Attic & Crawl Space Ventilation
~10 min read · Apply ventilation ratios and clearances that keep concealed spaces dry.
Attics and crawl spaces breathe by design — 1/150 net free area for attics, ground-cover-plus-vents or full encapsulation for crawls — and the inspection judges whether the design chosen is actually complete, connected, and working.
Attic systems
The vented-attic recipe repeats from the roofing section at system level: 1/150 net free area base ratio, balanced intake (soffit) and exhaust (ridge/high), air paths open (baffles at eaves), no short circuits (pick one exhaust strategy), and no moisture injectors (exhaust ducts terminating inside). Unvented (conditioned) attics are legitimate by design — spray foam at the roofline — judged by completeness of the foam envelope and the absence of condensation, not by missing vents. The finding is a MIXED system: foam here, vents there, insulation on both planes.
- 1/150, balanced, baffled, single-strategy
- Unvented-by-design is valid — judge the foam envelope
- Hybrid confusion is the defect
Crawl space systems
Two coherent designs: vented crawl — ground vapor retarder plus operable vents (the old sizing rule-of-thumb ~1/150 reduced with ground cover), insulation in the FLOOR above; or encapsulated crawl — sealed poly up the walls, vents closed, walls insulated, and conditioning/dehumidification provided. The half-converted crawl is the standard finding: vents sealed but no conditioning (humidity builds), or encapsulation poly laid but vents left open (outside humidity condenses on cool poly). Humid-climate summer venting physics — warm moist air condensing on cool ducts and framing — explains why encapsulation wins there.
- Vented: ground cover + vents + floor insulation
- Encapsulated: sealed + wall insulation + conditioning
- Half-conversions are the defect pattern
Reading performance
Symptoms outrank design theory: attic frost/mold/staining, crawl condensation on ducts and pipes, musty odor rising into rooms (the stack effect carries crawl air UP into the house — why crawl condition is indoor-air condition), wood moisture readings, and rust on hangers and nails. The report names the system, judges its completeness, and ties symptoms to the missing piece.
Worked example
A humid-climate house: the crawl's vents were foam-sealed by an energy contractor; poly covers the ground but stops at the walls' base; no dehumidifier or supply duct exists; ducts below drip with condensation and the subfloor reads 19% moisture; upstairs smells faintly musty. Judge the design.
Diagnose the half-conversion: sealing vents committed the crawl to encapsulation, but the poly never sealed up the walls, no conditioning was added — so ground and wall moisture accumulate in still air: duct sweat, 19% subfloor (approaching decay range), and the stack effect delivering the musty evidence upstairs. Two coherent exits: complete the encapsulation (seal poly up walls, insulate walls, add dehumidification/conditioning) — the right answer in this climate — or revert to a true vented design (reopen vents, full ground cover, floor insulation). The report prescribes completion, not blame: the finding is a system half-built, with the moisture readings as its consequences and indoor air as its reach.
Common exam pitfalls
Writing up unvented designs as missing vents.
Conditioned attics and encapsulated crawls are legitimate systems — judge completeness, not conformity to the vented recipe.
Blessing sealed vents without conditioning.
Sealing is half of encapsulation — without dehumidification/conditioning it's a humidity trap; report the missing half.
Treating crawl air as separate from house air.
Stack effect lifts crawl air into living space — crawl moisture findings are indoor-air findings.
Vent it fully or seal it fully — half-done breathing spaces grow the moldiest findings.
Recap
- Attic: 1/150 balanced, or complete unvented foam design
- Crawl: vented (cover+vents) or encapsulated (sealed+conditioned)
- Half-conversions are the defect pattern
- Humid climates favor encapsulation physics
- Stack effect makes crawl air house air
- Symptoms (frost, sweat, moisture readings) rule the verdict
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