Because of the relationship between temperature, moisture and RH, ventilation of a house in the winter tends to dry it out. When you bring cold outside winter air into a house and warm it up, the RH of that air drops significantly. For example, air at 30 degrees Fahrenheit and 50 percent RH when warmed to 70 degrees will be at 10 percent RH. To get the RH of this air back up to something respectable, we would need to add moisture. The more ventilation that is occurring, the more this dry air is drying out your customer’s house, and the more moisture she needs to add. The solution to this part of winter drying is to reduce ventilation. Ventilation of a house is measured in air changes per hour (ACH). As an example, a house that is 1,800 square feet with 8-foot ceilings has a volume of 14,400 cubic feet (1,800 x 8 = 14,400). Changing all the air in this house with fresh air once an hour would be one ACH. Current building codes and standards recommend home ventilation rates near 1/3 ACH. Not all states enforce these codes or standards. Average homes have ventilation rates near 1 to 2 ACH, while some old, leaky homes are near 7 to 10 ACH. Weatherization and home energy audits typically measure ventilation rates. These programs can also pinpoint leakage sites and direct sealing efforts to reduce excessive ventilation rates. Old windows are often major leakage sites, as are recessed lights and other holes in ceilings and floors.
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As I mentioned above, bringing in 30-degree air at 50 percent RH, then warming it to 70 degrees causes its RH to drop to 10 percent. To raise the RH, we need to add moisture. The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) publishes charts showing moisture and air relationships. Using these charts with our example house from above, we need to add about 3.3 pints of water per hour to raise the inside RH to 30 percent. If the ventilation rate is higher, we need to add more water. If it’s colder or warmer outside, the amount of water needed changes. This same house, if located in Duluth, Minn., would need almost 5 pints per hour during common winter conditions. If we want the RH to be even higher, we need to add more moisture. The colder outside air requires more moisture. Higher ventilation rates require more moisture, and higher target indoor RH levels require more moisture. Since the ventilation rate and moisture needed are related, an economical approach is to reduce ventilation rates, then add moisture. Moisture is added to indoor environments from normal household activities and use. When this moisture is not sufficient to meet the needs, a humidifier can be added. A family of four contributes about ¾ pint of moisture per hour. This number is likely smaller than that shown, because people aren’t home all day and don’t clean every day. So I would suggest ignoring household sources when determining moisture needs. Adding moisture then boils down to using humidifiers. Humidifiers can either be stand-alone or attached to a central forced air furnace. Typical residential systems can provide up to about 6 pints per hour. This is an important number: 6 pints per hour, maximum. More than 6 pints per hour are necessary to get to 40 percent RH when it is real cold outside in a relatively tight, 1,800-square-foot house. We can’t even get to 30 percent RH in a somewhat leaky house when it’s moderately cold outside, or in a larger, moderately tight house. (By moderately cold, we mean the kind of weather in South Carolina. By real cold, we mean the type of weather in Minnesota or New Hampshire.) To make matters worse, moisture output from some humidifiers depends on furnace air temperature. According to Aprilaire, a large manufacturer of whole-house humidifiers, their humidifiers produce a maximum of about 3.6 pints per hour when connected to a heat pump. With that number, we can’t even get to 30 percent RH in a moderately tight, moderately sized house in a moderate climate