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I am trying to finalize my order for insulating my attic. I know I want fiberglass batts. I have an old house in the southeast with a vented roof.

there is currently R4 or something silly in there now(it's like 2 inches high and compressed).

I plan to swap it out with R21 unfaced. This will give me the option to plus up with a perpendicular run when/if I decide to since my roof is 2x6.

My question is two fold.

  1. What is the difference in R21 offerings? I see 15" width and 16" width, and my joist spacing ranges between 15.5" to 16". I also hear of stiff vs. hard but they don't seem to be identified clearly on Lowes/hd etc.

  2. Is unfaced r21 listed as 'not for attics' simply because the manufacturer assumes you need a moisture blocker? My vented attic is the source of any moisture, and is generally always going to have very humid air in it. My Current insulation has facing on the roof side, which appears to really only be preventing rain and dust from entering the insulation(I have a ridge vent, whirly vents, and decent wind in my area).

I don't want to order a bunch of r21 and need to return it all because of a big gap in my joists and insulation.

I have done a good bit of searching around, and see many references to 'crossing' insulation over 2x6's, but no one really addresses using r21 etc. for this in what I've read of the code or home energy guides. this has me doubtful.

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  • If your joists are spaces 16" on center (OC), that means you've got 14.5" between them, so you'd want 14.5 or 15" wide insulation batts to squeeze in between them. TBH, I've never seen 15" and 16" insulation at Lowes or HD. I've seen insulation for 16" OC (14.5" actual width) and 24" OC (22.5" actual width).
    – FreeMan
    Jan 5 at 12:22
  • Also, please take the tour, then read through how to ask a good question, then edit this down to one question at a time. This site prefers 1 question per post so that you can select the "best" answer for your situation - doesn't work well if I answer one part of your question and someone else answers the other part. Feel free to ask your other question right now - nobody will look down on you for doing so...
    – FreeMan
    Jan 5 at 12:24
  • Cellulose loose fill, but you already decided on fiberglass batts, so whatever...
    – Ecnerwal
    Jan 5 at 17:19

1 Answer 1

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Do you know what your total heating degree days (HDD) and cooling degree days (CDD) are your your location? I'm going to assume that cooling, rather than heating, dominates your energy usage.

Since you're in the Southeast part of the country (hot, humid weather much of the year), you want a vapor barrier/retardent on the exposed attic side of your insulation. This is to prevent the high humidity in your attic from migrating through the insulation and possibly condensing on the cooler, lower part of your insulation (the ceiling side).

If you put down multiple layers of insulation, the lower one(s) do not have to be faced. Only the top one does, as I described above.

Also if you put down multiple layers, I would run the first, lowest layer inside of and so parallel to the ceiling joists. I would then run the second layer perpendicular to the joists, so that a thermal break is created between the conditioned/living space and the attic itself.

I do not know the difference between your 15" and 16" width insulation you are proposing, aside from the width.

Finally, you should aim for a total R value that's recommended for your part of the country.

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  • The difference in width is so that the insulation does not get compressed in between the joists. Compression mean lower R-value, though some claim this is not a thing. The compression is pretty minor IMO and most home owners buy what is cheapest at their local store. The width that the store stocks them most can be drastically cheaper.
    – Evil Elf
    Jan 5 at 12:31
  • The crisscrossing of two layers is to combat the thermal bridging that allows heat to migrate through the joists. Running the top layer perpendicular covers the joists.
    – Evil Elf
    Jan 5 at 12:32
  • Appreciate the comment. I was assuming the 15 vs. 16" was mainly in case you had thin steel studs(wall application). But I still want the 15" to fit my wood studs tight not loose lol and I can't just 'buy a sample' apparently
    – dummy
    Jan 5 at 20:46

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