Insulation for Your Home
Department of Agricultural Engineering
Insulation is an essential part of home heating and cooling systems. It adds to the comfort level in the home and reduces fuel costs.
A well insulated home has lower fuel bills for heating and air conditioning than a similar home without insulation. Bills may be as much as 50 percent less.
How much insulation?
Determining the proper amount of insulation for a home is basically an economic decision. Fuel savings are balanced against the cost of insulation to determine optimum levels. In Missouri, considering fuel cost and expected weather conditions, the following minimum insulation R-values are recommended.
"R" = 30
- Exterior walls
"R" = 20
"R" = 13
- Floors over unheated crawl space
"R" = 20
The "R" value is a measure of a material's ability to resist the flow of heat. The higher the "R" value, the better the insulation qualities of the material.
"R" is an additive quality. Two inches of a given material will have approximately twice the "R" value of 1 inch. Also, the individual "R" values for all materials in a section of a structure can be added together to obtain a total "R" value.
Several years ago, most insulation material was either mineral wool or fiber glass. These materials have similar insulating properties, and it became common practice to specify insulation in terms of inches of thickness. Today, many different insulations are on the market, and each has its own characteristics. The only fair way to compare their insulation ability is to compare "R" values.
Table 1 contains a list of the more common insulating and building materials with their respective "R" values. These "R" values are for the material only. Some manufacturers quote "R" values on an installed basis, which allows them to include assumed value for other components of the ceiling, wall or floor section. When shopping for insulation, consider material insulating value, "R", per dollar cost of the installed insulation.
Insulate where heat flows
Heat flows from warm to cold areas. Insulation should be placed where you are trying to slow the heat flow. Obvious places are in all exterior walls and the ceiling immediately below the attic area. Remember to insulate walls between heated and unheated rooms such as a garage, basement, or storage room.
When energy costs were low and supplies were plentiful, floor insulation generally was not recommended because heat loss through the floor helped keep the basement area warm and comfortable. This is still a good idea if the basement is used frequently. If it is used only occasionally, you save fuel dollars by heating it only when you use it.
Care should be taken to insulate concrete slab-on-grade floors and walkout basement floors to lower fuel bills and have warmer floors. The earth under the floor achieves some temperature between earth temperature (about 55 degrees Fahrenheit) and air temperature in the home. Therefore, not much is lost through the floor.
Most of the floor heat loss is to the outside air through the foundation wall or to the cold ground just outside the foundation wall. Slab-on-grade floors usually are insulated around the perimeter rather than under the floor. These figures also show how to insulate under a door, often an area of significant heat loss. Structural details in these figures will need to be changed if unusually high loads or low soil strength is encountered.
Insulating concrete or masonry basement walls is important if it is planned to heat and cool the basement.
If the insulation is installed on the outside of the basement wall, the thermal mass of the concrete or masonry wall will help maintain a more stable temperature in the basement. If the insulation is installed inside the concrete or masonry wall, the finish of the wall can be varied to match esthetic considerations. Structural details in these figures should be changed if unusually high loads or low soil strength is encountered.
Several areas are often neglected when a home is insulated. Some of the more common ones are:
- Walls separating living area from the attached garage. Keeping a garage warm is nearly impossible because of air leakage around the garage door. Insulating the wall between house and garage reduces heat flow from the warm house to the cold garage.
- Walls and ceiling of basement garages for the same reason as above.
- Walls and ceiling of dormers. These exterior walls frequently are overlooked during the insulation process.
- Sloping ceiling areas in upstairs rooms where the ceiling has been "clipped" to accommodate the roof rafters.
- Narrow cracks around window and door framing. This represents a relatively small area of total exterior surface of a home; however, careful filling with insulation will reduce air leakage into the home.
- Between closely spaced studs at corners of exterior walls or at junctions of exterior and interior walls.
- The ceiling near the exterior walls.
Seal cracks around doors and windows because heat required to warm cold outside air that leaks into a home may represent up to half your annual fuel bill.
Two major problems associated with insulation in the home are poor workmanship during installation and moisture condensation.
Failure to properly cut and fit insulation into all areas where you want to reduce heat flow will decrease its effectiveness. You pay for this decreased effectiveness every time you purchase fuel for as long as you own the home.
Moisture condensation is a frequent problem, but it can be prevented. All air contains moisture. Warm air can hold more moisture than cold air. During winter months, air inside the house is warmer than outside air, and moisture is added continually to air in the home by normal household activities such as washing and cooking, and purposely with humidifiers.
Water vapor moves from areas of high concentration (inside the house) to areas of low concentration (outside). If it encounters a cold surface during this migration, it condenses from a vapor to a liquid.
When condensation occurs inside an insulated wall, insulation becomes wet, building materials start to decay, and paint peels off the outside of the building. This can be prevented by using a vapor barrier material to stop water vapor from entering the wall or ceiling and condensing on cold surfaces.
Vapor barriers are installed over the face of the studs or joists on the side closest to the inside surface.
They should be used where insulation is used and temperature differentials of 10 degrees or more are expected.
A 4- to 6-mil (1 mil = 1/1,000 inch) thick clear polyethylene sheet stapled to the studs is an excellent vapor barrier if care is taken to eliminate seams and tears. Clear polyethylene has good transparency, so carpenters or dry wall installers can see stud faces easily when they apply the interior wall covering.
Any insulation board on the outside of a wall that acts as a vapor barrier can trap moisture in the wall if it is a better barrier to moisture than the inside vapor barrier. Foamed plastic or metal foil covered boards can cause this problem if they are installed so tightly that vapor penetrating the inside barrier cannot escape. Make sure the exterior insulation as installed is a poorer vapor barrier than the inside vapor barrier.
The well insulated homes of today are often blamed for moisture problems when poor-quality construction and building materials are the real causes. Older homes with poor-fitting windows and doors have so much air leakage that moisture never has a chance to build up inside the home. This is fine from the standpoint of moisture problems, but is expensive in terms of fuel costs.
Moisture need not be a problem in your home if you recognize it as part of the home environment and manage it accordingly.
R-value for insulation and building materials (per inch or for thickness given)
Glass or mineral wool batt
- Approximately 3-4" thick
- Approximately 5.5-6.5" thick
- Approximately 9-10" thick
|Expanded polystyrene-extruded per inch
|Expanded polystyrene-molded per inch
- Unfaced per inch
- Foil faced per inch
|Cellulose (shredded paper or wood pulp1)
Glass or mineral wool loose fill
- 3.75-5" thick
- 6.5-8.7" thick
- 10.2-13.7" thick
|Concrete per inch
|Brick per inch
|Plywood per inch
|Fir or pine boards per inch
|Gypsum or plaster board per inch
|Fiber board sheathing per inch
- With fiber pad
- With rubber pad
Concrete blocks, for given thickness
Siding, for given thickness
- Hardboard, 0.4375"
- Wood, drop, 1 by 8"
- Wood, bevel, 0.5 by 8" lapped
- Luminum or steel, given thickness
- No backing
- Insulation board backed, 0.375"
- Board as above foil backed
- Vertical and sealed, 0.5-3.5"
- Foil faced one surface
- Foil faced both sides
- Single glass
- Double glass
- Triple glass
- Double glass, low emissivity
1Recycled products preferred for their environmental benefits.
2Resistances of air films and frames are included. Low values are for double hung aluminum frames with no thermal break, high values for wood or vinyl single pane frames with thermal break.
Reference — 1989 ASHRAE Handbook-Fundamentals
G1721, reviewed October 1993