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Vented and Unvented Roofs
Dr John Straube Building Science Corporation
University of Waterloo
Building Science Corporation
Pitched Roof Types • Vented Attic
– Insulation/air barrier at ceiling plane • Cathedral Ceiling
– Insulation/air barrier at roof plane
Building Science Corporation
Building Science Corporation
Pitched Roof Types • Unvented Cathedralized Attic
– As cathedral but no venting above insulation
• Unvented Cathedral
Building Science Corporation Building Science Corporation
Ventilated Attics
Vented and Unvented Roofs
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Vented Attics
• Ductwork placed in ventilated attics! • Complex roof shapes hard to vent
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Building Science.com 8
Air leakage through ceiling into roof
Vented and Unvented Roofs
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Building Science 2008 Joseph Lstiburek – Roofs 9
Condesation forms on nails first (coldest) then sheathing (cold) framing (warmest)
Building Science 2008 10
Building Science 2008 Joseph Lstiburek – Roofs 11
Building Science 2008 Joseph Lstiburek – Roofs 12
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Vented Attics
• Air leakage thru ceiling is a problem in colder weather – Ducts are a disaster in both
• Condensation and heat gain on ducts in warm climates
• Modern roofs often hard to vent
Building Science.com Building Science Corporation
Limitations
Building Science Corporation
Limitations of Ventilated Cathedral
Building Science Joseph Lstiburek 16
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Building Science.com
How do I leak?
• Let me count the ways …
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Big Air Leakage Holes
AHUSheet metal and high-temperature
caulk
Caulk electrical
fixtures to
drywall
Seal HVAC
penetrations
Seal
bottom plate
Seal electricalpenetrations
Seal plumbing
penetrations
Seal and
insulate dropped
soffit
Seal chases
• Ductwork • Partitions • Dropped soffits • Cabinetry
• Ceiling lights • Rimjoists • Plumbing stacks • Attic hatch
Airflow Control No. 19/79 Building Science.com Building Science.com
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Paritition leaks (1/8”x 100 ft)
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Air sealing
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Ventilated Cathedral Ceiling
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Building Science Corporation
Ventilated Cathedral Ceilings
Building Science.com
Simple Vented roof Air leaks! Ventilation path?
Air Leaks – cathedral ceilings
Building Science.com
Air Leaks – cathedral ceilings Hard to Vent
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Unvented Cathedralized Attics
• Move air and insulation control from ceiling plane to roof plane
• Allows attic space to be used • Or HVAC into conditioned space
– Saves lots of energy, reduce problems with comfort, extends life of equipment
• Avoids wind blown rain, snow, and burning wildfire embers
Building Science 2008 – Roofs 30
Building Science 2008 Roofs 31 Building Science Corporation
Unvented Cathedral Ceiling spray foam insulation (only ccSPF Zone 5 and above)
Occupied or non-occupiable space
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Full ccSPF roof
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R6+R30
R6+R16
Cold Climate Zone
2” HD spray foam
3.5” spray cellulose/fiberglass
2x6 wood frame
6” (R38) - 8” (48) ccSPF spray foam
Note: rafters insulated on the inside
Zone 4-5: 6” Zone 6-8: 8”
Building Science.com – Roofs 34
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Hybrid Unventilated Cathedral Ceiling
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Roofs
Building Science 2008 Airflow Control No. 36/79
R5- R20
R15- R40
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Air Leaks: Monthly Calculation
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Compare interior air dewpoint to exterior sheathing temperature
Hybrid Roof Insulation IRC
• IRC 2009
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IRC 2009
… or all air impermeable insulation
1” 2” 2.5” 3+” 4+” 5” 6”
ccSPF
Hybrid Code R & fire protection
Building Science No. 39/65
Cold Climate R30: Zone 3 R38: Zone 4
1.5” - 2” ccSPF (R9-12)
5.5” ccSPF (R20) 7” (R25)
Building Science 2008 Airflow Control No. 40/79 Building Science
Airflow Control No. 40/79
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Low Slope Roofs
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US Code: IRC R806.4
• “Cathedralized attics” explicitly allowed – Require air impermeable insulation in contact
• No additional vapor control needed in Zones 1-4
• Insulation needs to be – Zone 4 Marine, Zones 5 and higher need retarder – All of Canada
Hybrid Roof Insulation IRC
• IRC 2009
Building Science.com 44
IRC 2009
… or all air impermeable insulation
1” 2” 2.5” 3+” 4+” 5” 6”
ccSPF
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Building Science Corporation
Roof Temperatures
• Shingle life is affected by high temperatures
• Ventilation does little to cool shingles! – Extensive field research
• Roof color has big effect • Orientation has a big effect
Building Science Corporation
Temperatures in Georgia
Building Science Corporation
Temperatures in Vancouver
Building Science Corporation
Temperatures in Vancouver
Building Science Corporation
Large Vent Areas= Cooling
Building Science Corporation
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Conclusions • Highly ventilated open attic with R50+ of low
cost fibrous insulation is best – Requires good air sealing of ceiling – No duct work! (less than 5% leakage?)
• Unvented roofs provide more options – Airtightness is again critical to success – Hybrid foam /fibrous
• Venting does not cool shingles noticeably
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Ottawa/Toronto/Montreal: 6 Vancouver / Windsor: 5 Calgary/Edmonton/Quebec: 7 Halifax/St Johns: 6
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