Attic Insulation Payback Formula
drydem
was
looking at possibly increasing my attic insulation up to R76 - and I
wanted to know what was the payback (return on investment) - after
much surfing I found the formula below. HTH
The following webpage coves estimating
the payback for adding Insulation to the Attic
http://apps1.eere.energy.gov/consumer/your_home/insulation_airsealing/index.cfm/mytopic=11360
(incase the above webpage dissappears ....)
the equation is as follows:
Years to Payback = (C(i) × R(1) × R(2) × E) ÷ (C(e) × [R(2) -
R(1)] × HDD × 24)
To calculate the payback, you must supply the following information:
C(i) = Cost of insulation in $/square feet.
Collect insulation cost information;
include labor, equipment, and vapor barrier if needed.
C(e) = Cost of energy, expressed in $/Btu.
* To calculate the cost of energy, divide the actual price you pay
per gallon of oil, kilowatt-hour (kWh) of electricity, gallon of
propane, or therm (or per one hundred cubic feet [ccf]) of natural gas
by the Btu content per unit of fuel.
* To figure the price you pay per unit, take the total amount of your
bills (for oil, electricity, propane, or natural gas) during the
heating season, and divide it by the total number of gallons, kWh, or
therms you consumed during those months. Use the following values for
fuel Btu content:
Fuel Oil = 140,000 Btu/gallon
Electricity = 3,413 Btu/kWh
Propane = 91,600 Btu/gallon
Natural Gas = 103,000 Btu/ccf
or 100,000 Btu/therm
E = Efficiency of the heating system. For gas, propane, and fuel oil
systems this is the Annual Fuel Utilization Efficiency or AFUE.
Typical AFUE values are 0.6 to 0.88 for oil or propane furnaces, and
0.7 to 0.95 for natural gas furnaces. Older systems are usually less
efficient. Use E = 1.00 for baseboard electric systems. For heat
pumps, use the Coefficient of Performance or COP for E; where E = 2.1
to 2.5 for conventional heat pumps, and E = 3.2 to 3.5 for geothermal
heat pumps.
( an energy star non-geothermal HVAC is typically going to have an
E=2.5 )
R(1) = Initial R-value of section
R(2) = Final R-value of section
R(2) - R(1) = R-value of additional insulation being considered
HDD = Heating degree days/year. This information can usually be
obtained from your local weather station, utility, or oil dealer.
( e.g. for HDD for MD-DC-VA see [1])
24 = Multiplier used to convert heating degree days to heating hours
(24 hours/day).
Example:
Suppose that you want to know how many years it will take to recover
the cost of installing additional insulation in your attic. You are
planning to increase the level of insulation from R-19 (6-inch
fiberglass batts with moisture barrier on the warm side) to R-30 by
adding R-11 (3.5-inch unfaced fiberglass batts). You have a gas
furnace with an AFUE of 0.88. You also pay $0.87/therm for natural
gas. Let's also suppose that you supply the following values for the
variables in the formula.
C(i) = $0.18/square foot
C(e) = ($0.87/therm)÷(100,000 Btu/therm) = $0.0000087/Btu
E = 0.88
R(1) = 19
R(2) = 30
R(2) - R(1) = 11
HDD = 7000
By plugging the numbers into the formula, you obtain the years to
payback:
Years to Payback = (C(i) × R(1) × R(2) × E) ÷ (C(e) × [R(2) -
R(1)] × HDD × 24)
Years to Payback = (0.18 × 19 × 30 × 0.88) ÷ ($0.0000087 × 11 ×
7000 × 24)
90.288 ÷ 16.077 = 5.62 years
[1]HDD
district of columbia
http://www.efficientwindows.org/codes/DC-IECC.pdf
maryland
http://www.efficientwindows.org/codes/MD-IECC.pdf
Virginia
http://www.efficientwindows.org/codes/va-IECC.pdf
CJT
I don't see how the units work out right in that formula.
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Jim Wilkins
> Two years ago I upgraded my attic insulation to R38 --- recently I
> was
> looking at possibly increasing my attic insulation up to R76 - and I
> wanted to know what was the payback (return on investment) - after
> much surfing I found the formula below. HTH
>
> The following webpage coves estimating
> the payback for adding Insulation to the Attic
The infrared thermometer and snow melt pattern show that most of my
remaining attic heat loss is around the edges where I couldn't stuff
in more insulation due to restricted space and protruding roofing
nails. More insulation in the middle wouldn't help much if that isn't
where the larger losses occur.
ransley
> Two years ago I upgraded my attic insulation to R38 --- recently I
> was
> looking at possibly increasing my attic insulation up to R76 - and I
> wanted to know what was the payback (return on investment) - after
> much surfing I found the formula below. HTH
>
> The following webpage coves estimating
> the payback for adding Insulation to the Attic
>
Research how Fiberglass batts loose R value as temps drop. Depending
on where you live and winter lows you can loose 20-30% R value as the
attic gets colder. Also settling. I put in R 100-110 fiberglass batts
and it settled to R 70. Best is Foam. What is your Zone. Settling and
R value loss at -f temps increase payback.
Mark F
I assume that the R values that are used are the initial values.
Therefore, instead of using initial R(1) = 19, use R(1) = 14.
I'm not sure if R(2) - R(1) should be 11, the initial value of
the added insulation or 8, the expected final added value.
Eeyore
drydem wrote:
> Years to Payback = 5.62 years
Apologies for snippage.
Precisely what I've been saying for years.
Graham
drydem
wrote:
Yes. One of the reason I've looking at insulation
has been due to comments like those you have
made in the past. I realized that any home
power generation and/or storage system requires
a much more energy efficient house than the
current standard home.
For the past several years I've been looking at
methos to reducing my average monthly electric consumption.
I've been able to lower it to as low as 290 kwh/month
in the summer but the winter is still a challenge
(my highest monthly usage this year has been
almost 1080 kwh/month) and my average kwh/month
for the last year has been 560 kwh/month - which is
way too high for a 1.6Kw solar array ( the max array
size that I estimate could be install on the southwest
roof is -10*160w/PV panels). I suspect that in the
DC area - with a solar panel facing southwest at
a 45 degree fixed pitch - that such a system would
be about 25% efficiency - that is a 1.6Kwh PV array
would generated about an average of 400watts/hour
for 7 hours a day or 2.18w/h/day. The problem with
using 560kwh/month is that comes down to 18kwh/day!
My lowest usage 290kwh/month is equal to an
average of 9.3kwh/day - which while considered
low for an all-electric home in the DC Area -- is
still too high for any PV system. Thus the
the performance of current PV systems is still
too low to be installed on a townhome like
mine - so for now it looks like my best option is
to lowering my energy usage.
drydem
Interesting I will do some for investigating...
fyi...
About three years ago...I replaced the original losse blown insulation
in
the attic of my townhome because it had compacted and settled
down over time so it had lost much of its insulating qualities over
time.
I replaced it with a single layer R-38 fiberglass batting (with craft
paper facing
down) - at the time the recommended attic insulation for my area was
R-30 - so I thought was doing a more than adequate job. That was
the last time I was going to go up into the attic for the rest of my
lift
I said to myself (famous last words).... However, a few months
later (at the Solar Decathlon down at the Mall in Washington DC by
the Department of Energy Kiosk) I learn from a new DOE handout
that the recommended attic insulation for my area had been
raised to R-49! As of today, nobody makes an R-49 fiberglass
batting so that means putting down a second layer of R-11 fiberglass
batting on top of the current R-38 batts to get the attic insulation
up to
DOE recommended specs. One of the problems with Fiberglass batts
is that putting on a second layer would lead compression of the
lower batt and thus lower its insulation performance/qualities.
Hence, I thought maybe if I increased the second layer to R-38
that I could more than compensate for this effect ( or maybe
it would make things worst?) Surely - the DOE isn't going
to change the recommend attic insulation that much higher
in the future?
A friend of mine said forget that - get a stormdoor for my front door
instead - she said that was the weakest insulated point of my home.
It would be alot easier and cheaper...Yeah - I'm tempted to go
this route instead....
Well - each R-38 batt is 12 inches high - addiing an additional 12
inches
of fiberglass batting poses a problem in that the batting could block
the air from venting-circulating from the soffits/eaves at the edge of
the
roof up to the top of the roof. Drier cooler air needs channel from
the
bottom of the soffit/eave vents has to push more humid and hotter
air in the attic through the vents at the top the roof to prevent
ice damming outside of the roof and prevent condensation inside
the attic. Because the house was not designe for 24 inches of
attic insulation (it has only about 18-20 inches of clearance
for insulation at the edge of the roof line) soffit vent channels
would
be needed to secure the needed to allow air circulation ( maybe that's
why the DOE recommenndation is only for adding R-11 since
that would be about 4 inches more of fiberglass batting and hence
not require soffit venting channels to be installed) - for
a while I couldn't find a local hardware store that had them...
however,
just recently a found a home depot store that had them...
well - then I thought what would be the payback period? My local
electric
company said they had energy expertise I could ask - but it turns out
they were about as clueless as I was. So I surfed the net and
the initial formula that I posted was what I found. Using that
equation
upgrading the attic insulation from R-38 to R-76 for an all electric
house
townhouse in the DC Area would have a payback period of about 26
years
if the cost of electricity stays at the cost of 18 cents/kwh. Those
are
alot of ifs though. ...
drydem
Sound like a great idea! The farmer's almanac predicts
lotsa of snow this winter for the Mid-Atlanic area so maybe
I'll get a chance to do just that....
Eeyore
drydem wrote:
> Eeyore wrote:
> > drydem wrote:
> > > Years to Payback = 5.62 years
> >
> > Apologies for snippage.
> >
> > Precisely what I've been saying for years.
>
It's highly unlikely anyone will ever realistically heat a home in
average locations with PV solar.
Solar thermal might help more, but as ever you get the least when you
need it the most. AFAICS, typical US home construction doesn't lend
itself well to extra thermal insulation in walls either.
Graham