Building Science Roofs
John Straube, Ph.D., P.Eng
presented by www.buildingscience.com
This presentation
• Roofing – Low-slope – Pitched – Membrane, metal, etc.
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Roofs
• Roofs are significant proportion of the area of low-rise buildings – Significant to total cost – Major area for heat loss / gain – Low-slope membranes usually need
replacement every 15-25 yrs. • Wide range of membrane choices • Insulation on top or bottom of membrane • Green roofs = organic ballast
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Inverted Roof
• Pros: – Preferred approach – Exterior insulation
eliminates thermal bridges
– Protects membrane • Cons:
– Added weight of ballast
– Can only use XPS
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Exposed membranes
• Pros: – Lightest weight – Wide variety of
insulation and membranes
• Cons: – Exposed membrane!
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“Money spent on roof slope is one of the best bargains in the construction business” Wayne Tobiasson, CRRL
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• Why fully adhered?
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Low-level air barriers
• Roof membranes can be air barriers • In practise, a deck level air barrier is more
reliable, easier to control – Other benefits include secondary roof during
construction and re-roofing
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A cheap air barrier
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Combine multiple penetration at doghouses
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Avoiding penetrations is always best
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Rain leaks at parapet Air leaks everywhere Heat leaks everywhere
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Parapets
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White roofs: the coming storm
What air barrier
White roofs
• Lower heat gain: great! • Reduce stress on exposed roof membranes • But: Reduces drying out of roof • Thus: Require better moisture control!
– Air barrier – Construction moisture
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Green Roofs
• Green roof acts as thermally massive, low solar absorption surface – Not insulation
• Save energy compared to black roof of same R-value
• White roof with more insulation is less expensive alternate
• Don’t feel good
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“Perfect” Green Roof
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Fully-adhered membrane!
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Energy: Dirt is not insulation
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Source: Ray, S., Glicksman, ASHRAE Buildings XI 2010
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Stormwater Management
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NRC Roof Test Ottawa, Canada ¾” (19 mm) rainfall over 6.5 hrs 150 mm soil over membrane
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Green Roofs
• To save energy, use white roofs with lots of insulation
• Soil does store lots of stormwater – Retention mats can do the same
• Likely cleans water somewhat • Likely absorbs particulates
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Sloped roofs
• Basic Physics are the same • Slopes allows non-waterproof materials to
be lapped “shingle fashion” • Air – vapor impermeable insulation needed
OR ventilation of the sheathing
• Air leaky roofs require more ventilation
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Vented Pitched Roof Types
• Vented Attic – Insulation/air barrier at ceiling plane
• Cathedral Ceiling – Insulation/air barrier at roof plane
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Simple Vented Cathedral Beware Air leaks! Ventilation path?
Hard to Vent
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Limitations of Ventilated Cathedral
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Limitations
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Pitched Roof Types
• Unvented Cathedralized Attic – As cathedral but no venting above
insulation • Unvented Cathedral
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Temperatures in Georgia Roof Surface Temperature Distribution - Comparison of Different Roof Strategies (w/ ZIP)
(2005.04.11-2005.11.07)
0
200
400
600
800
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1200
-10 -5 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90
Temperature (°C)
Hou
rs a
t Tem
pera
ture
Attic, Poorly VentilatedAttic, Well VentilatedCathedral, VentilatedCathedral, Unvented
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