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

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)
1x4 vertical furring strips
1.5" Roxul ComfortBoard IS
WRB - Tremco EnviroDri Fluid Applied
OSB
2x6 16" OC (lilkely insulated with Fiberglass Batt)
Drywall
Paint

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

Asked by Shane Pollin
Posted Mon, 12/02/2013 - 16:59

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3 Answers

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1.
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Shane,
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

Answered by Martin Holladay, GBA Advisor
Posted Tue, 12/03/2013 - 06:14

2.
Helpful? 0

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

Answered by Trevor Trainor
Posted Tue, 12/03/2013 - 09:53

3.
Helpful? 0

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.

Answered by Patrick Walshe
Posted Tue, 12/03/2013 - 22:13

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