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How to attach cladding to outside of house when using rock wool for exterior insulation?

Jeff Newman | Posted in General Questions on

I just found this website from an Fine Home Building podcast. I really enjoyed the podcast.
I have a 1902 home that is two story and uninsulated. I thought about insulating this house from the outside by using rock wool because of its impervous to moisture and i could strip off the existing vinal siding and place a water control layer ( peal and stick or some sort of barrier) then take Rock Wool sheets to insulate over the outside of the existing walls. My problem is that Rock wool has an R value of R3 or R3.3 per inch. If i want R15 wall, I will need 5″ of Rock wool. I know shear is not effected but how do i put this much insulation on an exterior wall, add lath, and then add clading. If i built out the wall with a 2X6, I will have thermal briding to the inside from the 2×6, so would would be best or is there another way to add the clading to the outsid of this Rock wool ( thermal layer) of the house. thanks in advance. great website!

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Replies

  1. Expert Member
    Dana Dorsett | | #1

    Open blown very low density rock wool might be R3- R3.3 per inch, but that's not what you'd be putting on the exterior.

    Very high density rigid rock wool sold in North America designed to be installed on the exterior runs R4/inch, so you'd beat R15 on the exterior with just 4".

    Mid density rock wool batts designed for 2x4 cavities run about R4.3/inch an are rated R15. But they are only 3.5" thick, not 4", and thus not a great fit if the house was built with rough sawn full dimension 2x4 framing (as many houses were during that time period).

    Drilling and filling the cavities from the exterior with blown fiber (cellulose fiberglass, or rock wool would allow even thinner exterior rock wool to hit the same thermal performance numbers. The amount of exterior R necessary to keep the moisture content of the sheathing under control varies by climate. Where are you located?

  2. Jeff Newman | | #2

    Dana,
    I am located in Kentucky. Climate zone 4. The thought of using exterior rock wool sheeting, was the ability to insulate and break the thermal bridging from all of the wood framing. This house is 2 story and has a crawl space as well, so in my thinking, the exterior insulation would solve the problem on how to insulate the Rim joist of the 1st floor and also the 2nd floor. I just dont know the best way to do lath and clading.

    1. Expert Member
      Dana Dorsett | | #4

      Unlike colder climates, in zone 4A there is no minimum R value for the exterior R to have reasonable dew-point control at the sheathing. Even with no exterior R standard latex interior paint is a sufficient vapor retarder to avoid excessive wintertime moisture accumulation in wood sheathing of framed wall with insulation in the cavities.

      Empty stud bays are huge thermal bypass and fire-spread channels, stack-effect infiltration drivers, and only worth about R4 "whole wall" even when including the thermal performance of interior & exterior cladding & air films, even when made air-tight. Filling a 2x4 framed wall with R13 fluff cuts the heat transfer by more than half even before adding exterior insulation. Filling the stud bays with cellulose and adding even 1.5" of exterior rock wool (or an inch of foil faced polyiso) would beat current IRC code minimums, delivering a "whole-wall R" of R17 or more.

      1. Jeff Newman | | #5

        Dana,
        Thanks for your replys. Just wanted to touch on something. I saw a youtube of Matt Risinger talking about the 500 year wall (house). Im sure you have seen this. He had a wood frame house, used a peel and stick on the outside sheeting, and then applied his thermal control layer outside of the peel and stick. applied lath and then clading. He said it was like adding a wool sweater to the house with the thermal layer on the outside of the home. The inside of the home was left an open frame wall, which was painted. In my mind, i thought of doing this same concept to my 1902 home. My only difference is i have an interior wall covering ( sheet rock). As you stated above, there would be stack affect in my walls because of my exterior sheathing and the interior sheathing making a hollow wall, ( which do have fire stops BTW) . What do you think of this method for the 500 year wall (house) that Matt is demonstrating?

        1. Expert Member
          Dana Dorsett | | #6

          Matt Risinger has a lot of videos up. I've seen some of his videos, but not that one.

          Building methods with all the insulation on the outside have acronyms such as REMOTE or PERSIST. It's been covered here fairly often, eg:

          https://www.greenbuildingadvisor.com/article/getting-insulation-out-of-your-walls-and-ceilings

          It's an expensive way to build, but it works. For retrofitting an antique in a milder climate than central AK it's cheaper and easier to hit thermal performance targets making use of that dead space in the cavities.

          Even with fire blocking those cavities aren't perfectly sealed. (Just 18" of cellulose would suffice as a fire block.) Dense packing the cavities with cellulose would render the stack effect moot, and would provide a moisture buffer drawing moisture from the structural wood. Just a couple inches of exterior insulation offers significant wintertime dew point margin in zone 4A (or even zone 5A), more than enough to protect the sheathing & studs, and would outperform 4" of insulation on the outside with no cavity fill.

          Hanging the siding out 4-5" moment arm of screws on furring with a somewhat compressible rigid rock wool needs many more fasteners to be as mechanically robust as doing it with 2" of rigid rock wool.

  3. user-7022518 | | #3

    Hi Jeff,

    Hammer and Hand has a video with a rainscreen cladding over rockwool that should explain things: https://www.youtube.com/watch?v=XEiFCW9Tbbw

    We are doing something similar.

    Lisa

    1. Expert Member
      Dana Dorsett | | #7

      I noticed in the video that the vertical furring in contact with the rock wool is 1x6, not 1x4 (the horizontal girts are 1x4), probably for less compression of the rock wool.

      With thicker layers there may need to be some fine-tuning with the screws to achieve a flat surface to the siding.

  4. AlexPoi | | #8

    There is pretty extensive documentation on Rockwool Canada website :
    https://cdn01.rockwool.com/siteassets/o2-rockwool/documentation/technical-guides/residential/comfortboard80-installationguide.pdf?f=20180718133916

    Basically you just put the insulation between some furring strips and your sheeting and screw it (make sure to use ss screws as they are more resistant to heat conduction). Here, in Canada, some people put up to 8 inches on their wall so no worry with only 4 inches as long as your cladding is not too heavy. Just make sure everything is straight, otherwise you'll run into trouble when installing your cladding.

    Rodenhouse, Heco and Cascadia also sell specialized fasteners just for that purpose. They are expensive but they will speed the process.

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