Part 3: Mechanical Ventilation – Now Required for All Homes
“Build Tight and Ventilate Right”: over the past decade, anyone exposed to building science circles has repeatedly encountered this mantra. Do a google search for the phrase, and you’ll find 15,600 hits – admittedly, a few shy of Justin Bieber’s 484,000,000, but enough to show that somebody’s paying attention. And now, count the IECC and soon, your local building code officials among those who are.
Part 2 of this series profiled the “build tight” provisions of the 2012 IECC: ≤ 3.0 ACH 50 in climate zones 3-8 and ≤ 5.0 ACH 50 in climate zones 1-2. By decreasing air leakage across the building envelope, homes are able to save energy that would otherwise be spent on heating and cooling air that is ultimately lost to the outdoors.
While reducing air leakage can save energy and money, it can also restrict the passage of indoor pollutants to the outdoors – hence the need to install mechanical ventilation to provide for indoor air quality. Recognizing that tight homes need mechanical ventilation, the IECC mandates it for all new homes in section R403.5:
“R403.5 Mechanical ventilation (Mandatory). The building shall be provided with ventilation that meets the requirements of the International Residential Code or International Mechanical Code, as applicable, or with other approved means of ventilation…”
So, the IECC establishes the requirement (mandate) for mechanical ventilation and sends us packing to other sources to find out HOW to supply mechanical ventilation. Now, there’s been some confusion over whether this section really requires mechanical ventilation or if natural ventilation can be used instead. And while code officials always have the final trump card on what’s “approved”, the IRC’s version of this same language (Chapter 11 of the 2012 IRC) clarifies that mechanical ventilation is in fact required:
“N1103.5 (R403.5) Mechanical ventilation (Mandatory). The building shall be provided with ventilation that meets the requirements of Section M1507 of this code or with other approved means of ventilation.”
The tip-off here is the reference to Section M1507, titled “Mechanical Ventilation.” Connect the dots, and we see that mechanical ventilation is mandatory (based on the title), and that Section M1507 is the primary source for saying HOW that mechanical ventilation is to be provided. There are two components of mechanical ventilation required in Section M1507: local exhaust and whole-house mechanical ventilation.
The location and flow rate capacity for local exhaust systems in new homes is as follows:
- Bathrooms and Toilet rooms: 50 CFM intermittent or 20 CFM continuous
- Kitchens: 100 CFM intermittent or 25 CFM continuous
Whole-House Mechanical Ventilation (WHMV)
WHMV is a new concept in the 2012 I-codes that has its roots in ASHRAE 62.2, the standard for Ventilation and Acceptable Indoor Air Quality in Low-Rise Residential Buildings. The IECC defines WHMV as follows:
WHOLE HOUSE MECHANICAL VENTILATION SYSTEM. An exhaust system, supply system, or combination thereof that is designed to mechanically exchange indoor air with outdoor air when operating continuously or through a programmed intermittent schedule to satisfy the whole house ventilation rates.
So what does it mean? It means that builders have a lot of options for the equipment selected to serve WHMV needs. A simple bath exhaust fan designed to provide not only local exhaust but also WHMV is typically the easiest and least expensive solution. Other options include a supply-side solution (usually an outdoor air duct with damper connected to the return trunk of the central air handler, including controls that govern operation), or heat or energy recovery ventilators (H/ERVs).
The equipment that is selected must have the capacity to meet the WHMV flow rate requirements of Table M1507.3.3, based on the number of bedrooms and the dwelling unit’s floor area. For example, a 2-3 bedroom house between 1500-3000 sqft requires a minimum of 60 CFM of continuous WHMV. As an alternative to continuous ventilation, the builder can provide an intermittently-operating WHMV system that is designed to cycle on at least once every 4 hours. Whether operating continuously or intermittently, the WHMV system must have controls that allow the system to be turned off manually; this is as simple as a wall switch that gives the homeowner the final say as to whether the WHMV system operates or not. A label that lets the homeowner know that the switch is for the WHMV system is not required, but it’s a good idea.
While the IECC punts on most of the WHMV design requirements (letting the IRC handle these instead), it does address energy efficiency requirements of the WHMV system. Hang in there – this is the last thing the code requires you to know. Section R403.5.1 sets the minimum efficiency requirements for WHMV systems, referred to as “efficacy” in the case of fans. Here’s Cliff’s version of the requirements:
- If you’re using a local exhaust fan for WHMV, make sure that it’s ENERGY STAR qualified; ENERGY STAR has identical efficacy requirements as those listed in Table R403.5.1 (a minimum of 1.4 CFM/Watt).*
- If you’re using an outdoor air duct connected to the return trunk of the centrally ducted heating/cooling system (and associated dampers and controls), then the central fan must be powered by an electronically commutated motor (also known as an “ECM” – an upgraded motor that is designed to be more efficient than traditional permanent split capacitor motors).
All you Need to Know…
- Mechanical ventilation (local exhaust and whole-house mechanical ventilation) is now required for all dwellings
- Equipment: WHMV systems can be exhaust only, supply only, or a combination of the two
- Bath or kitchen exhaust fans can serve double-duty; providing both local exhaust and WHMV
- Flow rate requirements: IRC Table M1507.3.3 for WHMV, IRC Table M1507.4 for local exhaust
- Controls: Manual override controls must be provided
- Efficiency: Choose an ENERGY STAR rated exhaust fan for the WHMV system; an H/ERV; or if using a central-fan integrated system, be sure to specify an ECM fan.
*Fans with much higher efficacy levels than ENERGY STAR minimums are now available, at small incremental costs. Look for an efficacy of 8.0 CFM/Watt or higher for the best performance at the lowest cost.