Homeowner’s Energy Workbook Part 10 Component U Values

Now that we have the total U value for the house, we can start looking more carefully at individual component U Values.  P1000274Considering the single story house we just analyzed, it is about 1,000 square feet.  It has a nice, flat 1,000 square foot attic floor insulated with fiberglass batts that have an R-value of 19.  There is also sheetrock covering the ceiling that also has a small R value, and there are the ceiling joists that hold the structure together.  Would it be worthwhile for the homeowner to have a contractor come in and blow in another 24” of cellulose insulation?  How much would it save?

To keep things simple, let’s say that R19 is the average R value[1] of the whole ceiling.  The U value = 1/R, so the U value is 0.053.  To get the U value of the whole ceiling element, multiply the U value times the area: 0.053 x 1000 or 52.6 UA.

We have to multiply the UA value times the Heating Degree Days (4151 in this case) times 24 hours in a day, we get 5,243,368 BTUs for the year or 52.43 Therms (because there are 100,000 BTUs in a therm). And since a therm costs $1.47 it costs $77 per year for the heat going through the attic floor or just about 32% of the cost of heating the whole house.

What would happen if we had someone blow in 24” of cellulose insulation into the attic?  The R-value of loose fill cellulose is about 3.7 per inch, so that would be about R-89 plus the existing R-19.  So we would have R-108 in the attic.  That would reduce the heat loss (going through the same calculations as we just did for R-19) to 9.25 therms or $13.56 for the year!  The insulation itself would cost about $900.

What about a window?  Let’s say we have a single pane wooden window without a storm window.  Would it be worth it to replace it?  Two issues to think about here.  One is the conductive loss through the window and the second is the convective loss, the drafts around the edges and between the sashes.  Before we had special tools like blower doors to use to test for air leaks, we used to estimate the leakage of the windows by inserting a quarter in the gaps!  The problem is that we can’t really get a good average number for the convective savings, but we can determine the conductive savings.

Let’s say it’s a thirty-two inch wide by forty-five inch high window or ten square feet.  If it has a single pane, its U value would be 0.90 or a UA of 9 (10 x 0.9).  That would mean that window would require 8.9 therms or $13.18 to heat for the year.

If we replaced it with a new double paned window with a 0.30 U value, it would cost just a third of that or $3.95 in heat for the year.  We would have to replace about seven windows to get the same savings as we would in adding some insulation to the attic.

We are neglecting the convective savings from the windows, however.  But the process works for estimating savings for any energy improvement.  Before adding insulation to the attic, it is very important to seal all the holes and cracks and gaps where wires and pipes and chimneys come through.  It is important to mark where electrical connections are that will need to be serviced in the future.  It is important to carefully seal the tops of the exterior walls.

Now you know, however, that if someone tells you they can sell you a window that will double the U value of the windows you have, that that would be a bad deal indeed!

Next Time: Fuels and their Ratings


[1] To calculate the average R value, one needs to determine the R value in the space between the ceiling joists, the R value through the joists themselves, and calculate how much of the ceiling is the space and how much area are the joists.  This is a really worthwhile calculation when one looks at things like attic hatches where the R value can be really poor and have a major impact on the overall average R value of the whole ceiling, but we’re going to keep it simple here.


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