Rain-Screen Walls: a Better Wayto Install SidingSpacing the siding awayfrom the housewrap promotes ventHation anddrainage for long-lasting siding and paint

BY MARK AVERILLSNYDER

D sensed trouble as soon as I saw the

house. I was there to bid on paint-ing the eight-year-old, multimil-

lion-dollar home. What first struck me was

the sheets of paint dangling from the cedarsiding (photo below left). Even more alarm-ing, the paint peeling from the walls wasfrom the house's third paint job.

When I learned that the previous paint jobwas done only two years before, I knew thatthe problem ran deeper than the paint. Jit-tery about hiring yet another painter, thehomeowners agreed when I suggested call-ing in a consultant, Joseph Lstiburek.

When Joe saw the house about a week lat-er, he said, "I'll tell you in two minutes what's

THE PAINT ON THIS HOUSE NEVERSTOOD A CHANCE

wrong if we can pull one clapboard." Pullingoff one clapboard revealed tea-colored Tyvekhousewrap, soaked like blotter paper (photobelow right). Both the clapboards and the ply-wood sheathing were saturated. I was stunned.

Joe and I checked for possible moisturesources, from the attic to the basement.There was no evidence of any interior mois-

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Tannins leaching from the back of the cedar siding, which had not been back-primed, ruined the housewrap's water

repellency. Any rain that got past the siding soaked the sheathing, creating a reservoir that kept the siding wet and the paint

peeling. See sidebar, p. 89, for more information about housewrap failure.

86 FINE HOMEBUILDING

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DETAILING A RAIN-SCREEN WALLFurringstrips space the clapboards away from the wall, creatinga vent space that helps to dry the back of the siding. Layeredtar paper and flashings behind the furring strips create what iscalled a drainage plane, which alone would protect the housefrom water, even if there were no siding. This belt-and-suspenders approach can contribute to the longevity of boththe house and its paint.

Clapboards

Tar-paper spliriesprotect joints.

A lx2 rippedto an anglecaps the water

,table.

Window screenrepels insects.

Cor-A-Vent

Drawings: Rick Daskam FEBRUARY/MARCH 2001 87

r-I

First,flash vulnerable spots such as rough sills to drain water. Note how the tar pa-per to the rightof the opening laps over the top of the Ice&WaterShieldflashing.Inturn,the Ice&WaterShieldlapsthetarpaperbelow. .

r ~ --~

Counterflashing covers step flashing, Ice &WaterShield and tar paper.The counter-flashingis layeredover the lower clapboardto drain,while the upper clapboardstopsabove it to defeat capillaryaction.

88 FINE HOMEBUILDING

ture source that could account for the dam-

age and peeling paint. Furthermore, wefound no flashing deficiencies that couldlead rainwater behind the housewrap. Thepattern of wetting, coupled with the lack ofany interior moisture source, indicated thesource of moisture to be rain that found its

way past the siding and the housewrap.None of this surprised Joe, who explained

to me the science behind the failure and the

reason why the one sure way to correct it was tobuild a rain-screen wall (sidebar facing page).

The key is to isolate the housewrapor tar paper from the siding

Rain-screen walls have an airspace betweenthe siding and the building paper or house-wrap. (Installed behind a rain screen, plastichousewraps can work. I prefer #15 tar paperbecause of its century-old track record. Ei-ther one makes up what is called thedrainage plane). This airspace is created byapplying vertical furring strips over thedrainage-plane material. The siding, typical-ly clapboards, is nailed to these strips.

The airspace does several duties. First, itspaces the siding away from the drainageplane, minimizing any chance of extractivebleed (sidebar facing page). Back-primingthe siding would do the same thing, and it isa good practice. But it does nothing to ad-dress capillary action. As demonstrated bythe grade-school experiment of putting astalk of celery into a glass of colored water,capillary action can draw water upwardalong a continuous surface. Water can movelong distances in this way, so a raindropclimbing the back of one clapboard to thetop of another, then to the wall sheathing, isnot an imaginary event.

The airspace in a rain-screen wall providesa capillary break, stopping water driven be-hind the siding by capillary action from everreaching the drainage plane. Vented top andbottom, the siding has a way to dry from thewater that gets behind it. Finally, this ventedairspace helps to equalize the air pressure oneach side of the siding, reducing the chanceof wind-driven rain being forced or drawnbehind the siding.

You might argue that rain should never getpast a properly detailed, standard siding job.I once would have argued the same. Howev-er, when my crew stripped this house's wallsof their trim and clapboards, we stood backand marveled (photo right, p. 86). The worstdamage was around door and window open-ings and under clapboard joints, exactlywhere water had leaked under the unprimedsiding. The funny thing is that even thoughwe looked carefully at the siding's installa-

tion, it was hard to see how water had leaked

in. The workmanship was perfectly acceptable.

Layers of membranes, flashing andtar paper protect the sheathing

The existing clapboards, trim and house-wrap of this house had to be removed, anddamaged sheathing and framing repaired.Once these tasks were done, the only differ-ence between this job and a new house wasthat I was locked into existing window anddoor trim, and soffit details.

Had this been new construction, I wouldhave furred out the door and window units,

thereby keeping all the trim on one plane.The 84 windows and 18 doors were alreadyinstalled with their 5/4x4 casings appliedagainst the original housewrap. The home-owners drew the line here; they did not wantthese units removed. The best that my crewcould do was to pry the casings away fromthe house enough to slip double-thick #15tar-paper splines under the casings.

"Be the raindrop," an old-timer once told me.Although this phrase might sound like a hazybit of Eastern philosophy, imagining the pathof a raindrop is key to understanding water-proofing details. Basic to this idea is startingat the bottom, and layering upper flashingsand drainage planes over those below.

On a new home, the first layer would be apan of self-adhering bituminous membraneat the rough sills of doors and windows. I useGrace Ice & Water Shield (Grace Construc-tion Products; 800-444-6459; www.grace-construction.com). There were only a fewdoors on this house that had to be taken out

to repair the surrounding framing, so wecould use this detail only in these places (topphoto, facing page).

We also adhered a 3-ft. wide course ofIce

& Water Shield to the sheathing at the bot-tom of the walls, all the way around thehouse, to guard against dripping watersplashing back to the house.

Other spots that we covered with Ice &Water Shield were walls that rose above an

intersecting roof On a new house, we wouldhave extended half the width of the Ice &

Water Shield onto the roof deck and half upthe wall. This option was closed to us on thishouse because we weren't tearing off theroof. The best that we could do was to bend

down the existing step flashing to expose thewall sheathing, which we then covered withIce & Water Shield.

The next layer in the rain screen is the capflashings above windows, doors and otherwall penetrations such as dryer vents andlight-mounting blocks (photos p. 90). Theseflashings are installed with vertical flanges

How housewraps and building papers failby Joseph Lstiburek

"Marketingclaims to the

contrary,housewrapsdo little toreduce air

infiltration."

The primary function of a housewrap(Tyvekand Typar are two commonexamples) or building paper (#15or #30tar paper) is to drain rain that pene-trates the siding through leaky jointsor capillary action. Marketing claims tothe contrary notwithstanding, house-wraps do very little to reduce anyair infiltration.

The problem with housewraps, andbuilding papers to a lesser extent, is lossof water repellency. Contaminants re-ferred to as surfactants (tolearn how surfactantswork, visit www.chemistry.co.n2/surfac-tants.htm) degrade the wa-ter repellency of buildingpapers and housewrap, al-lowing the wetting of thehousewrap or building-pa-per surface by water. Wa-ter-soluble extractives inwood, such as tannins andwood sugars in Douglas fir,redwood and cedar, as wellas detergents and soaps, are surfactants.Once the surface is wet, pores in thehousewrap or building paper fill, allow-ing liquid-phase water to pass through.Once water penetrates the housewrapinto a wall, peeling paint and rotten sid-ing often follow.

Back-primingwood clapboards andtrim helps to isolate surfactants in thewood from the housewrap or building-paper surface. Similarly,providing anairspace between wood trim and sidingand the housewrap or building paperusing furring strips (Cedar Breather is a

over the tar-paper splines that we slid behindthe casings directly against the wall sheathing.

I used copper and lead flashings. Wherethe flashing would show, I used copper.Where it wasn't particularly visible or whereI needed a flexible flashing, I used lead.

Tar paper has worked for a century;why change?

The final step before nailing up the furringstrips was to wrap the house in #15 tar paper.

commercial product for this applica-tion; BenjaminObdyke Inc.;800-346-7655;www.obdyke.com)reduces thepotential of surfactant movement. Irecommend both practices.

Stucco should never be installed in

direct contact with any plastic-basedhousewrap. Stucco can adhere or bondto the housewrap surface, allowinghousewrap pores to become wettedand subsequently establish capillaryflow. Also,many stuccos add surfac-

tants to improve workabil-ity and freeze-thaw resis-tance. Adrainage spacebetween stucco and

housewrap is essential tocontrol water.

Stucco does not typicallybond with building papers.However,most stucco ap-plications over buildingpaper result in insufficientdrainage. I recommend us-ing at least two layers ofbuilding paper under stuc-

co to allow some drainage between thetwo layers.

Plain dirt can also cause housewrapto leak, just as Scotchgard-coated fab-ric that becomes dirty must be cleanedand re-treated to re-establish water re-

pellency. Don't let your housewrap be-come muddy or dirty.-Joseph Lstiburekis a principalinBuilding Science Corporation (www.buildingscience.com)in Westford,Massachusetts,and the author ofBuilder'sGuideto ColdClimates(TheTaunton Press,2000).

Recently, I've switched to Shingle Mate byGAF (800-234-4285; www.gaf.com). afiberglass-reinforced tar paper that stays flatand doesn't pucker up when it becomes wetas regular tar paper does. We started at thebottom, covering the layer of Ice & WaterShield. We applied the tar paper horizontal-ly, lapping each succeeding course a mini-mum of 4 in. over the lower course. This lay-er of tar paper also laps both the stepflashings at roof intersections and the cap

FEBRUARY/MARCH 2001 89

.,

Capflashing protects door and window casings. Note how tar papercoversthe fur-ringstrip by the side casing,and how the cap flashingandtar paperare all layered todraindown.

If siding leaks at joints, let it drain.Thetar-papersplines underthis light-mountingblock both coverthe furringstripsand extend over the next lower clapboardto drainwater awayfrom the house.

90 FINE HOMEBUILDING

flashings over doors, windows and the like(photo left).

We made the furring strips from 3fs-in.ACfir plywood. We ripped most of the strips 2 in.wide, plus a few 61/2in. wide for under cornerboards. (The corner boards were to be 3/. in.

by 5 in., so that left 11/2in. of furring strip tosupport the abutting siding.) On new con-struction, we would have furred out the win-dows and doors, but as I said before, theowners didn't want us pulling and rein-stalling all those units.

As luck had it, the door and window cas-ings were of 5/4 stock. This good fortune al-lowed us to add the 3fs-in.spacer parallel to theside casing and still have the clapboards beflush with the casings. For added protection,we covered the furring strips next to the win-dow casings, a spot we knew from the tearoffto be vulnerable, with tar paper (top photo).

To provide the greatest possible chance forwater behind the siding to dry out, the topsand bottoms of the channels between the

furring strips were left open to the air. Wescreened these channels' bottoms and tops toprevent bugs from entering. This step was asimple matter of stapling 12-in. wide stripsof screen along the bottom edge of thebuilding so that 6 in. of it hung below thebottom of the sheathing (drawing p. 87). Wethen nailed the furring strips, with their bot-toms flush to the bottom of the sheathing,over the screen. The furring strips werenailed to the house directly over studs withstainless-steel 7d ring-shank nails.

I wanted to back up the water table so thatit didn't flex if someone leaned on it, yet stillallow ventilation. To accomplish this, wefilled the spaces between the furring strips'bottoms with Cor-A-Vent (800-837-8368;www.cor-a-vent.com), a plastic roof ventthat resembles corrugated cardboard. Weripped the Cor-A-Vent to 3 in. wide andpeeled its layers apart so that they matchedthe furring strips' thickness.

We repeated this detail at the tops of thewalls, finally folding the screen over the fur-ring strips and Cor-A-Vent, top and bottom.The Cor-A-Vent fills the void between fur-

ring strips; it's rigid and waterproof, and itsupports the screen so that bugs can't crawl in.

Above the windows and the doors, we ran ahorizontal furring strip over the flange of thecap flashing. To vent these spaces, the verti-cal furring strips stop 1/2in. shy of the hori-zontal strip.

The final step on this job before beginningto nail on trim and clapboards was to installcounterflashing where roofs met sidewalls.We used 8-in. wide lead, nailed to the fur-

ring strips and bent 90°, to go 3 in. over the

VENTING THE TOP OF A RAIN-SCREEN WALL UNOBTRUSIVELY

The air from below (drawing p. 87) needs to exit somewhere. Rabbeting the trim board that caps the siding completes

the path.

'"

............

cedar roof shingles. This flashing wascrimped down over the butts of the shinglesto minimize the intrusion of wind-driven

rain (bottom photo, p. 88).

Even on a rain screen, back-primingis good practice

After we built the rain screen,skimping onthe trim and the clapboardsmadeno sense.Ihave found that of the commonly availablewoods, only two are suitable for harsh cli-mates such as New England's: cedar and red-wood. With a minimum amount of mainte-

nance, these two woods will last a lifetime.We chose cedar.

I sanded and dusted all six sides of everytrim board that I used. This task may seemto be overkill-it's certainly tedious-but Iaccomplish several ends at once. The processallows me to read the boards for cups andcracks, dings and dents; it removes dirt andwax film from the board; it levels planermarks; and it scuffs up the surface for supe-rior paint adhesion.

After sanding, I primed all surfaces ofevery board with an alkyd primer, Pratt andLambert Suprime 8 (800-289-7728;www.prattandlambert.com). Alkyd primertakes at least 48 hours to dry on cedar, so I

always needed to be two to three days aheadof the installation crew.

Because I could buy clear, preprimed cedarclapboards, I avoided the tedium of sandingthem. To be sure, though, we primed everycut end before installation. .

Siding goes on conventionally

The first trim boards up were corner boards.Before nailing up the corners, we coveredtheir furring strips with #15 tar paper.

Next came the water table, a lx8 cappedwith a lx2 that we ripped to pitch 150 todrain water outward (drawing p. 87). Thewater table is installed level with the bottom

of the plywood sheathing. We avoided jointsin the water table as far as was possible.Where they were unavoidable, we scarf-joined the boards and protected the furringstrip behind the joint with a tar-paper spline.

The clapboards were installed convention-ally (see FHB #47, pp. 48-52, and #112,pp. 62-67), but on top of the furring strips.They were face-nailed with small-head 6dstainless-steel ring-shank nails. Every jointwas backed up by a 3-in. by 5-in. tar-paperspline that laps the clapboard below (draw-ing p. 87). All the joints were tight, a razor-knife blade thicknessor less,but not so tight

Soffit

Rabbeted lx4

Cor-A-Vent

J/.-in. by 2-in.plywood furringstrips

that they would bow the corner boards orwindow casings.

We lapped the lower tar-paper splines at in-terruptions such as windows or light-mount-ing blocks over the top of the clapboards (bot-tom photo, facing page). The final trim pieceis a lx4 installed in place of a final clapboard,tight to the soffit. This board's lower edge israbbeted 1/4in., creating a weatherproof, al-most-invisible vent (photo, drawing above).

Even on a rain-screen wall, good paint isworth the cost. I ordered clapboards pre-primed with Cabot's Problem Solver (800-877-8246; www.cabotstain.com). I spot-primed cut ends with Pratt and LambertSuprime 8; then the entire house got threecoats of Pratt and Lambert's best acrylic ex-terior paint, Accolade, in an eggshell finish.We'd made the joints tight enough that nocaulk was needed. Caulk doesn't last, and itdoesn't make up for poor carpentry.

I've been back recently to look over thishouse. After three years, the paint still looksnew-on a house that previously wouldn'thold paint for half that time. 0

Builderandwriter MarkAverill Snydersplits histime betweenGreensboroBend,Vermont,and

Carlisle,Massachusetts.Photosby the author.

FEBRUARY/MARCH 2001 91