Should I worry about mold in my basement?

[jpluncford21]

We have an unfinished, cinderblock basement w/ a concrete floor. It's a wet basement when it rains heavily for some reason has had open vents to the outside. These vents are basically 20"x13" slots covered with mesh to prevent any animals from getting in. The problem was that this obviously lets in the outside temp and makes the basement either hot or cold, which in turn affects the temp of my house.
About a month ago I plugged these holes with some insulation and styrofoam to try and raise the temp. Now I'm worried about mold taking advantage of the raised temp and the wet conditions. Is that something I need to be worried about? My thinking was that since the temp fluctuates with the warm and cold seasons, if mold was an issue it would have already been one. Thoughts? 

[Welmoed]

You need to decide whether the basement will be conditioned or unconditioned space. To make it conditioned space, there needs to be a vapor barrier and insulation against the cinderblock foundation, the vents need to be completely sealed, and there needs to be a way to connect the basement to the house's ventilation system to heat, cool and dehumidify the space. To make it unconditioned space, you need to put a vapor barrier up against the subfloor between the first floor joists, then insulate the floor. Leave the vents open.
If you're having issues with a wet basement, 90% of the time the real problem is the gutters. Make sure the downspouts are extended at LEAST six feet away from the foundation, otherwise the rainwater will simply seep into the soil and get into the basement. The next time you have a heavy rain, walk around outside and take a careful look at where the water is coming from. Are there gullies or low spots? Are the vents level with the grade, allowing water to pour in? Is the yard graded towards the house?
If you are having real water intrusion issues, you need to contact a drainage specialist, NOT a basement waterproofer. A waterproofer will just deal with the water when it's already in the house; your goal is to prevent it from getting into the house in the first place, and a drainage specialist will do that.
Mold growth requires a humidity level of 40% or above. Properly ventilating the basement will help keep the humidity down, and insulating the main floor will prevent moisture intrusion into the house and heat loss into the basement.
--Welmoed Sisson
(Maryland Home Inspector)

[paddedhat]

. Properly ventilating the basement will help keep the humidity down, and insulating the main floor will prevent moisture intrusion into the house and heat loss into the basement.
--Welmoed Sisson
(Maryland Home Inspector)

 WOW, just wow.

 I built new homes for a living and work in a textbook area when it comes to difficult moisture, and mold issues in basements and crawl spaces. I generally build sealed, conditioned crawl spaces.  Just a few things that I have learned using everything from building inspector input, published research, personal trial and error, and observing countless sucesses and failures in local post war construction.

First and foremost, ventilating a basement in many (most) climates WILL NOT keep humidity down. In many mixed climates it will greatly increase humidity and do serious damage to the structure and the health of the occupants. It's basic common sense. The exterior air being exchanged needs to be dry <50% M.C. driven by wind pressure at an adequate rate of exchange, and of a suitable temperature that it neither heats or cools the interior space at undesirable levels. Fact is that this magical set of conditions doesn't happen too often. Ventitation in cold conditions can be both an expensive waste, as the heating system works to compensate for the loss, and a disaster if pipes freeze. In hot, humid summer conditions, full ventilation leads to serious moisture and mold issues as the wet, hot air condenses on cool interior walls and causes the interior moisture level to spike.

Second, using the homes HVAC system to control basement or crawl space conditioned can lead to serious health issues. If conditions deteriorate in the below grade area, ( ie, moisture levels climb, and mold growth proliferates) and it isn't noticed or controlled by the occupants, the HVAC system will distribute mold throughout the home. I HAVE had it happen.

Third, insulation DOES NOT control moisture, it isn't designed to do so, and it shouldn't. If you are refering to the practice of installing kraft faced fiberglass with the "vapor barrier" toward the warm side (up in this case) in the false belief that it is somehow preventing vapor transfer, well good luck with that. In a basement where moisture issues are properly addressed, floor insulation can be beneficial. In a wet basement or crawlspace, floor insulation, particularly fiberglass batting, can be an absolute disaster. IF the O.P were to follow your advice to install a vapor barrier, then insulation, under his floor joists and maintain the space as vented, he will end up with a mess. I have seen a case when the floor joists were rotted, full of carpenter ants and the floor was collapsing into the crawl space because it was fully vented with the vapor barrier and insulation installed per your recommendations. Given the hot humid conditions in TN, I would seal the space tightly, get a dehumidfier cranking 24/7 and not worry about insulating, or installing a vapor barrier anywhere. Naturally the underlying issues of grading, gutters, etc need to be addressed, asap.  As for the "wet when rains" issue, you may be in a situation where it will be impractical, or too expensive, to really eliminate the issue entirely. Especially if the issue is a water table that rises temporarily. Thoroseal brand masonary waterproofing done with their acrylic additive can give some amazing results when used on the interior of the block wall. Good luck

[strider3700]

I'm really not a fan of treating the inside of the masonry for water proofing although if it's a very rare event I'd consider it.  Waterproofing from the outside is usually not insanely expensive  if you're willing to do the awful, dirty, possibly dangerous digging all the way around the house.   While you're at it  insulate the foundation if you're in a cold enough area to be worthwhile. 

As for venting the basement I'd never do it  but it's also wet and wetter here. perhaps there are places where climate would cause it to make sense.

In general though blocking up venting without taking into consideration why it was there and dealing with that issue is usually a bad idea.

[paddedhat]

I'm really not a fan of treating the inside of the masonry for water proofing although if it's a very rare event I'd consider it.  Waterproofing from the outside is usually not insanely expensive 

Full exterior excavation of an existing home is generally not done for waterprooofing, as it can be extremely expensive  and disruptive. Destroying everything from existing yards, planting, landscaping and decks, can be unavoidable. In many cases decks, porches and garages prevent the job from being done at all.  Masonary based interior waterproofing coatings can be a low cost and extremely effective solution, if all other recommended measure of exterior water diversion fail. Thoroseal is regularly used to maintain and waterproof water storage tanks and dams.

In general though blocking up venting without taking into consideration why it was there and dealing with that issue is usually a bad idea.

I have found that typically foundations are vented simply because " that's the way it done in these parts". There is a 40 year history here of a few locals building, and retrofitting, sealed conditioned crawl spaces using thin concrete floor slabs and sprayed urethane insulation on the interior face of the foundation wall. It works exceptionally well. That said, there are still a few morons (builders mostly) that create brand new moldy disasters, since they are too lazy, or dishonest to do the right thing.

[Snow White]

I work as a registered nurse and I'd be leery of living in a house with mold, even in the basement.  There are to many known health affects to risk it.  I'd do whatever is necessary to not have a moldy basement!