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Question DetailsAsked on 11/29/2015

I keep getting mold in my basement walls

My husband and I purchase a house 6 months ago, it was build in 1915, we live in New Jersey, we are having a problem with our basement, basically every once in a while we get some mold in the concrete walls, close to the floor, we extended the downspouts to redirect the water far away from the house,, we have a French drain and a sump pump. The weird thing is that I have never see water coming into the basement, sometimes I bring the humidity and temperature meter to the basement, and humidity is really low, so we have no idea what is going on, have anyone have had this problem

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I would guess that you have not put the meter right at the base of the wall, tight against it. The air humidity may be relatively low (at say 60-70 degrees), but the foundation and slab are likely more like 40-50 degrees, so the humidity at the surface of the foundation is much higher (humidity percentage rises as temperature cools, until at 100% humidity it condenses - like on a bathroom mirror).

So, sounds like at times (commonly in summer when outdoor humidity is higher) and especially if air conditioned as that drops the air temp so raises humidity, your basement walls reach the humidity needed to sustain mold growth - commonly 50-60% relative humidity.

Bear in mind that the concrete is probably wet outside, at the french drain. When that was put in (assuming you meant an outdoors one rather than under-slab), the wall should also have been coated with bitumastic waterproofing. If not, then you have wet concrete at the backfill, and concrete wicks and transmits water vapoir very well, so it is probably damp to the inside surface, where that moisture then evaporates into the air - and if the wall is cold, immediately condenses some of it on the concrete surface.

If you are never seeing dampness (darkening) of the concrete, then you are probably just barely getting there. You can probably solve it just by putting in a dehumidifier to reduce the humidity, and make sure there is a cover over the sump pump pit if in the baasement.

For bare concrete walls, painting them with a vapor retarding concrete paint with mold inhibitor additive added can also go a long ways to retarding this moisture flow to the warmer basement.

Another more positive but commonly more expensive solution is sometimes to increase the basemetn air temp (which lowers humidity) and/or to increase air circulation with the rest of the house (presuming it is relatively low humidity) with a "Robin Hood" type fan circulating household air into the basement, provided it is not cold. Or circulating the return air duct into the basement en route to the HVAC air handler can commonly solve a minor humidity issue like this.

You do NOT want to bring in humid outside summer air - that will ADD moisture to the basement and aggraavate the problem. Low humidity winter air can reduce the problem, but wastes energy - in that case you are better off mixing the return air through the basement, as long as that does not introduce enough air upstairs to cause window condensation.

Here is a link to another similar question with response - more links to other related questions below that, and so on below each of them -

Answered 4 years ago by LCD

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