Insulate Ceiling or the Roof? What Works Better?

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Properly insulating a home is critical in keeping the roof from causing damage and saving energy costs. You may be faced with the choice of insulating either the ceiling or the roof. How do you know which is best for your home?

Should you insulate the ceiling or the roof? Which one works better? Your number one priority should be insulating your ceiling for a number of reasons, including:

• Keeping the temperature of your living space regulated
• Saving energy costs by preventing heat loss due to heat conduction
• Preventing the roof from accumulating moisture damage
• Ease of construction- insulating the ceiling can be easier than insulating the roof, particularly with older homes

There is a set of circumstances in which it makes more sense to insulate the roof, such as when your attic happens to be part of your living space. Here, I will compare the methods most commonly used to insulate the roof and ceiling of your home.

What is the Difference Between Ceiling Insulation and Roof Insulation

Before we get started on discussing which insulation method is the best, I need to clarify a few terms.

• Roof insulation refers to the practice of installing insulation on the roof slope, both above and below the rafters.
• Ceiling insulation refers to any type of insulation installed within the floor nearest to the roof structure of the home.

Your home is most likely insulated at the ceiling because this is easier to do, according to this insulation contractor. It may be insulated to some degree at the roof, but it is much more likely that your home is insulated at the ceiling and not the roof than vice versa.

Later on, I will discuss the differences between a cathedral ceiling and a vaulted ceiling. A cathedral ceiling requires some degree of roof insulation. A vaulted ceiling does not necessarily require roof insulation because there is an air cavity separating the interior of the home (or conditioned space) from the roofline.

Ceiling Insulation

Typical ceiling insulation involves spreading either loose fill or batt (the term commonly used for blanket insulation within the attic or airspace of a home. Your home needs at least some type of insulation to keep the interior temperature of your home regulated, regardless of the specific climate you live in.

This Old House recommends not depending on your attic floor space to be an extra storage space. This is because you can make your home much more energy efficient by removing plywood flooring if you have any, and adding an extra layer of insulation.

Insulating your home in this manner is a cheap and easy way to go about it because you can easily buy bags of insulation and spread it yourself using either a blower that you can rent at your local hardware store or spread it manually by yourself.

Either way, the addition of insulation to your attic floor is a simple DIY project.

Insulating the Roof

Insulating the roof is a priority if the ceilings of your home happen to be part of the roof deck, as is the case with cathedral ceilings. You may also insulate your roof to help regulate the temperature in an attic that is a part of your living space.

Materials commonly used for the insulation of the roof deck include:

• Rigid Foam Insulation Boards
• Radiant Barriers
• Spray Foam in some cases

What are the Advantages of Roof Insulation?

Energy Performance Advantages

A sealed attic or insulation at the roof of a building offers distinct advantages in terms of energy efficiency, according to the Florida Solar Energy Center.

Slab-on-grade homes located in hot and humid climates have air conditioning ducts located within the attic of the home. These ducts will remain cooler if you have installed insulation at the roofline.

An insulated roof will lead to energy savings because the air conditioning will be more effective. Duct leaks are also common in attics that are not sealed.

Prevents Moisture Damage to Roofs in Cold Climates

Ice dams and frost under the roof deck can lead to moisture damage to roofs, especially for 1-½-story Cape Cod homes.

The design of these types of homes often prevents thorough insulation, air sealing, and ventilation from the interior, according to the U.S. Department Of Energy. 

As a result, the U.S. Department Of Energy created the External Thermal Moisture Management System (ETMMS) to analyze cost data related to installing an air/water membrane underneath the roof of such homes.

So far, the study has led researchers to believe that the air/water membranes will successfully serve their purpose in the long run.

The study is still ongoing, and it may be some time before The researchers are able to draw conclusions, but things seem to be on the right track to taking care of ice damming and other moisture-related issues.

Advantages of Insulating the Ceiling

Insulating an Unfinished Attic is a Simple DIY Project

Insulating an unfinished attic can be a simple DIY project. An unfinished attic is an attic that is not considered part of the living space.

They are the easiest to insulate because the insulation is installed between the framing members (or joints) of the floor, according to the University of Tennessee Extension.

Rolls of either glass fiber or loose-fill cellulose are the most popular materials used by DIY installers.

Insulating a Ceiling is Affordable

Cellulose or wood fiber insulation is made from used newsprint, other paper feedstock, or wood. Chemicals are then added to the insulation in order to provide fire resistance. Fiberglass insulation is made from glass that has been heated and drawn out to form workable fibers.

You can find fiberglass batt insulation relatively cheap in bulk quantities such as is the case with this Owens Corning R-38 Fiberglass Batt Insulation Roll designed to fit up to 341.36 square feet of attic space. Keep in mind that this is the roll and not the loose fill.

Loose-fill materials commonly used include the Owens Corning Atticat Blown Insulation.

Installing insulation in the roof deck, as is necessary in some cases, is a much more difficult process that will likely require a contractor, according to the North Dakota State University Extension.

In the next section, I will discuss the process of installing rigid foam board insulation in the roof deck.

Rigid Foam Board Roof Insulation

There are a couple of scenarios in which it would be recommended that you install insulation in the slopes roof assembly of a home rather than on the attic floor, as is so commonly done.

One such scenario in which this becomes practical is that the home has a cathedral ceiling over a great room. If you are unfamiliar with the differing insulation requirements of a cathedral ceiling vs. that of a vaulted ceiling, you can read more about it here.

To summarize:

• Cathedral Ceiling: this ceiling type involves exposing the sheetrock facing (or underside of the roof) to the interior of the home. The roof cladding (a portion of the roof at the exterior) runs parallel with the sheetrock facing, separated only by the rafters.
• Vaulted Ceiling: this ceiling type involves the installation of a ceiling that is not attached directly to the roof rafters. This type of ceiling is insulated in the same manner that you would insulate a standard attic space.

If you decided to use a rigid foam board to help insulate your cathedral ceiling, you would install the board on top of the roof sheathing and underneath the plywood at the top of the roof.

Rigid foam board installation in the case of a cathedral ceiling involves installing three different layers that are each 1.5 inches thick, according to the procedure discussed in this article appearing in Fine Homebuilding.

The rigid foam layers are installed with staggered seams.

Advantages of Rigid Foam Insulation Under the Roof

There are several key advantages to installing rigid foam underneath the roof of a home, including:

• Reducing air leaks that can compete with your air conditioning and heating systems to regulate the interior temperature of a home
• Limits condensation at the sheathing layer that can lead to problems with mold, rot, and insects
• Interrupts thermal bridging through the rafters
• Makes ice dams in the roof unlikely

You can achieve high R-values, even if your home does not have deep rafters. This means that the rigid foam can help you increase your capability to keep either cooled or heated air from escaping your home.

What is Rigid Foam Insulation Made out of?

There are three types of materials used for rigid foam boards that are installed above roof sheathing. Any type of rigid foam can be used for the purposes discussed in this section, but there are some differences in the materials to be mindful of.

The three types of rigid foam materials commonly used for roof insulation include:

• Expanded Polystyrene
  • R-Value: 3.8 per inch
• Extruded Polystyrene
  • R-Value: 5 per inch
• Polyisocyanurate(XPS)
  • R-Value: 6.5 per inch

Polyisocyanurate (often referred to as XPS) is the most environmentally friendly of the three different options since it is manufactured in a way that does not contribute to global warming, and it does not contain potentially harmful brominated flame retardants.

How Thick Does the Rigid Foam Layer have to be?

The thickness of the rigid foam layer depends on the climate zone you live in. Consult with a map developed by the International Residential Code (IRC) to make informed decisions.

In general, though, you will want to make sure that the layer is thick enough to prevent condensation during wintertime in colder climates.

Rigid Foam Board Insulation can be used in the Ceiling as well

The installation of loose-fill insulation often renders the attic unusable for storage purposes due to the loose-fill on the floor.

As a result, some homeowners may find it more desirable to place rigid board insulation on the floor and cover it up with plywood.

What are Some Drawbacks to Rigid Foam Board Insulation?

There can be drawbacks to using rigid foam board insulation, depending upon what material the boards are made of.

Rigid foam boards used in ceiling insulation are often made of either cellulose fiber, fiberglass, polystyrene, or polyurethanes.

Most local building codes require that rigid foam boards made from polyurethane or polystyrene be covered with a 15-minute fire-rated thermal covering if they are used on the inside of the house.

This added treatment is often required by code because these materials are flammable and will release toxic chemicals while burning.

Examples of Rigid Foam Board

• Owens Corning Foamular 150 XPS
• R-Tech Insulating Sheathing 
• Cellofoam Polystyrene Foam Insulation Board

What is a Radiant Barrier?

If you happen to live in a warmer climate, then a radiant barrier can help you save energy costs by helping to keep the interior temperature of your home regulated.

The U.S. Department Of Energy defines radiant barriers as layers that are made of a highly reflective material that reflects radiant heat rather than absorbing it.

Three types of heat travel are all cause for concern when it comes to keeping the interior temperature of a home regulated:

• Heat Transfer By Convection: Occurs when heated air becomes less dense and rises. As it becomes cooler, it becomes less dense and falls.
• Conductive Heat Flow: this is the flow of heat from a warmer area to a cooler area separated by a solid material until both areas are at the same temperature. Most insulation materials are designed to slow down conductive heat flow.
• Radiant Heat: This is the flow of heat in a straight line away from any surface that will then heat anything solid that absorbs its energy

Reflective insulation systems such as radiant barriers are effective in reducing heat gain on a surface. The sun’s radiant energy is the main reason your roof gets so hot, especially during summer.

Where does all this heat go? The heat from the sun’s radiant energy then travels via thermal conduction through the roof materials and into the home’s attic space.

Cooler attic surfaces affected by this heat transfer include the air ducts and the attic floor. Basically, a radiant barrier is designed to keep the heat from the sun’s radiant energy from reaching the interior of your home, as discussed above.

In order for a radiant barrier to be effective, its reflective surface must be exposed to the air.

Where Do You Put a Radiant Barrier?

There are three viable locations for a radiant barrier, according to the Florida Solar Energy Center.

The three locations are:

• On top of the ceiling insulation with the foil surface facing upwards
• Up towards the rafters below the bottom chord truss
• Under the roof sheathing

The details involving the logistics of installing a radiant barrier go beyond the scope of this article comparing roof and ceiling insulation. However, you should at least know that the most effective radiant barrier location is either below the bottom chord truss or underneath the roof sheathing.

If your radiant barrier is on top of the ceiling insulation, it will become prone to collecting dust over time. Installing the radiant barrier beneath the bottom chord truss is particularly effective because this location offers the opportunity for both sides of the foil to be exposed to airspace.

If the radiant barrier is in this location, it will keep the attic at a temperature that is closer to the temperature of the rest of the home.

Should I get a Radiant Barrier for my Home?

A radiant barrier is most effective in hot climates, especially in situations where cooling ducts are located near the attic, as is commonly the case in slab-on-grade homes.

If you live in either the Southern U.S. or an area with a similar climate, you will probably want to invest in a radiant barrier if you don’t already have one.

According to the U.S Department Of Energy, studies show radiant barriers decrease filling costs by 5-10% when used in warm, sunny climates.

Are Radiant Barriers Worth the Cost?

Like just about any other major home construction project, it is much easier to install a radiant barrier during the original home construction process rather than adding one later on.

Making sure that a brand new home includes a radiant barrier can be very beneficial to those who live in climates such as the one found in the southern United States, according to the Florida Solar Energy Center.

The research team at the Florida Energy Center notes that you need not install much more than one radiant barrier layer in your home. This is because the first barrier surface already eliminates about 95% of the radiant heat transfer. Adding any more than one layer of thermal foil insulation may be overkill. There are many examples of products that can be useful for this purpose.

The most expensive and effective radiant barriers are rigid insulation materials that have an aluminum surface, such as SmartSHIELD Thermal Foil Insulation.

This type of insulation material includes both an aluminum radiant barrier and a polyethylene foam that can act as a barrier to thermal conduction. Keep this in mind when you look at costs.

The more affordable option is a radiant foil barrier, such as this one manufactured by AES. This material comes double-sided and is a little bit easier to handle than the rigid foam radiant barrier. The downside is that it may not be as effective as a thermal conductor, though it will likely perform its intended purpose quite well.

Consider the R-Values of Ceiling Insulation – It’s Important

You may have noticed an “R” value displayed on the insulation at your local hardware store. This value is an important piece of information in helping you select the correct insulation for your needs.

Heat always travels from areas of greater heat to areas of lesser heat. Insulating materials are measured by their ability to restrict this movement of heat. The ability of a material to restrict the movement of heat is described in magnitude by the material’s coefficient of heat transfer resistance, often referred to as the “R” value.

Materials with a higher R-value are more effective at keeping heat from escaping through an area such as a wall or ceiling.

You may be thrown by a loop if you notice a U-value on the package of insulation rather than an R-value. The U-value is just the inverse of the R-value, showing how easy it is for heat to pass through a material.

The Best Insulation for Your Home Depends Upon the Climate You Live in

Localities typically have their own energy efficiency codes. For example, the Florida Energy Efficiency Code requires that insulation has a rating of at least R-19 to handle the hot and humid climate.

When you are consulting with your contractor to install insulation in your home, make sure you are aware of what type of insulation best suits the climate zone you live in.

Researchers with the International Code Council have provided the public with a U.S map showing the minimum recommended R-values for insulation, depending on which climate you live in. You may view the map here.

Sign up for an Energy Audit Test

If you own an older home that you are looking to make more energy-efficient, you are encouraged to sign up for an energy audit test to help you determine what your best option is moving forward.

An energy audit is an affordable analysis of a home’s ability to conserve energy, according to the North Dakota State University Extension.

If you are concerned about high utility bills, this is a good way to find out what’s going on.  You can arrange for a basic audit from your gas or electric utility provider or for a comprehensive energy audit from a private contractor who specializes in home performance reviews.

Energy audit procedures may include the use of infrared cameras in order to pinpoint voids in insulation as well as air and moisture leaks. These diagnostic tests will go a long way in helping you determine how to solve your energy efficiency issues.

The Final Word – Ceiling Insulation is the Most Important

Earlier in this article, I referenced this contractor, who said that homeowners commonly insulate their ceilings but not necessarily their roofs.

If you are deciding between insulating the roof deck or insulating the ceiling, it is most important that you insulate the ceiling. The ceiling I am referring to, in this case, is the dividing line between your living space and the attic or airspace beneath the roof deck.

Good Reasons to Insulate Ceilings

• In order to avoid damage to the roof deck, you should insulate ceilings to create an air barrier between the living space and an unfinished attic or airspace underneath the roof deck.

Attic moisture problems are often the result of heated air from the living space rising up into the attic, which is not heated. This leads to moisture damage in the roof from ice damming and condensation in climates where it gets cold enough to require heating for at least a portion of the year.

• You also need to insulate ceilings in order to prevent heat due to conduction. Heat conduction is a form of heat transfer in which heated air moves across a solid barrier to an area of lesser heat until both areas are heated the same.

The process of thermal conduction could be giving you high utility bills as your air conditioning or heating unit has to work that much harder to keep the interior temperature of your home regulated.

Don’t Forget the Roofline

• If your attic is part of your living space, or you live in a home with a cathedral ceiling or other low-pitch type ceiling, then you will probably need to insulate at the roofline.
• You also may need to insulate at the roofline if you live in a hot and humid climate. Radiant heat from the sun beats down on the roof cladding all day.

The heat from the roof then travels into the attic. A radiant barrier, discussed in further detail within this article, will stop the radiant heat from reaching the attic.

Using a Radiant Barrier

If you live in a slab foundation home, then your air ducts are probably located near the ceiling and attic. Radiant heat can affect the air ducts.

The most effective place to install a radiant barrier is underneath the bottom truss of the roof deck. This is particularly important if you are looking to regulate the air temperature within your attic.

Hopefully, this gave you some insight as to why insulating your ceiling well is so important, as well as your best options for getting the project started!