Insulation and R-value
Adding insulation to walls, floors and ceilings can reduce heating and cooling costs. We can estimate these savings if we know the R-value before and after insulating. (R-value is a measure of resistance to heat movement.) Insulation slows heat as it moves toward cold. Insulation makes the warm side of your home warmer, so insulation also improves comfort. An insulating material that is twice as thick has twice the R-value. Some insulation materials have twice as much resistance as others of the same thickness. For instance, polyiso foam board has about R-7 per inch while blown fiber has about R-3.5 per inch. Common construction materials have some R-value too, though not much compared to insulation. Without insulation, typical walls, floors and ceilings have about R-4. We’ll use this as a baseline or “0” point in measuring the R-value on the insulation graph.
Using the Insulation Graph of Cost Savings
The insulation graph (below) shows the cost of heat that will move through 1000 square feet of a home’s exposed surface, e.g. walls, floors and ceilings. The graph is set for our Chicago climate for an entire heating season.
The insulation chart shows heating cost for uninsulated surfaces as $390 per year per 1,000 sq ft. Our graph counts the insulation material that is present, or that you might add, starting at “0.” What is not found on the graph is the actual total wall R-value of R-23 (R-19 + R-4), but which is taken into account in the heat loss calculations.
We know the cost to heat 1,000 sq ft of uninsulated wall is $390. Let’s see how much we can save by adding R-19 to that wall. Slide across the horizontal axis to R-19 and move straight up to the curved line. Now slide straight left from this point to find the cost, which is $68. Subtract the R-19 cost from the R-0 cost to see the savings. In this case, the savings is $322 per year per 1,000 sq ft. ( $390 – $68 = $322 ) To find out how much you would save in a house with 2500 sq ft of uninsulated walls you would simply multiply $322 by 2.5. (2500/1000 = 2.5 ). In this house, the savings would be $805 dollars per year!
