Gary R. Lloyd wrote:

At what outdoor dewpoint temperature do we need to start adding
moisture in order to maintain our 70F and 40% RH?

When 160x0.075(wi-wo) 0.0116, ie wo wi - 0.000966 = 0.005319 and Po
= 29.921/(0.62198/wo+1) = 0.2357 "Hg and Td = 9621/(17.863-ln(Pi))-460
= 40.2 F...

Considering that our family of 4 needs 15CFM of fresh air each... I don't
think we want to tighten a house up much more than that.

I think we do, with positive ventilation as needed.

Nick