insulating low slope epdm roof from ceiling below

Hi. I recently had an Epdm roof installed on my 140 year old house. It’s low slope and meets with a gable roof on the other part of the house. The Gable is standing seam. We had a tapered ISO board system installed as the underlayment to help with drainage and make the edge details easier to deal with. I reframed the roof and sheathed it myself before the roofing contractors started so I could save money. At the time the ceilings underneath the roof were all closed so I installed 4 inch rockwool batts right on top of the plaster/lath of the ceilings below. I was trying to maintain a 2 inch air space for ventilation and even though 4 inches doesn’t give me much R value it was better than nothing which is what it had been before.
I then created a continuous soffit vent with cor-a-vent and made sure there was at least 2 inches of space between the old skip sheathing of the gable so that it could all share the same vent space. The roofers installed a ridge vent on the metal gable roof.
Long story short. I have since torn open the ceilings of 2 of the rooms under the EPDM roof meaning all the insulation I did above those rooms needs to be redone. I started researching again recently because of the cold winter we’re having and realized that because I have ISO board on top of the roof sheathing, the roof assembly should probably be an unvented “warm roof” situation.
If so that’s fine for the two rooms I have open. The other room will just have to stay the way it is.
My question is…..is my thinking correct on this? Should I ignore the fact that I built a soffit vent and just fill the cavity between the drywall and the underside of the roof sheathing? And if so is rock wool a good product to use considering I can really pack it in there much better than fiberglass. Note- the space is very small….too small to access from gable attic in any way. 6 inches at the eaves maybe 11 or 12 where it meets the gable
Also, if I really need to go with some sort of foam insulation could I get away with cutting sheets of 2 inch rigid foam, pressing it to the underside of the roof sheathing, spraying the voids and gaps with canned spray foam and then filling the remaining cavity with Roxul? I really don’t want to rent a spray foam kit or hire a contractor for these relatively small spaces. Budget will not allow. Thanks.
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Low slope insulation
I can't advise you as to the best combination of materials but we did something similar in the vaulted ceilings of our house. In our case there is no attic and the roof is framed with 2 x 8s. We took out all the ceilings and glued 4 inches of Celfort XPS to the underside of the roof in each rafter bay, then used many tanks of two part closed cell spray foam to fill the gaps between the XPS and the rafters. I also sprayed about two inches of closed cell foam over the XPS and between the rafter bays. There is no ventilation or poly vapor barrier.
That was done about five years ago. In early November 2014 we took out a 6' x 35' section of roof and insulation to tie rafters for a kitchen addition to the existing roof and found no evidence of moisture or dark streaks from air movement. One of the objectives with a hot roof is to stop inside or outside air and moisture from moving into the area, and we seemed to have been successful. I also contacted a well-known building technology consulting firm about building envelope details for the kitchen addition, which included discussing insulation and vapor barriers in the existing house. They suggested that a thin layer of two part closed-cell foam on the underside of the roof, then a layer of XPS, filling the remainder of the cavity with rock wool, and using a smart vapor barrier would provide good insulation value, and the rock wool would reduce external noise better than just XPS. In our case, using the XPS is a way of achieving the required R value in the 7 - 8 inch depth of the rafter bays. If you have a greater space you may be able to use rock wool only as long as all the air leakage from outside is sealed off.
A few things I learned: I've used both a table saw and a hot-knife for cutting XPS. A table saw gives a cleaner, more even cut but produces a lot of foam particles. If you are doing the cutting in an enclosed area be sure to hook a vacuum with a container bag to the table saw, otherwise the vacuum filter will plug up quickly. Closed cell foam is denser than canned foam and seems to do a better job of sealing and insulating (purely subjective opinion), but it comes out a greater pressure than canned foam and takes a while to learn the best technique for dispensing. If you use canned foam to seal around the edges of XPS, dispense it with a professional gun because it is difficult to use the consumer canned foam with straws. I found that I needed a short barrel and long barrel gun to get into all areas of the roof. It's very difficult to tight fit XPS between rafters especially if you cut it with a hot knife (uneven sides), or the rafters are twisted. So leave a gap of about half an inch or more between the XPS and rafters so that you can get the gun in the gap and fill it, otherwise it will be difficult to do a good job of sealing around the XPS.
If you skip the rigid foam and use only XPS, you should ensure that the existing ISO is preventing air leakage otherwise you should flash the underside of the roof deck with a layer of spray foam. You would need a two-part system for this task because it would be tedious to cover the underside of the roof deck with bead after bead of canned foam.
Finally, when you apply drywall, use some of the airtight drywall techniques that have been suggested in several Fine Homebuilding articles and videos.
Thanks so much for the detailed response. I was looking for something I could do relatively easily and this sounds like it. I wasn't sure about what type of rigid foam to use. I was picturing the foil faced stuff but the XPS you're describing is more like the 2 inch thick pink kind they have at the box stores. Cool. I've also cut that on a table saw. Definitely the way to go. And then filling the remainder of the bay with rock wool sounds like exactly what I had in mind.
I'm a little confused on the use of a vapor barrier. Never used the smart vapor barriers before. That would probably be a special order item from the lumber yard. Is this a critical step in this sort of assembly? I feel like I remember an article about a warm roof being able to "dry to the inside" and therefore having no vapor barrier. Does that sound right? Thanks again for your help.
Thanks So Much
Hi. With respect to your vapor barrier question, my understanding from engineering reports and depending on your geographic location, 50 - 75 mm (2 - 3 inches) of continuous XPS acts as a vapor retarder. However, in the case where the XPS is going to be against the roof deck and then rock wool towards the drywall side, the vapor retarder (depending on how thick the XPS is), would be inside the insulation structure rather than on the warm side of the building (I'm assuming you are in a colder area of the country). In my area building inspectors insist on a vapor retarder such as plastic sheeting even if the insulation is 6 inches of closed cell foam, which is why the engineer suggested a smart vapor retarder (Google 'smart vapor retarders').