Rainscreens, Mineral Wool, Mold and Insects

Last Updated: Jun 6, 2011

The following message has been sent to the BEC-Chairs listserv.

To All:

I have a client who is interested in improving the exterior enclosure of his building by utilizing a rainscreen cladding with mineral wool insulation in the space between the cladding and the sheathing.

The rainscreen manufacturers provide closures of the rainscreen cladding joints to different degrees. Some do not close off the joints at all; some use gasketing to close off some of the joints; some use gasketing to close off all the joints.

The client has a reasonable concern about two things:

one, will continual wetting of the mineral wool eventually create a mold condition in the dust and debris that might accumulate in it?

And two, will insects or other creatures burrow into the mineral wool to live there?

Can anybody tell me what the long term experience is with regard to mold and insects in rainscreens with mineral wool insulation?

Judd Peterson, AIA Chair, BEC - Minnesota

Please see the response Judd received, below:

Hi Judd,

I have never heard of mold or insect problems, as described, in the past 12+ years we have been detailing exterior insulated rainscreens with mineral wool. Insect screens are important top and bottom, of course, and I typically don’t recommend open rainscreens, so gaskets or baffles should also keep the insects out. In the number of unitized curtain wall assemblies we have reviewed, I have never seen a big problem of insect nesting in the open gasketed joints. I have seen the odd cobweb or cocoon, but nothing that can’t been cleaned out with periodic building maintenance and nothing I’ve seen would have manifested into a problem.

In addition, if the rainscreen is detailed well to shed water, the amount of water getting to the mineral wool should be quite small. Dust that settles into the mineral wool over time would not provide much of a food source for mold to flourish.

Aside from less R-Value than XPS per inch, here are some typical benefits to using semi-rigid mineral wool:

• Easily cut • Good R-value/inch • Fire resistant • Water and moisture resistant; does not absorb moisture (maintains insulating value) and will drain • Chemically inert • Does not rot, promote mildew, fungi, or bacteria, or sustain vermin • CFC- and HCFC- free product and process • Made from Natural and Recycled materials

David C. Young, P.E. Principal, Building Science Specialist

Last updated: 11-Dec-2012 08:47 AM
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