Why Ventilation Matters
Ventilation serves two critical functions for mold prevention:
- Removes moisture-laden air from the building (exhaust ventilation)
- Dilutes airborne mold spore concentrations by introducing fresh outside air
Modern tight construction makes mechanical ventilation essential. Buildings no longer “breathe” naturally through gaps and cracks — they require intentional air exchange systems to maintain healthy indoor environments.
Types of Ventilation
Exhaust Ventilation
Removes moisture at the source — bath fans, kitchen range hoods, dryer vents. Most important for mold prevention. Creates slight negative pressure that can draw in outdoor air through building envelope.
Supply Ventilation
Brings fresh air into the building, typically through HVAC system. Prevents excessive negative pressure that can draw moisture through foundation or soil gas into living spaces.
Balanced Ventilation
Equal exhaust and supply. ERVs (Energy Recovery Ventilators) and HRVs (Heat Recovery Ventilators) exchange heat/moisture between incoming and outgoing air streams. Best for tight modern construction.
Bathroom Ventilation
Bathrooms are typically the primary moisture source in homes. Inadequate bathroom ventilation is a leading cause of mold problems.
| Ventilation Type | ASHRAE 62.2 Requirement |
|---|---|
| Continuous | 20 CFM per bathroom |
| Intermittent (during use) | 50 CFM minimum |
- Duct to outside: Never into attic or crawl space
- Run time: Continue 15-20 minutes after shower ends
- Timer or humidity sensor: Automated controls ensure proper operation
Common Problem: Many bath fans are undersized, noisy (so occupants don’t use them), or improperly ducted into attics where they deposit moisture. TrueSight frequently finds attic mold caused by bath fans venting into attic space.
Kitchen Ventilation
- Range hood ducted to exterior (not recirculating)
- Minimum 100 CFM for 30″ range, 150+ CFM preferred
- Use during all cooking, especially boiling/steaming
- Size hood to cover entire cooktop surface
- Consider makeup air for high-CFM hoods in tight homes
Note: Recirculating range hoods filter grease and odors but do NOT remove moisture. Only ducted hoods provide moisture control.
Attic and Crawl Space Ventilation
Attics
Vented attics allow moisture to escape through soffit and ridge vents. Inadequate attic ventilation causes condensation on roof sheathing in winter and accelerated shingle aging in summer.
- Standard: 1 sq ft net free area per 150 sq ft attic floor
- Balance between high (ridge/gable) and low (soffit) vents
- Clear soffit vents — insulation should not block airflow
Crawl Spaces
Traditional vented crawl spaces can introduce hot humid air in summer. Modern building science often favors sealed, conditioned crawl spaces with vapor barriers for humid climates like Oklahoma’s.
Oklahoma-Specific Considerations
Vented Crawl Space Problems
Oklahoma’s humid summers mean vented crawl spaces draw in 70-80% RH outdoor air. This condenses on cool floor joists and subfloor, creating ideal mold conditions. Encapsulation is often the solution.
Attic Bath Fan Issues
TrueSight frequently finds bath fans terminating in attics rather than outdoors. In Oklahoma’s hot attics, the flex duct often detaches. This deposits bathroom moisture directly onto roof sheathing, causing mold and rot.
Whole-House Ventilation
Newer Oklahoma homes are built tighter and may need mechanical whole-house ventilation. Exhaust-only systems are most common, but ERVs are increasingly installed in high-performance homes.
Research & Citations
- ASHRAE Standard 62.2: Ventilation and Acceptable Indoor Air Quality in Residential Buildings.
- EPA. “Moisture Control Guidance for Building Design.” EPA.gov.
- DOE Building America. “Ventilation Strategies.”
- Building Science Corporation. “Crawl Space Ventilation.”
- Home Ventilating Institute. “Certified Products Directory.”