Replies

1. Expert Member
   MALCOLM TAYLOR | Jan 20, 2023 06:16pm | #1

   Eric,

   Would it not make more sense to just put on a peel & stick membrane like Grace Ice & Water, and reserve the Zip for the roof sheathing above your foam when that goes in?

1. 
   symphyotrichum | Jan 20, 2023 07:29pm | #2

   Hi Malcom,
   Thanks for your quick reply. A potential issue with some peel & sticks such as Grace Ice & Water shield (although opinions appear mixed) is the issue of them chemically reacting with the knots in conifer woods, as mentioned in this post: https://www.greenbuildingadvisor.com/question/grace-ice-water-or-select-not-recommended-for-tongue-groove-boards
   I am also concerned with potential odors from these membranes if they are directly above my tongue and groove boards. I am not sure if this is realistic or not but I strongly dislike the "tar paper" smell in some older houses on warm summer days when you pick up smells from attics. I won't have an attic of course, but am wondering if the same phenomenon might occur.
   That same post mentions that many manufacturers recommend a sheet product anyway (and advise against placing the membrane directly on the T+G.
   These potential issues (which may not be valid? - I'm not sure on this) plus the fact that I have the zip on hand for about the same price that I can get Grace I+W for prompted me to consider this option. If I do go this route is anything required under the zip? Thanks for helping me think this through,
   Eric

1. Expert Member
   MALCOLM TAYLOR | Jan 20, 2023 09:02pm | #4

   Eric,

   Avoiding anything that might cause odors in the interior makes a lot of sense.

   If you have the Zip there is no downside. I just looked back at your older thread and see you intended to use strapping for the roof anyway. As maine_tyler said, nothing underneath the Zip would add much.

   Good luck with the rest of your build!

2. 
   maine_tyler | Jan 20, 2023 08:18pm | #3

   Hey Eric,
   Perhaps the mento product is out due to needing to order it? That combined with having the zip handy? Because having worked with mento 1000, I can say it is robust, tapes well, and definitely doesn't smell. The 3000 meant for roofs would surely be solid and provide lasting weather protection (how long a delay are you anticipating?).

   But if your really just hoping to use the zip, I don't see why you would need anything beneath it. At least based on what you described as your roof stack up previously. Where did you land on metal vs shingles, strapped vs sheet gooded, vented vs not?

   Your foam layers will be quite impermeable. If you're using a foil iso as your lowest(first) layer that could also be taped if you wanted belts&suspenders, but it's really air, not vapor, that's the main concern unless you've opted for plywood topped unvented(?).

1. 
   symphyotrichum | Jan 21, 2023 06:13am | #5

   Thanks maine_tyler and Malcom,
   I am leaning towards shingles and would like to go with the unvented roof as Malcom suggested in a response to my earlier thread (although my changes may alter his suggestions). I have access to a mix of foli-faced and kraft-faced polyiso. My plan was to place the foli-faced down first and tape the seams like you suggest, then stack up the kraft-faced layers, and top with an EPS layer. I'm shooting for an R60 roof so it will be pretty thick!

1. 
   maine_tyler | Jan 21, 2023 10:14am | #6

   I think as long as whatever you put directly on your spruce decking is a solid air barrier, you will be fine with relying on the taped iso as a vapor throttle. If you are placing plywood/osb directly on the foam for shingling without venting, then technically vapor could be a factor, but I think the foil iso should handle that. Best for it to be placed down lowest in the assembly as you plan.

   But if you are using shingles and thus need a sheet good on top, I do wonder if Malcolm's original suggestion to save your sheet goods for that layer wouldn't make sense ? Obviously that decision may depend on more factors than we are aware of.

   The only other thing I would add is that r60, when continuous as it will be, is quite a lot and could add logistical challenges with the super long screws you will be requiring. I'm all for lots of insulation, but I have come to understand that with a thermal bridge free assembly made of foam, going to that extreme may be well past ROI (economic and environmental both) for a lifetime or more perhaps*. There is the U-factor calculation to consider (meaning because you will have no thermal bridging your effective R is a good bit higher than R-60 placed between rafters.

   * I'm just guessing here, not running any numbers.

1. 
   symphyotrichum | Jan 22, 2023 06:23am | #7

   Maine_tyler,
   Thanks for these comments, particularly the U-factor consideration. This helps a lot in the decision making process,
   -Eric

3. Expert Member
   Michael Maines | Jan 22, 2023 01:46pm | #8

   The only concern I can think of is the potential for ice dams to form while you're waiting for insulation, especially if the house is heated but even if it isn't heated. I'd use a roof rake after every snow storm and be prepared to use a heat cable at the eave if you get significant icicles. You don't want that pretty spruce decking to get tannin stains!

1. 
   symphyotrichum | Jan 23, 2023 08:46am | #11

   Hi Michael,
   Thanks for this. I will make sure I am vigilant about snow removal. We were lucky (weather wise) to be able to treat the timber frame itself with Heritage Natural Finishes before the latest round of repeated ice/snow storms we have been getting in Kennebec county. Although expensive, we have also been using that same product to treat the undersides of the spruce decking prior to installing it.
   -Eric

1. Expert Member
   Michael Maines | Jan 23, 2023 09:13am | #12

   Sounds good! I'd still be careful about water stains, even with the finish.

   We're neighbors; I'm in Palermo.

4. 
   onslow | Jan 22, 2023 04:41pm | #9

   symphotrichum,

   There may be an infiltration problem lurking that hopefully is already addressed. If your roof decking boards extend past the timber framing to form the gable end eave, then there is a pathway from inside to outside at every joint. Shrinkage over time will tend to make the gaps between deck planks larger not smaller. Have these gaps been sealed?

   I believe there was a problem with decking gap infiltration with a brand of modernist homes sold in the northeast in the 70's and 80's. The bigger problem with the homes was insufficient insulation above the roof deck. The icicle towers were monumental.

1. 
   symphyotrichum | Jan 23, 2023 08:42am | #10

   Onslow, I am not extending my decking beyond the timber frame. I will build overhangs and attach them over the insulated shell. Thanks for your comments,
   Eric