be used in a building element according to the invention;

Figure 5 is a perspective view of a second embodiment of a building element in accordance with the invention, as used in Example 1 below;

Figure 6 is a graph of the data obtained in Example 1;

Figure 7 is a perspective view of a third embodiment of a building element in accordance with the invention, as used in Example 2 below;

Figures 8 and 9 illustrate comparative blocks as also used in Example 2;

Figure 10 is a graph illustrating the results of Example 2;

Figure 11 is a perspective view of a fourth embodiment of a building element in accordance with the invention, as used in Example 3 below; and

Figure 12 is a schematic plan view of a still further embodiment of a building element according to the present invention.

Figure 1 illustrates a brick 1 with facing surfaces 2 and 3. The brick is shown superimposed on an XYZ co-ordinate axis system in which, for a wall built of the bricks, the X axis represents the length (i.e. the horizontal extent) of the wall, the Y axis represents the wall height, and the Z axis the wall thickness. Therefore as shown in Figure 1 the facing surfaces 2 and 3 lie in the XY plane and the heat flow direction (when the brick is built into a wall) is in the Z direction. The facing surfaces 2 and 3 define the heat transfer area of the brick. More specifically, the brick is shown as having a length (X axis) of

L mm, a height (Y axis) of H mm, and a depth (Z axis) of B mm. The heat transfer area is thus HL mm ² .

As described more fully below, the brick includes a plurality of high aspect ratio voids. These voids are required to satisfy certain criteria for their dimensions and orientation within the brick, and these criteria may be understood by reference to the single void 4 illustrated in the brick 1. The void 4 has a notional plane of maximum cross-section of area A square millimetres and a greatest linear dimension d of not greater than 6 mm in a direction perpendicular to this plane.

The void 4 is illustrated as having a perpendicular P to its notional plane of greatest cross-section. This perpendicular P is illustrated (as is preferred) in being parallel to the Z- direction but in accordance with the invention may be within 30° of the Z direction. This is illustrated by the conical angle shown in dotted lines and it will be appreciated that the voids may be so orientated that P lies on the conical surface or within the conical angle.

Reference is now made to Figure 2 which illustrates a British Standard (BS 3921) brick 21 (with facing surfaces 22 and 23) embodying the present invention. In this brick the values of B, H and L are as follows:

B = 102.5 mm

H = 65.0 mm

L =• 215.0 mm

The heat transfer area (HL) is thus 13975 mm ² .

The brick incorporates twenty perforations 24', 24'' and 24''' in the form of open ended rectangular slots which extend between the top and the bottom surfaces of the brick. All of the illustrated slots have their notional plane of greatest cross- section (as defined by e and f dimensions) parallel to the XY plane so that the perpendiculars P to these notional planes are also perpendicular to the facing surfaces 22 and 23. All slots 24', 24'' and 24''' have a dimension d equal to 4 mm and a dimension f _. equal to 65 mm (i.e. equal to H) . The slots do however differ in their values of e.

The number of slots of each different type together with their respective e values are shown below

The slots are arranged in eight rows spaced apart in the Z direction with each row being parallel to the XY plane (see also Figure 3). The rows each extend over a length of 185 mm in the X direction so that there is a 15 mm margin at each end of the brick within which no slots are provided. Two 'different' types of row are provided. One row type R^ has two slots 24''' (e =

87.5 mm) with a spacing of 10 mm between the adjacent edges of the two slots. The other row type R ₂ has, at each end, a slot 24', with there being a slot 24'' in the row between the two slots 24', the spacing between the adjacent edges of slots 24' and 24'' being 12.5 mm. There are four rows R _x alternating in the Z direction with four rows R ₂ -

It will thus be seen that the slots in row R _x overlap with those in row R ₂ . More particularly each slot 24''' in row R _α fully overlaps with a 37.5 mm slot 24' (in row R,) and partially overlaps a slot 24' ' to an extent of 37.5 mm in the X direction. Thus in the direction of heat flow, i.e. from one facing surface 22 to the other surface 23, there are two paths in which there are four slots 24''' and four slots 24' (a total of eight voids in the path) and two further paths in which there are four slots 24''' partially overlapping slots 24''' (again a total of eight voids in the path). In each of these four paths, the area of overlap of the slots is 2437.5 (i.e. 37.5 x 65 (i.e. H) ) square millimetres. Thus the total area of these four paths is 9750 square millimetres.

Consider now that the value of B for the brick is 102.5 mm. In accordance with the invention there must be at least 7 (i.e. the next integer higher than 102.5/16) air gaps in the path of heat travel over at least 40% of the heat transfer area and at most 30% of the heat transfer area provided with no slots at all.

It can be seen from (i) and (ii) below that the brick of Figure

2 satisfies these relationships.

(i) the area over which there are eight slots in the path of heat travel is 9750 square millimetres. The total heat transfer area is 13975 (i.e. 215 x 65) square millimetres. Thus the percentage of the heat transfer area over which eight slots are provided in the path of heat travel is 69.77%.

(ii) the area over which there are no slots in the path of heat travel is 1950 square millimetres (i.e. 2 areas of 15 mm x 65 mm) . As a percentage of the heat transfer area this is 13.95%.

The perforations shown in Figure 2 are regular in the notional plane of greatest cross-section, but this need not be the case and shapes such as shown in Figure 4 are acceptable. The perforations of the example are all parallel to the XY plane which again is a simplified case. ^' The notional planes of greatest cross-section are ideally all parallel to the XY plane but need not be so, and the notional plane of largest cross- section of any perforation may lie anywhere within 30° of the XY plane of the brick or block. These variations are acceptable provided that there are at least N(=B/16) of the air gaps in series in the path of heat flow over at least 40% of the heat transfer area of the brick or block. The dimensions d need not necessarily be constant throughout a single void nor the same for each void provided that it is nowhere greater than 6 mm.

The conductivity of the voids may be reduced by providing a low emissivity coating to the surfaces of the voids This will have the effect of reducing still further the thermal conductivity of the air gaps and consequently the thermal conductivity of the resulting brick or block.

The manufacture of the brick may be undertaken by a variety of methods. Continuous regular voids ₀ f the type shown in Figure 2 may be introduced into clay bricks and blocks by extrusion, stamping or cutting, they may be introduced into clay or concrete by suitable forming. The irregular shaped vo ^id s Q Fig re 4 may be provided to the specification of the present invention by the introduction of suitably shaped combustible material such that the voids are left after firing the clay brick or block. The surface emissivity may be lowered by suitable glaze coatings to the surfaces of the voids _or by o her surface coatings or unfired block or brick materials.

This invention- is further illustrated with reference to the following non-limiting Examples.

Example 1

Test Sample L (Figure 5) was prepared by introducing thin rectangular sheets of combustible material into unfired clay so as to produce voids after firing. In this example d = 4.3 mm, the number of slots used in series was 8 and the percentage of

the heat flow area which placed 8 slots in series was 81%. The total volume fraction of perforations was 43.3%. The density of the perforated block was 1098 ^' kg/m ³ .

For comparison a further test sample M was prepared from the same clay as sample L. In this case the sample contained no perforation.

Thermal conductivity measurements for samples M and L are given in Figure 6. Specimen L (according to the present invention) shows that over the moisture content range 0 - 1% there are extremely large reductions in thermal conductivity of between 83% and 86% of the solid material. This example shows that where the number of thin air gaps in series per unit width is greater than that given by B/13 over a larger proportion of the heat flow area than 50%, in this case 81%, that the resulting thermal conductivity is very low, ca 0.1 W/m°C.

A comparison may be made with standard thermal conductivity data given for brickwork with conventional voids or homogeneous aggregate concretes containing conventional perforations in the form of pores. At 1% moisture content by volume the thermal conductivity of concrete work of density 1100 kg/m ³ is 0.28 W/m°K whilst that of brickwork of density 1200 kg/m ³ is 0.31 W/m°K. The thermal conductivity of a block of the present invention of density 1098 kg/m ³ at the same moisture content is at least 64% less than that of the above conventional materials.

Example 2

A test sample A of clay matrix material in accordance with the present invention was prepared. In this case H = 307 mm, L = 307 mm, B = 96 mm and N = B/16 = 6 exactly. The perforations, with d=3.63mm, are shown in Figure 7 and are again regular rectangular slots arranged to present 6 high resistance air gaps in series over 71.6% of the heat transfer area. The total volume fraction of slots is 18%.

Sample B (Figure 8) was prepared of the same factory extruded clay with the more usual 3 cylindrical perforations . In this case the cylindrical perforations amount to 15.6% of the total volume of the block.

Sample C (Figure 8) was prepared of the same factory extruded clay with no perforations. The thermal conductivity of these three specimens was measured to BS 874 at moisture contents (% volume) of 0% to 5% and the results are compared in Figure 9. Specimen A (in accordance with the present invention) shows for any given moisture content, a substantial reduction in thermal conductivity of between 53% and 62% of the conventionally perforated sample B is effected even with low (18%) volume faction and that conventionally perforated clay has little effect on the thermal conductivity of the solid matrix material (Sample C).

Example 3

A test piece was constructed from 6 flat plates 4 mm thick of a

material with a thermal conductivity of approximately 1.0 W/m°C. The plates were separated by bridges of the same material 2 mm thick as in Figure 11.

The measurement of thermal conductivity was accomplished by guarded hotplates method over the central area (204 mm x 204 mm) shown dotted in Figure 11. Within this area (the heat transfer area) the number of 2 mm slots in series is 5 (cf B/16 = 3 when rounded up to the next integral value) and the percentage of the heat transfer area where there are at least 3 slots in series is 90%. The percentage of the heat transfer where 5 slots are presented in series to the flow of heat is 75%. The volume fraction of slots is 26%. The measured thermal conductivity for the sample was 0.15 W/m°C, 85% less than that of the solid material.

Figure 12 illustrates a further embodiment of a building element according to the present invention, in the form of a brick of standard dimensions. More particularly, B = 102.5mm and L = 215 mm. The brick is made from fired clay which is initially extruded, and has' formed therein a series of rectilinear slots 100 which are arranged in a series of rows 101 across the brick, i.e. parallel to its facing surfaces 102 and 103. Each slot has a thickness d equal to 4 mm, which is substantially constant along its length.

As illustrated, there are eight rows of slots 100 in total. Starting from the facing surface 102,the first four rows each

have three slots, the fifth row has five slots, and the remaining three rows have four slots each. In between the slots in each row there are bridging portions 104, and the disposition of the slots is such that certain ones of these bridging portions centrally of the brick are arranged in a generally Y-shaped configuration. The pattern of slots is symmetrical about a centre plane of the brick, perpendicular to the surfaces 102 and 103.

The dimensions and spacings of the slots are as follows:

1 _: = 1 ₂ = 1 ₃ = 55 mm I* = 1 ₅ = 1 ₆ = 59 mm 1 ₇ = 1 ₉ = 65 mm l _β = 47 mm l _lx = 35 mm

1 ₁₃ = 1 ₁₇ = 42 mm

114 ⁼ J-16 ⁼ 30 mm

1 ₁₅ = 23 mm 1 ₁₈ = 1 ₂₁ = 42 mm

• ^■ ••26 ⁼ -i-27 ⁼ -1*28 ⁼ !29 = 2 mm

b _x = 18 . 5 mm b ₂ = b ₃ = b ₄ = b ₅ = b ₆ = b ₇ — b ₈ •= 10 mm b _Q = 10mm

It can thus be calculated that, for this particular embodiment, the voids occupy 24.6% of the volume of the brick, approximately 60% of the heat transfer area has a minimum of 7 slots in series in the path of heat flow (with approximately 45% of the area. having a minimum of 8 slots in series), and there are no voids in the path of heat flow over approximately 10% of the heat transfer area.