Followup to this question

one of the comments says that fiberglass can trap moisture and lead to mold/mildew. I did some research myself and found other sources saying the same.

This worries me -- since I live in Florida (very humid all year round) so should I avoid fiberglass insulation entirely in my 2x4 framed home, and opt for foam board, even at the sacrifice of r-value?

  • The question isn't very useful, as virtually anything can grow mold on it. You seem to be asking whether fiberglass is more likely to "trap" moisture. I'm not sure what that means. If it gets rained on? Due to condensation? Even extruded foam will fill with moisture in those situations. Please revise to ask something more specific.
    – isherwood
    Mar 7, 2020 at 15:55

3 Answers 3


Fiberglass itself will not support mold. Mold needs 3 things:

  1. Mold spores
  2. Moisture
  3. Organic matter (i.e. food)

With fiberglass itself you have #1 and #2 but are missing #3 since fiberglass is glass and not organic.

Note that paper backed fiberglass insulation adds the organic matter component but most is treated to inhibit mold growth. Check with your supplier to be sure the product you are considering is applicable for your application.


This question could get flagged as opinion based but here goes anyway. I live in South Florida and have been in hundreds of attics with fiberglass insulation and have never seen any problems. Yes, there is a lot of humidity but if the attics are vented with soffits, etc., then the humidity doesn't cause a moisture problem. I had a roof leak and some of my fiberglass insulation got wet. I fixed the leak and monitored the insulation and it dried out in a day or two and did not compress. fiberglass insulation has been proven to be an excellent insulator. I have read some articles that warn about fiberglass insulation but most are from companies that sell something different.


Less likely, but mold can grow also next to/on Fiberglas insulation, since there is always some organic matter in the air and on building materials. It is more a matter of due point, drying time, damp diffusion etc. Here is a reliable way to get all necessary information: Just input each layer into the professional "Ubakus"-Site, which is free for private use: Ubakus.de

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