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Community and Q&A

Rockwool outsulation: How does the condensate get out?

Richard_in_Maryland | Posted in Green Building Techniques on

I lived in Maryland, mixed-humid climate, and am thinking of a retrofit strategy for my solid masonry house from the 1930s.

Since I have plumbing between the 2-wythe brick and the plaster, and for the sake of keeping the thermal mass inside, I am thinking of the PERSIST strategy of wrapping the outside of the house with Grace Ice & Water Shield (the roof also needs replacing, so overlapping from bottom to top all around) and then outsulation. I’d then include a ventilated drainage space and use a lath to use thin brick. But for the insulation type I’d like to consider rockwool instead of rigid foam, because of insect control and because I do worry about long-term foam shrinkage.

Lstiburek’s “perfect wall” in Building Science Corp. does list rockwool as an option in this strategy.

But what happens in the summer? Wouldn’t the solar drive cause water vapor to go through the rockwool and condense against the Grace Ice&Shield? Since rockwool repels water, does that mean I’d get drops of water held against the wall? Or would it allow for any condensate to run down safely the walls behind the rockwool to the flashings at the bottom?

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  1. Riversong | | #1


    The semi-rigid rock wool board should allow water drainage, as long as flashings are properly integrated at the bottom and there is a weep space.

    But neither the rockwool nor the bituthene would be undermined by dampness (other than some loss of R-value), so solar-driven moisture should not cause a problem. And, with a drained vent space behind the brick cladding, moisture should have a place to either drain or evaporate before being driven through the insulation, and the vent cavity would offer an escape route for moisture within the insulation as well when ambient humidity changes.

  2. HDendy | | #2

    Make sure your details allow venting at the top and bottom of the drainage space to allow adequate air flow to facilitate the drying mechanism Robert described. Even if water did condense on the outside of the Ice/Water shield in the summer that would be fine- that's what it's there for.

  3. Richard | | #3

    Thank you, Robert. I'm glad you still frequent this site. And thank you for your consultation in the past, which was very helpful. As you can see, I am still working on the plans. Apparently the cost of building an addition doubled in the housing bubble and have yet to come down (people either are out of business or survive by asking for peak bubble prices). So I have time to further refine things as I search for a builder.

    I know from your other posts that you favor the Bau-biologie idea and do not like vapor barriers (perm<0.1). Does that still apply in your mind to my case where I have load-bearing brick, where the risk of moisture and the need for the structure to dry out is far less? The benefit I see to the Grace & Shield is to be able to dehumidify more easily in this humid climate.

  4. Richard | | #4

    Thank you. I have read the material on this site and at Building Science Corp detailing how ventilating the drainage space goes a long way towards controlling solar vapor drive off my brick into the home. The article that I believe Martin linked to with the science also showed how greater air flow rates improved this control. But I didn't see how these air flow rates translated to drainage space dimensions. Would you know? Is 1/2 inch good? Should it be 1"? Or 1.5"? What is the best practicable dimension?

  5. HDendy | | #5

    1/2" is good enough (possibly even less). I would think more about what materials you want to use to achieve it. 1x's (3/4") might be the easiest and most available, and fairly inexpensive.

  6. J Chesnut | | #6

    The function of the Grace Ice & Water Shield in the PERSIST system, in addition to serving as a vapor barrier, acts as the air barrier and WRB. If you forgo the Grace Ice & Water Shield you would want to consider another product for the air barrier or you would be relying on the performance of your existing assembly and that may not be continuous between the wall and roof planes.

    I have not seen your previous posts but it seems to me one of your most difficult challenges is incorporating windows into your proposed wall assembly.

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