Last fall we had insulation added to our attic. I thought it would be worthwhile to see what
kind of improvement in our heating bill this made and share that with people who might
be wondering the same thing. The insulation is blown-in cellulose, and
the installer added 4"-5" (100mm-125mm) thickness over what was already existing. This was done in the top floor of our house,
which is a tri-level. You can see in the other attic space on the
lower level what the insulation used to look like. I know there's a lot of junk up here, but
that's because I'm in the middle of some work in the attic. Our house was builtin 1976, and the energy
efficiency standards weren't what they are now. There is loose fiberglass insulation filling
in between the 2x6 joists, and at one point the old owners added another layer of 3-1/2"
(90 mmm) thick fiberglass batts, the type you would use in a 2x4 wall. Figure the attic has an R-value of maybe 28,
though likely a lot less because the fiberglass was old and kind of packed down. The addition of the cellulose added about
R-18 to the attic, so theoretically I'm at R-46 in the upstairs attic. Anyway, let's take a look at the numbers and
see how the heating was impacted. Since we have an ecobee thermostat, I have
access to detailed thermostat data going back 15 months, so I'll download some data here. To give a reasonably close comparison, I'm
looking at data from December 2020 through February 2021 against the data from those
same months a year later, after the insulation was installed. Before getting into the numbers, it's worth
discussing how heat is transferred through an attic. This is very much simplified, and ignores
a lot of things like air leakage and convective heat loss, but it will be good enough for
talking purposes. The general equation for conductive heat loss
through a surface is given on-screen. The Area (A) of the space has not changed,
so that's a constant. The temperature difference (delta T) can be
determined from the thermostat data. It's difficult to impossible to figure out
the heat transfer coefficient (U) given all of the confounding factors, but it's safe
to say that this value has gone down with the addition of the new insulation. The operating assumption here is that the
heat transfer (q) is directly proportionate to the furnace runtime - that is to say, the
more heat leaving the house, the more the furnace has to run to keep up. Since the ecobee app only allows data to be
downloaded in 31-day long chunks, I had to combine the data from the three months into
one sheet. I'm really only interested in three columns
of data here, the outdoor temperature, the master bedroom temperature, and furnace runtime. As such, I hide the columns with extraneous
data. The first data point I want to find is the
temperature difference (delta T) between the bedroom and outside. To get an idea of the heat loss over time,
I multiply the temperature difference by the time step, which is 5 minutes for this data. I had to make some adjustments in my formula
to account for the time rolling overnight, which was giving me some negative values. Next to sum the delta-T minutes and the furnace
runtime. I did this for both sets of data, and then
summarized each on this sheet for comparison. The idea here is that the delta-T minutes
divided by the furnace runtime will give me a number representing the heating efficiency
of my house. Comparing these two results from each year,
it looks like adding the insulation provided about a 9% increase in heating efficiency,
year over year. To give an idea of what this looks like in
dollars, I downloaded some more data from my natural gas utility showing my therm usage
per day. This usage isn't totally for the furnace as
our water heater and stove are both gas, but they use very little gas compared to the furnace. Summing the data over the last three months,
we spent $410 on gas heating the house. Assuming our efficiency increased 9%, that's
a savings of about $39 over the three months. This may not seem like a lot compared to the
cost of the insulation, but bear in mind that the insulation will also reduce our cooling
bills in the summer, and the benefits will continue to accrue through the life of the
house. There's undoubtedly a lot more we could do
to help with heat loss, like sealing for air leaks and that sort of thing. However, I'd say that this was a success. I know this was a very simplified analysis
of the heat transfer through my attic, but I think it's good enough to illustrate the
difference that the insulation made. Anyway, I hope this was helpful if you're
thinking of adding some insulation to your attic. Don't forget to Like and Subscribe, and I'll
see you next time.
