So You're Wondering Where to Insulate in a Home?


Examples of where to insulate.

Oak Ridge National Laboratory

For optimal energy efficiency, your home should be properly insulated from the roof down to its foundation. The illustration above shows all the areas of the home where there should be insulation. The numbered areas shown in the illustration are as follows:

1. In unfinished attic spaces, insulate between and over the floor joists to seal off living spaces below. If the air distribution is in the attic space, then consider insulating the rafters to move the distribution into the conditioned space.

(1A) Attic access door

2. In finished attic rooms with or without dormer, insulate (2A) between the studs of "knee" walls, (2B) between the studs and rafters of exterior walls and roof, (2C) and ceilings with cold spaces above.

(2D) Extend insulation into joist space to reduce air flows.

3. All exterior walls, including (3A) walls between living spaces and unheated garages, shed roofs, or storage areas; (3B) foundation walls above ground level; (3C) foundation walls in heated basements, full wall either interior or exterior.

4. Floors above cold spaces, such as vented crawl spaces and unheated garages. Also insulate (4A) any portion of the floor in a room that is cantilevered beyond the exterior wall below; (4B) slab floors built directly on the ground; (4C) as an alternative to floor insulation, foundation walls of unvented crawl spaces. (4D) Extend insulation into joist space to reduce air flows.

5. Band joists.

6. Replacement or storm windows and caulk and seal around all windows and doors.

In addition to insulation, consider moisture and air leakage control in each area of your house. If radon is an issue where you live, you’ll also need to consider radon and radon-resistant construction techniques as you research foundation insulation options. In addition, if you live in an area with termites, you’ll have to consider how termite protection will affect the choice and placement of insulation in your home.

Attic Insulation

Loose-fill or batt insulation is typically installed in an attic. Loose-fill insulation is usually less expensive to install than batt insulation, and provides better coverage when installed properly. See more on different types of insulation.

To find out if you have enough attic insulation, measure the thickness of the insulation. If it is less than an equivalent of R-30, you could probably benefit by adding more. Before insulating, seal any air leaks and make roof and other necessary repairs. If it is located in a conditioned part of the house, also remember to insulate and air seal your attic access.

Insulate and air seal any knee walls -- vertical walls with attic space directly behind them -- in your home as well. In addition, if you're building a new home or remodeling, make sure any attic decking that provides additional storage space or a platform for a heating and/or cooling unit or hot water tank is raised above the ceiling joists to leave room for adequate insulation. If the air distribution system is not within the conditioned space but within the attic, insulating the rafters will enclose the distribution system. Finally, if you live in a hot or warm climate, consider installing a radiant barrier in your attic to reduce summer heat gain.

Duct Insulation

If the ducts in your home are in unconditioned space, seal and insulate them. If you’re building a new house, place ducts in the conditioned space to avoid the energy losses associated with most duct systems.

Cathedral Ceiling Insulation

Properly insulating your cathedral ceilings will allow ceiling temperatures to remain closer to room temperatures, providing an even temperature distribution throughout the house. Cathedral ceilings must provide space between the roof deck and home’s ceiling for adequate insulation and ventilation. This can be achieved through the use of truss joists, scissor truss framing, or sufficiently large rafters. For example, cathedral ceilings built with 2x12 rafters have space for standard 10-inch batts (R-30) and ventilation. Unvented (hot roof design) cathedral ceilings are also an option. The hot roof design allows more insulation to be installed in the roof cavity as the need for a vent space is eliminated. It is important that the roof cavity be totally air sealed from the conditioned space below to prevent moisture intrusion and roof degradation.

Foil-faced batt insulation is often used in cathedral ceilings because it provides the permeability rating often required for use in ceilings without attics. A vent baffle should be installed between the insulation and the roof decking to maintain the ventilation channel.

Consider using high-density R-30 batts, which are as thick as R-25 batts, but fit into 2x10 framing. You can also add rigid foam insulation under the rafters, which adds R-value and eliminates thermal bridging through wood rafters. However, rigid foam insulation must be covered with a fire-rated material when used on the interior of a building. Half-inch drywall is usually sufficient, but check with local building officials before installing.

Exterior Wall Insulation

If your attic has enough insulation and proper air sealing, and your home still feels drafty and cold in the winter or too warm in the summer, chances are you need to add insulation to the exterior walls. This is more expensive and usually requires a contractor, but it may be worth the cost—especially if you live in a very cold climate. If you replace the exterior siding on your home, consider adding insulation at the same time.

In an existing home, consider using blow-in insulation, which, when installed with the dense pack technique, will provide significant air sealing. It can be added to exterior walls without much disturbance to finished areas of your home. If you’re remodeling and your wall cavities will be open, look into two-part spray foam or wet spray cellulose insulation. If your wall cavities are not going to be open, you could consider injectable spray foam insulation. If you’ll be doing the work yourself, blanket (batt and roll) insulation, while not capable of providing an air seal as dense pack and two-part spray foam will, is an affordable option.

In a new home, first see our information about insulating a new home, which will help you choose from among the many types of insulation on the market. If you’re in the design phase of planning your new home, consider structural insulated panels, insulating concrete forms, and insulated concrete blocks. These materials literally have insulation built in, and houses built using these products often have superior insulating qualities and minimal thermal bridging.

If you’re building a traditional framed house, consider using advanced wall framing techniques. These techniques improve the whole-wall R-value by reducing thermal bridging and maximizing the insulated wall area.

Also consider using insulating wall sheathing rather than wood sheathing products, as they provide superior R-value. Foam sheathing:

  • Provides a continuous layer of insulation, which reduces thermal bridging through wood studs, saving energy and improving comfort.

  • Is easier to cut and install than heavier sheathing products.

  • Protects against condensation on the inside wall by keeping the interior of the wall warmer.

  • Usually costs less than plywood or oriented strand board (OSB).

If you replace plywood or OSB with foam sheathing, your walls will require bracing or other structural reinforcement.

Insulating Floors Above Unheated Garages

When insulating floors above unconditioned garages, first seal all possible sources of air leakage. This strategy has the added benefit of minimizing the danger of contaminants (from car exhaust, paint, solvents, gardening supplies, etc.) in the garage migrating into the conditioned space. Also install an air barrier to prevent cold air in the garage from "short circuiting" the insulation underneath the subfloor.

Foundation Insulation

In addition to reducing heating costs, a properly insulated foundation will keep below-grade rooms more comfortable and prevent moisture problems, insect infestation, and radon infiltration. In new construction, consider construction techniques that provide both foundation structure and insulation, such as insulating concrete forms and insulating concrete blocks.

During the construction process, many builders insulate the outside of foundation walls before backfilling. This strategy is possible but impractical and disruptive for existing homes. Optimal foundation insulation materials and placement vary by climate, so consult a local insulation professional if you’re planning a new home.

Basement Insulation

A properly insulated basement can save you money on heating and provide a dry, comfortable living space. In most cases, a basement with insulation installed on its exterior walls should be considered a conditioned space. Even in a house with an unconditioned basement, the basement is more connected to other living spaces than to the outside, which makes basement wall insulation preferable to ceiling insulation.

In new construction, adding insulation on the exterior of the basement walls will do the following:

  • Minimize thermal bridging and reduce heat loss through the foundation

  • Protect the damp-proof coating from damage during backfilling

  • Provide some protection against moisture intrusion

  • Make the foundation part of the thermal mass of the conditioned space, thereby reducing interior temperature swings

  • Reduce the potential for condensation on surfaces in the basement

  • Conserve room area, relative to installing insulation on the interior.

In an existing home, adding insulation to the exterior of the basement walls is impractical. Interior basement wall insulation has the following advantages:

  • It is much less expensive to install than exterior insulation for existing buildings.

  • Almost any insulation type can be used.

  • It eliminates the threat of insect infestation.

Here are a few things to consider when installing interior basement wall insulation:

  • Many insulation types require a fire-rated covering because they release toxic gases when ignited.

  • Interior insulation reduces usable interior space by a few inches.

  • It doesn't protect the damp-proof co