Tips for Keeping a “Green House” Healthy

December 6, 2019 Bob Krell

By Jeff May — Healthy Indoors Magazine November 2019

Regrettably, summer will soon come to an end. In New England where I live, the leaves will turn colors and nights will be cooler. Soon we’ll be shutting up our houses and turning on the heat. With a reduction of fresh air in the house as cooler weather approaches, the quality of our indoor air becomes an even more important consideration.

“Green houses” are tight houses, which saves energy. We all want to preserve fuel and lower energy costs, but we don’t want to breathe contaminated air. Luckily, there are some simple things homeowners can do to help keep airborne allergens and contaminants at a minimum indoors in the heating season in all homes, including “tight” ones.

Air it Out:

  • Turn down the heat or turn off the air conditioning on a temperate day, and open up windows to air out the house.

The Attic:

  • Pull-down stairs or an attic hatch leading to an attic should be covered above with an airtight, insulated box to prevent moist house air from flowing into the attic and fueling mold growth in cooler weather, when the moisture condenses on the sheathing.

Attic sheathing with mold growth
May Indoor Air Investigations LLC

A Finished Basement: I worry about finished basements, because they are prone to developing mold-growth problems. Here are a few tips to help you maintain such a space as mold-free as possible.

  • If the relative humidity (RH) in below-grade (below-ground level) spaces is not controlled (kept under 60% in finished basements), mold problems usually develop. As air cools, its RH rises, and some species of mold can grow when the RH is over 80%. In the heating season, a finished basement must be kept consistently warm, with the thermostat set at a minimum of 60oF, whether the space is being used or not.


  • Be sure that your central vacuum system exhausts into the exterior rather than into the basement or garage.
Leaky central vacuum system
May Indoor Air Investigations LLC
  • Conventional, portable vacuum cleaners can spew out particulate matter in their exhaust stream. If you don’t have a central vacuum system, use a HEPA vacuum (one with a high efficiency particulate arrestance) filter for all household cleaning.
  • Do not use a bagless vacuum unless you intend to empty it outdoors.
  • If you hire outside cleaners, insist they use your vacuum and not their own; otherwise, mold spores and pet dander from another home will be emitted into the air in your home.

Exhaust Fans:

  • A bathroom fan that vents into the soffit or attic can result in attic mold growth; be sure your bathroom exhaust vents directly to the exterior.
  • Use a kitchen exhaust fan (preferably one that vents to the exterior) when cooking or baking. This will reduce moisture levels and cooking odors in your home.

Excess Humidity:

  • Air in the winter can feel dry, so people add central humidifiers to their furnace and/or run portable humidifiers in bedrooms. The moisture may feel great on your skin, but it also can fuel microbial growth (like bacteria and mold). Monitor the relative humidity and keep it under 40% in above-grade (above ground level) rooms (lower on very cold days).
  • If you have a wood-burning stove, don’t have that pot of water simmering on top hour after hour. Again, this can introduce excess moisture into your home.

Mechanical Systems:

  • Use the best filtration possible (with an efficiency rating of at least a MERV 8; MERV-11 is better) in your hot-air heating and/or central air conditioning system.
  • If you have a hot-water or steam heating system with a separate central air- conditioning system in the attic, close returns and supplies when you shut down the A/C system for the season. This will help to prevent moist house air from passively flowing into the ducts, providing moisture for mold growth.
  • Vacuum radiators before turning the heat on. You can use a 36-inch vacuum crevice tool to get to hard-to-reach areas.
Convector covered with biodegradable dust
May Indoor Air Investigations LLC
  • Wipe clean the tops of baseboard heating convectors. If you’ve never cleaned the convectors, remove the covers and damp wipe top and bottom cover surfaces. HEPA vacuum the tops, bottoms and sides of the fins. Then treat them with steam vapor from a steam-vapor machine. Put rags or old towels under the fin tubing to protect the flooring and catch the grime. Once you’ve cleaned the convectors in this way, you need only wipe the covers before or shortly after turning the heat on.
  • If you have an air-to-air heat exchanger in your home, follow the manufacturer’s directions for cleaning; filters should be cleaned several times a year. A poorly maintained unit can become a source of mold spores!
  • Upgrade filtration in an air-to-air heat exchanger. Remove the existing filters and install two Fantech FB6 in-line filter boxes: one for the fresh-air intake and one for the house-air intake. Replace the MERV-12 filters that come with the FB6 with MERV-8 filters.

Vacation Homes

  • If you owna vacation home, whether a condo at the base of a ski area or a spot near the beach or a lake, you mayuse the property primarily during the summer, on weekends and holidays, or during school vacations. In such circumstances, itisall too common that people turn the air conditioning or heat down the rest of the year. This can lead to elevated relative humidity and mold growth.

I inspected one home that sat on a bluff overlooking the ocean. The views were dramatic. The owners had bought the home for their future retirement, and until they retired they only occupied it on weekends and vacations.

The year they moved into the home on a permanent basis, they both began to experience respiratory symptoms. They called me because the basement smelled musty and they were worried that mold growth there was causing their symptoms.

While we were talking at the site, they admitted that when they used the house on a part-time basis, they turned the heat way down – just high enough to prevent the pipes from freezing. Then they’d open up the house on weekends and fill it with moisture from cooking and showering. They also used two wood-burning stoves on the first floor, and on top of each one they placed a pot of water to keep the air in the house moist.

I found mold growth in their heirloom carpets, on their family antiques, in the couchwhere they sat together in the evenings, and even on some of their personal possessions such as clothing in closets and books on shelves. Yes, the basement had a minor mold problem, but the couple’s greatest mold-exposure threats were in the rooms where they lived.

The expression “penny wise and pound foolish” is credited to the 27th century scholar Robert Burton. In this case, the couple saved money on heat but now faced an expensive mold clean-up and remediation.

Rather than deal with an indoor-air-quality problem once it’s developed, it’s worth investing in proactive steps that help keep our indoor environments healthier in the first place.

I can’t list all the tips I can offer you in an article of this length, so I encourage you to refer to our  book Jeff May’s Healthy Home Tips, which offers space for you to keep a written record of  steps you’ve taken to improve the quality of the indoor air in your home.

Read the full article in the November issue of Healthy Indoors Magazine at:

Available on

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