Presentation from 30th RCI Annual Convention and Tradeshow in San Antonio, TX - March 9, 2015.
Peer reviewed paper and presentation covers review of current issues with ventilated attics in the Pacific Northwest with case studies, the latest research and potential solutions to address mold growth and other moisture issues.
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The Problem With and Solutions for Ventilated Attics
1. The Problems With and Solutions for
Ventilated Attics
RCI 30TH INTERNATIONAL CONVENTION - SAN ANTONIO, TX
MARCH 9, 2015
GRAHAM FINCH, MASC, P.ENG – RDH BUILDING ENGINEERING LTD.
2. Presentation Overview
Current Issues with Ventilated Attics
Case Study of Repairs
Attic Roof Hut Research & Monitoring Study –
Key Findings
Performance of Potential Solutions
Ongoing Research & Field Trials
5. Influencers of Moisture Problems in Attics
What influences attic moisture issues &
what can we control by design?
Roof orientation (solar radiation)
Roof slope (solar radiation)
Roofing material/color
Adjacent buildings – shading
Trees – shading & debris
Outdoor climate
Indoor climate
Roof Leaks
Insulation R-value
Air leakage from house
Duct leakage in attic
Duct discharge location
Vent area and distribution
Sheathing durability
Roof maintenance
Other things
6. Where are we Seeing the Biggest Issues?
• Air leakage (ceiling details)
• Exhaust duct leaks & discharge location (roof, soffit, or wall)
• Inadequate venting provisions (amount, vent location, or materials)
• Outdoor moisture: night sky condensation on underside of sheathing
• Wetting through shingles/roofing (tipping the moisture balance)
8. Industry Trends – Less Heat Flow into Ventilated Attic Spaces
<1970’s attic construction with
excessive air leakage and heat loss
into the attic
1980’s to 1990’s attic construction with
moderate air leakage and heat loss into
the attic
2000’s attic construction with minimal air
leakage and heat loss into the attic
9. Standard Faith-Based Ventilation Approach
Air-sealing details, duct
exhaust details often not
provided & left up to the
contractor
10. Alternates to Ventilated Attics
Unvented Hot Roof
(Sprayfoam applied to
underside)
Exterior Insulated &
hybrid approaches
21. Case Study: Two Steps Forward, One Step Backwards
• 2007 investigation of 5 yr old large
townhouse complex
• Was experiencing all of the problems we
were and still are currently seeing
41. Key Findings from Field Investigations & Field
Monitoring
Seeing widespread issues with
mold growth in newer wood-frame
attics in Pacific Northwest
Wetting is exceeding drying
capacity provided by ventilation
Problem is most often NOT due to
a lack of ventilation
Usual culprits of air-leakage
condensation (leaky ceiling, leaky
ducts & discharge point)
Also seeing supplemental exterior
moisture sources (night sky
condensation, rainwater seepage)
43. Research Study Premise
Controlled field monitoring study to
isolate exterior wetting
mechanisms from interior sources
(air, vapour)
To specifically evaluate impact of
orientation, slope (3:12, 4:12 and
6:12) & shingle underlay
Remove influence of air leakage or
heat gain from house
Monitor the performance of
surface treatments
Typical ventilated attic
construction with air
leakage and heat loss into
the attic
Theoretical attic with no air
leakage or heat loss into the
attic and unrestricted
ventilation – Study setup
here
44. Concurrent Companion Research in Pacific Northwest
Homeowner Protection Office of BC
RDH – monitoring field conditions & controlled field
study (covered here)
MH – monitoring of attic moisture levels and air-
leakage from indoors into attic spaces
FP Innovations – monitoring of effectiveness of various
surface treatments
BCIT – modeling of attic ventilation rates & moisture
movement
45. Roof Test Hut Field Monitoring Setup
a) 3:12 Slope roof with roofing felt underlay b) 4:12 Slope roof with roofing felt underlay
c) 6:12 Slope roof with roofing felt underlay d) 3:12 Control roof with SAM underlay
50. Seasonal Average Moisture Contents
0
5
10
15
20
25
30
Fall 2012 Winter
2012/2013
Spring 2013 Summer
2013
Fall 2013 Winter
2013/2014
Spring 2014
MoistureContent(%)
MC-FULL-N-CONT MC-FULL-S-CONT MC-FULL-N-312 MC-FULL-S-312
MC-FULL-N-412 MC-FULL-S-412 MC-FULL-N-612 MC-FULL-S-612
51. Long Term Impacts of Elevated Moisture Contents
North 3:12 after 1 yr North 4:12 after 1 yr
52. Tracking Mold Growth
TABLE 2: VITTANEN’S MOLD GROWTH INDEX DESCRIPTIONS
INDEX GROWTH RATE DESCRIPTION
0 No growth Spores not activated
1 Small amounts of mold on surface (microscope) Initial stages of growth
2 <10% coverage of mold on surface (microscope) ___
3 10% – 30% coverage of mold on surface (visual) New spores produced
4 30% – 70% coverage of mold on surface (visual) Moderate growth
5 >70% coverage of mold on surface (visual) Plenty of growth
6 Very heavy and tight growth Coverage around 100%
56. When Does it Occur?
0
100
200
300
Oct 02 Oct 03 Oct 04 Oct 05 Oct 06
Sol
Solar Radiation CONDENSE-Plywood-312
Increasin
0
5
10
15
20
25
30
Oct 02 Oct 03 Oct 04 Oct 05 Oct 06
MoistureContent[%]andTemperature[°C]
Condensation
0
5
10
15
20
25
30
Oct 02 Oct 03 Oct 04 Oct 05 Oct 06
MoistureContent[%]andTemperature[°C]
MC-FULL-N-312 MC-IN-SURF-N-312 T-IN-N-312-Plywood
T-N-312-Embedded Outdoor - Temperature Outdoor - Dewpoint
COND-N-312 Sheathing Solar Radiation
Condensation
57. When Does it Occur on the Underside of the
Sheathing?
0
100
200
300
400
Oct 02 Oct 03 Oct 04 Oct 05 Oct 06
SolarRa
Solar Radiation CONDENSE-Plywood-312
IncreasingSurf
0
5
10
15
20
25
30
Oct 02 Oct 03 Oct 04 Oct 05 Oct 06
MoistureContent[%]andTemperature[°C]
Condensation
0
5
10
15
20
25
30
Oct 02 Oct 03 Oct 04 Oct 05 Oct 06
MoistureContent[%]andTemperature[°C]
MC-FULL-N-312 MC-IN-SURF-N-312 T-IN-N-312-Plywood
T-N-312-Embedded Outdoor - Temperature Outdoor - Dewpoint
COND-N-312 Sheathing Solar Radiation
Condensation
0
100
200
300
400
500
600
700
800
900
1000
Oct 02 Oct 03 Oct 04 Oct 05 Oct 06
SolarRadiation[W/m2]
Solar Radiation CONDENSE-Plywood-312
IncreasingSurfaceCondensation
25
30
perature[°C]
25
30
perature[°C]
0
100
200
Oct 02 Oct 03 Oct 04 Oct 05 Oct 06
Solar Radiation CONDENSE-Plywood-312
Incre
0
5
10
15
20
25
30
Oct 02 Oct 03 Oct 04 Oct 05 Oct 06
MoistureContent[%]andTemperature[°C]
Condensation
0
5
10
15
20
25
30
Oct 02 Oct 03 Oct 04 Oct 05 Oct 06
MoistureContent[%]andTemperature[°C]
MC-FULL-N-312 MC-IN-SURF-N-312 T-IN-N-312-Plywood
T-N-312-Embedded Outdoor - Temperature Outdoor - Dewpoint
COND-N-312 Sheathing Solar Radiation
Condensation
Dry
Heavy Condensation
Light Condensation
Daily Solar
Radiation Cycles
61. Shingle & Sheathing Temperature Depressions –
Night Sky Cooling
2.6 2.5 2.5
2.6
2.4 2.5
0.9 0.9 0.9 1.0 1.0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
312-N 312-S 412-N* 412-S 612-N 612-S
TemperatureDepressionfromAmbient
(°C)
Avg Shingle T Avg Interior Sheathing Surface T
*412-N sheathing temperature unavailable due to sensor malfunction
4.5°F
On top of
shingles
<2°F
At interior
of roof
sheathing
Average Shingle and Sheathing Temperature Depression compared to Ambient
Temperature for Winter (December 1, 2013 to January 31, 2014).
62. Hours of Potential Condensation - Sheathing
0
100
200
300
400
500
600
700
800
0.00
0.25
0.50
0.75
1.00
1.25
1.50
North South
HoursofPotentialCondensation
TemperatueDepressionfromAmbient(°C)
3:12 Temp 4:12 Temp 6:12 Temp 3:12 Hours 4:12 Hours 6:12 Hours
<2°F temperature drop
150 to 300 hours per year
63. On the Flip Side: Solar Heat Gain & Drying during the
Day
64. On the Flip Side: Solar Heat Gain & Drying Potential
0
5
10
15
20
25
30
35
40
45
50
55
60
Aug 21 00:00 Aug 21 06:00 Aug 21 12:00 Aug 21 18:00 Aug 22 00:00
Temperature[°C]
Temperatures - 3:12 and 6:12 Slope Roofs - Early Spring Conditions
T-OUT-N-612-Shingle
T-OUT-S-612-Shingle
T-IN-N-612-Plywood
T-IN-S-612-Plywood
T-OUT-N-312-Shingle
T-OUT-S-312-Shingle
T-IN-N-312-Plywood
T-IN-S-312-Plywood
Outdoor - Dewpoint
Outdoor - Temperature
South Shingles
130 -140°F
North Shingles
100-110°F
Ambient Air
up to 79°F
South Sheathing = 98°F
North Sheathing = 90°F
65. Shingle and Sheathing Temperature Rise – During the
Winter
1.9
5.8
1.7
6.5
1.7
6.6
0.9
1.7
0.9
2.0
1.7 1.8
0
1
2
3
4
5
6
7
312-N 312-S 412-N 412-S 612-N 612-S
TemperatureRiseFromAmbient(°C)
Avg Shingle T Avg Interior Sheathing Surface T
Shingle Surface
Sheathing
~11°F
~4°F
67. 0
5
10
15
20
25
30
Oct 17 Nov 14 Dec 12 Jan 09 Feb 06 Mar 06 Apr 03 May 01 May 29 Jun 26
MoitsureContent(%)
MC-OUT-S-CONT MC-OUT-S-312
South Slope - Fall to Spring
Impermeable SAM vs Permeable Roofing Felt Underlay
0
5
10
15
20
25
30
Oct 17 Nov 14 Dec 12 Jan 09 Feb 06 Mar 06 Apr 03 May 01 May 29 Jun 26
MoitsureContent(%)
MC-OUT-N-CONT MC-OUT-N-312
Permeable Felt
Impermeable SAM
North Slope - Fall to Spring
68. Okay But.. What Happens when Shingles Start to
Leak/Seep - Long Term?
69. Key Findings – The Cause of the Problem in the
Pacific Northwest
Roof sheathing in well ventilated attics (also soffits, canopies)
experiences elevated moisture levels in winter
Occurs despite elimination of typical wetting mechanisms within
attics (air leakage, duct leakage, rain water leaks etc.)
Moisture level is above equilibrium level indicating additional wetting
sources
Night sky cooling causes wetting when sheathing drops below ambient
dewpoint (few hundred hours per year) in attic
Difficult to stop it from occurring
Fungal growth occurs due elevated moisture and condensation on
underside of sheathing
More fungal growth on north than south – drying matters, the heat
from a ceiling above a house will also help too
70. Monitoring of Potential Mitigation Strategies
Vented underlayment – de-
couple night sky radiation
cooling effects
Surface treatments to kill &
prevent fungal growth
71. Night Sky Radiation De-Coupler?
Vented shingle underlay
installed in one roof in 2nd year
of study
Purpose: try to de-couple
night sky cooling effects
from sheathing
72. Vented Underlay – The Double Edged Sword
South Orientation – Vented Underlay vs Direct Applied Shingles
0
50
100
150
200
250
300
350
400
450
-5
0
5
10
15
20
25
Jan 16 Jan 17 Jan 18
SolarRadiation(W/m2)
Temperature(°C)
T-OUT-S-VENT (Shingles) T-IN-S-VENT (Sheathing)
T-OUT-S-312 (Shingles) T-IN-S-312 (Sheathing)
Outdoor T T-Drainmat-S-VENT
Solar Radiation
Shingles – direct applied
Shingles – vent mat
12°F drop in
shingle
3°F drop in
sheathing
73. Vented Underlay – The Double Edged Sword
0
2
4
6
8
10
12
14
16
6
8
10
12
14
16
18
20
22
Day Night Avg
Temperature(°C)
MoistureContent(%)
Spring
VENT-N-MC VENT-S-MC 312-N-MC 312-S-MC
VENT-N-T VENT-S-T 312-N-T 312-S-T
Vent Normal
North
South
North
South
Spring (March 1, 2013 to April 30, 2013) Diurnal Moisture Content (SURF)
and Temperature Averages for North and South Oriented Vented and
Control (Direct Applied Shingles) Roof Assemblies
77. Visual Assessment of Surface Treatment Efficacy
VISUAL ASSESSMENT OF SURFACE TREATMENT EFFICACY
Test
Roof
Surface Treatments (north left, south right)
Sansin
Boracol®
20-2
Copper
Naphthenate
Bleach
Thompsons
WaterSeal®
Kilz®Paint
Zinc
Naphthenate
Sansin
Boracol®
20-2BD
Control
3:12
4:12
6:12
VISUAL ASSESSMENT SCALE
Pristine or very light fungal growth
Moderate fungal growth
Significant fungal growth
Hence need for duplicate samples – slope not a big factor (heartwood vs sapwood is)
In our experience Kilz® & Boracol® 20-
2BD while okay here after 2 years may
not be best long term for fungal growth
78. Wood Preservative & Fungicide Surface Treatments
The best decay fungicide (Boracol 20-2BD) & surface
paint (Kilz) looked okay in years 1 and 2 – but not in year
3
82. Summary – Mitigation Strategy Performance
Thermally de-coupling the exposed shingles from the sheathing did
not work well
Did not significantly “warm” sheathing temperature at night
On flip-side – the sheathing did not get as hot during the day, so
less drying and resulting prolonged wet periods
Surface treatments appear to be a potential viable solution if right
product is developed - current products not quite effective (nor
developed specifically for this application)
Concurrent work by FP to perform accelerated testing of some new
biocides/fungicides
Ongoing monitoring at our huts to monitor long-term field
performance of next generation treatments
83. Concurrent Fungicide/Biocide Research –
FP Innovations
Images courtesy FP Innovations
Developed an
accelerated 12 week test
method to evaluate new
fungicides applied to
wood products
Have already tested a
handful of newly
innovated & proprietary
fungicides & coatings
A few promising
formulations completely
prevented mold growth
Follow-up with field
testing
84. Round 2 - Field Trials & Monitoring of New Surface
Treatments
87. Ongoing Monitoring of New Surface Treatments
Most Promising
fungicide/biocide is
water repelling &
contains several “active”
ingredients to prevent
long term mold growth.
Note it is not a wood
preservative it is a
fungicide
Currently undergoing
environmental testing &
available soon?
88. What A Real Roof Leak Ends up Doing to Sheathing
(6 Months)
89. Final Thoughts
Ventilated attics & roof assemblies ‘built to code’ are experiencing
mold growth on underside of sheathing (plywood or OSB)
Wetting from night sky condensation and may be exacerbated
by air leaks & water leaks
Hard to reliably stop night sky condensation
More ventilation makes it worse, less may also
Mold growth may be minor but perceived as risk
Could build other attic assemblies but unlikely to replace
ventilated attics any time soon
Need to address durability & sensitivity of wood based sheathings
to mold growth
Just make sure the fungicide is no more harmful to humans
that the mold is