Replies

1. GBA Editor
   Martin Holladay | May 17, 2017 02:24pm | #1

   Dave,
   Really, the answer depends on your goals, your budget, and your level of experience.

   In general, I would agree that attaching furring strips through 4 inches of rigid foam will be easier than attaching furring strips through 6 inches of rigid foam. But plenty of people have successfully added 6 inches of rigid foam to the exterior of their wall sheathing.

2. Expert Member
   Dana Dorsett | May 17, 2017 03:56pm | #2

   If your goal is hitting Net Zero in a zone 6 climate, your wall is probably overkill. Take a look at Table 2, p10 of this document:

   https://buildingscience.com/sites/default/files/migrate/pdf/BA-1005_High%20R-Value_Walls_Case_Study.pdf

   They're suggesting R35 whole-wall for zone 6 as the likely sweet spot, and that was back in 2009 when rooftop PV was only 15% efficiency(requiring more rooftop real estate) costing USD $7-8/watt (installed) and cold climate mini-split heat pumps were in the HSPF 10-ish range. In 2017 the efficiency of the PV is in the 20% range (less rooftop needed) and cold climate heat pumps are running HSPF 12+ (20% more heat per kwh used.)

   With current class rooftop PV & heat pumps, odds are that you can hit Net Zero with only 3" of exterior polyiso with a PV array that fits comfortably on the roof.

   An R35 wall would be 2x6/R20 + 3" of exterior polyiso at the labeled R value, 3.5-4" of polyiso if derated for climate. Even fully derated 3" of polyiso is sufficient for dew point control at the sheathing without resorting to interior vapor barriers or vapor retarders tighter than standard latex paint on wallboard. The quick & dirty way to estimate the whole-wall-R is to assume the 2x6/batts plus sheathing, siding, wallboard, and interior & exterior air films is roughly R15 whole-wall, give or take (depending on actual framing fraction, siding type, etc.), then add the R-value of the continuous insulating sheathing layer.

   You won't be able to hit PassiveHouse with a 2x6 + 3" polyiso wall, but you might with a 2x6/R20 + 6" of exterior polyiso, if you're willing to spend a lot of time on designing the house to that standard, and a lot of money on super-performance windows.

   Another approach to consider is 2x4 framing using R4 fiberboard sheathing and 3" of exterior polyiso. The additional R4 of thermal break over the framing from the fiberboard makes up for the lower center-cavity R. That requires the timber screws to be an inch longer than with conventional structural sheathing, but the sheathing itself is far more moisture tolerant. The overall wall thickness is an inch thinner than the 2x6 + 3" foam solution. Higher R fiberboard sheathing isn't available at a reasonable price in the US, but in Canada MSL's R4 SONOclimat ECO4 can be had for CDN$30-35 per 4'x8'x 1.5" sheet:

   http://www.mslfibre.com/Produits/Fiche/13/SONOclimatECO4

   Pay attention to the structural capacity specifications and the fastener schedules when using any fiberboard sheathing. (It's not a direct swap-in for OSB or CDX in those regards.)

   Here is a peek of what it looks like when cap-nailed into place as well as more description:

   http://guideperrier.ca/revetement-isolant-sonoclimat-eco4/

3. 
   canadianexpy | May 17, 2017 04:28pm | #3

   Not trying for Passive house , too much $$$ I don't see the pay back
   Most likely vinyl windows maybe triple pane, and vinyl siding
   Just a good build that will do for the rest of my life! (I'm only in my forties)

   It seems that product is only available in Quebec,

   As for only 3" Polyiso I though it rated around R5.6 per inch cold weather, so at 3" plus a 2x6 whole wall at R15 I am below the R35. or did I miss something.

   Thanks Dana for the input

4. 
   canadianexpy | May 17, 2017 04:36pm | #4

   Martin
   Goals pretty good house with Net zero possible, that my family can live in for a long long time. Never plan to move.
   Budget is like everyone else ...tight ..but trying to invest in the house and not the granite counter top.
   I have no experience installing exterior insulation , and I may have the builders do it, just trying to get an idea so I don't get told it takes $$$ to install because of the difficulty.
   I just not sure why the issue with lining up long screws and studs??? As I said I'm not a builder.. just trying to understand .
   Thanks for your input.

5. Expert Member
   Dana Dorsett | May 17, 2017 05:22pm | #5

   "As for only 3" Polyiso I though it rated around R5.6 per inch cold weather, so at 3" plus a 2x6 whole wall at R15 I am below the R35. or did I miss something."

   What you missed is that it no longer actually takes R35 whole-wall performance to hit Net Zero with PV that fits on the roof, the way it did in 2009 when that document was drafted. R32 is plenty, even R30 if the rest of the house is up to snuff.

   Using 18-20% efficient PV panels instead of the circa 2009 12-15% efficiency panels makes a difference. Adding another inch of foam may be less cost effective than buying a few more panels, and the array will STILL take less space than it did in 2009.

   Even with the polyiso derated to R5/inch it would still be in the R30-ish range. With some attention to framing details the raw 2x6 + other-stuff can hit R16-R17, and with the seasonal average performance of the polyiso at R5.6/ inch that would add another R17, bringing it pretty close the R35 recommendation back in 2009. Pay attention to the other columns in Table 2. Another R2-R5 of wall-R isn't necessarily going to be the lowest hanging fruit, and going at it with a 6" foam-over just doesn't make sense if Net Zero is considered "good enough".

   *The recommended R75 attic has to have 20-22" of insulation depth that extends all the way out over the wall's top plate and then some. That much insulation also weighs something. It takes some forethought & design work to get there, but R75 on the attic floor is still going to be pretty cheap.

   *The recommended R20 basement/crawlspace wall can be 2.5" + 2.5" EPS insulated concrete forms, or it can be 1.5" of polyiso or 2" of EPS trapped to the foundation with a rock-wool insulated non-structural 2x4 interior wall, but it needs to be at least R15 whole-wall (the recommended level for zone 5) at the above grade section, and R20-R25 still makes sense if the fully above grade walls are R30+ whole-wall.

   * The R10 sub-slab insulation can probably be backed off to 2" of Type-II EPS (R8.4) for basement slab , but would need to be higher if the slab is a heated radiant floor.

   * With all of the above Net Zero might be do-able with the very best double-pane windows, if window size & placement are optimized for wintertime solar gain, and west facing window area is limited to avoid high peak cooling loads. Double pane glazing with low-E on surfaces #2 & #4 and argon fill would be pretty good, but could be prone to wintertime condensation during cold snaps. With low-E triple panes wintertime condensation becomes a non-issue, but it's usually a big step up in price.

6. 
   canadianexpy | May 17, 2017 05:41pm | #6

   And this is why I follow this site. The expertise and willingness to share , even if it cost a couple hundred it is well worth it.
   I will now have to do some re thinking. The plan was to use mini split, so I want to make sure I get the insulation up enough to eliminate having to use I large unit
   thanks

7. Expert Member
   Dana Dorsett | May 18, 2017 02:21pm | #7

   The cost of up-sizing a mini-split is really pretty small compared to the cost of a substantially higher performance building envelope. It's probably worth modeling the whole thing in BeOpt to find the financial sweet spot.

   https://beopt.nrel.gov/downloadBEopt2

8. 
   canadianexpy | May 19, 2017 12:48pm | #8

   I was looking at doing that, as soon as I have some down time.

   Thanks for the input Dana