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flash and batt clarification/decision assistance please

Bill Vesely | Posted in Energy Efficiency and Durability on

The great/living room in our “California ranch” is vaulted and finished in a post-and-beam fashion. Turns out that the ceiling boards are also the roof sheathing, so there’s literally no insulation there (figured this out when getting a new roof last year). Based on reading the great info on this site I reached out to an insulation contractor about options. It’s a smaller area than his minimum cost, so we are going to insulate the wall between the kitchen and garage as it’s quite cold there.

The contractor proposed spraying 2-inches (we’re in Louisville, KY, so region 4 I believe) of closed-cell on the living room ceiling. Then I am thinking that I would use batts to fill to the bottoms of the beams followed by finishing the ceiling with shiplap boards. The gap between beams is about 4 feet, so hanging batts is going to be an interesting DIY project. From reading on this site it seems like I need taped drywall between the batts and shiplap, but I’m not 100% there.

We are doing the kitchen/garage wall with the same flash-and-batt approach, but I am wondering if adding more closed-cell in the living room ceiling would make my finish-up work easier and maybe just do the kitchen/garage wall myself with batts. Is there a reason not to fill to the full depths of the beams (about 5-1/2 inches I believe) and trim off anywhere it would be bulge against the finished ceiling (or just compress it?). The living room heat loss/gain is the bigger issue here. Any suggestions/knowledge transfer would be greatly appreciated.

Thanks, Bill.

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Replies

  1. DCContrarian | | #1

    With 4' rafter spacing I would consider 4x8 sheets of polyiso foam. It would be cheaper than spray and less work and more insulation than batts.

    It's often available used for very attractive prices.

  2. Bill Vesely | | #2

    Thanks for the response. I was originally thinking to go that route, but information here about the "cut and cobble" (probably got that wrong sorry) approach not being recommended lead me towards the foam and batt approach. I could do the foam board approach myself, which with everything going on right now is appealing. Thanks again for the advise. Would welcome more perspectives. Best, Bill.

  3. Sam S | | #3

    Using just closed cell in the living room ceiling is going to save you a lot of aggravation playing around with batt insulation in oversized structural bays. I think that's the right call.

    You need a thermal barrier between the closed cell and the interior space. IRC/IBC if those are your governing codes will point you towards gypsum board as a solution, but you can use 'alternative' materials or assemblies if they perform the same way. Talk to your code enforcement officer about what alternative thermal barriers they will approve, as that will get you an answer faster than trying to decode the various test protocols for equivalent thermal barrier performance.

  4. Bill Vesely | | #4

    Thanks for the input Sam. If they spray to the beam depth and it expands beyond, is it okay to shave down or should the finished boards just compress the material? Is filling a 5-1/2 inch deep cavity with closed-cell foam a one step process for a pro? Thanks again, Bill.

    1. Sam S | | #6

      Talk it through with your contractor regarding your expectations on insulation depth in the cavity. It's normal to 'overspray' the cavity a little bit and shave it off to account for the unpredictability of closed cell expansion. However, expecting a perfect installation where everything is exactly 5-1/2 inches deep is unrealistic unless you want to pay for perfection. There's a reason the contracts always say 'average' depth or 'nominal' depth (I think 'nominal' isn't the best word to use, but I've seen it a couple of times - I think it's installers riffing off the language used for dimensional wood products).

      A 5-1/2 inch cavity will likely require more than one 'pass' by the insulator due to the maximum install thickness per pass requirement set by most manufacturers for most closed cell products (I mean, there's probably an exception out there, so I'm hedging). Talk to your installer about what product they will be using and the 'pass' or 'lift' depth.

  5. John Clark | | #5

    You could supplement the ccSPF with rockwool (aka mineral wool) batts. Rockwool batts are semi-rigid and approx 47" long. Friction fit them end-to-end between the 4' wide beams. In any case with only 6" of space it's going to be difficult to reach code (R49) for the ceiling.

    [ Is there a reason not to fill to the full depths of the beams (about 5-1/2 inches I believe) and trim off anywhere it would be bulge against the finished ceiling (or just compress it?). ]

    - Cured closed cell spray foam (ccSPF) is difficult to shave so installers don't typically fill the void entirely. It will not compress. Open cell spray foam (ocSPF) is easier to cut so installers will fill the void entirely and then shave it flush with the framing. If you're wondering I don't know if you could get away with only using ocSPF on that ceiling. There are those around here (aka Dana Dorsett) who reside in colder climate zone that would know the answer.

  6. Expert Member
    Zephyr7 | | #7

    You should ask your spray foam contractor about a hybrid approach -- closed cell directly against the underside of those roof boards, with open cell to fill the remainder of the cavity. This gets you the vapor protection of the closed cell foam, but cheaper open cell for the rest that can then be trimmed flush. This will avoid the problems of supporting nearly four-foot spans of batts, which even mineral wool is going to fall out of. This will likely cost a bit more than spray foam + batts, but with the contractor already on site, it won't be as much as a seperate installation of spray foam, and it gets everything done with one contractor so there is a big time savings.

    There is also no need for a seperate air barrier with an all-spray foam assembly, but you still need a thermal barrier (typically 1/2" drywall) since this is a living space. Basically that means you need to put up drywall and mud/tape the joints, but you don't have to make it pretty since it's getting covered with shiplap anyway. Rules for thermal barriers with spray foam can be found here:
    https://ncfi.com/content/uploads/2014/07/Thermal-Barriers-for-SPF-AY-126-Dec-2011.pdf

    I wouldn't trust only open cell foam in this assembly myself. Open cell can have moisture issues in some cases with assemblies like this, similar to what you read about in attics insulated with open cell spray foam. Closed cell spray foam is safer here where it will be directly against the underside of the sheathing. If you were to go with all open cell spray foam, I would want a vapor retarder on the interior behind the shiplap ceiling boards. I wouldn't try to completely fill those cavities with closed cell spray foam due to the difficulty with trimming and the cost of the material.

    Bill

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