“Why is a Passive House airtight, but vapor permeable? Don’t I want to keep the moisture out and let the air circulate? I don’t want my house to fill up like a fishbowl! I can’t breathe under water!!!”
How science works can be a surprise to what many would consider common sense. Passive Houses are indeed based on being airtight, but not without air, and certainly not without fresh air. Passive Houses are also designed to let moisture migrate, but not accumulate. This means you will not be living in a fishbowl!
Every time you breathe out you are adding moisture to the air. Ever cook a pan of spaghetti? All that boiling water puts a lot of steam into the air. I am certain you have taken a shower and had to wipe the mirror off afterwards because it was all fogged up. Those are just examples of how you can put moisture into a building. Other moisture can come from external sources, like rain or humidity in the air that you are circulating through your ventilation system.
This moisture migrates through building materials both heading into the building and heading out of the building depending on heat flow and pressure. When humid air comes into contact with a cooler surface the moisture in the air can condense into water again. This point is called the dew point and depends on the amount of moisture in the air, the temperature differences between the air and the surface, as well as the barometric pressure. Most of us have seen water puddled on a window ledge inside a house during winter. That is because the moist warm air hit the cold window and condensed into water again. That puddle can eventually cause a problem for the sill and the wall assembly, but what about the water that condenses inside your wall assembly that is absorbed by the wall materials and doesn’t leave a puddle to proclaim its presence? That can be a HUGE problem for a building and can even happen where there is not a window.
This is why Passive Houses are planned to allow moisture migration. Water vapor molecules are smaller than air molecules so Passive House planning allows the water vapor to leave through the walls, but the air can’t. Even in a Passive House, if that vapor condenses into water again, it too can be trapped and then mold, rot, & rust can begin forming. This is one reason why Passive Houses are designed to eliminate thermal bridges. As heat leaves through the thermal bridge, it can result in a temperature change that would allow the dew point to be reached at that location. If you eliminate the thermal bridges between the different areas of your construction and plan your structure to be within a temperature range that will not allow the air to reach its’ dew point you are preserving the integrity of your structure.
Ideally then, the plan is to keep water out and let any water vapor that works its way into a structure have a way to work its way back out again. In some areas eliminating thermal bridges and keeping the temperatures at levels to prevent the air from reaching its dew point within the structure is enough. In particularly humid areas, or structures that will have an increase in moisture created inside the building, like restaurants, indoor pools, etc. Additional moisture controls might be necessary. An ERV (Energy Recovery Ventilator) can be placed instead of an HRV (Heat Recovery Ventilator). The ERV has a humidity control feature that allows interior air humidity to be regulated as the ventilation process occurs. This reduces the occurrence of a higher dew point within the structure.
To have a better idea of how your building is expected to operate, many high-tech software modeling programs are available to use. Passive House consultants can use products like THERM for 2D conduction heat transfer analysis, and WUFI for coupled heat & moisture calculations. These programs measure the effects temperature and moisture conditions have on the building and are just another way for your Passive House consultant to plan and test for performance success, not just cross their fingers. Many Passive House projects are going a step farther and actually installing moisture sensors in their buildings to take actual measurements to make sure the modeling resulted in the right performance. As more of these houses are built and these measurements shared greater accuracy will come from the planning stages. This care and planning before construction is key to a structures successful performance after construction, and isn’t that what you want when someone is planning your home?
Here is a video explaining how the WUFI program basically works. I am on my way to take WUFI training and will have a more in-depth article about that particular program in the future.