Replies

1. 
   Matt_Millham | Dec 22, 2022 09:45pm | #1

   The back of the roof sheathing doesn’t need insulating; it needs to be dry. Venting is one way to help keep it dry. Insulating over the top of it is another, since keeping it warm enough to stay above the dew point keeps water vapor from condensing on or in it. While your suggested approach might be on the riskier side, I’d think you’ll be alright so long as your roofing and ice and water shield are able to keep the sheathing dry from above and you do what you can to prevent vapor from getting into the insulation from below - and what you propose is right on: a layer of sheathing with taped seams on the bottom of the trusses with zero penetrations in it should be a pretty good vapor retarder as well as an air barrier. You can mitigate risk further by keeping the humidity down in the heating season. The questions I have are 1: why dense pack and not loose and 2: how are you going to get the insulation in there- especially if it’s dense packed? If you were to use the typical netting, that might be more weight than it could hold, and even if it did, you’ll have a difficult (not to say impossible) time pushing any bulges up to attach your OSB below. If you install your OSB before you insulate, I’m not sure how you’d verify that you filled the space, and your inspector might not sign off on that approach, since insulation is something they typically check prior to it getting covered.

1. 
   caprahomeconcepts | Dec 23, 2022 02:39pm | #3

   Thanks for the thorough reply Matt. (Miss you on the Podcast!)
   I had dense-pack in my mind, but I think as you suggest, just blowing it as full as we can, and knowing it'll settle a little bit is still fine. I just want as much R-value between the conditioned ceiling and roof deck as possible. 28' of loose fill, would still put me in the neighborhood of R-95 though, which seems great.
   I was planning to have it netted so we could make pretty sure that everything gets filled, but I'll have to check with the insulator if that's feasible with the weight.
   I wanted to run the interior OSB ceiling and then insulate from the top, but we're building this right now, so the roof will be going on in a few weeks, and our North Idaho weather isn't very conducive to exposed fluffy stuff.
   If the bib can't hold it up, I'll have to talk with my inspector (he's awesome and very open to reasonable ideas.) I too would be worried about nooks and crannies not getting insulated so that's definitely a backup plan.
   Thanks again for your time!

1. 
   Matt_Millham | Dec 24, 2022 04:11pm | #6

   Thank for your kind words. Because this is kind of a risky assembly, consider installing sensors. The trusses will be at risk as well - perhaps more than the sheathing - and are considerably harder to replace. Ben Bogie covered this a couple years ago. It’s good to know one way or another if something is working, whether for peace of mind or to head something off before it becomes a serious problem. https://www.finehomebuilding.com/2020/11/09/why-we-need-building-sensors

2. 
   unclemike42 | Dec 23, 2022 09:04am | #2

   You might want to take another look at insulation over the roof decking.

1. 
   caprahomeconcepts | Dec 23, 2022 02:44pm | #4

   Mike, thanks for the reply. I know I should probably give this method more weight, but it's a 81-sheet roof, putting that option at about $4,000 per 2" layer. (that's just a Home Depot price, obviously I could whittle that down, but still probably looking at $10-11k

1. 
   unclemike42 | Dec 23, 2022 07:07pm | #5

   If any moisture gets into that mass of cellulose, it will migrate to the bottom of the roof deck.

   I have a neighbor who recommends doing things the second time first.