Condensation and moisture problems.

These last few questions raise issues related to dampness or moisture control. While moisture control is not explicitly part of a zero-carbon retrofit, I think it is very important to pay attention to it or you will get mold and eventually rotten timbers. This is not a trivial problem. A house in our town was condemned by the board of health because mold issues sent the occupant to the hospital with asthma. The risk of condensation/mold/rot increases if you do a lot of air sealing because air sealing reduces the flow of air in the house. It is this air flow that evaporates the condensation and prevents mold. Early-generation PassiveHouses, which rely on exceptionally tight building envelopes, often developed mold issues. These problems can be overcome with good humidity control.

In the winter, drafts rob you of heat (and dollars), but they are also a source of dry air that evaporates any moisture that has condensed in your walls, basement or attic. Condensation happens when the temperature of the air falls below the dew point. This is what causes dew on the grass in the morning. Overnight the temperature dropped below the dew point of the air. The dew point of the air varies with its humidity, but it is often around 40-45°F. This is why you see dew on the grass in October and April because we have fairly warm days with lots of humidity in the air, followed by cold nights. If there is dew on the grass, there is dew in your walls.

At these times of year (spring and fall), condensation in your walls and roof is inevitable because if the house is at 70°F and the outside is at, say, 40°F. Somewhere in the wall, the temperature is at the dew point and condensation happens. Air flow up your cavity walls and through your roof evaporates this condensation. This air flow is not driven by wind. It is driven by the stack effect which creates vertical air flow in the house caused by warm air rising. Condensation does not cause mold as long as it evaporates within about 24 hours. Condensation is inevitable, but mold is not. 

The best way to prevent mold in a zero-carbon retrofit (or any house) is to control the humidity. If the humidity in the house is lower than that in the walls (which it will be as long as the house is warmer than the dew point of the air, which it almost always will be unless you keep your house at 50°F) then condensation can dry to the inside of the house by diffusion. Even drywall allows moisture to dry through it by diffusion. This provides moisture a way out of the wall cavity even if there is very little air flow up the wall cavity. Air-sourced heat pumps come with humidity control. On mine, I can set the humidity level as easily as I can set the temperature. I set it to 40% year-round which feels comfortable because it does not leave your skin feeling dry, but the air is still dry enough to dry out the bath towels or the laundry.

In addition to the humidity control on the heat pumps, I also have a heat-pump hot-water tank in the basement. This dehumidifies the air in the basement. Humid air is less dense than dry air and so it rises. Warm air is also less dense than cold air, so it rises too. If the air is both warm and humid, it really rises. Basements are often damp and are often warm (at least in places) because of the furnace or boiler. The warm humid air in the basement rises through the house, reaching the roof where the moisture often condenses on the cold roof surface. The source of mold and rot in cathedral ceilings can often be traced to dampness in the basement. Hence, if draft sealing is part of your zero-carbon retrofit, I think it is essential to control the humidity in the house and the basement. 

Heat pumps with humidity control, heat-pump hot-water tanks and draft sealing play very nicely together.