Insulation:  The Basics

Introduction

Electricity bills, oil bills, gas bills--all homeowners pay for one or more of these utilities, and wish they paid less. Often many of us do not really know how to control or reduce our utility bills. We resign ourselves to high bills because we think that is the price we have to pay for a comfortable home. We encourage our children to turn off the lights and appliances, but may not recognize the benefits of insulating the attic.
Why Should You Insulate?

Heating and cooling ("space conditioning") account for 50 to 70% of the energy used in the average American home. About 20% goes for heating water. On the other hand, lighting and appliances and everything else account for only 10 to 30% of the energy used in most residences. It makes good sense to turn lights and appliances off when they are not needed, and you'll save even more on your energy costs if your reduce the amount of energy needed for heating and cooling.

Unless your home was constructed with special attention to energy efficiency, adding insulation will probably reduce your utility bills. Much of the existing housing stock in the United States is not insulated to the best level. Older homes are likely to use more energy than newer homes, leading to very high heating and air-conditioning bills. Even if you are building a new home, choosing upgraded insulation will save enough money in reduced utility bills to pay for itself within only a year or two, continue to save you money for as long as you own the home, and increase the resale value of your house.

The Crucial Role of Thermal Insulation

Inadequate insulation and air leakage are leading causes of energy waste in most homes. Insulation saves money and our nation's limited energy resources. It can also make your house more comfortable by helping to maintain a uniform temperature throughout the house. Walls, ceilings, and floors will be warmer in the winter and cooler in the summer. Insulation can also act as a sound absorber or barrier, keeping noise levels down.

It is possible to add insulation to almost any house.  To get the best possible result, it is important to choose a licensed, certified contractor who will install your investment properly.

The amount of energy you conserve will depend on several factors: your local climate; the size, shape, and construction of your house; the living habits of your family; the type and efficiency of the heating and cooling systems; and the fuel you use. Once the energy savings have paid for the installation cost, energy conserved is money saved--and the annual savings will increase if utility rates go up.

Insulation Priorities

It is most important to:

• Insulate your attic to the recommended level, including the attic door, or hatch cover.
  

• Provide the recommended level of insulation under floors above unheated spaces, around walls in a heated basement or unventilated crawl space, and on the edges of slabs-on-grade.
  

• Use the recommended levels of insulation for exterior walls for new house construction. When remodeling or re-siding your house, consider using the levels recommended for new construction in your existing walls.
  

How Does Insulation Work for You?

Heat flows naturally from a warmer to a cooler space. In the winter, this heat flow moves directly from all heated living spaces to adjacent unheated attics, garages, and basements, or to the outdoors; or indirectly through interior ceilings, walls, and floors--wherever there is a difference in temperature. During the cooling season, heat flows from outdoors to the house interior. To maintain comfort, the heat lost in winter must be replaced by your heating system and the heat gained in summer must be removed by your air conditioner. Insulating ceilings, walls, and floors decreases this heat flow by providing an effective resistance to the flow of heat.

Insulation is rated in terms of thermal resistance, called R-value, which indicates the resistance to heat flow. The higher the R-value, the greater the insulating effectiveness. The R-value of thermal insulation depends on the type of material, its thickness, and density. In calculating the R-value of a multi-layered installation, the R-values of the individual layers are added. Installing more insulation in your home increases R-value and the resistance to heat flow. It is important to note that true results are directly related to completely filling all gaps and voids.

The effectiveness of an insulated wall or ceiling also depends on how and where the insulation is installed. For example, insulation which is compressed will not give you its full rated R-value. Also, the overall R-value of a wall or ceiling will be somewhat different from the R-value of the insulation itself because some heat flows around the insulation through the studs and joists. That is, the overall R-value of a wall with insulation between wood studs is less than the R-value of the insulation itself because the wood provides a thermal short-circuit around the insulation. The short-circuiting through metal framing is much greater than that through wood-framed walls; sometimes the metal wall's overall R-value can be as low as half the insulation's R-value. With careful design, and an upgraded system, this short-circuiting can be reduced.  

Some Things You Should Know:

If you are buying or building a new house, make sure that recommended energy-saving features are included. The Federal Trade Commission (FTC) home insulation rule requires the seller of a new home to provide information on the type, thickness, and R-value of the insulation that will be installed in each part of the house in every sales contract. Insulation contractors are required to give their customers similar information. Many state or local building codes include minimum requirements for home insulation. Be sure that your new home or home addition meets these building codes. Also, some government home financing programs require that the home be built to meet the model energy code. You may wish to install insulation beyond the minimum specified in such codes, especially if those minimum levels are below those recommended in this fact sheet.

To keep initial selling prices competitive, many homebuilders offer standard (not optimal) levels of insulation, even though additional insulation would be a good investment for the buyer. Guidelines are published by several organizations. Following these guidelines will provide you with a more energy efficient home. These guidelines also describe methods you can use during house design to compensate for energy lost through metal studs in the walls or a large amount of windows. You should find out if your builder constructs homes in accordance with these guidelines. It is almost always more economical to install the recommended levels of thermal insulation during initial construction rather than adding insulation later.

How Much and Where?

When both insulative sheathing and cavity insulation are specified for walls, it's important to use them together as a system. Also, these recommendations assume that the insulative sheathing will be placed outside a wood sheathing product. If you choose to replace the wood sheathing with a combination of insulative sheathing and necessary bracing, you should choose sheathing with a slightly higher R-value.
The band joists, or outside edges of frame floors, should be insulated while the house is under construction. Foundation insulation options for new construction are broader than for existing homes. The builder may, for example, choose to insulate the exterior of a basement or crawlspace wall. You should discuss termite inspection and control options with your builder when choosing your foundation insulation method. Special sill plate (the joint between the top of the foundation and the bottom of the house frame) mineral fiber sealing products are designed to reduce air leaks if installed during the initial house construction.

Does Your Home Need More Insulation?

To begin to answer this question, you must first find out how much insulation you already have and then determine how much more would be cost-effective. Many older homes have less insulation than homes built today. A qualified home energy auditor will include an insulation check as a routine part of an energy audit. For information about home energy audits, call your local utility company. State energy offices are another valuable resource for information. An energy audit of your house will identify the amount of insulation you have and need, and will likely recommend other improvements as well.

Your home may have one or more of several different insulation materials. Mineral fiber insulation, including fiberglass and rock wool, is produced from either molten glass, slag, or rock. Fiberglass insulation is usually very light-weight, and yellow, pink, or white in color. Fiberglass can be found in loose-fill and blanket, either batt or roll, forms. Rock wool loose-fill is usually more dense than fiber glass, and is most commonly gray with black specks. Some rock wool products, however, are near-white. Loose-fill cellulose insulation is commonly manufactured from recycled newsprint, cardboard, or other forms of waste paper. Most cellulose is in the form of small flat pieces rather than fibers. However, some cellulose products are so finely divided they look fibrous as well. Vermiculite- and perlite-loose-fill products are no longer commonly used as home insulation, but you may find them in an older home. They are produced by expanding naturally occurring minerals in a furnace. The resulting granules are non-combustible and are commonly poured-in-place.

First, check the attic; then check walls and floors adjacent to an unheated space like a garage or basement. In these places, the structural frame elements (the ceiling joists or wall framing boards) are often exposed, making it easy to examine the insulation (if any) and to measure the depth or thickness of the insulation. It is more difficult to inspect finished exterior walls. One method is to use an electrical outlet on the wall, but first be sure to turn off the power to the outlet. Then remove the cover plate and shine a flashlight into the crack around the outlet box. You should be able to see whether or not insulation is in the wall. You may need to pull a small amount out to determine which type of material was used. Also, you should check separate outlets on the first and second floor, and in old and new parts of the house, because wall insulation in one wall doesn't necessarily mean that it's everywhere in the house. An alternative to checking through electrical outlets is to remove and then replace a small section of the exterior siding.

Next, inspect and measure the thickness of any insulation in unfinished basement ceilings and walls, or above crawl spaces. If the crawl space is not ventilated, it may have insulation on the perimeter wall. If your house is relatively new, it may have been built with insulation outside the basement or foundation wall. However, this insulation would not be visible because it would be covered by a protective layer of stucco, plastic, fiber glass, metal flashing, or a rigid protection board. The builder or the original homeowner may be able to tell you if such exterior insulation was used.

Making Your Decision

The amount of money you are willing to invest in insulation will of course depend on your personal finances. But remember that the initial investment will pay for itself in reduced energy consumption, particularly where the amount already installed is substantially less than recommended. If fuel and electrical power costs rise, it will make even more sense to invest in insulation. If you are financing a new home, or a major home improvement, you may wish to check to see if banks in your area allow larger loan amounts for energy efficient housing.
You Must Control Air Leakage

Most homeowners are aware that air leaks into their houses through what seem to be small openings around doors and window frames and through fireplaces and chimneys. Air also enters the living space from other unheated parts of the house, such as attics, basements, or crawl spaces. The air travels through any openings in your walls, floors, or ceilings, such as cracks where two walls meet, where the wall meets the ceiling, or near interior door frames. Other openings may also be found, such as gaps around electrical outlets and switch boxes, recessed fixtures, recessed cabinets, pull-down stairs, furred or false ceilings such as kitchen or bathroom soffits, behind bath tubs and shower stall units, floor cavities of finished attics adjacent to unconditioned attic spaces, and plumbing connections. These leaks between the living space and other parts of the house are often much greater than the obvious leaks around windows and doors. Since many of these leakage paths are driven by the tendency for warm air to rise and cool air to fall, the attic is often the best place to stop them. It's important to stop these leaks before adding attic insulation because the insulation may hide them and make them less accessible. Usually, the attic insulation itself will not stop these leaks and you won't save as much as you expect because of the air flowing through the insulation. Sometimes these leak locations are visible because the existing insulation has been stained by dust carried by the air flow. A qualified insulation contractor can help you with the following steps:

• Top openings of interior partition wall cavities: staple a plastic sheet over the opening and seal it around the edges with a high quality caulking material.
  

• Around the chimney: pack gaps around an insulated chimney with unfaced fiber glass insulation. Do not insulate bare, hot flue pipes. DO NOT USE ANY COMBUSTIBLE PRODUCTS, SUCH AS CELLULOSE INSULATION OR PLASTIC FOAMS, HERE.
  

• Around the attic trap door or entry door: weatherstrip the edges.

• Areas above staircase ceilings and dropped ceilings: staple a plastic sheet over the opening and seal it around the edges with a high quality caulking material.
  

• Around pipes (look under your sinks and behind your toilets) and ducts penetrating a wall or attic floor.
  

Sometimes joints between walls and floors allow open passage of air between the heated part of the house and the attic area or outdoors. Look for such joints in your attic or in the space over a porch ceiling. This air leakage path is commonly found in Cape Cod-type houses, or if attic space has been converted to living space.  A similar arrangement occurs when the second floor of a two-story house is larger than the ground floor and has an overhang over the outdoors. Another major source of air leakage can be the joint between a porch roof and a side wall. If you can reach these areas, you can stop the leaks by carefully covering the openings with plywood. If the areas are more difficult to reach, you can greatly reduce the air leakage by blowing high-density insulation into these joints, thus reducing these energy-gobbling air paths.

Ventilation

Adequate ventilation in your house is important for two reasons:

• Moisture Control - Ventilation will prevent elevated moisture levels within the conditioned space during the heating season. These elevated levels can lead to condensation on window surfaces and give rise to surface mold and mildew, as well as concealed condensation within walls and roof spaces.
  

• Avoiding Indoor Air Pollution - When natural ventilation has been sharply reduced, as in super-energy-efficient houses, it may be necessary to provide fresh air ventilation to avoid build-up of stale air and indoor air pollutants. Special air exchange units with heat-saving features are available for this purpose.
  

A well-insulated attic should be adequately ventilated to prevent moisture accumulation. Attics may be ventilated with a combination of soffit vents at eaves and continuous ridge vents. Attic vents may also be installed in gable faces. Many codes and standards require one square foot of unobstructed ventilation opening for each 300 square feet of attic floor area if a vapor retarder is included in the top floor ceiling. Twice as much ventilation is recommended if there is no vapor retarder. The net free area of a vent is smaller than its overall dimension because part of the vent opening is blocked by meshes or louvers. The openings should be equally distributed between the soffit and ridge vents or between each gable face. Never cover or block vents with insulation. Take care to prevent loose-fill insulation from clogging vents by using baffles.

Whether or not to ventilate a crawl space has been a controversial issue. Most building codes presently require installation of vents to provide ventilation with outside air, but a recent symposium on crawl space design organized by the American Society of Heating, Refrigerating and Air Conditioning Engineers concluded that there is no compelling technical basis for crawl space ventilation requirements. However, if the crawl space is not ventilated, it is crucial that all of the crawl space ground area be covered with a durable vapor retarder, such as heavy-weight polyethylene film.

You Must Prevent Moisture Accumulation

There is always some moisture in the air around us. An indoor relative humidity of about 50% is usually considered a healthy level because it is comfortable for humans and because many molds and mites are unlikely to thrive in that environment.

When is Moisture a Problem?

Even though you need some moisture in the air you breathe, too much moisture in your home can cause problems. When moist air touches a cold surface, some of the moisture may leave the air and become liquid, or condense. If this happens on a cold pane of window glass, you will see the water run down and collect on the window sill, where it may ruin the paint or rot the wood trim. The water may even freeze, producing frost on the inside surface of the window. If moisture condenses inside a wall, or in your attic, you will not be able to see the water, but it can cause a number of problems. For example, mold and mildew grow in moist areas, causing allergic reactions and damaging buildings. Structural wood may rot and drywall can swell (see Figure 1). If moisture gets into your insulation, the insulation will not work as well as it should, and your heating and cooling bills will increase.

How Does Moisture Come into Your Home, and How Does it Move Around Indside the Building?

The most obvious way that moisture enters your home is through rain, either falling on a leaky roof, wind-driven against a poorly-sealed wall, or collecting against (and eventually leaking through) the walls of your basement or crawlspace. Roof leaks are usually noticeable and must be repaired immediately. Rain coming through a wall may be less apparent, especially if it is a relatively small leak and the water remains inside the wall cavity. These kinds of leaks may occur around window or door frames, so it is important to replace any missing or cracked caulking. Rain seeping through the ground into your basement or crawl space may appear as damp, moldy walls or may be handled by a sump pump. In any event, you want to be sure that all rain coming from the roof, gutters, or across the landscape is directed well away from your house.

You also generate moisture when you cook, shower, water your indoor plants, use unvented space heaters, do laundry, even when you breathe. More than 99% of the water used to water plants enters the air. If you use an unvented natural gas, propane, or kerosene space heater, all the products of combustion, including water vapor, are exhausted directly into your living space. This water vapor can add up to 5 to 15 gallons of water per day to the air inside your home. If your clothes dryer is not vented to the outside, or if the outdoor vent is closed off or clogged, all that moisture will enter your living space. Just by breathing and perspiring, a typical family adds about 3 gallons of water per day to their indoor air.

Because air always contains some moisture, any air movement carries moisture with it. Did you know that your house breathes? We inhale and exhale through our noses, but your house inhales through one air pathway and exhales through another. Usually houses inhale around their bottom half and exhale around their top half. These air pathways include all available openings, both small and large. Back when homes had central fireplaces or open furnaces, the chimneys took care of most of the exhaling. Now, however, much of that job is handled by small leaks through your walls, floors, or ceilings. Remember that if any air is leaking through electrical outlets or around plumbing connections into your wall cavities, moisture is carried along the path.

The same holds true for air moving through any leaks between your home and the attic, crawl space, or garage. Even very small leaks in duct work can carry large amounts of moisture, because the airflow in your ducts is much greater than other airflows in your home. This is especially a problem if your ducts travel through a crawlspace or attic, so be sure to seal these ducts properly (and to keep them sealed!). Return ducts are even more likely to be leaky, because they often involve joints between drywall and ductwork that may be poorly sealed, or even not sealed at all.

Moisture also moves through a process called diffusion. Diffusion occurs if some part of your home has a higher moisture level than another part, such as the movement of moisture from the bathroom to the bedroom after a hot shower has filled the bathroom with steam. Another example of diffusion is the movement of moisture through a floor above a damp crawl space and into the house above. Diffusion happens even if there is no air movement at all. Just as heat travels from a hot space to a cold space, even if it has to go through a wall, water vapor will travel from a space with a high moisture concentration to a space with a lower moisture concentration, again, even if it has to go through a wall. Cold air almost always contains less water than hot air, so diffusion usually carries moisture from a warm place to a cold place.

Liquid Movement can also happen within your walls, such as when water runs down an internal wall surface, or seeps through your insulation. Capillarity is another kind of liquid movement, and it can carry moisture from the ground up into your walls. This is the same process used by trees to carry water from their roots to their leaves. (Did you ever put a stalk of celery into a glass of colored water and watch the color climb to the leaves?). This process can carry water through concrete slab floors into your home. It can also carry water from the foundation into your walls, so your builder should include a vapor retarder between the foundation and the walls.

Moisture can also enter your home during the construction process. The building materials can get wet during construction due to rain, dew, or by lying on the damp ground. Concrete walls and foundations release water steadily as they continue to cure during the first year after a home is built. During the house’s first winter, this construction moisture may be released into the building at a rate of more than two gallons per day, and during the second winter at a slower rate of about one gallon per day.
We have talked about moisture moving through your house, but your house is also able to store moisture. All building materials, including the wood studs within your walls and the gypsum wallboard, can hold a certain amount of moisture and still do their job properly. So if your weather alternates wet times with dry times, the building materials may be able to hold the moisture until drier air carries it away, as illustrated in Figure 2. But if the drying times are not long enough, or often enough, the extra moisture will cause problems.

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 (these retarders are described later in this fact sheet), 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 crawlspaces 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.

Click Here For Insulation Basics: Part 2