Duct and Envelope Performance
Duct and Envelope Performance tests the shell and the delivery system together: how much air and heat escape the ducts, how tight the building is, and how load calculations and duct design connect to measured field performance.
Duct performance
- Duct leakage is measured with a duct blaster at 25 Pa and reported two ways: total leakage and leakage to outside. Only leakage to outside is a true energy penalty; supply-to-return leakage inside the envelope largely cancels.
- Rough-in code targets commonly fall near 4 cfm per 100 ft² of conditioned floor area at 25 Pa for leakage to outside; a mastic-and-mesh sealed system tests well below that.
- Duct insulation R-value only matters when ducts run outside conditioned space; R-8 is typical for attic ducts. Compressed insulation loses its rated R-value.
- Manual D sizes ducts from available static pressure and a friction rate in inches w.c. per 100 ft of equivalent length. Long runs, tight elbows and sagging flex all add equivalent length and starve airflow.
Envelope performance
U-factor is the reciprocal of R-value, and assembly U includes thermal bridging through studs, joists and slab edges. Air sealing usually pays back faster than added insulation, because a leaky pressure boundary wastes energy and wets the insulation.
Worked example
A 2,000 ft² house with 8 ft ceilings has 16,000 ft³ of volume and tests at 1,200 CFM50. ACH50 = (CFM50 × 60) ÷ volume = (1,200 × 60) ÷ 16,000 = 4.5 air changes per hour at 50 Pa - moderately leaky. Current cold-climate code targets are around 3.0 ACH50 or tighter, which forces deliberate mechanical ventilation design.
Traps
Reporting total duct leakage as if it were leakage to outside; confusing friction rate with velocity; and judging attic insulation by depth while ignoring settled or compressed material.