The debate about interior vs. exterior waterproofing has been going on for a long time. If you’re tired of evaluating arguments from both sides of this issue, that’s not surprising. Let’s see if we can get some clarity here. 

We can start by acknowledging that there are advantages and limitations to both waterproofing strategies. We’re all aiming for the same goal: a dry basement. How you get there in your house depends on a number of factors – not just the condition of your basement and its surroundings, but also on your budget.

Exterior waterproofing: Best done during new construction

You’ve heard the argument before: “The best way to avoid water inside the basement is to keep it outside the foundation.” In theory, this makes a lot of sense. That’s why a number of companies have developed a wide range of exterior waterproofing products: plastic membranes, special paints and coatings, and drainage systems, for example.

All of these products are designed to be applied or installed when the basement foundation is fully exposed, dry, and free of dirt and debris –in other words, during new construction. The window of opportunity for installing an exterior waterproofing system is narrow: after the forms have been stripped from a poured concrete foundation (or after a concrete block foundation has been built), and before backfilling covers the foundation walls with soil.

Exterior waterproofing systems also include the installation of drain lines located at the base of the foundation wall or alongside the footing. This system of plastic pipe is supposed to carry excess ground water away from the foundation, to a drainage location lower on the property. Unfortunately, these exterior drain lines eventually get clogged with silt or plant roots over time, losing their effectiveness as part of the waterproofing system.


“Damp-proofing” is not waterproofing

The black asphalt coating you see close to grade level on many house foundation walls is not a waterproofing treatment. This thin coating is technically known as a “damp-proofing” treatment, and it is required by most building codes. Damp proofing is supposed to limit the penetration of soil moisture into foundation walls, to reduce the amount of moisture that is absorbed by the concrete foundation and released into the basement in the form of dampness or even water seepage. In contrast, a foundation waterproofing treatment is supposed to stop soil moisture penetration as well as ground water penetration. If your basement walls feel damp or show signs of water seepage, you know that these exterior barriers aren’t doing their job.


Interior waterproofing: The best retrofit solution

If your house has a wet basement, it doesn’t matter what exterior damp-proofing or exterior waterproofing system was installed in the past. The water problem simply indicates that the exterior waterproofing methods aren’t working. 


Excavating around the foundation to install a new exterior waterproofing system isn’t a viable option for most homeowners. For starters, it’s going to be very expensive. It will also involve bringing heavy equipment onto your property and digging up some of your favorite bushes and plantings and possibly removing your porch, deck, or patio. Because of the major expense and major disruption to your property, retrofitting an exterior waterproofing system is rarely a good solution to basement water problems. 


In contrast, an interior waterproofing system is a very attainable solution for most homeowners. To install an interior waterproofing system, the contractor will create a shallow trench at the perimeter of your basement floor. This trench may extend around the complete perimeter of the basement, or just in the areas where water is leaking into the basement. Drainage lines are installed in the trench; this arrangement is sometimes referred to as an interior French drain. The drain lines are connected to a sump pit in the basement floor where a sump pump is located. When the water level in the sump pit reaches a predetermined level, the pump comes on automatically to expel the water outside. 


One of the major differences between interior and exterior waterproofing has to do with the hydrostatic (water) pressure exerted against foundation walls. Water weighs about 60lbs. per cubic foot. During wet weather, the pressure of water against the base of a foundation wall can approach 600lbs. per square foot. Exterior waterproofing wall solutions are supposed to resist this pressure, but that’s often an impossible task. With that amount of pressure, water will find its way through seams in plastic membranes and through even the smallest leaks in concrete or protective coatings.  


The drain lines in an interior waterproofing system give water an escape route, effectively reducing hydrostatic pressure against the foundation. By lowering the hydrostatic pressure, there’s less chance of water being forced through leakage points that will admit water under pressure. 


What about DIY basement waterproofing?

Obviously, homeowners can’t easily excavate around a foundation to install an exterior waterproofing system. Nor is it practical for a DIYer to install an interior French drain and sump pump system. But there are some improvements homeowners can make to help keep basements dry. One improvement is to upgrade gutters and downspouts, so that roof runoff is directed well away from the foundation. Also, you’ll want to make sure that this roof drainage system is kept free of debris to avoid clogs that can cause gutters to overflow. 


For the same reason exterior wall protectants fail, so do interior ones that don’t address draining the water away via a gravity drain or sump pump. Filling cracks in the basement floor and walls using hydraulic cement, “waterproof” paint, or a filling material formulated specifically for this purpose is a short-lived solution if water remains against the walls and hydrostatic pressure is allowed to build. 

So what is the best way to waterproof a basement?

An exterior waterproofing system is certainly worth considering if you are building a new house. Sealing, protecting and draining the outside of the foundation is faster, easier, and more affordable when your basement walls are newly built and completely exposed.


When an existing house has a wet basement, it makes sense to complete some of the DIY waterproofing measures mentioned above. These improvements can be done quickly and affordably. If you still get water in your basement, contact an experienced basement waterproofing contractor to have an interior waterproofing system installed.