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The US rule for a vented crawl space, with or without a ground cover, or the English rule for a suspended floor.
Covering the ground with a Class I vapor retarder cuts the IRC's vent area to a tenth. An unvented, sealed crawl space is a different design (R408.3); the page gives its air rate beside the vented answer.
Outside to outside of the foundation walls around the space.
The plan dimensions of the space under the floor, taken as a rectangle. Any other shape has a longer perimeter than the rectangle of the same area, which matters under Approved Document C's wall-run rule.
The other plan dimension.
Length times width is the under-floor area both rules divide; twice their sum is the wall run Approved Document C multiplies.
Vents as each market sells them; the count divides by the rating printed on the vent.
US foundation vents are rated by net free area, UK airbricks usually by equivalent area, which is measured and normally smaller than the free area of the same grille. A screen, a damper or a louvre makes the net figure smaller than the hole in the wall, so use the rating, not the opening size.
Net vent area required
1,123 in²
1 sq ft for each 150 sq ft of under-floor area, with one vent within 3 ft of each external corner — so never fewer than four on a rectangular plan.
- The same area
- 7.8 sq ft
- Vents at the chosen rating
- 25 vents
- Under-floor area
- 1,170 sq ft
- Unvented instead: continuous exhaust (R408.3)
- 23.4 cfm
- Least vents by position (one near each corner)
- 4 vents
They open the calculator with your figures already in it
Crawl Space and Subfloor Vent Calculator: 1,123 in² — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- International Code Council, 2021 International Residential Code, R408.2: net ventilation opening area at least 1 sq ft (0.0929 m²) for each 150 sq ft (14 m²) of under-floor area, one opening within 3 ft (915 mm) of each external corner; exception 1, 1/1,500 of the area where the ground is covered with a Class I vapor retarder; exception 2, cross ventilation in place of the corner rule with that retarder
- The same code, R408.3 unvented crawl spaces: a continuous Class I vapor retarder and, among the options, continuous exhaust or a conditioned supply of 1 cfm (0.47 L/s) for each 50 sq ft (4.7 m²) of floor area
- HM Government, Approved Document C (2004 edition incorporating 2010 and 2013 amendments), 4.14(b) and 4.19(b): openings in two opposing external walls of not less than 1,500 mm² per metre run of external wall or 500 mm² per m² of floor area, whichever gives the greater; a void of at least 75 mm to wall-plates and 150 mm to the floor; ducts at least 100 mm in diameter
- Vent ratings: Gibraltar Building Products ventilation catalogue (2022), 16 × 8 in foundation vents at 45 sq in (models ALSL, PLSLBL) and 64 sq in (PLDPBL) net free area; Timloc 1201 airbrick datasheet (2025), 6,170 mm² equivalent area; Rytons Periscope PUFV datasheet (2010), 7,750 mm² with airbrick fitted
Inputs used
- Rule
- IRC R408.2 — open ground (United States)
- Under-floor space length
- 39 ft
- Under-floor space width
- 30 ft
- Vent
- 16 × 8 in foundation vent, 45 sq in (slider)
- Net free area of one vent
- 45 in²
Intermediate steps
- The same area
- 7.8 sq ft
- Vents at the chosen rating
- 25 vents
- Under-floor area
- 1,170 sq ft
- Unvented instead: continuous exhaust (R408.3)
- 23.4 cfm
- Least vents by position (one near each corner)
- 4 vents
Confidence note: 1 sq ft for each 150 sq ft of under-floor area, with one vent within 3 ft of each external corner — so never fewer than four on a rectangular plan.
What this calculation does not cover
- The IRC's area rule is a minimum for the openings' NET area — after screens, louvres and dampers — and each opening must be covered with one of the meshes or grilles R408.2 lists. A closed damper counts for nothing.
- On a plan with more than four external corners the corner rule asks for one vent near each of them, which can exceed the count by area.
- Crawl spaces also need an access opening (R408.4), cleared of vegetation and organic material (R408.5), and flood openings where the building is in a flood hazard area (R408.7). None of these is worked here.
- The count assumes every vent is the chosen rating and none is blocked by planting, a raised patio or a later extension; a vent that cannot breathe adds nothing to the total.
Add the equipment this sizes
This result is a specification — 1,123 in² — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-25 · v1.0.0
Regulatory standards & verification citations4
- International Code Council, 2021 International Residential Code, R408.2: net ventilation opening area at least 1 sq ft (0.0929 m²) for each 150 sq ft (14 m²) of under-floor area, one opening within 3 ft (915 mm) of each external corner; exception 1, 1/1,500 of the area where the ground is covered with a Class I vapor retarder; exception 2, cross ventilation in place of the corner rule with that retarder
- The same code, R408.3 unvented crawl spaces: a continuous Class I vapor retarder and, among the options, continuous exhaust or a conditioned supply of 1 cfm (0.47 L/s) for each 50 sq ft (4.7 m²) of floor area
- HM Government, Approved Document C (2004 edition incorporating 2010 and 2013 amendments), 4.14(b) and 4.19(b): openings in two opposing external walls of not less than 1,500 mm² per metre run of external wall or 500 mm² per m² of floor area, whichever gives the greater; a void of at least 75 mm to wall-plates and 150 mm to the floor; ducts at least 100 mm in diameter
- Vent ratings: Gibraltar Building Products ventilation catalogue (2022), 16 × 8 in foundation vents at 45 sq in (models ALSL, PLSLBL) and 64 sq in (PLDPBL) net free area; Timloc 1201 airbrick datasheet (2025), 6,170 mm² equivalent area; Rytons Periscope PUFV datasheet (2010), 7,750 mm² with airbrick fitted
Which documents these citations point at
- International Residential Code (United States)One- and two-family dwellings and townhouses up to three storeys — structure, fire safety, energy, plumbing, mechanical and electrical in a single document.
- Approved Documents to the Building Regulations (England) — C (United Kingdom)Practical guidance on meeting the Building Regulations, one lettered part per subject — Part A structure, Part B fire, Part K protection from falling, Part L conservation of fuel and power, and the rest.
A code or standard has force only where a jurisdiction has adopted it, usually with local amendments. This site holds no adoption data for any authority, so check what is in force with the authority where you build. Any section cited above without an edition should be checked against the edition in force where you build. What it would take to know.
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