About Insulation

Think of heated air as heated water. When cold water is poured into it, the temperature of the combined water, when thoroughly mixed, will be cooler than the heated water, but warmer than the cold water.

Now, consider a one room building with heating and air conditioning exposed on all sides to the outside air. When air is contained in this building, the inside air will conduct energy through the four walls, floor, and ceiling.  If the temperature on the outside of each surface is higher than the air inside the room (as in the summertime), heat will conduct through the walls and heat the room air via convection.  Over time, the room temperature will rise to the level of the outside temperature.

Likewise, in the cold months of the year, heat flows from inside the heated room, through the walls, to the outside. If the furnace is turned off, over time, the room will become as cold as the outside air.

Basically, if a temperature differential exists between the inside and outside of a building, you can't stop the flow of heat, but you can slow it down. 

The rate at which the heat flows through the building's walls, in either direction, is a function of the following items

- Resistance of the walls, floor, ceiling, windows, and doors to heat flow (R value) 

- Air leaks between the room interior and the outside air 

- Amount of time the room's doors and windows are open

Of these three items, only the last one can be managed every day by the occupant of this one room building.  With some investment, the first two items are likely to produce substantial savings.  Item one is further discussed in this section, while item two is covered under weatherproofing.

Another property of heat is that, in fluids, heat rises.  It does so because its warmer fluids, such as air or water, have a lower density than cold fluids.  A hot air balloon is an illustration of this principle.  As heat is added to the air in the balloon, the air in the balloon has a lower density than the outside air, which creates a buoyancy effect. In a room without air circulation, the warmer air will rise to the ceiling. For this reason, more heat is conducted through the ceiling than through the walls and floors.

The application of insulation to a surface increases the resistance of its heat flow, or "R value".  Among the materials most commonly used in building construction are:

Fiberglass

Fiberglass insulation is typically sold in the form of batts, which are fluffy strips of fiberglass cut to certain widths and thicknesses. To accommodate the standard spacing of wall studs and ceiling joists, they are sold in widths of 15" or 23". The thickness of the batt determines its R-value.

For horizontal applications, like attics, the fiberglass can also be applied as loose fill. To save on installation labor, loose fiberglass can be pumped through a blower.

Polystyrene

Because it is water resistant, it is often used for lining exterior basement walls and sheathing exterior above-ground walls of houses.

Cellulose

Cellulose is commonly used for insulating attics because it can be blown in for easy installation. Due to its granular consistency, it can also be blown into wall cavities through small holes drilled from the outside, making it an ideal material for retro-insulating older home. When mixed with a binder, it can be blown into open wall cavities during a home's construction before the sheetrock is installed.

Urethane

Urethane sold in spray cans is typically used to seal penetrations through walls, floors, and the foundation.  It is also used to fill voids between door and window frames and wall studs. 

Comparison of Different Types of Home Insulating Materials

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