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I'm pretty sure my attic needs some additional insulation (it looks pretty sparse and my house is freezing!).

Here are some images of my attic: http://imgur.com/a/pdsXF

Two questions:

  1. There is some existing blown-in insulation (looks like the recycled kind), can I lay down batt insulation over that?
  2. I have a skylight that is not insulated at all, how can I insulate the drywall around it? Is this a good idea.

*Some additional Info: I'm in CA (East SF Bay Area). Winters are not too cold, Summers can be hot (90s-100s).

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Might help if we know your location -- how cold does it get in the winter? how hot in summer? –  shufler Dec 10 '11 at 0:29
    
Thanks, added some more info –  christo16 Dec 10 '11 at 0:31
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3 Answers

up vote 6 down vote accepted
  1. You can, but there's a few issues to be aware of. First, your blown-in insulation requires space to stay "fluffy" and work well. Compressing the blown-in with batts will diminish the former's effectiveness and compromise the overall r-value.

    Secondly any vapor barrier (the paper on the roll) should be against the drywall. If you don't have a vapor barrier in place, I would address that (see this article from this answer for more info). If you do have a vapor barrier, make sure you buy unfaced (like your link), or remove the paper from your batts before installing.

    Your best value will likely be buying more blown-in insulation. If you buy a reasonable amount most of the big box stores will let you rent the machine for free. It is a messy two person job, however.

  2. It looks like you have standard 2x4s on the edges where the drywall is attached, you could staple faced fiberglass batts to it. Measure between the "studs" and buy a roll that you can cut to width. I've seen skylights both with and without insulation, I don't know if there is a downside to insulating them.

Edit

If you put plywood over the cellulose, you're going to have a similar compression problem. This table shows you what kind of r-value you can get per inch. So the depth of your joists (they didn't look very tall) is going to limit the r-value you can get if you're covering the joists with plywood. Since it appears the insulation you already have is near the top, you won't see a huge gain in efficiency if you add just another inch or two of blown insulation. You might try asking another question once you figure out what r-value you have and what r-value you'd like to have. If you're near the recommended r-value for your climate and your house is still too cold, you might be better served by looking for air leaks first.

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Thanks for the answer, gave me something to think about. Can I lay plywood over the blown insulation? –  christo16 Dec 10 '11 at 1:37
    
@christo16 My comment was too lengthy so I added it to the answer. –  Steve Jackson Dec 10 '11 at 4:01
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I had a similiar issue with my house in New England (lack of insulation), basically I laid unfaced batts over the existing insulation to increase the R value.

Some things I did:

  • Make sure your vents still have access to the outside air, do not cover them with the newer insulation.
  • I laid the batts perpindicular to the joists and over existing installation

If you DIY, try not to put your foot through the ceiling (easier said then done) and I recommend adding the insulation during the fall or spring or else it will be very uncomfortable up there.

I brought a few 2x6s to the attic so I could stand on them when I was up there.

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Laying the batts perpendicular seems like a good idea for keeping the bottom layers uncompressed. Just make sure whomever goes in the attic knows which way the joists actually run! –  Steve Jackson Dec 10 '11 at 4:03
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For the skylight, instead of insulating the shaft, you can get a lot of improvement by adding a plexiglass panel to the bottom of the skylight, level with the interior ceiling. You seat the plexi sheet on top of strip molding nailed to the bottom of the skylight shaft.

You sacrifice about 10% of the light, and materials cost around $30 for a 24x48" skylight (lowes.com part# 126161 and 11220).

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