Replies

1. Expert Member
   Dana Dorsett | Sep 11, 2019 06:23pm | #1

   They're all good, if done well.

   The 2x6 + exterior foam is an easier build unless going for 4" + on the foam. Dense packing double studwalls is hard to do well, but it's a lot greener (cellulose insulation is sequestered carbon, with a negative carbon footprint.)

   A 2x6/R20 wall with 3" of exterior polyiso comes in at about the same ~R30 "whole-wall" thermal performance as a double-studwall with 9.5" thick cellulose, either of which is approaching the limits of financial rationality on an lifecycle energy cost basis.

   See walls 2b & 4 , and the corresponding discussions of Case 2 on p34 & Case 4 on p37 of this document:

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

   See Table 2, the zone 5 row of this document:

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

   It might even be rational to back off to the climate zone 4 row, given the now higher efficiency of heat pumps that 10 years ago, and the dramatic reductions in the cost of solar, and the fact that Indianapolis will be in zone 4 well within the lifecycle of a house at the current rate of climate change.

2. 
   Jon_R | Sep 11, 2019 06:26pm | #2

   I'd use MemBrain, cellulose filled 2x6s, tape or liquid sealed plywood, DrainWrap, 1.5" or more unfaced EPS, pressure equalized rain-screen, substantial overhangs. But there are lots of designs that work.

1. Expert Member
   Dana Dorsett | Sep 11, 2019 06:47pm | #3

   1.5" of cellulose doesn't quite cut it for dew point control a the sheathing in zone 5, but the cellulose + MemBrain + the higher vapor permeance of EPS offer a lot of forgiveness on that, and it'll be already be in zone 4 before 2050. From a whole-wall-R point of view (about R20 or a hair more) it's probably nowhere near the financial rationality limit either.

1. 
   Jon_R | Sep 11, 2019 07:10pm | #4

   > ... offer a lot of forgiveness on that

   So much so that it will outperform (moisture wise) most walls with more exterior R value and Class I exterior perms. Increase EPS thickness as you see fit for R value.

3. Expert Member
   MALCOLM TAYLOR | Sep 11, 2019 07:34pm | #5

   Adam,

   A lot depends on that the common practices are in your area. As Dana said, dense-packing cellulose needs experienced installers, and similarly, properly detailed exterior foam is something best specified where builders are familiar with it. The first thing I'd do is feel out some local contractors to see what they prefer.

4. GBA Editor
   Brian Pontolilo | Sep 12, 2019 09:22am | #6

   Hi Adam,

   This article takes a pretty close look at the assemblies you are considering, with some variations as well:
   How to Design a Wall