Need Roxul cover be entire to act as a rain screen?
In Zone 4, (Lawrence Kansas) we are rebuilding an 1883 house. For reasons of cost we have chosen not to take off the exterior clapboard, instead we are fitting 1.5″ of high density Roxul between the studs and against the back side of the sheathing to provide a rainscreen. On top of this we will spray a thin layer of high density foam before blowing in another layer of dense pack cellulose to slightly built out studs.
My question is: do we need to entirely cover the sheathing with the Roxul to prevent moisture problem and thus risking mold problems? Cutting the Roxul by hand to insert in unevenly placed studs is labor intensive and time consuming. Much piecing is required. Are we inviting problems if leave gaps in the Roxul covering?
Thanks, Sacie
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If you haven't read this article yet, it sounds like you should! https://www.greenbuildingadvisor.com/articles/dept/musings/insulating-walls-old-house-no-sheathing
Thanks Brendan. It is my understanding Roxul, a mineral wool material, serves to allow moisture to dissipate in the same way a gap does. At the same time it has an insulation value. Good article though.
Sacie,
Strictly speaking, if your house has existing sheathing, the Roxul mineral wool that you intend to install between the studs is not a rainscreen. (A rainscreen is located directly behind the siding, not behind the sheathing.)
In a house with no sheathing, some type of drainage gap on the interior side of the siding is a good idea. But if your house has existing sheathing, your approach isn't really necessary.
If I were you, I would skip the mineral wool, and just use the flash-and-fill approach. For more information, see Flash-and-Batt Insulation.
-- Martin Holladay