Insulation in Ceiling Question

Discussion in 'Remodel Forum & Blog' started by Thomas K, Feb 10, 2021.

  1. Thomas K

    Thomas K Member

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    Opelika, al
    I'm about to install drywall ceiling in bathroom and insulate ceiling. I'm not sure which Owens Corning roll fiberglass insulation is needed. Ceiling joists are 2 x 6. It had blown-in insulation, but I removed it all. Can anyone tell me which thickness and R-rating? I live in Opelika, AL.

    Someone told me I needed to fill the joists with R-15 paper faced, and then use R30 batts above the joists. They didn't say what thickness R-15, and all of it that I see is 3 1/2" thick. So if they meant two layers of R-15, that would put level of insulation above the joists.

    Thanks for any replies!

    -Thomas
     
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    Alabama is in zone three, and code now requires a minimum of R-30 there...how you achieve that doesn’t matter. If you use one with a vapor barrier, you only want one on the ceiling side, not multiple ones that could trap moisture in between.
     
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  4. Dana

    Dana In the trades

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    Facet or unfaced, high density R21s or mid-density R20s will fit 2x6 framing well, and are sufficiently dense to limit performance losses due to convection within the insulation. (The cheap R19s designed for 2x6 walls would be half-inch or so too much loft and convect freely.) R21s between the cavities with "contractor roll" R13s (unfaced)rolled snugly on top would beat R30 performance (if that's the state code), but the IRC calls out R38 or U0.030 (=R33 "whole assembly", with all other layers and thermal bridging of the rafters factored in) in IECC climate zone 3.

    Using only using cheap contractor roll R13s with kraft (not aluminum) facers one could double-layer the R13s in the 2x6 bays, stapling the facers to the tops of the rafters for a slightly bulging 5.5". At the compressed density the total cavity R would run about R22 (not the labeled R13 + R13= R26) due to the loft being 5.5", not 7".

    Then adding a layer of uncompressed R13s over that brings it up to R35 for the insulation layer, but with the R13 thermal break over the rafters, the topside & bottom side air films, and the ~R0.5 of the sheet rock it would meet the U0.030 code requirement of the IRC.

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    Installation quality is everything. With even a 5% gap the performance is cut by about half, so take the time to sculpt the batts around vents, pipes, electrical boxes etc with a batt knife (an 8-10" bread knife is almost as good), and when rolling out the continuous top layer be sure to snug up the adjascent layers as tight as possible, leaving no gaps, voids or compressions.

    In a zone 3 climate as long as the ceiling has been made air-tight it won't matter what side of the assembly (or within the assembly) kraft facers are located. Kraft facers are "smart" vapor retarders, and become vapor open when there is sufficient moisture in the adjacent air to support mold, but they are NOT air barriers- it's literally impossible to make them truly air tight even with generous caulk at the facer tabs and taping all cuts & knicks in the facer. If the ceiling leaks air there can be enough wintertime moisture accumulation in the insulation to create a problem, and if the batts aren't snugged up enough to limit convection or worse, big bypass gaps & voide there is potential for summertime moisture accumulation at the ceiling gypsum in an air conditioned house.

    How deep was the blown insulation that got removed, and what type of fiber was it- cellulose? fiberglass? Rock wool?
     
    Last edited: Mar 27, 2021
  5. Thomas K

    Thomas K Member

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    Thanks for the info! Blown in insulation removed was cellulose, and didn't quite come up to the top of the ceiling joist.
     
  6. Dana

    Dana In the trades

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    So it was less than 5.5" (= the depth of milled 2x6 lumber)?? That is WOEFULLY under insulated for any US climate!

    Replacing it all with 10-12" of low density blown cellulose would far outperform low density fiberglass solutions, and is at least on par with high density batts (R15s & R21s), but it's likely to be more expensive than contractor roll R13s. Cellulose is far more air-retardent than R13s, and unlike R13 batts does not need air barriers on all sides to meet the labeled R-performance. Blown insulation also self-conforming to all obstacles & irregularities, leaving no gaps or thermal bypass channels, whereas batt installation requires obsessive attention to details to get there.

    But I get that you're going for a lower cost batt solution, which is fine- it'll still do great if you do it right. Before installing ANY batts, go around and air seal every crack seam & penetration in the ceiling below with polyurethane, can foam, or even cardboard (sealed at the edges). It can easily take an entire day to do a 1500 square foot attic, but it will be worth it.

    The seams of the ceiling gypsum & top plates of partition walls usually leak much larger than anticipated volumes air along the edges, as do where duct boots or electrical boxes meet the ceiling gypsum. Recessed cans can be a HUGE air leak, which can become a serious indoor air quality issue when fiberglass insulation is in the attic, especially in homes where the HVAC is in the attic (= most homes in AL). Even minor air leaks in the ducts and normal duct imbalances can pressurize the attic (& outdoors) relative to individual room pressures when the air handler is running, and air leaking in through recessed lights goes directly into rooms, dragging tiny shards of fiberglass with it that can hang in the air for hours. This is less of a health issue with cellulose, since the cellulose itself isn't hazardous, and the borate fire retardent dust has very low toxicity (less toxic than table salt.) But with any new attic insulation it's important to air seal between the attic & conditioned space BEFORE installing the insulation for whole-house and HVAC system performance reasons.
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    Mastic sealing every seam and joint on the duct boots and hard ducts also becomes more important when there's fiberglass in the attic.
     
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  7. Thomas K

    Thomas K Member

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    So I'm actually going about this the wrong way. I need to have drywall guy drywall the ceiling and bathroom before insulating.
    Someone took their time installing ac ducts in the house, and did it right, but natural gas/ac unit sits in the crawl space of middle floor of split level home.
     
  8. Dana

    Dana In the trades

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    I'm assuming it's an unvented crawlspace?

    If the ducts and air handler are all inside the pressure & insulation boundary of the house it's a good thing, since it lowers the risk of air handler driven infiltration.

    Even without ducts & air handlers in the attic it's still important to air seal the attic floor prior to insulating, since some rooms will still be pressurized, others depressurized (albeit to less extreme levels than with leaky ducts in an attic), and the 24/365 stack effect pressure infiltration drive still needs to be blocked.
     
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  9. Thomas K

    Thomas K Member

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    Crawlspace has two outside vents, but that's all. All upstairs rooms have vents coming in from attic, as whoever installed this system ran a large air handler from the ac unit up to attic, where it splits into individual branches to each room. I will air seal ceiling before insulating.
     
  10. Dana

    Dana In the trades

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    In your climate (IECC climate zone 3) crawlspace vents to the exterior allow a lot of humidity into the crawlspace and house, increasing the potential for condensation on the air handler & ducts during the cooling season, and adds to both the heating & cooling loads. If there is a slab or heavy ground vapor barrier on the floor it is usually beneficial (for efficiency & indoor air quality) to insulate & seal any exterior crawlspace walls. IRC code minimum for basements & crawlspaces zone 3 is R5 continuous insulation, which is easy to achieve with 0.75-1" foil faced polyisocyanurate foam board glued to the walls with foam board construction adhesive. The bottom edges of the polyiso need a capillary break from any soil or concrete contact (contact with the ground vapor retarder is fine.) It's hard to get 4x8 sheets into crawlspaces, but there is always some reasonable size to cut them to for getting them in. Just tape the seams with foil HVAC tape once they're up.

    Sometimes it's easier in a retrofit to seal ceiling duct boots to the ceiling gypsum from the room side with mesh-tape + mastic or UL 188 type foil HVAC tape (eg Nashua 324A, sold at most box stores) or even housewrap tape, reinforced at the edges with mastic. The bigger holes where the plenums punch through may or may not be be a bit harder to get at, usually from the attic side.
     
  11. Thomas K

    Thomas K Member

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    Thanks for the info, Dana! Crawlspace does have heavy plastic covering the bare ground. What I meant by sealing the ceiling before insulating is using fire foam around any cracks and around light cans (have only LEDs in them) from above. I thought about using fiberglass batts, but what you were saying about using cellulose insulation makes more sense, even if I have to install it by hand. I do have one question about that. I installed 2 of those Globe Electric clip-in LED lights and am wondering how to insulate around them.

    One thing I did find out about this house. The entire lower floor has concrete block walls, and someone screwed 2 x 4 framing on inside of exterior walls and covered framing with paneling. I found out by tearing out a couple of panels to see how the wiring was done. Old NM wire comes down pipes that were set into the concrete block and exit through outlet boxes. My point is that no one bothered to insulate these concrete walls. I've tried to learn the best way to do this, and was told to cover the concrete block with 1" rigid foam insulation, build walls over insulation with 2 x 3"s, and then add batt insulation between the studs.

    Is this the right way to go about this?
     
    Last edited: Apr 6, 2021
  12. Dana

    Dana In the trades

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    If you can lap the edges of the ground vapor barrier plastic three inches or more above grade onto the walls and seal it to the wall with caulk, mastic or construction adhesive, you can then lap the wall foam over the plastic without concern, since the bottom edge of the foam is on the vapor barrier, thus protected from ground moisture.

    If the lighting fixture cans are not both air tight & rated for insulation contact (there is usually a label on the fixture for those ratings) you need to establish a 3" clearance on all sides to full meet the letter of the code, even with low wattage LEDs installed. An easy way to do this is to find a cardboard box of sufficient size, tape all the seams with housewrap tape, then seal it to the ceiling gypsum with caulk & mastic, (or can foam if it's easier). Sometimes codes require a non-combustible material (such as half-inch wallboard) to be used rather than cardboard, but for a low wattage LED the risk of using a cardboard box is very low. Where the wiring enters the box you may need to use mesh tape & mastic over the wire to get a good seal. There are commercially made fireproof buckets for sealing over recessed lighting fixtures out there as well. (Box stores near me keep them in stock for $15-20/each- YMMV. )

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    ^^ "InsulMax" brand can cover made of fire retardent EPS foam^^



    CMU block walls are considered "mass walls" under the IRC. If you look under the MASS WALL column of TABLE N1102.1 2 in the zone 3 row you'll see two numbers for R value: 8 / 13 . The R values refer to a continuous layer of insulation, the lower number (8, in this case) meets code if half or more of the insulation is on the exterior side of the structural concrete, the bigger number (13) is if more than half the insulation is on the interior side.

    If you're going to gut the interior, the easiest way to get there would be 2" of foil faced polyiso foam board (typically labeled R12 or R13) glued to the wall, secured in place with 1x4 strapping through-screwed into the CMU with TapCons, mounting the wallboard on the furring. That takes up 2.75" of depth from the wall, buying back 3/4" of space for the room. If that complicates re-mounting of the electrical boxes there are other ways of getting there.

    Cellulose packaged for insulation blowers is sold in tightly compressed bales, and needs to be fluffed up to do it by hand. (Insulation blowers break it up mechanically with the feed from the hopper.) For small areas it's easy enough to break a chunk off the bale, put it in a 5 gallon bucket, and fashion a lid with a hole in it big enough to accommodate the shaft of a paint or mud mixing drill, the way this guy did:

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  13. Thomas K

    Thomas K Member

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    Location:
    Opelika, al
    I installed 2 IC can lights with Cree bulbs two 3" clip in led lights and a combo fan/light unit over the shower on a GFCI breaker circuit in the bathroom.

    Exterior of lower floor block walls is covered by tar paper and bricked up. I have noticed no water infiltration in the block. I like the idea of insulating with the rigid foam insulation. I will have to upload a photo to show the mess someone made of the wall between the crawl space and lower floor where someone had ac vents installed at one time . I don't believe it myself. :)
     
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