0 Helpful?

Is external insulation necessary for moisture control?

My new home will be in climate zone 4A, in the mountains of Virginia - a mixed humid climate. My budget is somewhat limited. Our house plans have 6" walls, 24" on center, with R23 mineral wool batts. Taped plywood sheathing, Tyvek, and vinyl siding on the outside. The main air barrier will be sealed exterior sheathing and the drywall ceiling. The house will be pretty air tight.

I will also be doing 2 additional things:

1) Sealing the top and bottom of the drywall, to reduce interior air penetration and air circulation inside the walls (though not to the point of doing full ADA, since my full air barrier is on the exterior)

2) My goal is to keep our interior humidity levels to no more than 30-35% (not usually a problem in winter if I don't use a humidifier)

These walls will be able to dry out on both sides, and not much interior air should get inside them. Do you think this wall design will do a decent job of controlling moisture inside it, in my moderate climate?

Asked by Debra
Posted Jan 10, 2018 12:04 AM ET



There shouldn't be any reason for the wall you describe to have moisture problems, as long as you pay attention to flashing at windows, doors, deck ledgers, and other penetrations.

Answered by Martin Holladay
Posted Jan 10, 2018 7:47 AM ET


Excellent! Thanks. Yes, I'll be paying great attention to the flashing details.

Answered by Debra
Posted Jan 10, 2018 10:04 AM ET


Debra, a goal of 30-35% RH in a place that is "pretty airtight" may not be as easy to achieve as you expect. Depending on the actual airtightness and the occupants' lifestyle, low wintertime RH will be a thing of the past and it may even be a struggle to get the RH down into the low 30s. In one sense it's a vindication of the energy-saving effort and expense you put into your house's envelope, but high RH might lead to sleepless nights worrying if it might be condensing somewhere you don't want it to. Make sure your HRV or ERV installation is well designed and and professionally installed--don't skimp on this. DAMHIKT.

Answered by Andy Chappell-Dick
Posted Jan 10, 2018 1:40 PM ET


Yes, I plan to have good mechanical ventilation with the ability to adjust for various rates of cfm airflow.

Answered by Debra
Posted Jan 11, 2018 2:15 AM ET

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