VAMOS - Casement windows with vacuum glazing: Performance-Monitoring for Building retrofit

Knowledge consolidation of the exploratory project VIG_SYS_RENO; This project focused on the application of vacuum glass in existing casement windows for purposes of energetic performance improvement of buildings. Expected results include new insights about and a guideline for the application and utilization of vacuum glass products in existing window systems.

Short Description


completed (September 2022)

Starting point, contents and results

The project VAMOS („Vakuumglas-Kastenfenster: Performance-Monitoring in Sanierungsprojekten" which more or less corresponds in „Vacuumglas-Casement-Windows: Performance – Monitoring in building retrofit projects") focused on the thermal retrofit of casement windows via implementation of vacuum glazing products. Toward this end, different casement window typologies, such as "Alt-Wiener-Kastenfenster" and "Wiener Kastenfenster" as well as different degrees of retrofit intervention were considered. These different degrees included minimal-invasive restoration but also ranged until total reconstruction and replacement of the old casement window by a facsimile. These different retrofit modes resulted in construction concepts, which were evaluated both virtually via building performance simulation tools and as full-scale mock-ups within laboratory test-beds. Moreover, the retrofitting approaches were applied to real casement windows of case study buildings, in part being of historic value due to their building history. The idea behind was to study the different retrofit concepts in real life buildings, and subjected to real-life conditions. To be able to evaluate the different retrofit concepts later on, the test sites were equipped with comprehensive monitoring equipment, tailored to capture performance data such as surface temperature at critical points, dry-bulb temperature indoor, in interstitial spaces, and outdoor, and levels of air humidity and thus dew point temperatures (to assess condensation risk). Moreover, subjective and objective performance criteria were collected utilizing comfort and condensation assessment by the building occupants. To the knowledge of the authors, this venture can be considered a pioneering one, given that no other casement window retrofit effort has been conducted so far that includes both the implementation of vacuum glazing products and accompanying scientific monitoring. Moreover, the present project might be also the first in Austria that implements vacuum glass panes in windows of historically relevant buildings and thus the sensitive building stock.

By vacuum glazing products regularly two parallel glass panes are understood that feature a small, evacuated interstitial gap, a vacuum tight edge seal, and a set of distance pillars to keep the parallel positioning of the panes against the air pressure from outside. Due to the minimizing of conductive and convective heat transfer, such glass products feature very good thermal insulation at very small system thickness. The Ug-values of such glass panes can be found to be between 0.4 and 0.6 W.m-2.K-1. Due to the thermal bridges caused by the edge seal and the distance pillar grid, such glass products require an in-depth planning and evaluation (e.g. risk assessment toward condensation risk). In the previous exploratory project VIG-SYS-RENO, which also forms the basis of the present project, a proof of concept was delivered that illustrated the principle possibility and suitability of vacuum glass as mean of retrofit for casement windows. However, due to time and economic restraints of this project no real-life performance inquiry could be done at that time. However, such a real-life performance assessment is a crucial condition sine qua non to consider potential and risk of vacuum-glass based window retrofit.

Thus, building upon the findings of the previous projects VIG-SYS-RENO, MOTIVE, and FIVA, different casement window typologies and different construction specifics of casement windows were examined, and tailored retrofit solutions utilizing vacuum glass products were envisioned. These construction solutions were subjected to a rigorous series of test and analysis runs, both virtually via suitable simulation tools and physically via prototype testing in the laboratory. As a result, decisions toward the development of solutions for the specific window typology could be made. Typical crucial aspects of such retrofit concepts encompass the question, if vacuum glass should be applied to the inner or the outer layer of the casement window system, or the question, how frame geometries and connection joints have to be designed. These questions need to be worked upon under consideration of the architectural appearance, which, in most cases, may not be affected by the implementation of vacuum glass or any other retrofit measure. Moreover, performance aspects pertaining to building physics need to be considered as well. These questions were answered for the case study windows within the project. Furthermore, a wide range of laboratory and simulation-assisted evaluations were conducted to explore performance aspects of the windows (e.g. air tightness, tightness against wind-driven rain, hygro-thermal performance, acoustics, etc...).

Six demonstration sites could be utilized for demonstration purposes. In all but one case, test windows (regularly two different vacuum-glass integrated windows) and control windows (regularly one non-modified window still equipped with the original window panes) were selected and subjected to monitoring. The monitoring equipment was tailored to the specific requirements of each situation. The test windows of the different sites showed variations in the improved layer and in the specific boundary conditions (microclimate outdoor conditions, usage of the rooms, wall-material and construction typology of the casement windows). In one case, no control window was available due to the client's wish to update all of the existing windows with vacuum-glass-equipped windows. The monitoring was conducted in different levels of detail, which were named "standard" and "detailed". Subjective aspects and observations by the occupants were additionally collected via a logbook for the specific windows. This aimed at thermal comfort assessment and observation of surface condensate on the windows.

The results of the project illustrate that – despite the non-optimal integration possibilities of the vacuum glass in the windows (regularly the edge seal cover was by far smaller than 40mm) in contrast to new window constructions as reported about in the project FIVA (see – no negative effects could be monitored in most cases. In detail, no condensate could be found, if the window construction and mounting followed the correct principles. Rather, a significant reduction of heat losses through the specific window could be monitored. However, the integration of vacuum glass into casement windows required specific knowledge in the case studies of this project, and the same is true for any other casement window. Given the multitude of combinations of different construction parameters, such as size, indoor and outdoor conditions, position and material of windows, walls, and casement frame, it seems obvious that any retrofit planning should be accompanied by a detailed evaluation by domain experts on building construction and building physics, as well as by experts of window carpentry. This is a condition sine qua non to reach the goals of longevity, energy savings, and upkeep of architectural appearance.

The present project emphasized that vacuum glazing is a viable option for thermal retrofit of casement windows and might play an important role in reduction of energy demand and connected emissions of the building stock. The findings of the project have been published in this report, as well as in numerous scientific and non-scientific publications. Moreover, a rudimentary decision tree for interested stakeholders was created.

Project Partners

Project management

Department of Building Physics and Building Ecology, TU Wien

  • Univ.Prof. DI. Dr.techn. Ardeshir Mahdavi
  • Univ.Ass. DI. Dr.techn. Ulrich Pont (contact person)

Project- or cooperation partners

  • Holzforschung Austria - Österreichische Gesellschaft für Holzforschung
  • Ernst Prohaska - Gepr. Restaurator im Tischlerhandwerk, Tischlermeister
  • Schaden Fenstersanierung GmbH
  • Alois Svoboda GmbH
  • Tischlerei Alois Winkler
  • Zoller-Prantl Gesellschaft m.b.H Tischlerei
  • AGC Glass Europe saDept. New Business Development - Fineo

Contact Address

Univ.Ass. DI. Dr.techn. Ulrich Pont
Department of Building Physics and Building Ecology, TU Wien
Karlsplatz 13/4
A-1040 Vienna
Tel.: +43 (1) 58801 27033