Most houses built before 1940's were constructed without insulation. From World War II till about 1955 many houses were built with attic insulation but little or no wall insulation. Since 1955 most houses have been built with both wall & ceiling insulation, though not necessarily enough. Regardless of age, houses with masonry or brick walls usually have no wall insulation. 

The effectiveness of insulation materials is not only related to thickness, but also to resistance to heat flow, known as the R-VALUE. (R stands for resistance to heat flow.) The higher the R-VALUE rating of insulation, the better it will resist heat loss & heat gain. The minimum R-VALUE for walls is 11/ for ceilings 19/ and for floors over crawl spaces 13/:

In colder climates & for homes heated by electricity, higher R-RATINGS may be needed. In general, R-19 should be sufficient for walls/R-33 for ceilings/R-22 for floors. Check with your local authorities for insulating needs in your area (the area of your property). In colder climates, higher R-ratings may be needed. (Contractors & Builders sometimes suggest thicker insulation for homes heated by electricity.)

TYPES OF INSULATION: 

Bats or Blankets: used to fill air spaces in walls, floors, attics or roofs. 

Loose-fill: used to fill wall cavities & flat areas above ceilings. 

Board or Sheet: used as wall sheathing for cavity fill, rigid roof insulation and perimeter slab insulation. 

Reflective: aluminum foil combined in layers with air spaces and used for roofs, walls & floors above vented or unheated spaces. 

Two kinds of insulation to be avoided are asbestos & urea formaldehyde (UFFI) If you suspect a house has either asbestos or urea formaldehyde insulation, bring in a qualified inspector to examine all questionable areas. It would probably be in your best interest to avoid homes or properties with these kinds of insulation. 

Good insulation means energy savings. Whenever there is a temperature difference between inside and outside, there will be a heat flow. Insulation helps retard the process of summer heat entering the home, which helps keep the inside cool & lessens the need for continuous air conditioning. Similarly, in winter insulation helps keep the house warm by reducing the escape of interior heat.