Homeowners with problematic basements often post questions on GBA. They generally complain that their basements are damp, and that basement moisture problems aren’t easy to fix.
Solutions to damp basement problems are only indirectly connected to the topic of green building. In recent years, however, green builders — that is, builders who focus on energy efficiency — have become experts on moisture problems in buildings, in part because insulation problems are intimately tied to moisture problems.
In short, green builders are more likely to build a dry basement than a run-of-the-mill builders — and in this way, building a dry basement has become a green building concern.
One elegant and simple way to reduce the chance of moisture problems in basements is to install a dimple mat on the exterior side of the basement walls.
What is a dimple mat?
A dimple mat is a sheet of semi-rigid plastic (usually high-density polyethylene) that comes in a roll. The plastic has an egg-carton shape, with protruding dimples that make the membrane fairly thick — generally between 1/4 inch and 1/2 inch thick.
When a dimple mat is placed against a concrete wall, the dimples make the plastic stand out from the wall, leaving a free-draining air space between the plastic and the concrete. If any water reaches this air space, it easily flows to the bottom of the dimple mat.
It’s part of a system
While a dimple mat helps convey water to the bottom of a basement wall, it’s not enough to keep a basement dry. The dimple mat is intended to be part of a drainage system that includes a footing drain (see Image #2, below). Ideally, the footing drain system includes perforated 4-inch piping surrounded by crushed stone and a filter-fabric “burrito.” The footing drain needs to lead to daylight or a…