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Nearly every foundation wall is rolled or sprayed up to the grade line with a thin layer of an asphalt-based coating called dampproofing. Some people mistakenly think this is a waterproofing layer, but it is not designed to stop liquid water and will not bridge small cracks where water can enter.
For what it’s worth, the building code requires at least dampproofing on all foundations and waterproofing only on foundations exposed to hydrostatic pressure. When the foundation drainage system is working well and its capacity is not exceeded, all is fine. But if the system malfunctions or you are hit with an inch of rain in one hour, all bets are off, and you may wish you had paid extra for a fully waterproofed foundation.
The thin layer of asphalt applied to most foundations is called “dampproofing”. The purpose of dampproofing is to slow down the transmission of water vapor through the concrete walls into the house. If you backfill with a well-drained, granular material next to the foundation wall, or a drainage board that provides a capillary break, then dampproofing will do a reasonably good job of keeping water vapor from entering. However it will not stop liquid water under pressure and it will not bridge the small shrinkage cracks that develop in concrete walls and slabs. In short, dampproofing helps keep basements a little drier, but does not prevent basement leakage or flooding.
If you plan to use the below-ground construction as living space, or want to provide that option in the future, then you should consider upgrading from dampproofing to a full basement waterproofing system. There are many systems, but most start with a heavy spray-on elastomeric coating that can bridge cracks and keep out water under all but the most extreme conditions.
To work effectively, basement waterproofing coat must seal the joint between the foundation wall and the footing, as this is a common entry point for water. This requires attention to detail, such as cleaning the dirt out of this joint before applying the waterproof coating.
Most f0undation waterproofing systems include a layer of “protective board” over the elastomeric coating to protect it from damage during backfilling and settling of the soil. The protective board can a dimpled mat or a foam or fibrous insulation board.
Some elastomeric coatings claim that they do not need a protective layer, and some dimpled mats are marketed as complete waterproofing system without a membrane, but I am skeptical. One test I apply is “If it sounds too good to be true, it probably is.”
Drainage Boards & Mats
There are a host of products designed to provide a capillary break and vertical drainage against the foundation wall. Most are designed to eliminate the need for granular backfill around the foundation. Some are used in conjunction with a spray-on waterproofing membrane, while are sold as standalone waterproofing without the elastomeric (synthetic rubber) coating.
To add to the mix, some products include exterior insulation – either foam made with vertical drainage grooves, or fibrous insulation boards that provide both drainage through the open matrix of the fibers.
Exterior foundation products do not have an easy life. Excavation contractors are not always precision craftsmen. The boards or membranes can be damaged or pushed around during backfilling, be dragged down as the soil, attacked by insects (in the case of foam board), and damaged at grade level. So pay attention to the attachment details and overall durability.
With any of these products, it’s important to pay attention to the termination at the top. Some are more secure than others and all run the risk of getting damaged here or buried during landscaping. With exterior foundation insulation that continues up to the wood frame, as it should, the termination is not a concern. However, protecting the exposed foam above grade can be problematic.
Some of the more popular drainage boards and sheet products include:
Dimpled membranes: These are plastic sheets with the dimples facing into the wall, creating about a ½ inch gap for drainage and a capillary break. Widely available products include Platon Air-Gap Waterproofing Membrane, Delta MS, and GeoMat. It’s important to select a product with adequate compressive strength to resist being crushed by the backfill.
Dimpled membranes with filter fabric: On these, the dimples face outward and are laminated to a layer of geotextile to protect against clogging. Products include: Platon DoubleDrain, Delta-Drain , and CCW Miradrai.
Matrix panels: These form the air gap by using a tangle of thick plastic fibers. The best known is Enkadrain, which is available with filter fabric on one or both sides and uses 40% to 50% post recycled material. This product has been around for decades, which is always a good sign.
Insulating drainage panels: These products provide both exterior insulation and drainage. Foamular’s Insul-Drain is XPS foam with vertical grooves for drainage with a bonded filter fabric. Warm-N-Dri and Drain & Dry are both semi-rigid fiberglass panels that insulate and drain, although Warm-N-Dri is no longer available as a standalone product (available only as part of the Tuff-N-Dri waterproofing system). The new kid on the block is Roxul Drainboard. This high-density mineral wool insulation board has good drainage characteristics and excellent compressive strength. Like fiberglass, mineral wool is impervious to water, insects, fire and pretty much anything else you can throw at it.
Some materials do double-duty as both a drainage layer and exterior basement insulation. Two I am familiar with are Insul-Drain Drainage Board (XPS foam with drainage grooves and filter fabric) and Roxul Drainboard (rigid rockwool panels). Rigid rockwool is currently marketed as a commercial product and is not available in all areas. But it has some advantages over foam – it drains water well and is impervious to water and insects. Foam below grade can get excavated by insects for a nice, warm home.
Drainage Mats. As mentioned above, a dimpled or matrix-type drainage mat can go over the waterproofing to create a drainage space and capillary break, which means that it is less important to use granular backfill. Some companies market these products as the primary waterproofing material. This can work with a well-functioning footing drain system. However, if the footing drains silt up over time or cannot drain fast enough, and water backs up around the foundation, the drainage mat may not be enough.