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Community and Q&A

When should wet fiberglass insulation be replaced?

chicagofarbs | Posted in General Questions on

Hi Everyone –

I have posted a few times now about what we are experiencing in our home.  A quick recap is: we unfortunately purchased a historic brick home that was remodeled: 3 wythe brick -> 2×4 wood stud wall with fiberglass batt -> 5/8″ gyp.  We just had our first major freeze/thaw cycle and all fiberglass that is up against the inner brick face is wet.  It was wet back when the inner brick face was frozen, and now its even more wet after the thaw.

We removed a section of the batt and it is wet enough to saturate a piece of paper if you press it down lightly on the fiberglass surface that was facing the wet brick surface. 

My question is, is there any reason to believe that the fiberglass could dry out in the current assembly?  It has been wet to the touch for as long as 3 weeks since we started to investigate in the walls.  We are concerned about both degraded thermal performance, but more importantly the potential for mold growth.

I’m doing my best to try and make the case to the home builder that the fiberglass insulation needs to be replaced (ideally with ccSPF based on the Building Science articles).



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  1. user-2310254 | | #1


    If you haven't done so already, read Martin's article on insulating old brick buildings ( For your situation, here is a key quote:

    "Clearly, fiberglass batts should never be used to insulate the interior of a brick wall. (Since fiberglass batts are permeable to vapor and air, they permit interior moisture to condense on the cold bricks. That’s bad.) Most experts agree that the best insulation for the interior of an old brick building is closed-cell spray foam."

    To your specific questions (and hopefully the experts will weigh in), I think humidity in the wall cavity is one big concern.) High humidity will increase the chance of mold growth on the back of the drywall (at least that is my assumption).

  2. Expert Member
    AKOS TOTH | | #2

    I recently had a largish leak and a fair bit of water got into sections of floor insulation. This was kraft faced fiberglass batts. I tried for a week to blow a heater and fan at the insulation and it barely did anything to dry it up. Maybe if I left it for longer it would have helped, I ended up removing the wet sections. If the insulation is soggy to the touch, best is to get it out, mold risk is not worth it.

  3. Expert Member
    BILL WICHERS | | #3

    I would replace it immediately. The material is a pain to dry, the kraft facing will probably come apart after being wet, and the material isn't very expensive to begin with. While the insulation is in there wet, it's keeping other stuff wet too, which can lead to further problems as well as mold.

    Replacing the fiberglass with a more suitable type of insulation for your wall assembly is a very good idea.


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