Drying Out a House in New Orleans, and Why Opening a Window Fails

Close the house, run the air conditioning and a dehumidifier, and get the material dry inside 24 to 48 hours. Do not open the windows. The EPA says material dried within 24 to 48 hours will in most cases not grow mold, and to keep indoor humidity below 60 percent. NOAA's normals for this station put mean relative humidity at 76 percent for the year and 88 percent at 6am, with a July mean dew point of 75.2F, so in summer the outdoor air brings water in rather than taking it out.

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The 24 to 48 hour clock, and when it starts

The EPA states that wet or damp building materials and furnishings should be cleaned and dried within 24 to 48 hours to prevent mold growth, and that if they are dried within 24 to 48 hours after a leak or spill happens, in most cases mold will not grow. The CDC says the same thing about flooding: clean up and dry out your home fully and quickly, within 24 to 48 hours. The important part is when the clock starts. It starts when the water arrives, not when you find it, which is why a slow supply leak under a cabinet is already a different job by the time anybody smells it. Anything still wet after two days is not the same job dried on day one, and the difference is measured in what has to come out of the building rather than in hours of labor.

There is no hour of a summer day when a window gets you under 60 percent

This is the single most useful local correction on the site, and it contradicts the most common piece of advice given here after a leak. The EPA's target is to keep indoor relative humidity between 30 and 60 percent. NOAA's normals for the New Orleans airport station put mean relative humidity at 76 percent for the year, 88 percent at 6am and 63 percent at noon, with a July mean dew point of 75.2F. Air carrying a 75.2F dew point is already saturated at 75F, which is roughly what a cooled room is held at. So bringing that air into a wet building in summer imports water rather than removing it, and it is worst in the morning, when instinct says to open everything up. Letting it air out is not a cheap version of drying here. It is the opposite of drying. Reaching the EPA number in this city is mechanical dehumidification, which is a purchase decision rather than a habit.

Sixty percent, fifty percent, and which number to work to

Two federal agencies give two numbers and they are not in conflict, they are a target and a ceiling. The EPA advises reducing indoor humidity to between 30 and 60 percent, by venting bathrooms and dryers outside, using air conditioners and dehumidifiers, increasing ventilation and running exhaust fans when cooking, dishwashing and cleaning. The CDC is tighter: keep humidity as low as you can, no higher than 50 percent, all day long. In a July with a 75F outdoor dew point, holding 50 percent indoors usually takes a dehumidifier working alongside the air conditioning rather than the air conditioning alone. Work to 50 and treat 60 as the point at which something is wrong. A hygrometer that costs less than a takeout dinner will tell you which of those you are living in, and it is the cheapest useful instrument in this whole field.

The mornings when everything outdoors is already wet

The station normals record 16.8 days a year with heavy fog, defined as visibility of a quarter mile or less. Heavy fog means the outdoor air has reached its own dew point, so on those mornings every surface in contact with outdoor air is wet whether or not anything leaked: the underside of a raised floor, an uninsulated slab edge, ductwork in a vented space. If you are mid-way through drying a house and you open it up on a morning like that, you have undone the previous day. The monthly dew point table shows how much of the year this applies to: 68.0F in May, 73.5F in June, 75.2F in July, 75.1F in August, 71.5F in September, 63.0F in October. Seven months at or above 60.8F. Passive drying is a winter option here, if it is an option at all.

What comes out of the building and what stays in it

The EPA states that absorbent or porous materials such as ceiling tiles and carpet may have to be thrown away if they become moldy. It also advises that in areas where there is a perpetual moisture problem, carpeting should not be installed, and specifically advises against carpet on concrete floors subject to leaks or frequent condensation, which names the standard slab-on-grade floor in half this metro. That second point matters after a flood, because the instinct is to put back what was there. If the slab has sweated or taken water before, carpet over it is a recurring problem rather than a finish. Hard materials, non-porous surfaces and framing can usually be dried and kept if they are reached inside the window. Soft materials are a decision, and making it early is what keeps the job small.

Spraying is not drying, and it is not remediation either

The EPA states that the use of a chemical or biocide that kills organisms such as mold, chlorine bleach for example, is not recommended as a routine practice during mold cleanup. It also states that there is no practical way to remove every mold spore from an indoor space, and that the way to control indoor mold growth is to control moisture. Read together, those two lines settle a lot of arguments. Anything sold as treating the air rather than finding and stopping the water is selling the wrong product, and a surface that is sprayed and left wet will grow again. Dry it, then decide what stays.

Under a raised house, and inside a slab

Where the water goes next depends on which kind of house you have, and this metro has both on the same streets. On a raised pier and beam house the space underneath is the problem and the drying problem: the Building America guidance says the vents can encourage condensation by letting warm humid air in to condense on joists that are cooler when the rooms above are air conditioned, so opening everything up under there in August is the same mistake as opening the windows. On a slab the plumbing is cast into the concrete and the water wicks up through it into the bottom plate and the drywall, showing as a base-of-wall stain a foot from the actual break. Neither can be diagnosed from the mold you can see, which is why drying and assessing are two separate purchases rather than one.

Rain that beats the drainage, and what that means for timing

It is worth knowing how much warning you get. The Sewerage and Water Board of New Orleans states that once the system fills it can move about half an inch of water an hour, and that rainfall exceeding an inch in an hour will most likely outpace the system's capability. The same utility reports 120 pumps in 24 drainage pump stations, more than 68,000 catch basins, 1,500 miles of lateral pipe and 200 miles of canals, and that fully running the system takes 52 megawatts of power. Rainwater does not leave this city by gravity, it is lifted out by machinery that needs electricity, which is why a long outage plus heavy rain is the flooding scenario here rather than rainfall volume alone. Against a normal annual precipitation of 63.35 inches and 73.4 thunderstorm days a year, the half-inch-an-hour figure is exceeded routinely. Plan to be able to start drying without power on the street.

When you can do all of this yourself

Often, and it is worth saying plainly. If the wet area is small and you reach it inside the window, extraction, lifting soft material, closing the house and running a dehumidifier is a homeowner job, and the EPA's own line is that below about 10 square feet of visible growth a homeowner can in most cases handle the cleanup too. Louisiana expressly exempts a residential property owner doing this work on their own property from licensure, so nothing stands in your way. Where it stops being yours is when the water has been there longer than two days, when it came from a sewer line, when it is under a floor or inside a slab you cannot see, or when the area is large. At that point the useful purchase is a moisture assessment with the readings written down, before anyone prices removal, and Louisiana requires that to come from a different firm than the one doing the removal on jobs above $7,500. We arrange them separately at every price. If your address has flooded before, ask whether it is on the federal multiple loss list: there are 24,518 such properties across these six parishes, and it is an answerable question.