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Home Insulation: Preventing Moisture Issues

With today’s focus on energy efficiency, proper home insulation is a priority for contractors and homeowners alike. However, moisture infiltration in a home can spell disaster. Moisture infiltration drastically reduces the effectiveness of insulation. Even worse, it can lead to mold growth that renders a home uninhabitable if left unchecked. Moisture can also rot out trusses, studs and joists, weakening and damaging the structure of a home. 

3 Elements that Affect Insulation Performance

To keep its structure, contents and occupants optimally protected from the elements, every home needs three types of barriers: an air barrier (air sealing), a thermal barrier (insulation), and a moisture barrier or vapor barrier (which can be supplied by various materials, including exterior paint). 

These barriers are three distinct concepts, but it’s possible for one material to fulfill multiple functions. For example, when cellulose insulation is blown in as dense pack, it functions both as a thermal barrier and an air barrier. By the same token, housewraps and other similar products function as both an air barrier and a moisture barrier. 

Ideally, all three barriers should cover the exterior of the building without any gaps. Keep the following things in mind about how air, moisture and heat interact because they all contribute to the performance of your insulation: 

     • Moisture can enter a home in two ways—directly or carried as water vapor through the air.

     • There is no such thing as a perfectly sealed home. There will always be some air migrating into and out of the structure through cracks in the cladding or through the cladding itself.

     • As the air drops in temperature, the relative humidity goes up until you reach the dew point, in which water vapor turns liquid and you get condensation. 

How to Avoid Insulation Moisture Problems 

Understanding the principles above will help you proactively avoid the leading causes of moisture damage to insulation and the surrounding structure:

1.    Direct water penetration. Driving rain, ice dams and leaky roofs can allow liquid water to enter the insulation cavity. Unlike airborne moisture infiltration, it is often possible to completely prevent liquid water penetration. Inspecting the home’s exterior and sealing any leaks or penetrations should do the trick. 

2.    Lack of an air barrier. The biggest moisture problems occur when you have insulation without an air barrier. Air that is allowed to move freely through insulation will readily condense as it moves from a cooler temperature zone to a warmer one. This can happen in both heating and cooling situations; however, the most extreme condensation happens in very cold weather—when there is a large temperature differential—and warm, moist air exiting the home cools down and condenses inside the wall. Air sealing the home will go a long way toward reducing or even eliminating this problem. In extremely cold climates, it is also often preferable to go with non-air-permeable insulation (such as polyisocyanurate board insulation) installed on the exterior framing rather than insulating the cavity. 

3.    Lack of a moisture-escape mechanism. Some moisture will inevitably reach your insulation layer. In order to prevent damage to insulation and the building structure, it is critical to manage that moisture and do two things: 

     • Slow it down to reduce or eliminate condensation; and 

     • Create a path for moisture to leave the structure via evaporation (i.e. drying). 

The No. 1 rule here is to never place a vapor barrier on both sides of your insulation. You want to allow any moisture that does get into the insulation to escape. 

Also, it’s generally a bad idea to use a completely waterproof moisture barrier such as sheet plastic. You should really think of the vapor barrier more as a moisture retarder—a material that will slow down the infiltration of moisture where you don’t want it, but still allow any moisture that does get into the insulation cavity to escape. 

If you find that moisture infiltration is causing issues in your client’s home, check to see if any of the above conditions exist. Fixing them could be as simple as sealing the exterior penetrations, or as complex as a complete mold remediation project. However, it’s very important to take the steps to do it right in order to avoid moisture problems in the future, which could have even more costly results. 


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