If you’re looking at adding additional information to your home, you will see a number of options available (e.g. fiberglass batts, blown-in cellulose, spray foam). R-value is the metric used to evaluate just how much insulating ability each option can provide. For engineers who want to skip ahead, the governing formula is:
To understand what R-value means, you have to know a little bit about building heating and cooling. In freedom units, heat is measured in British Thermal Units (BTUs). 1 BTU is the amount of heat required to raise the temperature of 1 lb of water by 1°F. You have likely seen a furnace or water heater rated in BTU/hr. For example, a 80,000 BTU/hr furnace could generate 80,000 BTU in an hour, or 24 x 80,000 BTU = 1,920,000 BTU in a day. Often you buy natural gas in units of therms, which is really just 100,000 BTU of heat.
R-value tells you the number of BTU/hr that will get across a wall of a given area for a given temperature difference. Let’s run through an example for a tiny log cabin in the middle of Alaska. We want to evaluate what size furnace we will need to buy and whether upgrading the insulation is worth it or not.
STEP 1: What is the geometry of the cabin?
We ultimately need to determine the surface area of the cabin exposed to the outdoors. That includes four walls and a roof. If the cabin is 30′ L x 15′ W (and 10′ tall), we get:
STEP 2: What is the temperature difference across the walls?
In Anchorage, AK, the 5-year coldest day is -20°F (ASHRAE Fundamental 2005 ed.). If the owner wants to keep his cabin at a balmy 70° year-round, the temperature difference across the cabin walls would be 90°F.
STEP 3: What is the R-value of the insulation on the cabin?
Let’s start off cheap. Only R-13. If you want to maintain the temperature inside the cabin at 70°F, you could calculate the energy required:
STEP 4: How much does that cost to heat?
Let’s say that the Alaskan Wilderness stays at the -20°F design temperature for an entire month. You would need:
Now, we convert that into therms of propane:
The national average price of propane is $2.30/gallon and propane delivers 0.91 therms/gallon.
STEP 5: Would we get value from upgrading the insulation?
When it comes to first installation, Dow Corning R-13 is $0.460/sq ft and R-19 is $0.683/sq ft right now at Home Depot. We can multiply by wall area to get the material cost for each.
(R-13) ($0.460 / sq ft) x (1350 sq ft) = $621
(R-19) ($0.683 / sq ft) x ( 1350 sq ft) = $922
So the additional insulation to upgrade from R-13 to R-19 would cost $301.
STEP 6: When would we hit break even on the upgrade?
We need to run through the calculations from STEP 3 and STEP 4 again but instead for R-19. That would give us 50.6 gal/mo or $116/mo.
Now our Alaskan cabin isn’t going to experience design conditions month after month, but if it did it would take ($301 / ($170 – $116)) = 5.5 months to pay off the added insulation cost. Probably well worth the upgrade if the design life of the cabin is 40+ years!
A Valiant Effort 