Remodelers Guide

Fireblocking (and Draftstopping)

TLDR

  • Locate every concealed cavity > 10 ft tall or 1,000 ft².
  • Block stud bays at tops, bottoms, and every 10 ft.
  • Add draftstops to divide open-web floor systems ≤ 1,000 ft².
  • Seal plate penetrations with orange fireblock foam or mineral wool.
  • Use red intumescent firestop caulk/collars on rated walls & floors.
  • Photograph all work before insulation for inspector & client records.

Overview

Fireblocking, draftstopping, and firestopping are three related—but not interchangeable—requirements aimed at slowing hidden flame spread inside wood-framed homes. Fireblocking cuts off vertical and concealed draft paths; draftstopping subdivides large horizontal cavities; and firestopping protects the integrity of assemblies that already carry a fire-resistance rating. All are mandated by the 2015 Michigan Residential Code (2015 MRC) and the 2021 International Residential Code (2021 IRC).

Prerequisites

  • Framing complete; mechanical rough-ins drilled but not yet insulated.
  • Blocking materials and sealants staged on site.

Materials / Tools Needed

  • 2 × sawn lumber cut-offs (2015 MRC R302.11.1 #1)
  • Two 1 × boards with lap joint (#2)
  • ¾-in plywood/OSB or ½-in gypsum board (#3, #5)
  • ¼-in cement millboard or ⅜-in particleboard (#6)
  • Mineral-wool or unfaced fiberglass batts (packed tight) (#7)
  • Orange one- or two-component “fireblock” polyurethane foam (R302.11 Item 4)
  • Red intumescent firestop caulk / collars tested to ASTM E814 or UL 1479

Process

  1. Install Fireblocking
    • Cut lumber or drywall to fit snugly and flush with framing.
    • Insert mineral-wool or fiberglass tightly where boards are impractical.
  2. Add Draftstops - install drywall or osb to cut ceilings or soffits into less than 1,000 sq ft sections (R302.12).
  3. Seal Plate Penetrations - Seal pipes and cables at top and bottom plates with intumescent caulk, foam, or mineral wool (R203.11 item 4).
  4. Firestop Rated Penetrations – In garage walls, townhouse separation walls, or floor assemblies carrying a required rating, apply red intumescent caulk or collars per listed UL system (R302.4).

Inspections

  • Rough framing inspection: confirms fireblocking & draftstopping before insulation.
  • Fireblocking will be inspected during the rough framing inspection
  • Some electrical and plumbing inspectors will look for fireblocking measures, but if your MEP contractors have missed these you can usually correct and ask them to inspect on the rough building inspection.
  • If fireblocking is missing during the rough building inspection, you can often ask the inspector to check those spots on the insulation inspection which follows.
  • Special focus areas: garage common wall (R302.6), open-web floor trusses over finished basements, stair stringers (R302.7), and soffit chases.
  • Garages: Garage is required to be separated from the residence and its attic area with no less than 1/2” gypsum board applied to the garage side, 5/8" Type X on ceiling of garage with living space above. Doors between house and garage are required to be a minimum of 1 3/8” thick solid wood or honeycomb steel, or a 20-minute fire-rated door.

Client Interaction / Communication Notes

  • Fireblocking is an important part of the build that means safety for the occupants. Some homeowners are not interested, but it is critical to get this right.

Common Issues & How to Avoid Them

  • Dropped soffits unblocked – Pre-rock with drywall strips or block with lumber.
  • Rated Walls - Make sure to install firestopping caulk in rated wall assemblies per the instructions, and not fireblocking foam which is not compliant.
  • Draftstopping forgotten in open-web floors – Add to framing punch list before calling for inspection.

Fireblocking Materials

Blocking / Draftstop MaterialCode Reference
2 × sawn lumberR302.11.1 #1
(2) 1 × boards with lap joint#2
¾-in plywood or OSB#3
½-in gypsum board#5
¼-in cement millboard#6
Mineral-wool / fiberglass batts (packed)#7
Orange “fireblock” PU foam*R302.11 Item 4
Red intumescent firestop caulk / collars*R302.4

*Foam and caulk must be specifically labeled for the intended use; orange foam is not a substitute for listed firestop caulk.

Resources

  • 2015 Michigan Residential Code: R302.4, R302.11, R302.12 (ICC, 2015)
  • 2021 International Residential Code: same sections (ICC, 2021)
  • ICC Residential Code Commentary Vol 1, pp. 139–147 (ICC, 2021)
  • Hilti Firestop Systems Guide, System CFS-SL GA 0017 (Hilti, 2024)
  • APA Technical Note “Blocking & Bridging” (APA – The Engineered Wood Assoc., 2023)