In the manufacture of exterior wall elements, generally a technique is used whereby the elements are manufactured in a continuous process wherein the first step is to produce the outer panel of the element in a mould. After this, reinforcements connecting different parts of the element together are mounted in the outer panel. Thermal insulating material is placed between the reinforcements so that these remain in the seams of the insulating material. The reinforcements for the inner panel are mounted above the insulation and tied to the connecting reinforcements. After the thermal insulating material and reinforcements have been mounted, the concrete for the inner panel is cast upon the thermal insulation. In the element structure, the outer panel, the insulation and the inner panel are in direct contact with each other.

However, the prior-art structure involves problems. The element becomes a continuous structure which has been found to be susceptible to problems re- sulting from the accumulation of moisture in the structure, mainly in the thermal insulating material.

It has been established via research and it is also known from experience that the accumulation of mois- ture can be eliminated by providing sufficient venti- lation between the outer panel and the thermal insu- lating material of the element.

Thus, a previously known technique is to pro- vide the insulating material with ventilation grooves into which an air current is supplied from the joints between the outer panels of the elements. There are also solutions in which the outer panel of the element is produced as a separate part in connection with be- ing mounted in a wall. In another known method, a

layer of granular rock material is placed between the outer panel and the insulating material, which layer is removed after the casting has hardened. In this way, a ventilator slot is created between the insula- tion and the outer panel.

A problem with prior-art solutions is how to achieve sufficient ventilation. Compression of the in- sulating material and blockage of the grooves in it together with its relatively small volume result in problems and consequent disadvantages regarding qual- ity. The method of making a separate outer panel is expensive and complicated in respect of installation.

The use of a granular filler to produce a ventilator slot is in principle a good solution, but because moisture and cement are partially absorbed into the filler, it is generally not possible to remove the filler completely from the clearance, which is why the ventilator slot remains partially blocked and is in- adequate for its purpose.

The object of the invention is to eliminate the above-mentioned disadvantages. A specific object of the invention is to disclose a new type of method for manufacturing a prefabricated element, allowing effective ventilation and drying of the element and the insulating material in it by a simple and cheap method.

As for the features characteristic of the in- vention, reference is made to the claims.

In the method of the invention for the manu- facture of a prefabricated element, an outer panel is first cast into a mould, reinforcements connecting different parts of the element are mounted in the outer panel, thermal insulating material is mounted upon the outer panel and an internal panel is cast on top of the thermal insulating material. In addition, naturally depending on the element to be produced, the required reinforcements, window and door openings,

pipes etc. are mounted both in the outer panel and in the inner panel before the casting. According to the invention, a slot insert of adjustable thickness in an expanded state is placed between the outer panel and the thermal insulating material. After the inner panel has been cast, the slot insert is shrunk in thickness, whereupon the slot insert is extracted from between the outer panel and the thermal insulating material, the open space thus created forming a ventilator slot between the outer panel and the thermal insulating ma- terial.

The thickness of the slot insert is prefera- bly controlled using a flowing medium. The flowing me- 'dium used to expand the slot insert may consist of a suitable liquid or gas, preferably air.

The slot insert is preferably shrunk, i. e. the medium is let out from it after the concrete has reached a sufficient hardness. In other words, the outer panel and inner panel need not necessarily be completely hardened, but it is sufficient that they have become hard enough to remain in shape when the slot insert is removed.

In a preferred embodiment, the slot insert consists of a two-layer insert made of rubber or a suitable plastic, in which the clearance between the insert layers forms a space into which the medium, a liquid or gas, is pumped, causing the slot insert to expand in thickness. It is also possible to use a slot insert consisting of a more rigid structure, e. g. two plates which can be moved farther apart and brought closer to each other using either a flowing medium or other means. However, this type of rigid structures involve a drawback regarding their detachment from the surface of the casting. It has been found that in rub- ber inserts, although they mainly expand in the thick- ness dimension, some stretching of the structure also occurs in the direction of the plane. As a result,

even a slight movement in the planar direction dis- rupts the bonds between the concrete and the rubber insert so that the slot insert is completely detached from the cast surface, thus allowing it to be easily removed from the space, where a ventilator slot per- mitting air circulation is formed.

Slot inserts as provided by the invention can be used in both vertical and horizontal orientations, extending from one edge of the element to the other.

It is also possible to have slot inserts placed only within a given limited area in the element.

The method of the invention for the manufac- ture of a prefabricated element has significant advan- tages as compared with prior art. The method of the invention permits easier production and makes it pos- sible to create a ventilator slot of a desired size, extending through the entire element. By this method, exterior wall elements can be produced as a continuous process with no need to wait until the outer panel has dried. The elements can be produced economically as the ventilator slot is formed using repeatedly the same slot insert, leaving no permanent structural parts between the insulating material and the outer panel that could later obstruct ventilation in the element and transfer of humidity in its structures.

Thus, the method of the invention for the manufacture of a prefabricated element makes it possible to pro- duce an element structure that prevents the absorption of any significant amounts of moisture into the insu- lating material while allowing the small amount of moisture introduced into the insulating material dur- ing manufacture to be ventilated away before the final installation of the element.

In the following, the invention will be de- scribed in detail with reference to the attached draw- ing, wherein

Fig. 1 represents a first step in the method of the invention, Fig. 2 represents a second step in the method of the invention, Fig. 3 represents a third step in the method of the invention, Fig. 4 represents a fourth step in the method of the invention, Fig. 5 represents a fifth step in the method of the invention, and Fig. 6 represents a finished prefabricated element produced by the method of the invention.

As illustrated in Fig. 1, an outer panel 1 is cast in a mold, which is not shown in the figures. Be- fore the outer panel has hardened, reinforcements 4 connecting different parts of the element are mounted in it as illustrated in Fig. 2. It is also possible to place the reinforcements 4 in the mould before casting the outer panel, e. g. by connecting them to other re- inforcements in the outer panel.

As shown in Fig. 3, slot inserts 5 of elastic material connected via a suitable valve 7 to a suit- able source of compressed air are mounted on the outer panel 1 between the reinforcements 4. The slot inserts are placed on the outer panel 1 so that they together extend over the entire planar surface defined by the outer panel, from one edge to the other, in both ver- tical and horizontal directions of the element. After this, compressed air is supplied via the valve 7 into the slot inserts 5, which thus expand mainly in the direction of thickness, determining the thicknesswise dimension of the ventilator slot formed in the ele- ment.

Next, as illustrated in Fig. 4, thermal insu- lating material 3 of e. g. mineral wool is placed on top of the slot insert 5 over the entire area of the element. At this point, the reinforcements 4 still ex-

tend through the thermal insulating material, project- ing outside it.

The next step, as shown in Fig. 5, consists in casting an inner panel 2 upon the thermal insulat- ing material. The inner panel covers the reinforce- ments 4 connecting the inner panel and the outer panel. Naturally, various reinforcements as needed can be provided in the inner panel as well, and they can be e. g. fastened to reinforcements 4 before the inner panel 2 is cast.

Once the concrete of the outer panel 1 and inner panel 2 has hardened sufficiently, the valve 7 may be opened to let out the air from the slot inserts 5. As a result, the slot inserts will shrink mainly in thickness, so they are detached from the cast surface of the outer panel 1. Thus, they can be easily ex- tracted from the element. In this way, an open venti- lator slot 6 is formed in the element between the outer panel 1 and the insulating material 3, with only the reinforcements 4 going through the slot and con- necting the parts of the element together.

In the foregoing, the invention has been de- scribed by way of example with reference to the at- tached drawings while different embodiments of the in- vention are possible within the scope defined in the claims.