Replies

1. GBA Editor
   Brian Pontolilo | Mar 13, 2020 09:43am | #1

   Hi Jeff.

   You are sure to get lots of opinions on this because there's more than one way to build for Climate Zone 4 that can be durable and efficient. Get the details right and many assemblies will work. But there are limiting factors to consider.

   If you are considering the complexity of details in relation to knowledge and skill, that may limit things. For example, installing exterior rigid foam and a rain screen complicates flashing details. Using ZIP R, may make it possible to do a good and efficient job for a builder how has never built a high-performance home with exterior continuous insulation.

   Similarly, if you want to consider environmental issues and embodied carbon, options are also limited. In this case, you'll choose cellulose instead of mineral wool.

   These are just two examples. I suggest you start with the articles on this page: All About High Performance Wall Assemblies.

   And here's a new article about working with ZIP R: Working with ZIP R-sheathing. And no, the insulation isn't on the wrong side of the sheathing. You just have to understand the implications of the insulation's location.

2. 
   user-723121 | Mar 13, 2020 11:13am | #2

   I will throw this out once again. Heating degree days for your location divided by 180 for wall R-value and and 120 for ceilings. This would equal superinsulation and (PGH) pretty good house. An ACH50 of 1.5 or less is very achievable and will give good performance results with the above HDD climate based R-values. Foundation and sub slab insulation is all part of the package.

1. 
   DCContrarian | Mar 13, 2020 01:07pm | #3

   I'm in Zone 4 and my HDD is 3800. So that would give about R20 for walls and R30 for ceilings. Code is 20 for walls and 49 for ceilings.

   I read somewhere (can't remember where) this rule of thumb: Take HDD and divide by 180 for your walls. Double for the ceiling. Take half of walls for basement walls, half of that for below the slab, and half of that for windows. Here's what that gives, code requirement is in parentheses:

   Windows 2.5 (2.85)
   Below slab 5 (NA)
   Below Grade 10 (10 continuous, 13 cavity)
   Walls 20 (20 or 13+5)
   Roof 40 (49)

   The idea is that if you want to go beyond code it gives you a metric for increasing proportionally and not over-insulating.

2. Expert Member
   Dana Dorsett | Mar 13, 2020 01:56pm | #4

   > "Heating degree days for your location divided by 180 for wall R-value and and 120 for ceilings. This would equal superinsulation and (PGH) pretty good house. "

   That's a bit overkill, and unless the insulation is a carbon sink (like cellulose or denim), may be more damaging over it's lifecycle than a code-min house.

   In Table 2, p10 of BA-1005 the middle-of-the road "whole-assembly-R" values from a lifecycle economic perspective would be something like:

   Walls- R25

   Attic- R60 (unvented insulated roof -R45 )

   Foundation walls- R15 (R7.5 slab-edge, if slab on grade)

   Exposed floors- R30

   Windows- U0.30, SHGC <.35

   But how you get there makes a HUGE difference. A wall assembly of 2x8 24" o.c. stud wall full of cellulose has a center-cavity R of about R27, and a couple inches of fiberboard sheathing would bring the whole-wall R to something around or over R25, and it would have a NEGATIVE carbon footprint (it's all sequestered carbon.)

   A 2x6/R23 with 1.5" of exterior polyiso would also be in that range, but a significantly larger carbon footprint.

   If blown cellulose isn't an option, using denim batts instead of rock wool is substantially lower-carb than rock wool, and has shares similar beneficial hygric buffering & thermal diffusivity characteristics to cellulose, but is more carbon-neutral rather than nearly carbon-negative, depending on how the accounting is done:

   https://havelockwool.com/wp-content/uploads/2018/03/Carbon-emmission-comp.png (<note, the extruded polystyrene in this comparison wood only apply to the CO2 blown goods used in Europe, not the HFC blown stuff XPS found in North America.)

   https://materialspalette.org/wp-content/uploads/2018/08/CSMP-Insulation_090919-01.png (, the XPS here is blown with HFCs.)

   Denim R30s have a manufactured loft of 8", and would perform at about R28 when compressed to 7.25" in a 2x8 stud cavity. That would only require ~R3 continuous insulation to hit an R25 whole-wall value, though more would be fine. Half inch or 3/4" foil faced polyiso would be greener than (but not as fireproof as) 1" rigid rock wool. The high density per R of rigid rock wool makes it higher per R polyiso (about half the impact of HFO blown polyurethane per R).

   The 2x6/R23 + 1.5" polyiso approach is pretty much splitting the difference between case 2a & case 2b analyzed in this document, with a slightly higher center-cavity R (See Table 3 for the comparative whole-wall R performance):

   https://www.buildingscience.com/sites/default/files/migrate/pdf/BA-0903_High-R_Value_Walls_Case_Study_rev_2014.pdf

   The hygrothermal analysis of case 2a/2b starts on page 31 (p.35 in PDF pagination.)

   Case 3 would also you there using only cellulose (or denim batts) if the cross framing is 2x4 rather than 2x3, using the same 2x6 studwall.

3. 
   user-723121 | Mar 13, 2020 07:38pm | #5

   As I think about it my recommendation is more of a cold climate recipe. It was devised at the 2nd Annual Passive House Conference by Harold Orr. It is interesting to see R-49 ceilings for a zone so far south, I suppose heat saturation and AC are a large factor.

4. 
   andrewofsaintlouis | Jun 16, 2023 04:44pm | #6

   I am an amateur building scientist, I live in Zone 4, and what you describe is nearly exactly what I recommend and have built for myself...2×6 studs (16 oc), rock wool cavity insulation (R21), zip system sheathing, 1 1/2 to 2 in thick exterior insulation (EPS), furring strips and cladding.

   Cheers!