Choosing an Insulation Strategy
We are in Deshler, NE – on the southern edge of 5A.
I was pretty set on 2×6″ walls with Comfortbatt R23 with 2″ of exterior insulation, hopefully Comfortboard. My supplier just got back to me that 2″Comfortboard would be $40k while rigid foam is $6k. I also learned that Zip R3 is only $6500 more than Zip. Last but not least, he misunderstood what I was asking and quoted me for 2×8 walls with Comfortbatt R30 and it was only $10k more than 2×6 w/R23. So for an extra cost of $10k I will have the same R value and less framer headaches (which will save me money, likely $4-6k).
Option A – 2×6″ walls with Rockwool R23; Zip or plywood; 2″ rigid foam; rain screen; siding
Option B – 2×8″ walls with Rockwool R30; ZipR3; rain screen; siding
Option C – 2×6″ walls with Dense Pack Cellulose (R21?); Zip or plywood; 2″ rigid foam; rain screen; siding
Option D – ???
I keep going round and round. Would appreciate your thoughts.
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Replies
I build in zone 6. I doubt that R3 will be an ideal thermal break; I'd suggest a thicker insulation. My recommendation would be 2" of Comfortboard, Polyiso or, a new product coming on the market- TimberHP in Maine is gearing up to sell their wood fiber exterior insulation board - made from wood chips. With an R20 2x6 & R 12 exterior, you should be good.
You could look into used Polyiso instead of the Comfortboard for the exterior rigid. This could get your cost down and you could still persue the higher R value.
I recently purchased used 1-3/4" thickness for around $0.40 per sqft.
Where did you source this?
Look on Facebook marketplace or Craigslist. Type in "rigid insulation." By me, in Detroit, there are lots of options. A 2" 4x8 sheet is $30 for example.
Option A or B with 24” stud spacing would likely be the best performing wall as your exterior insulation is the best (2”) on those two options.
From a strict performance standpoint (making some assumptions):
Uaverage = (Acavity · Ucavity) + (Aframing · Uframing)
Choice A: Uaverage = (0.75 · (1/32)) + (0.25 · (1/15.875))
1/ans= R = 25.52
Choice B: Uaverage = (0.75 · (1/33)) + (0.25 · (1/12))
R=22.96
Choice C: Uaverage = (0.75 · (1/30)) + (0.25 · (1/15.875))
R=24.54
---------------
2x6 R=6.875
2inch foam R=9
2x8 R=9
6.875+9= 15.875
23+9= 32
9+3= 12
30+3 = 33
6.875+9 = 15.875
21+9 = 30
I think I would lean option C (2" polyiso would have higher R than I used in the calc).
Or 2x8 but possibly drop the zip-R, for the dead simplest wall to build, but yes lowest performance. Can R-3 even be used? I have to admit I don't understand all the rules surrounding zip-R-- it seems people use it in ways that bucks the rules...
R3 is not enough exterior R to control condensation in your climate.
I also agree with poster #1 though: if you aren't building for a year or so, TImber HP may have some rigid wood-fiber insulation on the market, and hopefully it will be cost competitive with foam. Hopefully.
Option C means you need a competent contractor that can install dense pack cellulose properly. In some areas, such conctractors are relatively easy to find, but in other areas no one wants to do it. That might rule out option C.
Option A is easiest if you want to insulate yourself, since mineral wool batts install pretty easily, and it isn't difficult to do a good job even on your first try. I would probably go this route myself.
Zip R3 is really too thin to help you all that much here, so I wouldn't go that route. 2" and thinner is usually not enough to count as "thick" exterior rigid foam, and doesn't usually complicate the build all that much. I would try to find reclaimed or factory second polyiso for this to save you some money -- vendors for this material often advertise on craig's list.
Bill
If you're doing it yourself, Rockwool is by far the easiest to install. Fiberglass is cheaper, and typically installed poorly from what I've seen.
Houses typically lose up to 30% of their heat through thermal bridging - the 25% of the wall that is studs and plates have a far lower R value than the insulated bays, and you can often see this difference on the outside of the house in very cold weather. (unless you have vinyl siding) because the solid wood areas are warmer. Exterior insulation - when installed at the proper thickness - which depends on climate, but probably 2" of polyiso in zone 5, will insulate the whole wall - studs and bays, and warm up the interior wall so the batt insulation works better. So a good level of exterior insulation is worth far more than a thick wall surrounded by studs of any thickness. R value is one metric but not the only one by far.
In addition to reducing thermal bridging, the exterior insulation increases the durability of the envelope by reducing the risk of condensation on the interior face of the sheathing by keeping the sheathing warm. However, in your climate zone, to really get this benefit, you'd probably need more exterior insulation (~41% of the total R-value of the wall)
andy,
In zone 5, you need R-7.5 for a 2x6 wall.
Woops, I think I pulled that ratio from a table for roofs. my bad
Seems to me you are not considering price in making your choice.
Consider damp sprayed cellulose often this is the low cost option in terms of Rs per dollars by the square foot and you get a better fill than with rockwool.
I think the smart move is to get bids for the different walls convert that number $ per SQF enter those numbers into a computer model of your home and find the wall with the lowest cost to own given your local variables over the next 20 years. BEopt is a no cost modeling software but will require 20 hours or so for training and data entry.
https://www.nrel.gov/buildings/beopt.html
https://www.youtube.com/playlist?list=PLHC0xDtkdjgec8QhVt7exJY3tpSLEFk-d
Walta
Option B won't work in zone 5. Even with a warm side vapor retarder you need more exterior insulation, take a look at table 2B here:
https://www.appliedbuildingtech.com/system/files/abtg_rr_1701-01_moisture_control_guidelines.pdf
With R30 batts, you need to bump up the Zip to R6 at least.
A 2x8 24" OC wall without any exterior insulation is actually a pretty decent high R value wall, not sure if it is worth while to complicate your build by adding Zip R to it.
Is it just me, or are the ratios listed in that table different than the prescriptions in the current IRC?
IRC: R 7.5 for 2x6 wall (R 23?) = 0.25
whereas table 2B with a class III interior lists 0.35
Table 2b is listed as "minimum insulation ratio." Am I doing the math wrong since it's 'ratio'? 7.5/23 instead of 7.5/30.5 ?
I don't know what the source data is for the ABTG report. Their values for a wall with Class III vapor retarder seem to be more conservative than the IRC. Nothing wrong with a bit of extra insulation.
I like the fact that it specifically deals with walls with different VR ratios especially ones with no VR that sometimes come up when dealing with T&G wood paneling.
I see now that since the ni VR ratio is above 1, it is in fact a ratio (as they said!) and not a percentage like I'm used to. So it doesn't look that different after all.
If you wanted to pursue the 2x8 wall, but wanted a bump in R value, the Tstud 2x8 is an option. About $20 for a 10' board by me right now, so it definitely costs more than a base 2x8. But if it eliminates more labor and the costs of foam to get you close to a true R30 wall, it might start to balance out. You'll need to use loose fill of some sorts though.
2x8 + 2" Rockwool.
Foam insulation, most notably poly iso has performance issues in colder climates unless it's covered by another layer such as mineral wool. My favorite combo is 2.5" poly + 1.5" rock wool on the exterior. 2x6 wall cavity with rock wool. Bomber wall assembly for cold climates. You can add a service cavity to the interior if you'd like, another R-15. Rainscreen it all.