There are several reasons to consider bricking up basement windows. Old basement windows may have leaks that allow cold air and water into your basement. Windows that allow water in can be a danger to your home's foundation as well as any furniture or stored items in the basement. If you plan to change your exterior landscaping and place plants or build up the soil around a basement window, you should brick it up before proceeding. Finally, bricking up a basement window can be a cosmetic decision that allows you to remove old windows that hurt your home's appearance.
The process for bricking up a basement window begins with removing the old window. First remove the window pane, then pry out the wooden, vinyl or aluminum window frame. For painted windows that were installed before 1978, lead paint may be an issue. Check your state's environmental protection code to ensure that you can perform the work yourself; some states require that licensed contractors perform most work on older houses that might have lead paint. Once the window is out, you can install bricks using lime mortar that you mix yourself just before applying it. Cut the bricks to size, and lay them out in the order you plan to install them so that you can work quickly before the mortar dries.
While you can brick up most small basement windows without major consequences, larger egress windows are a different story. Egress windows in a basement allow people to escape in the event of a fire or other disaster. Local building codes may require your basement to have egress windows if it contains a bedroom or other living space. Bricking up an egress window may violate building and fire codes, which can subject you to a citation or an order to reinstall the window at your own cost.
If insulation is your primary reason for bricking up a basement window, you may first want to try sealing the window with caulk or foam insulation. This may be enough to keep water and cold air out without the cost and effort of bricking up the window. Bricking up a basement window also eliminates a source of natural light for the basement, making it impossible to grow indoor plants and increasing your reliance on electric lighting.