Seal old windows

Thermal insulation is more important today than ever. Some of the biggest weaknesses are explicitly the windows in older buildings. In this guide you will learn how to seal old windows and what has to be considered in relation to the modern window seal.

People always try to insulate needed windows

Since time immemorial, people have been building window openings in buildings. Just as long as they try to dam these windows as efficiently as possible. First with skins and leather, later with transparent natural stones, then with leaded glass. With industrialization, large glass surfaces were possible for the first time. In the 19th and 20th centuries, the focus shifted to double-glazing. Due to the numerous old and existing buildings, we distinguish between the following types of windows today:

  • single glazing
  • Double glazing (two wings, also box windows)
  • Double glazing (insulating window with vacuum or gas filling between the panes)
  • Heat-insulating windows (gas-filled and specially coated glass)

Seemingly leaky windows are part of a centuries-old building experience

In the future, windows with special solar foils will be added. The windows have also always been part of the house ventilation and dehumidification concept. Because first, an exchange of air must take place. In addition, rising from the soil moisture, the temperatures usually generate a pressure in the house.

The warm, water-enriched air now literally pushes it out of the house. The typical weak points are the windows here. Especially between the soffits and the window frame. Ultimately, however, this was also part of the ventilation concept for centuries.

Towards the end of the 20th century, fundamental changes took place

In the 1990s, for the first time, highly efficient heat-insulating windows came onto the market. Everyone wanted to save energy and obstruct such new windows. However, the window assembly had not yet adapted to the new window. Although the windows themselves were now completely sealed, the frames were mounted with gas-permeable PU foam. As a result, it pressed the humid air into the wall (between soffit and window frame) and here cooled the air.

The result: Mold infestation to such an unprecedented extent

Cold air can not absorb so much water, so the cooling air has to give off water. Sweat or condensation forms in the walls, ie in the building fabric. Exactly here lies the problem with old existing buildings, which should get highly modern heat-insulating windows. Thus, the prerequisites must first be created so that such heat-insulating windows can be installed:

  • Window and door mounting according to RAL
  • separate house ventilation (preferably)
  • thermal insulation window
  • appropriate facade insulation (no thermal bridges between window and facade)

The sealing of old windows according to different techniques

In a refurbishment, an old or existing building can therefore be modernized accordingly. However, this is not always intended. For historically preserved and listed buildings, the owners often want to retain the original old windows. However, these windows must be sealed differently.

Sealing in half-timbered houses

Old windows must be sealed in such a way that they are denser on the inside than on the outside. Inside, therefore, no air may come between window frame and soffit. In old buildings such as half-timbered houses, cords (hemp) are used. These are stuffed between soffit and frame. From the outside then plastered with clay or clay and grouted. Thus, the sealing joint remains a working and maintenance joint.

Seal windows in stone old buildings

However, you can also apply a sealing tape to ordinary old stone buildings, followed by PU foam. Inside is then completely sealed with silicone or acrylic. The outside is sealed in such a way that the joint remains permeable to the outside, ie it can vent. However, you must otherwise adequately vent the house in this type of sealing with otherwise completely dense windows.

If you rehabilitate airtight, the total cost is greater

Either integrate a separate vent (then you can also convert the typical cold roof into a warm roof) or you must consistently adapt the venting (ventilation) via the windows to the structural requirements. Incorrect airing can otherwise cause mold very quickly. Even omitted airing leads to, because somewhere in the house are colder areas (corners, outer walls, etc.), in which then forms condensation.

Tips & Tricks

If you want to seal your old windows to modern standards, you should also tackle the thermal insulation of the facade. In addition, the ventilation must be guaranteed. Only in combination with an efficient facade insulation and equally effective house ventilation does the sealing of old windows result in a clear insulating effect, which protects the building fabric over decades.

Video Board: DIY How to seal drafty old windows