Avoiding MoistureRelated Issues in Modern LowSlope Roofs Presented - - PowerPoint PPT Presentation

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Avoiding MoistureRelated Issues in Modern LowSlope Roofs Presented - - PowerPoint PPT Presentation

Dec 16, 2022 277 likes •795 views

Avoiding MoistureRelated Issues in Modern LowSlope Roofs Presented by: Dr John Straube, P.Eng. Professor University of Waterloo Principal Building Science Consulting Inc BuildingScienceLabs.com Modern High Performance Roofs ! What

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Avoiding Moisture‐Related Issues in Modern Low‐Slope Roofs

Presented by: Dr John Straube, P.Eng. Professor University of Waterloo Principal Building Science Consulting Inc BuildingScienceLabs.com

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Modern “High Performance” Roofs

! What are high performance roofs?

– They don’t leak – Enable buildings with low operational energy – Provide comfort & safety – Ensure good Indoor Air Quality – Last a long time (economy, sustainability)

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How to achieve

! Don’t leak

– No holes

! Insulation

– Easy. Continuous ASHRAE 90.1, says R30 plus

! Airtightness

– Provide an air barrier! A focus of this seminar.

! Durability

– Material selection and system design

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Building Science 2008 Joseph Lstiburek – Roofs 4

Roof Enclosures … They put up with a lot.

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The classic “Perfect” Enclosure

Building Science.com 5

1. 2. 3. 4.

www.BuildingScience.com

From: Straube, J.F. High‐Performance Enclosures, Building Science Press 2012.

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Building Science 2008 Joseph Lstiburek – Roofs 6

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Building Science Enclosures No. 7 /

The “Perfect” Roof?: Protected Membrane Roof

Some owners insist on it. Eg. US Federal Government (GSA)

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Building ScienceLabs .com Roofs 8

Details matter to get full durability Experience has taught us….

From: Straube, J.F. High‐Performance Enclosures, Building Science Press 2012.

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PMR “Inverted” Roof

! Pros:

– Preferred approach – Exterior insulation eliminates thermal bridges – Protects membrane

! Cons:

– Added weight of ballast is major limitation – Can only use XPS

Building Science Labs Enclosures No. 9 /

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Exposed membranes

! Most common solution ! Pros:

– Lightest weight – Wide variety of insulation and membranes

! Cons:

– Exposed membrane! – Need separate air and vapor barrier membranes

Building Science Labs Enclosures No. 10 /

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Building Science – Roofs 11

It’s a roof, not a pool

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Building Science – Roofs 12

It’s a roof, not a ice rink

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Building Science – Roofs 13

“Money spent on roof slope is one of the best bargains in the construction business” Wayne Tobiasson, US Army CRRL

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Slope

! 2% is a good design value for slope ! Laps still resist drainage a bit ! Account for roof sags: long span steel often sags more than slope ! Be careful of roof drains at columns

– Roofs 14

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Vapor Control: What is the permeance of the insulation? Self‐Drying roof? Construction Moisture?

(Water control) (Thermal control) (Support)

From: Straube, J.F. High‐Performance Enclosures, Building Science Press 2012.

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Vapor Control

! Vapor barriers do not need to be supported against wind or be perfectly continuous ! Polyiso insulation usually has low enough permeance to manage vapor diffusion ! Stonewool insulation usually needs additional vapor control ! Very low permeance vapor barrier at deck will often trap water in roof assembly

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Durability & Membranes

! What causes most materials to fail?

– Heat – Moisture (corrosion, biological, etc.) – Ultra‐violet radiation – Others, oxygen, ozone, specialty chemicals, etc.

! Classification of materials

– Polymer – Mineral – Metals

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Exposed Membrane Selection

! Material? Asphalt, TPO, EPDM, PVC, etc. ! Single‐ply vs Multi‐ply? ! Factory made or site made? ! Polymer durability limited by high temperature and UV exposure

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Best UV Protection: rocks

! But, you need to hold down to resist wind

A “traditional” roof

and Building Science .com

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Building Science 2008 Joseph Lstiburek – Roofs 20

Roof membrane ! UV protected by rocks and ! kept cool by white ….

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Building Science.com – Roofs 21

New and retrofit solution

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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

Building Science.com Roofs 24

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Building Science – Roofs 25

White roofs: the coming storm

Where is the air barrier?

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!

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White Membranes stay cold

!

Black Membrane White Membrane

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ORNL: Rocks+White save Energy!

!

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No deck Air Barrier + No fully‐adhered membrane + White Roof = accumulation of moisture & failure

Building Science – Roofs 29 From: Straube, J.F. High‐Performance Enclosures, Building Science Press 2012.

Problems

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Even fully‐adhered can fail

Building Science Enclosures No. 30 /

Insulation

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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

31

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Vapor Control: What is the permeance of the insulation? Self‐Drying roof? Construction Moisture?

Stonewool, polyiso, XPS, EPS

Solution

From: Straube, J.F. High‐Performance Enclosures, Building Science Press 2012.

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High‐performance, air and vapor barrier

Photo courtesy of Garland Industries

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Deck Cover Board

! Seal the joints, or cover the whole surface to form an air barrier

! Note: Gypsum board is vapor permeable

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Deck‐level Air Barriers

! Stonewool hybrid roofs ! Low‐cost: Seal joints of rigid polyiso, mechanically attached, to form an air barrier ! Use the deck‐level polyiso as vapor control

Drawing courtesy Roxul Inc.

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Hybrid Roof

Building Science Enclosures No. 37 /

Polyiso

Air barrier tape / mastic

Rockwool

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Cheap Air Barrier: Sandwich taped/sealed membrane between two polyiso layers Photo courtesy: Andy Shapiro

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Special Applications

! Swimming pools

– Esp in cold climates

! Freezer buildings

– Esp. in hot humid climates

! Badly (normally) operated hospitals ! MUST have a deck level air barrier

– Sealed at all edges, penetrations!

– Roofs 39

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Details

! Penetrations and edge

  • f roof

! Continuous air barrier

AIR

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Building Science .com – Roofs 41

Air sealing details May need plastic pipe section to reduce thermal bridge

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Parapets

From: Straube, J.F. High‐Performance Enclosures, Building Science Press. 2012. Roofs 42

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Parapets

! Common failure location ! Usually wrap roof membrane

  • ver

Building Science – Roofs 43

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!

From: Straube, J.F. High‐Performance Enclosures, Building Science Press 2012.

Vapor Control?

Roofs 44

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!

Vapor Control?

From: Straube, J.F. High‐Performance Enclosures, Building Science Press 2012.

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Building Science 2008 Joseph Lstiburek – Roofs 46

Condensation can still

  • ccur.

From: Straube, J.F. High‐Performance Enclosures, Building Science Press 2012.

Risk for cold‐weather tall parapets

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Drying cold parapets

!

Vapor permeable peel‐stick diffusion often most practical

From: Straube, J.F. High‐Performance Enclosures, Building Science Press 2012.

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!

From: Straube, J.F. High‐Performance Enclosures, Building Science Press 2012.

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Conclusions / Summary

! Good slope is critical ! All low‐slope roof membranes benefit from lower temperatures and UV protection ! Cool roofs are cooler... so condensation risks increase ! Now need a deck‐level air barriers ! Fully‐adhered membranes are best ! Vapor Control, not Vapor Barriers

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BuildingScienceLabs.com BuildingScience.com