Cut and cobble insulation question

Hi,
I have a question about using the cut and cobble insulation method for an attic. Martin described the method generally in the following article: https://www.greenbuildingadvisor.com/article/cut-and-cobble-insulation
Here is our situation:
-Simple gable-style roof with a ridge vent
-Roof is fairly new, so replacing it and adding exterior insulation is not cost viable
– North/Central IL – Zone 5A
– Rafters are 5.5″
– Existing (attic floor joist) insulation is minimal and there is a wood floor overtop of it. Ideally, insulating the attic would address the current insulation deficiency and provide more usable space. There are no signs of rodent activity. The poured concrete foundation is still in great shape, which likely helps significantly.
– I came into a nearly free and massive supply of used polyiso. It is primarily 1/2″.
I would like to use the cut and cobble method to add 3.5-4″ of polyiso/spray foam to the rafter cavity, leaving a 1.5-2″ ventilation cavity. This part seems relatively standard. If it is 4″, then presumably I would have R26, but also thermal breaks and significant potential for cracks/air leakage to occur, especially with thermal/moisture cycles. I would like to add 2″ (4 offset and taped layers) of polyiso under the rafters. This would bring me to ~R39, with continous layers. My understanding is that the greatest risk with this system is moisture from the interior air infiltrating and reaching a cold surface. I am hoping to minimize this risk with several offset and taped layers. The interior ceiling geometry is very very simple (continuous slope to the floor), so I believe it will be straightforward to seal, especially from the bottom of the rafters.
The interior height is limited, so my strong preference is to make use of some portion of the rafter cavity, rather than add all of the insulation below the rafters.
I know this will be labor intensive. I have older teenagers that would love to help with the prep :). As far as the pratical approach, I was thinking about cutting the sheets into strips (8ft by 13.5″ for example). This could be done fairly quickly, especially by my cheap labor. I could then stack sets of 8 together (to get 4″), but offset the ends to develop the lengths that I need. I believe the total distance on each side of the ridge is ~18 ft. I could use a little adhesive to glue them together, I don’t think it would take too much. When they are ready for installation, I can attach temporary wooden strips across the bottom of the rafters that will hold the 4″ assemblies in place for spray foaming. This will leave 1.5″ above the assembly for ventilation. Once the spray foam has dried I can then remove the wood strips and trim any extra spray foam, so I have a flat, flush surface to mount the first layer of polyiso to the bottom of the rafters. I think the rest is fairly self explanitory, but I wanted to share the potential method as well. Although this whole process will take a lot of time, it does save the step of creating a separate baffle system, since it is somewhat integrated in the process.
The basic question is: Does this route minimize the risks or cause other potential concerns that I have overlooked?
Separately, I am very open to any thoughts, suggestions, criticisms, etc.
Thanks!
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Replies
Matt,
You will no doubt get good advice on whether this assembly makes sense compared to alternatives that others will suggest. I'll limit my comments to what you are proposing.
The risk that comes with cut & cobble is if moist air does get by the foam to the sheathing it can accumulate there and cause damage - but that's when the foam is in direct contact. You are incorporating a vent space, so that risk disappears. As described your assembly will work well.