Build a Massachusetts home tight enough to pass the required blower door test (3.0 ACH50 or better) and it no longer breathes on its own — mechanical ventilation isn’t optional, it’s the code-required lung of the house. Here’s how many CFM you actually need, and why nearly every new Massachusetts home now gets an ERV or HRV.
The CFM formula
Whole-dwelling ventilation follows the ASHRAE 62.2 approach used in HERS ratings:
Continuous CFM = (0.03 × conditioned floor area) + 7.5 × (bedrooms + 1)
Worked examples:
- 2,000 sq ft, 3 bedrooms: (0.03 × 2,000) + 7.5 × 4 = 60 + 30 = 90 CFM continuous
- 2,500 sq ft, 4 bedrooms: 75 + 37.5 = ~113 CFM
- 1,200 sq ft ADU, 2 bedrooms: 36 + 22.5 = ~59 CFM
Your HERS rater verifies the installed system actually delivers these flows at the final inspection — a system that’s ducted poorly can spec 90 CFM and measure 60.
Local exhaust still applies on top
Kitchens need 100 CFM intermittent (a ducted range hood) or 25 CFM continuous; bathrooms need 50 CFM intermittent or 20 CFM continuous. Recirculating range hoods don’t count.
Why it’s almost always an ERV or HRV in Massachusetts
The 2025 stretch and specialized codes make balanced, energy-recovery ventilation effectively unavoidable: exhaust-only fans dump heated air outdoors with no recovery, and the HERS math at a 42–45 target rarely closes without recovering that energy. An HRV recovers heat only; an ERV recovers heat and moderates moisture — a real advantage in Massachusetts, easing summer humidity and winter dryness. When comparing units, look at two numbers: sensible recovery efficiency (higher is better; quality units recover most of the exhaust heat) and watts per CFM (lower is better — fan energy shows up directly in your HERS score).
Design choices that make or break the install
- Fully ducted beats simplified: dedicated supply to bedrooms/living and exhaust from baths/kitchen area delivers fresh air where people are; tying an ERV into the furnace ductwork often needs the air handler running to distribute, burning fan energy.
- Locate intakes away from exhausts, dryer vents, and driveways.
- Commission it: flows must be measured and balanced — supply within about 10% of exhaust — or the “balanced” system quietly pressurizes or depressurizes the house.
Home Energy Efficiency Consultants verifies ventilation flows as part of every HERS rating and energy code compliance package across Massachusetts — ENERGY STAR® partner, U.S. DOE Efficient New Home partner, 5.0 stars across 135 Google reviews from 4 offices. Sizing a system? Call (617) 501-6788.
