Building Performance
Solving Indoor Humidity Problems
Nathan Shirai explains how a revamping the air barrier and mechanical ventilation systems solved high indoor humidity problems on two recent projects.
18 MIN READ
Indoor Air Quality
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Health-Based Ventilation Rates
2 MIN READ![1. International Residential Code (IRC) ventilation requirements are calculated using the formula: [7.5 cfm x (number of bedrooms + 1)] + (sq. ft. x .01). 2 The ASHRAE 62.2 (2013) ventilation rates are calculated using the formula: [sq. ft. x .03] + [(number of bedrooms + 1) x 7.5 cfm]. This air-flow rate remains in the 2016 and 2019 versions of the 62.2 Standard. 3. Formula for Hayward Score (HS) recommended ventilation rates: 20 cfm x (number of bedrooms + 1) + [(sq. ft. - 1000) x 0.3]. a. Formula assumes 9-ft. ceilings b. Occupancy dictates ventilation rate for the first 1,000 sq. ft., then add 30 cfm per 1,000 sq. ft. (at 9-ft. ceiling height), as smaller volumes are quickly overcome by occupant CO2 c. It matters where you put the air: Bedrooms must get their 20 cfm per occupant, as healthy air at night is priority number one. d. As home square footage increases, two or more systems will be required. Focus on the "casita" (core area) first - the main bedroom and primary occupancy area, which includes the kitchen, baths, and home offices. As occupancy increases (families grow) additional systems can be added. e. Supply air to bedrooms, home office, living space, fitness/wellness room. Exhaust air from kitchen, laundry room, bathrooms and closets. Balance by zone. f. The Hayward Score (HS) recommendations assume stand-alone, balanced, energy-recovery ventilation (ERV), and are based on experience with high-efficiency ERV at 90% energy recovery and 60% moisture recovery with additional dehumidification where climate dictates. g. For best ventilation performance, consider the following: i. Design ventilation system to deliver HS recommended ventilation at 50% of capacity to optimize energy consumption. ii. In winter climates with increased levels of ventilation, supplemental humidification may be necessary to maintain 35-55% RH. iii. Installing a 4-inch MERV-13 filter in the main forced-air HVAC system can assist significantly in reducing dust and particle count indoors. It helps if the HVAC system is a variable-speed system that can operate continuously between heating and cooling cycles. iv. Infiltration air (natural leakage) is typically contaminated and needs to be diluted as well as cleaned. v. In the first two years after construction, chemical off-gassing from building materials and new furnishings is higher and may require running the ventilation system at higher than 50% operating speed. vi. Ideally the envelope of the house should be built to an air tightness of less than 2 ACH @50p. (Code maximum is 3 or 5 ACH, depending on climate.) It doesn't make sense for healthy, conditioned air to compete with infiltration air. Airtightness must be central to this discussion. This example that illustrates the importance of the envelope: A home built to airtightness limit of 3 ach @50Pa. has a natural leak rate (with no wind or significant temperature differential) of .17 ach - more than 50% of the air being introduced and conditioned by the ERV. 4. While four and five bedrooms are unlikely in these smaller houses, air-flow numbers have been provided here because the number of bedrooms often serves as a proxy for house occupancy. It is not uncommon for smaller houses to have 5, 6 or more occupants, and as we have learned human occupancy is a more important driver of ventilation need than house size.](https://jlconline.dev.zonda.onl/wp-content/uploads/sites/4/2021/hs-recommended-corrected.jpg?w=232)
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