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Help with rainscreen options and cost for insulation on 1970s remodel

espiritiv | Posted in Green Building Techniques on

Hi, I am in Zone 4, Portland, Oregon.
My existing 1970’s home is two stories with 2×6 framing with ) insulated walls(probaby still R13 in a daylight basement. Top floor main living room has 2×4 walls (R13). Existing Sheathing is Fiberboard sheathing (Cellulosic Fiber Insulating Board,).
I am tearing off existing combination of T111 and some other wood composite lap siding and installing a new rainscreen + fiber cement cladding, + new windows.

My Questions are:
1) What are the best cost effective option for a rainscreen layering if my fiberboard is intact and re-usable?
WRB+ 2″ Roxul CIS + furring+ fibercement cladding?

2) what if my fiberboard needs replacing? Should i then go with something with integrated insulation and WRB? What is least labor overall?
Option 1) Looking at Zip R System that includes Polyiso layer + furring + cladding.
Option 2) demo any damaged Fiberboard, install OSB or Plywood + 2″ Roxul CIS + Cladding

Any issues you can think of?

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  1. GBA Editor
    Martin Holladay | | #1

    It's tricky to determine cost-effectiveness. To determine cost-effectiveness, we would need to know a lot more about your energy prices, as well as the cost of your labor and materials. Suffice it to say that retrofitting continuous insulation on a wall in your climate is rarely cost-effective. However, since you need to replace the siding anyway, it makes sense to do the work.

    Using mineral wool in this location (as you suggest) is an option, but is probably not for the faint of heart. Because mineral wool is squishier than rigid foam, it is harder to get all of the furring strips co-planar when using mineral wool than when using rigid foam. That's why rigid foam is much more common in this location.

    You are correct that installing Zip-R sheathing will require less labor than installing OSB plus mineral wool.

    For more information, see:

    How to Install Rigid Foam Sheathing

    Wrapping an Older House with Rock Wool Insulation

    Installing Roxul Mineral Wool on Exterior Walls

  2. espiritiv | | #2

    Thanks Martin,
    You have confirmed a lot of what i have guessed was correct. Our energy prices are low for electricity, and natural gas is not too bad either. It might take many decades to pay back on insulation cost. Comfort and sound control might only be the other added benefits, or maybe resale appeal that my house is "up to code"

    One contractor told me making sure everything was coplanar would be required so that things looked good in the end and that would add more time. He suggested new sheathing regardless to help in that effort. If i can add some additional insulation with a Zip R sheathing that might be the best bet. Any additional options you could recommend?

    Any issue you can see with my walls breathing, and dewpoint with either insulation method (poly iso, or Roxul)?

  3. GBA Editor
    Martin Holladay | | #3

    Walls don't have to breathe, but they do need to be able to dry out in at least one direction. In climate zone 4, you have a wide latitude in the thickness of your exterior rigid foam (if indeed you decide to install some). For more information, see Calculating the Minimum Thickness of Rigid Foam Sheathing.

    The most important factors in your work are:
    (a) Conscientious attention to quality installation work -- in other words, careful installation of your chosen insulation, and careful attention to flashing and water management details.
    (b) Attention to airtightness in all aspects of your work.

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