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Wood flooring is a natural product that responds to humidity variations by expanding and contracting. Hardwood is kiln dried & milled with 6%-9% of moisture content (MC) and is suitable for indoor applications with relative humidity (RH) in the air 30%-50%.
Moisture and humidity inside the house and in the crawl space varies in every house, it can even change from one part of the house to another.
Here are some factors:
House location and size, height of the crawl space, type of soil, drainage, insulation, sun exposure, etc. For example, two houses in the same neighborhood can have completely different environments for hardwood flooring installation.
Flooring installer's responsibilities:
The homeowner should keep these conditions consistent in the house and crawl space to avoid dimensional changes and moisture related issues with hardwood flooring.
Over the course of a year, the temperature typically varies from 30°F to 89°F and is rarely below 18°F or above 96°F.
Daily High and Low Temperature
The daily average low (blue) and high (red) temperature with percentile bands (inner band from 25th to 75th percentile, outer band from 10th to 90th percentile).
The warm season lasts from May 26 to September 16 with an average daily high temperature above 81°F. The hottest day of the year is July 17, with an average high of 89°F and low of 70°F. The cold season lasts from December 1 to February 25 with an average daily high temperature below 57°F. The coldest day of the year is January 19, with an average low of 30°F and high of 49°F. (Source: www.weatherspark.com)
The relative humidity typically ranges from 37% (comfortable) to 95% (very humid) over the course of the year, rarely dropping below 21% (dry) and reaching as high as 100% (very humid). The air is driest around April 11, at which time the relative humidity drops below 45% (comfortable) three days out of four; it is most humid around August 18, exceeding 92% (very humid) three days out of four.
The average daily high (blue) and low (brown) relative humidity with percentile bands (inner bands from 25th to 75th percentile, outer bands from 10th to 90th percentile). (Source: www.weatherspark.com)
During the summer, even the most carefully installed wood floors tend to expand and sometimes cup or even buckle due to high moisture content (MC) or high relative humidity (RH) in the air.
To control summer-related expansion of flooring and associated issues, you have to control (usually reduce) the moisture. Cooling indoor air increases it's relative humidity. Combine that with moisture entry through a crawl space or basement foundation walls, or from exposed soil in a crawl space, or from activities in a building that keep the doors/windows open and we have a significant potential for issues with our hardwood floors.
The best solution for real humidity control in many homes is a central, or whole-house, dehumidifier. Good systems put close to 100% of their energy into removing moisture, regardless of mild weather or cool mornings when an AC won't run as much. They will also shut down when an AC can handle the moisture loads, since dehumidifiers turn on and off because of humidity levels. Portable dehumidifiers can also help, but they typically don't remove much moisture. Since they are usually located in one room, humidity control in other rooms or the whole house may not be as good.
NOTE: If your home is vacant during hot summer months, DO NOT turn your A/C system and dehumidifier off. The relative humidity in the home will be increased, resulting in hardwood floor cupping. This is commonly known in the trade as the "greenhouse effect."
The air outside is much drier in the fall and winter. Your hardwood flooring will go through seasonal movements during this time. Wood flooring will give up or lose some of it's moisture and contract as a result of the dry, heated air. Often times, noticeable gaps can appear between planks. This is normal, but can be avoided.
Installing a humidifier in the furnace or placing a moveable humidifier into a room with good air circulation will help to keep your flooring from developing gaps between the planks. The optimal humidity level in your home should be consistent, between 30%-50% year round. Keeping the humidity level stable in this range will help to avoid dimensional change and movement in the hardwood flooring. You can monitor and keep the humidity level in your home consistent year round by installing a simple humidity meter (hygrometer).
NOTE: Early fall months will bring rapid outdoor temperature changes from comfortable upper 70's down to low 40's and back up to 70's again. By rapidly heating the house with warm and dry air, the moisture content in the upper layer of the wood flooring will decrease and dry out the top of the floor. The moisture content in the lower part of the hardwood remains higher. This imbalanced MC in hardwood may cause gaps along with light cupping. To avoid this, it's important to know when to switch from dehumidifier to humidifier.
Crawl space soil needs to be covered 100% with 6 Mil plastic. Crawl spaces and their venting have been a subject for decades. Vent or not to vent?
Soil under a building is often cooler than the interior. So we now have warm, humid summer air and cooler crawl space surfaces. When warm outside summer air enters a crawl space and cools, the RH of that air increases. For example, air at 90°F and 50% RH, when cooled to 70°F, will be at 100% RH. (This air has a dew point of 70°F.)
To get the RH back down to normal, remove moisture by reducing outdoor air entering into a crawl space and using a dehumidifier to remove moisture from the crawl space. Venting very often during summer months makes a crawl space cool, not just humid. It also will make it wet in states like North Carolina. An unvented crawl space with a dehumidifier is a great, efficient way of controlling humidity levels below wood floors that are over a crawl space. (Advanced Energy's study performed in Raleigh, NC found that the wall-vented crawl spaces stayed above 70% relative humidity during the summer, while the closed crawl spaces stayed below 60%).
WARNING: No one should do what I recommend before reading Advanced Energy's recommendations (see below) and then talking to a licensed contractor. There are potentially life threatening consequences to closing off your crawl space (for instance, some homes have gas furnaces in their crawl space, and having any unvented furnace in a closed crawl space can be dangerous).