Vented and Unvented Roofs

Dr John Straube Building Science Corporation

University of Waterloo

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Pitched Roof Types •  Vented Attic

–  Insulation/air barrier at ceiling plane •  Cathedral Ceiling

–  Insulation/air barrier at roof plane

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Pitched Roof Types •  Unvented Cathedralized Attic

– As cathedral but no venting above insulation

•  Unvented Cathedral

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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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Air leakage through ceiling into roof

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Building Science 2008 Joseph Lstiburek – Roofs 9

Condesation forms on nails first (coldest) then sheathing (cold) framing (warmest)

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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

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Limitations

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Limitations of Ventilated Cathedral

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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

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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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Ventilated Cathedral Ceilings

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Simple Vented roof Air leaks! Ventilation path?

Air Leaks – cathedral ceilings

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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

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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”

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Hybrid Unventilated Cathedral Ceiling

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Roofs

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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

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Cold Climate R30: Zone 3 R38: Zone 4

1.5” - 2” ccSPF (R9-12)

5.5” ccSPF (R20) 7” (R25)

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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

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IRC 2009

… or all air impermeable insulation

1” 2” 2.5” 3+” 4+” 5” 6”

ccSPF

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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

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Temperatures in Georgia

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Temperatures in Vancouver

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Temperatures in Vancouver

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Large Vent Areas= Cooling

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Vented and Unvented Roofs

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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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