R11 or R13, or does it matter?

[saichele]

so I went to my local bigbox to buy insulation, and I can get r11 or r13 fiberglass for 2x4 framing. Need about 400 sq ft (doing all 4 walls).

The r11 is 23c/sq. ft, the r13 is 33c. So a diff. of $40. Both unfaced, both 15.5 in wide (for 16" framing).

It's an older house, and we're stepping up from r0. Someone give me an argument why the extra r2 (not even that, since it;s non-linear) is worth 50% more.

Steve

 

[saichele]

Short, concise, and weighs the pros and cons of each approach.

Good response.

Steve

 

[Michael6268]

What part of the country do you live in? What type of room is it going in? If you are in a cold (or hot) climate you should go with at least the R-13. They make a R-15 for 2x4 framing too, but it is a little pricey. Should pay for itself down the road though....

 

[elkimmeg]

$40 and getting tax credits? If one compares R11 and R13 R11 looks chinzy. It looks more like 2.5 " thick The reason it is cheaper. R11 today will not satisfy any energy code. I am supprised it is even for sale. But you made a valid point R11 beats R-0. Its your call and wallet

 

[wg_bent]

Well, I've got a basement project going where I already have R10 foam and I'll add the r13 in the studs. The room will nice an comfortable when done!

 

[saichele]

So this wasn;t exactly going as I had intended...

I found the appropriate reference information, including a couple handy equations at a website (http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/heatloss.html#c1) and that I can expect 7200 base65 heating degree days in my location.

A couple minutes in Excel revealed that in it's current shape, my 10x15 room with 2- 8 sq.ft.windows, no wall insulation and R50 inthe attic would take a whopping 22.5 million BTUs to maintain 65 all winter.

Put R11 in the walls, and it drops to 7.3mill, R13 takes it to 6.7mill BTUs/yr.

From Hearthnet, I can tell 1 MBTU's of electric is $32, 1 MBTU gas is $17, and 1MBTU pellet/corn (the eventual solution) is $12.

The difference I'm interested in is 0.6 MBTU, so $19.20/yr electric, 10.20/yr gas, and 7.20 pellet/corn. Over a 5 yr horizon, the first year much of the heat inthat room will come from electric, the remaining 4 probably from pellet/corn. So 19+(4*7) = $47

Which means the difference between the R11 and R13 probably is worthwhile, and that the difference of 16 MBTU annually makes the cost of the remodel pretty trivial as well - savings of $500 in that first year of electric heat. More like $200 once we switch to pellet/corn.

Steve

 

[Rhone]

I'm bummed I haven't been around for a few months or so this stuff is right up my alley! I have all sorts of this stuff.

Your savings/year are going to be different than quoted, your numbers appear to be very conservative. The $32/MBTU's electric, it costs me $57.89/MBTU's right now where I live. Pellet prices are all over the place depending where you live, where I live today pellets are $250/ton from what a friend just told me, that's $18.38/MBTU. My program includes heating & cooling degree days so you can tell how much electricity you'll save keeping it cool in summer as well. Choosing to go with R13 instead of R11 for someone in a 7200 heating and 450 cooling, and 400 sq ft will save you 966,713 BTU's/year in heating and 60,420 btu's/year cooling. That translates into

Electricity at $0.19757/kw: $55.96 heating + $3.50 cooling = $59.46/year
Oil at $2.69/gal and 75% efficiency: $24.77 heating + $3.50 cooling = $28.26/year savings
Pellets at $250/ton & 85% efficiency: $17.77 heating + $3.50 cooling = $21.27/year savings
Wood at $185/cord & 70% efficiency: $9.46 heating + $3.50 cooling = $12.96/year savings

So, the R13 is the wiser choice but you already figured that out! It should pay for itself within 2 years except for wood, there's a federal tax credit for insulating the envelope of your house of 20%, I think it's still active and 20% if you live in the USA so going to R13 over R11 after the credits are factored the difference is actually $32 in the end instead of $40. Also, one can't account for the savings of wear & tear on equipment that doesn't have to work as hard because you have more insulation. Interesting about going to R50 in the attic. It would take me 17 years to recoup the price increase of going to R50 instead of R38 in my attic. That's a lot less than when I ran it a few years ago, it told me it would take 29 years so I opted to stop at R38.

Since you have the info in Excel, for some fun calculate the total square feet of your windows and their R-Value. Most likely, if they're windows & storms, or double-pane they're around R2. If new high-efficiency replacement windows with Low-E and Argon they're around R3. Then, determine the effect of adding a plastic film over them for winter which increases their R-Value by about R1. Amazing isn't it? Watch that the tape doesn't ruin your trim/finish. Next, run it for adding R 2.5 insulating blinds instead.

 

[saichele]

Hey Rhone - Nice to see you back.

You're paying the "joy of New England" premium. (I think I remember you were from NE). Anyway, Electricity is $0.102 here, wood about $135, and gas $1.04/ccf. Pellets are 205/ton (for now).

I catually did the windows, and they're bad. After insulating I'll lose more through 16 sf of glass than through 184 sf of wall. The R50 was also about ease and timing - we had the machine, and the trailer full of cellulose, and a babysitter. So I blew about 65 bags into the attic because that was what I could get home. We were coming up from nothing, and frankly I don't want to do it again. Shouldn;t have too.

Next we can start the old convective/radiant debate.

Steve

 

[Sandor]

Nice to see you analyze again Rhone.

Rather than beat the numbers to death, I choose to go full bore whenever you cannot go back and do it again easily.

So, I would order R15 and go that way. God only know what a therm will cost in another 10 years; whether its electric, gas, oil or wood.