We've got moisture/mold issues after spraying closed cell foam in the rafters of our 1925 home's unfinished attic.

Asked by Gena Dennis
Wausau, WI

We decided to insulate and finish off our unfinished attic in our 1925 bungalow home. We installed closed cell foam insulation (which covers the soffit area) then drywalled. The floor isn't finished yet. We also installed new windows in the attic. Issues that are now occuring: the main floor wood exterior screen doors no longer close. The storm & screen windows are difficult to install - very tight. Mold is growing on the interior window sills and the exterior paint is peeling excessively. How do we fix this issue? What is happening to our house? Thank you for any advise.


Lucas Johnson

Answered by Lucas Johnson

Seattle, WA

Cascadia Consulting Group

March 30, 2012

After my last few years as a building scientist, I would assume your primary issue is that you made your house "too tight."

  • Homes need a certain amout of air exchange (typically 0.35 Air Changes per hour under natural conditions) to maintain the right moisture balance and indoor air quality (IAQ).
  • Closed cell foams, while offering great insulation with an integrated air barrier, will also block the transmission of moisture out of your home.

Sources of moisture in the air

Now, the important question: where is the moisture coming from?

  • On average, roughly 90% of all indoor moisture comes up from the crawl space (raised foundation).
  • Since you have not finished the floors, this may be the root of the problem. 
  • In addition, the other leading sources are breathing, showering, cleaning, and cooking.


This now leads into some potential solutions.

  • First, air sealing the crawl space (or subfloor) will go a long way towards limiting the amount of moisture that enters your way.
  • Second, you will most likely need mechanical ventilation to exhaust the moist air and introduce fresh air into your home. If you live in a moist climate, I recommend using an Energy Recovery Ventilator (ERV) or a dry climate is better paired with a Heat Recovery Ventilator (HRV).
  • Third, installing "spot ventilation" like bath fans and kitchen hoods will be really useful.
  • Lastly, and most importantly, I recommend hiring a Building Performance Institute (BPI) certified Building Analyst to come perform a "home performance assessment". They will use scientific equipment and expert knowledge to truly determine the issues and a scope of work to fix them.

Best of luck and please feel free to reply with any additional questions!

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