A Crash Course in Roof Venting

Home , Attic, Eaves, Roof

A Crash Course in Roof Venting Understand when to vent your roof, when not to, and how to execute each approach successfully by joseph Lstiburek

o much information has been devoted to the subject of roof venting that it’s easy to become confusedS and to lose focus. So I’ll start by saying something that might sound controversial, but really isn’t: A vented attic, where insulation is placed on an roof Venting 101 air-sealed attic floor, is one of the most underappreciated building Vent the attic space assemblies that we have in the To ensure that a vented attic performs at its best, history of building science. It’s you need to get the details right. Here are five hard to screw up this approach. rules that are critical to the success of this simple A vented attic works in hot cli- roof design. These rules must guide design and mates, mixed climates, and cold climates. It works in the Arctic construction no matter where the roof is being built. and in the Amazon. It works absolutely everywhere—when executed properly. Unfortunately, we manage to entire attic or if you’re venting ventilation serves either role, top level of the house being screw it up again and again, and only the roof deck. depending on the season. absolutely airtight before any a poorly constructed attic or roof In a cold climate, the pri- insulation is installed. (See “Attic- assembly can lead to excessive mary purpose of ventilation is Vent the attic Insulation Upgrade” in FHB energy losses, ice dams, mold, to maintain a cold roof tempera- A key benefit of venting the attic #200.) It’s also important to rot, and lots of unnecessary ture to avoid ice dams created is that the approach is the same ensure that there isn’t anything homeowner angst. by melting snow and to vent regardless of how creative your in the attic except lots of insula- Here, I’ll explain how to con- any moisture that moves from architect got with the roof. Be- tion and air—not the Christmas struct a vented attic properly. I’ll the conditioned living space to cause the roof isn’t in play here, decorations, not the tuxedo you also explain when it makes sense the attic. (See “Energy Smart it doesn’t matter how many hips, wore on your wedding day, to move the thermal, moisture, Details” in FHB #218 for more valleys, dormers, or gables there nothing. Attic space can be used and air-control layers to the roof on ice dams.) are. It’s also easier and often less for storage, but only if you build plane, and how to detail vented In a hot climate, the primary expensive to pile on fiberglass or an elevated platform above the and unvented roofs correctly. purpose of ventilation is to expel cellulose insulation at the attic insulation. Otherwise, the insu- solar-heated hot air from the floor to hit target R-values than lation gets compressed or kicked Theory behind venting attic or roof to reduce the build- it is to achieve a comparable around, which diminishes its R- The intent of roof venting var- ing’s cooling load and to relieve R-value in the roof plane. value. Also, attic-access hatches ies depending on climate, but it the strain on air-conditioning The success of this approach are notoriously leaky. You can is the same if you’re venting the systems. In mixed climates, hinges on the ceiling of the build an airtight entry to the

68 FINE HOMEBUILDING Drawings: John Hartman COPYRIGHT 2011 by The Taunton Press, Inc. Copying and distribution of this article is not permitted. 4. ProViDe PLenTY of AirSPACe The IRC calls for 1 in. of airspace, but I call for a 2-in.-minimum airspace between the back of the roof sheathing and the top of 3. VenT THe SoffiT the insulation. This will ensure sufficient ConTinUoUSLY airflow through the roof assembly. The vent should be placed as far to the outside edge of the soffit as possible. Otherwise, warm air next to the heated siding can rise, enter the vent, melt snow, and cause ice dams. This is especially a concern on cold-climate homes with deep eaves.

1. SeAL THe ATTiC fLoor CoMPLeTeLY Make sure the attic floor is absolutely airtight before any bulk insulation is installed. Air leaks in these critical areas are major contributors to energy loss in all climates and cause ice dams in cold climates.

2. BULK UP THe inSULATion 5. SLigHTLY PreSSUriZe THe ATTiC ABoVe THe ToP PLATe Building codes suggest balancing the intake and exhaust ventilation. The code, however, is Make sure the amount of wrong, and I’m working hard to get it changed. More ventilation at the eaves than at the ridge insulation (typically fiberglass or will slightly pressurize the attic. A depressurized attic can suck conditioned air out of the living cellulose) above the top plate space, and losing that conditioned air wastes money. is equal to or greater than the For best results, provide between 50% and 75% of the ventilation space at the eaves; a R-value of the wall assembly, 60/40 split is a good sweet spot. The code specifies 1 sq. ft. of net free-vent area (NFVA) for never less. every 300 sq. ft. of attic space. (Keep in mind that different vent products have different NFVA ratings.) Here’s how to do the math for a 1200-sq.-ft. attic.

step 4

Apply it to the particular soffit and step 1 step 2 step 3 ridge vents that you are using. Calculate how much Convert that Divide it up between the Soffit vents

NFVA you need. to inches. soffit and the ridge. 345.6 sq. in. ÷ 9 (NFVA-per-ft. 1200 sq. ft. 4 sq. ft. of NFVA 60% of 576 sq. in. = rating of vent) ÷ 300 sq. ft. x 144 (in. per sq. ft.) 345.6 sq. in. (soffit vents) = 38.4 lin. ft. of intake, or = 4 sq. ft. of NFVA = 19.2 ft. of intake per side of roof = 576 sq. in. of NFVA 40% of 576 sq. in. = 230.4 sq. in. (ridge vents) Ridge vents 230.4 sq. in. ÷ 9 = 25.6 lin. ft. of exhaust

www.finehomebuilding.com AUGUST/SEPTEMBER 2011 69 COPYRIGHT 2011 by The Taunton Press, Inc. Copying and distribution of this article is not permitted. Drywall ceiling, provision R806.3 in the fire barrier International Residential Code calls for the roof deck above the space to be vented continuously from the eave to the ridge. This is easy to accomplish in simply constructed roofs and difficult, if not impossible, to accomplish in roofs that have hips, valleys, dormers, or skylights that inter- rupt the rafter bays. If you choose to vent the roof deck, then be serious about it and really vent it. The code calls for a minimum of 1 in. of airspace between the top of the insulation and the back of the roof sheathing. That’s not enough. For best performance, R-42 assembly the airspace in the vent chute should be a minimum of 2 in. deep. Unless you’re bulk-filling 6½ -in. closed-cell foam rafter bays between 2x10 or 2x8 Prefab vent baffle rafters with closed-cell spray foam, this approach will likely require you to fur out the raf- roof VenTing 201 ters to accommodate additional insulation to achieve desired Vent the roof deck R-values. That can be a pain, but If the attic space is conditioned or if a room has you won’t run into the problems associated with having too little a vaulted ceiling, the roof deck should be vented air circulating under the roof. from eave to ridge. I suggest a 2-in. airspace To be sure your roof is getting between the sheathing and the insulation. The enough ventilation, there are IRC calls for a minimum of 1 in. of airspace; the simple calculations that you can AccuVent baffles shown here comply. follow (sidebar p. 69) Beyond the decreased capac- ity for insulation when venting attic, but you should know that Don’t think you can get away to take some of the responsibil- the roof deck, venting the roof the more it is used, the leakier with putting ductwork in an ity out of the builder’s hands and deck or the attic has some other it gets. unconditioned attic just because think of other options. drawbacks worth considering. How do people get this simple you sealed and insulated it. Duct- In a situation where mechani- In cold climates, snow can enter approach wrong? They don’t sealing is faith-based work. You cal systems or ductwork has to be the soffit and ridge vents, melt, follow the rules. They punch can only hope you’re doing a in the attic space or when there and potentially cause rot. Simi- a bunch of holes in the ceiling, good-enough job. Even when are lots of penetrations in the larly, in coastal environments or they fill the holes with recessed you’re really diligent about air- ceiling below the attic, it’s best to in regions with lots of rain and lights that leak air, and they sealing, you can take a system bring the entire attic area inside wind, moisture can be forced stuff mechanical systems with with 20% leakage and bring it the thermal envelope. This way, into the vents and into the roof air handlers and a serpentine down to maybe 5% leakage, and it’s not as big a deal if the ceiling assembly. In hurricane-prone array of ductwork in the attic. that’s still not good enough. leaks air or if the ducts are leaky zones with frequent high-wind The air leakage from these holes With regard to recessed lights and uninsulated. events, vented-soffit collapse and systems is a major cause of and other ceiling penetrations, it can pressurize a building, which ice dams in cold climates and a would be great if we could rely Vent the roof deck can cause windows to blow out major cause of humidity prob- on the builder to air-seal all these If the attic space is going to be and the roof to be blown off. lems in hot climates. It’s also an areas. Unfortunately, we can’t be conditioned, either for living Finally, in wildfire zones, floating unbelievable energy waste no sure the builder will air-seal well or mechanical purposes, or if a embers can enter the vents and matter where you live. or even air-seal at all. So we have home design calls for a vaulted cause roof fires. If any of these

70 FINE HOMEBUILDING COPYRIGHT 2011 by The Taunton Press, Inc. Copying and distribution of this article is not permitted. 2-in. XPS R-58 assembly rigid foam R-10 Two-in. XPS option 2: insulate above rigid foam R-30 reduces thermal and below the roof bridging. Not all homes have deep- enough rafters to insulate to desired R-values easily. Similarly, not everyone can afford to spray nearly 10 in. of open- or closed- cell spray foam in their roof or to stack a half-foot of rigid foam on the roof deck. An alternative option is to insulate the rafter bays with a less expensive insulation like fiberglass or cellulose and to control the temperature of the roof sheathing with rigid foam on the exterior. This approach works well in nearly any assembly, but especially on cathedral ceilings. The approach also remedies thermal-bridging issues.

Drywall fire barrier

7½ -in. closed-cell foam

option 1: insulate below the roof 6-in. The most conventional approach to insulating polyiso a roof is to put all the insulation below the roof foam deck. This approach is especially prevalent in retrofits when the existing roof is in good shape but the attic is being conditioned. R-39

option 3: insulate MASTer CLASS above the roof By adopting this approach, you’re essentially building a Unvented roofs site-made SIP (see “Built for Foul Weather,” pp. 76-81). Three 2-in. layers of polyiso Unvented roofs aren’t nearly as common as vented rigid insulation are stacked on top of each other with assemblies, and builders may not be familiar with detailing their seams staggered and taped. This approach is them correctly. While there are certainly a variety of ways most popular on timber-frame structures, on vaulted ceilings, or on roof assemblies where you want the to build an unvented roof assembly that performs well, rafters to be exposed from below. Installing all the here are three examples worth considering. insulation above the roof deck helps to eliminate thermal bridging through the roof.

PreVenT ConDenSATion WiTH Zone 7 THe rigHT AMoUnT of inSULATion Zone 6 r-30 An unvented roof assembly is possible r-25 r-49 Zone 4c r-49 only if you keep the roof sheathing warm r-10 enough to prevent conditioned air from r-38 Zone 5 condensing against it. the map at right, r-20 which is based on table r806.4 of the irC, r-38 lists the minimum r-values required to prevent condensation in unvented assemblies in various climate zones. the thickness of Zones 4a, 4b the insulation will vary depending on the r-15 r-38 type. these r-value requirements are in- Zones 2b, 3b tended only to prevent condensation and None required for tile roofs don’t supersede the code-required r-values Zones 1, 2, 3 Condensation control: r-5 for energy efficiency, which are also listed. Climate overall roof: r-30 zones 1 2 3 4 5 6 7

www.finehomebuilding.com AUGUST/SEPTEMBER 2011 71 COPYRIGHT 2011 by The Taunton Press, Inc. Copying and distribution of this article is not permitted. eXTrA CreDiT issues are of concern, there is works in all climates, but espe- another option. cially well in climate zones 5 Site-built or prefab baffles? through 8, where high R-val- Create an unvented roof ues are desired and where air- The success of a vented attic or roof deck relies on its Through provision R806.4, the impermeable insulation also airtightness. The space above the top plate of exterior IRC also allows you to build must be a vapor retarder. Low- walls—at the bottom of each rafter bay—is especially an unvented roof assembly. density, open-cell foam is permis- important. Baffles placed in this area channel intake air into Unvented assemblies work par- sible, but in climate zones 5 and either the attic space or vent chutes, and also prevent ticularly well on complex roofs above, it has to be covered with insulation from falling into the soffit and blocking airflow. that would be difficult or impos- a vapor-retarder coating, like sible to vent properly or on roofs rigid foam or painted drywall. Site-built: 2-in. chutes and baffles where it would be difficult to Also pay attention to roof- Cut 1-in.-thick rigid polyiso insulation into 2-in.- wide spacer strips, and glue them to the inside insulate properly if the roof ing materials. Asphalt shingles face of each rafter with a spray-foam adhesive were vented. require special attention when like Pur Stick (www.todol.com). Cut the polyiso insulation to fit snugly in each It should be noted, however, installed on unvented roof rafter bay, and foam it in place against that in high-snow-load areas, assemblies in hot-humid, mixed- the spacer to create a 2-in. chute or you still need a vented over-roof humid, and marine climates due baffle. to deal with ice damming. In to inward vapor drive. To keep Size: Custom-cut polyiso foam essence, you’re creating a hybrid moisture out of the roof assem- Cost: $23 per sheet Source: Dow vented/unvented roof system. bly, a roofing underlayment www.dow.com The goal in an unvented roof is with 1 perm or less (class-II to keep the roof deck—the prin- vapor retarder) must be installed cipal condensing surface in roof under the shingles. Also, check 1-in. rigid assemblies—sufficiently warm to be sure that you are in com- foam through the year to prevent pliance with the manufacturer condensation from occurring. warranties when installing In most climates, builders have shingles over an unvented roof Rigid-foam furring strips to insulate the roof sheathing in all climates. Some manufac- to prevent condensation from turers don’t warranty or offer occurring within the assembly. only a limited warranty when Spray-foam sealant The exception is hot-dry cli- their products are used over an mates such as in Phoenix, where unvented roof assembly. condensation isn’t as big an issue. Shingles that are installed Prefab: fast and functional Condensation control is most on unvented roof assemblies The AccuVent soffit insulation baffle is made of rigid recycled plastic. It’s more durable than often accomplished by installing operate at slightly higher tem- other foam-based products and installs rigid foam above the roof deck peratures, roughly 2°F to 3°F quickly with staples. These baffles should still be air-sealed with spray foam, or by installing air-impermeable warmer than shingles on vented but they’re a good option if you’re spray-foam insulation directly assemblies. This can reduce looking for a stock product. against the underside of the roof their service life by roughly 10%. Size: 41 in. by 22 in. deck. The code also allows for You can vent the roof cladding, Cost: $1.68 each air-permeable insulation, such which will increase its longev- Source: Berger Building Products as fiberglass or cellulose, to be ity, but the expense of fastening www.bergerbuilding used under the roof deck as battens over the roof sheathing, products.com long as rigid foam is used above then adding another layer of the roof sheathing. Flash-and- plywood over the battens as a Spray-foam batt (or flash-fill) assemblies nail base for the shingles, may sealant are also allowed. Any of these not be worth the expense. After approaches can adequately all, the shingle color and the roof Baffle prevent condensation from orientation are much more sig- occurring within the roof when nificant concerns when it comes the rigid foam or spray foam is to shingle life. □ installed at the appropriate thickness (chart p. 71). This article was excerpted Go to Finehomebuilding.com for a If you’re spraying foam on from a class taught by Joseph video of Joseph Lstiburek the underside of the roof deck, Lstiburek, Ph.D., P.Eng., of speaking about roof be sure you’re using the right Building Science Corp. in venting. product. Closed-cell spray foam Westford, Mass.