What Does Insulation Have to Do with Moisture Problems? Adding insulation can either cause or cure a moisture problem. When you insulate a wall, you change the temperature inside the wall. That can mean that a surface inside the wall, such as the sheathing underneath your siding, will be much colder in the winter than it was before you insulated. This cold surface could become a place where water vapor traveling through the wall condenses and leads to trouble. The same thing can happen within your attic or under your house. On the other hand, the new temperature profile could prevent condensation and help keep your walls or attic drier than they would have been.

So how do you know what to do? Your home's moisture performance will depend on the type and position of the insulation, whether you install a vapor retarder, and where the vapor retarder is located. We used to think that the best insulation approach only depended on your weather. But now we know that it is more complicated than that. Moisture problems and their solutions depend not only on your climate, but on the type of house construction, the amount of moisture you produce inside the house, the way you ventilate your house, and the temperature conditions you maintain inside the house.

Why does the climate change the way you should use insulation? Remember that diffusion usually carries moisture from a warmer space to a colder space, and that moisture will condense to a liquid, or even solid, form if it contacts a cold surface. The location of the cold surface, and the location of the higher moisture concentration both vary with climate and season. If the outside air is colder than the inside of a home, then moisture from inside the warm house will try to diffuse through the walls and ceiling toward the cold, dry outside air. If the outside air is hot and humid, then moisture from outside will try to diffuse through the walls toward the dry, air-conditioned inside air. In both of these cases, what's important is the difference between the inside and outside climates. So next-door neighbors could install the same insulation and vapor retarder but get very different results, depending on what temperatures they maintain inside their homes and how much moisture their lifestyles generate.

How does house construction impact moisture problems? Different materials will hold and transport moisture differently. For example, a brick surface will allow more moisture to pass than does aluminum siding, but the brick is also capable of storing moisture. And the house design will make a difference too. For example, attics, basements, and crawl spaces can be vented, or can be sealed and act as a part of your conditioned space. Insulation can be placed inside a wall, or on the inner or outer surface of the wall. These configurations obviously require different approaches if you want to avoid moisture problems.

  • So How Can You Avoid Moisture Problems? Here are six things you should consider:

  • You need to stop all rain-water paths into your home by making sure your roof is in good condition and by caulking around all your windows and doors. If you are planning a new house, choose wider overhangs to keep the rain away from your walls and windows. You can also keep rainwater away from your basement walls or crawl space by making sure that all water coming off your roof is directed away from your house and by sloping the soil around your house so that water flows away from your house. Be sure that dripping condensate from your air-conditioner is properly drained away from your house. You can place thick plastic sheets on the floor of your crawl space to keep any moisture in the ground from getting into the crawl space air, and then into your house. These actions can also help reduce capillary water flows from the ground into your walls. You should also be careful that watering systems for your lawn or flower beds do not spray water on the side of your house or saturate the ground near the house.

  • You need to ventilate your home to remove the moisture that results from human activities within your home, such as breathing, bathing, cooking, etc. You especially need to vent your kitchen and bathrooms. You may be able to see mold that grows around your bathtub, but you will not be able to see mold growing inside the walls or in the attic. To see a close-up view of mold see Figure 61. So, be sure that these vents go directly outside, and not to your attic, where the moisture can cause problems. Remember that a vent does not work unless you turn it on; so if you have a vent you are not using because it is too noisy, replace it with a quieter model. Some of the better vents are available with timers or moisture sensors so that you can be sure that the vents run long enough to remove all the excess moisture from that room. (While you are in the bathroom, it is also a good idea to check the caulking around your tub or shower to make sure that water is not leaking into your walls or floors when you bathe.)

  • When you think about venting to remove moisture, you should also think about where the replacement air will come from, and how it will get into your house. Many older homes have relied on leaky construction, drawing in their fresh air around window and door frames, building corners, wall-foundation joints, wall-roof joints, chimneys, etc. Air coming into your home through these pathways often travels through places you can not see, such as wall cavities, the space between floors, the crawl space, or attic. If this outside air contains moisture, you will effectively be pumping moisture into these unseen places. As we seal up our homes to save energy, we need to replace these uncontrolled air pathways with energy efficient pathways. Air-to-air heat exchangers can keep the indoor air at a healthy moisture level without increasing your energy costs. In humid regions, attic ventilation may also be a moisture source because you may be pulling air into your attic that has more moisture in it than the air in your home.

  • It is very important to seal up all air-leakage paths between your living spaces and other parts of your building structure. Measurements have shown that air leaking into walls and attics carries significant amounts of moisture.

  • Plan a moisture escape path. Some moisture will always be present in your home. You can help this moisture escape with well-planned ventilation or by careful selection of your building materials. Typical attic ventilation arrangements are one example of a planned escape path for moisture that has traveled from your home's interior into the attic space. You can also use a dehumidifier to reduce moisture levels in your home, but it will increase your energy use and you must be sure to keep it clean to avoid mold growth. If you use a humidifier for comfort during the winter months, be sure that there are no closed-off rooms where the humidity level is too high.

  • You can use vapor retarders to reduce moisture diffusion through your walls, floors, and ceilings. This is relatively easy to do when building a new house, but there are a few things that you can do for existing houses as well. The kind of vapor retarder you should use, and where you put it, depends on whether moisture is more likely to be moving into or out of your house. If moisture moves both ways for significant parts of the year, you may want to avoid the use of a vapor retarder completely.

What is a Vapor Retarder? Vapor retarders are special materials including treated papers, paints, plastic sheets, and metallic foils that reduce the passage of water vapor. Tests are made to measure how much water vapor can travel through each material, and the results are called permeance, or perms. The lower the perm, the better the vapor retarder.

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Figure 61: A Magnified View Of Mold Spores

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