Home Energy Conservation

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Home Energy Conservation

• The Art of Insulation

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Conduction, Convection and Radiation

• Conduction Heat enters or escapes a house
  through the walls, floors and ceiling
• Convection Heat enters or escapes through
  drafts, air current move heat around inside the
  house
• Radiation House absorbs radiant heat from sun,
  house cools by radiating at night

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Conduction and Insulation

• Insulation is designed to prevent the conduction
  of heat through the walls, floor and ceiling
• The rate of heat flow through a wall of area A is
• R is the R-value and ?T is the temperature
  difference between inside and outside

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R-value

• R-value is a measure of the insulating value of a
  material
• R-value is equal to the thickness divided by the
  thermal conductivity
• R-value increases as the thickness of the
  insulation increases
• Example 6 fiberglass insulation R-19
• 12 fiberglass insulation R-38

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Some Typical R-Values
Note the standard units used to measure R-value
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Example Calculating R-Value

• The R-value of layered insulation is the sum of
  the individual R-values
• What is the R-value of a wall with 5 of brick,
  6 of fiberglass and 1 of plywood?
• Answer (From previous table)
• Brick 5 x 0.2 1.0
• Fiberglass 19
• Plywood 2 x 0.62 1.24
• Total R-value of insulation 21.24

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Example Calculating Heat Loss

• How much heat is lost (in Btu per hour) through a
  wall constructed like the previous example? The
  wall is 14 ft. by 10 ft., the inside temperature
  is 70? F, and the outside temperature is 5? F.
• Answer Area 10 x 14 ft.2 140 ft.2
• Temperature difference 70-5 65? F
• R-value 21.24 ft.2 ?F hrs/Btu
• Heat loss 140 x 65 / 21.24 428 Btu/hr

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Comments on Insulation

• Air is a good insulator provided it is still
• Insulation with lots of air pockets (like
  fiberglass) has a high R-value
• A window conducts about 20 times as much heat as
  an insulated wall of the same area
• About 35 of heating costs go toward replacing
  heat lost through windows

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Convection in the Home

• A home will lose more heat on a windy day because
  of convection
• Convection near windows is a large source of heat
  loss
• Cold air leaks into a house (air infiltration)
  through convection
• Convection can be beneficial in cooling a house
  during the summer

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Air Infiltration

• In winter, cold air leaks into a house through
  cracks, through the foundation, etc.
• Air infiltration can be prevented by sealing
  cracks with caulking, foam rubber, etc.

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Convection and Windows

• Convection near windows can be prevented by using
  valences and drapes that reach the floor
• Still air acts as an insulating layer

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Radiation

• In the summer, the attic of a house is heated by
  the suns radiation
• By placing reflective foil under the shingles and
  above the ceiling, heat will be reflected rather
  than absorbed

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Example Heating a Home

• How much heat is necessary to heat a home for one
  day? Its dimensions are 30 ft. by 60 ft. by 20
  ft. and the R-value of the walls is 20 ft.2 F /
  Btu. The outside temperature is 10 F and the
  inside temperature is 65 F.
• Answer Q (A x ?T / R) x 24 hours
• Note The total surface are of the house must be
  calculated

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Comments on Heating a House

• Only the conduction losses are included in our
  calculations
• Air infiltration losses are roughly equal to the
  conduction losses for an insulated house
• Air infiltration losses can be calculated by
  comparing conduction losses to the actual energy
  used to heat the home

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Heating a Home for a Year

• The calculate how much heat is needed to heat a
  home for a year, calculate the heating
  requirements for each day (using the average
  temperature for that day) and add them up
• An easier way is to use degree days, which is a
  sum of the average temperature difference
  (relative to a room temperature of 65 F) for
  every day of the year
• Using degree days, the total annual heat required
  is

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Degree Days in the U.S.
The greater the number of degree days, the more
home heating will cost
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Example Heating a Home for One Year

• How much heat is required to heat a home for one
  year? Its total surface area is 7,200 ft.2 and
  the R-value of the walls is 20 ft.2 F / Btu.
  The number of degree days in Stevens Point is
  7500.
• Answer Q A x (degree days) / R
• x 24 hours per day