Energy Conservation

• Burden on Industry --> make more efficient devices
• Burden on the Consumer --> Accountability behind closed doors

• Force consumer to pay the real cost of Energy --> high energy costs promote conservation

Protecting your home against heat loss:

Insulation is the Key:

Energy loss through a wall is given by:

Q_e = (kA/L)*T_d

where

• Q_e = energy lost in units of BTUs per hour
• A = area of the wall
• l = wall thickness
• k = conductivity of the wall which depends on the material in the wall
• T_d = difference in temperature between inside and outside

T_d is defined in units of degree days.

A degree day = T_in - T_out where in and out refer to the inside and outside temperatures.

By convention: T_in = 65^o F

So, if T_out averaged over one 24 hour period is, say, 30^o F, then that day is equivalent to

T_in - T_out 65 - 30 = 35 degree days

When you go to the store and buy insulation the label tells you its R-value

R = (1/k)*L

R-values of materials just add. In other words, the total insulation in a wall would be

R_t = R₁ + R₂ + R₃ + ...

Where the individual R values are associated with different kinds of materials in your wall.

General Rules:

• Insulation usually pays for itself in reduced heating bills relatively soon (2-3 years).
• Many houses and buildings were constructed when energy was cheap and therefore are not adequately insulated most insulation is therefore retrofitted (meaning its more expensive).

Nationwide Distribution of Heating Degree Days

Note, a heating season is just the cumulative number of degree days. Consider the following 5 day heating season:

Day	Average Outside Temperature	Degree Days	Total Degree Days in Season
1	40	25	25
2	30	35	60
3	35	30	90
4	0	65	155
5	50	15	170

so the total heating season in this case is 170 degree days and the one coldest day (0 degrees) makes up about 40% of the total heating season.

Example Insulation Calculation:

Inputs:

• assuming a heating season of 6000 degree days
• assume an 8x20 foot wall

  The wall has the following components:

  • 3/4 inch slab of insulating sheathing on the outside with R=2.06
  • 1/2 inch gypsum board with R = 0.45
  • Outside and Inside air layers give R = 0.85

• Total R for these components is 2.06 + 0.45 + 0.85 = 3.36

What is the Heat Loss through our Wall

Q_e = (kA/L)*T_d = A/R_t*T_d

Since Q_e is in units of heat loss per hour or BTUs per hour we need to multiply T_d which is in degree days, by 24 hours per day to get the total heat loss in BTUs.

Other variables are:

• A = 8x20 = 160
• R_t = 3.36
• T_d = 6000

So our heat loss averaged over the heating season is:

Q_e = (160/3.36)*6000*24 = 6.85 million BTUs

So in our uninsulated 8x20 foot wall we lost 6.85 million BTUs in the heating season.

Approximate cost of natural gas is $4 per million BTUs which would be about $28 for this wall.

Now lets add some fiberglass insulation with R=12.95.

Now R_t = 3.36 + 12.95 = 16.31 so the total heat loss is reduced to:

Q_e = (160/16.31)*6000*24 = 1.41 million BTUs

Total cost savings to you for this one wall is then

(6.85 - 1.41) * $4 = $20

If it costs you $60 in insulation for this one wall then you recover that cost in 3 years and you really win big should you have a particularly severe heating season.

Hence it pays to have insulation and insulation technology is getting better (spray on expanding foam, etc).

What else can the home owner do?

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