Insulation Installation Failure

We usually assume homes built around the 1920s in cold climates will have inadequate insulation by today’s standards. When and how can that insulation be considered a major failure?
Inspecting Older Homes
While inspecting older homes like the one shown in Photo 1, we often recommend additional insulation because they were built with limited or no insulation. If you make this recommendation, always note insulation “with proper air sealing.”

I was called out to this home because the owner had an extensive insulation project completed and now, for the first time in 100 years, there was moisture and mold in the attic. The insulation contractor followed their contract standards for installation and believed they did not cause any problems.
A Look in the Attic
This beautiful old home had a typical walk-up attic (Photo 2) with wood boards covering the floor. Wooden plugs are visible, indicating where insulation was blown under the attic floor. There’s also cellulose surrounding a plumbing vent pipe at an overhang. We can’t see any ventilation openings at the lower edge of the roof. We can see lots of water
stains and, perhaps, mold growth.

With a closer look (Photo 3), we see extensive mold growth on the underside of the roof framing. Growth and stains appeared throughout the attic on the lower edge of the roof, with heavier growth on the north side.

What Happened?
The insulation contractor ignored the basic tenet of a thermal boundary/envelope, as shown in Illustration I008C. Homes must have a vapor barrier (or air seal/barrier) on the warm side of the insulation. This barrier prevents moist warm air from escaping the heated space.

All homes leak lots of air through the attic, as depicted in Illustration I005C. In the 1920s, there were no vapor barriers or any attempts to seal the envelope. Lots of warm air leaked into the attic and kept it warm. Since it was warm in the attic year-round, there was no condensation. Snow melted off the roof, and there were no ice dams, but there may have been large icicles.

So now we have a well-insulated barrier between the attic and the heated space, and the attic is cold.
Some moisture-laden air still leaks into the attic, and moisture condenses on the cool inner roof surfaces. The combination of moisture and dirt on the wood results in mold.
The Takeaway
If you recommend additional insulation for a home, always note insulation with proper air sealing and/or evaluation by a qualified energy consultant to design a proper insulation system.
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