With the Baltimore area’s humid summers and unpredictable winters, good weatherization is essential for reliable comfort. Because insulation plays a major role in your home’s weatherization, knowing some basic facts about insulation can help you stay more comfortable and lower your energy bills year round.
1. Insulation Slows Heat Transfer, Not Airflow
One of the most fundamental facts about insulation is what this material actually does. Because insulation is often added to parts of the home where air leakage can occur, it’s logical to assume insulation does something to block these air leaks. In reality, insulation does next to nothing to stop unwanted airflow.
It’s not just a matter of choosing an insulation that blocks more airflow than most. Insulation is designed to slow heat transfer, not to block air. It helps your home act like a Thermos, keeping hot things hot and cool things cool. In winter, when you’re running the furnace, insulation reduces the rate at which heat radiates out through the walls and other surfaces. In summer, when you have the air conditioner on, insulation keeps out the sun’s heat.
Due to some common misunderstandings, certain types of insulation have gained an undeserved reputation for stopping air leaks. One of these is spray foam insulation. Some homeowners apply this insulation to cavities in the attic, walls and floors assuming this foam will stop any air seeping in. While spray foam reduces heat loss and gain as it was designed to do, air can flow through this type of insulation just like any other.
Stopping air leakage is important for your comfort, indoor air quality and your home’s energy efficiency, but there are better ways to do it. What actually stops air from blowing into your home is gypsum board, which stops nearly 80 percent of the air, as well as siding or similar material, which stops another 12 percent. The remaining air leakage should be blocked by applying caulk and weatherstripping.
2. Proper Air Sealing Is Necessary Before Adding Insulation
Also among the important facts about insulation are the preparation methods for installation. Air sealing is the most critical of these. Air leakage can occur in many different parts of your home, but when you’re planning on upgrading your insulation, you’ll want to focus on the parts of the house where insulation will be added.
- Walls – In an existing house, air sealing inside the walls usually isn’t practical, but you can still take care of leaks around them. Use an all-purpose caulk, such as acrylic latex, to seal along the baseboards and crown molding. To seal electrical outlets and light switches, install foam insulating gaskets designed for these areas. Outdoors, seal the gap between the siding and foundation, corner joints of your siding, and any other gaps in the exterior. Use a caulk designed for outdoor surfaces, such as butyl or polyurethane.
- Attic – Because they contain so many potential leaks, attics should be thoroughly sealed before insulation is added. Run a bead of caulk under and behind knee walls and around the cavities inside dropped soffits. Add flashing around the furnace flue and seal the space between the flue and the flashing with silicone caulk rated for high temperatures. Install weatherstripping around the attic hatch. If you have older recessed lights that aren’t IC-rated, consider replacing them with IC-rated models.
- Basement – Seal under the sill plate, which is between the sill plate and the foundation, with caulk or sill seal. The rim joists are an exception to normal weatherization guidelines in that they should be insulated before they’re air sealed. This means cutting rigid foam to fit against the rim joist in the spaces between the floor joists. The edges of these foam sections should then be sealed with caulk.
3. Vapor Barriers Aren’t Always Necessary
A vapor barrier, more appropriately referred to as a vapor diffusion retarder, is a layer of material designed to control the amount of water vapor that seeps into an enclosed area such as the attic or crawl space. They’re commonly made of polyethylene (plastic) sheeting or rubber, but can be made of a wide variety of material depending on the local climate and the type of insulation you plan to use.
One of the lesser known facts about insulation is that these barriers aren’t always necessary. A high-permeability vapor diffusion retarder is usually added during construction in humid climates or ones that see extreme temperatures. Homes in a mild and relatively dry climate are often built without these layers because typical building materials, such as painted drywall, are enough to control moisture in these climates.
In existing homes, adding vapor diffusion retarders is hard to do without tearing open the surfaces where the material is to be added. The good news is that vapor diffusion retarders are most important in extremely cold climates, and the Baltimore area is mild enough that our homes are usually just fine without them. This is particularly true if you choose cellulose insulation, which manages moisture efficiently by itself.
4. R-Value Reflects Insulation Efficiency
One of the most useful facts about insulation to know is how its heat resistance capability is measured. Every type of insulation has an R-value that tells you how efficient the material is at slowing the transfer of heat, also known as thermal resistance.
Although thicker layers of insulation tend to have higher R-values, the value isn’t a measurement of thickness. It’s possible for a thicker layer of one material to have a lower R-value than a thinner layer of a different material, just as a thin layer of down insulates better than a thick layer of cotton.
The U.S. Department of Energy’s guidelines for how much insulation each part of your home needs are based on R-value. For example, per these guidelines, a Baltimore home should have a layer of attic insulation that’s between R-38 to R-60. Exactly how much insulation that is depends on the type used. If you use fiberglass batts, you’ll need a layer around 12 inches thick. If you use rigid foam, you’ll need only a 7- or 8-inch layer.
Don’t be mislead into thinking two insulation products with the same R-value are essentially the same thing or that the material with the higher R-value is always better. Different insulation products have different properties that make them better suited to some situations than others.
For example, cellulose is eco-friendly, and blown-in cellulose can be installed in walls without the need to tear out the whole wall. On the other hand, it’s not ideal for high-humidity areas such as the crawl space. Rigid foam is a better choice for this area.
5. The Advantages of Cellulose
Knowing some basic facts about insulation materials can help you choose the best type of insulation for each part of your home. Many insulation materials are available, including fiberglass, cellulose, mineral (rock) wool, foam, cotton and wool. Cellulose, in particular, is a good choice for many parts of the home, but common misconceptions about this material discourage some homeowners from choosing it.
Cellulose insulation is made primarily from recycled newspapers broken down and turned into fibers. It offers a slightly higher R-value per inch than fiberglass, so you’ll need less material for the same insulating power.
Loose-fill (blown-in) cellulose consists of small chucks of these fibers that are blown into place with a special machine. This form fills in small crevices better than batts and is easier to install in existing walls and floors. It doesn’t settle or shift inside the walls because it’s packed too tightly to move.
Although it originates from paper, cellulose isn’t a fire hazard because it’s treated with the fire-resistant mineral borate. Although it’s prone to rot if it gets wet, this is true of any insulating material. If you’re not sure which insulation is right for each area of your home, a heating and cooling technician can help you decide.
For more useful facts about insulation, check out Griffith Energy Services, Inc’s insulation assessment and installation services or call 888-474-3391.
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