Roof Ventilation for Coastal Homes

Roof Ventilationfor CoastalHomesby Mike GuertinFaced with driving rain that leaks throughattic vents, plus uncompromising codes, aRhode Island builder relies on his ingenuityIn the days after a nor’easter or tropical storm hits Rhode Island’s south-facingcoast, I invariably receive a flurry of calls to investigate “leaking roofs.” Most ofthose “leaks” are the result of wind-driven rain entering the vents high on theroof — typically through the ridge vent and gable-end vents, though occasionallythrough the soffit vents as well.In my efforts to reduce these callbacks, I’ve tried a variety of products, likelow-profile or baffled ridge vents and aggressive louvered gable vents, but noneof these consistently keep out moisture. A product that may resist water penetrationon one house inevitably leaks on another. This may be due to differencesin roof pitch or in the angle of the roof to the storm’s force. I haven’talways been able to determine the exact cause, and I’ve yet to find a foolproof,one-size-fits-all solution. However, in the last eight years of working on thisproblem, I’ve come up with some workable strategies, while also dancingacross the lines between what building codes and shingle manufacturers wantand what actually works.NAVIGATING CODES AND WARRANTIESSince most of the storm-driven water intrusion I’ve seen has come through theholes we purposely put in roofs (gable, ridge, and roof vents), I avoid themwhenever possible. In other words, I often prefer not to vent coastal roofs. Butthat’s not always so simple in practice. The International Residential Code(IRC) has two provisions that “require” roof ventilation. The first, in the “RoofVentilation” section R806, explicitly requires ventilation for attics and cathedralceilings. The second, R905.1 in “Requirements for Roof Coverings,”instructs builders to follow the manufacturer’s installation instructions (which,of course, usually require ventilation). To calculate the amount of roof ventila-CoastalContractor~Spring 2005 1

tion required, see “Roof Ventilation by theNumbers,” page 3.Roof ventilation is required by buildingcodes and manufacturers for a few reasons.For starters, ventilation cools the undersideof the roof sheathing, which prolongs shinglelife and reduces heat gain in the livingspace. Ventilation also reduces the chanceof ice damming and flushes out moisturevapor entering the attic or roof assemblythrough air leaks from inside the house.In order to install an unvented roof, Ineed a cooperative building official who’s intune with building science research onunvented “compact” or “hot” roof designs.Unvented roof designs aren’t new, andthey’ve been used successfully for manyyears. The key to their problem-free installationin a heating climate is to create anairflow barrier/retarder that prevents leakageof warm, moist air into the roof assemblyduring the heating season. Stop thevapor-laden air from reaching a condensingsurface (the underside of roof sheathing,for instance), and you won’t have condensation.I use a two-layer system to accomplishthis: After insulating, I cover the ceilingjoists and truss bottom chords or undersideof the roof rafter drywall with an airtightvapor barrier. Then I carefully seal any penetrationsthrough the drywall.Most officials to whom I’ve presentedunvented roof designs have been amenableto the idea and have approved my plans.When officials have concerns and don’tapprove my alternative designs, I go to PlanB: I meet the code by using soffit vents at a1:150 vent space-to-ceiling area ratio. And Istill install an airtight ceiling, knowing thatthe soffit vents alone will do little to evacuatemoisture vapor.Even with an obliging code official,there’s still the sticky point of shingle manufacturewarranties. If you don’t follow theinstructions, you void the warranty, despitea large body of evidence that climate, roofexposure, and shingle color have a muchgreater effect on roof temperature thanventilation does. So I resign myself to thefact (and explain to my clients) thatinstalling an unvented roof may void theroofing material warranty.SOFFIT LEAKSIn my experience, soffit vents let waterinside only when the plane of the soffit isat a level equal to, or above, the wall topplate and ceiling inside, and when thevents are strip or rectangular style. This ismost common with raised-heel trusses, butmay also occur when a bottom chord overhangsa truss. I’ve never seen water entersoffit vents on cut roofs or top-chord overhangtrusses in which the soffit is belowthe top plate and the ceiling plane. I suspectthat some wind-driven rain gets intothe soffit in the two latter cases, but there’snot enough air pressure to drive it up andover the wall. Instead, the water drainsback out through the vents or at the soffitjoints.With this in mind, I detail vents differ-Spring 2005~ CoastalContractor 2

Roof Ventilationfor Coastal HomesNext, I cut one or two tapered 2x “rafters”with a base that matches the width of thebow. The rafter supports the middle of a1/4- to 3 /8-inch plywood cover that forms theshape and structure of an eyebrow. This customgable vent provides a right-angle ventpathway for air that proves reasonablyweather resistant. Still, such a high gablevent needs to be coupled with soffit ventsto provide effective attic ventilation.HOT-CLIMATE ROOFSMy experience is primarily in coastalheating climates, but what aboutcoastal roofs in cooling climates?“Bow” cut from1x10 or 1x12FIGURE 2. On remodels where theceiling plane is too porous toensure a safe “hot roof,” theauthor builds this eyebrowgable vent.FIGURE 3. To create this gable vent, Guertinbegins by cutting a bow like this from a3-foot-long 1x10 or 1x12. About 8 inchesabove the bow, the author cuts a 12-inchhole through the sidewall sheathing, thenframes an eyebrow with triangular 2xrafter blocks. This structure will bewrapped with 3 /8-inch plywood andshingled over, as seen in the photoabove.PROMISING FUTUREOne product I have not used, but lookforward to trying, is X-Treme X-5 fromCor-A-Vent. The X-5 has a hingingbaffle that closes and seals the ventopening when winds exceed 10 mph,reportedly preventing wind-driven rainand snow from entering. The manufacturerclaims that the harder the windblows, the better the vent seals. Andwhile a storm’s force closes the windwardside of the vent, the leeward sideremains open. — M.G.Typically, builders don’t worry as much aboutmoisture vapor entering the roof assemblyfrom within the house. Instead, they have toworry about thermal moisture drive.Moisture condenses on the roofing asnighttime surface temperatures drop. Capillaryaction wicks water between the shingle laps,and when the sun returns the next morning,and roof surfaces heat up, driving the waterbetween the shingles away from the surface,through the underlayment, and into the roofsheathing. To prevent moisture from beingdriven into the roof, Joe Lstiburek of BuildingScience Corporation, a leading building-scienceconsulting firm, recommends using a permeableroofing underlayment, particularly overunvented roof assemblies where thermal-drivenmoisture can get trapped and is slow to dry.(For more information, see the BuildingScience Corp. article “Unvented Roofs, Hot-Humid Climates, and Asphalt RoofingShingles,” www.buildingscience.com/resources/roofs/unvented_roof.pdf.) Severalnew roofing underlayments meet the permeabilitycriteria for this application, includingTri-Flex 30 (www.flexia.ca/products/building_construction. asp), Typar RoofWrap 30(www.typarhouse wrap.com/roofwrap.html),and Titanium-UDL (www.interwrap.com/titanium/Ti_FCPhysical.html). ~Mike Guertin is a builder and remodeler fromEast Greenwich, R.I., and the author of thebook Roofing with Asphalt Shingles (TauntonPress, 2002).Insect screenCoastalContractor~Spring 20054