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Dense pack cellulose and vapor barrier

smokey11106 | Posted in Energy Efficiency and Durability on

Hi. I live in NY Zone 4. I have exterior XPS foam and dense pack cellulose on all exterior walls drying inward with hopefully air tight drywall. The house is a side split with two bedrooms over attached two car garage. I am tempering the garage with a modine water heater w/ its own zone set to 55 degrees. I would like to use dense pack cellulose for the garage ceiling ( 2×8 joists) and the rear garage wall (2×6 studs) which separates the family room from the garage to complete the insulation. One, are there any particular considerations for the use of cellulose in these locations. Two, would I need any type of vapor barrier on the inside of the rear wall on the family room side of the cellulose and any such barrier in the attic ceiling? Also, not to start a second thread, but the basement has 2″ xps foam board against the block wall seems taped with my framing against that. Would it be a problem putting Roxul insulation (unfaced/ no vapor barrier) in the open bays of the wall? You can walk out to the rear yard from the rear of the basement, because the property is lower in the back which exposes half of the block walls to the exterior. Thank you.

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  1. smokey11106 | | #1

    Any ideas?

  2. jklingel | | #2

    Eric: I am not one of the pros here, but I sure have read a ton in the last few years. I see no reason to not use cellulose where you plan to, but you won't dense pack it in the lid. "Dense pack" means packed into a "container" to about 3.5 lbs/cf. Loose in the lid I think it will be about 1.8 lb/cf (not sure on that number, but blow it in as thick as you can afford to). Make sure you ventilate over the cellulose in the lid; 2" minimum. My lid will have 25" over the walls, and building up to 30" as the trusses allow room. The walls will have 19". You have to run your own numbers and see how thick you can pay for in a reasonable time. As for the true VB, they are being shied away from in most places. Check your local code for the required perm rating of your wall assembly. Do not trust someone to tell you "you need 6 mil poly", because that means they probably have not read the code; I've recently found that to be true w/ two inspectors. The code "probably" says "one perm or less", ( 6 mil poly is 0.06, or essentially impermeable) and you can get one perm or less with methods like the ADA (airtight drywall approach), and others. These methods will let the wall pass vapor in both directions, letting a wall dry to the inside, too, if it gets wet. Poly keeps any exterior vapor/liquid water to the exterior; sometimes that is not good, esp if you ever run air conditioning. That can be disastrous. The exterior should be 5x as vapor open as the inside, if possible. The basement sounds like what I read a lot about; no vb, and no fiberglass batts (though they are OK, other materials appear to be better). Search here, and on, and draw your own conclusion, but it sounds to me like you are most of the way there already. Hope this helps.

  3. GBA Editor
    Martin Holladay | | #3

    Don't worry about a vapor barrier in either of the assemblies you are talking about.

    However, wherever you have a wall or a ceiling that separates a garage from a living area, you need to be very careful about air sealing. You certainly don't want any carbon monoxide from your garage to reach your living area.

  4. smokey11106 | | #4

    Thanks Martin, Thanks John. I appreciate the input. I just found out that the contractor has been using green board instead of regular drywall for the basement. Hope thats not an issue and doesnt trap anymore moisture than regular drywall. I only wish my contractor and Architect had known more about every single thing I had to research and I would have found this site before having the plans drawn up. Would have saved me a lot of trouble and money to get where I am now. I would have had more choices. Well...thank you for the information and thank you to this sight and all that contribute to it. I have applied a number of things I have learned here all the way down to my separately ducted HRV. We'll see how this big experiment works.

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