Moisture in Manufactured Homes
Manufactured homes are a popular home option with approximately 8% of the US population living in them full time. There are more than 8 million manufactured homes across the country, with the most popular regions being Southern and Western United States. Manufactured homes have gained popularity as a more cost effective option than a traditional home build, with the average cost being less than $35 per square foot.
Part of the reason manufactured homes are more cost efficient is that they are constructed in sections on an assembly line. The home is assembled around a steel frame and with an integrated floor system design, including a plastic cover known as a bottom board. Once constructed, homes may be shipped across the country to a variety of temperature and humidity conditions. Furthermore, manufactured homes are often designed to be installed quickly. In addition, the experience of the on site building crew may vary greatly and there are often looming deadlines from the manufacturer, homeowner, and retailer. These circumstances combine to create an environment that is prone to moisture issues.
By Riverview Homes, Inc. (Own work) [CC BY-SA 3.0 (http:// creativecommons.org/licenses/by-sa/3.0)
By Riverview Homes Inc (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)
Because of their unique construction, manufactured homes have their own set of regulations, the Manufactured Home Construction and Safety Standards, which is monitored by HUD. Many of these regulations are designed to prevent moisture problems but HUD is still looking for ways to improve the efficiency of manufactured homes. In 1999, the Florida Solar Energy Center was selected to investigate manufactured homes and how to improve their overall efficiency. As part of this investigation, 25 manufactured homes, constructed by 5 different builders, were examined. During testing, various moisture issues were found in all of the homes and could be traced to 6 main sources.
1. Temperature Inside the Home
One potential source of moisture problems is
the temperature inside the home being below the
outdoor dew point. This was actually found to be an
issue in all 25 of the homes tested. If the air
conditioning is set to a temperature much lower than
the dew point, it can actually lead to condensation
as the outside air meets interior surfaces.
Furthermore, if the HVAC system is oversized for
the home, it can lead to short cycling in which case, the unit will never have enough time to provide dehumidification.
• The obvious solution to this issue would be to maintain the temperature above the dew point so condensation doesn’t occur.
• Alternatively, if you'd rather not depend on your air conditioning for dehumidification, you could install a dehumidifier in your home. This would give you the freedom to keep your home any temperature you prefer, while still maintaining a consistent humidity level.
2. Negative Pressure
Another cause of moisture problems in manufactured homes is when there is negative pressure breaking the building envelope. Again, this was found in 100% of the homes tested. Negative pressure is what brings moisture into the home through leaks, cracks, crevices, and other areas that are not properly sealed. The building envelope includes all portions of the home that protect it from the outside environment, such as the walls and roof. It is designed to maintain a constant pressure within a home, however, certain factors can alter this.
One reason pressure differences might be felt across the building envelope is because of the stack effect. The stack effect explains that warm, moist air isn’t as dense as cold, dry air causing the warm air to always rise to the top of a house. This means the top portion of the home will have higher pressure and there will be lower pressure at the base of the home. In the center of a home is a neutral pressure. The high pressure will then be forced out through any cracks or leaks at the roof, thereby, creating a reverse pressure at the base of the home and drawing air in.
Another reason for negative pressure in a manufactured home could be because of the placement of a return duct. In general, a manufactured home will only have one return duct. This is the spot where air is sucked into the air handler and then distributed to the supply ducts in the various rooms. The problem occurs when air can’t circulate throughout the home and the balance of pressure becomes negative. For example, if the return duct is in a hallway that distributes air to supply ducts in bedrooms but all of the doors are kept shut. This can make it difficult for air to circulate, causing a build up of pressure.
The pressure inside a building envelope can also be easily disrupted by open windows or exhaust fans blowing interior air outside. Just like with the stack effect, excess outgoing air will cause the home to draw in air through any small crack or openings possible. Whether the negative pressure is in a single room or throughout the entire home, it can still lead to moisture issues.
• Refrain from using fans, such as exhaust fans, continuously as it will only aid in building negative pressure.
• When possible, leave doors open to allow air to circulate.
• Make sure supply ducts are not blocked by furniture or other belongings.
3. Air Leakage
All of the tested homes also had issues with leaks in the ducting and return air pathways, subsequently letting humid air into the home. Leaks in the ducting often occur during the construction process and could be a result of rushing during assembly. Duct leaking is an especially an issue if it happens to the ducting in the floor, which would leak into the belly board and eventually outside. This scenario will create a negative pressure inside the home and a positive pressure in the belly board.
Air leakage is also common in ceilings. Ceilings in manufactured homes typically start out airtight, however, once all of the necessary fixtures, outlets, and cables are installed, there is plenty of opportunity for leaks and holes. This will allow the humid air to rise into the attic which can become a haven for mold and condensation.
• While installing items that require a hole in the ceiling, floor, or walls, be sure it is done correctly and properly sealed. This includes items such as outlet plates, cables, and fixtures.
• Make sure ducts are installed properly with no blockages, disconnections, or leaks. Joints in the ducting should be sealed with mastic or tape.
By Riverview Homes Inc (Own work) [CC BY-SA 3.0 (http:// creativecommons.org/licenses/by-sa/3.0)
4. Moisture Diffusion
As mentioned earlier, the belly board and floor assembly can be the site for many moisture issues. In fact, 100% of the tested homes showed moisture diffusion through the floor. If the belly board has any tears or holes, moisture will be able to easily pass into the home, through the subflooring. Tears in the belly board material often occurring during transportation of the home.
• Patch any holes or tears that appear in the belly board using sheathing material and sealant on the interior of the hole, then cover the exterior with scrap belly board material.
5. Vapor Retarder
An additional source of moisture problems is the improper use of vapor barrier. HUD requires that homes have vapor retarder installed (excluding certain Southern states), however, if done improperly it can cause issues. The most economical method of vapor retarder is a vinylcoated wallboard. Unfortunately, the vinyl coating is also prone to moisture issues. In the tested homes, it often seemed to occur when air registers caused the vinyl to be below the dew point leading to condensation.
• Choose a different vapor barrier than vinyl coated wall board
• Avoid having air registers blow directly on the vinyl board.
6. Improper HVAC Equipment
A final source of moisture problems is inadequate moisture control. If the air conditioning system isn’t properly sized or maintained, it will be difficult to control moisture levels. For instance, two of the homes in the test were built by the same manufacturer and located a short distance apart. Both homes were the same size yet they had different sized air conditioners. The home with a 2.5 ton air conditioner had less moisture problems than the home with a 4 ton air conditioner. The oversized air conditioner will simply short cycle and not provide necessary dehumidification for the home. Keep in mind that short cycling of the HVAC can also occur if the home is too airtight.
In addition to having proper equipment, it’s also important to maintain the equipment. For example checking to make sure ducting isn’t disconnected or blocked. Additionally, it’s a good idea to verify that other portions of the HVAC are working properly, including everything from the thermostat to the condensate drain line.
• The easiest solution to this problem is to ensure that your HVAC system is properly sized for your home.
• Check the HVAC on a regular basis for issues.
By Riverview Homes Inc (Own work) [CC BY-SA 3.0 (http://creativecommons.org/ licenses/by-sa/3.0
By Riverview Homes Inc (Own work) [CC BY-SA 3.0 (http:// creativecommons.org/licenses/by-sa/3.0)
In addition to the solutions mentioned above, there are also many ways to help moisture control during the planning, building and installation process. One factor to consider during planning is the placement of supply and return ducts to allow for adequate air circulation. It is also important to specify the locations of electrical, plumbing, and other connections to make it easy for the installers on site. When it comes to the assembly process, some tasks are better done ahead of time instead of on site. This includes utility connections and sealing the marriage line gasket. Both are crucial for moisture control and are easier to install in the factory versus out in the field. Another helpful idea for the factory is to include all necessary set up materials for the installation so workers aren’t scrambling to find the right tools on a deadline.
There are also some tasks that can addressed on site before the home arrives. For instance, make sure the yard is properly graded so water flows away from the home. Furthermore, make sure the foundation piers are installing properly and are level. Once the home arrives on site, keep the rain guard on as long as possible to protect it.
During the building process, always use proper insulation techniques. Having correctly installed insulation with no missing or condensed areas will help avoid cold spots. Pay attention to other details as well, such as wall flashings and window seals. In addition, installing items such as the HVAC system may create holes in the exterior walls or bottom boards. These need to be carefully sealed to prevent future moisture issues.
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