Calculation: insulation cost vs. energy cost
I’m using http://www.builditsolar.com/References/Calculators/HeatLoss/HeatLoss.htm to compare insulation to BTU loss.
I have a hard time putting my data to use. For example, I am comparing the added cost of insulation versus the return on investment. When I change the R-Value of a wall, I get more or less BTU/hour loss.
However, I don’t know how to apply that number when I don’t know what the btu loss is compared to. Is that assuming the designed outside temperature I plug in is what the calculator assumes is the constant temp, or is that assuming it will only get that cold and naturally as the day passes heat and cool, using my HDD hours, and therefore show an overall average per hour/per day/per year of BTU loss?
So say I know I am losing 2,400BTU/Hour, and I know the cost of energy to provide said BTU per hour, but I do *not* know how long per day/week/month I am supposed to multiply out. I can’t do a cost basis on extra expense of insulation because of this. I know the larger the delta in temp the larger the BTU loss will be, but the delta in temp is not a constant, so I can’t wrap my head around how it’s even possible to calculate the BTU loss for such a large variable.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Nicholas,
The software you are using is intended to help you calculate your design heat load. The purpose is to help you size your heating equipment (usually a furnace or a boiler). It calculates heat loss on (simplifying somewhat) the coldest day of the year.
This software isn't designed to calculate your annual energy use or annual fuel bills. For that you need software like PHPP or REMrate.
Martin,
That's refreshing and embarrassing at the same time. I'll check out those. I'm trying to find something fairly simple such as that Heat Loss calc that I can just formulate the total loss in BTU per day.
Nicholas,
Look up the heating degree days (HDD) for your geographical location.
HDD provides a simple metric for quantifying the amount of heating that buildings in a particular location need over a certain period (e.g. a particular month or year). In conjunction with the average U-factor for the building envelope, the HDD number provides a means of roughly estimating the amount of energy required to heat the building over that period.
More info from Wikipedia in the image below.
.
The calculator linked provides outputs including the following:
Design Loss (BTU/hr): That's for sizing the heating equipment.
Year Loss (Million BTU/yr) : That's the annual energy use.
Fuel Cost (US dollars)
It's a simple calculator, so it won't be as accurate as Martin's suggestions, but for seeing the impact of a particular change it should work fine.
Thanks, Charlie. I didn't realize that the Build It Solar calculator provided those outputs. Since it calculates the annual energy use, Nicholas should be all set.
I'm glad I wasn't a fool then and that I was using it for an average. I put in as many numbers I could get, narrowed down to the gnat's rear end. On my size of test home, a 30x40 with 8' walls and 2x6 framing I am coming up with a cost analysis of roughly $88 savings a year for a differential in two materials. For those interested I ran the surface area of my exterior wall (1120) with two options.
Option 1 was my initial idea, 2x6 wall 16" O/C, with R23 Mineral Wool. I calculated 1 stud per lineal foot of wall to include plates. This correlates to exactly 1 square foot of wood per foot of wall. I assumed a 2x6 is R6 (maybe a bit generous) and the mineral wool was R23. This gave me two surface areas. 980 sq ft @ R23 and 140 sq ft @ R6.
Option 2 was assuming same wall, with 2" foam exterior. I just eliminated the 140 R6 and assumed a total 1120 sq ft @ R30. This comes in at a minimum 17 year ROI if 2" foam is used.
FYI I did a -5 Design Outdoor Temp with 6657 HDD.
I have a feeling my main flaw is in R Value of lumber and potentially there is more surface area of wood per foot of wall? I do want to note, I would have rigid foam (PolyISO or XPS) sandwiched in my headers. It would only probably be 2" if (2) 2x6's + (1) 1/2" OSB piece or 2.5" if just between (2) 2x6's header pieces.
I also am running at 175% efficiency on the calculator. I read heat pumps are 200% but I also know some days it will not be enough so my 97-100% efficient toaster oven (furnace) will have to run instead. Should I drop my 175% efficiency even lower because of that? That's about the only other variable that should change.
Take a look at a free program from the US government BEopt
If you input the data the program will consider local weather data from the nearest airport, the size and orientation of every wall, window and door. If you enter your cost per square foot of several different R value walls along with the interest rate of your loan. The program will graph the options that use the most energy to least energy from left to right and each option vertically lowest monthly cost at the bottom.
It is safe to assume the wall with the highest R value will be on the right but it will also most likely to be at the top of the chart and will have a higher monthly cost than code built option on the left.
My guess is to make the chart in this image keeped someone computer at 100% CPU cycles for 10 or 20 hours. This graph is one I found with Google’s help.
Walta
Any GBA readers who are interested in BEopt may want to read my 2011 review. Here is the link: BEopt Software Has Been Released to the Public.
Of course, the review is out of date -- so some of the glitches I mentioned in my review may have been fixed by now.