One question almost everyone asks is about how I’ll control for mold and mildew inside the house and the greenhouse.

Mold and mildew growing conditions

Although mold and mildew are distinctly different organisms, and there are many different species of each, I’ll use the term “mold” to include both molds and mildews. Both molds and mildews are generally harmful causing respiratory health effects including wheezing, coughing, and upper respiratory issues. Some molds are also toxic causing neurological and other effects including cancers and death. It is critically important to prevent their growth in the home.

Molds are fungus, they are living organisms and therefore have conditions required for life such as food, moisture, and temperature. Molds feed on organic materials such as paper, gypsum, fabric, and wood. They also thrive in high humidity (62% to 93%) and high temperatures (roughly 75 to 85 degrees F). Within a home they grow best in places where food, water, and temperature are optimum, such as:

• in or around HVAC systems and ductwork.
• under sinks and around tubs in the bathroom and kitchen,
• in or around dishwashers, clothes washers, and refrigerators,
• other areas high in moisture.

The best way to prevent mold growth is to disrupt one or more of its requirements such as moisture by keeping the home dry, or in moist areas by limiting an organic food source and/or maintaining a cooler temperature. Since the greenhouse will act as a second enclosed space for mold to grow and/or effect growing conditions within the house, I’ll need to control for mold within the house and within the greenhouse. This will be through a multi-layered approach.

Controlling for temperature

Local climate

The climate is moderate in the Pacific Northwest region of the United States. Average summer temperatures are cooler than the U.S. average and average winter temperatures are warmer than the U.S. average. Even though, on average, we have more days of precipitation than the U.S., the moderate temperature helps control mold growth from what might be expected in an area with so much rain.

Home and greenhouse temperature control

I will not be relying on the climate, alone, to keep the temperature low enough to limit mold growth. Not only are there plenty of days that are warm enough for mold growth, but solar gain through the greenhouse will raise the average air temperature inside the greenhouse and, potentially, within the house. The main heating and cooling inside the house will be from ductless mini-splits that can maintain the indoor air temperature below 75 degrees F. I’ll be curious to see what effect on the indoor temperature can be achieved, if any, from the positive air pressure created by the mini-splits when the windows are open and/or the roll-up doors are open. I may need to keep the doors and windows closed during the hottest days as protection from the solar-gain in the greenhouse.

There will not be any supplementary heating in the greenhouse as its purpose is mainly for protection from the elements rather than to grow commercial and/or temperature-sensitive plants. Therefore, the temperature inside the greenhouse will still be below mold growing temperature for a good portion of the year. During the warmer months moisture control will be essential.

Controlling for moisture – minimizing humidity

There are several sources of moisture inside the house and inside the greenhouse. This is the plan for mold control at each source.

‘Life sources’ – cooking, breathing, laundry, etc.

Normal every day activities such as cooking, running the dishwasher, showering, and even breathing create a lot of moisture inside the house. The range hood, laundry dryer, and bathroom fans will vent outside the house (but into the greenhouse) to control moisture from these sources inside the home.

Greenhouse humidity from plants and irrigation

“The fall and spring are times when humidity related diseases usually peak in greenhouses. Sunny days increase the transpiration of moisture from leaf surfaces and evaporation from soil. The warm air holds the moisture in the vapor form. At night as the air cools to the dew point, condensation occurs and water droplets are formed on cooler surfaces such as the leaves and glazing.” “Proper watering and adequate plant spacing, having well-drained floors, warming plants, moving air and venting moisture are ways to reduce humidity in greenhouses.”¹

The plan to control moisture in the greenhouse is to water only through drip irrigation, early in the day, and only as needed. Fans will provide air movement, and the ground to air heat exchange will provide convective movement of moist air out of the greenhouse through the ridge vents. The ground to air heat exchange will also heat the ground under the plants to maintain their temperature above the dew point for as long as possible during the night. Fortunately there is a wealth of free information available online on how to minimize and/or respond to molds and mildew in the greenhouse from agricultural colleges like UMass Amherst, one of my alma mater’s.

Pool and pond surface

None of the other house-inside-a-greenhouse elsewhere in the world include a lap pool and/or ponds so mine will be the first. This means there’s no other directly applicable model to learn from and I’ve had to extrapolate from near-models such as swimming pools and fish ponds. The pool was added to the plan mainly for year-round exercise and the ponds as a method for natural water cleaning and conditioning, as well as to be landscape features. However, as the greenhouse ventilation design developed they became beneficial as sources for cooling in the ground to air heat exchange (more on that in a future post).

The pool and ponds’ main impact will be to atmospheric moisture from surface evaporation. The conditions effecting evaporation are surface area, temperature, and air movement. I’ve tried to minimize evaporation through design as follows:

• Surface exposure – the greater the exposed surface area, the faster the evaporation. I won’t be able to cover the lap pool when not in use, so there isn’t a way to reduce the exposed surface area. However, to minimize the evaporation due to exposed surface area, there will not be any decorative features in the pool or ponds that increase the surface area like fountains, water falls, or streams.
• Temperature – water evaporates faster as heat increases so it is slower in the cooler weather and faster in warmer weather. The pool will not be heated except by the heat transferred from the air to ground heat exchange tubes run under and along the sides of the pool. Also, the pool will be on the north side of the house so it will not receive any direct sun, and one pond will be on the east side of the house so it will receive direct sun only in the morning.
• Air movement – this factor is hard to estimate its contribution to evaporation inside the greenhouse since there won’t be any wind like there would be if the pool and ponds were outside. On the warmest days, however, there will be air movement from the fans and convective air movement from ground level out through the greenhouse ridge vents. I’m looking forward to finding out whether the narrow space (7 feet wide) for the pool between the house and greenhouse wall causes a wind tunnel effect with the rising air.

In the end, when weighing the pool and ponds’ estimated ‘cost’ to the project in dollars and atmospheric moisture versus their benefit in entertainment and summer cooling, they’re estimated as a net benefit.

Controlling for moisture – other considerations

There are a few other sources of mold-supporting moisture in ‘standard’ homes that I evaluated to decide if they’d impact my home or be at least slightly different due to the greenhouse.

Gutters and downspouts

Improperly installed or maintained gutters can cause moisture and rot between the gutter and house or possibly even leak into the house allowing mold to grow in the attic or inside the walls. In my case, the house will not be exposed to rain or snow so there will not be gutters on the house.

There will, however, be gutters on the greenhouse and the carport to collect rainwater for the greenhouse’s irrigation system. I do not expect appreciable mold on the greenhouse since the metal frame and polycarbonate panels won’t provide any organic material for the mold to feed on. The risk from mold on the carport will be minimal since the walls will not be insulated so any leaks will be easily detected.

Since the gutters will drain to large tanks away from the house and carport, there will not be any downspouts on the greenhouse with the attendant risk of backing up and flooding the foundation. Even if there is an overflow from the rainwater storage tanks, they will be outside the greenhouse so the leak will not reach the house’s foundation and cause mold in the crawlspace under the house.

Floods and groundwater

Watching the news of floods in so many areas of the world I have at least considered that were I to experience a flood inside the house it would be more difficult to dry out due to the greenhouse acting as an additional barrier to exhausting the moisture away from the house. Fortunately my property is on a hill that is at least a hundred feet above any water body, including the ground water table so it’s pretty safe from natural floods.

Material considerations

The house exterior and interior finishes are selected for ease of maintenance and longevity but they are also good choices to control mold growth. The house is designed as 3 ‘modules’ that will be clad in metal around the livingroom module, Western Red Cedar siding around the entry and bedroom module, and cedar shou sugi ban siding around the kitchen and bathroom module. While mold can grow on cedar siding, cedar has been a material of choice in the Pacific Northwest for generations that demonstrate its ease of maintenance in our wet climate. I do not expect mold to be an issue on the house or the greenhouse wall from splash while swimming as that side of the house will have metal siding.

All in all, I think we’ve done the best we can to anticipate mold sources and minimize the weaknesses but only time will tell how effective the plan is. Stay tuned as my intent is to continue to write about my actual experience, good and bad, through at least a full year of living inside the greenhouse.

¹ Reducing Humidity in the Greenhouse, UMass Extension Greenhouse Crops and Floriculture Program. https://ag.umass.edu/greenhouse-floriculture/fact-sheets/reducing-humidity-in-greenhouse

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