Energy Conservation: Heating and Insulation

Energy Conservation

Protecting your home against heat loss:

Energy loss through a wall is given by:

Qe = (kA/L)*Td

where

Td is defined in units of degree days.

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

By convention: Tin = 65o F

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

Tin - Tout 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

Rt = R1 + R2 + R3 + ...

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

General Rules:

  • 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:

    What is the Heat Loss through our Wall

    Qe = (kA/L)*Td = A/Rt*Td

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

    Other variables are:

    So our heat loss averaged over the heating season is:

    Qe = (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 Rt = 3.36 + 12.95 = 16.31 so the total heat loss is reduced to:

    Qe = (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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