What is a comfortable indoor humidity level, and is that the same as the ideal humidity level for the house? How much window condensation is too much?

When we finally got our new house finished and turned on the heat recovery ventilation unit, we were amazed at the amount of water the HRV removed from the air in the house. Two months later we are still seeing signs that the house is drying out: gaps appearing in trim joints, disappearance of cupping in parts of the hardwood floor, condensation on the windows in the mornings. The amount of water draining from the HRV has, however, slowed significantly. It's time to fine tune.

What is a comfortable indoor humidity level? Somewhere between 30 and 50% relative humidity is recommended for comfort. For me it would be near the top of that range. (My sinuses are most comfortable in a tropical rain forest!) A document from the Maryland Cooperative Extension (Relative Humidity for Comfortable Living) says that below 35% relative humidity, people are more prone to head colds, scratchy throats and dry skin. This source says 40% is optimal for health.

The humidity level that’s best for the house is a different matter. Here the aim is to prevent condensation on windows and in walls, and prevent molds from growing. (In short, we want to prevent rot.) There’s no magic number because the degree of condensation depends on inside and outside temperatures, humidity levels, and the energy efficiency of windows and walls. The colder it is outside, the lower the indoor relative humidity must be to prevent condensation. Natural Resources Canada provides a condensation chart (Maximum Humidity Before Window Condensation Occurs) that indicates that an indoor relative humidity of 40% should be okay to -20ºC outside, as long as the windows are energy efficient (and presumably the walls have good vapour barrier).

My indoor relative humidity is always in the 40 – 50% range, and we haven’t seen temperatures below 15ºC yet this winter, so why do I have condensation on my windows in the mornings? It’s because of my insulating window blinds. Though these blinds prevent some heat loss through the windows at night, they do greatly reduce air circulation across the surface of the glass, and Natural Resources Canada writes: “Any restriction of air movement will increase condensation on the window.”

If my windows were made of wood, or the condensation was sufficient for water to run down the glass onto windowsills, there would be a problem, but the windows are vinyl and the water simply beads on the glass. The water is always gone a few hours after I open the blinds in the morning. When the house finally finishes drying out, I expect the condensation will be even less, so the present humidity level in our house is probably okay.