GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter X Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Furring Strips on Rock Wool + 4 Stories = Plumb Cladding?

spollin_psvdev | Posted in Green Building Techniques on

Based upon much research including John Straube’s “High Performance Enclosures” figure 4.11 I am interested in using rock wool insulation to the exterior of my exterior walls in an upcoming four story multi-family structure in Maryland (Zone 4).

So the sandwich from out to in would be:

Horizontal Cladding (Vinyl or hardie)
1×4 vertical furring strips
1.5″ Roxul ComfortBoard IS
WRB – Tremco EnviroDri Fluid Applied
2×6 16″ OC (lilkely insulated with Fiberglass Batt)

I think I have everyone on board to bear the additional expense of building what is a better performing and more durable wall but no one will take kindly if my “new” wall looks like wavy disaster.

Some on my team are concerned that the either: a. the weight of the siding in 4 stories and/or b. difficulty in consistently fastening the furring strips to achieve a plumb installation (because the rock wool does not have the compressive strength of board foam and may be compressed inconsistently as applied) could cause noticeable and inconsistent deflection in the exterior cladding.

We are forced to use XPS in sections of the wall (mostly 1st floor) where we have stone cladding (b/c of weight) but I like the vapor open, non combustible, and insect resistant characteristics of the mineral wool.

Anyone have thoughts or experience using mineral wool (i.e. Comfortboard IS) in taller walls of up to 4 stories with horizontal siding? I’m planning to build a mock up but do you have concerns about vertically “wavy” furring strips & cladding up to 4 stories?

Thank you for your consideration,

Shane Pollin

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. GBA Editor
    Martin Holladay | | #1

    First of all, a disclaimer: if you are talking about details for a 4-story multifamily building, you need to satisfy your local building inspector and follow the local building code for commercial buildings. I am not an expert in commercial buildings.

    Second, whether or not your siding ends up co-planar depends entirely on the skill of the installers. It can end up looking fine, or it can end up so wavy that it makes onlookers seasick.

    For more information on this topic, see:

    Wrapping an Older House with Rock Wool Insulation

    Installing Roxul Mineral Wool on Exterior Walls

  2. user-1120647 | | #2

    Shane - Look into Roxul CIS (commercial insulated sheathing) - it is heavier, but significantly more resistant to compression.

  3. pwalshe | | #3

    We used the Roxul Comfortboard IS for 2 storeys - but with a 12:12 roof the gable ends go up about 3 storeys. We used 2" thick roxul, 1x4 strapping and fiber cement boards. Installing the strapping took some extra care. That CIS sounds good, I did not know it was available when researching up here.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |