The crawl space is one of the house's biggest risk areas for moisture damage. It is an exposed part of the house's construction where moisture-related stresses come from several different directions.
When problems arise, it is important to think carefully and act quickly.
Moisture problems in the crawl space
Having a completely dry crawl space is virtually impossible. However, it is important to keep moisture levels low enough to prevent damage or mold growth. The main problem with excessive moisture levels is that the moisture will eventually damage the materials in the house's construction. A critical moisture condition begins when the relative humidity (Rh) exceeds 75%.
Factors affecting crawl space moisture levels:
- Airtightness in the crawl space
- Air circulation in the crawl space
- Ground temperature & outdoor humidity
- Air and surface temperature in the crawl space
Airtightness in the crawl space
Poor airtightness in the crawl space is a common cause of moisture problems. Sometimes, incorrect airtightness occurs already during the foundation's construction or during a renovation. In other cases, the seal has worn out and needs to be replaced.
If you experience moisture problems, you should immediately inspect the foundation's airtightness. A common problem is that moisture has managed to migrate up through the foundation wall, behind the plastic film. Check that the plastic film's seal is correct and that the plastic film does not have holes or cracks.
Air circulation in the crawl space
Air circulation in the crawl space is affected by the extent of air leakage in the foundation wall, against the ground, and in the floor structure itself. The ventilation must be dimensioned for the size of the crawl space, and the openings placed in such a way that the air does not become stagnant inside the crawl space.
Here, you should also keep in mind that too much air supply can also cause problems, just as too little air supply can.
Ground Temperature & outdoor humidity
Since the crawl space is usually significantly colder than outdoors, condensation easily occurs. This is especially true during the warmer half of the year. Because warm air carries more water than cold air, condensation will occur when the warm outdoor air enters the colder crawl space. The warm air will then release its water molecules onto the cooler area, and condensation will form.
When this happens, it also creates a perfect breeding ground for dry rot and mold to take hold in your house.
Air and surface temperature in the crawl space
The crawl space vents are designed to ventilate dry air into the crawl space. The problem is that outdoor air is humid approximately half the time of the year. Depending on where you live in the country, you can expect that from May to October, the air introduced into your crawl space will be more humid than the air already present there.
In other words, there is a greater risk of moisture during the warmer months of the year than during winter.
Preventing moisture in the crawl space
Good sealing of the foundation and proper ventilation that circulates the air are the basic prerequisites for a healthy crawl space. Unfortunately, this is not enough in a country like Sweden, where we have large temperature differences throughout the year.
One of the first things you should do is measure the humidity in your crawl space. Use both a hygrometer and a moisture meter.
You use the hygrometer to get an idea of the relative humidity (Rh) in the space. Let the measurement take place over a few days, preferably under varying weather conditions. Note the values for the placement and then move the hygrometer to a new location and repeat the procedure. This may seem like a time-consuming and cumbersome procedure, but it can give you valuable information about how air and moisture interact in your particular crawl space. The relative humidity should be below 75% Rh in the crawl space.
You use the moisture meter to test the material in the crawl space. Check the floor structure, frames, and other organic material. Most materials should have a moisture content below 16%.
Install a dehumidifier in the crawl space
Having a dehumidifier in the crawl space is a good way to keep moisture and its associated problems at bay. There are primarily two different dehumidification techniques suitable for crawl spaces;
- Desiccant dehumidifiers
- Thermal dehumidifier
Desiccant dehumidifiers
Using a powerful fan, the desiccant dehumidifier draws in warm, humid air. Once inside the dehumidifier, the humid air is driven through a rotor, coated with moisture absorbers, which draw in moisture. The moisture then vaporizes as warm air is driven through the rotor and is subsequently discharged from the space via a hose.
If you want to get the most out of your desiccant dehumidifier, you should first check the sealing in the crawl space. This type of dehumidifier provides the best results when the space is properly sealed.
A major advantage of desiccant dehumidifiers is that they work well even in cold weather. The disadvantage is that in some cases they can require a rather cumbersome installation in the crawl space. Older models can also be relatively expensive to run.
If you have suffered water damage in the crawl space, a desiccant dehumidifier is exactly what you need. The technology is both powerful and effective for large water and moisture damages.
Thermal dehumidifier
Uses natural thermal movement to remove moisture. By gradually adding warm air to the crawl space, a thermal movement is induced that causes the humid air to expand. This expansion, in turn, forces the humid air out through the ventilation.
Since the thermal dehumidifier keeps the crawl space heated, it also prevents the formation of new moisture.
The advantage is that the technology has relatively low energy costs and works well even during the winter months. However, it is important that you have ventilation that works with the dehumidifier.
The disadvantage is that thermal dehumidifiers require professional installation, which must be performed correctly. An incorrectly installed thermal dehumidifier can, in some cases, catch fire. The installation can also be extensive, with some models requiring heating coils to be installed in the crawl space to be effective.
Crawl space - comfort & complications
For a long time, the crawl space was by far the most common foundation in Sweden for new single-family houses. The crawl space builds on the 19th-century cottage foundation, but they have some essential differences that also explain the moisture problems specifically in crawl spaces.
The old cottage foundation's construction usually worked well, as the foundation was kept warm by having the fireplace running all year round. The heated chimney then pushed moisture out of the house.
During the 1960s, when what we know today as crawl spaces began to be built, natural stones were no longer used, but more modern materials. During this time, the floor structure itself also began to be insulated. This, of course, led to greater comfort, but it is somewhere around here that the complications with moisture problems really began to accelerate. The insulation of the floor structure also made the foundation under the floor colder, and thus more susceptible to moisture.
During the 1970s, even more insulation continued to be added to save energy. Often with different plastic materials, which led to an even greater increase in problems with moisture and mold. By then, people had also completely stopped burning wood to heat their homes. Electric heaters replaced fires, and the chimney, which had previously kept the foundation warm, was now more often cold than warm.
Actually, it is perhaps not the design of the crawl space that is the problem, but rather the fluctuating weather throughout the seasons here in Sweden. We go from warm summer days to icy winter nights. We need houses that can withstand the harsh cold, while during the summer we live in a climate where we do not need to heat our homes.
There is an imbalance between cold and warm air, which in turn can create significant moisture problems in our crawl spaces. However, it is possible to keep the crawl space dry enough to avoid dry rot and mold.
By reviewing your crawl space and measuring moisture levels, you can get a good handle on the moisture in your crawl space. A good dehumidifier, adapted to your needs, can then help you keep the foundation safe and healthy.
