Slight basement seepage through floor -- what should I try first?

My 55 year old home does not take any water in through the walls during heavy rain, but I do notice small reoccuring wet spots on the carpet in the finished half. I've never seen standing water, only slightly damp carpet. The other half is unfinished and does not appear to be taking water anywhere.

In general, the basement is pretty humid (dehumidifier runs a lot) and seems to be more humid after it rains.

My questions are:

1) If this was a drainage problem, wouldn't I be seeing water in the basement walls first?

2) Would I benefit from the installation of a sump pump?

3) Would ripping up the carpet and sealing the floor help resolve the humidity problem?

 

There is a simple test you can do to determine where the humidity is coming from. In the unfinished part of the basement, take a piece of clear plastic and tape it to the floor. The next day, check the piece of plastic to see if condensation is under it. If there is condensation, then there are two possible causes for it. One is heat and the other is a high humidity level in the cement floor.

If the basement is fairly new, it takes several months for the cement to cure. One of the things that cement does when it is curing is give off heat. Even though several months may have passed and the cement does not feel warm, it will still give off a little heat. In the unfinished portion of the basement, this heat dissipates into the basement causing the high humidity level in the air, because all heat has moisture in it. In the finished portion, it gets trapped under the carpet and condenses. It is just a matter of time before the cement cures to the point where this will no longer occur. Because of the carpet being damp, you are going to have to remove it.

If it is a high humidity level in the cement, you could water-proof the walls and floor. I do not believe a sump pump will resolve your problem. There is a debate on the causes for this high humidity level and the possible solutions. One such organization is known as Indoor Air Quality (IAQ) experts. They base most of their arguement on high pressure areas being attracted to low pressure areas. There has been considerable amount of research concerning this. If this interest you, you can go to the U. S. EPA website and search IAQ.

 

In a 55-year old house there is a good chance that the basement slab is 1" or 1 1/2" of concrete cast directly on dirt. During a heavy rain the water runoff would build up under the slab and seap thru the fine cracks. Such cracks are unavoidable in a thin slab on grade. Even new concrete will have cracks, its a matter of how deep and wide and trying to control them.

 

Thanks for the help guys, let me add a few more details.

I've never found the carpet to be wet to the touch. I've only noticed this because of the recurring COLOR in a few spots. I plan on walking barefoot over all of the carpet next time it rains big. I did the aluminum foil experiment (walls & floor in the unfinished and finished portions) and everything came up dry.

If/when I remove the carpet, is it possible to seal the cracks and use something like Dry-lock on the floor?

I do find the humidity annoying. The basement seems to be naturally warm.

 

You will never be able to seal all of the cracks if your slab has fine cracks forming throughout. I doubt drylock will really do much in this situation. To really solve the problem, a new slab on a stable, well graded subbase is required.

 

Fantastic. Neither of those would be a lot of money.

I guess I'll stick with the dehumidifier.