BACKGROUND TO THE INVENTION Field of the invention The present invention relates to construction panels and to a method of manufacture of construction panels. The invention aims in particular to provide construction panels with improved thermal performance over corresponding known construction panels, and/or a simplified manufacturing process.

Related art Steel is widely used in the construction of multi-storey buildings but traditionally steel construction has been seldom used and was considered uneconomical for low-rise properties where timber or structural brickwork or blockwork was preferred.

Over the years, various improved systems have been developed for use in low-rise properties and an economical light gauge steel frame system is increasingly being used across the world. This light gauge steel frame is developed through a cold-formed process without the use of heat from sheet-like starting materials. Lightweight but high tensile strength steel sheets are readily available from steel manufacturers. The sheet surface is coated with a zinc alloy that completely covers the steel surface and protects it from corrosion. The cold-forming process provides a frame that is lightweight and economical and can be used in construction panels for buildings having benefits in terms of solidity, strength, durability and ease of construction.

As such, the light gauge steel frame is an attractive alternative for use in low-rise properties, with the main benefits usually seen as:

• Buildability - the use of prefabricated and preassembled steel components

reduces site works, reduces material waste and improves quality.

• Speed - this system requires a shorter construction period compared to that for a conventional system. • Strong but lightweight - steel has one of the highest strength-to-weight ratios of any construction material; this results in savings in the foundation required and makes for easier on-site handling.

• Safety - steel's inherent strength and non-combustible qualities enable light steel frame buildings to resist such devastating events as fires, earthquakes and hurricanes. Homes can also be designed meet the highest seismic and wind load specifications in any part of the country.

• Quality - more preparation can be done off-site resulting in a better quality

finished house that is durable and low in maintenance.

• Easy to remodel - remodelling can be easily accomplished. Non-load bearing walls can be readily relocated, removed or altered.

• Design flexibility - because of its strength, steel can span longer lengths, offering larger open spaces and increased design flexibility without requiring intermediate columns or load bearing walls.

• Recyclable - all steel products are recyclable.

• Thermal performance with normal wall thickness: external walls can have low U, Ψ and Y values, with the potential for excellent air-tightness without very thick walls.

The U-value of a building element e.g. a wall or window describes how well that building element conducts heat from one side to the other, per unit area. It can be calculated

AT x A

Here, Q is the rate of heat flow across the building element, AT is the temperature difference across the building element, and A is the area of the building element in consideration. U is typically measured in watts per square metre per kelvin (W/m ² K). It is noted that the U-value is also equivalent to the thermal conductivity of the building element per unit volume of the building material.

The t/i-value (linear thermal transmittance) gives the heat loss associated with a (non- repeated) thermal bridge in a building element. A thermal bridge is a highly thermally conductive element which passes (at least partially) through a more insulating material, effectively forming a "path of least resistance" for heat to flow along, thus increasing the overall thermal conductivity of the entire arrangement. The i >-value is given by the heat flow per degree temperature difference between the two sides of the building element per unit length of the thermal bridge itself. Its units are watts per metre per kelvin (W/mK). The ψ-value of a thermal bridge can then be used to adjust the U-value of the entire building element.

This recalculated U-value, taking into account any thermal bridging in the building element is known as the Y-value, also measured in W/m ² K. An example of a light gauge steel frame is shown in Fig. 1.

In this drawing, the external wall construction panel 1 has a frame including two side beams 2, each with a U-shaped profile in cross section. Perpendicular to these, and joining the two side beams 2 are cross beams 3.

Conventionally, construction panels can be formed from light gauge steel frames such as those shown in Fig. 1. This can be done, for example, applying a rigid insulation sheet 7 across the back of the panel, which forms a flat surface. The thermal performance of this panel can be improved by filling the recess defined by the frame and the insulation sheet with an insulating foam. To the knowledge of the present inventors, this is only done at present by spraying a foam-forming liquid into the recess while the frame is upright.

However, this produces a messy finish, which means that the U value of the foam, and therefore of the panel is not uniform across its area, with some areas receiving a far thicker coating of insulating foam than others. Some parts appear not to have been covered at all. In this way, either the thermal properties of the panel are not satisfactory, or the insulating foam is not efficiently used. An indicative illustration of a construction panel formed in this way is shown in Fig. 2a. A construction panel 30 includes a backing layer 14 attached to a frame 12. A recess 15 is formed by the backing layer and frame. Insulating foam layer 31 is formed by spraying a foam-forming liquid into the recess while the frame is upright, i.e. already in place within the building. The foam layer 31 is thereby formed on the backing layer. As shown in Fig. 2, the upper surface of the foam layer formed in this manner is inevitably of uneven height above the backing layer. As a result, the thermal properties of the panel may not be satisfactory.

An illustration of a panel prepared according to the spray method is shown in Fig. 2b, where again the uneven application of insulating foam is evident. SUMMARY OF THE INVENTION

The present inventors have realised that it is possible to address the issues identified above, in particular those relating to the thermal performance of construction panels. The inventors have aimed to achieve low thermal bridging, excellent thermal

performance, and excellent air-tightness whilst retaining a conventional wall-thickness. Building regulations impose targets for thermal performance, and in order to meet these targets with conventional building material such as timber or masonry, thicker, heavier walls are required. Clearly this is undesirable for reasons both to do with cost and living space. The present invention has been devised in order to address at least one of the above problems. Preferably, the present invention reduces, ameliorates, avoids or overcomes at least one of the above problems. In particular, the present invention allows for increased thermal performance while maintaining wall thickness, or alternatively allows the same thermal performance with reduced wall thickness. Furthermore, it is an aim of the present invention to ensure a substantially uniform thermal performance of the panel across its area.

Accordingly, in a first preferred aspect, the present invention provides a method of manufacturing a construction panel, the construction panel having a frame, and a backing attached to said frame, wherein the backing and the frame define a recess. The method includes the steps of:

(a) laying the construction panel horizontally, such that the recess faces upwards, with the backing forming a base of the recess,

(b) applying a predetermined quantity of an insulating foam precursor liquid to the recess,

(c) allowing the insulating foam precursor liquid to flow and expand to fill the lateral extent of the recess

(d) allowing the insulating foam precursor liquid to set to form a foam layer.

The insulating foam precursor liquid may be a combination of different compounds, of which at least one is in the liquid phase at room temperature. The insulting foam precursor liquid may expand to fill the lateral extent of the recess before it begins to expand, or it may spread out and expand simultaneously.

In this way, a construction panel as shown, for example, in Fig. 2c can be formed. Fig. 2c includes contour lines 17, showing elevation of a main surface of the foam layer relative to a base of recess 15. In comparison to Figs. 2a and 2b, it is clear that the layer of insulating foam which is applied to the construction panel has a far neater, more level finish. As a result, the thermal properties of the whole construction panel are more uniform. Furthermore, by allowing the insulating foam precursor liquid to fill the lateral extent of the recess, air-tightness can be achieved, because the liquid makes contact with the frame along its entire perimeter. Clearly, the spraying method, the result of which is shown in Fig. 2b, does not typically result in an even finish so there is a significant likelihood of gaps in the foam which would then result in a non-air-tight panel.

The spraying method used previously requires the presence of certain components in the sprayed liquid which allow it to cure quickly enough to stick to surfaces so that no dripping of the liquid occurs after spraying. The present method does not necessarily require such additional components. This reduces the cost and complexity of the process.

In a second preferred aspect, the present invention provides a method of manufacturing a batch of construction panels, each construction panel having a frame, and a backing attached to said frame, wherein the backing and the frame define a recess, the method including the steps of:

(a) laying a first construction panel horizontally, such that its recess faces upwards, with the backing forming a base of the recess,

(b) applying a predetermined quantity of an insulating foam precursor liquid to the recess,

(c) allowing the insulating foam precursor liquid to expand to fill the lateral extent of the recess, and

(d) laying a second construction panel horizontally on top of the first construction panel, and repeating steps (b) and (c).

The method used to manufacture the construction panel shown in Fig. 2a requires one panel to be sprayed at a time. However, in the method according to the second aspect of the invention, the manufacture of a second panel can begin before the manufacture of a previous panel is completed, in the same working space, in this way, more panels can be produced in a given time in the same working space. It is preferable, therefore, that the second construction panel is laid horizontally on top of the first construction panel before the insulating foam precursor liquid is allowed to set. Furthermore, the present method allows the panels to be stacked during manufacture. As a result, the invention is beneficial not only from a time-saving point of view, but also from a space-saving point of view.

In a third preferred aspect, the present invention provides a construction panel which is obtained or is obtainable by the manufacturing method of the first aspect of the present invention.

In a fourth preferred aspect, the present invention provides a batch of construction panels which is obtained or is obtainable by the manufacturing method of the second aspect of the present invention.

In a fifth preferred aspect, the present invention provides a construction panel having a frame, a backing attached to said frame, wherein the backing and the frame define a recess, and wherein the recess contains a layer of insulating foam with a substantially level main surface. A sampling area of the foam layer is defined as a region of the main surface which excludes a perimeter of 10% of the maximum width of the main surface around its entire periphery. An average height, h _avg of the insulating foam is defined as the mean height value of at least 100 sampling points which are taken in a regular square lattice configuration within the sampling area. According to the fifth aspect of the present invention, it is required that:

10% or fewer of the sampling points have a height, ho.g < 0.9h _av g, and

10% or fewer of the sampling points have a height, h-i.i≥ 1.1 h _avg . A "square lattice", in this case, refers to a two dimensional lattice wherein the points lie on the intersection points of two orthogonal sets of equidistantly spaced, parallel lines. The outside shape of the entire lattice need not be square. Preferably the lattice is such that it covers as great a proportion of the sampling area as possible. "Height" refers to the distance between a point on the upper surface of the foam layer and the point on the upper surface of the backing which forms the base of the recess which is directly below. In the case where the backing has surface features such as screws or holes, if a sampling point falls on one of these points, it may be disregarded, as it does not give a true representation of the height of the foam layer. Alternatively, the position or lattice constant (defined as the length of an edge of one of the constituent squares) of the lattice may be adjusted to exclude this feature from the calculation of h _avg .

The requirements on the geometry of the foam layer of the construction panel of the fifth aspect of the present invention ensure that the surface of a central region of the upper surface of the foam layer, i.e. the sampling area, is sufficiently level that it confers the advantage that the panel has substantially uniform thermal properties across its area. The outer edges are excluded from the sampling area, as these are likely to have an increased height variation. For example, during formation, the insulating foam precursor liquid may set more quickly, or expand more slowly when it comes into contact with the walls, and as a result, the overall upper surface may have a convex, domed appearance.

By taking an average over a large number of sampling points, conditions are imposed only on the overall, gross shape of the surface of the foam layer, while still allowing for a porous surface as a result of bubbles forming as the insulating foam precursor liquid expands. It is possible, alternatively, for the surface not to be porous. The first, second, third, fourth and/or fifth aspect of the invention may have any one or, to the extent that they are compatible, any combination of the above-described or following optional features.

The insulating foam precursor liquid of the first and second aspects of the present invention may be allowed to set with a free main surface. In other words, the insulating foam precursor liquid may not contact with a covering layer as it sets. By allowing the liquid to set as a free main surface, the manufacturing method is simple, with few steps required to arrive at a final construction panel product. In particular, a covering layer does not have to be laid over the insulating foam precursor liquid before it sets, or as it sets. By allowing the foam to set as a free main surface, rather than under the influence of a covering layer, there is no covering layer to adhere to the foam layer as it sets.

Accordingly, the foam layer of the third, fourth and fifth aspects of the present invention may have a main surface that is a free main surface.

It is noted here that even in cases where the foam layer may be obscured from view, for example by a plasterboard layer, the main surface will still be a free surface, with a layer of air interposed between the insulating foam and the layer obscuring the insulating form from view.

It is preferable that the insulating foam precursor liquid is applied to the base of the recess. It is preferable that the viscosity of the insulating foam precursor liquid is sufficiently low that when it sets, the upper surface of the resulting foam layer is substantially level. This is because, when the viscosity is sufficiently low, the liquid is able to flow over and/or around surface features within the time available before setting, and therefore form a substantially level surface quickly, before the liquid has set. Furthermore, it is possible for a substantially level surface to form, without the need to apply a levelling force to a main surface of the foam precursor liquid, as discussed above.

The viscosity of the insulating foam precursor liquid, at room temperature (taken here to be 25° C), is preferably at most 200 Pa s, more preferably at most 100 Pa s, still more preferably at most 50 Pa s, and most preferably, at most 20 Pa s. Even more preferably, the viscosity of the insulating foam precursor liquid at 25°C is at most 1000 mPa s, more preferably at most 800 mPa s, more preferably at most 600 mPa s, more preferably at most 400 mPa s.

The lower limit of the viscosity of the insulating foam precursor liquid is not particularly limited, but may be at least 0.5 mPa s, more preferably at least 1 mPa s, more preferably at least 2 mPa s, more preferably at least 3 mPa s, more preferably at least 4 mPa s.

The properties of foam precursor liquids may be determined by measuring the cream time (the time taken between application of the liquid and the initial formation of bubbles in the liquid to initiate expansion), rise time (the time taken between application of the liquid and the complete expansion of the foam) and tack-free time (the time taken between application of the liquid and the point at which the expanded foam is no longer "tacky" - also known as the "cure time").

It is preferable that the insulating foam precursor liquid has a rise time of at least 3 minutes, for example at least 4 minutes or at least 5 minutes. In this way, the liquid has sufficient time after application to flow to fill the recess properly before expansion of the liquid is complete.

It is preferable that the insulating foam precursor liquid has a tack-free time of at most 30 minutes, for example at most 25 minutes or at most 20 minutes. This allows the process to be completed within a reasonable amount of time, for the sake of efficiency. It is preferable that the insulating foam precursor liquid has a tack-free time of at least 30 seconds, for example at least 60 seconds or at least 90 seconds. This ensures that liquid can be added to the entire area to be covered before the foam is fully set, which provides a smooth upper surface of the set foam. The insulating foam precursor liquid is preferably configured to form a polyurethane foam when it has expanded and set. Polyurethane foam, in this context, refers to a foam composed, at least in part, of a polymer composed of organic units containing urethane links. Polyurethane is very useful in this context since it is light, rigid, and has excellent insulating properties. The polyurethane foam is preferably formed from a precursor liquid which contains, at least in part, a di- or poly- isocyanate compound and a polyol compound. These two compounds are able to react together to form the desired polyurethane compound. More preferably, the precursor liquid also contains water. When water is present in the precursor liquid, the isocyanate compound reacts with the water to form a urea linkage and carbon dioxide. The carbon dioxide released forms the bubbles in the resulting foam. It is preferable that the liquid also contains a catalyst, in order to increase the rate of the reaction to a desirable value. The amount of catalyst may be controlled to ensure that there is sufficient time for the precursor liquid to spread out to fill the recess before rising a significant amount. The thermal properties of the precursor liquid are not particularly limited, as long as the liquid sets to provide an insulating foam. It is preferable that the insulating foam has a k-value (thermal conductivity) of around 0.023 W/mK.

It is preferable that the predetermined amount of insulating foam precursor liquid is such that the foam sets before it expands to such an extent that it overflows over an edge of the recess. In this way, it is possible to lay a second construction panel on top of the first, before the precursor liquid in the first panel has set, without the risk that the insulating foam precursor liquid will expand and contact the bottom of the second construction panel, which could lead to the panels being stuck together, or the foam not setting in a desirable manner. In the case where the second construction panel is laid on top of the first after the precursor liquid has set, if the resulting foam layer extends above the edges of the frame, it will not be able to stack as securely.

Preferably, the internal surfaces of the frame which form the peripheral walls of the recess are not completely covered by the foam layer, after it has set. This is beneficial, for example, in situations where there are holes, e.g. service holes or panel fixing holes, in these surfaces. In order for these holes to be used, it is required that the surface in which they are formed is exposed. Preferably, the foam layer covers at most 90% of the total area of these internal surfaces. More preferably, the foam layer covers at most 80% of the total area of these internal surfaces. Still more preferably, the foam layer covers at most 70% of the total area of these internal surfaces. Still more preferably, the foam layer covers at most 60% of the total area of these internal surfaces. Most preferably, the foam layer covers at most 50% of the total area of these internal surfaces.

Advantageously, the foam layer may not cover the area of these internal surfaces, in which holes (e.g. service holes or panel fixing holes) are formed. Accordingly, in embodiments in which the construction panel has multiple recesses defined by the frame, the foam in each recess may be isolated from the foam in adjacent recesses by the frame.

The foam layer may be in contact with at least 10% of the total area of these internal surfaces. Preferably, the foam layer may be in contact with at least 20% of the total area of these internal surfaces. More preferably, the foam layer may be in contact with at least 30% of the total area of these internal surfaces. Most preferably, the foam layer may be in contact with at least 40% of the total area of these internal surfaces.

Moreover, the foam layer may be in contact with at least 90% of the total area of the internal surfaces covered by the foam layer. In exemplary embodiments, the foam layer may be in adhesive contact with these internal surfaces. Through adhesive contact, it is possible for the foam layer to be secured to these internal surfaces, without the requirement for additional fastening or a separate adhesive layer.

It is preferable that at least 80% of the total area of the base of the recesses in the construction panel is covered by insulating foam in the finished panel. More preferably, at least 85%, 90% or 95% of the total area is covered. Most preferably, 100% of the total area is covered, i.e. no part of the backing is exposed through the foam layer. In this way, the thermal properties of the panel are improved and the panel is airtight.

The foam layer may be in contact with at least 75% of the total area of the base of the recess in the construction panel. More preferably, the foam layer may be in contact with at least 80%, 85%, or 90% of the total area of the base of the recess in the construction panel. Most preferably, the foam layer may be in contact with at least 95% of the total area of the base of the recess in the construction panel. Moreover, the foam layer may be in contact with at least 90% of the total area of the base of the recesses in the construction panel covered by the foam layer. In exemplary embodiments, the foam layer may be in adhesive contact with the base of the recess. Through adhesive contact, it is possible for the foam layer to be secured to the base, without the requirement for additional fastening or a separate adhesive layer. As will be clear to the skilled person, adhesive contact of the foam layer with the internal surfaces and/or base of the recess may be achieved by manufacturing the construction panel according to the method of the first or second aspect of the present invention.

In preferred embodiments, the set foam layer in the construction panel has a thickness of at least 20 mm, for example at least 25 mm, 30 mm, 35 mm or 40 mm. In some embodiments, the set foam layer in the construction panel has a thickness of up to 90 mm, for example up to 80 mm, 70 mm, 60 mm, 50 mm or 40 mm.

In preferred embodiments, the foam layer has a thickness of 35-45 mm, for example 40 mm. The frame is preferably made, at least in part, from a metal. More preferably, for the reasons discussed earlier in the application, the frame is made from steel. Most preferably, the frame is made from light-gauge steel. Here, light-gauge steel refers to steel which is formed into a frame from a steel sheet precursor in the absence of heat. Most commonly this is done by cutting galvanized sheet steel into shapes, and using dies to shape the steel. It is also preferable that the steel frame is at least partially hollow. This results in a lighter panel, which is easier to form, and also to transport. A hollow steel frame also has a lower thermal conductivity than a solid steel frame of similar overall outer dimensions.

Because the construction panels are intended for use in walls of buildings and the like, it is preferable that the frame is substantially rectangular, for ease of transport and installation. It is therefore preferred that the invention also provides a kit for constructing a building, comprising a plurality of construction panels as set out above, being adapted to be fixed together.

The backing may comprise one or more sheets of rigid insulation. In this way, the assembled panel has a degree of rigidity which is further enhanced by the rigid insulation which forms the backing.

In preferred embodiments, the backing layer has a thickness of at least 40 mm, for example at least 50 mm, at least 60 mm, at least 70 mm or at least 80 mm. In some embodiments, the backing layer has a thickness of up to 150 mm, for example up to 140 mm, up to 130 mm, up to 120 mm or up to 1 10 mm. In this way, the backing layer provides adequate thermal resistance of 1.8 to 5 m ² .K/W, in addition to the physical support provided to the panel. It is preferable that the backing is composed, at least in part, of a material which is a good thermal insulator. This improves the overall thermal performance of the whole construction panel. Specifically, the backing material preferably has a thermal resistance of at least 1.8 m ² .K/W and up to 5 m ² .K/W. The backing may be a rigid insulation panel including an insulating core of rigid thermoset insulation. The core may be sandwiched between two layers of low emissivity composite foil.

Preferably, the core comprises a rigid thermoset polyisocyanurate (PIR) insulant.

For example, a backing of Kingspan Thermawall (RTM) TW55 or similar may be used. Such backing offers good rigidity and thermal resistance.

Preferably, when considering only the gross structure of the upper surface of the foam layer and excluding any small pores which open onto the upper surface, a cross section of the foam layer is such that the foam layer has a substantially dome-like upper surface extending away from each edge of the frame. In preferred embodiments, the construction panel of the present invention is an external wall construction panel. In this way, the insulating foam layer may be used to its full potential to more effectively insulate the internal environment of a structure from the external environment.

Further optional features of the invention are set out below. BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which:

Fig. 1 shows a plan view of a conventional light gauge steel frame.

Fig. 2a shows a schematic representation of a cross section through a construction panel, filled in with insulating foam according to a method from which the present invention was developed.

Fig. 2b shows an illustration of a construction panel, filled in with insulating foam according to a method from which the present invention was developed. Fig. 2c shows an illustration of a construction panel formed using a method according to an embodiment of the first aspect of the present invention.

Figs. 3a-c show schematic, cross-sectional views of different stages in the method of improving the thermal performance of a construction panel, according to an embodiment of the first aspect of the present invention.

Fig. 4 shows a diagram illustrating a method for determining the relative height variation of the foam layer for an embodiment according to the fifth aspect of the present invention.

Fig. 5 shows a schematic plan view of a panel with a representation of a possible method for application of the insulating foam precursor liquid.

Fig. 6 shows a schematic, longitudinal cross section view through a panel according to the present invention, in place against a wall.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS. AND FURTHER OPTIONAL FEATURES OF THE INVENTION Figs. 3a-c show steps of a method according to an embodiment of the first aspect of the present invention.

Fig. 3a shows the construction panel 10a before the insulating foam precursor liquid is added. The panel 10a has side beams 12, and a base 14. The side beams 12 and the base 14 form a recess 15. In Fig. 3b, a bead 16b of insulating foam precursor liquid has been metered onto the base 14 of the recess 15. The quantity of insulating foam precursor liquid which is added depends on the size of the recess, the properties of the insulating foam precursor liquid itself, and the desired height of the foam layer.

The liquid used may be Envirofoam 16.506, produced by IFS Chemicals Ltd. This liquid has desirable viscosity as well as good cream and rise time for use in the present application.

Envirofoam 16.506 is an insulating foam precursor composition including a resin component and an isocyanate compound component. The resin and isocyanate are mixed in a ratio of 100:110 to form the insulating foam precursor liquid. The viscosities of the resin and isocyanate at 25°C are 350 mPa s and 240 mPa s respectively. The Envirofoam 16.506 precursor liquid has a cream time of 55 s and a tack-free time of 340 s, measured for 50g of the composition at 20°C.

In Fig. 3c, the bead 16b has been allowed to expand, and now forms a foam layer 16c. Because of the viscosity, and foam-forming properties of the insulating foam precursor liquid, the bead shown in 16b expands laterally to be in contact with both of the side beams 12, and also has sufficiently low viscosity, that before setting, it forms a level upper surface 18. In the case where the upper surface of the base panel has any protruding surface features, the foam-forming liquid can flow over these and still achieve a level upper surface. In this way, the thermal performance of the construction panel is substantially uniform across its area. In the present embodiment, the base panel has a height h ase of 70 mm, the foam layer has a height hf _0a m Of approximately 40 mm, and the side beams have a height hf _ra me of 90 mm. The height of the foam layer 16c, hf _0a m is sufficiently low at the leftmost edge of the foam layer 16c that the foam does not cover the opening of service hole 20. In an optional next step, a second construction panel of the kind shown in Fig. 3a may be laid on top of the panel at the stage shown in 3b, and the process repeated. This takes advantage of the fact that the panels are laid flat during the forming process, and so can be stacked one on top of another.

Fig. 4 shows an example of a lattice arrangement for measuring the height of the foam layer.

Here, a plan view is shown of a rectangular construction panel 10, with a frame 12, and a foam layer 16. This panel could, for example, be prepared according to the method illustrated in Figs. 3a-c. The sampling area A is defined by the outer dotted line. The maximum width of the main surface of the foam layer 16 is given by w. The sampling area A is a region of the main surface which excludes a perimeter of width 0.1w all the way around the periphery of the main surface. Covering the sampling area A is an array of 10 by 15 lattice points L, which are arranged in a regular square lattice. In the fifth aspect of the present invention, the height of the foam layer would be measured at each of these points, and the mean of these 150 values would give the value h _avg . In the preferred embodiment of the invention, the following requirements are satisfied, ensuring a suitably level upper main surface of the foam layer. In this particular example, where there are 150 measurement points:

• 15 or fewer points have a height less than or equal to 0.9 h _avg , and • 15 or fewer points have a height greater than or equal to 1.1 h _avg .

Fig. 5 shows one advantageous method which may be employed to apply the insulating foam precursor liquid to the construction panel of the present invention. Fig. 5 is a schematic plan view of a construction panel including five recesses defined by the frame 12 and the backing 14 of the panel. The panel has a length I of 3 m and a width w of 2.4 m, defining a total area of 7.2 m ² . Each individual recess has l _re cess = 0.6 m, thereby defining an area of 1.44 m ² . Arrows denote the direction of movement of the operator as the precursor liquid is applied within each recess in turn.

The arrows are labelled A-H according to the order in which the operator applies liquid to each recess. Thus the operator begins at the end of the arrow marked "A" and walks in the direction of the arrow along the edge of the panel, applying liquid in a line along the centre of the first recess. After completion of the first recess, the operator immediately moves on to the arrow marked "B" to apply liquid there, then the arrow marked "C", and so on. The liquid used (Envirofoam 16.506) has a cream-time long enough that liquid applied along the arrow marked Ή" will flow and mix with the liquid applied previously at "B", and no joint will be evident where the two portions of liquid meet at the marked area 50. This method therefore allows the operator to apply the liquid in one complete circuit around the periphery of the frame. This is only one exemplary method of application and the skilled person will understand that any suitable application method may be used. In one exemplary embodiment, a hand-held portable pump is used to deliver insulating foam precursor liquid to the recesses of a construction panel. The pump may be calibrated by the operator to deliver a predetermined volume of liquid over a given period. In the present example, the pump is calibrated to deliver 0.3 kg of liquid per second. The pump operator can monitor the amount of liquid delivered because the pump makes an audible "click" sound at regular intervals, for example one "click" per second. Therefore, in the time between each "click", 0.3 kg of liquid is delivered to the panel. Calibration may be performed by, for example, pumping 5 "clicks" of liquid into a polythene bag, weighing the deposited liquid and then adjusting the nozzle pumping rate accordingly depending on the result. A further weight check can then be performed, followed by further adjustment where necessary.

In the present example, to achieve a layer of insulating foam 40 mm thick, 1.5 kg of liquid must be applied to each square metre of backing within the panel. The operator walks along the side of the panel, delivering liquid at a constant rate from the pump. The amount of liquid delivered to the panel therefore depends on the pump calibration (already predetermined) and the speed at which the operator moves alongside the panel. The operator can adjust his speed accordingly to ensure the delivery of the requisite 1.5 kg rrr ² of liquid. In the present example, an area within the panel which is 0.6 m wide by 1.66 m long will require around 5 seconds for liquid application, with the pump calibrated to deliver 0.3 kg s ^~

After sufficient liquid has been applied to the entire panel, the panel is left horizontal for around 20 minutes to allow time for the liquid to cream and rise, thereby setting to form a foam layer which properly fills each recess. During or after this 20 minute period, a second panel may be laid on top of the first and the process described above is carried out in an identical manner for this second panel. In practice, the second panel can be laid on top of the first immediately after the operator has finished applying liquid. This saves time in the application process. It is envisaged that, for panels according to the present example, up to 6 panels may be stacked in this way for manual processing (to a total height of around 1 m).

The embodiments described above assume that precursor liquid is applied manually, but of course the liquid could also be applied in an automated process for example by a nozzle connected to a computer which controls the calibration, liquid delivery and movement of the nozzle. Fig. 6 shows a schematic cross sectional view of a finished construction panel 10 fixed in place against a wall of a building structure 60. The wall is made up of a layer of masonry 61 such as brickwork which is of thickness m = 100 mm. Inside the masonry there is an air-gap 62 of thickness a = 50 mm. The air-gap is bridged by wall ties 64 which link the masonry 61 to the frame 12 of a construction panel 10 according to the present invention. The construction panel 10 includes a rigid foam backing layer 14 and a steel frame 12. The recesses within the panel defined by the backing layer and the frame are filled with a layer of foam 63 formed from precursor liquid, as described above. The insulating layer thus formed is of thickness f = 40 mm.

The backing layer of the panel is a layer of Kingspan Thermawall (RTM) TW55 of thickness t = 110 mm. A self-drilling screw 65 fixes the Thermawall backing to the light- gauge steel frame stud 12.

The stud 12 is of thickness s = 90 mm. The total thickness of the construction panel is therefore (110+90) = 200 mm. The total thickness of the wall and construction panel combination is 363 mm. This thickness is comparable to standard block and brick construction, but the layer of insulating foam formed from precursor liquid allows improved thermal properties over standard arrangements.

A layer of plasterboard 66 of thickness p = 13 mm is placed against the insulating panel to complete the wall construction.

While the invention has been described in conjunction with the exemplary embodiment described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiment of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiment may be made without departing from the spirit and scope of the invention.

All references referred to above are hereby incorporated by reference.