Designing and building high performance homes, and especially Passive Houses, becomes far more challenging if dehumidification is needed. And even if dehumidification is not yet needed in your climate today, it may be needed in the future. Most climates are warming; some rapidly. Warmer air holds more moisture, and even parts of North America which never suffered humid days in the past may get muggy, sticky days in the future.
Uncomfortable summer humidity is often accompanied by high outdoor temperatures. Exterior shading, night flush cooling, and other passive design strategies may keep your home cool, but not reduce humidity. Fortunately, if you need an “active” mechanical cooling system like a mini-split heat pump or air conditioner (AC), that system usually incidentally removes some humidity from the air at the same time it cools it.
However, cooling air and removing humidity from it occur differently. Removing moisture from air requires cooling the air well below the dew point or temperature at which the moisture will condense into water and drip into the collection pan. That temperature may be uncomfortably low. In short, your mini-split/AC may cool and dehumidify at the same time, but not in the desired proportions.
You high-performance home may be a challenge for your mini-split/AC system. Here’s why. A cooling system is designed to run when a high indoor dry bulb temperature is reached. A high-performance passive house typically may only need cooling for a few hours of the day. The cooling system may not run long enough to remove the enough moisture to maintain comfort. Running the cooling system at a lower indoor temperature to achieve the necessary dehumidification may make your house uncomfortably cold. Some mini-splits/AC systems have settings allows you to vary their cooling and dehumidification function, but those settings may not be sufficient for your mini-split/AC system to produce both the desired temperature and the desired humidity.
A separate dehumidifier may be the best solution. Like ventilators providing fresh air from the outdoors and removing stale, polluted air from your home (see Heating/Energy Recovery Ventilators), dehumidifiers can be for the whole-house or just for a single room. There are pros and cons to both, and your HVAC designer/installer will guide you on these options. We have listed Vendors for both systems in two categories.
It is worth noting that many HVAC system designers/installers still do not have experience with high-performance or Passive Houses. They may not realize that the passive comfort features of your home radically reduce your heating and cooling loads. Thus, do not allow an HVAC installer who is not familiar with your Passive House to “over-design” your system as they commonly will do to assure there are no comfort complaints. HVAC systems which are unnecessarily large may provide too much cooling or heat, likely will operate sub-optimally in other respects, cost more than necessary, and waste substantial electricity over their lifetime.
Just as cooling and heating systems may be over-sized for a Passive House, you’ll want expert advice on whether a whole-house system is necessary. Or a single-room humidifier is sufficient. For example, if you have an ERV, it should tend to maintain consistent, comfortable humidity inside your home. Potentially well below the high outdoor air humidity level. In that case, perhaps you’ll need relatively little “active” dehumidification from a dehumidifier. If the ERV is running continuously, then operating a dehumidifier in just one or two rooms may be sufficient to bring the humidity throughout your house to comfortable levels.
Thanks to Matt Bowers at Rochester Passive House for assistance on this article.