is there such a thing as too much insulation?

Is there such a thing as too much attic insulation? I have R38 in the attic at the moment, but the local building inspector says they like to see at least R50 in new homes. Would adding another layer of R38 (total R value would then be R76) perpendicular to the existing layer of fiberglass be overkill? Or will it make a difference? Does attic insulation really cut down on heat gain in the summer and heat loss in the winter? Or is there some other place to look to cut down on heat gain?
Thanks,
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Go for it. Did the same in mine, and I'm sure it helps. Don't have a vaopr barrier in the middle though. Buy unfaced.
Jason wrote:

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When I put in the first bat of R38 I checked and there is no vaper barrier anywhere. It goes something like this: ceiling wallboard (1/2 inch thick), ceiling joists (2x4), 2 -3 inches of probably rockwool insulation (at least 35 years old) between the joists, R38 bats of insulation laid perpendicular to the joists to cover them.
The first layer is unfaced and I was planning on purchasing unfaced again. Is the lack of a vaper barrier a problem?
Thanks,

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A good paint job on the ceiling below acts as a vapor barrier. Especially with a "vapor barrier" rated paint. Ask about it at a good paint store. HD _might_ carry such.
R38 is nothing to sneeze at. Going to R76 would be overkill methinks, and not going to make much difference over something slightly more modest. I have seen a modern house done to R70. Almost scary ;-)
If you're doing this for a sale, it might be worth it. If anybody notices.
A single layer of 3.5" (R16) would be easy, cheap, and get you to R54.
By the time you're at that level, other issues will end up being the biggest losses.
A house that started with 3-4" of rockwool (about R16.5 with modern rockwool, probably somewhat less with older stuff) is going to need attention to other things before going to R76.
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Chris Lewis, Una confibula non set est
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Yes, I went from no insulation to an attic full of it. It made a huge and immediate difference in the winter.
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On 14 Jul 2006 19:05:20 -0700, snipped-for-privacy@gmail.com wrote:

I went from damaged/settled original r30 to r60(layed r30, unfaced batts), and my bills are low enough to brag to my friends about. They really don't want to work with fiberglass, but today's fuel costs will change that. ;)
later,
tom @ www.Consolidated-Loans.info
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Jason wrote:

That depends. If you aren't concerned with cost, then no. However, since there are diminishing returns, at some point, adding insulation beyond a certain point is not cost effective. That point is dependent on what future energy costs are.

Yes.
If the only insulation is in the attic, then all you are doing is losing heat/cool through the walls and basement. As well, too much airflow through the walls, doors, windows etc makes the attic insulation ineffective.
The optimum is to balance it all out. You have to ensure that you have insulation in the walls and basement as well. Your windows and doors should not leak air excessively. Ditto walls and such. If you restrict air flow too much, you should then look at a heat recovery ventilator to ensure proper ventilation of living space without heat/cool losses.
In general, retrofitting an older house to the max is very expensive and not always worth the trouble. Sometimes something as simple as a few tubes of acoustic caulking is very cost effective - close up the biggest air leaks. You have to investigate all the options and decide when the cost exceeds the benefit.
Mike
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Jason wrote:

Doing a heatloss calc is not that difficult. I would say that r76 is overkill, but what if oil hits 5 bucks a gallon?
in general, with good windows, you still lose half your heat out the windows, more or less. Around half the remaining goes out the roof[both because of square footage and heat rising]
Look up heat loss calcs on the web, it is an interesting exercise if nothing else.
You can estimate pretty damn close
My blind screaming 2 minute guess on a 20x40 little ranch with walls/window/ceiling of r19/3/38 loses over 16 percent of its heat through the roof, not counting differences in heat movement, so that means the most you could gain would be $111/1000 spent heating, and probably more like half that. But if oil went from 2.50 to 7.50 a gallon......
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Is it overkill, well my R110 attic was worth every penny. In zone 5, I had a yearly Ng bill of 460 for 1800sq a few years ago. Mine has settled to R70 it is fiberglass bats. If you are zone 8 or less R 70 + is a good idea. Standards are outdated, insulation is cheap compared to Oil. I had a load calc done , 1800sq ft and only 50000 btu.
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Jason wrote:

The more the better. As noted you should not add a vapor barrier. After so much additional there are diminishing returns, but it would appear for the comments of the building inspector that you live in an area where the extra would be a good idea.
I don't know exactly what I have, but I know I doubled what the builder put in 12 years ago.
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Joseph Meehan wrote:

Read an article today on blown in foam insulation. 90% of benefit is covered in the basic application (i.e. r38 for attics), adding 50% more gets an extra 5% of benefit, adding another 50% more gets an extra 2.5% of benefit.....
Plus there are articles a plenty that describe what happen INSIDE fiberglass insulation in cold climates. R76 drops to R25!!!!
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