Window rebate ventilators are small air vents that can be installed in new and existing windows. They ensure a minimum air exchange that is intended to remove some of the moisture from living spaces. In practice, however, their purpose is controversial. We explain why this is the case and provide information about the functionality, areas of application, and costs of window rebate ventilators.
Window rebate ventilators are small air vents
Window rebate ventilators (short FFL) are small valves that can be easily installed in the frame of windows. They are almost invisible and create a planned leak in the building envelope. They get their name from the place of installation: the ventilators are located in the so-called window rebate between frame and sash. When cold air flows in from outside, it enters through the window frames. In doing so, it already absorbs some heat and its temperature rises. To limit the air exchange rate, window rebate ventilators are equipped with a so-called wind pressure safeguard. The small ventilators operate purely mechanically and consume no electricity. They ensure a minimum air exchange but allow only a little air inside compared to other ventilation systems. Craftsmen can install window rebate ventilators today in new and old windows made of plastic, wood, or wood-aluminum.
Air exchange rate depends on numerous factors
For window rebate ventilators to let air flow through, there must be a pressure difference between the windward and leeward sides of a building. This is possible when the wind presses against one side of the building. Because of this, a negative pressure forms on the other side, which sucks air out of the house. In calm wind conditions or when all windows face the same direction, hardly any air flows through the small ventilators. Used and moist air remains inside the house, relative humidity rises, and mold in the apartment could be the result.
Shock ventilation is mandatory even with a window rebate ventilator
It is important to know that the small ventilators always allow only a little air to circulate through a house or apartment. The air exchange is just enough to expel moisture loads independent of users. However, when occupants are at home, they release much more moisture through their skin, cooking, or bathing. To safely remove this moisture, regular shock ventilation is required even when using window rebate ventilators.
Areas of application and limits of window rebate ventilators
New or energetically renovated buildings are usually especially airtight. Outdoor air no longer flows through by itself, and moisture or pollutant loads can accumulate over time. While this disturbs comfort, mold problems are especially possible with very humid air. Window rebate ventilators help to prevent this at least partially. They ensure a certain minimum air exchange independently of users. Typical areas of application for this technology are:
[*]retrofit solutions for existing mold problems (the cause of the mold must be clarified beforehand)
[*]moisture protection ventilation in new and old buildings (as cross- or shaft ventilation)
[*]burglary-proof minimal ventilation solution
[*]combustion air supply for room air-dependent heating appliances
Window rebate ventilators are usually unsuitable for use in a decentralized exhaust air system. This technology extracts air from the house using a fan and lets fresh air flow in from outside through special outdoor air inlet elements (ALDs). The performance of the small ventilators is rarely sufficient for this purpose.