Located underground, a crawl space is naturally cooler than outside during the summer. What will happen when hot, humid summer air is vented in? Crawl spaces often house many pipes and utilities that need to be hot.
This includes your furnace or HVAC system, your water heater, air ducts, and hot water pipes. When cold winter air enters a crawl space , it will cool these utilities, ducts, and pipes, forcing them to work harder than ever just to do their daily work.
This means they're using more energy, and costing you more on your monthly bills. Snow trapped in the wells around crawl space vents , as well as puddles formed by melting snow, can also lead to water leaking through. While it is possible to seal the crawl space vents during the winter, there are other times that crawl space ventilation simply makes no sense at all.
For example, venting a crawl space with air during rainy weather is not likely to keep the space dry. And what will happen during foggy, damp, or humid weather? Sometimes the solution is worse than the problem. And sometimes the solution is not a solution and is worse than the problem it's failing to solve! During the summertime, the air outside is hot! Let's set our temperature for a hypothetical summer day:. Hot air is able to hold more moisture than dry air.
A portion of this outside air now enters the crawl space through the vents. Despite the hot summer weather, the crawl space is located underground and remains cool throughout the year. Remember that hot air can hold a lot of moisture? As this hot air is cooled in the crawl space, its ability to hold moisture diminishes.
For every one degree that it's cooled, its RH level rises by 2. If you drop the temperature by 20F, then the RH rises by 20 x 2. But wait -- that's impossible! On an especially humid day, with all that condensation everywhere, the crawl space looks like it's raining!
Moisture will soak everything -- including wood, fiberglass insulation, and any other porous materials. Nonporous materials will be dripping with water. What are those numbers again? Given the right conditions, mold will grow within 48 hours, growing on wood, cardboard, and any other organic surfaces in the space. Over time, it will cause severe damage to the space -- including structural issues! Mold Damage: While fiberglass itself does not support mold growth, there are many resins and dyes within the material that do fiberglass in not naturally pink!
Additionally, fiberglass that's been installed in a crawl space often has a paper backing -- which easily supports mold growth. Vented crawl spaces are often provided with operable vents that can be closed to reduce winter heat losses, but also potentially increase radon infiltration.
Although not their original purpose, the vents can also be closed in summer to keep out moist exterior air that can have a dew point above the crawl space temperature. This approach, however, requires a high level of informed occupant participation to be successful. Unvented conditioned crawl spaces are generally preferred in most cases, except where flood risks are exceptionally high, as in coastal zones subject to hurricane flooding.
The principal disadvantages of a vented crawl space over an unvented one are that 1 pipes and ducts must be sealed and insulated against heat loss cooling loss in the summer and freezing, 2 a larger area the crawl space ceiling typically is larger than the area of the crawl space walls usually must be insulated, which may increase the cost, 3 under hot humid conditions warm humid air circulated into the cool crawl space can cause excessive moisture levels in structural wood components especially floor joists that can cause mold and decay, and 4 an airtight, continuous thermal envelope at the crawl space ceiling is very difficult to achieve in practice.
It is not necessary to vent a crawl space for moisture control if it is open to an adjacent basement, and venting is clearly incompatible with crawl spaces used as conditioned air distribution plenums. They are a go-to for most households because they are cheaper than full basements but more functional than slab foundations.
These spaces offer a convenient location for plumbing, electrical lines and ductwork for heating and cooling systems. However, they also pose a major problem, and for several decades they have been built and maintained incorrectly. Building codes and conventional wisdom have insisted that crawl spaces be ventilated with outside air to control moisture and improve damage within the space. However, wall-vented crawl spaces can actually be the cause of moisture buildup and other complications in households, especially in the Southeast.
Many unpleasant symptoms are associated with wall-vented crawl spaces, and although they are most often noticed in the humid spring or summer, they can occur any time of year:. With high costs and time-consuming repairs to resolve the moisture concerns and other difficulties, homeowners, property managers, tenants and the construction industry became more aware of the importance of finding ways to stop these symptoms from the start.
This awareness led to studies demonstrating that closed crawl spaces, insulated spaces without vents to the outside, can significantly improve moisture control and provide energy savings. Research from Advanced Energy has shown that three main properties of air contribute to the failures of wall-vented crawl spaces: temperature, relative humidity and dew point temperature.
Temperature is the measure of heat in the air. Relative humidity is the ratio of water vapor in the air to the maximum amount of water vapor the air can hold, and there is a limit to how much water vapor can be held before it condenses into condensation or rain. Dew point temperature is a direct indication of how much water vapor is in the air.
Putting it together, dew point measurement states the temperature at which the water vapor in the air will condense into liquid water. Instead, the outside air ends up contributing water vapor to the crawl space. To state it simply, using outside air to ventilate a crawl space only adds more moisture to the area — it does not dry it out.
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