LIVING TINY OFF-GRID HOUSE IN SUMMER
Photo courtesy of iStock
Elevated summer temperatures can raise indoor humidity levels which can make living conditions uncomfortable for the occupants. Inaddition to the lack of comfort of the occupants and their animal companions, a more heighten concern is controlling indoor relative humidity (RH) to maintain health and the quality of the structure. Elevated levels of RH can facilitate the growth of indoor mold / mildew, bacteria, dust mites and odors. Indoor molds have been known to impact the health of the occupants and cause structural damage to the dwelling. Dust mites thrive in humid environments with a RH > 50 by absorbing moisture through their body from the moist ambient air. Since dust mites do not drink water orally, maintaining an indoor RH < 30 causes dust mites to dehydrate and die.
Safe and comfortable indoor RH levels should ideally remain between 30% - 50%; at indoor temperatures of 68 - 76°F / 20 - 24°C. If the RH exceeds 50% some of the above mentioned health & structural concerns can occur. However, a RH below 30% can also cause discomfort to Humans exhibited by signs of dry skin, nose bleeds, with symptoms of respiratory irritation, along with static electricity discharge; with wooden floors, furniture and musical instruments warping even cracking.
The Tiny Off-Grid House design incorporates an air-tight, insulated, vapor barrier frame consisting of closed cell (Not to be confused with open cell) foam insulation and the Huberwood ZIP System; which has an unintended consequence of retaining indoor moisture, preventing evaporation out through the walls and floor.
What is the solution to indoor humidity ?
In an air-tight structure “mechanical ventilation” is necessary to provide fresh filtered air exchanges and dehumidification. A 12,000 BTU Mini Split provides: filtration of the inlet outside air of contaminants, heating in winter, cooling in summer and “dehumidification.” The focus of this discussion will be on the latter. But first we should appreciate that a mini split can be more than 100% efficient; delivering more thermal energy than the electricity consumed to create it. This is because a mini split does not create heat; it simply transfers existing heat from one space to another using a pressurized refrigerant to accomplish the second law of thermodynamics (Heat naturally travels from hotter to colder regions of matter).
In the Evaporator (aka air Handler), section of a mini split, a low-pressurized cold liquid refrigerant absorbs hot indoor heat through the indoor coils. Then, the now gaseous refrigerant transfers the absorbed heat to the outdoor condenser where the refrigerant condenses back to a liquid releasing the heat through the condenser coils; cooling the indoor space. The cycle is repeated as the liquid refrigerent passes through an expansion valve that lowers the pressure & temperature of the refrigerant as it returns to the evaporator to absorb more heat. Indoor cooling temperatures have the potential to drop to 60 °F / 15.5 °C; depending on the outside temperatures and the R rating of the structure insulation.
In winter, this cycle reverses with the cold pressurized refrigerant gas, inside the condenser coils, absorbing latent heat from the outside transferring it indoors. Even at 0°F / -18°C there is latent heat in the winter air.
Dehumidification is performed similar to the way warm indoor moisture condenses on the cold glass surface of a window. The indoor air handler, of the mini split, pulls hot humid indoor air in to the outdoor condenser where it comes in contact with the evaporator cold coils which condenses the moisture in to a liquid that is removed by a drain pan / tube to the outside.
Another mechanical ventilation used to provide filtered fresh air exchanges are a pair of Lunos e²60, Heat Revovery Ventilators (HRV’s); one located in each loft space. The Lunos e²60 is not considered a primary dehumifier because it typically only removes 20% - 30% of moisture from the ambient air compared to an Energy Recovery Ventilator (ERV) which has a higher moisture removal rate of 50% - 60%; which is why the Lunos e²60 is designated as a HRV.
Points-of-humidity generation: like a kitchen range-top and bathroom shower, deserve a dedicated moisture removal mechanism. In the bathroom, a humidistat control allows the Lunos eGO to operate automatically when the bathroom RH exceed a set point. The Lunos eGO removes 20% of moisture with a heat recovery efficiency of 81% to > 86%; short & standard length fans, respectively. The family of Lunos mechanical ventilators work in unison to avoid depressurization of the indoor space. While an exhaust fan located directly above the range-top evacuates fumes, heat and moisture to the outside.
Household plants can minimally contribute to indoor humidity through a natural process called transpiration—exhalations of oxygen & water. However, a properly designed indoor mechanical ventilation system can mitigate any potential impact of humidity by indoor plants; while the health & biophilic benefits of indoor plants contribute to the health and well being of the occupants.
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