Dehumidification vs. ventilation in an existing house
I have done significant air sealing of our 1930s cape cod house in Virginia (climate zone 4) and it only recently dawned on me through reading various GBA blog posts that this air sealing is probably the cause of our high indoor winter humidity levels (ranging from 55% to 70%)
I understand that adding ventilation is the correct way to go, and the bathroom exhaust fan approach seems the most cost effective. However, before doing this I have a few questions.
1. I wonder if this humidity level poses a threat to the structure of our house because of moisture build-up inside the walls. The original wood lap siding was covered with a foil-type house wrap, which was then covered with aluminum siding at some point. I assume the foil would inhibit drying to the exterior. However, since the humidity level seems to be caused by lack of air flow, I’m guessing that there isn’t an overly large amount of air passing through those walls to cause such problems.
2. Is there any possibility that running a dehumidifier to deal with the high humidity could be more energy-efficient than running an exhaust fan? A dehumidifier burns a lot of electricity, but it also puts off heat that would help in space heating. I am installing a somewhat over-sized PV system, so that electricity would be mostly, if not totally, covered by the PV output. By contrast, bringing in more cold air would make our oil furnace run more, and oil is not free. (I hope to at some point switch over to a heat pump, but that is not the case as of yet.) I know that there are other reasons to introduce fresh air, but purely from an energy efficiency perspective, how might dehumidification compare with ventilation in this case?