Replies

1. 
   pjmeg | Apr 13, 2017 01:24pm | #1

   I have the same situation in my house. So I'll be following the conversation closely.
   I'll go into some of the alternatives I have been mulling over. In my situation, I do not have attic access to the area above the ceiling plane, and I want to maintain a vented roof assembly.
   1. gently pull down T&G, install drywall and tape seams to create air barrier, re-install T&G. This alternative doesnt allow me to appreciably increase my insulation levels because I would only have access from below.
   2. remove roof from above, rake out existing fiberglass batts, lay-in drywall strips cut to fit between bottom chord of trusses to act as a ignition barrier (not sure this is code compliant as an ignition barrier for SPF), and then spray ccSPF a minimum depth over the drywall and bottom chords to get a continuous air barrier. Blow cellulose over ccSPF to necessary depth, then re-construct the roof. I have concerns about the duration of time this option requires. I also do not get the full fire resistance from the drywall because it isn't continuous.

   Why have you discarded sheet rock as an option?

   To answer your questions from my limited knowledge.
   Question 1. Yes, I've heard that spray foam will expand past any small gaps in the T&G. I've seen it sneak past gaps in ceiling drywall where the ceiling electrical box isnt perfectly tight against the drywall.

   Question 2. I have a tough time believing that the weight of the cellulose will cause the fasteners in the T&G to pull away from the structure. If youre that concerned, use trim screws to supplement the existing fasteners.

   Question 3. Sounds like very fussy and time intensive work. Potential for lots of small holes in your air barrier, also the foam will need an ignition barrier, because I am not sure if the FG facer on the reclaimed foam will satisfy the code.

2. Expert Member
   Dana Dorsett | Apr 13, 2017 02:36pm | #2

   It doesn't take closed cell foam to seal it up as an air barrier from the attic side. If using a spray foam for airr sealing is your desired method, 3" of open cell foam will air seal it more reliably and half the price of 2" of closed cell foam, while using less polymer than just 1" of close cell foam. Since its blown with water open cell foam has none of the environmental impacts of the HFC or HFO blowing agents used for closed cell foam. At 3" you'll already have about R11 in the foam, so it then takes only another R38 of cellulose to hit the R49 code min, which is less overall weight.

   And with open cell foam, trimming any overspray bits that get through the seams of the t & g or electrical penetrations is infinitely easier than with closed cell foam.

   A t&g wood ceiling has far more weight bearing capacity than the more typical gypsum ceilings used under cellulose. There should be no "...pushing down the ceiling..." unless the fasteners are already failing.

   A cut'n'cobbled rigid foam approach is OK, but a lot of work, and a less reliable air seal than broad-sheet goods or a 3" open cell foam.

3. 
   Greenconfusion | Apr 13, 2017 02:49pm | #3

   Only reason for wanting to go closed cell is i purchased two dow froth packs 200 sq ft. @ 1inch. I wanted to seal just where i can see light from down stairs into the attic. Some of fasteners have loosened up over time, may have to use trim screws , also some boards are cupped from previous roof leak. Thank you for open cell foam idea and code min.

4. Expert Member
   Dana Dorsett | Apr 13, 2017 03:40pm | #4

   Using only light penetration as your guide is a lousy way to determine the level of air leakage in a t & g assembly in any season, and seasonal changes in humidity will make the cracks bigger & smaller at different times of year.

   Assume that ALL of the seams leak, it's only a matter of degree.

5. GBA Editor
   Martin Holladay | Apr 14, 2017 04:09am | #5

   Green,
   The best way is what Pete suggested first: temporarily remove the ceiling boards, install drywall, and then re-install the ceiling boards. If you can't do that (or don't want to do that), this is probably the second-best way:

   1. Install new fasteners as necessary to secure the ceiling boards.

   2. Install cardboard rectangles between the joists to prevent spray foam from entering the living space.

   3. Install 1 inch of spray foam as an air seal, covering the cardboard and the seams between the cardboard and the joists.

   4. Install cellulose insulation.

   -- Martin Holladay

6. Expert Member
   Dana Dorsett | Apr 14, 2017 10:02am | #6

   The inch of spray foam is complete overkill for a cut up cardboard rectangle air barrier. Tape + duct mastic at the seams would work just fine, using spray foam just for sealing the cardboard to the joists at the edges.

7. 
   Greenconfusion | Apr 15, 2017 08:29am | #7

   Not sure I would install cardboard in a hot humid attic. The tapetape idea with mastic sounds good. I will have to see if sticks to rough cut . Would tyvek stapled in between joists be better than cardboard .

8. 
   Greenconfusion | Jul 28, 2017 07:52am | #8

   I decided against cut up cardboard in the attic. Does anyone see a problem with wood paneling ripped to fit in between the joists in attic . The wood paneling would be laid against tongue and groove ceiling and then top coat of closed cell . Open to other ideas also .

9. Expert Member
   Dana Dorsett | Jul 28, 2017 10:29am | #9

   The ripped paneling detailed as an air barrier is fine, but the environmentally & financially costly closed cell foam overcoat buys you nothing. Using a bead of closed cell foam or can-foam to air seal the wood to the joists is fine.

   Plywood and OSB are "smart" vapor retarders, and will allow any accumulated moisture to dry in either direction if need be. Closed cell foam is close to Class-II vapor retardency at 1", and doesn't change much with humidity or moisture content.

   Asphalted fiberboard is both moisture tolerant and fairly vapor open (also a smart vapor retarder, about 5 perms when dry, 15+ perms at higher humidity) and may be a good choice for the paneling. Asphalted one side with the asphalt side facing the attic limits any outgassing issues (but houses have been sheathed in double-side asphalt fiberboard for decades without outgassing complaint.)

10. 
    Greenconfusion | Jul 28, 2017 01:02pm | #10

    When you say closed cell spray foam buys me nothing , i have cracks everywhere along edges and interior load bearing wall . I thought closed cell spray foam would buy me alot of air sealing. Is the asphalt board you speak of sold at lowes or HD ? I would like to use the best product to prevent mold build up between tongue and groove and product i chose. One other thing would caulking all seams be a viable choice ? I have seen habitat do this on some retrofit jobs on walls .

11. Expert Member
    Dana Dorsett | Jul 28, 2017 01:53pm | #11

    You don't need to cover the entire surface of your ripped paneling air barriers with closed cell foam- you only need to seal up the cracks & edges of the panel, as well as other other penetrations. If you're going for a full-coverage foam air sealing method, 3" of open cell foam seals as-well or better than an inch of closed cell at the same price, uses less polymer, and uses water rather than HFC245fa (an extremely powerful greenhouse gas) as the blowing agent.

    http://www.homedepot.com/p/1-2-in-x-4-ft-x-8-ft-Southern-Pine-Asphalt-Impregnated-Board-A11230848096/206086210

    https://www.menards.com/main/building-materials/panel-products/construction-panels/fiberboard-panels/1-2-x-4-x-8-fiberboard-sheathing/p-1444435987716.htm

    See also:

    https://www.greenbuildingadvisor.com/community/forum/energy-efficiency-and-durability/31065/air-sealing-exterior-celotex-fiberboard-sheat

12. 
    Greenconfusion | Jul 28, 2017 03:03pm | #12

    Just curious if lauan plywood would be a better option without off gassing risk ? or is perm rating a big deal. Is vapor perm only a concern from living space to attic or does hot humid attic air get stopped by blown in cellulose ?

13. 
    tommay | Jul 31, 2017 12:07pm | #13

    Silicone caulking.....