attic insulation R30 to R60 cost effecitve?


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Old 01-16-12, 09:04 AM
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attic insulation R30 to R60 cost effecitve?

HI Folks,

This is my first post here, thanks for having me. I wanted to reply to this thread but I was not permitted to do so because that thread was too old.

I live in the marine climate of the Pacific Northwest region. My 3300 sq ft home was built in 2006 but I am the second owner. The attic has about R30 insulation. I am considering whether or not its cost effective to insulate to R50 or R60 for the cost of about $1000 (blown cellulose)

I have had a blower door test performed and will also perform some air leak sealing. The basement is finished and crawlspace insulated. The results of the blower door test are not detailed enough to my liking however the gov't of Canada will rebate me about $200 if I do do this upgrade.

There is not much more I can do accept insulate the attic. At times our upper floor does feel cooler than downstairs. I'm hoping to save 10% in natural gas per year- but not sure if that is realistic as I have been googling a lot about the diminishing returns of adding insulation.

Our total heating bill for living space is about $500. I'm hoping that adding R60 might save me $80 per year but that is looking doubtful.

I welcome your comments.

Thanks,

David
 
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Old 01-16-12, 09:59 AM
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Hi David and welcome to the forum. The folks who did the energy audit have that information in detail, but often don't share. The details needed are the various heat loss numbers throughout the house. For a peak at just the ceiling you can calculate an estimated annual loss with r-30 and then with r-60 (one half). Heat loss = U A (HDD x 24) where U is 1/R A is the 3,300 sq ft and HDD is the Heating Degree Days where you live. There are other variables, but this will give you a number for energy lost. If we use 4,000 HDD, the r-30 gives us 10.6 million BTU's. If we estimate the gas at 1,000 BTUs per cubic ft we get 1,000 cubic ft of gas used to heat just that ceiling. Now, we don't use natural gas in my neighborhood so I'm a bit rusty on how you buy it, but adding another r-30 to what is there will cut that number in half. Savings 500 cubic feet of gas per year or 50 therms, however you meter it. Now, I may have slipped a link in there somewhere, but with your HDD and fuel cost per whatever units we/you can use the equation above to answer your question.

Bud
 
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Old 01-16-12, 10:41 AM
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Thanks Bud, I ran some numbers and will post some values below:

I pay about $13 per GJ
1.055 GJ = ~ 1 x 10^6 BTU
HDD for me average of 25 years looks to be about ~2850

UxAxHDDx24x1.055 GJ/10^6 BTU = (1/30)(3300)(2850)(24)(1.055)= 7.9 GJ x $13=$103 this number seems low but then again you are talking energy LOST so we are calculating $lost RIGHT?

So adding to R60 would save $50 per year it looks, or about 10% of my space heating bill making this project ($1000) about a 20 year payback. ahrg. seems border line to me...

Should the calculation really use 3300 sq ft if we are talking only energy lost through the ceiling?
 
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Old 01-16-12, 12:00 PM
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If your home is a two story, then use the sq ft for the attic, I assumed without asking a big ranch Duhh. That would cut that number in half if ceiling area is half and as we often encounter, the savings above a certain point are minimal. I have to run, but will check tonight if you need more. Do you have the option to improve elsewhere, ie air sealing, if not already done, there could be more for less there.

Bud
 
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Old 01-16-12, 03:30 PM
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My home is 2 story + basement. The upper floor/attic area is ~1000 sq ft. Therefore if one runs the calcs you provided the heat loss $ value is $34 at R30 and $15 at R60. Doesn't appear to be a winning proposition- but somehow intuition tells me it should actually be better- perhaps not.

I do need to do some airsealing. One attic insulator said no need to airseal in the attic (as Gov't Canada recommends to do) as the cellulose will take care of it...

Its mind boggling how much myth and misinformation there appears to be in building non-science...
 
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Old 01-18-12, 02:29 PM
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Just wondering if I've done these calc's correct and that doubling the insulation will provide such poor savings?

Any additional opinions?
 
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Old 01-18-12, 03:20 PM
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Math is close, the initial number at 30 is about $31 which agrees with the $15 and change at 60. The 60 number should be exactly half the 30 number.
As for a reality check, these numbers would actually be rather close to what it costs to heat that ceiling area. The only fudge factor might be to increase the HDD to account for increased temperatures on the ceiling, but it would be minor, like $3 or $4 extra savings.

Here is a small calculator that uses the same equation and lets you fill in walls, floor and other areas to account for all heat loss. When you get it working and the number agrees with your total real world use it adds confidence to a single calculation. I've done this many times so my confidence is high. Home Heat Loss Calculator

Remember, r-30 is great insulation, and r-60 is over the hill. Another way to estimate the savings is a bit crude, but there are 5 loss areas, windows and doors, walls, ceilings, floors and foundation, and air leakage. Assign 20% to each and you get $100 for each area. Cut that is half with the extra insulation and you save $50 per year. Except your ceiling is already less than half the total, so take say 40% of the $50 and you save $20 for the extra insulation. Crude, but a reality check that shows the above calculation is in the ball park.

Bud
 
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Old 01-26-12, 06:12 PM
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Old 01-27-12, 08:14 AM
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Thanks GBR. At a cost of $1000 for a contractor to add insulation this looks like a dead deal. The savings don't add up.

However, after checking at home depot the cost of adding insulation myself- the materials are about $275 include use of the blower.

Of course I am sure this will take me and a buddy a whole day whereas the contractor would probably be 3h or less.

For a cost of $275 the savings are actually a worthwhile %age. But I'm not sure if I'd screw things up in the attic all the more doing it.

What are the top ten mistakes for blowing your own insulation?
 
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Old 10-08-13, 11:11 AM
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First thing you've gotta do, David, is get your numbers right. You said originally you have a 3300 sq ft house.........(Remember, basements don't count when you're calculating sq ftage of a home.) ....then later you said your second story attic is 1000 sq ft. So, does that mean you have 2000 more sq ft on the first floor (with its own attic) than you have on the second floor? (Remember, basements don't count when you're calculating sq ftage of a home. So that means if your second story has a 1000 sqft attic........AND you in fact have a 3300 sq ft house...that means you have another 2000 sq ft of above-ground living space.
Do you? (Remember, basements don't count when you're calculating sq ftage of a home.)
 
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Old 10-08-13, 11:23 AM
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David hasn't been back since his last post and since the thread's almost 2 years old, I'm sure he's long since finished the job.
 
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Old 10-09-13, 08:32 AM
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Hi Folks,

I just wanted to thank again the kind members at this site for their willingness to help those of us that drop by from time to time with just a question or two. It is very generous of you folks to share your knowledge freely when you must get asked the same things many times over. Thanks to you guys for your generosity and willing spirit and kindness in doing so.

I chose not to move forward with this project at the moment. The time and $ didn't appear to be a significant motivating factor at the moment.
 
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Old 10-09-13, 05:09 PM
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You're welcome, David, and thank you for posting a compliment! Let us know if needing anything else; in answers that is, lol.

Gary
 
 

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