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merlanvp 07-25-2010 08:08 PM

Remove old attic insulation and critter mess?
Do I need to remove old attic insulation before putting in new stuff?

Live in a 1969 house with two layers (approx 6 in) of insulation in the attic. I live in the vicinity of Pittsburgh, PA. The second layer was paper-backed and simply laid on top of the first layer with the 2nd layer's paper facing the living area. 1st layer is also paper backed and facing the living area.

While in the attic to extend storage flooring and fix some electrical work, I also noticed LOTS of OLD evidence of animal infestation (nesting materials, droppings, rat poison bags, etc).

Should I remove all the old insulation as well as the animal infestation stuff before blowing in new insulation? I plan on blowing in 13 inches for approx R38 of blown cellulose (assuming R value of 3 per in) in the next year or so.


Just Bill 07-26-2010 06:11 AM

Remove the second layer with vaopr barrier, or resue the insulation after pulling off the vaopr barrier. You mention critter mess, but don't elaborate, how much, what kind??? If a lot of poo and such , I would remove it. Whatever, use a good quality dust mask!!!!

merlanvp 07-26-2010 08:18 AM

Critter mess specifics
Not sure what kinds of critters. All the old straw and nesting material (including bunched up insulation along the edges of the house (no soffits, just where the roof meets the facia holding the gutters) is a pile-of-crud. I have found 4 areas that were probably nests at one-point or another...but nothing current. Lifting insulation to do an electric-wiring trace, I found small black pellets that remind me of am inclined to thing squirrels.

I did not wear a mask when moving the insulation that time...a mistake I will not repeat---that stuff stayed with me for hours in my coughs (ugh...)


Gary in WA 07-26-2010 01:43 PM

Remove both because only removing one adding 1.7# per sq.ft. of cellulose will compress the fiberglass batt negating it's value:

That's like putting a 12ounce water bottle on every square foot of batt insulation. Then you will be able to air seal the attic/living space:

Make sure you have either 16"o.c. ceiling joists with 1/2" drywall or 5/8" drywall on 24"o.c. 1/2" drywall on 24"o.c. will not work, cellulose is too heavy and could cause ceiling sag. Blow the cellulose 20% more to achieve projected R-value due to settling:
It should state on the bag installed weight per sq.ft.--- per inch.

Good time to check your balanced attic ventilation, now:

Add some roof venting above the fascia, if needed--- to protect against ice dams:

Be safe, Gary

merlanvp 07-27-2010 09:32 AM

Removing insulation and critter mess?
Good point about venting...redoing roof next week and removing power attic fan, replacing with ridge vent for outflow, adding smartvents for inflow at a mid-roof installation (no soffets, and insulation needs to run to the edge of the roof joist/facia connection), and retaining the gable vents.

Had not really thought about the weight of the cellulose. Should have. The joists are 16 OC and not sure about the dry wall. In the bathrooms, where I just replaced ceiling fans, it appears to be 1/2" sheet rock, 1/2" rock board. Is blown fiber glass a better option if the sheet rock is 1/2"?

Proby 07-27-2010 09:51 AM

I would definitely get rid of all that crap. That would give you an nice open area to find out where the critters could have come in and seal it and also seal all penetrations into the house. Around caulk around wires, caulk or spray foam around pipes, spray foam over electrical boxes.


While in the attic to extend storage flooring
3/4" plywood has an R value of less than (1) so try to limit your flooring. If your joists are 2x8's than you have about 7" of space to put insulation which will only be about R-20.

However, the law of diminishing returns applies here, I've found that after about R-20 or so the energy savings drop off significantly.

Gary in WA 07-27-2010 04:36 PM

Block off the gable vents as they will short-circuit the ridge vents, page #606:

Rip the plywood into 12" pieces to allow the moist air from below to by-pass it and not deposit the water on the ply bottom facing the insulation, causing mold. It has a low permeability and will stop the water there. Space the pieces 1-2" apart.

Proby made a good point on value vs. cost:

Be safe, Gary

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