Replies

1. 
   fingol | Apr 10, 2025 04:37pm | #1

   A day later and a lot more reading :) and I think maybe I'm a little closer to an answer.

   It seems like as far as condensation on the back of the metal roof panel goes, as long as the metal roof is sitting directly against the underlayment, and the insulation cavity right below it (above the decking) is as airtight as possible, that I shouldn't really have to worry about condensation on the backside of the metal roofing panel, and therefor don't need a drainage plane in this location.

   The other location in my assembly within my exterior insulation cavity where I'm adding the slits in the bottom of my EPS foam panel for drainage is where I haven't landed on a solid answer just yet. It seems like if moisture does condense and drain downward towards the lower framing member below the insulation that it would sit up against this wood. One idea is to add some sort of pathway through the very bottom of this 2x6 so that condensed liquid water could escape, but since I need to keep this insulation cavity as airtight as possible maybe I should use a smart vapor tape over this opening, so that if moisture inside the cavity reaches a certain threshold it would be open and allow water out, but would limit or prevent airflow in. Sounds overly complex though and potentially hard to implement, so I guess the search continues.

2. 
   climbing_carpenter | Apr 10, 2025 09:01pm | #2

   Drainage UNDER the foam? New to me. Your assembly is nothing like the one you referenced at buildingscience.com.

   A vented air gap here will compromise the performance of the foam, and water issues here -bulk or vapor- are indicative of significant installation failures elsewhere.

   Any vapor drive from the interior would condense beneath the Adhero- but given the ratio of permeable to impermeable insulation, the Adhero should be warm (which is the goal) and condensation shouldn’t be an issue.

   Carefully detail the Intello, don’t compromise it installing the pine boards- that might involve strapping, and the standing seam will take care of bulk water. If not, you have a back-up there too.

1. 
   fingol | Apr 10, 2025 09:37pm | #3

   Thanks for the reply and information!

   Sorry, I should have made it more clear that the assembly in the picture was just to try and reference the foam on top of the Adhero with the framing below. I have an updated version that I think references the version from my previous comment without having the framing in the middle of my insulation, I've attached that one.

   The drainage under the foam was new to me as well and I had not really ever seen that anywhere, but I did more research and I did stumble across this article by Joe Lstiburek https://buildingscience.com/documents/insights/bsi-038-mind-the-gap-eh#P09 (picture 10) talking about this very thing, which is what turned me on to the slitting my foam for drainage. Maybe I'm just misunderstanding what is being referenced in this article though, but it matches what the Adhero manufacturer advised me about that I needed a drainage plane between the foam layer and Adhero.

   The most recent version I attached shows the switch in my foam layers to accommodate easier fastening and a better thermal break. I have been searching for days on EPS foam creep and shrinkage though and I can't find anything great other than EPS will shrink over time, which gives me pause over having the new roof deck resting on the EPS and having it shrink and cause the metal roof to buckle or warp. I had considered maybe adding some small sections of 2x6 supports within the foam at the plywood edges, so basically reduce down from a full length 2x6 all the way along the edge and just have some "island" supports of a 1 foot section of 2x6 at the plywood corners. This way there would be some solid support that won't collapse down even if the foam does over time, but I'd have significantly less thermal bridging this way.

   
3. 
   climbing_carpenter | Apr 11, 2025 07:15am | #4

   Spaced out, Adhero will allow drying to the exterior.

   The article you’re referencing, however, discusses walls. Apples and oranges. Bulk water is a concern with walls as most claddings allow its penetration. Bulk water is not a concern, except in catastrophic failure, with roofs. If you were to have a gap, geometry would come into play.

   You do not need a gap beneath the foam- you don’t even need the Adhero. Carefully detail the inbound Intello; vapor diffusion and air movement through the assembly shouldn’t be a concern. What is your intention with the Adhero? A vapor impermeable membrane instead here would isolate the foam and new sheathing from interior vapor drive while the interior of the cavity has good inward drying potential.

   Embed 2x4 strapping at 24 in centers in an outer layer of 1.5” foam for better roof deck nailing. Whatever membrane installed directly under the metal should be high temperature rated.

1. 
   fingol | Apr 11, 2025 11:05am | #5

   Thanks, honestly I hadn't ever thought of the drainage between the foam and the Adhero, it was only brought to my attention when I was discussing the project with tech rep for Adhero who said that I needed a drainage gap in this location because the EPS was essentially a vapor barrier and would prevent moisture from diffusing, and that any moisture that might condense on the EPS needs a place to escape. This led me down the path searching for how I'd accomplish getting a drainage plane inside my insulation cavity since I didn't have any space to add a physical layer because of the stack up height.

   The reason for the Adhero on the original roof deck is because it's 1x12 boards with decent gaps between, so I was using the Adhero as my air barrier to seal the deck and control air/vapor movement. Also I wanted a self sealing membrane for ease of driving fasteners through and having it seal around them once I've got the foam and layers on top. Because of the coverage needed in rolls for the original deck (just a tiny amount more than 2 rolls so I need 3), it ended up being simpler and the same cost to just add a 4th roll and do the layer under the metal roof as well. I spent time searching for an alternative peel and stick metal roofing underlayment and found multiple options, but there were not much difference in price than the Adhero.

   The most current design (attached) is the one I think that fits kind of what you're saying in terms of embedding the 2x4s into the 1.5" layer over the 4" layer, this way I can attach the 2x4s to the original roof deck, and then fasten the new sheathing to the 2x4s. The only thing I'm still wondering about here is the foam compression/creep/shrinkage over time and if I'll lose a small amount of height in the foam and the roof deck will slowly bow in the middle where there isn't any direct support between the original roof deck and the new sheathing. I have found articles talking about how EPS does shrink over time, but I haven't found anything conclusive that shows how much that works out to be in the real world.

   
4. 
   climbing_carpenter | Apr 11, 2025 01:43pm | #6

   Strapping should be horizontal.

   Your air control layer isn’t at the existing roof deck, it’s at the new Intello. No concern of air movement outbound of the roofers due to the EPS. You can, however, fill the gaps at the end of the board courses.

   Have you considered building a vented assembly over the existing? I do think getting all of the layers to plane out with the gambrel angles, as you have proposed, will be finicky work.

1. 
   fingol | Apr 11, 2025 10:57pm | #7

   Thanks for clarifying, I'll work on the horizontal strapping configuration then.

   Okay, I think I originally was thinking because I had an open board decking that I couldn't seal very well that I needed to use a self adhesive membrane to control airflow/vapor at the deck level, and the intello was going to manage moisture on the inner assembly.

   I had considered a vented assembly, but there are a couple of dormers not shown in my drawing that make the roof thickness a challenge because the window placement is so low I have to transition any additional height I have into a lower area in front of those dormer windows, so the easier thing it seemed was to just add insulation across almost the whole deck. Also there is a porch on the one side of the gambrel that is unconditioned and low slow which makes venting a bit more challenging. I agree getting all the layers even is going to be bit tricky, but I'm going to do my best to get it flush.

5. 
   AxDesign | Apr 12, 2025 04:47am | #8

   In this assembly, if your Intello is compromised at all in the fastening of the planks, it won't cause any problem.
   The Adhero above the 1x12 planks is what absolutely needs to be uncompromised. You've got R22 of EPS above that. Your 1x12 planks will never be a cold condensing surface.

   That's quite odd that you were told to put a drainage gap between the Adhero and the foam. Are you certain there was no miscommunication there?

1. 
   fingol | Apr 12, 2025 11:25am | #9

   Thanks, I'm going to strive for as close to perfect of an install on the Intello as possible on the interior, but I know there will be some tricky locations because the 2nd floor was built in such a way that it intersects the gambrel knee wall so I'm going to need to detail around a lot bends.

   For my Adhero layer I'll similarly aim for total sealing, and detail between the planks at the soffit edge with spray foam to seal that gap as well.

   I too was confused about the drainage gap with the Adhero, I had to clarify a couple of times to understand what was being conveyed. I was told that the EPS foam is (basically) impermeable and therefore no moisture can move through it, so if moisture is present inside the internal wall assembly and moves outward through the wood planks and Adhero that it will hit the back of the EPS layer and have nowhere to go and would potentially drive vapor back inward into the wall assembly. Therefore I need a drainage plane between the Adhero and EPS to allow a path of vapor to move freely.

   I can't say I totally understand this though. In my original understanding I thought that my interior wall assembly of rockwool and Intello allowed for vapor to be actively controlled through the Intello dynamically adjusting to the moisture level in the wall, allowing moisture to pass out when it reaches a certain threshold, and with the Adhero allowing vapor to pass through it as well. Within the exterior insulation cavity I thought any moisture that was present would be passed out through the layer of Adhero on the plywood deck that sits above the insulation cavity.

   I thought I was aiming to keep the original roof deck as airtight as possible with the Adhero, then my insulation cavity and foam would be staggered with taped seams and then sealed with plywood and another layer of Adhero. My intention was to make the insulation cavity as sealed as I could, and any moisture that was present could pass outward through the Adhero layer above it. But at this point I've thought so much about this I might have lost the thread.

1. 
   AxDesign | Apr 12, 2025 11:49am | #10

   re: "if moisture is present inside the internal wall assembly and moves outward through the wood planks and Adhero that it will hit the back of the EPS layer and have nowhere to go and would potentially drive vapor back inward into the wall assembly."

   What you are describing here is a fundamental rule for wall assemblies. The wall needs to be able to dry in at least one direction. Your assembly provides for that.

   No outside water should ever reach that lower layer of Adhero. Think about it. You have the metal roof shedding bulk water, then a drainage gap, then Adhero, then a plywood cover board, then 5.5" of rigid foam, then another layer of Adhero! At that point the lower layer of Adhero is just an air barrier preventing any warm moist air from entering the roof assembly.
   Anyone claiming you need another ventilation gap below the foam is just an example of uneducated product support at a company tbh.

   Due to the ratio of exterior to interior insulation, you could also leave out the intello and this assembly would be fine. There would never be condensation at the roof deck.

1. 
   fingol | Apr 13, 2025 02:33pm | #11

   Thanks, that's helpful to know and makes sense to me.

   I felt like I was largely taking a belt and suspenders approach with the various layers, and that as you pointed out, my metal roof and Adhero layer means no water should even make it into the insulation layer below, and then the insulation layer is protected from moist air coming from the internal house, plus adds another layer of water barrier to the internal house. Between that and the amount of exterior insulation I have that the original roof deck should never be at a temperature where condensation can occur anyways.

   I think at this point the last major design decision to make is how I approach framing or supporting the insulation layers. I've got the option of putting the 4" EPS down as the first layer and then embedding 2x4's as strapping in the 1.5" EPS layer above, attaching those 2x4's to the roof deck, and then putting the plywood over the top and attaching the plywood to the perimeter 2x6 frame and then the 2x4 strapping underneath. This version gives me the best thermal break, and ease of fastening.

   The other option is to keep the internal 2x6 framing with the foam layers inside of them and the plywood edges (mostly) sitting on the 2x6 framing. This adds a fair amount of thermal bridging, but if the foam shrinks over time I've still got the 2x6 framing to support the plywood and metal roof above so that my outer finish roof doesn't distort.

   Pros and cons either way, one gives up performance due to thermal bridging in exchange for dimensional stability of the roofdeck over time, while the other gives up that longterm stability in exchange for better thermal performance. I intentionally spec'd 25psi foam, so it should be more resistance to compression, but shrinkage will occur for different reasons no matter what.

6. 
   AxDesign | Apr 14, 2025 03:53am | #12

   Sounds like you're planning to use OC NGX 250 XPS.

   That product is highly dimensionally stable, but is rated for up to 2 % linear shrinkage over its service life under extreme conditions. Emphasis on "extreme conditions". In typical building applications, expect much lower movement—often less than 0.5 %—and negligible change in thickness.

   This does not compromise your first assembly above, which is the better approach here in several ways. In your second assembly, you would need to cut every piece of insulation in order to fit between a 24" OC framing layout to provide for fastening of the 4x8' coverboard to the 2x6 members. In your first assembly, you install full sheets of XPS (I recommend the T&G variant) then fasten the 2x4's on the flat (easier than 2x6 on edge), and end up with a wide nail base to fasten the coverboard to. You'll just need to rip the 1.5" XPS to the needed width, which is of course much easier than ripping 4" thick sheets!

7. 
   fingol | Apr 14, 2025 08:27am | #13

   Thanks, sorry for not clarifying on the foam type. I'm using 25 PSI EPS foam, I've attached the product sheet. It's type IX in this sheet.

   For the two plans I've added the different mockups with labels.

   Design #1 - This was my original version that used 2x6 as a perimeter and internal frame that the foam layers would sit within. I'd fasten the 2x6 to the original roof deck and rafters, then I'd fill in between the framing with my foam insulation. The internal 2x6 framing is spaced so that full 4x8 sheets fit within to minimize any cutting. Then I'd add plywood over the top and fasten the plywood edges to the 2x6 framing, and then fasten the middle sections of plywood directly through the foam layers below into the roof deck with longer 8" screws.

   Design #2 - This was the modified version like seen in the previous post, with only the 2x6 perimeter framing, then filling in the inside with the 4" foam and then adding the 2x4 strapping over it, then fastening the 2x4s to the rafters, then cutting and filling that 2x4 layer with foam in between. Then plywood over the top and fasten the plywood to the 2x4 beneath.

   All your previous comments still make sense, design #2 will be much easier to implement and install since I don't have to install extra 2x6's on edge, plus driving long screws through the middle of the assembly will be tricky. I guess the question is do you think the same holds true with my EPS foam as the XPS you previously referenced? If my EPS foam shrinks 1% in thickness this would be about .055, which is probably so small that in the entire stack of the assembly I'd never notice it in the finished roof deck. So maybe I should just go with design #2 for its merits and forget about design #1.

   Thanks for your help, much appreciated

   
8. 
   AxDesign | Apr 14, 2025 09:20am | #14

   No difference.

   The question here is shrinkage of the 4" of EPS beneath the 2x4 strapping, and 1% shrinkage of that would be 0.04". That's 3/64".

   In particular, that 3" is secured away from UV exposure, moisture, and extreme temperatures. As a buffer it has 1.5" of EPS above it, plus a plywood cover board, gap, and metal roof.

   0.5% is probably the extent of how much it may shift over time.

9. 
   fingol | Apr 14, 2025 11:59am | #15

   Thanks, that makes sense. I think I'll go with design #2 with just the 2x4 strapping in the middle as that will make things much easier and the performance will be better.

   Thanks for all the help!