IAQ & Moisture Control Issues


Home ventilation comes in a variety of forms for different purposes, including cooling, drying, moisture control, air circulation, and fresh air. It’s used in all parts of the home, including the attic, crawlspace, basement, and garage.

When a house is made more energy efficient, moisture control and fresh air needs must be carefully managed for a healthy home with good indoor air quality (IAQ). The goal is to tighten the house and add mechanical fresh-air ventilation. It’s a lot cheaper to condition and ventilate a tight house than it is to condition a leaky one!

Discussions in this article include

  • Current State of Ventilation
  • Contributing Issues to Ventilation Difficulties
  • Diagnosing Moisture Control and Indoor Air Quality Conditions
  • Home Assessment Details for IAQ and Moisture Control
  • Customer Performance Targets for Fresh-Air Ventilation
  • Improving Mechanical Fresh-Air Ventilation
  • Post Mechanical Ventilation Testing

Most existing houses adequately exchange air with the outdoors to keep the air indoors relatively fresh. The problem with using air leaks for ventilation is that it either too much or not enough during various times of the day or year.

Air leaks also don’t ventilate all parts of a house equally. Some parts are drafty while others are stagnant and polluted. Air leakage is also the greatest during severe whether when outdoor temperatures are extreme and the weather is most active, leading to increased heating & cooling costs.

In the past, windows were treated as a key part of meeting the ventilation needs for the home as well as cooling. They can and should be used for those purposes now. The problem is windows can’t or shouldn’t be used for ventilation purposes all the time. When they are shut on tight house, mechanical fresh-air ventilation is essential.

Current State of Ventilation

  • Most houses leak too much outdoor air into and out of the house
  • Ductwork leaks too much air
  • Windows aren’t or can’t used for a variety of reasons
  • Unhealthy pollutants frequently build up in modern houses
  • Many exhaust fans are rarely used
  • Many exhaust fans don’t work as intended
  • It’s common for exhaust fans to be under or over sized
  • Most tight houses have inadequate fresh-air exchange

Contributing Issues for Ventilation Difficulties

There really isn’t a profession who takes responsibility for residential ventilation like there is for commercial properties. Ventilation is part of the mechanical code controlling heating & cooling. However, when you look closely at a residential heating & cooling contractor’s logos, ventilation is missing!

Ventilation also means different things to various companies and homeowners.

We surprised ourselves while writing these articles on indoor air quality and moisture control. There are a lot of things to consider.

Improving indoor air quality amounts to these principles:

  • Remove or relocate pollutants
  • If pollutants can’t be eliminated, reduce them
  • When pollutants can’t be removed or reduced, manage them
  • Ventilate for what remains

To accomplish these objectives, a variety of professionals may be needed to implement them.

Home Energy Audit: Diagnosing IAQ & Moisture Control

The process for reducing improving window performance follows this path:

  • Interview Homeowners
  • Conduct Visual Assessment
  • Select Indoor Air Quality & Moisture Control Options
  • Prepare the house for Ventilation and Better IAQ
  • Install Ventilation Systems
  • Post Ventilation System Installation Testing

Interview Homeowners. Improving home performance is about providing a healthier, longer lasting, more comfortable, and energy efficient house.

As they relate to ventilation, the following questions are asked during the homeowner interview:

What’s your indoor air quality like?
Does anyone living here have allergies?
Does anyone living here have respiratory issues, such as asthma?
Are there certain times of year when indoor air is better or worse?
Any outdoor odors or pollution coming into the house?
Is there mold, including mildew, anywhere you’re aware of?
What have you tried to do to improve indoor air quality?
Do you use your bathroom and kitchen exhaust fans? How often?
Do you use your windows? Why?
Does anyone living here regularly smoke?
How long do odors linger after cooking?
Are paints, air fresheners, cleaning agents, or other chemicals used often?
How do you operate your ventilation systems?
Do you have problems with static electricity?
Is there anything else you feel is relevant?

Answers to these questions help us evaluate your home in terms of moisture control, indoor air quality, and energy transfer: the golden home performance triangle.

Conduct Visual Assessment. While investigating air leakage, insulation, windows, and installations of the water heater and heating & cooling systems, moisture control and indoor air quality issues are assessed.

The following are some of the areas we consider in the course of a home energy assessment:

  • Chronic Moisture Problems
  • Unusual Indoor Moisture Sources
  • Local Exhaust Fans
  • Building Tightness
  • Duct Tightness
  • Man-Made Outdoor Pollution Sources
  • Man-Made Indoor Pollution Sources
  • Unvented Space Heaters & Fireplaces
  • Attic Air Sharing
  • Crawlspace Air Sharing
  • Attic Fans
  • Attic Entrances
  • Whole House Cooling Fans
  • Condensation
  • Humidifiers / Dehumidifiers
  • Ductwork Layout
  • Heating & Cooling System Ventilation
  • Continuous Ventilation
  • Soil Gases

Our visual assessment is discussed in more detail.

Diagnostic Testing. Some diagnostic testing for moisture control and indoor air quality is routinely done during a home energy assessment. Other types are considered additional services. With the exception of testing for carbon monoxide at a gas stove, most home performance assessors are not trained to perform other tests.

Our company, Home InSight, tests gas ovens and local exhaust fan performance as a routine part of the home performance assessment. When it’s appropriate, moisture meters are used to check structural moisture content. Testing for radon and mold are additional services. Referrals are made for other types of environmental testing, such as lead, asbestos, mercury, etc.

Our diagnostic testing is discussed in more detail.


Ventilation recommendations rely on a variety of factors. Reducing or eliminating pollution sources and installation bath area and kitchen exhaust fans are always recommended. When the house is air tight enough, fresh-air ventilation recommendations depend on climate, type and severity of indoor air quality problems, and budget.

Pollutant Source Control. Mechanical ventilation cannot effectively deal with many types of moisture and indoor air quality issues by themselves. Where possible, sources of pollutions should be eliminated or reduced.

A common form of elimination is relocating fertilizer and gasoline to areas away from the house, such as a shed or detached garage.

Reducing pollution comes in several forms. One is relocating vents. Another is repairing equipment. A favorite is air sealing.

The methods for controlling pollution at its source are too numerous to elaborate here. Specific approaches are part offered in the home performance assessment report.

Spot Exhaust Ventilation. Every room with plumbing in it should have an exhaust fan in it to remove moisture directly outdoors. New homes require them in bathrooms and kitchens. Someday, laundry rooms may also be added.

Bath Area Fans. Bath area fans are sized by the floor area of the bathroom up to 100 sqft. After that, the quantity of air depends on the number and type of plumbing fixtures.

An ENERGY STAR model is always recommended for better energy efficiency. The noise levels needs to be 2 Sones or less. When there is a lamp, it’s usually a fluorescent bulb.

A quiet fan that no one has to remember to turn off is likely to be used. A timer or humidistat is recommended.

Once installed, they need to be tested for actual air movement.

Kitchen Exhaust Fans. Normally, a 100 cfm fan is sufficient. When they get larger, considerations for makeup air are needed. One reason is to avoid causing draft problems for water heaters and furnaces.

Fresh-Air Ventilation. The need additional mechanical fresh-air ventilation depends on building tightness, climate, types & severity of IAQ issues, filtration needs, pre-conditioning air, and homeowner budget.

Building Tightness. The first consideration is how air tight the house is. Until the air-filtration rate is 0.35 natural air changes per hour (ACHnat) or less, fresh-air ventilation is not recommended. This is the Building Tightness Limit.

When the house is made tighter than the Building Tightness Limit, Home InSight always recommends fresh-air ventilation. We’ve researched all sides of the debate to settle on this policy.

The evidence suggests that after the Building Tightness is exceeded, it’s best to provide a reliable 0.35 ACHnat exchange rate. For a variety of reasons, people are healthier for it.

Fresh-Air Ventilation Types. There are three types of mechanical ventilation: exhaust only, supply only, and balanced. We will recommend balanced ventilation only when the house is made tight enough to make a difference. It is expensive to install, easy to install wrong, and is overwhelmed by air infiltration.

Unless the house is in freezing climates, our preference is for supply-only fresh air ventilation. The source of air is from a known location, the air is filtered, and stale out is continuously pushed out of the building. In humid environments, there are models that dehumidify the air only when necessary.

Exhaust-only fresh-air ventilation is an option. It’s advantage is that it can be cheaper to install and operate than a supply-only system.

There are a number of things to consider when this option is chosen. First, source of air is infiltration from unknown locations, perhaps the garbage, crawl space, or attic. Second, incoming air cannot be filtered or pre-conditioned (tempered) in any way. Finally, air distribution is easily interrupted by closed doors.

This type of fresh-air ventilation is best suited for small, open plan, tight homes or where installing ductwork is not an option.


Climate. Expect for very cold places, supply-only fresh-air ventilation is preferred. Supply-only ventilation should supply filtered, pre-conditioned air before use.

In cold places, moisture-laden air will condense and freeze behind the walls. When it thaws, the moisture will cause a variety of indoor air quality issues. Under this condition, it’s best not to force indoor air through building air leaks.

IAQ Issues. Some types of fresh-air ventilation are unsuitable under certain conditions. For example, exhaust-only ventilation is unacceptable where gas water heaters and furnaces are present.

Air Filtration. Outdoor air is dirty. It contains pollen, dust, mold, and other debris. People sensitive to dirty air need to have it cleaned before using it.

Exhaust-only fresh-air ventilation cannot be used for this purpose. Only supply-only or balanced ventilation is capable of filtering air.

Pre-Conditioning Air. Depending on climate, outdoor air may need to be tempered before using it. Humid air needs to be dehumidified. Cold air needs to be warmed. Hot air needs to be cooled.

Homeowner Budget. Some fresh-air options are more expensive than others. The type of fresh-air ventilation supplied largely depends on homeowner preferences.

Note: For your health’s sake, we will not recommend tightening the house beyond the Building Tightness Limit when dependable fresh-air ventilation is unreliable or unavailable.

Air Filters. A popular way to improve indoor air quality is to install better air filters on the central heating & cooling system. This is a great method, but make sure the equipment is adjusted to cope with the additional air restriction. Air filters are discussed in the articles about central heating & cooling systems.

Ultra-Violet Lights. Ultra-violet lights installed in central heating & cooling systems are reportedly capable of dealing with microorganisms, such as bacteria and viruses. Before installing them, do your research.


Ask a Home Performance Question



Phone #

Call Time