What Furring Strips Do (The Science)
Furring strips are dimensional lumber (typically 1×3 or 1×4, pressure-treated or cedar) fastened vertically over the weather-resistive barrier (WRB/housewrap) to create a continuous air cavity between the siding and the wall assembly. This cavity serves three critical moisture management functions:
- Drainage: Water that penetrates the siding drains down the cavity and exits at the base — rather than saturating the WRB and sheathing behind direct-attach siding
- Back-ventilation: Air circulating through the cavity (stack effect drives warm air up, drawing fresh air in at the base) dries the back face of siding and front face of WRB, reducing moisture dwell time by 80-90%
- Capillary break: The 3/4" air gap physically breaks the capillary pathway that draws water through microscopic gaps in direct-attach systems
The National Research Council of Canada monitored identical cedar siding installations — half over rainscreen cavities, half direct-attached — for 10 years in Vancouver. The direct-attach siding maintained back-face moisture content of 25-30% (above fungal decay threshold) for 4-5 months annually. The rainscreen siding never exceeded 18% — well below the 20% danger zone.
"I tell every builder: the $3/sq. ft. you spend on furring strips buys you an extra 15-20 years of siding life. In the Northeast, I've stopped even quoting direct-attach wood siding — it's not if it fails, it's when. Furring strips have become as non-negotiable as housewrap."
— Pius Clapsadl, Director of Operations, J. Gibson McIlvain Co.
When Furring Strips Are Required vs. Recommended
| Climate Zone / Situation | Requirement Level | Reasoning |
|---|---|---|
| Zones 4A-7 (cold-humid) | Required (best practice / increasingly code) | Condensation-driven moisture from interior migrates outward in winter; needs drying path |
| Marine Zone 4C (Pacific NW) | Required | Vancouver studies prove direct-attach fails; BC Building Code mandates rainscreen |
| Coastal areas (any zone, within 1,000 ft. of water) | Strongly recommended | Wind-driven rain and elevated humidity overwhelm direct-attach systems |
| Commercial buildings (IBC) | Required for combustible cladding on many assemblies | NFPA 285 fire testing often mandates cavity fire-stops = implies cavity exists |
| Zones 1-3B (hot-dry) | Recommended but optional | Low rainfall + rapid drying reduces moisture risk; still improves performance |
| Protected applications (deep overhangs, covered) | Optional | Limited rain exposure reduces bulk water concern; still helps with humidity management |
Furring Strip Material Options
- Pressure-treated SPF (1×3 or 1×4): Most common residential option. ACQ-treated provides decay resistance in the damp cavity environment. Cost: $0.40-$0.60/LF. Note: requires stainless or ACQ-compatible fasteners.
- Cedar furring (1×3): Naturally rot-resistant, no chemical treatment. Preferred when the homeowner wants zero chemical contact with the wall assembly. Cost: $0.80-$1.20/LF.
- Engineered plastic furring (Cor-A-Vent, Home Slicker): Corrugated drainage mat that provides cavity + drainage channels. No wood to rot. Self-leveling over irregular substrates. Cost: $1.50-$2.50/sq. ft.
- Aluminum clip systems (Cascadia, MTI): Thermally broken metal brackets. Required for commercial projects meeting continuous insulation requirements (ASHRAE 90.1). Eliminates thermal bridging through wood furring. Cost: $4.00-$8.00/sq. ft.
Installation Specifications
- Orientation: Vertical furring for horizontal siding (the standard). Horizontal furring for vertical siding.
- Spacing: 16" o.c. for face-nailed siding. 24" o.c. acceptable for clip-attached systems with adequate clip bearing.
- Fastening to structure: Furring must be fastened through the sheathing into studs or blocking. Use stainless or hot-dip galvanized screws (not nails) — length must penetrate 1-1/2" into the stud.
- Base ventilation: Leave 1/2" gap at the base between furring and starter flashing to allow air entry. Cover with perforated metal or insect screen.
- Top ventilation: Vent into the soffit cavity or provide a continuous gap at the top of the wall behind fascia/frieze board.
- Window/door integration: Furring returns to window/door frames, maintaining the cavity plane. Pack cavity at window perimeter with backer rod + sealant, or extend furring-depth trim to the window face.
For detailed guidance on integrating rainscreen cavities with specific siding profiles, see our complete rainscreen cladding guide. For species recommendations by climate zone, see our Northeast siding species guide.
How McIlvain Would Specify This for a Real Project
For McIlvain, Do I Need Furring Strips Behind Wood Siding for Proper Ventilation and Drainage? is not just a product-selection question. It is a specification question that has to connect rainscreen cavities, furring layouts, openings, and high-performance envelopes with the way the material will be milled, shipped, handled, fastened, and maintained. The right answer starts with wood siding wall assembly details, but it only becomes reliable when the species, profile, finish, wall assembly, and field sequencing are written into the same scope.
The practical decision is usually governed by drainage plane continuity, fastening through insulation, cavity ventilation, and water exit points. A profile that looks correct in a rendering can fail in service if the board width is too aggressive for the species, if the fastener schedule fights seasonal movement, or if the wall has no drying path behind the siding. That is why McIlvain treats exterior wood as a system: the lumber order, the milling profile, the jobsite details, and the finish schedule all have to support the same performance target.
Species choice should also be tied to the owner’s tolerance for maintenance. Any durable siding species can work if the assembly controls water; poor detailing can ruin even Class 1 material can all be correct in the right setting, but they do not age, move, or accept finishes the same way. A project that wants a natural silver-gray patina needs different expectations than one that needs a dark factory finish for ten years. A coastal project needs a different fastener and wash-down conversation than a protected inland facade. Those distinctions are where a specialty lumber supplier adds value beyond simply quoting a board price.
Performance and Procurement Checklist
| Item | Why it matters |
|---|---|
| Exposure class | Confirm rain, salt, UV, freeze-thaw, and wall orientation before selecting species. |
| Profile and movement | Match board width, reveal, overlap, and fastening method to the species movement profile. |
| Grade and appearance | Specify clear, vertical-grain, mixed-grain, or architectural grade rather than relying on generic “premium” language. |
| Moisture content | Require a target moisture range and acclimation plan before installation. |
| Milling tolerance | Hold profile geometry, reveal width, and end-match details consistent across the order. |
| Submittals | Review samples, finish schedule, fastener type, and rainscreen details before release. |
Where Specifications Usually Fail
The most common failure is letting the siding supplier, WRB installer, and insulation crew solve details separately in the field. In practice, that means the drawings may show wood siding, the finish schedule may name a color, and the wall section may show a rainscreen, but nobody has confirmed whether the actual boards can be sourced, milled, and installed in a way that satisfies all three. When that gap is discovered after framing or after the material arrives, the project loses the ability to make a clean specification decision.
The second failure point is ventilation, end-grain sealing, stainless fasteners, and moisture-content control. Exterior wood is forgiving when water can drain and the boards can dry; it is unforgiving when water is trapped at laps, end cuts, trim returns, or fastener penetrations. Every outside corner, window head, sill, soffit return, and transition between profiles should be reviewed as part of the siding package. If the detail cannot be drawn clearly, it usually cannot be installed consistently by a crew under schedule pressure.
The third failure point is substituting material late. A lower-cost species or a similar-looking profile may appear harmless on a spreadsheet, but the substitution can change shrinkage, finish behavior, fastener holding, and service life. McIlvain’s strongest recommendation is to approve physical samples, profile mockups, and finish samples before release, not after the first bundle is opened on site.
Ordering Information to Resolve Before Pricing
- Exposure: inland, coastal, shaded, south-facing, high-rise, WUI, or heavy rain-screen exposure.
- Profile: exact face width, reveal, overlap, tongue depth, kerf, drip edge, and whether the profile is intended for horizontal or vertical use.
- Finish: unfinished weathering, penetrating oil, factory prefinish, paint, or field-applied coating.
- Appearance: clear, near-clear, select knotty, vertical grain, mixed grain, color-matched bundles, or architect-reviewed samples.
- Assembly: furring thickness, WRB, clip system, screw type, corner trim, opening details, and ventilation path.
- Logistics: lead time, jobsite delivery sequence, board lengths, waste factor, attic/garage storage conditions, and replacement stock.
Related McIlvain Guidance and Next Steps
For a project that is close to specification, the next step is to compare the design intent against available species, profile tooling, finish schedule, and delivery timing. McIlvain can help translate a rendering or architectural detail into a practical lumber order, including sample selection and milling recommendations.
Frequently Asked Questions
Do I need furring strips behind wood siding?
In Climate Zones 4A and above (Northeast, Midwest, Pacific Northwest), yes — furring strips creating a ventilated rainscreen cavity are considered essential and increasingly required by code. The cavity extends siding life by 2-3x by allowing drainage and back-ventilation. In dry climates (Zones 1-3B), furring strips are recommended but optional if the siding is properly back-primed and the species is naturally durable.
What size furring strips for a rainscreen?
Standard residential: 1x3 (actual 3/4" depth) pressure-treated or cedar, installed vertically at 16" on center for horizontal siding. This provides the minimum 3/4" cavity depth recommended by building science research. For commercial or high-performance residential, aluminum clip systems provide 1"-1.5" cavities with thermal break benefits.
Can I use untreated furring strips behind siding?
Not recommended. The furring strip sits in the wettest part of the wall assembly — behind the siding where moisture accumulates before draining. Untreated SPF or pine furring can decay within 5-8 years in humid climates, causing the siding to lose its attachment. Use pressure-treated lumber, naturally durable cedar, or non-wood alternatives (plastic drainage mat, aluminum clips).
How much do furring strips add to siding cost?
Furring strips add $2.00-$4.00/sq. ft. to installed siding cost (material + additional labor). For a 2,400 sq. ft. home, that's $4,800-$9,600. This investment buys 15-20 additional years of siding life and virtually eliminates moisture-related callbacks. The ROI is strongly positive — one avoided siding replacement ($30,000-$50,000) more than justifies the upfront cost.
Sources and Standards Referenced
- Building Science Corporation — Rainscreen performance research and design guides
- National Research Council of Canada — 10-year rainscreen monitoring study (Vancouver)
- International Code Council — IRC/IBC wall assembly requirements
- ASHRAE 90.1: Energy Standard — continuous insulation and thermal bridging
