Home & Garden Columns

About the House: The Trouble with Damp Basements

By Matt Cantor
Friday June 01, 2007

Some things are always a bad idea. Karaoke with your boss, bell bottoms on chain driven motorcycles, long-haired thoracic surgeons or pesto-flavored ice-cream. 

Another thing that is nearly always a bad idea is converting a basement into somebody’s bedroom. 

I’ve seem this a lot and there is a special red flag (animated with flashing stripes) that goes up in my head whenever I walk down stairs in a house and see a “staged” bed over new carpeting and can just make out the flue from the furnace sticking out from behind the lacy curtain. As they say, “What could go wrong?” 

Long ago, when I was young, basements were places where people drank and shot pool. They were places where a boy could take apart the family TV and be threatened with a spinal transplant by a red-faced father (clearly this was way before Child Protective Services). They were, however, not living spaces. They tended in many parts of the country to be damp or even wet. They often housed a sump (a well recessed into the slab) and a pump for management of subsurface waters. 

You see, basements violate nature’s laws when it comes to grade (ground level), erosion patterns and the design of the watershed. If you’re lucky enough to live in a fairly dry part of the country, this is far less of an issue, but most places suffer as we do, with water that insists on filling up spaces that we dig out of the earth. They used to call them wells. Big ones might be called quarries. Whatever you call them, they’ve always tended to get wet. 

Regardless of this truth, in our modern age of common senselessness, we’ve forgotten all the basic stuff. While it was well understood a hundred years ago that a cellar would be damp and that action would have to be taken to keep one dry, we have forgotten these things and end up carpeting concrete floors 8’ below ground level. 

Basements ARE excellent spaces for crafts and storage and occasional short-term belching and cursing but they do not work well as bedrooms. 

One thing that is not commonly understood is that typical concrete is quite porous to water and much like a wick will transmit substantial volumes of water into a basement, even across a slab or wall of 6” or more. As concretes go upward in strength they also become less porous and so, when designing a basement, this is one of many strategies that can be employed in producing a dryer environment. 

It’s common to see a white crystalline precipitate (powder) on the surface of concrete floors and walls in basement as evidence of this slow water movement. We call this efflorescence. As water moves through the concrete it carries evaporative salts such as calcium chloride to the dryer side and as the water evaporates from the exposed side of the concrete, it leaves these salts behind. These are largely benign but do illustrate a problem with drainage.  

If the concrete is kept relatively dry, or if there is a low-pressure path to allow the water to move easily along the other side of the concrete (the soil side), the water would not make the more difficult journey through the concrete, at least not much of it. 

The pressure of water on the back side of the concrete wall or floor is known as hydrostatic pressure and the higher it is, the more it wants to push through the concrete and dampen the rug and grow the mold colony in the new bonus room. 

On retaining walls, we usually create holes through the walls to relieve this pressure. Without these “weep” holes, large walls can be slowly toppled. Water is amazing stuff. It always wins unless you let it through. There is no opponent more dangerous than the one that is patient and moves very slowly. 

In basements, it is possible to arrest some of the infiltration of water in liquid or vapor form by the use of sealants. The problem is that they cannot resist high levels of hydrostatic pressure. If there’s a lot of push to the water on the other side of the wall, the surface of the concrete including the sealant can “spall” or exfoliate in thin chips releasing the moisture and damaging the surface. 

If there is just a little weeping going on and the concrete quality is good, a sealant can be quite effective. There are two common sealants that have been in use for quite a long time, UGL Drylok and Thoroseal. Thoroseal is a cementitious sealant with an acrylic component and seals over the concrete. 

It comes in a range of colors and can be painted when installed. Drylok is a clear latex sealant that impregnates the concrete surface. In my experience, Thoroseal is the better choice for weeping concrete although Drylok is a nice choice for maintaining the appearance of brick or other masonry (being a clear sealer). 

Better than either of these is a line of products by Aquafin™. Included in these are epoxy sealants that can solve major problems where escaping water vapor has made living spaces uninhabitable and also cementitious sealants that can seal rough, highly porous concrete. 

I don’t want to create any illusions here. Damp basements can be restored to relatively dry conditions with these methods as well as ventilation, heat, dehumidifiers and drainage systems. But when pushing water comes to shoving humidity, nature often wants these basements to stay damp. 

The key to using this data properly is not in the utter abandonment of the basement but in reasonable expectations and appropriate use of space. To those of you who are now carpeting and painting that basement in preparation for sale, keep in mind that the next owner will assume that these finishes guarantee dry, cozy space. 

If nothing else, take some time to write down what you know and what you don’t know about the basement (“it’s seemed mostly dry these last four years but it wasn’t carpeted or painted”). It could mean the difference between a nasty phone call next January and a clear conscience combined with reduced liability. 

If you’re a buyer, look twice and three time at that basement and don’t start planning the office layout just yet. Give yourself a winter to assess the real utility of the space and be prepared to take some special measures (or to use it as a …. basement). 


Got a question about home repairs and inspections? Send them to Matt Cantor at mgcantor@pacbell.net.