Quick answer
A siding rainscreen is a wall design that leaves a narrow drainage gap between the back of the siding and the water-resistive barrier, so rain that gets behind the siding can drain out and the wall can dry instead of staying wet.
The gap is usually made with furring strips or a drainage mat. It can be as small as 1/8 inch where a code requires a drainage space, or 3/8 inch or more for airflow.
Quick facts
The key facts about Siding Rainscreen, with sources:
- Wall type
- A drained and screened wall 1
- Capillary break
- A gap of about 1/4 inch stops water bridging across 1
- Common airspace
- 3/8 inch is usually quoted as the minimum for an air space 1
- Oregon rule
- At least 1/8 inch space between WRB and siding 2
- IRC stucco, moist or marine climates
- A 3/16 inch drainage space or 90% drainage efficiency 3
Key takeaways
- All siding leaks a little; a rainscreen gives that water a path out.
- The gap also lets air move, which dries the siding and the wall behind it.
- A gap of about 1/4 inch stops water bridging across; 3/8 inch is the common airspace figure.
- Some codes require a drainage space, such as Oregon’s 1/8 inch rule and the IRC’s stucco rules in wet climates.
- Re-siding is the cheapest time to add one.
The rainscreen in plain English
Siding is not waterproof. Wind pushes rain through laps, joints and nail holes. Building Science Corporation’s John Straube writes that lap siding and brick veneer have been shown to leak significant amounts of water 1. The question is what happens to that water.
In a wall with siding nailed tight to the house wrap, water can get trapped between them. In a rainscreen wall, there is a gap. Water runs down the back of the siding or the face of the house wrap and drains out at the bottom. Air in the gap helps both surfaces dry.
Building scientists call this a drained and screened wall. Straube lists what every drained system needs: a screen or cladding, a drainage gap, a drainage plane, flashing at the base to direct water out, and weep openings 1. The siding is the screen. The house wrap or other water-resistive barrier is the drainage plane.
A rainscreen is not a product. It is the gap, plus the flashing and screens that make it work.
How a rainscreen handles water
Straube describes rain control as the three Ds: deflection, drainage and drying 1. A rainscreen serves all three.
- Deflect. The siding sheds most rain off its face.
- Drain. Water that gets behind runs down by gravity, clinging to surfaces. Straube notes water can drain through gaps as thin as a dime 1.
- Break capillary flow. Water wicks across tiny gaps. A gap of about 1/4 inch is roughly what water cannot span, so it acts as a capillary break 1.
- Ventilate. Small gaps of 1/8 inch allow a little airflow; gaps over 3/8 inch are usually needed for significant ventilation 1.
- Exit at the base. Flashing at the bottom directs water out, and an insect screen keeps pests out of the gap 14.
- SidingThe screen that sheds most of the rain
- Drainage gapAir space from furring, mat or drainage board
- Water-resistive barrierThe drainage plane, lapped shingle-fashion
- SheathingPlywood, OSB or rigid foam
- Framing and insulationKept dry by the layers outside
Insect screen closes the top and bottom of the gap, and flashing at the base sends water out. Source: [1] [4]
Ways to build the gap
| Method | How it works | Notes |
|---|---|---|
| Wood furring strips | Vertical 1x strips over the WRB, siding nailed to them | BASC lists 1x4 wood or 20-gauge steel furring over foam 4 |
| Drainage mat | A plastic mesh or dimpled sheet behind the siding | Forms a thin, even space 2 |
| Drainage board or textured WRB | A wrap or board with built-in channels | Oregon allows a WRB meeting 75% drainage efficiency instead of a space 2 |
| Stucco drainage layer | A space or drainage material behind lath | IRC asks for 3/16 inch in moist and marine climates 3 |
| Furring over exterior foam | Strips through foam into the studs | Fasteners must fully engage the framing 4 |
Drainage vs ventilation
A very thin gap can drain water. Ventilating the wall, so air dries it, takes a bigger gap. Straube puts significant ventilation at gaps over 3/8 inch 1.
Example: gap sizes in codes and building science
Four figures come up again and again. Each comes from a different source and serves a different purpose.
| Gap | Source | What it does |
|---|---|---|
| 1/8 inch | Oregon Residential Specialty Code R703.1.1 2 | Code minimum drainage space between WRB and siding |
| 3/16 inch | 2021 IRC R703.7.3.2 for stucco in moist or marine zones 3 | Drainage space behind stucco |
| About 1/4 inch | Building Science Corporation BSD-013 1 | Capillary break |
| 3/8 inch | Building Science Corporation BSD-013 1 | Common minimum for an air space; ventilation needs more |
Pick the size by goal. If the code asks for a drainage space, meet it. If you want a capillary break, plan about 1/4 inch. If you want the wall to vent and dry, plan 3/8 inch or more, often with standard furring.
Where a rainscreen shows up
On a siding quote. Look for a line for furring strips, drainage mat or a drainage wrap, and for insect screen at the top and bottom. Our whole-home renovation guide covers re-siding scope.
On plans for an addition or ADU. Wall sections show the cladding, gap, WRB and sheathing. See our pages on home additions and ADUs.
At the bottom of the wall. A rainscreen wall has a vented starter or screened opening at its base. If you can see a screened gap below the first course, that is it.
At windows and doors. Trim gets thicker to cover the gap, and head flashing must drain into it.
Ask how the gap ends
Ask your siding contractor how the gap is closed at the top and bottom and around windows. An open gap without insect screen invites pests, and a gap without base flashing has nowhere to drain.
Benefits and trade-offs
Benefits
What a rainscreen cannot do
A rainscreen does not replace flashing or the water-resistive barrier. It only works if water reaching the gap stays on the outside of the WRB and is flashed out at windows, doors and the base. Straube lists flashing and weep holes as required parts of every drained system 1.
It also does not fix leaks at the roof, gutters or deck ledgers, which dump far more water than siding lets through.
Finally, a gap that is blocked by mortar, debris or trim stops working. Oregon’s code says the parts must combine to drain water entering the assembly to the exterior 2.
What drives the cost of a rainscreen
We do not publish siding prices. The main cost driver is whether the siding is coming off anyway. Adding a gap during a re-side or new wall is a modest step; adding one alone means removing sound siding.
Other drivers: the method (drainage wrap, mat or furring), gap depth, window and door build-outs, insect screen and base flashing, and longer fasteners 4. Walls with exterior foam need furring fastened through the foam into the studs 4. Stucco and stone veneer use different drainage details 3.
A rainscreen extends the life of siding, paint and sheathing by keeping them drier, but no source we found gives a lifespan figure. Plan the job with the whole-home budget tool and read our cost guide.
How a rainscreen is installed
Siding work often needs a permit, especially with window or structural changes. Follow the siding maker’s instructions, which may set their own gap and fastener rules.
- Install and lap the water-resistive barrier shingle-fashion, upper layer over lower 5.
- Flash windows and doors into the WRB, with sill pans where needed 5.
- Install base flashing and an insect screen at the bottom of the wall 4.
- Fasten furring strips or a drainage mat over the WRB; locate studs and fasten into them 4.
- Fold the insect screen over the furring ends before siding goes on 4.
- Build out window and door trim to match the new wall depth 4.
- Install siding with fasteners long enough for the furring and framing 4.
- Leave the top of the gap screened and vented, not caulked shut.
- WRBLapped shingle-fashion
- Flash openingsWindows and doors
- Base flashing and screenWater out, insects out
- Furring or matFastened into studs
- Trim build-outsWindows and doors
- SidingLong enough fasteners
Keeping the gap working
- Keep the bottom vent clear of mulch, soil and debris.
- Check insect screens at the base and top for damage.
- Do not caulk the bottom edge of siding or trim where water should exit.
- Keep gutters working so roof water does not overwhelm the wall.
- Repaint or reseal siding on schedule so it sheds most rain.
Warning signs of trapped water
- Paint peeling or blistering on wood siding.
- Stains below windows or at the base of walls.
- Soft sheathing or swollen siding near the ground or under windows.
- Musty smells or mold inside exterior walls.
- Stucco staining or cracking at window corners.
Ladders, mold and old paint
Re-siding means ladder and scaffold work, so leave high walls to equipped pros. Wet walls may hide mold. In homes built before 1978, removing painted siding needs lead-safe work by an RRP-certified firm 7.
These signs call for a siding or building envelope contractor to open a section and look. EPA advises drying wet materials within 24 to 48 hours to prevent mold 6.
Codes that address rainscreens
The model IRC. The IRC requires a weather-resistant exterior wall envelope with flashing and a water-resistive barrier. For exterior plaster (stucco) over wood-based sheathing, the 2021 IRC adds climate rules.
In moist or marine climate zones, Section R703.7.3.2 calls for a space or drainage material at least 3/16 inch deep outside the WRB, or drainage efficiency of at least 90% under ASTM E2273 or ASTM E2925 Annex A2 3.
In dry zones, R703.7.3.1 allows a two-layer WRB or a heavier WRB separated from the stucco by foam or a drainage space 3.
Oregon. The 2021 Oregon Residential Specialty Code R703.1.1 requires a space of at least 1/8 inch between the WRB and siding, formed by noncorrosive furring, drainage mat or drainage board.
Exceptions include a WRB meeting 75% drainage efficiency under ASTM E2273 and sills with pan flashing that drains to the outside 2. Oregon kept this amendment in its 2021 code 2.
Your town. Most states do not require a gap behind lap siding in the model code. Check the edition and amendments your building department enforces.
Rainscreen vs related wall terms
| Term | What it is | Main difference |
|---|---|---|
| Rainscreen | A drained, often vented gap behind siding 1 | The space, not a material |
| Water-resistive barrier | House wrap or felt behind the siding | The drainage plane the gap protects |
| Drainage plane | The surface water runs down 1 | Part of the rainscreen system |
| Barrier wall | A wall meant to stop all water at its face 1 | Relies on one perfect seal; rainscreen expects leaks |
| Air sealing | Closing gaps that let air leak | Controls air, not rain |
Common rainscreen myths
- Myth Good siding does not leak.
- Reality Straube writes that lap siding and brick veneer leak significant amounts of water 1.
- Myth House wrap alone is a rainscreen.
- Reality Wrap is the drainage plane; a rainscreen adds the gap in front of it 1.
- Myth Bigger is always better.
- Reality The right size depends on the goal: drainage, capillary break or ventilation 1.
- Myth It is required everywhere.
- Reality Oregon requires a space; the model IRC targets stucco in moist and marine climates 23.
- Myth A rainscreen replaces flashing.
- Reality Flashing at the base and openings is a required part of every drained wall 1.
How climate changes the case for a rainscreen
The regional variable is rain and its climate zone. The 2021 IRC ties its stucco drainage rule to moist (A) and marine (C) climate zones, with a different set of options in dry (B) zones 3.
Straube’s digest notes driving rain is the largest single source of moisture for most walls 1. Oregon, in the wet Pacific Northwest, requires a 1/8 inch drainage space under siding statewide 2.
In dry, low-rain areas the benefit is smaller. Ask your building department which climate zone and code rules apply.
When a rainscreen is worth it
It is worth the most in rainy, windy or marine climates, on walls with little roof overhang, behind stucco and stone veneer, and over exterior foam. It also suits wood siding, which lasts longer and holds paint better when it can dry.
Add it when the siding is coming off anyway. Ask each bidder how they would build the gap, screen it and flash it, and compare answers in the quote comparison tool. Our guide to choosing a contractor lists more questions.
- If You live in Oregon
- Then A 1/8 inch space or approved drainage WRB is required
- If Stucco in a moist or marine zone
- Then Plan a 3/16 inch space or 90% drainage-efficiency layer
- If Wood siding in a rainy climate
- Then Consider furring for a 3/8 inch vented gap
- If Dry climate, vinyl siding
- Then A drainage-enhanced wrap may be enough; check code
Questions about Siding Rainscreen
What is a rainscreen gap behind siding?
It is a narrow air space between the back of the siding and the water-resistive barrier. Water that gets behind the siding runs down and drains out at the bottom, and air in the gap helps the wall dry. It is usually made with furring strips, a drainage mat or a drainage wrap.
How big should a rainscreen gap be?
It depends on the goal. Oregon’s code requires at least 1/8 inch. The IRC asks for 3/16 inch behind stucco in moist and marine climates. Building Science Corporation says about 1/4 inch makes a capillary break and that 3/8 inch is usually quoted as the minimum air space; ventilation needs more.
Is a rainscreen required by code?
Sometimes. Oregon requires a drainage space of at least 1/8 inch under siding. The 2021 IRC requires a 3/16 inch drainage space or a high drainage-efficiency layer behind stucco over wood sheathing in moist and marine climates. Many places do not require one behind lap siding, so check your local code.
Do I need a rainscreen for vinyl siding?
Vinyl siding already hangs loosely with gaps behind it, so it drains better than tight siding. Building Science Corporation lists vinyl siding among drained wall systems. A full furred rainscreen is less common under vinyl, but a good WRB and flashing still matter. Check local code and the siding maker’s instructions.
What is the difference between a rainscreen and house wrap?
House wrap is the water-resistive barrier, the surface water runs down. A rainscreen is the gap in front of it that lets that water drain and the wall dry. You need both. Some drainage wraps have built-in texture that creates a small gap, which Oregon accepts if it meets 75% drainage efficiency.
Can a rainscreen be added without removing siding?
Not really. The gap sits between the siding and the WRB, so the siding has to come off to add furring or a mat. That is why the best time is a re-side, an addition or new construction. Fixing flashing at windows and the base can help an existing wall in the meantime.
Do rainscreens need insect screens?
Yes. An open gap at the top and bottom is an easy path for insects. Building America guidance says to install insect screen at the top and bottom of the furring strips and fold it over the strip ends before siding goes on. Vents must stay open so water can drain.
Does a rainscreen help with insulation?
Not directly. The air gap is vented, so it adds little insulating value. Its job is moisture control. It often pairs with exterior foam insulation, though, because furring strips give a place to fasten siding over the foam. That combination improves both drying and energy performance in one wall upgrade.
Sources
- Building Science Corporation, BSD-013: Rain Control in Buildings (John Straube, 2011), retrieved .
- Oregon Building Codes Division, 2021 Oregon Residential Specialty Code Significant Changes (includes R703.1.1 text), retrieved .
- Pennsylvania Housing Research Center (Penn State), Adapting Stucco and Stone Assemblies to Changing Codes, handouts quoting 2021 IRC R703.7.3, retrieved .
- U.S. DOE Building America Solution Center (PNNL), Continuous Rigid Insulation Sheathing/Siding (furring and insect screen), retrieved .
- TechNote: Window and Door Flashing, Code Requirements and Best Practices (September 2015), hosted by Ocean City, MD, retrieved .
- U.S. EPA, A Brief Guide to Mold, Moisture and Your Home, retrieved .
- U.S. EPA, Lead Renovation, Repair and Painting Program, retrieved .
Expert review
Written by the HyreRemodel Research team. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.
How the numbers were checked: Rainscreen principles and gap sizes come from Building Science Corporation’s BSD-013. Oregon’s R703.1.1 text was read in the state’s 2021 significant-changes document.
2021 IRC stucco WRB rules are quoted from Penn State PHRC handouts, because the ICC page would not load for us. Furring and insect screen steps come from the Building America Solution Center. The gap comparison is a side-by-side of these sourced figures. No prices or lifespans are given.
Suggest a correction. We fix errors and say what changed.