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Exterior insulation vs. rainscreen

Owen D | Posted in Energy Efficiency and Durability on

Hello,

We are building a single family home in zone 2 in Marin County and trying to decide whether to have a rain screen or exterior insulation. For some reason our building department will not allow us to do both, they say since we are in a high fire zone (WUI) we can not have insulation between the furring of a rain screen and wall sheathing.

In any case,  we have 2×6 walls with blown in Fiberglass that should have R value of R23.

If we were to choose between either rain screen or R6 of exterior insulation which would be recommended?

From what I can find reading online  both can be omitted in our  climate zone 2 in Marin County CA, but not sure if there is a economic/building science case to install them anyway.

To me it seems that a rain screen might be the better choice given it would be less expensive  and possibly extend the life of our siding, less maintenance, increased drying,  less chance of water issues migration into wall assembly. These are general benefits but don’t know how to evaluate to my particular climate.

The exterior insulation may not be as crucial as we don’t need to worry as much about condensation in our wall assembly in our climate, and we have R23 already in our wall, with thermal bridging might have a real R value close to R20 which seems good enough in our climate. And our contractor who hasn’t done it before thinks it will be expensive to install.

Am I thinking about this correctly?

Thanks,

Owen

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Owen,
    Assuming you are correct about the regulations -- and assuming that vented rainscreens are permitted, as long as there is no rigid foam adjacent to the vented rainscreen -- you might want to inquire whether it's legal to have a rainscreen that is adjacent to a continuous layer of semi-rigid mineral wool insulation. (Mineral wool is fireproof.)

    For more information on this approach, see "Installing Roxul Mineral Wool on Exterior Walls."

  2. Expert Member
    Dana Dorsett | | #2

    For the record, Marin County CA is in US climate zone 3C. It's in CA Title 24 zone 2.

    https://www.title24express.com/s/cc_images/cache_2599443704.png?t=1331164014

    https://beta.greenbuildingadvisor.com/app/uploads/sites/default/files/images/DOE_climate_zone_map.preview.jpg

    In climate zone 3 "whole wall R" of any more than R20 (thermal bridging & other layers all factored in) would be about the limits of what's financially rational an a lifecycle energy cost basis especially if using more expensive insulation. See Table 2, p10, row 3:

    https://buildingscience.com/sites/default/files/migrate/pdf/BA-1005_High%20R-Value_Walls_Case_Study.pdf

    A 2x4/R23 wall with typical sheathing & siding options comes in at about R16-R17. An inch of rigid rock wool would bring it to R20-R21. There is a real non-energy use value add to having that inch of rock wool arguably worth paying for in your situation. Unlike rigid foam it's completely fireproof, and will protect the structural sheathing from ignition (for awhile, anyway). Taking it to 2" won't necessarily be more protective- if it's hot enough for long enough to ignite the sheathing through 1" of rock wool the glass in your windows would have already failed.

    I'm not sure how available 1" rigid rock wool is in your area, Rockwool Comfortboard 110 (R4) comes as thin as 1" thickness, the thinnest ComfortBoard 80 is 1.25". Some of the box store chains carry Comfortboard 80, but you can probably do better through a regular distributor.

    https://cdn01.rockwool.com/siteassets/o2-rockwool/documentation/technical-data-sheets/residential/COMFORTBOARD-80-Non-Structural-Sheathing-Continuous-Insulation-Techdata.pdf?f=20181016101508

    https://cdn01.rockwool.com/siteassets/o2-rockwool/documentation/technical-data-sheets/commercial/COMFORTBOARD-110-Continuous-Insulation-Techdata.pdf?f=20180618131156

    (Comfortboard 110 is quite a bit denser than the -80, and could be more expensive at 1" than than 1.25" Comfortboard 80. Get pricing on both.)

    Rigid rock wool is air permeable and not strongly wicking. While not as good a capillary break as a rainscreen, at an inch or more its still a decent drying path. If you're allowed to rainscreen it, do, but it's the right stuff even if you're not allowed rainscreen construction.

  3. Owen D | | #3

    Thanks Martin,
    we actually did specify a rain screen with Rockwool as the exterior and got this comment back

    "The furring strips are to be directly attached to the structure and not over the insulation"
    .....
    "the assembly as shown, with the furring strips and air gar on the outside face of the Comfortboard 80 insulation does not meet the WUI requirements of having the exterior siding directly over the structural sheathing. "

    we could appeal to State Fire Marshall but we have had too many other "fires" we are fighting, at this point.

    Dana, thanks for correcting me on my climate zone and the real Rvalue of my 2x6 wall assembly. Sounds like if we had to choose between rain screen and Rock wool you would go with rock wool for reasons above, and we will be OK. Many thanks!

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