Roofs, Attics, and Cathedrals - Consulting & Architecture14 Vented vs. unvented shingle temperatures Building Science.com – Roofs 53 South-facing shingle temperatures Jacksonville,

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Roofs

Roofs, Attics, and Cathedrals

John Straube, Ph.D., P.Eng

www.buildingscience.com

Building Science Corp

© 2008!

Building Science

Corporation!

Building Science.com

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© buildingscience.com

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Joseph Lstiburek – Roofs 5


– Roofs 7

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Vented Attics

•! Air leakage thru ceiling is a problem in colder

climates

–!Ducts are a disaster

•! Condensation and heat gain on ducts in

warm climates

•! Modern roofs often hard to vent


How do I leak?

•! Let me count the ways …


Big Air Leakage

Points AHU

Sheet 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. 11/79 Building Science.com Building Science.com


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Paritition leaks (1/8”x 100 ft)



Air sealing

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Air Leaks – cathedral ceilings



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Hard to Vent


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Unvented Cathedral Ceilings

•! Not absolutely necessary to vent if airtight and

vapour tight material,

–! e.g. spray foam.

–! Or insulated sheathing

•! May be most practical in retrofit

•! If no wetting, little drying required

–! Demands very high performance

–! >R40, no penetrations

–! spray foam is a practical solution

–! beware thermal bridges


Hybrid: Air, thermal, fire

Building Scienc.com Insulation and Thermal Bridges No. 30/65

R6+R30

R6+R16

Warm Climate Zone!

Hybrid: Air, thermal

Building Science.com Insulation and Thermal Bridges No. 31/65

R6+R30

R6+R16

Cold Climate

Zone

2” HD spray foam

3.5” spray cellulose/fiberglass

2x6 wood frame

4” (R30) or 6” (R38) HD spray foam

Note: rafters insulated on the inside

R40+ roof, fire protection

Building Science 2008 Insulation and Thermal Bridges No. 32/65

Cold Climate (Zone 5/6) Solution


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Calculation


Hybrid Roofs with code levels of insulation

•! IRC 2007 Supplement


… or air impermeable insulation


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Simple Vented roof Air leaks! Ventilation path?

Unvented roof for more complex roof lines


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Low Slope Roofs


– Roofs 37


Low Slope Commercial


Best Practise Exposed Membrane


40


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– Roofs 41

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Unvented Sloped Roofs


– Roofs 43


– Roofs 44


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Roofs 45


Roofs 46


– Roofs 47

Zone 4 or lower



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– Roofs 49


– Roofs 50




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Vented vs. unvented shingle temperatures


– Roofs 53

South-facing shingle temperatures

Jacksonville, FL 16-Sep to 18-Nov 2000

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Temperature (F)

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mb

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url

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rva

tio

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unvented S shingle vented S shingle


Ventilation & Roof Temperature

Cathedral 5C/9F hotter than attic

Vented cathedral is same as Unvented


Roof ventilation could provide cooling Ice Dams

•! Cold Climate problem

•! Need snow and temperatures around freezing



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Conclusions

•! Highly ventilated open attic is with R60+ of low

cost fibrous insulation is best

–! Requires exceptional air sealing of ceiling

–! No duct work!

•! Unvented roofs provide more options

–!Airtightness is critical to success

–!Requires spray foam or full adhered

membrane

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Building Science 2008 Joseph Lstiburek – Roofs 69





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Roofs, Attics, and Cathedrals - Consulting & Architecture14 Vented vs. unvented shingle temperatures Building Science.com – Roofs 53 South-facing shingle temperatures Jacksonville,