Fenestration refers to the openings in the building envelope, including the installation of windows, doors, and skylights. The inclusion of windows, doors and also other openings in the home is necessary as long as they improve the overall environment inside the home, however they do work to break the seal of the home, providing possible places for air, water, and cold air to enter the home. Thus, the number of openings included in a home must be taken into account, as well as the quality of the materials used to create these openings along with the placement.
Functions of Windows and Doors
Windows and doors have several functions, including giving access to the building, providing outlook, and letting in daylight. In specifying window performance for a specific region, it is necessary to consider both heating and cooling loads to maximize performance and achieve the lowest total annual energy impact, or best energy balance. In some climates, a positive energy balance – or energy gain – can be achieved using advanced static glazings combined with well-insulated window systems and architectural shading optimized for seasonal impacts, for example, a triple-glazed window system with two layers of Low-E glass, high solar heat gain, low-conductive frame, exterior shading, in a moderate European climate.
Although most cold-climate OECD member countries are making a significant effort to promote high ‑ performance windows, triple-glazed windows, have not achieved full market share in any country. Triple glazing with clear glass was more prevalent in Northern European countries but then diminished because manufacturers were able to achieve comparable performance using modern, double-glazed, Low-E coated windows.
This trend is changing, however, with the promotion of the Passivhaus program and recent more stringent building codes. Austria, Germany, and Switzerland have the highest market share for triple glazing usually with two Low-E surfaces, at 54% of total window sales. New construction and the residential sector have the highest market penetration. Overall, the majority of windows sold in the European Union are still double glazed. Unfortunately, windows are still being sold in many regions of the world that are only single-glazed, with clear glass and poorly insulated frames. These have U-values of approximately 4.5-5.6 W/m2K. The majority of OECD member countries in cold climates have moved to double-glazed windows with Low-E coatings, low-conductive frames, and inert gas for the residential sub-sector, with U-values of approximately 1.8 W/m2K. As shown in Fig. 2.8, highly insulated windows such as the ones discussed above for the European Union, have U-values around 1.1 W/m2K.
Benefits of Fenestration
The inclusion of openings in the building envelope is necessary for people to live comfortably inside a building (or even get in and out of them in the first place). Openings such as windows allow natural light into the home and can promote passive solar heating and cooling, as well as providing natural points of ventilation, air circulation, and fire exits. Doors are also vital to the design of a building to provide a point of entry and exit to the building. Skylights provide similar benefits as windows, however they generally do not open. Aside from the increased energy efficiency, the benefits of fenestration include:
- Increase comfort by reducing drafts and making a house feel warmer during the cold season.
- Improve indoor air quality by reducing condensation, preventing mold growth. As well, a tighter seal into the home prevents external contaminants from entering the home.
- Saving money by minimizing air leakage, improving the performance of furnaces and air conditioners, and improving the thermal performance of windows.
- Control condensation due to the fact that energy-efficient windows stay warmer on interior surfaces, allowing for a higher level of relative humidity without condensation. This in turn allows windows to last longer, decreasing the need to replace them as frequently.
Issues due to Fenestration
All fenestration products experience a degree of heat loss due to:
- Radiation - heat energy is absorbed by the material of the door or window and radiates toward the cooler side.
- Conduction - heat energy moves through the solid materials that make up the frame or other parts of the window, door, or skylight.
- Convection - heat energy is transferred to the air between parts of the window or door.
- Air leakage - heat energy is transferred to air moving through seals or gaps in the door or window
The first issue in this arises due to improperly installed windows, doors, and skylights. When large drafts enter a home it can lead to overworked heating or air conditioning systems as outside air penetrates the building envelope, allowing untreated air to flow into the home. A simple way to check for leaks such as these is to hold a candle or lit stick of incense around the frames of doors, windows, or skylights. Once the smoke swirls, it will indicate a draft point. High-efficiency, weatherstripping, and caulking fenestration products are used to increase the seal of the window or door and reduce the costs due to air leakage. And also windows, doors, and skylights that let air in can let water in as well. This leads to condensation and mold. Poorly sealed windows and doors can also let in noise and dust, negatively affecting air quality in the home.
The second issue with including windows, doors, and other forms of fenestration is that simply including these products decreases the efficiency of the insulation in a home. By putting in a window of a door and removing part of the insulation in the wall of a home, the effective R-value of that wall is reduced. This can cause a reduced ability to resist the flow of heat within a home. To avoid this it is vital to ensure that quality insulation is used and installed properly.