The present invention relates to a ceiling structure including a plural- ity of ceiling joists with insulating batts or slabs arranged between them, said insulating batts or slabs being retained by a plurality of wires spanning across the ceiling underneath the joists and the insulating batts or slabs, each of said joists having a lower surface, an upper surface facing away from the lower surface and two sides interconnecting the lower and upper surfaces. The in- vention further relates to a method for retaining insulating batts or slabs in such a ceiling structure.

Traditionally the insulating batts or slabs are made slightly oversize and arranged between ceiling joists, so that they are held by a clamping force between the ceiling joists. When the insulating batts or slabs have been mounted, a thin steel wire is secured to the lower surfaces of the joists and spans across the ceiling in order to ensure that the insulating batts or slabs do not fall down. Conventionally, the steel wire is fastened to the lower surfaces of the joists by staples.

A ceiling insulated in this way is structurally very simple and hence both cheap and easy to construct, but when using wooden joists it will fail relatively quickly during a fire, as the staples come loose when the lowermost area of the joists burns away. The wires and insulating batts or slabs then fall down, allowing the fire to spread more rapidly to the construction above the joists.

Other structures where insulation material is secured between joists or like framing are known from US4,023,323, DE3106299, US6,487,825 and US4,735,026. These methods all rely on metal members spanning the space between two neighbouring joists or beams and holding the insulation in place and these metal members thus functionally correspond to the wires in the tra- ditional method described above. The m eta l m em ber known from US4,023,323 is of a very sturdy design and in DE3106299 and US6,487,825 means are provided for adjusting the length of the metal members, thus making all of these solutions relatively expensive. Similar considerations apply to US4,735,026, where the insulation is surrounded by a complex lattice, which is suspended between joists.

Another disadvantage common to the ceiling structures and methods described in these four pieces of prior art is that they span only a single space between joists. This means that a local heating caused by a small fire and causing the attachment at one joist to fail will result in the insulation in the adjacent space falling down. Moreover, a heat-induced softening of the metal members may contribute to the failure of the attachment, particularly when using a relatively thin and loosely supported member as in DE3106299. When using a continuous wire as initially described, the attachment at neighbouring joists will keep the wires, and consequently also the insulation, in place for a while, thus making this structure less sensitive to local heating.

It is therefore the purpose of the present invention to provide a ceiling structure and a method for insulating a ceiling structure, which allow the insu- lating batts or slabs to be installed in an easy and cheap way, while at the same time having a good resistance to fire.

This is achieved with a ceiling structure, where the wires are held by a plurality of wire holding devices, where each of said wire holding devices is arranged with a central portion resting on the upper surface of a joist and two legs extending from the central portion along the sides of the joist, said legs being substantially aligned with a mutual distance that corresponds to the distance between the sides of the joist and having a length that corresponds substantially to the distance between the lower and the upper surfaces of the joist, and where the wires are attached to connectors at the ends of the legs opposite the central portion.

In practice this means that the staples are replaced with wire holding devices that extend above the joists, where they are protected from the heat of a fire. The advantage of having a continuous wire is maintained, but the connection to the joist will not fail until the joist has deteriorated completely.

The two-legged design means that the weight on the wire holding device is balanced and that it is less likely to slide off the joist than when using a one-side embodiment as in the prior art documents described above. This effect is particularly pronounced if one wire is connected to both legs of a wire holding device.

The material of the wire holding device itself is located between the side of the joist and the insulating material and is thus protected from the heat. In addition, when the wire holding device is made from a material having a high thermal conductivity, such as steel, heat will be led away from the ends, which are most exposed.

The connection between the wire and the wire holding device may be achieved in any desired way, including twisting the ends of the legs around with the wire or the use of clamps or wire locks. A very simple connection is, however, achieved when the wire holding devices are provided with hook shaped ends, possibly by in-situ bending, and the wires are connected to the wire holding device by being laid into the hooks and kept in place by the weight of the insulating batts or slabs. If extreme agitation of the wires can be expected it may be advantageous to flatten the hook upon insertion of the wire so that the wire is clamped in place.

It is noted that the length of the legs of the wire holding device at delivery will of course depend on the type of connectors used, as some of them will result in the legs being shortened. In addition, the wires need not be arranged so that they abut the joists. On the contrary, a distance between the lower surface of the joist and the wire will allow the introduction of insulating material, which may protect the joist in the event of fire.

As mentioned above the two legs of the wire holding device are intended to extend from the central portion along the sides of the joist, which is traditionally of a rectangular cross-sectional shape, and they should therefore be substantially aligned with a mutual distance that corresponds to the distance between the sides of the joist and having a length that corresponds substantially to the distance between the lower and the upper surfaces of the joist. This, however, does not mean that the legs must necessarily be straight. They may be curved or bent so as to conform to the cross-sectional shape of the joist. Hence the indication that the legs are "aligned" is only to be seen as an indication that they are substantially of the same shape and following the same direction. In addition it is to be understood that if using H-section joists, the legs will extend from the upper flange of the H-section to the lower flange of the H-section, thus not following the outer contour of the joists tightly. Likewise, the legs and central portion of the wire holding device may span over recesses in the joist.

In ceilings of a very limited size, a single wire may suffice, but this is rarely the case. It is therefore preferred that a number of wire holding devices are arranged at regular intervals along the joists. Preferably, each batt or slab of insulating material should rest on two or more wires.

The wire holding device may be made from a wide range of materials including polymers and composites, which may display excellent resistance to fire. For most purposes, however, a thin steel rod, which has been bent into shape, will pose the perfect combination of durability and price.

A ceiling structure according to the invention may be made by a method comprising the following steps:

a) arranging wire holding devices with a central portion resting on the upper surface of a joist and two legs extending from the central portions along the sides of the joist, said legs being substantially aligned with a mutual distance that corresponds to the distance between the sides of the joist and having a length that corresponds substantially to the distance between the lower and the upper surfaces of the joist,

b) arranging insulating batts or slabs between the joists,

c) drawing wires between wire holding devices and along lower surfaces of the joists so as to span across the ceiling underneath the insulating batts or slabs, and

d) attaching the wires to connectors at the ends of the legs opposite the central portion of the wire holding device.

It is noted that the sequence of the steps may be different. For example step b) of the method may be performed after step c) and possibly also step d) depending amongst others on the type of insulating material used and the distance between the wires. The insulating material can then be inserted between two wires and shifted into place on top of the wires. However, as this solution will be more complicated in practice, it will rarely be preferred.

It is also noted that even though the present invention has been developed for use in ceiling structures, where the joists are made from wood, it may of course also be used with other types of joists. In fact the method according to the invention is very versatile as there is no longer a need for penetrating the joist material.

In the following the invention will described in more detail with refer- ence to the drawing, where:

Fig. 1 is a perspective, partially cut-away illustration of a pair of joists mounted with wires and wire holding devices according to the invention, and

Fig. 2 is a cross-sectional, partially cut-away illustration of a pair of joists mounted with wires, wire holding devices and insulation as well as with battens and roofing laid out on top.

A ceiling structure according to the invention includes a plurality of ceiling joists 1 with a plurality of wires 2 arranged at lower surfaces of the joists and spanning across the ceiling. Here the joists are made of wood and have a rectangular cross-sectional shape with a lower surface 1 1 , an upper surface 12 facing away from the lower surface and two sides 13, 14 interconnecting the lower and upper surfaces. The cross-sectional shape need, however, not be rectangular, but may for example be provided with recesses and/or bevels (not shown). In other embodiments the joists may be made from other materials, such as metals, polymers or composites, in which case other shapes, such as for example an H-section, may be more suitable.

Riding on the joists 1 are wire holding devices 3 arranged a distance d1 from each other corresponding to the distance between the wires 2. As may be seen, the wire holding devices each comprises a central portion 31 resting on the upper surface 12 of a joist and two legs 32 extending down- wards from the central portions along the sides 13, 14 of the joist. The legs 32 are substantially parallel with a mutual distance d2 that corresponds to the distance between the sides 13, 14 of the joist and having a length I that corresponds substantially to the distance between the lower and the upper surfaces 1 1 , 12 of the joist. Here the legs are straight and substantially perpen- dicular to the central portion, but in other embodiments, they may be curved or bent so as to conform to the cross-sectional shape of the joist as mentioned above.

The symmetrical design of the wire holding devices 3 and the mount- ing with one leg on each side of the joist means that wire holding device is effectively kept in place by the loads affecting it. This is particularly so, when both legs are connected to the same wire as shown in Figs. 1 and 2 and the load is thus substantially the same on both legs 32.

Throughout this application the term "lower surface" and "upper surface" has been used to indicate the intended orientation of the respective surfaces of the joists, when insulating a flat ceiling structure. It is, however, to be understood that the joists need not necessarily be arranged horizontally and that they may have complex shapes with more than one surface facing up- wards and/or downwards without thereby departing from the scope of the claims.

In the embodiment shown in Fig. 1 the ends of the legs are bent to form hooks 33, which serve as connectors, and the wires 2 are held in these hooks. Here the hooks are left open, but it is also possible to flatten the hooks upon insertion of the wire so as to fixate it. When the wire holding device is made from a thin steel rod, this is easily done by means of a pair of flat nose pliers or may even be done by hand.

In other possible embodiments of the connectors a part of the outermost end of the leg is twisted around the wire 2 (not shown). This connector obviously has to be made in-situ. A hook-shaped bracket 34 may also be attached at the end of the leg 32 as shown in Fig. 2 or wire locks may be used (not shown) if a particularly good hold of the wire 2 is required. This may for example be the case if the wires are also used to carry other components than the insulation 4, such as ceiling panels (not shown). In that case, it may also be necessary to make both the wires 2, which are typically thin steel wires, and the wire holding devices 3 stronger than what would otherwise have been required. For economic reasons the wires and wire holding devices should, however, never be made stronger or more complex than what is required and it may therefore prove expedient to provide several wire holding devices of different design and even to use them at different places in one and the same construction.

As is clearly seen from Fig. 2, only the outermost ends of the wire holding devices 3 including the connectors will be exposed in the event of a fire, whereas the remaining parts will be protected by the insulating material 4. Combined with the symmetrical design, where the weight is distributed substantially evenly on the two legs 32 on each side of the joist 1 , this means that both the wire holding device 3 and the insulating batts 4 will stay in place as long as the ceiling joists 1 are there or at least until softened by the heat. This in turn means that the fire is prevented from, or at least delayed in, reaching other construction part above the joists, such as the roof battens 5 and roof covering 6 shown in Fig. 2.

The material used for the wire holding device 3 may vary depending on demands, where issues such as resistance to fire, strength and resistance to corrosion may be taken into considerations. For most purposes, wire holding devices made of a thin, rigid steel rod that is bent to the shape will provide a good balance between price, durability and strength, but other materials including other metals, polymers or composites may also be used.

The preferred insulating material for use in this invention is mineral wool, such as stone wool or glass wool, but various foam materials are not excluded. The insulating material has been described as being in the shape of insulating batts or slabs and these terms should be understood in their broadest sense as also including blocks and mats, which may be delivered in a rolled-up state.

In this specification, a limited number of embodiments of the invention have been shown and described and it is to be understood that several variations, which will be readily imaginable to the skilled person, are possible without departing from the scope of the invention as explained in the claims.