Under-slab ductwork in Alberta basements is governed by stringent requirements for materials, insulation, vapor barrier integration, and rigorous documentation. Permitting authorities and inspectors focus on how these elements are represented in construction documents-requiring explicit code call-outs, dimensioned layouts, and cross-referenced details. Achieving compliance under the National Building Code - 2023 Alberta Edition (NBC(AE)), declared in force on May 1, 2024, hinges on drawing conventions that make material, assembly, and sequencing unmistakably clear, reducing RFIs and field errors. The depth and specificity of these requirements ripple through every drawing sheet, note, and detail within a plan set, from schematic duct routing to slab-edge sealing cross-sections.

Duct Material & Insulation Specifications: Precise Drawing Details Mandated by NBC(AE)

Material Call-Outs and Code Requirements: NBC(AE) 2023 Part 9.32.3.6

All under-slab ducts must use materials resistant to corrosion and moisture. According to NBC(AE) 2023 Part 9.32.3.6, call-outs on mechanical sheets must specify either:

  • Rigid Non-Metallic Ducting: For example, PVC SDR-26, or other approved plastics. The notation must include wall thickness and product listing (e.g., “100mm Ø PVC SCH 40 Under-Slab Duct, ULC-S636 Listed”).
  • Sealed Sheet Metal: Galvanized steel (minimum 26 gauge, typically 0.55 mm) with all seams and joints explicitly sealed. Common drawing annotation: “26ga Galv. Steel Under-Slab Duct, All Joints Sealed - NBC(AE) 9.32.3.6”.

Material specification boxes and mechanical schedules must indicate location-specific material choices, using symbols that correspond to a mechanical legend on each sheet.

Insulation Detailing: R-Values and Sectional Graphics (NBC(AE) 6.2.4.2)

Insulation is required for all under-slab ducts to reduce heat loss and condensation risk. Per NBC(AE) 2023 Part 6.2.4.2:

  • Minimum R-Value: Drawing notes to state "R-10 min. Closed-cell spray foam insulation around duct (thk=50mm), typ. under-slab".
  • Material Annotation: “2” Closed-cell foam (min. R-10), vapor-impermeable, installed continuously.”
  • Continuous Application: Plan cross-sections must illustrate insulation wrapping all exposed duct surfaces, with detail call-outs for tape or foamboard joints. A typical section call-out: “R-10 insul. cont. over duct, overlap min. 50mm at seams, all joints taped/sealed.”

Failure to provide this information in both notation and cross-section is a frequent source of redlines, especially when specifying only the insulation thickness but omitting R-value or continuity details.

Vapor Barrier Integration: Location and Sealing (NBC(AE) 9.13.4.7, CAN/CGSB-51.34-M)

Concrete slab vapor barriers must cover under-slab ducts, with Alberta building officials expecting this to be shown with:

  • Material Notation: “0.15 mm (6 mil) polyethylene sheet, CAN/CGSB-51.34-M, lapped min. 300mm, all seams taped.”
  • Placement in Sections: Drawings to show vapor barrier extending over insulated duct, beneath slab, with details for all vertical penetrations. Detail bubble note: “VB cont. over duct, tape at all penetrations and overlaps.”

A common review comment: “Revise slab section to show vapor barrier continuity over duct and at bulkhead penetrations per NBC(AE) 9.13.4.7 and 9.32.3.6.”

Sealing Methods: Joints, Penetrations, and Notes

Meticulous sealing is demanded where duct joints, insulation, and vapor barriers intersect. Mechanical plan notes must specify:

  • “All duct joints and seams sealed w/ UL-listed mastic or HVAC-approved tape (ref. NBC(AE) 9.32.3.11).”
  • At slab penetrations: “Seal annular space between duct and slab at risers w/ compatible polyurethane sealant, tool flush.”

Typical redline: “Provide detail showing duct/vapor barrier/sealant assembly at slab penetration.” Lack of explicit annotation results in requests for additional detail sections or manufacturer’s installation instructions.

Drawing-Sheet Conventions: Notations, Graphics, and Schedules

Plan and section sheets utilize double lines for duct extents, dashed lines for concealed portions, and color-coded insulation overlays. Project notes should include:

  • Tag: “U/S Duct (PVC, R-10 CI, 6 mil VB)” on plan and in schedules.
  • General Note: “All under-slab ducts per NBC(AE) 9.32.3.6, 6.2.4.2, insulated and vapor sealed as detailed.”

Absence of such call-outs is frequently cited as “Insufficient documentation of duct material or insulation value.” Alignment of plan legend, mechanical schedule, and section detail reduces confusion and inspection delays.

Under-Slab Duct Routing & Clearance Standards: Layout, Structure, and Clash Avoidance

Routing in Plan: Symbols, Layering, and Critical Clearances

Mechanical plans layer under-slab ducts beneath structural and civil references, using bold double-linework. Duct routing must avoid interfering with slab stiffeners, footings, or grade beams; cross-sections or enlarged details are mandatory where routes pass near these features. Required drawing attributes include:

  • Clearance Annotation: “Maint. 25mm min. clear (edge of duct to footing), typ. at all crossings - per NBC(AE) 9.32.3.11.”
  • Duct and Structure Coordination: Floor plans to indicate crossings (with bubble detail tags) and arrows for elevation transitions.

When ductwork traverses under thickened slab edges or near grade beams, enlarged sections must label the exact slab thickening, duct embedment, and insulation/vapor barrier layering. Comment often encountered: “Provide enlarged duct/footing crossing detail. Show VB and insul. continuity, and duct clearance from footing per NBC(AE) 9.13.4.7.1.”

Clearances from Utilities: Plumbing, Electrical, and Drainage Integration

Plumbing and electrical plans cross-reference mechanical layouts to confirm all under-slab ducts maintain required lateral and vertical clearances. Drawing notations required:

  • “Min. 50mm clear from U/S duct to plumbing waste stack-verify field as req’d.”
  • “U/S duct not to pass under sump pit, maintain 200mm off pit edge.”

Special attention in clash detection: plan overlays must clarify duct runs relative to floor drains, sump lines, and radon rough-in headers-failure to do so triggers review comments like, “Revise plans to coordinate U/S duct with plumbing mains and future radon piping.”

Cross-Section Detailing at Intersections

Complex intersections-such as at mechanical room drop slabs, structural columns, or service chases-require scaled sections (1:10 or 1:5). Each section must show:

  • Duct exterior with insulation thickness dimensioned (“Duct OD + 50mm CI each side”).
  • Finish slab above, annotated: “75mm conc. slab over, 6 mil VB, closed-cell foam, 100mm PVC duct w/ mastic-sealed joints.”
  • Reference elevation markers for both duct invert and top of insulation relative to UFFL (Upper Finished Floor Level).

Enlarged call-out example: “Section A/A - U/S Duct Passing Under Shear Beam, NTS. Min. 25mm clear duct to bottom of beam, continuous insulation, vapor barrier lapped and sealed at beam offset as shown.”

Code References On-Sheet

All these relationships are governed by NBC(AE) 9.33.6.7 & 9.33.6.8. Common notation in the title block or detail bubble: “All U/S duct install. as per NBC(AE) 9.33.6.7/8. Ducts watertight, corrosion resistant, w/ interior drainage at low points.”

Radon Mitigation & Basement Drainage Integration: Coordinated Detailing and Clash Prevention

Radon Rough-in and Under-Slab Duct Integration: Layout, Penetrations, Sealing

Alberta code (NBC(AE) 9.13.4.1 to 9.13.4.7) requires radon rough-in for all new residential basements. Under-slab duct drawings relate these components via:

  • Plan annotation: “100mm Ø PVC radon rough-in stack, coordinate with U/S duct layout. Min. 150mm clear to duct O.D., typ.”
  • Vapor barrier detail: “6 mil poly VB continues over ducts and around radon stub; all penetrations sealed per CAN/CGSB-51.34-M.”

Radon pipes and ductwork drawn with separate hatch patterns; crossing points marked with bubble “B” and detail reference. Typical detail shows slab, vapor barrier, insulation, duct, and radon pipe-with notation: “Seal vapor barrier at all U/S duct and radon penetrations using compatible lap seal and clamp, tool all seams smooth.”

Plan reviewers often redline: “Provide enlarged detail of duct/radon rough-in intersection showing required vapor barrier continuity and sealing.”

Basement Drainage Features: Sump Pits, Floor Drains, and Airtightness

Sump pits, floor drains, and associated piping must maintain both physical clearance and air-sealing integrity with under-slab ducts. Drawing protocols include:

  • Location note: “Sump pit F.F.E. 3500, U/S duct offset 200mm, typ.”
  • Section: “Sump pit cover gasketed and sealed (typ.), VB continuous to pit edge. Floor drain provides P-trap w/ airtight test plug prior to connection.”

Drawings should ensure every penetrant (duct or pipe) is called out with: “Seal penetration at slab and VB with compatible polyurethane caulk, min. 10mm bead, trowel finish.”

Clash Avoidance and Coordination Schedules

Civil, mechanical, and plumbing overlays must be coordinated to show no spatial interference between U/S ducts and radon or drainage elements. Clash detection notes on the composite plan:

  • “Coordinate U/S duct, radon stack, and plumbing runs-min. 100mm clear between all pipes/ducts at any slab penetration.”
  • Mechanical schedule: Column for “Radon/Drain Proximity (mm)” parallel to each U/S duct run to prompt installer review.

Lack of explicit clearance coordination generates redlines such as: “Revise composite plan: Avoid radon/drainage/duct overlap, clarify all clearances and sequencing.”

Sealing and Material Specification Call-Outs

For all penetrations through radon barriers, schedule note and section callout: “All U/S duct and radon penetrations through vapor barrier sealed with tape/caulk certified to CAN/CGSB-51.34-M, as shown - Ref. Detail C/C.”

Jobsite Layout & Inspection Dimensioning/Annotation: Eliminating Ambiguity in Under-Slab Ductwork

Dimensioning Duct Layouts: Centerlines, Offsets, and Elevations

Under-slab ducting is dimensioned in plan using gridlines and key reference walls for accuracy:

  • Plan note: “Duct C/L offset 1650mm from Grid A, typ. to centre of run. Measure from face of A/B wall; verify in field prior to pour.”
  • Register centerlines dimensioned in both plan (“Boxout for grille: 200x400mm, C/L 2200mm S of Line D”) and reflected ceiling plan.
  • Elevation section: “Duct invert 150mm above subgrade; top of insulation 25mm below slab soffit.”

Drawing conventions call for dual-dimensioning-centerline and closest wall/structural grid-so field crews have options for measurement when forming and rough-in tolerances are tight. Reviewer comments for plans lacking these notes: “Add dimensions from duct centreline to primary grid or FND wall. Elevation/tag required for riser location.”

Registers, Returns, Transitions, and Risers: Construction Call-Outs

Each duct transition, riser, register, and return is tagged on plan and referenced in a mechanical schedule, typically as:

  • “R1: Return grille 300x600mm, C/L as indicated, duct transitions up at Mark A.”
  • Section call: “Riser at 4/A - U/S duct transitions to vertical, note riser height, duct OD, and insulation wrap as shown.”

Annotations specify boxouts for slab risers (“Provide 200x300mm boxout, slab pourback post-duct install, seal per detail”) and transition details (“Duct transitions from 100mm PVC to 150mm x 300mm steel at intersection, slope for drainage per plan”).

Embedment Depth, Support, and Sealing: Requirements and Notation

Mechanical details stipulate embedment:

  • “Embed duct min. 50mm below slab soffit, insulation to continue over.”
  • “Support U/S duct at 2.4m max. O.C. on non-corroding brackets or slab chairs per Detail D1.”
  • “Seal all slab-to-duct interfaces with polyurethane caulk, min. 10mm bead.”

Sections must also reference adjacent slab reinforcement (“Duct clear of slab reinforcing by 25mm min.-coordinate per S1/S2 structural dwgs.”).

Reducing RFIs: Comprehensive Annotation Strategies

Each ambiguous transition, elevation shift, or intersection prompts RFIs. To preempt this:

  • Provide large-scale "slab duct embedment detail" per slab area, showing all clearances and insulation notes.
  • Include in General Notes: “All U/S ductwork to be field-verified prior to pour. Any conflicts to be reported before installation.”
  • Reference all duct runs in slab schedule: “Slab Duct #1: 6.5m length, 100mm dia., insulation R-10, centerline 1450mm from Grid B, min. invert 125mm below finished slab.”

Lack of such annotation is the number one cause of mid-construction clarification requests.

Plan Set Organization & Code Referencing for Review: Clear Documentation and Typical Redline Checklist

Plan Set Structure: Integration Across Sheets, Schedules, and Notes

Under-slab ductwork requires full representation across the plan set:

  • Mechanical Plans (ME): Show all duct runs, register locations, transitions, with direct reference notes (e.g., "See Section 3/M-401 for duct embedment").
  • Mechanical Schedules: Tabulate every under-slab duct instance-size (ID and OD), insulation rating ("R-10 Closed-cell Foam"), length, and material.
  • General/Mechanical Notes: State universally applied specs (“All duct joints sealed & tested per NBC(AE) 9.32.3.11. Ducts insulated to R-10. Vapor barrier installed cont. over all under-slab ducts per NBC(AE) 9.13.4.7.”)

Cross-referencing is essential: “Duct insulation detail see 5/M-401. Vapor barrier detail see 7/A-403.” Omission of such links appears as “Lack of cross-referenced detail for U/S duct insulation or VB assembly” in typical municipal redlines.

Explicit Code Call-Outs: On-Sheet Section Numbers

Permit reviewers expect direct citations of relevant NBC(AE) sections in critical call-outs:

  • Plan note: “Under-slab ducts installed per NBC(AE) 9.32.3.6 (material), 6.2.4.2 (insulation), and 9.13.4.7 (vapor barrier).”
  • Mechanical legend: “Sealing to be completed ahead of concrete pour per NBC(AE) 9.32.3.11.”

Direct code references reduce back-and-forth queries with authorities, especially when paired with enlarged details and referenced drawing locations.

Common Plan-Review Redlines: Checklist and Drawing Corrections

  • “R-value missing in duct schedule or insulation detail.” Correction: “Add R-10 (or R-7.5/R-5 as required by zone) insulation rating at every U/S duct and in all schedules.”
  • “Insufficient clearances to footings, beams, or sump pits.” Correction: Dimension and bubble detail every instance of less than 100mm clearance; adjust routing or provide structural engineer signoff.
  • “Sealing detail at slab penetration not provided.” Correction: Add enlarged sections at each riser and slab penetration.
  • “No vapor barrier continuity or radon barrier shown at duct crossings.” Correction: Revise plans to show continuous 6 mil poly, taped/lapped at all crossings, and sealing method for every duct/vapor barrier interaction-reference CAN/CGSB-51.34-M.
  • “No direct code references on drawing.” Correction: Annotate on all relevant sheets the applicable NBC(AE) or STANDATA variance number governing the detail.

Municipal redline comments from Calgary and Edmonton routinely cite these omissions; correcting them at the outset minimizes approval delays.

Material, Insulation, and Sealing Schedule Exemplar

How these would be represented in a typical schedule or general notes block:

  • Location: “Basement Under-Slab, Living Area”
  • Duct Material: “PVC SDR-26, watertight, corrosion resistant, ULC-S636 listed”
  • Insulation: “Closed-cell polyurethane foam, R-10 min. (50mm thk), tape all seams.”
  • Vapor Barrier: “6 mil (0.15mm) poly per CAN/CGSB-51.34-M, extend 300mm beyond duct perimeter, all laps taped.”
  • Sealing: “Mastic and UL181 tape to all joints. Polyurethane sealant at slab penetrations.”

Best Practices for Sheet Organization and Legend Use

Under-slab ducting components should be listed in a dedicated legend, with corresponding graphic symbols. Mechanical plans to reference all detail callouts (“See Detail 6/M-403 for transition at slab thickening”) and sheet indices updated to include all applicable details.

Through systematic drawing conventions and full alignment with the National Building Code - 2023 Alberta Edition, every detail of under-slab ductwork design, layout, material, and sealing carries through from plan sheet to field installation. Kingsway Drafting & Design executes this workflow to enable Alberta projects to proceed with speed, quality, and reliable permit approval.