Basement Egress: How to Avoid Moisture Issues

One of the biggest obstacles to properly installing a basement egress window is the potential for the accumulation of moisture either around the window or within the egress well itself. Proper construction of the egress window and well is key to avoiding this eventuality. The likelihood that the addition of an egress well will suddenly cause your otherwise dry basement to flood is low, but it is nonetheless important that you take the steps to ensure proper drainage. With the proper slope of the trench, the use of consecutive layers of gravel, treated wood and flashing around the window, the most common moisture problems can largely be avoided. 

Slope of Trench

When excavating the trench for the installation of the egress well, it is important that the bottom of the trench slope away from the foundation. Soil, especially soil with a high clay content, holds water very well. If the trench is flat or worse yet, sloped unknowingly towards the foundation, it will encourage the gathering of water up against it. After time, this can be very bad for the home’s support structure. By sloping the trench away from the home, you provide a means for water to run away as well. 

Layers of Gravel

On top of the trench you dig for the well should go two consecutive layers of gravel, 3 inches deep a piece. The bottom layer should be ¾-inch gravel while a 3-inch layer of pea gravel is laid atop the first. You may choose to dig all the way down to the bottom edge of the foundation and fill the entire hole with gravel, or you may stop at the downward sloping trench approximately 1 foot below the bottom edge of the window. Either way, layers of gravel ensure the maximum amount of drainage. Important too is that you lay down a sheet of landscape fabric below all gravel to prevent the soil below from clogging up the gravel drain. 

Treated Wood

The wood used to frame in the window should be pressure-treated to withstand moisture damage. The wood will be capped on the outside, but it is important that it not be allowed to rot after time. Basements are typically more moist than other areas of the house. This fact combined with the new exposure to the outside will eventually damage the frame around the window. 

Window Flashing, Drip Cap and Vapor Barrier

On the sides of the outside edge of the egress window, you need to install some kind of metal or plastic flashing strip which blocks moisture from entering the basement between the window frame and the wall. Flashing may also fit across the head or top of the window frame, but you may also find that using a drip cap along the head increases moisture protection. Those two items together with an internal vapor barrier installed with the frame and window should prevent moisture from seeping in unnoticed. 

Avoiding moisture issues when installing a basement egress window and well is a matter of proper planning and utilization of the available materials. By not cutting corners and taking the time to install the right components, moisture accumulation should never be a problem.