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Louisiana perfect wall system cost? Roxul exterior paneling questions.

user-7188790 | Posted in General Questions on


I am in the planning stages of building a 500 sq  foot home in Southern Louisiana. We have a limited budget so we are only hiring a few of our carpenter friends to do the basic construction work. 

Because of the budget we are looking to price everything we can out well in advance. I was intrigued by Matt Risinger’s perfect wall farmhouse video since it eliminates the need for drywall and keeps heat from flowing through the studs (we are more concerned with heat getting in than out most of the year). Plus it looks very nice and makes renovations easier. We can run most services under the house. We wont have a gas line, plumbing is all in a 10 sq ft area and we wont have overhead lighting so exposed studs wont be a challenge for us. 

Roxul exterior insulation seems like a great product for the south due to the insect resistance, vapor permeability, mold and water resistance etc. I can’t find an insulation company who installs this locally. How complicated and expensive is this to install with a few handymen friends? Because of its vapor permeability can it be installed “sandwich” style with cavity insulation (unlike other exterior foam board). Is it possible for this to replace cavity insulation? Anyones experience with this product would be very helpful!

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  1. Expert Member
    Dana Dorsett | | #1

    Rigid rock wool is quite expensive compared to foam board alternatives (especially reclaimed roofing foam).

    Rock wool sheathing also quite a bit LESS than ideal in a southeastern US climate from a moisture point of view if you're going to be using air-conditioning. It's quite vapor permeable, and outdoor moisture would condense or adsorb into in the cooler layers inside your wall assembly. With no interior insulation or wallboard the structural sheathing will be below the outdoor dew point for much of the summer if air conditioned, taking on substantial amounts of moisture if insulated with air & vapor permeable rock wool on the exterior.

    I'm not sure what the cavity insulation sandwich problems would be with exterior foam and something else in the cavity. With the low-permeance foam on the exterior the air conditioning keeps the sheathing and studs dry.

    In US climate zone 2A/southern LA an IRC code minimum wall is 2x4/R13 or a U-factor less than U0.084 (= "whole wall R" of R12). You can get there with 1.5-2" of reclaimed roofing polyiso on the exterior, and NO cavity insulation or 2" of continuous rock wool if you also sheath the interior sides of the studs- wallboard, plywood, anything, as long as it's a reasonable air barrier to be able to count on the additional air-films in the U-factor calculations.

    There is a reasonable long term financial rationale for a whole wall-R of R15 (more if it can be done cheaply) which could be achieved with 1" of exterior polyiso and R13-R15 cavity fill. Alternatively 2"of foil faced polyiso or (for quite a bit more money) 3" of rigid rock wool and open studs comes close.

  2. Expert Member

    Matt Risinger's house is trickier than it at first looks to build. Because the framing is the finished interior, the lumber needs to be high-graded, and all and cuts sanded prior to assembly. Unlike houses with drywall, you only get one shot at things. Pulled-nails, misplaced holes for services and the like all will be visible forever. This is particularly true of the fasteners used to attach the thick exterior insulation. Consistently hitting the studs is a challenge.

    Detailing the exterior, getting the strapping coplanar and planning service runs are all a lot more involved that most builders are used to. I don't want to discourage you, but I'd reticent to taking o0n building a house like that even though I have a lot of experience. It just isn't a very forgiving way to build.

    1. user-7188790 | | #3

      Very very good info. Thank you. We are mostly looking for ease of build i.e cost effective building methods. I was originally looking for a cheaper alternative to drywall but this obviously isn't it.

  3. GBA Editor
    Martin Holladay | | #4

    First of all, can you tell us your name? (I'm Martin.)

    Q. "Because of its vapor permeability, mineral wool can be installed 'sandwich' style with cavity insulation (unlike other exterior foam board)."

    A. Actually, I'm not sure why you wouldn't want to use rigid foam as exterior insulation. Your "sandwich" comment is probably based on a misconception. For more information on using rigid foam on the exterior side of your walls, see these two articles:

    "Calculating the Minimum Thickness of Rigid Foam Sheathing"

    "How to Install Rigid Foam Sheathing"

    For more information on installing semi-rigid mineral wool on the exterior side of wall sheathing, see these three articles:

    "Installing Mineral Wool Insulation Over Exterior Wall Sheathing"

    "Installing Roxul Mineral Wool on Exterior Walls"

    "Wrapping an Older House with Rock Wool Insulation"

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