Some folks think that if the double the thickness of attic their attic insulation that it will reduce their heating and cooling bills by 50%. Doubling the R-value of the insulation in your attic does not cut your home heating bill in half. Here is why.
First, what is R & U all about?
Insulation is rated by its resistance to heat flow and is measured in units of R-value. The R-value is used in selling insulation because bigger is better.
It is easier to sell a product when bigger numbers mean it is worth more. It is harder to sell stuff to people, when lower is better.
R-value is an accepted product labeling number to make it easier for folks to understand that thicker is better. And, worth a higher price.
The inverse of thermal resistance is conductance, referred to as the U-value. The U-value is equal to one divided by the R-value (U = 1/R). Your remember fractions and algebra from school right?
The U value is important to engineers because it describes how fast energy is traveling in or out of your house. It is the number we use in our heat loss calculations.
In the English speaking part of the world, we count U-values in Btu/square foot-Fahrenheit-hour. You don’t need to remember that. Just hold on to the idea that it is the speed of escaping energy, so lower is better.
So how do you use R-value?
R-value is intuitively easier to understand, the higher the R-value, the better the insulation. R-values of different components can be added. As an example if you have three layers of R 7 insulation in your attic the total R-value = 7 + 7 + 7 = 21
You cannot add U-values as easily. Watch out here come the fractions and decimals. Total U-value = 1/(7 + 7 + 7) = 1/21 = 0.048
Let’s see that visually
This graph shows the speed of escaping energy on the left. Squint so you can see the decimal point. The highest value is point 3 with zero at the bottom of the graph. The R-value is shown across the bottom of the graph.
Look at the graph and notice two things.
One. The speed of escaping energy is slowed a lot for each little bit of insulation until you reach R 14 where it turns the corner.
Two. From R 28 onward, there is very little improvement.
If you were to improve attic insulation from R-7 to R-21, for example, you could cut you energy losses through the ceiling by 65%.
Going from 1/7 versus to 1/21 U-value (or from 0.14 to 0.05) is a 65% improvement.
In most homes about 35% of the heat is lost out the roof, the rest gets out through the walls, windows and by air leaks. Therefore, the overall energy bill for the heating season would be reduced by about 23% (a 65% improvement of 35% of the heating bill.)
In this example, three times as much insulation saves less than one quarter of the heating bill.
The myth persists that, if R-11 insulation in your house saves heat and money, R-22 insulation saves twice as much. However, the energy and money you save by adding more insulation diminishes quickly.
Look at the graph again. Starting with an R-value of 3.5, every doubling of units in R-value cuts the speed of heat loss through the wall or ceiling by about half. The R-value points shown on the graph are selected because they are double the previous number.
As the thickness (R-value) gets higher, the speed of heat escaping through the ceiling soon becomes tiny. It reaches a thickness where adding more insulation costs more than it is worth.
So what do I recommend?
If the insulation in your attic is less than R 30 (9 inches or less of fiberglass) then you should add more.
Because the amount of labor to do the job does not change much with thickness, it is best to do the job once. By that, I mean add as much as you need in one new layer. Make it thick enough so that you will not have to do it again when the price of energy goes way up.
You should target a total of about R-50, plus or minus 10. The number does not need to be exact. R 60 is good for far North America. R 40 is good where the snow melts between winter snowfalls. If it hardly ever snows where you live, R 40 insulation will save on your summertime air conditioning bill.
I recommend going big and do it just once by adding one new layer of fiberglass. Use the thickest available single layer of unfaced fiberglass batt insulation that will get you near R 50. That is about 16 inches of total thickness.
The cost to bring in and set up the equipment for blown in loose fill insulation is the same no matter how deep you fill your attic. So, I suggest you shoot for a total of 16 inches deep.
Yes the material cost for thicker is higher, but by going with one new layer that takes you to a total of about R 50 you only pay the labor cost once. Or, if you do it yourself you only risk falling through the ceiling once.
That will give you plenty of insulation now and still do the job when the price of home heating fuel goes up.