Hello energy pros.

I need your combined wisdom before I proceed to help a client of mine address what appears to be mold growth and draftiness after I completed attic air sealing and other miscellaneous measures on her home.

The HO owns a split level in Northern VA. She, her husband, and three children have lived in this house for three years.

The house has existing exterior wall insulation, and some attic insulation. No rim joist air sealing or insulation and no foundation wall insulation. 

We sealed the entire attic above the two-story section of their split level (above the bedrooms), and replaced the existing blown-in fiberglass. We also insulated a section of exterior wall in their laundry room that was driving discomfort in their house with closed-cell spray foam. Finally, we insulated the floor between the 1st and 2nd story (between bedrooms) for sound attenuation purposes.

About one month after completing the work the HO e-mailed me to say that surface mold had begun to form on the exterior wall of the 1st story bedroom that we insulated the ceiling above and adjacent to the wall we spray foamed. This is the main concern at this point.

Before I run over there later this week and try to determine what is going on, I'd like to tap the knowledge that you all bring to the table.

My sense is that there is cold air rushing into this exterior wall through the rim joists in the basement. The cold air is mixing with warm conditioned air that is leaking into the walls through outlets and baseboards, and the air sealing of the top plates is keeping the air in the walls, but also allowing cold air to mix with warm, introducing condensation within the wall.

The HO also mentioned condensation on some of the glazing. One final detail I want to mention is that they have a gas insert in the 2-story section of the house (on the 1st floor).

Any ideas on what may be taking place, or tips on how best to proceed? 

Thanks in advance!

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Thanks Kevin.

You leave me with a lot to consider.

I will likely need to follow-up on the matter.

Good point on the gas insert. Those do put out a lot of moisture.


The condensation on the windows is a clear indicator that humdity in the house is high enough to expect moisture accumulation on cold walls.


The question is whether the cause is a damp fall or a source in the house.


In my experience, DC houses that are weatherized don't typically have summertime humidity problems. AIr sealing sharply reduces the added humidity from infiltrating humid air. Since many projects don't improve wall insulation or glazing, AC loads don't go down all that much and dehumidification is adequate. In many cases the reduced humid infiltration seems to more than offset the lost dehumidification of lower runtime. In my experience anway.

Patrick, what is ACH50? ACHnat?

Where is the dryer vent located? Has it been moved or damaged during work? Was it inspected? Is the hose full of lint?



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