Messages and Online Forum Q&A

Re: Foundation waterproofing and insulation

From: info@builderswebsource.com
Category: Foundations and Basements
Remote Name: 61.8.254.14
Date: 08 Feb 2001
Time: 10:48 PM

Comments

Basement waterproofing is an exacting science. If done incorrectly, you'll have nothing but trouble down the road.

While the method your contractor proposes may work, our experience shows there are better methods.

First, CMUs are notoriously porous. So the first thing is to be sure that they are fully filled with reinforcing steel and grout. You can even add a waterproofing agent to the grout called anti-hydro that seals the molecules of the concrete. Do not leave any air pockets inside the CMUs. Vibration, tamping and proper slump are important to ensuring a tight waterseal.

Next, you must thoroughly prep and seal the external surface. Excellent results have been attained with products such as Bituthene, from the Grace company. This rubberized membrane, when applied over basement foundations, creates an impermeable surface. It must be applied properly, however.

Next, EPS foam or special drainage channels should be placed against the Bituthene to ensure that any water due to hydrostatic pressure can easily flow down the vertical surface to the sub drain. The foam or panels provide added protection to the waterproofing membrane when you backfill. In fact, improper backfilling is a large cause of wet basements, since the waterproofing membrane can easily get damaged with rocks during the process. If you live in a cold climate, using the foam as you descibe is a good idea as it provides added insulation. It may even be required by code. Some foams are available with drainage channels exactly for this application.

Be sure that the foundation is keyed, which helps to block water from entering into the inside of the basement near the top of the footing. Sometimes special waterproofing shields are used between the footing and the stem wall to provide additional security against water infiltration.

Finally, be sure that the perimeter drainage depth (known as the invert)is lower than the basement floor with the holes pointing DOWN...this way any water will tend to go into the drainpipe, which should be the path of least resistance. If the soil is sandy or silty, you'll need a filter fabric to avoid clogged pipes.

Attention to detail is key to ensure a clean, safe and dry basement.

For more information on Bituthene, see:

http://www.gcp-grace.com/products/waterproof/applications/fwbs.html

Understand that there are often multiple ways to solve a problem and that your contractor may have had good luck with his method. If I were building this house for myself, I would spare no expense to ensure that the foundation and basement are done right...and that full consideration has been given to a good drainage system, including contingency scenarios such as "what happens if a pump fails." Since the basement is underground, if you have to fix it later, the cost is enormous and it will disrupt the entire exterior of your home.

Let us know which method you ultimately use. Good luck!

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