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Old 10-18-2013, 05:31 PM   #1
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Improving attic insulation: remove old fiberglass or just add cellulose/fiberglass?


I have a mid-1940's house with a very simple roof structure. It's basically a Cape Cod with no upstairs.

The attic has 5-6" of fiberglass insulation. The first layer is very old, perhaps original to the house. Whenever I've had to do work in the attic that involved disturbing the original layer of insulation, it has proved to be very crumbly and clumpy.

The top layer looks newer, perhaps 1970's or 80's, and claims to be R11/3.5". Both layers are kraft-faced and the top layer has the kraft paper on top, which I realize is improper installation. Actually just two faced layers is improper, as I understand it. The insulation has largely settled to meet the height of the top of the 2x6 ceiling joists (5.5" high).

I'd like to improve the attic insulation before winter. Clearly, in places where I've moved the existing insulation (I've run wires and added some recessed lights), that first layer has basically been ruined. It gets clumpy and falls apart, then trying to get it back into place makes it quite compressed. This seems to leave a lot of voids closest to the conditioned space.

My plan has been to
  1. Roll up the top layer of fiberglass and save it
  2. Take out and dispose of the lower layer of fiberglass
  3. Go through the attic and seal obvious air leaks with Great Stuff and caulk
  4. Blow in cellulose up to the top of the joists
  5. Put the saved fiberglass on top of the cellulose, running perpendicular to the joists to block some of the thermal bridging through the joists

This makes sense to me, though I realize that the standard advice when adding attic insulation is just to put it right on top of whatever's there.

The easiest thing to do would be to simply add more fiberglass batts. I'm sold on the benefits of cellulose (especially in cooler climates), though.

I calculate that the cellulose + fiberglass combo will total about R30, assuming the newer layer of old fiberglass still maintains its R11 rating (is this reasonable to assume?). Ultimately, I would prefer to have a higher R-value, but I also want to preserve reasonable access to the ceiling from above, since I'm still (slowly) remodeling the house and have been replacing and adding wiring and light fixtures.

My questions are as follows:
  1. Am I insane for thinking that the old, clumpy fiberglass needs to be removed? It's worst in areas where I've had to move it. I believe that in other areas it's in better shape, but it seems problematic in about 30% of the attic now.
  2. From what I understand, cellulose forms a fairly decent air barrier when installed correctly. Doesn't it make sense to put that barrier closest to the conditioned space, rather than on top of the old fiberglass?
  3. Will the standard fire resistant level of borate also provide enough insect/pest protection, or should I opt for the "pest control" cellulose? I have not had any problems with rodents and only minor insect issues.


Last edited by DrSteveBrule; 10-18-2013 at 05:35 PM.
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Old 10-18-2013, 05:56 PM   #2
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Improving attic insulation: remove old fiberglass or just add cellulose/fiberglass?


Toss the old junk and start out new with Cellulose. Have it blown in a deep (12"-16"?) as possible without interfering with air circulation out your roof vents. You are not paying for the insualtion, but the set-up and a little extra material. In your climate extra fluff in the ceiling is a cheap investment.

The old fiber glass is probably dirty and not really effective.

Dick

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Old 10-18-2013, 10:41 PM   #3
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Improving attic insulation: remove old fiberglass or just add cellulose/fiberglass?


Thanks for the reply.

I'm actually hoping to finish this project over the weekend, so I've been in and out of the attic over the past few hours.

The bottom layer of fiberglass is compressed to around 2", but in areas where I haven't been mucking around with electrical/remodeling, it still seems to be in otherwise reasonable shape. That said, I have been taking it out.

I know fiberglass insulates worse in walls when compressed. Is the same true when it's lying flat in an attic installation? I would assume so.

There's no access to the attic from the outside, so it's looking like a pretty big job just to take out half of what's there. Everything has to be carried through the house in garbage bags. I really can't see removing all of it, so I still plan to keep the newer fiberglass and layer it on top of the cellulose.

I feel like I'm kind of wasting this opportunity if I don't put down more cellulose, but I'm still thinking that I'd only blow in enough cellulose to match the height of the joists. In some parts of the attic, I don't anticipate needing to access the ceiling from above and could do more. I'll see if I can add to the joists in places to hold back additional cellulose from the areas I still need to access.
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Old 10-18-2013, 11:07 PM   #4
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Improving attic insulation: remove old fiberglass or just add cellulose/fiberglass?


Dont forget to staple in you air vents . I was really happy after I completed this project at my house instant gratification. YOU WILL FEEL THE DIFFERENCE.
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Old 10-19-2013, 06:21 AM   #5
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Improving attic insulation: remove old fiberglass or just add cellulose/fiberglass?


6" would be about 1/2 of the insulation needed in your area.
http://www.energystar.gov/index.cfm?...sulation_table
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Old 10-19-2013, 09:28 AM   #6
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Improving attic insulation: remove old fiberglass or just add cellulose/fiberglass?


i had old FG insulation in my attic. i removed it, put it in box's. i removed the paper as i did so. after i finished my ceiling work, i took temps of the ceiling from around the house.
i then put the old insulation back on the ceiling. it did in fact insulate, as the temps of the ceiling rose. i then blew cellulose in over that. i am very happy with it, and i didn't even put enough (yet)
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Old 10-19-2013, 10:03 AM   #7
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Improving attic insulation: remove old fiberglass or just add cellulose/fiberglass?


If you have a consistent application of an older material, going over top is not bad.

If you have a mix of a whole bunch of different crap, I usually recommend yanking it out.
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Old 10-19-2013, 11:13 AM   #8
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Improving attic insulation: remove old fiberglass or just add cellulose/fiberglass?


Going over the top is good and necessary because it cuts off the "thermal short-circuit" cause by the wood joists. Heat loss follows the easiest direct path and joists or studs are where much happens if you want to get the money out of your insulation $$. - That is why you never will get the fictitious R-value out of the insulation.

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Old 10-19-2013, 02:26 PM   #9
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Improving attic insulation: remove old fiberglass or just add cellulose/fiberglass?


IMHO, leave the old insulation in place, carefully lifting sections where the wiring/plumbing holes are to canned foam air seal them first. Compressed R-11 is divided by 3-1/2" = R-3.14 per inch (low density- useless at slowing air movement); used to be "the biggest loser";The "biggest loser" in fiberglass insulation....

Now it is R-19, another story, lol. Compressed to 2" thick your low density (R-11) has now become a medium density (a good thing) at R-7.55 or from R-3.14 to R-3.78! Congratulations!http://www.greenbuildingadvisor.com/...fiberglass.JPG It will stop air better because of the increased density but the drywall/plaster is still the air barrier in contact with your thermal barrier insulation. Move the material you don't want/need rather than exit the attic by sliding it together in a close puzzle piece fit in same bays to omit lifting/breakage. If able financially (I start all my personal projects with that) add more than to the joist tops, meeting/exceeding minimum code for your area; Ramsey county- R-49; http://energycode.pnl.gov/EnergyCode...tate=Minnesota

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Old 10-19-2013, 04:21 PM   #10
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Improving attic insulation: remove old fiberglass or just add cellulose/fiberglass?


Quote:
Originally Posted by Windows on Wash View Post
If you have a consistent application of an older material, going over top is not bad.
Well, I've already removed the worst of the original fiberglass. As I've said, it had gotten fairly inconsistent in areas where I'd had to move it for lighting/electrical work. There were also rows of blocking between joists where the batts were just laid on top of the blocking, creating significant voids.

Quote:
Originally Posted by Windows on Wash View Post
If you have a mix of a whole bunch of different crap, I usually recommend yanking it out.
What about creating a mix of crap? At this point, I could leave the remaining fiberglass and end up with areas of fiberglass on top of cellulose and areas of cellulose on top of fiberglass. This would seem to be quite a bit easier than making it entirely consistent.
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Old 10-19-2013, 04:24 PM   #11
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Improving attic insulation: remove old fiberglass or just add cellulose/fiberglass?


Quote:
Originally Posted by Gary in WA View Post
IMHO, leave the old insulation in place, carefully lifting sections where the wiring/plumbing holes are to canned foam air seal them first. Compressed R-11 is divided by 3-1/2" = R-3.14 per inch (low density- useless at slowing air movement); used to be "the biggest loser";The "biggest loser" in fiberglass insulation....

Now it is R-19, another story, lol. Compressed to 2" thick your low density (R-11) has now become a medium density (a good thing) at R-7.55 or from R-3.14 to R-3.78! Congratulations!http://www.greenbuildingadvisor.com/...fiberglass.JPG It will stop air better because of the increased density but the drywall/plaster is still the air barrier in contact with your thermal barrier insulation. Move the material you don't want/need rather than exit the attic by sliding it together in a close puzzle piece fit in same bays to omit lifting/breakage. If able financially (I start all my personal projects with that) add more than to the joist tops, meeting/exceeding minimum code for your area; Ramsey county- R-49; http://energycode.pnl.gov/EnergyCode...tate=Minnesota

Gary
Thanks, Gary. That's interesting. So, despite the lower R-value, compressed fiberglass may perform better than uncompressed? Are there any nuances to this when considering attic installations? Most of the data seems to be based on walls.

I do plan to do some air sealing with Great Stuff and will work on getting a better seal at the attic hatch, too.
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Old 10-19-2013, 07:54 PM   #12
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Improving attic insulation: remove old fiberglass or just add cellulose/fiberglass?


Air seal, box out any can lights, weatherstrip and insulated the access hatch with rigid foam, build up the area around the hatch so the loose fill doesn't come in, provide for soffit venting if you decide to do that in the future via proper baffles, loose fill cellulose blow up to R-50.

Compressed fiberglass is completely fine like Gary said. Don't bother putting it in a landfill as it does add to the total.
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Old 10-21-2013, 11:42 PM   #13
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Improving attic insulation: remove old fiberglass or just add cellulose/fiberglass?


" So, despite the lower R-value, compressed fiberglass may perform better than uncompressed? Are there any nuances to this when considering attic installations? Most of the data seems to be based on walls."----------------- the denser insulation stops air movement better that would rob you of R-value as the rating is given by being tested with no wind or air currents as in an attic; https://docs.google.com/viewer?a=v&q...M8l_EeA2F-qvvA

You don't read much against fg because after a particular "cubed" was tested and failed badly (and since removed), and has been under control since then....:-).....; http://www.homeenergy.org/show/article/year/1992/id/895

But they removed that low density cubed insulation from the market; https://docs.google.com/viewer?a=v&q...mAAqxY_9UrVORg

So let's blow-in more loose-fill fg over the top to stop those pesky convective loops, nope--- how about adding more fg with a higher density? nope, how about some cellulose; s://docs.google.com/viewer?a=v&q=cache:O6npU3elGFoJ:http://www.ecp1.com/science/Oak-Ridg...WhJRhM2V4M5EQ;

2004 in Sweden, they tested a low density fg product and found room air= 68*F and -18*F outside attic air- convection is stopped best by increasing the density of product; http://web.byv.kth.se/bphys/reykjavik/pdf/art_085.pdf

Here is an early test of fg vs. cellulose in walls/ceiling (R-30 fg is low density, right down there with R-19); http://www.builditsolar.com/Projects...n/Colorado.pdf

Look how they make it (density); pp.41; (wait for it- big file); http://www.huduser.org/Publications/pdf/walls.pdf

See a pattern on the compression chart, lol; http://numsum.com/spreadsheet/show/21111
Enough of that..... have 50 more links.....here is a good discussion; http://www.greenbuildingadvisor.com/...erglass?page=1

Gary

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