How much insulation should we buy?

February 10, 2010 9:42 AM

At what point is adding more insulation R-value overkill?

My partner and I are going to use our tax refund to bring our attic insulation R-value up. The new guidelines for our area recommend that our attic insulation have an r-value of 49.
We've gotten two bids for the job. One of the bidders, the one who seemed more observant and on top of things, offered to bring our r-value up to 60 for an additional $300.
Is it worth it to spend the extra cash? Or is an r-value of 60 overkill? Would it be better to keep that money in our fund to replace our antique gas furnace?

posted by pickypicky to home & garden (17 answers total)

From an HVAC designer standpoint, R-49 is great. I would be happy if my attic was that well insulated! While going up to R-60 would save some additional energy, I would be more inclined to save the money for a high-efficiency furnace. If your furnace is older and not in great shape, you can have your contractor give you a quote on a furnace "tune-up" for starters, if you won't be replacing it for awhile.

Assuming you are in the US, don't forget to check into the tax credits for energy efficiency projects.
posted by beandip at 9:56 AM on February 10, 2010

If your furnace is really old, it's probably better to put the money into replacing it. The difference in efficiency between a good modern furnace and an older one can be dramatic. Is your "antique" furnace a forced-air model, or a (really old) gravity type?
posted by jon1270 at 10:01 AM on February 10, 2010

Would it be better to keep that money in our fund to replace our antique gas furnace?

Yes. There have been very significant advances in efficiency for home HVAC systems in the past 10 years. Depending on where you live, you might even get that $300 back in a year or less from reduced utility bills.
posted by Burhanistan at 10:01 AM on February 10, 2010

I only have an n=1 contribution, but our energy bill decreased $97 (with roughly the same number of heating degree days) in January 2010 compared to January 2009. The only change was a new 95% AFUE furnace to replace one from 1991, which hardly qualifies as ancient. I think you're better off replacing the furnace than increasing your insulation from R-49 to R-60, and I agree that it'll pay for itself pretty quickly.
posted by brozek at 10:08 AM on February 10, 2010

Well, let's see.

R = dT / q

$/time = ($/energy) * (dT / R) * A

where dT is the temperature differential across the layer, and q is the heat loss per square meter per time.

If you're in the US, then an R value of 49 is 49 ft^2 * deg fahrenheit * hour / Btu

If you're elsewhere, it's probably m^2 * deg kelvin / watt

Let's assume you're in the US. If we assume a winter temp. differential in the attic of 40 deg. F, and an attic area of 3000 square feet, and a heating cost of $1.3 per Therm, then your heat loss through the attic amounts to:

R = 49: $23 / month

R = 60: $35 / month

I.e. if my assumptions were correct about your situation (they probably aren't), it would take around 2 years of 40 degree differences to recoup the $300 installation cost. Of course, if the temp differences are higher in the summer or winter than 40 deg Fahrenheit, or if your heat is more expensive, then you'll recoup the cost faster.

Then there are also the environmental costs that the R-value of your attic incurs on society that are much harder to calculate. But I'll try. My calculations indicate (using the same assumptions, including that you heat your house with natural gas, which is the most carbon-efficient fossil fuel) that:

R = 49: 95 kg CO2 / month

R = 60: 78 kg CO2 / month

I would do it.

If you want to post your approx. location or climate, roof area, house heating/cooling method, and the price of that heating method, I'll recalculate those numbers.
posted by Salvor Hardin at 10:19 AM on February 10, 2010 [3 favorites]

To calculate: R value of 49 = U factor of 1/49 = .0204. R factor of 60 = U factor of 1/60 = .0167. If you have 1000 square feet of attic space, and a temperature in the attic of 25 degrees, inside temperature of 68, the heat loss through the insulation with R49 will be .0204 x (68-25) x 1000 x 24 hours = 210,528 BTU per day. With R60, it's .0167 x (68-25) x 1000 x 24 hours = 172,344. The difference, 38,184, is the amount of BTUs saved on a 30 degree-day day (65-25). If your area experiences about 7000 degree days (typical midwest or inland northeast US), that means your seasonal savings (moving from R49 to R60) are (7000/30) x 38,124= 8,895,600 BTUs. Now, assume that your crappy gas furnace has a relatively poor efficiency of 70% (a new unit would be more like 95%), that means you'll burn 8,895,600/.70 = 12,707,999 BTUs worth of gas, which is 12,707,999/102000 = 124 CCF of gas. You can check your bill to see what you pay per CCF or therm. Typical rate might by $.85, so your incremental savings moving from R49 to R60 = 124 x .85= $99.20 per year. Obviously, adjust for attic area, local degree days, gas price and actual furnace efficiency, if known. But based on these numbers, it's a three-year payback.

By comparison, replacing your furnace and moving from 70% to 95% efficiency would mean that instead of losing 30% of your fuel cost up the chimney, you lose only 5%, and represents an improvement of (95-70)/70 = 35.7 percent off your fuel bill. You can figure out the payback period from that.
posted by beagle at 10:20 AM on February 10, 2010 [2 favorites]

Oh, I missed the part about the "antique gas furnace". If you have to choose between updating the furnace and upping the attic R-value to 60, I'd probably replace the furnace. As others have said, old furnaces are way less efficient.
posted by Salvor Hardin at 10:21 AM on February 10, 2010 [1 favorite]

Salvor, you beat me to it.
posted by beagle at 10:23 AM on February 10, 2010

One of the reasons houses in the U.S. are not as well insulated as they could be is that it is a long-term investment: that insulation might be up there for 50 years or longer. If the extra $300 would get you a new furnace, get a new furnace. But, R60 is probably the maximum for your roof; it would be worth it to me to do the extra insulating. It will pay for itself, it will just take a little while and I would have the satisfaction of knowing I did the best job possible.
posted by ennui.bz at 10:23 AM on February 10, 2010

We live in Colorado, and temperatures really vary from winter to winter. Last winter hardly had any snow at all, while this winter we've spent days snowed in with subzero temperatures.
I don't know the area of the roof, but the attic is around 1300 square feet. It's an asphalt roof pitched over it at a pretty standard angle.
$300 wouldn't make or break a furnace purchase, since we've been saving for it pretty aggressively since we bought the house. The furnace is forced air, and original to the house (1959!), so we've been making plans for the day it gives up the ghost. Still, as new homeowners it's hard for us to tell when to invest in certain upgrades, and when it is better to focus our resources elsewhere.
posted by pickypicky at 10:50 AM on February 10, 2010

PS we don't have air conditioning in our house, because at our elevation we wouldn't use it enough to make it worth expense of purchasing it.
posted by pickypicky at 10:52 AM on February 10, 2010

The furnace is forced air, and original to the house (1959!), so we've been making plans for the day it gives up the ghost.

Oh, man. Replace that sucker ASAP. It might be mechanically sound for years to come, and in the mean time you could have bought all kinds of stuff with the money you would have saved.
posted by Burhanistan at 10:53 AM on February 10, 2010 [1 favorite]

R = 49: $23 / month

R = 60: $35 / month

Something is flipped there, Salvor. R60 should cost less. Using your number to calc the month, I get (((40/49) x 3000 x 24 hrs x 30 days)/102000 BTU/therm)x $1.3 per therm= $22.47 for R49, and $18.35 for R60, a savings of $4.12 for 3000 square feet in a 1200 degree-day month.

Correcting my own calc, I should have said:
...The difference, 38,184, is the amount of BTUs saved on a 40 degree-day day (65-25). If your area experiences about 7000 degree days (typical midwest or inland northeast US), that means your seasonal savings (moving from R49 to R60) are (7000/40) x 38,124= 6,671,700 BTUs. Now, assume that your crappy gas furnace has a relatively poor efficiency of 70% (a new unit would be more like 95%), that means you'll burn 6,671,700/.70 = 9,531,000 BTUs worth of gas, which is 9,531,000/102000 = 93.4 CCF of gas (saved per year)...

And on preview,
- it's the attic area, not the roof area, that counts (assuming a completely unheated attic).
- Colorado heating degree days vary widely; find yours here.
- based on my calcs, your savings for the 1300 sqft attic would be $121.42. Adjust that for actual degree days relative to 7000. (So, 7700 would be 10 percent more, 8400 would be 20% more, etc.)

It does sound like you'll have a decent payback for the extra $300, I would do it. But then, go to the bank, get a home eq loan, and replace that furnace, since you'll probably cut out at least 1/3 of your heating bill.
posted by beagle at 10:56 AM on February 10, 2010 [2 favorites]

Dammit, what is wrong with me. Thanks for catching that beagle. Fixing my previous math...

R = 49: $23 / month (through the attic roof)

R = 60: $18 / month (through the attic roof)

Grumble stupid arithmetic error.
posted by Salvor Hardin at 11:14 AM on February 10, 2010 [1 favorite]

How much insulation do you have up there now?

If it was a question of R49 -> R60 vs New Furnace, yeah, that's easy, New Furnace wins.

But it's really a question of R?? -> R49 or R60 vs New Furnace.
posted by dalesd at 3:07 PM on February 10, 2010

If you have forced air, maybe both up the insulation and upgrade equipment. I do not have forced air, so my house is insulated OK, but I know it's a bit leaky. I keep it that way because, while less efficient, it's a source of outside (fresh) air. Plus, it's hard to measure efficiencies or inefficiencies with wood heat.
posted by Danf at 3:12 PM on February 10, 2010

If you're going to replace the furnace anyway, spend the extra $300 on better insulation now. The fuel you waste by delaying furnace replacement for an extra few weeks will be more than offset by what the extra insulation saves you over its service life.

In other words: the choice is not better insulation vs. new furnace (which would indeed make the new furnace win). It's (a) good insulation and a new furnace soon vs. (b) better insulation and an even newer furnace a little later.

Only during a small window between "soon" and "a little later" does the new furnace's increased efficiency have a chance to lower the fuel consumption cost of (a) compared to (b). Better insulation has its entire service life to lower the fuel cost of (b) compared to (a), and will surely lower it by more than its $300 upfront cost.

In my opinion, (b) wins.
posted by flabdablet at 4:50 PM on February 10, 2010