Restoration Guide: Dampproofing and Waterproofing Foundations

Roger Diez

Editor's Note: This is article 5 of 9 in Chapter 1: Foundations of the Old House Web Restoration Guide. This guide was developed and edited for old homes from original materials in the U.S. Housing and Urban Development (HUD) Rehab Guide.


Section 1--Overview

Basement dampness is a problem. Not only does it render the space unusable, but it can cause health problems as well. Mold, mildew, dust mites, and bacterial toxins thrive in dampness. Moisture can enter a basement in a number of ways, including leakage of ground water through cracks in walls or slab floors, dampness penetrating through concrete, and moist air from outside.

There are two main ways to combat dampness in a basement: dampproofing and waterproofing. Dampproofing protects the interior of foundation walls from water vapor penetration or moisture coming through walls via capillary action. Waterproofing is necessary when there is ground water present above the basement slab, and involves sealing tiny cracks.

Building codes in most areas require dampproofing or waterproofing, particularly since basements are now more frequently used as living space. When considering a home renovation project that includes a basement, remember that waterproofing is considerably more costly than dampproofing.

Section 2--Dampproofing

Dampproofing generally involves applying some kind of coating or sealant to the interior or exterior of the foundation wall. Here are some of the most common materials for dampproofing.

2.1: Crystallization Products

There are a number of commercial coatings designed to dampproof the interior side of basement walls. These are relatively inexpensive and can protect the interior basement wall surface from water wicking. However, they are often not effective in the long term, and may require covering with more attractive material such as paneling if the basement is to be used as living space.

2.2: Cementitious Coatings

This technique, known as "parging" is used primarily on concrete block walls. These coating should be applied to the exterior wall of the foundation. It is effective where there is good soil drainage, but has no crack-spanning ability.

2.3: Asphalt Coatings without Modifiers

These coatings can be applied hot or cold and come in a variety of formulations. They can be troweled, brushed, sprayed, or rolled onto the exterior foundation wall. Cold-applied coatings can be used on slightly damp walls. These coatings are economical and easy to use, but do not span cracks and require excavation.

2.4: Acrylic or other Approved Polymer Sealers

These spray-on sealers are easy to apply and can be used on the exterior foundation wall or surface of the basement slab. They can also be used as dampproofing on interior foundation walls. They qualify as waterproofing when used in conjunction with rubberized asphalt sheet on exterior walls. Excavation is required.

2.5. Polyethylene Sheet

This barrier is applied below the floor slab. When installed according to code (minimum 6-mil thickness, minimum 6 inch overlap, sealed with double-sided asphaltic tape), this provides an impervious barrier to moisture migration from below the slab. It can also be used under a new slab poured over an old one.

Section 3--Waterproofing

For stronger protection against moisture, waterproofing is recommended. Unlike dampproofing, waterproofing materials are nearly always applied to the foundation's exterior wall and sometimes to the basement slab. Here's a look at six common waterprooofing materials.

3.1. Asphaltic-Based Product

These products are similar to those used for dampproofing, but are combined with membranes of polyester, fiberglass, or fabric. They are an older technology, less environmentally friendly, and more labor intensive than newer products.

3.2: Rubberized Asphalt Coating

These coating products have recently come into use in residential applications. Because of their history in commercial building, they often carry material and performance guarantees. They have good crack-spanning performance, but are more expensive than alternative methods.

3.3: Asphalt-Modified Urethane Coating

These are also commercial products becoming more common in residential use. They use asphalt as filler in a urethane base, thereby reducing cost while maintaining performance. They can be sprayed or brushed on.

3.4: Urethane Coating System

Urethane coatings are solvent-based and typically come in black. They are great for covering irregular areas or protrusions, but the cost can be higher than other materials, and the coating should be applied by a trained professional.

3.5: Rubber-Based Coating

These coatings are formulated from neoprene, butyl, and hypalon. Rubber-based coating are best for horizontal surfaces, and they are also available in sheet form in areas where there is significant pressure from ground water. These materials have extremely good performance, but they are expensive. Rubber-based coatings are typically used only when serious foundation water problems exist.

3.6: Clay-Based Waterproofing System

Natural clay, also known as bentonite, is available in panels, sheets, and in combination with other products based on urethane, rubber, and asphalt. Clay is an excellent waterproofing material, because it swells 10-15 percent when exposed to water and becomes impervious. It is environmentally friendly, but care must be taken to provide room for expansion so it does not crack or raise concrete.

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