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Community and Q&A

Do we need to overlap rock wool?

Elfo | Posted in Green Building Techniques on

It seems like it’s common practice to seams in mineral wool, as seen in this Eco Home video:

However, in looking at Rock Wool (Roxul), a claim appears on page 3 of the following pdf that the material will knit together when pressed.

So, if one using a thickness that can be accommodated by a single product, say 3″ of Comfortboard, is there any benefit to going with two layers of 1.5″ insulation vs a single layer of 3″?

I cannot find this Duo Slab item on the US Roxul website, and when I reached out to them about the knitting claim there was no response.


  1. Elfo | | #1

    As an addendum, this is with respect to external insulation.

    However, if the answer is that there is no benefit to dual layers, then a followup question is if there is any benefit in going with dense pack cellulose inside a wall assembly as opposed to rockwool batts? The assumption is that there is an air barrier such as Cat 5.

  2. GBA Editor
    Martin Holladay | | #2

    I think that the benefit, if any, of using two layers with staggered seams is small if you are talking about mineral wool. After all, mineral wool is air-permeable.

    Two layers with staggered seams makes sense with rigid foam, since the seams between rigid foam can leak air. But with mineral wool, you've got the potential for air leakage everywhere, so staggering the seams isn't really needed.

    Q. "Is there is any benefit in going with dense-packed cellulose inside a wall assembly as opposed to rockwool batts?"

    A. In most cases, the dense-packed cellulose will do a better job of filling nooks and crannies than any type of batt.

    With batts, the quality of the installation depends on the skill and conscientiousness of the installer, who has to spend a lot of time doing fussy work at electrical outlets and anywhere there are electrical cables. A blown product like dense-packed cellulose reduces the need for fussy work and fills the space more completely.

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