Help me decide on insulation – Zone 5A
nlbailey
| Posted in General Questions on
I am working on finalizing my wall assembly in Zone 5A (Ohio). I have settled on doing 2×8’s with only cavity insulation in either fiberglass batts or mineral wool insulation. I originally thought of doing a bonfigliolo assembly but just couldn’t justify the time it would take to do properly over 2×8’s.
I’m not sure on how to balance cost / total r-value / ease of installation and what is “enough” r-value or if I should consider increasing it. Here are some assemblies I have calculated:
– $1.28/sq. ft: Two layers of R-13 Fiberglass for Total Wall Value of ~22
– $1.63/sq. ft: One layer of R-13 and one layer of R-15 Fiberglass for Total Wall Value of ~23
– $1.97/sq. ft: Two layers of R-15 Fiberglass for Total Wall Value of ~24
– $2.34/sq. ft: R-30 of Mineral Wool for Total Wall Value of ~24 (will also need something for vapor?)
If using fiberglass I will use faced batts for the interior layer. If using mineral wool I believe I will need to include a seperate barrier like MemBrain?
The full assembly will be:
– Vinyl cladding
– Either Zip or ForceField system
– 2×8 studs (with cavity insulation)
– Drywall
Going from R-22 to R-23 or R-24 seems like it would be a good idea but the jump to mineral wool seems hard to justify. I know that mineral wool will help with a more consistent installation but I will be doing in myself and can take my time making sure it is installed properly. Plus there would be the extra cost of a vapor retarder that would double the cost over R-22. Anything I’m missing?
Then is going higher than R-22 worth it? Will there really be a sizeable difference in comfort or reduction in HVAC needs from that increase?
Any thoughts to help me through this process and make a decision is appreciated!
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Replies
The R value difference between all 3 is pretty much noise. My suggestion is to use high density batts as it is much easier to get a class I insulation install.
You can also look at getting batts for bigger studs and compressing them a bit, might be cheaper and less work to install:
https://dcpd6wotaa0mb.cloudfront.net/mdms/dms/Residential%20Insulation/10017857/10017857-Building-Insul-Compressed-R-Value-Chart-Tech-Bulletin.pdf
In zone 5 you need a warm side vapor retarder, kraft facing work. With unfaced batts you can use poly or one of the smart vapor retarders.
Since you are using sheathing that must be taped, it will be your main air barrier but I would still put some effort in detailing an air tight drywall. Don't go crazy on getting it perfect but drywall gasket or caulk around the perimeter is pretty easy and takes no time.
Thank you! I kind of figured that was the case with a spread of R-2. The jump from 2x6 with a total wall value of R-18 to the 2x8 walls was a much easier decision. I have looked into using the R-30 batts and compressing them but that brings it down to R-27 which for a total wall value ends up being R-22.8 with a cost of $1.88/sq. ft. That can be matched by the r-13/r-15 combo and every penny saved I can use on another item to improve the house in another area.
I have begun thinking about the inside detailing as well and will take your advice for the drywall. I will be hanging it myself so this should be trivial to do. I found an article on Fine Homebuilding that I will take a look over.
It's nearly always worth it to detail the interior side drywall airtight, because about the only extra thing you have to do is to run a bead of sealant around the perimeter of each wall as you're hanging the drywall. The only real complication beyond that is that it's a little fussier to getthe drywall sheets into position since you don't want to smear the sealant too much. I do recommend one of the electric caulk guns though, to save your hands. I used to think those things were a joke until I tried one for this exact purpose...
I agree with Akos that a few points of R value here isn't going to make any noticeable difference. The only advantage mineral wool has here is that it would likely be a little easier to install well. I like using mineral wool for that exact reason. High density fiberglass batts can sometimes act as a sort of happy medium though, partway between mineral wool and "regular" fiberglass batts in terms of cost, but with the high density fiberglass having a somewhat more rigid form, making it easier to install properly.
I nearly always include MemBrain on the interior of exterior walls as some extra insurance against potential moisture issues. If you're not using any exterior rigid foam, then the interior side smart vapor retarder becomes even more of a plus.
Bill
Has anyone thought to cut a notch in the center of the 2x8, maybe full length minus 6-9" inches from each end for a thermal break? Bottom plate might could do the same. Top double plate, bottom the same, 2nd top plate non notch 2x6 center over the notch would allow 1.75" foam to add to a 2x6 2nd top plate. Maybe instead a 2x8 notch so that neither double top plate notches line. If this was a good idea, what size notch for a thermal break would be a benefit? Maybe a chainsaw notch?
woodreader,
It wouldn't make an appreciable difference. Wood has an R value of about 1.4 per inch, so the 2"x8" yields about R-10. The air gap might give you another R-1, but it comes with all sorts of complications.
Thermal breaks are a bit of a misnomer. They don't eliminate thermal bridging, they just replace a low R value material (in this case wood) with something higher (air) for part of the thickness of the assembly.
What you're describing sounds a little like a T stud (https://www.tstud.com/). I've never used these, but they are similar to what you're describing, and would probably work fairly well in a double stud wall. My personal preference is to just use exterior rigid foam though.
If you gouge out a lot of all the studs, I think you'd run into potential issues at inspection time. T studs have engineering data to back up their structural performance, similar to what Huber's Zip has for the Zip-R panels. If you have a chainsaw massacre of your studs, the inspector might frown on the result and red tag your project.
Bill
Thanks Malcom and Bill, ..nlbailey did a good job on home work to decide where to budget his money. Wishful thinking hoping your replies would state plunge cut .75-1.5" fill gap with some sheet foam and maybe stop height at a one floor ranch. ha ha nlbailey was not interested in bonfigliolo assembly so ignore the https://maineindoorair.org/a-practical-easily-built-wall-system-r-30-and-beyond/ , I love this but I have around 2700 annual heating hours where I live in zone 3 make 2x8 walls attractive, leaves more money to budget for air sealing, window shading and a good widows.
Woodreader,
Yeah, I thought long and hard about going with a bonfiglioli assembly but just couldn't justify it for myself. I did read the article you linked to and it is interesting. I actually feel like I may prefer that over the bonfiglioli assembly as it is more like a double wall. I'm curious what the reasoning in the 2015 IECC is for not allowing this assembly anymore? I was interested in the rim board being held back 2" to allow for rigid insulation to be added. I may have to look more into that and if it would be beneficial or not for me.
At the moment I've decided to go with using two R-15 batts for the insulation... though who know, I could change my mind :)
nlbailey,
What if we call the interior framing in particular around doors and windows interior bucks. What if around windows and doors instead of 2x4 we go 2" foam with 1.5x2 wood. Would we have plenty of wood for our interior trim to attach too? I think so. Also if true the article about windows perk my interest. https://www.zolawindows.com/post/how-windows-make-or-break-wall-performance-illustrated
Consider getting a bid for damp sprayed cellulose insulation.
You will get a better fill in the cavities at a lower cost using greener materials.
Walta
Hmmm... no one mentioned BIB Fiberglass. Blown-in-blanket is R4.2 per inch and using 2x8 framing would yield R31. The price tag is higher than batts but you avoid voids around electrical boxes, wiring and plumbing. Any thoughts from the crowd?
My current project is planning 2x6 with ZIP sheathing and then 1" XPS or Rockwool. BIB fiberglass R23 with R5 exterior for R28ish. I'm in climate zone 4C Western Oregon.