Insulation is rated by its thermal resistance, called R-value, which indicates its resistance to heat flow. Higher R-values indicate greater effectiveness at reducing heat flow. The R-value of thermal insulation depends on the type of insulating material, its thickness and its density. In calculating the R-value of a multiple-layered wall, floor or roof assembly, the R-values of the individual layers are added together. Installing more insulation in a home increases its R-value and helps keep heat from moving through the building envelope.

The R-value of a wall assembly is also affected by the quality of the installation and the properties of the insulation material. More tightly-packed wall cavities will allow less air-flow through the wall assembly, which reduces the amount of heat flow, since air carries heat.

Insulation packed too tightly will lose some of it's effectiveness because most insulation works by trapping air in microscopic air pockets. When these tiny pockets are crushed, R-value is reduced.

Insulation is also affected by thermal bridging. Thermal bridging commonly occurs where framing members in the building envelope interrupt the insulation. Wood studs have an R-value of approximately R-1 per inch. Fiberglass insulation is approximately R-3.3 per inch. This means that studs will conduct heat through the wall more quickly than the insulation, forming a thermal "bridge" between the conditioned-air interior and exterior.

Because heat rises, ceilings and attics typically have more insulation installed than walls or floors.

Areas Requiring Insulation

In unfinished attic spaces, insulate between the ceiling joists to seal off living spaces below.*
In finished attic rooms with our without dormers, insulate:

a. between the studs of "knee" walls,

b. between the studs and rafters of exterior walls, and

c. ceilings with cold spaces above.

All exterior walls, including:

a. walls between living spaces and unheated garages or storage areas,
b. foundation walls above ground level, and ,
c. foundation walls in heated basements (foundation can be insulated on inside or outside of wall).

Floors above cold spaces, such as vented crawl spaces and unheated garages, and:

a. any portion of the floor in a room that is cantilevered beyond the exterior wall below,
b. slab floors built directly on the ground,** and
c. foundation walls of crawl spaces and perimeter plates.
d. Add storm windows as recommended.

*Well-insulated attics, crawl spaces, storage areas, and other closed cavities should be adequately ventilated to prevent excessive moisture buildup.

**Slab on grade is almost always insulated, in accordance with building codes, when the house is constructed.

Source: Lincoln Electric System

Insulation Recommendations by Climate Zone

Here are examples of climate zones is use in various parts of the country. Check under #1 to see what practices are followed in your area.

Insulation Comparison Chart

Insulation Type

R-Value
per Inch

Density (lb per ft³) (for Loose-Fill Only)

Where Applicable

Advantages

Loose-fill:

Anywhere that frame is covered on both sides, such as finished walls or cathedral ceilings, unfinished attic floors and hard-to-reach places.

The only insulation that can be used in finished areas. Easy to use for irregularly shaped areas and around obstructions. Dense-pack provides air sealing as well as insulation.

Cellulose

3.1-3.7

1.5-2.0

(dense pack)

3.4-3.6

3.0-4.0

Fiberglass

2.2-2.9

0.5-1.0

(dense pack)

3.4-4.2

1.6-3.0

Rock wool

2.2-2.9

1.7

Batts:

All unfinished walls, falls, and attics

Fitted between frame studs, joists, and beams

Do-it-yourself

Suited for standard stud and joist spacing, if there are few obstructions

Fiberglass

2.9-3.8

Cotton

3.0-3.7

Sprayed insulation:

Unfinished walls, attics, and floors

Provides air sealing as well as insulation

Can provide complete coverage around obstructions

Polyurethane foam

5.6-6.2

Icynene foam

3.6-4.3

Wet-spray cellulose

2.9-3.4

Spray-in fiberglass

3.7-3.8

Foam board:

Basement masonry walls and floors

Exterior walls under construction

Exterior walls when adding siding

High insulating value for relatively little thickness

Covers wall framing, insulating studs as well as cavities

Expanded Polystyrene

3.9-4.2

Extruded Polystyrene

5.0

Polyisocyanurate

5.6-7.0

Polyurethane

5.6-7.0

Phenolic (closed cell)

8.2

Phenolic (open cell)

4.4

Sources: DOE Insulation Fact Sheet, 1993 ASHRAE Handbook of Fundamentals, Loose-Fill Insulations

Sprayed Insulation

The advantage to sprayed insulations like those shown below is that they reduce air movement through the wall dramatically by more effectively sealing around electrical boxes, and plumbing and electrical components.

Source: Kenton Shepard

Sprayed Polyeurethane foam

When inspecting homes with sprayed polyeurethane foam, inspectors should look for excessive crackoing and poor adhesion. This foam may be visible in attics or crawlspaces.

Source: Kenton Shepard

Sprayed cellulose

Blown-in Insulation

Source: Kenton Shepard

Mineral Wool

Source: InterNACHI library

Vermiculite

Insulating Crawlspaces and Basements

Get a recommendation for insulating crawlspaces in your area from the Builder’s Foundation Handbook, produced by Oak Ridge Laboratories and recommended by the US dept. of Energy as a reference for insulating foundations including slab-on-grade, crawlspaces and basements.

Proper Installation (fiberglass batt)

Foil or paper-backing on fiberglass batt insulation should always be installed with foil or paper backing toward the warm side of the wall to minimize condensation problems.

Insulation Baffles

In homes with soffit vents, baffles made of foam or cardboard are designed to maintain an air space between the insulation and the roof sheathing. Blockages are especially common when insulation is blown in.

Source: Kenton Shepard

Insulation baffles installed in a complicated roof

Checking for Insulation in Existing Walls

A number of methods exist for checking for the presence of insulation in the exterior walls of homes. Here are a few...

Infrared: Inspectors with an infrared camera or thermometer can use these instruments to check exterior walls for insulation levels. Significant differences between indoor and outdoor temperatures will make this method easier. Without the ability to create a baseline from an uninsulated wall, some practice will be required. These instruments may also be used to find areas of inadequate ceiling insulation above exterior walls which may encourage the formation of ice dams in cold climates.
Removal of switch or outlet cover plates may offer enough view to determine the presence and type of wall insulation.
Insulation contractors have been known to use a nail to punch a hole in a closet wall. Through this hole they insert a bent wire which they twirl to see whether it turns freely or catches on insulation.
In older homes which have had blown-in insulation added through holes drilled into exterior wall-covering materials, these holes may be visible.

Source: Kenton Shepard

Plugged holes drilled into brick to allow insulation to be blown in

Sources for More Information

Oak Ridge National Laboratory Building Technology Center
U.S. Dept. of Energy Insulation Fact Sheet

Peak to Prairie Home Inspection Service

Insulation

Insulation Comparison Chart