Wall & roof assembly questions for Zone 7/8 — new build
New build in zone 7/8, Colorado mountains at 8,600 feet elevation. Construction will start in a few months so I still have time to work through some details.
I have a few questions on the wall and roof assemblies that I was hoping to get opinions on.
The above grade walls will be 2×6 with dense pack cellulose cavity insulation, sheathed in plywood with a WRB and then 4″ of rigid mineral wool overinsulation.
The below grade walls will be 8-12″ CIP concrete, interior 2″ EPS foam with taped seams, then 2×4 stud walls
There is a cathedral ceiling in part of the house that will be 16-18″ TJI’s with cavity insulation. The interior finish is a structural tongue and groove.
The remaining ceilings are flat with traditional roof trusses and will be insulated with loose fill cellulose.
– For the above grade walls I was planning on using a smart interior membrane as both vapor and air barrier. I was not planning on using a service cavity, instead I was planning to use air tight electrical boxes and chase walls for plumbing. I am wondering if the interior membrane is necessary or if it makes sense to tape the seams in the sheathing instead and use that as the air barrier. I am not sure if a vapor barrier is required by code, I believe they are still on 2009 IBC.
– Detail for continuation of air barrier at rim and band joists – I was planning on using this detail from four seven five and inboarding the band joists by 2″ to allow the WRB to be wrapped around the joists and tied in to the interior membrane.
The 2″ cavity exterior of the band joist would be insulated with roxul and then sheathed over. I wanted to avoid using spray foam in the build but I wonder if it would be more cost effective and/ or fool proof.
– In the basement cavity walls, since they are separated from the concrete by 2″ of EPS can I use dense pack cellulose, or would it be safer to use roxul? No interior membrane would be used on these walls, the taped EPS would serve as air/ vapor barrier.
– For the cathedral ceiling – this part of the house is timber framed and the tongue and groove ceiling is structural. It would be built from the inside out by applying interior membrane to the top side of the tongue and groove, then over framing the ceiling with I-joists on top of that. The cavity will be insulated then WRB will be installed over the top of the I-Joists, furring will then be installed over this to create a vent space. Should I insulate this cavity with dense pack cellulose or would Roxul be a better choice?
– I was planning on using either Siga or ProClima products for the membranes and associated tapes and adhesives. Is there a clear better choice between the two? It appears to me that Proclima Intello plus may have an advantage over Siga Majpell for the interior membrane simply because it is a smart membrane. Both brands have very good reputations for the quality of their tapes. Siga is distributed by a local vendor so I would not have to mail order it, this has me leaning toward using Siga unless there is a good argument for Proclima. According to my take offs the cumulative cost for either brand will be very comparable.
– From anybody that has experience with mineral wool overinsulation, I am installing wood siding on furring strips as the wall cladding. Any input on the type of screws to use for attaching the furring strips? Heco topix therm are recommended, they are a double threaded screw and seem like a great option but they are expensive. I have also looked at pancake head SIPS and timber screws, are these a valid option and what other options are there that I haven’t considered?
Thanks for any input.
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