OLO Kranj

The importance of intervention

The restoration of the windows and hall glazing of the palace of the former OLO in Kranj, a monument of national importance, carried out between October 2025 and March 2026, represents one of the most important conservation interventions on modernist architecture of the post-war period in Slovenia. The project, led by the Municipality of Kranj and co-financed by the Ministry of Culture, is significant not only for the scale of its implementation, but above all for the premise that the building furniture is not considered as an interchangeable technical element, but as a protected component of the overall architectural design and a technical achievement of the period in which the building was created. The estimated value of the works was EUR 475 408,50 excluding VAT and the final value was approximately EUR 577 000, with the Ministry contributing just over EUR 175 000.

Change of approach and professional framework

The renovation was determined by the rejection of a previous solution from 2019, which had envisaged the complete replacement of the existing wooden windows and hall glazing with new triple-glazed glazing elements. The investor decided to change its approach and reapplied to the Ministry of Culture's call for tenders in 2025. On the initiative of the Slovenian Institute for the Protection of Cultural Heritage, Kranj Regional Unit, an interdisciplinary expert commission was formed in August 2023 to consider all interventions at the monument, which had already considered the project for the restoration of the square in front of the palace prior to the glazing. The basic starting point for the glazing was to preserve as much of the original material substance as possible, including the existing timber frames, and to limit the intervention to the minimum necessary for the windows to continue to function without significantly altering their appearance, structural logic and detailing.

Situation before renovation

The most critical condition was found in the large hall glazing on the south-west side of the building. The seals between the panes, made of an older sealant defined as bitumen or tar impregnated rubber, had almost completely disintegrated after almost seven decades due to ageing of the material and prolonged exposure to solar radiation. In some places, the seals at the joints with the supporting structure were no longer present at all. The consequences were leaks, increased temperature loads in the rooms, difficulty in using the halls and the gradual deterioration of the timber frames themselves. The conservation assessment was therefore based on the fact that, without timely intervention, it would not be possible to talk of restoration in the near future, but only of replicas.

Only a solution that preserves the original frames, their characteristic cross-sections, the way they open, the fixing of the slats with the same brass screws, the mechanism of the brass fasteners - butterfly fasteners - and the removability of the inner frame for cleaning was acceptable. The outer single pane of glass, attached to a tombak profile, has been preserved as original and, apart from cleaning with silicone putty of various colours, has not been tampered with.

On the recommendation of the expert committee, a sample of the glazing in one of the bays in Hall 14 was also made before implementation to verify the extent of the intervention, its visual impact in relation to the whole, the degree of preservation of the material and the functional implications of the proposed solution, including the issue of possible condensation. It was found that the proposed development does not alter the external appearance of the building.

A structural assessment of the entire building and the glazing in particular was also required before the final approval of the development. As the building was constructed before the introduction of modern standards and during a period of rationalisation of construction, it was necessary to check whether the existing structure could bear the additional load of the new double glazing. The existing timber frame weighed approximately 15 kilograms in one field with single glazing, while with thermopane glazing inserted it weighed 130 kilograms. A mechanical resistance and stability assessment by Elea iC, carried out in September 2023, showed that the existing structure could withstand the design loads of wind, earthquake and the additional weight of the new 6 mm thick double thermopane glass. It was also found that the load was correctly transferred to the steel structure, with local reinforcement required at the lower part of the frames with non-conspicuous metal plates within the timber frame. The structural analysis was also an important source of new information on the structural performance of the whole building and a starting point for the planned next interventions.

Understanding the glazing method

20th century bound windows represent a special category of windows that are not recognised as being of high quality and are unfortunately too often and too quickly replaced by modern insulating windows, perhaps even more quickly than historic 19th century windows. In the latter case, we have learnt through practice and good contractors that double-glazed thermopane glass can be inserted into the same thin wood profiles as the original single-glazed double-hung box windows, while retaining the single pane of glass in the outer sash.

The first plywood windows can be traced back to 1874 in the Carpenters» Handbook as «close-fitting double windows, in which the frame of the inner window also opens and the sashes within it», with the sashes also being separately hung. Three years later, in the next joiner's handbook, the first drawing of true double-glazed windows as plywood windows appears, where «lighter sashes must be fitted to the window sashes, containing a front hood and allowing all the panes to be cleaned permanently by opening them with an intermediate pin'. After the turn of the century, this detail gave rise to the plywood windows we know today, which were then standardised and dominated the market for about three decades after the Second World War. Bound windows therefore emerged as an improved successor to box windows, which would allow more light transmission, less material consumption and be more user-friendly.

Until the market introduction of the insulating glass window in the 1970s, the plywood window was an energy-efficient, user-friendly and cost-effective window. The double glazed window was the last serious competitor to the insulating glass window, the last impulse before the insulating glazed window became a mass-produced, consumable and expendable building element.

Standard modern insulating windows with a variety of frames do not retain the details of plywood windows, which are made of relatively thin profiles, nor the characteristic semi-industrial glass with a slightly wavy surface and an almost translucent white tint. The installation of modern thermopane windows thus also accompanies the external appearance of the architecture and can give the appearance of »fakeness«, of inconsistency with the concept, since modern windows in most cases have glass with a greenish tint, which is potentiated by the number of panes installed and the much wider profiles, while at the same time the glazing area itself is much smaller. This means not only a drastic change in the external appearance of the architecture, but also, as a consequence, less light in the interior spaces themselves.

Renewal

The hall glazing in the OLO Palace is one of the most ambitious elements of Ravnikar's design. The three large south-west facing glass walls are an essential part of the representative character of the palace; the central brickwork opening measures approximately 10 x 6 m and the two side openings approximately 6 x 6 m. The large span was achieved by means of a reinforced concrete structure with a slender steel grid attached to it, into which timber frames were inserted from the inside. The vertical steel profiles are spaced approximately 1,47 m apart and the horizontal profiles are spaced approximately 2,95 m apart. During the renovation, the steel mark »ST 27 11 88036 ŽJ« was also found on one of the profiles, indicating the Jesenice ironworks and the old German nomenclature code for structural steel. In each steel grid box is inserted a wooden frame measuring approximately 2,94 x 1,46 m, originally with two single panes of glass: an outer pane of 4 mm and an inner pane of 3 mm. The designer wrote that the outer glass had to be thicker »to like the wind«. This design has all the essential characteristics of a plywood window: two levels of glass, close-fitting, a thin profile and an exact match between the wood and steel.

Some of the glass has been replaced in the past due to mechanical damage. To improve the thermal comfort of the halls, only a limited recess into the existing timber frame, approximately one centimetre deep, was allowed to allow the insertion of double insulating glass without loss of essential details. The intervention was designed to be as reversible as possible: the removed interior glass was stored in a special wooden protective box in the basement of the building itself, and the thermopane glass installed can be removed in the future and replaced, if necessary, by a single pane of glass in the same plane.

The details of the fixings are also of particular conservation value. The outer glass is supported by a thin brass U-profile welded to a steel frame, and the joints between the glass panes are covered externally by profiled tombak cover elements. Between the two fields of glass, the tombac cover profile was cleaned with demineralised water and soapy water only, on the recommendation of a metal restorer, to remove impurities but preserve the patina. Inside, the wooden frame is held vertically by three brass butterfly-shaped fastenings in a steel grid. The steel had to be repainted in the horizontal area with anti-corrosion protection in the same dark colour as the original, and the bow ties had to be cleaned of impurities, as the rooms were once used for smoking. It is this almost filigree harmony between steel, wood, glass and metal details that is one of the key qualities of Ravnikar's architecture. The choice of the new glazing was determined by the question of its optical effect. The existing glass is almost colourless, slightly wavy and thickened in places, typical of older industrially produced flat glass. For the new thermopane glazing, OptiWhite, with low iron content and high light transmission, was chosen from several samples in order to minimise interference with the visual appearance of the façade, without the greenish tint of modern glazing.

The office windows have a completely different design - they open along a horizontal central axis, and the whole mechanism had to be rebuilt to open them as when they were built. The original hardware has two pins on each side, which (almost identical to the principle of modern burglar doors) are inserted into the outer frame after closing, ensuring the tightness of the element. In some cases, replicas of the aluminium handles had to be made, so moulds and precision castings were made to replicate the existing ones. The office windows are classically plywood windows, with the oak on the inside stripped and the oak on the outside having been originally treated with a more stable walnut-coloured overcoat. The windows in the toilets, which were secondary windows that had been replaced with plastic windows in the past, were replaced during these works with a single original one from the top floor. The secondary mask in front of the convector in the corridor of the top floor has also been adapted to allow access to the side boxes, which are opened by a handle on the lower transom. In front of the side windows in all corridors, Ravnikar has provided fixed tombak grilles on the shutter principle. All side windows have also been rehabilitated in their opening technology and any physical obstructions so that they can be used in their original function and design.

None of the frames were completely replaced in the restoration; only in two places a new piece of the same type of wood was inserted in the traditional woodworking union. The two glazing bars in the uppermost corridor, which had been reworked in the past, were made exact copies with the return of characteristic details. Moulds were also made for some of the important components: the brass fasteners, the brass plates next to them and the aforementioned aluminium handles for the office windows.

The renovation also produced important new findings on materials. In the carpentry workshop, it turned out that the wooden frames in the halls are not made of oak or larch, as the surviving inventory of the works would suggest, but of elm, while the frames in the offices, corridors and lobby are of oak. The observation about the use of elm is important from both a material and an interpretative point of view, as it is a quality hardwood with high toughness, elasticity and good resistance to moisture, which was probably suggested to the architect by the cabinetmaker himself. The original joinery marks of the windows with Roman numerals and the structural steel marks were also found.

New findings

The project has not only improved living comfort, airtightness and thermal performance, but also brought new insights into the building as a whole. During the renovation, an inspection of the roof's load-bearing steel structure was carried out, which showed a satisfactory condition, but also the need for a timely renewal of the corrosion protection in exposed areas. It was also noted that the original shutters in the hall area were not roller shutters as originally envisaged in the original plans and for the office windows, but textile blinds in the form of double curtains, translucent and blackout, where, due to the weight of the blackout blinds, they were upgraded with a motorised actuator similar to the one foreseen for the roller shutters. Ravnikar repeatedly referred to the curtains in the execution plans as »day and night curtains«. Ravnikar's bioclimatic design for natural cooling and ventilation, which works on the principle of directing cold air from ducts laid between the foundations and blowing it in just next to the glazing on the south-west side, has been further confirmed by the fact that it has been able to ventilate the building in this way, without any mechanisation, without affecting the external appearance of the building for almost 70 years.

The renovation of the OLO Palace thus goes beyond the level of a single technical intervention. It is one of the earliest and at the same time most extensive examples of a comprehensive renovation of modernist furniture in Slovenia, where the aim was not to replace historic components with new ones, but to preserve as much authentic material as possible, while improving functional properties and living conditions. The particular value of the project lies in the preservation of the plywood windows as an overlooked component of modernist architecture, in the preservation of the brass fastenings and details and, in places, in the restoration of their original function. The intervention is also important from the point of view of the development of conservation methodology and the interaction between the interdisciplinary profession, the structural engineers, the investor and the heir to the copyright. At the same time, the restoration has provided new information on the design, construction, materials, construction technology and degree of damage, which was not known before the intervention. The damage map and the accompanying reports and opinions not only document the situation as it was, but also provide an important technical basis for supplementing and upgrading the existing conservation plan and form the basis for the management plan. The project thus provides a valuable example of how a well-managed renovation can simultaneously preserve material authenticity, improve the usability of a listed building and generate new knowledge for future, related interventions in the modernist building stock. At the same time, the great understanding and trust of the investor and the entire City Council, which unanimously voted in favour of the proposed renovation at the end of August 2025, should not be overlooked, nor should the extensive experience in the field of renovation and thus the highly professional approach of the contractor himself.

Owner: Municipality of Kranj, on behalf of the Mayor Matjaž Rakovec

Project Manager at the IOC: Vid Krčmar, Tina Fleiser

The responsible conservator ZVKDS OE Kranj: Natasa Ülen

Contractor: Mizarstvo Ovsenik d.o.o., Kranj, for him Domen Ovsenik, Rok Murko

Construction supervision: Kamin konsulting, Jože Kamin, Anžej Kne

Static assessment: Elea iC d.o.o., for him Angelo Žigon, Dr. Jure Snoj

Safety at work: IVZ d.o.o., for Matic Jesenšek

Expert panel: Dr Martina Malešič, Dr Eva Sapač, Dr Tina Potočnik, Dr Aleš Vodopivec, Dr Andraž Keršič, and the rapporteurs Majda Kregar, Nataša Ülen

Recovery time: 10/2025 - 03/2026

Co-financing: Ministry of Culture