Structural insulated panels, methods for their manufacture and use

Abstract

A structural insulated panel is disclosed which allows the arrangement and attachment of a plurality of such structural insulated panels (SIPs) for constructing habitable buildings. Each SIP comprises

Description

Field of the Invention The present invention relates to structural insulated panels (“SIP”) for use in modular buildings. SIPs according to the invention are of particular, but not exclusive, interest for habitable buildings including garden offices, garden sheds and other such buildings. The present invention also relates to a method of manufacture of such a SIP. Background Known structural insulated panels (SIPs) are structural insulated panels that are used for constructing buildings by attaching the SIPs to each other. Known SIPs comprise an insulating foam core sandwiched between two structural facers. These facers can be made from sheet metal, plywood, magnesium oxide board or oriented strand board. The foam may be expanded polystyrene foam, polyurethane foam or similar. SIPs are common in efficient building design as they combine many structural and non-structural elements usually achieved by separate components. The use of a SIP avoids the need for dedicated insulation, studs, joists and air barriers and thus are versatile in application. SIPs can be used as exterior wall, roof and floor systems. The inventors currently offer a modular building system constructed from panels known as Bio-SIP™. See https: / / qubebuildings.co.uk / bio-sip-panel / [accessed 16 April 2024]. The Bio-SIP™ panels are sustainable, structural insulated panels. Bio-SIP™ offers a comprehensive solution that encompasses application-suitable: fire retardancy, insulation, structural integrity, weather resistance, airtightness, and decorative elements. Bio-SIP™ comprises an insulating foam core (made from 100% recycled PET or other sustainable insulation) sandwiched between structural facers. The structural facers are made of a biopolymer-infused plant fibre material, this may be hemp or flax or other suitable natural fibre. The fibre material of the structural facers is impregnated with polyfurfuryl alcohol bioresin (PFA) and the facers are formed by by single-shot hot pressing. The Bio-SIP™ is a structural insulated panel suitable for constructing a building by the arrangement of a plurality of such structural insulated panels and their attachment to each other. This is achieved by using a double-width insert that is fixed (screwed or glued for example) into a vertical channel formed in a side face of one SIP and is configured to be fixed within a similar channel of an adjacent Bio-SIP™ using screws. Summary of the Invention The present inventors have realised that further improvements to Bio-SIP™ panels are possible. In particular, it is considered that it would be advantageous to provide an environmentally friendly SIP system with increased strength and fire retardancy and structural utility. The present invention has been devised in light of the above considerations. Accordingly, in a first aspect, the present invention provides structural insulated panel for constructing habitable buildings by the arrangement of a plurality of such structural insulated panels and their attachment to each other, the structural insulated panel comprising an insulating foam core sandwiched between structural facers, wherein the insulating foam core has a central portion with a first density and a perimeter portion with a second density that is at least 1.5 times the first density, wherein the perimeter portion is disposed at least at one part of the perimeter of the panel. In a second aspect, the present invention provides a modular building system for constructing habitable buildings, the modular building system comprising at least two structural insulated panels according to the first aspect. In some embodiments, the system further comprises a jointing component that bridges from one panel to an adjacent panel in a tongue-and-groove arrangement and forms the perimeter portion of adjacent sides of the respective panels. In a third aspect, the present invention provides a method of manufacturing a structural insulated panel for constructing habitable buildings, the method including the steps: (a) hot pressing a pair of structural facers onto an insulating foam core with a first density to form a precursor board with foam core exposed at perimeter faces of the precursor board; (b) cutting the precursor board to a required shape to form a cut board with foam core exposed at perimeter faces of the cut board; (c) routing a vertical channel in each vertical perimeter face of the cut board; (d) fixing an insert into at least one of the vertical channels, wherein the insert comprises an insulating foam with a second density that is at least 1.5 times the first density of the insulating foam core; (e) routing lateral channels in each of the top side and the bottom side of the cut board; (f) fixing an insert into each of the lateral channels, wherein the insert comprises an insulating foam with a second density that is at least 1.5 times the first density; and (g) routing a channel in the insert fixed into the lateral channel of the bottom side of the board for receipt of a locator strip. Optional features of the invention are now set out, these being applicable single or in any combination with any aspect of the invention, unless the context demands otherwise. In some embodiments, the perimeter portions of the structural insulated panel may be formed along each perimeter side of the panel. The perimeter portions aid with the structural and thermal properties of the SIP and facilitate the connection of multiple panels. The perimeter portion formed along each perimeter side of the panel allows for more locations to connect adjacent panels, providing more versatility when constructing buildings. The composition of the central portion and of the perimeter portion may be substantially the same, except for the density difference. Using the same composition allows for a reduced variety of materials during the manufacture of the SIP, saving material costs and reducing manufacturing complexity. The structural insulated panel may further comprise a channel along a bottom side of the structural insulated panel for receipt of a locator strip. During construction, locator strips are fixed to a building base and each panel is fixed to the base by placing the channel over the respective locator strip and screwing the panel to the locator strip. The central portion of the structural insulation panel may have a density of not more than 60kg per cubic meter, not more than 55kg per cubic meter, or not more than 50kg per cubic meter. A low-density foam core is useful for increasing the insulative properties of the SIP. Lower-density foam reduces heat transfer by conduction as there is less solid to act as a transfer path. A lightweight SIP is also beneficial to reduce load during transport and construction when in use. A polyethylene terephthalate (PET) foam may be used in the central portion, particularly a low-density PET foam such as Eco50 may be used. The perimeter portion of the structural insulation panel may have a density of not less than 100kg per cubic meter, or not less than 105kg per cubic meter, or not less than 110kg per cubic meter, or not less than 115kg per cubic meter. A higher-density perimeter portion of the insulating foam core provides additional strength to the SIP, facilitating the building of larger structures. This also reduces or avoids the need for wood or metal reinforcement that may otherwise ultimately reduce insulation performance and promote thermal bridging. A material for the perimeter portion with a low thermal conductivity may be used to further prevent thermal bridging between panels. Further structural elements may be introduced with the perimeter portion to provide additional structural support. These may be needed particularly if the panels are used in larger buildings or buildings with more than one floor. These structural elements may be made from similar natural fibre-reinforced resin or GRP profiles (FST rated where required) or other suitable materials. The panels and system of the preferred embodiments avoid the use of metallic components because these can be are heavy (which makes transportation and installation more complicated and more expensive) and typically have a higher thermal bridge affect compared to non-metallic components so the heat loss is smaller. The panels and system of the preferred embodiments also avoid the use of wood elements because they may also be heavy (which makes transportation and installation more complicated and more expensive) and may also have a higher thermal bridge affect compared to non-metallics so the heat loss is smaller. Also, wooden structures are very susceptible to heat and moisture and can create problems with geometry stability. Wood also is much slower to grow than annually renewable natural fibres of the type described herein. In larger buildings, with more than 1 floor, a second base raft may be implemented on top the SIP panels to establish a new floor for the next storey and tie the base floor wall panels together. In some embodiments, the insulated foam core of the structural insulated panel may be made of made of polyethylene terephthalate. The polyethylene terephthalate may be 100% recycled PET. The structural facers of the structural insulated panel may be made of a biopolymer-infused plant fibre material. The biopolymer-infused plant fibre material may made of an annually renewable natural fibre such as hemp, flax or jute. The biopolymer may be polyfurfuryl alcohol bioresin. Optionally the structural facers of the structural insulated panel may be coated or filled with flame retardant material. For example, the SIP may be coated with intumescent paint. Alternatively, panels of fire-resistant material, such as magnesium oxide (MgO) boards, may cover the structural facers of the SIP. Flame retardancy is required for habitable buildings and such coatings aid with sustaining structural integrity at high temperatures for prolonged periods of time. The invention includes any combination of the aspects and optional features described except where such a combination is clearly impermissible or expressly avoided. Summary of the Figures Embodiments and experiments illustrating the principles of the invention will now be discussed with reference to the accompanying figures in which: Figure 1 shows a schematic cross sectional view of the structure of a known SIP panel. Figure 2 shows schematic cross sectional views indicating the stages of production of a SIP panel. Figure 3 shows a plan view of a SIP panel, the top of which is cut to match a roof pitch angle. Additional lines are included to indicate the depth of channels routed in the SIP panel at the perimeter. Figure 4 shows the stages of the joining process of two SIP panels using a monolithic jointing component. Figure 5 shows an isometric view of two SIP panels joined with a monolithic jointing component. Figure 6 shows the attachment of two SIP panels using a monolithic jointing component. Figure 7 shows additional structural elements to strengthen the jointing component. Figure 8 shows the stages of a SIP panel being fixed to a floor locator strip. Figure 9 shows an isometric view of a SIP panel fitted onto a floor locator strip. Figure 10 shows an isometric view of two Bio-SIP™ panels forming a corner. Figure 11 shows an isometric view of the walls and floor of a building, the walls being SIP panels. Figure 12 shows an isometric view of a window cutout in a SIP panel with inserts to mount a window frame. Figure 13 shows a schematic cross-section of three joined SIP panels. Detailed Description of the Invention Aspects and embodiments of the present invention will now be discussed with reference to the accompanying figures. Further aspects and embodiments will be apparent to those skilled in the art. All documents mentioned in this text are incorporated herein by reference. Reference numbers are included in the drawings. Similar features in different drawings are provided with similar reference numbers. Some features are described only in respect of one or more drawings and may not be provided with a reference number in all drawings. For the purposes of the detailed description, it should be known that the ‘insert’ and ‘locator strip’ structures form part of the perimeter portion of the SIP panel and that ‘jointing components’ and ‘monolithic jointing components’ are synonymous with the ‘inserts’ discussed below. Known SIPs As illustrated in Figure 1, known SIPs 100 consist of an insulating foam 104 core sandwiched between two structural facers 102a,b (skins) which are most commonly Oriented Strand Board (“OSB”). OSB is an engineered wood that is similar to particle board, in that it is a mixture of wood shavings and adhesive compressed together to form a strong but lightweight board. The insulating foam core is typically made from polyisocyanurate (PIR) or expanded polystyrene (EPS). The OSB is bonded to the insulating foam core, without any gaps between. Buildings constructed from SIPs can expect less air leakage and fewer drafts, reduced penetrations for noise and superior insulation properties, leading to lower energy bills and a significantly more comfortable and ambient indoor environment. Known SIPs are prefabricated to be integrated partially or completely into the design and construction of building projects. Each known SIP is produced to the required size and the SIPs can then be stacked together ready for shipment. Off-site manufacturing contributes to a reduction of waste and helps to minimise transportation energy and costs. Once on-site, installation is straightforward and the result is a building system which can be erected in a short time, reducing construction, labour and materials costs compared to buildings which do not incorporate SIPs. Known SIPs usually have a thickness of 142mm or 172mm, a width of 1220mm, and a height of up to 7.5 meters which is chosen according to the requirements of the project. The OSBs are usually 13mm thick. This allows architects and their clients the flexibility and creative freedom to create aesthetically pleasing builds using this system. Bio-SIP™ The garden studios previously sold by the applicant (see https: / / qubebuildings.co.uk / accessed on 16 April 2024) are constructed of a sustainable SIP. Figure 2 shows the overall format is similar to known SIPs in that the Bio-SIP™ 200 consists of a foam core 204 sandwiched between two structural facers 202a, b. They differ from known SIPs in that they use sustainable materials. The foam core 204 is made from recycled polyethylene terephthalate (PET), typically sourced from used plastic water bottles. The structural facers 202a,b are made from a biopolymer-infused plant fibre material. The plant fibre material is typically hemp or flax and the biopolymer is typically polyfurfuryl alcohol (PFA), a bio-resin derived from crop waste such as sugar cane bagasse. The natural fibre composite of the facers has a density of around 1.14 g / cm3 and its mechanical properties are given in the table below: Table 1 - Properties of the facer natural fibre composites (flax fibre with PFA resin) Modulus (GPa) Strength (MPa) Tensile 4.8 ±0.5 22 ±3 Compression 25 ±7 Flexural 2.7 ±0.2 20 ±3 Interlaminar Shear Strength (ILSS) 2.8 ± 1 Figure 2 shows the stages of production for a Bio-SIP™ panel 200. The structural facers 202a,b are be hot pressed on opposing faces of an insulating foam core 204 to produce a completed Bio-SIP™ panel 200. An example of a structure that may be made from Bio-SIP™ is a garden shed, or a leisure building. The panels for each building are prefabricated and delivered to the site for installation. Each sustainable SIP is manufactured in one shot by hot pressing, to produce a sandwich board with, for example, a standard width of 1.2 meters and a standard height of 2.4 meters. The thickness of the panel varies according to the requirements of the project - but the thickness of each structural facing is typically fixed at 3mm. As such, the thickness of foam core is varied in order to set the required thickness of the panel. With a panel thickness of 250mm, for example, it has been found that the Passivhaus standard can be achieved. The Passivhaus standard is a voluntary standard for energy efficiency in a building, with the aim of reducing ecological footprint. It focuses on reducing a building's energy consumption for heating and cooling by using passive measures rather than relying on active systems such as heating and air conditioning. The Bio-SIP™ panel adheres to the insulation efficiencies required in the standard. Method of construction using Bio-SIP™ Figures 4 and 6 illustrate the joining process for two SIP panels that are modified Bio-SIP™ wall panels. During construction, adjacent panels are connected using a double-width insert 210 that bridges the two panels 200L,R. The insert is fixed into a vertical channel 208b formed in the side face of one panel 200L (using an adhesive with good sustainability and fire performance properties such as green epoxy or polyurethane glue) and is configured to be inserted into a similar channel 208a of an adjacent panel 200R as shown in Figure 6. Screws are used to fix the panels together. Figure 8 shows the process for attaching a Bio-SIP™ wall panel to a building base. A channel 308 is provided along the bottom of each panel for receipt of a locator strip 510. The locator strip 510 is a protruding structure from the building base configured to mate with the bottom channel 308. The locator strips 510 may be made from a similar material to that of the SIP core 204 and perimeter portions 210. However, in some embodiments the locator strip may have even greater density than the inserts (e.g. 200kg / mA3 density). This may be preferred due to its superior screw pull out strength and shear strength. During construction, locator strips 510 are fixed to a building base and each panel is fixed to the base by placing the bottom channel 308 over the respective locator strip 510 and screwing horizontally the panel to the locator strip as illustrated in Figure 9 (screws not shown). Figures 10, 11 and 12 show that a building (a garden studio for example) produced by this approach has a frameless structure in which adjacent panels are fixed in position without using metal or wood joints that would, if present, provide significant thermal bridging effects. Further detail of the joining method for the SIP panels The joining method of SIP panels described above is now further explained. It uses higher-density inserts and locator strips that have the same composition as the core insulating material, thus providing various advantages. By reinforcing the foam core 204 using higher-density foam inserts the panel strength is sufficiently increased, such that the panel is suitable for constructing habitable buildings such as glamping pods and larger garden office structures. Removing the need for wood or metal reinforcements removes problematic thermal effects associated with these materials, notably reduced insulation performance and increased thermal bridging. Furthermore, this also makes the buildings much lighter and therefore cheaper and easier to transport and install. The modified inserts and locator strips of the SIP are made using the same sustainable material composition as used for the main core of the panel, albeit with higher density. This improves the ease of recycling the structure and allows for simpler material supply routes. The modified SIP inserts may be made from PET foam with a density of 115 kg / m3 or 200 kg / m3. The higher-density PET foam would provide better structural support, however, it would increase the effects of thermal bridging, thus lowering insulation performance. The balance of these properties may be decided upon depending on the needs of the structure. For example, a larger two-storey building may require a higher density foam, whereas a garden studio may favour better insulation. The locator strips 510 of the modified SIP system preferably have a density of 200kg / m3 due to its superior screw pull-out strength and shear strength. Fire testing performance Unexpectedly, the use of framing foam inserts 210 made of the same composition as the foam of the panels but with higher density, provides an improvement in the fire performance. Fire testing - Bio-SIP panels according to an embodiment of the invention have been shown to be able to obtain a category D classification in Small Flame ignitions test EN iso 11925-2. Furthermore, Bio-SIP panels according to an embodiment of the invention have been tested to EN13501-1 and obtained a category E rating. Still further, in accordance with an embodiment of the invention, Bio-SIP panels with MgO boards fitted as external facers have passed EN1365-1:2012 Reaction to Fire (loaded wall test) 30-minute and 45 minute test. To further improve the SIP fire performance the panels may be coated with intumescent paint. Additionally or alternatively, as mentioned above in relation to further improved fire performance, magnesia (MgO) boards may be laminated on the panel facers, modifying the SIP panels. An example cross-section of three joined modified SIP panels using magnesia boards is depicted in Figure 13. There are two layers of magnesia boards, the first 514a,b is a continuous piece acting as the outermost layer of the panel. The second layer 512a,b directly underneath is made from the same material but is discontinuous, e.g. in the form of battens, providing air gaps for better insulation performance. Adjacent to this are two laminated layers of biopolymer-infused (PFA for example) plant fibre material 502a,b and 506a,b. Two layers are considered to be useful because this allows better wetting and penetration of the PFA resin into the nature fibre mass. It was observed that is only one layer is used, with equivalent thickness to two layers, then the middle of this thick layer may not be adequately penetrated by the PFA resin. This also may have the effect of making the layer more hygroscopic, which in the presence of water or water vapour leads the layer to be softer, reducing the stiffness of the panel. In some embodiments, for example for larger panels, three or more layers of bio-polymer impregnated natural fibre composites can be used. The insulating foam core 504 is made from recycled polyethylene terephthalate (PET). The inserts 510 are made from the same PET as the core albeit with a higher density to provide better structural support. In this embodiment, the high-density inserts 510 comprise a 3mm natural fibre and PFA skin 516 to further improve their mechanical properties. It is considered that the air gaps provide not only better insulation, but also allow air circulation to prevent moisture build up and to avoid close coupling of the Bio-SIP panel to the facer board. This is considered to reduce heat transfer (conduction) from the outside, which is important for fire performance. The fire performance is heavily dependent on the material properties of the layers in the SIP panels. The structural integrity of the material layers degrades under high temperatures. Tables 2 to 4 simulate multiple levels of fire degradation by changing the residual strength of different SIP layers. Table 2 gives the compressive strength of a modified SIP board under ambient conditions, calculated by taking into account the compressive strength and relative proportions of each layer of structure similar to Figure 13. When all structural features are intact the panel has a compressive strength of 1008788 N. This is 5 contextualised with its ‘Factor of Safety’, a value indicating how much stronger the system is than it needs to be for an intended load. The test used to evaluate the Factor of Safety is set at 10000 N. Table 2 shows the modified SIP to be 101 times stronger than required. Table 2 - Compressive strength test of SIP panels under ambient conditions with all structure features intact (100%) Layers of modified SIP Panel Thickness (mm) Width (mm) Area (mmA2) Residual strength Remaining Area (mmA2) Compressive Strength (MPa) Compressive Strength (N) Compressive Strength (kg) Magply 12.0 3000 36000 100% 36000 6.00 216000 22018 Magply 18.0 370 6660 100% 6660 6.00 39960 4073 PFA 1.5 3000 4500 100% 4500 18.00 81000 8257 PFA 1.5 3000 4500 100% 4500 18.00 81000 8257 PET ECO-50 150.0 2730 409500 100% 409500 0.17 67568 6888 PET-GR115 75.0 300 22500 100% 22500 1.80 40500 4128 Insert PFA skin 3.0 1200 3600 100% 3600 18.00 64800 6606 PFA 1.5 3000 4500 100% 4500 18.00 81000 8257 PFA 1.5 3000 4500 100% 4500 18.00 81000 8257 Magply 18.0 370 6660 100% 6660 6.00 39960 4073 Magply 12.0 3000 36000 100% 36000 6.00 216000 22018 Panel Compressive Strength = 1008788 102833 Panel thickness = 210 mm N kg Test Set At 10000 1019 Factor OF Safety 101 101 Table 3 simulates the modified SIP under 850°C for 30 minutes. In this case, it is proposed that there are only the high-density inserts, PFA facers and Magply layers remaining at 25% and 33% of their rated compressive strength respectively. Under these conditions, the compressive strength is calculated to be 151292 N. Given the same test load of Table 1 (10000 N) the Factor of Safety is 15 and so is still 5 structurally reliable. Table 4 also simulates the modified SIP under 850°C for 30 minutes. In this case, it is proposed that there are only the Magply layers remaining at 25% of their rated compressive strength. Under these conditions, the compressive strength is calculated to be 63990 N. Given the same test load of Table 1 10 (10000 N) the Factor of Safety is 6. This shows with even a small proportion of the modified SIP available, the structure still remains structurally reliable. Table 3 - Compressive strength test of SIP panels under 850°C for 30 minutes - the high-density insert is at 25% and back Magply is at 33% load carry Layers of modified SIP Panel Thickness (mm) Width (mm) Area (mmA2) Residual strength Remaining Area (mmA2) Compressive Strength (MPa) Compressive Strength (N) Compressive Strength (kg) Magply 12.0 3000 36000 0% 0 6.00 0 0 Magply 18.0 370 6660 0% 0 6.00 0 0 PFA 1.5 3000 4500 0% 0 18.00 0 0 PFA 1.5 3000 4500 0% 0 18.00 0 0 PET ECO-50 150.0 2730 409500 0% 0 0.17 0 0 PET- GR115 75.0 300 22500 25% 5625 1.80 10125 1032 Insert PFA skin 3.0 1200 3600 25% 900 18.00 16200 1651 PFA 1.5 3000 4500 25% 1125 18.00 20250 2064 PFA 1.5 3000 4500 25% 1125 18.00 20250 2064 Magply 18.0 370 6660 33% 2197.8 6.00 13186.8 1344 Magply 12.0 3000 36000 33% 11880 6.00 71280 7266 Panel thickness = 210 mm Panel Compressive Strength = 151292 15422 N kg TestSetAt 10000 1019 Factor OF Safety 15 15 Table 4 - Compressive strength test of SIP panels under 850°C for 30 minutes - the back Magply is at 25% load carry Layers of modified SIP Panel Thickness (mm) Width (mm) Area (mmA2) Residual strength Remaining Area (mmA2) Compressive Strength (MPa) Compressive Strength (N) Compressive Strength (kg) Magply 12.0 3000 36000 0% 0 6.00 0 0 Magply 18.0 370 6660 0% 0 6.00 0 0 PFA 1.5 3000 4500 0% 0 18.00 0 0 PFA 1.5 3000 4500 0% 0 18.00 0 0 PET ECO-50 150.0 2730 409500 0% 0 0.17 0 0 PET- GR115 75.0 300 22500 0% 0 1.80 0 0 Insert PFA skin 3.0 1200 3600 0% 0 18.00 0 0 PFA 1.5 3000 4500 0% 0 18.00 0 0 PFA 1.5 3000 4500 0% 0 18.00 0 0 Magply 18.0 370 6660 25% 1665 6.00 9990 1018 Magply 12.0 3000 36000 25% 9000 6.00 54000 5505 Panel thickness = 210 mm Building with SIP panels and modified inserts As shown in Figure 3, the SIPs are CNC trimmed according to the requirements of the project (which may include cutting the top to match a roof pitch angle). Figure 3 shows vertical channels are routed in the left and right sides of the foam core (which has a density of 50 kg per cubic metre). A high-density (e.g. 115 kg per cubic metre) PET foam SIP insert is fixed into one of the vertical channels. The remaining vertical channel is configured to receive another double-width SIP insert such that it is fixed to an adjacent panel. In larger or higher load buildings the modified SIP inserts can include structural elements to improve loadcarrying ability. These may be necessary for larger buildings and structures with more than one story. Glass-reinforced plastic (GRP) profiles (preferably made with recycled material content) provide increased structural integrity whilst being lightweight and having low thermal conductivity to mitigate thermal bridging. These are illustrated in Figure 7. High-density insert 310 is a configuration with a hollow square tube of GRP running along the length of the insert, whereas high-density insert 410 is a configuration wherein the high-density foam insert is sandwiched by two plates of GRP. Figure 8 shows horizontal channel 309 is routed in the upper side of the foam core in which a modified SIP insert with a higher density (of 115 kg per cubic metre) (not shown in Figure 8) can be fixed within the top channel and a bottom channel 308 is routed so that locator strip is can be fixed in the bottom channel. In one embodiment the SIP insert has a thickness of approximately 50% of the panel thickness (i.e. 50mm insert into 100mm panel) and the respective channel is cut with 1mm of clearance to a width of 26mm or 51 mm regardless of the thickness of the foam core. The top of the panel is cut to match a roof pitch angle and the insert in the top channel is trimmed after glueing to be flush with the structural facings. Each insert is fixed to the panel using an adhesive such as green epoxy or polyurethane glue. As shown in Fig 11, by adding a plurality of panels with different shapes and including the high-density inserts, the inventors have therefore modified their known Bio-SIP™ panels to be suitable for constructing habitable buildings such as glamping pods and garden office structures and other larger structures (e.g. park homes, forest lodges, micro houses and larger buildings and houses). As with the known SIP joining method, a channel is routed in the bottom of the panel for receipt of a locator strip fixed to the base of a building system. Additionally, as shown in Figure 12, channels may be routed around any window cutouts within which modified PET foam inserts 610 (which have a density of 115 kg per cubic metre) are fixed to provide structural support for the window. Using the same type of high-density inserts that join the SIP panels on the window frames simplifies manufacturing and sourcing material for building modular structures. The features disclosed in the foregoing description, or in the following claims, or in the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for obtaining the disclosed results, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof. While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention. For the avoidance of any doubt, any theoretical explanations provided herein are provided for the purposes of improving the understanding of a reader. The inventors do not wish to be bound by any of these theoretical explanations. Any section headings used herein are for organizational purposes only and are not to be construed as limiting the subject matter described. Throughout this specification, including the claims which follow, unless the context requires otherwise, the word “comprise” and “include”, and variations such as “comprises”, “comprising”, and “including” will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps. It must be noted that, as used in the specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Ranges may be expressed herein as from “about” one particular value, and / or to “about” another particular value. When such a range is expressed, another embodiment includes from the one particular value and / or to the other particular value. Similarly, when values are expressed as approximations, by the use of the antecedent “about,” it will be understood that the particular value forms another embodiment. The term “about” in relation to a numerical value is optional and means for example + / -10%.

Claims

1. A structural insulated panel for constructing habitable buildings by the arrangement of a plurality of such structural insulated panels and their attachment to each other, the structural insulated panel comprising an insulating foam core sandwiched between structural facers, wherein the insulating foam core has a central portion with a first density and a perimeter portion with a second density that is at least 1.5 times the first density, wherein the perimeter portion is disposed at least at one part of the perimeter of the panel.

2. The structural insulated panel of claim 1 wherein respective perimeter portions are disposed along two or more, or each, perimeter side of the panel.

3. The structural insulated panel of claim 1 or claim 2 wherein the composition of the central portion and of the perimeter portion is substantially the same, except for the density difference.

4. The structural insulated panel of any one of claims 1 to 3, further comprising a channel along a bottom side of the structural insulated panel for receipt of a locator strip.

5. The structural insulated panel of any one of claims 1 to 4 wherein the central portion has a density of not more than 60 kg per cubic metre.

6. The structural insulated panel of any one of claims 1 to 5 wherein the perimeter portion has a density of not less than 100 kg per cubic metre.

7. The structural insulated panel of any one of claims 1 to 6 wherein the insulating foam core is made of polyethylene terephthalate.

8. The structural insulated panel of claim 7 wherein the polyethylene terephthalate is 100% recycled PET (post-consumer waste).

9. The structural insulated panel of any one of claims 1 to 8 wherein the structural facers are made of a biopolymer-infused plant fibre material.

10. The structural insulated panel of claim 9, wherein the biopolymer-infused plant fibre material is made of an annually renewable natural fibre such as hemp, flax or jute, and the biopolymer is polyfurfuryl alcohol bioresin.

11. The structural insulated panel of any one of claims 1 to 10, wherein the structural facers are coated with a fire-retardant and / or have an over-layer of a fire-resistant material.

12. The structural insulated panel of any one of claims 1 to 11, wherein the perimeter portion is an insert into a corresponding channel cut into the central portion of the insulating foam core and the insert further comprises a structural facer sandwiching the insulating foam core with the second density.

13. A modular building system for constructing habitable buildings, the modular building system comprising at least two structural insulated panels according to any one of claims 1 to 12.

14. The modular building system according to claim 13 further comprising a jointing component that bridges from one panel to an adjacent panel in a tongue-and-groove arrangement and forms the perimeter portion of adjacent sides of the respective panels.

15. A method of manufacturing a structural insulated panel for constructing habitable buildings, the method including the steps:(a) hot pressing a pair of structural facers onto an insulating foam core with a first density to form a precursor board with foam core exposed at perimeter faces of the precursor board;(b) cutting the precursor board to a required shape to form a cut board with foam core exposed at perimeter faces of the cut board;(c) routing a vertical channel in each vertical perimeter face of the cut board;(d) fixing an insert into at least one of the vertical channels, wherein the insert comprises an insulating foam with a second density that is at least 1.5 times the first density of the insulating foam core; (e) routing lateral channels in each of the top side and the bottom side of the cut board;(f) fixing an insert into each of the lateral channels, wherein the insert comprises an insulating foamwith a second density that is at least 1.5 times the first density; and(g) routing a channel in the insert fixed into the lateral channel of the bottom side of the board for receipt of a locator strip.s

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