My wife got new black out shades and now we have condensation in the lower corners on the windows in the morning.  NOT COOL.  Even with HRV on full.  Is the only solution basically just to not pull the window shades entirely down so there is more airflow? 

That’s probably the easiest remedy. The condensation is the result of the interior glass element getting below the dew point temperature. So you need some combination of increased drying effect (e.g., increase ventilation in this area) or increased heating effect (e.g., less heat shielding from the shade or different window glass element coatings that allow the interior glass element to stay warmer).

More air flow can increase condensation. The air films between the window shade and glass are insulating, so the glass drops to a lower temperature than it would when fully open. The more air that flows, the more moisture is being transported to that cold surface.

If there is enough exposure of the glass at the bottom that the bottom of the glass stays warm the condensation may re-evaporate into the room before puddling at the bottom, but it'll take more than just an inch or three for that to be the case.

Not too long ago I saw a window that had ventilation ports around the interior perimeter and I believe it used an internal fan to generate air flow that reportedly eliminated interior condensation issues. I would think this would work well because this could increase the temperature of the interior glass element via heat convection sufficiently so it could be above the dew point temperature. Don’t know if the additional complexity and power usage would make this worthwhile though... Anyhow, I suppose just placing a small fan that blows the warm and hopefully low humidity indoor air against the window could quickly confirm this. This is after all exactly how we deal with this issue for car windshields.

Posted By sailawayrb on 13 Nov 2018 05:37 PM
Not too long ago I saw a window that had ventilation ports around the interior perimeter and I believe it used an internal fan to generate air flow that reportedly eliminated interior condensation issues. I would think this would work well because this could increase the temperature of the interior glass element via heat convection sufficiently so it could be above the dew point temperature. Don’t know if the additional complexity and power usage would make this worthwhile though... Anyhow, I suppose just placing a small fan that blows the warm and hopefully low humidity indoor air against the window could quickly confirm this. This is after all exactly how we deal with this issue for car windshields.

That works, but it raises the heat load of the room by disrupting the air film next to the glass, reducing it's as-used U-factor.

Condensation is an even bigger issue when insulated shades are installed on the interior for the purposes of saving energy or reducing drafts when it's ultra-cold outside. Having an insulated shade adhered to the glass by ice in the morning isn't rare in cold climates.