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Envelope Design - Roof

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charles bagley

on 12 November 2012

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Transcript of Envelope Design - Roof

DESIGN THERMAL AIR MOISTURE Curriculum Session 1 - Foundation - August 14th, 2012 Session 2 - Walls - September 11th, 2012 Session 3 - Windows & Doors - October 9th, 2012 ENERGY AIR MOISTURE OPENINGS/PENETRATIONS WALLS PARAPET ROOF FINISHING
Roof/parapet membranes
Protection board
Air barriers
Drainage materials
Filter Fabrics
Waterproofing membranes
Protection board
Insulation materials
Drainage Pipe Session 4 - Roof/Parapet November 12th, 2012 “Architectural Nexus” is a Registered Provider with The American Institute of Architects Continuing Education Systems (AIA/CES). Credit(s) earned on completion of this program will be reported to AIA/CES for AIA members. Certificates of Completion for both AIA members and non-AIA members are available upon request.

This program is registered with AIA/CES for continuing professional education. As such, it does not include content that may be deemed or construed to be an approval or endorsement by the AIA of any material of construction or any method or manner of handling, using, distributing, or dealing in any material or product.

Questions related to specific materials, methods, and services will be addressed at the conclusion of this presentation.
Understand the common types of roof types systems, their advantages/disadvantages and specific considerations that need to be made for each as well as designing for proper drainage for the system.
Understand the critical points (material transitions, parapets, roof penetrations, roofing terminations, shifts in plane, etc.) that effect the overall performance of the system.
Understand the thermal performance of the different systems and the importance of having the insulation layers as continuous as possible, avoiding thermal breaks between the outside and inside of the building.
Understand the importance of an air barrier and how they can save energy and operational expenses when properly installed. Also how you can effectively connect your wall air barrier to the air barrier that is part of your roof system. At the end of this program, participants will be able to: Learning Objectives In order to maintain high-quality learning experiences, please access the evaluation for this course by logging into CES Discovery and clicking on the Course Evaluation link on the left side of the page. Course Evaluations "When you think you are done, ask yourself five hard questions?" Tim Thomas THERMAL BARRIERS MOISTURE BARRIERS VAPOR BARRIERS AIR BARRIERS ` AIR MOISTURE BARRIERS MOISTURE BARRIERS MOISTURE ENERGY Historically, so many problems have occurred with parapets that we have a name for it: “parapetitus.” They have a long history—which of course is not always clear—that allows me to embellish without threat of peer review reversal.1 Their major function today, aside from confusing architects, is to protect the edge of roof assemblies from wind uplift forces. Not so in the old days where they were useful in fire protection.
When wind blows against a building it produces vortices at the roof edges (Figure 1) that create huge pressure differences (Figure 2) at roof perimeters that can suck roofs off buildings. Parapets dramatically reduce these pressure differences at roof edges (Figure 3). Neat eh? All this from a University of Toronto guy, go Varsity Blues (Leutheusser, H.J., 1964(2)). The easiest thing to get right about parapet construction is to keep rainwater from getting into the top of them. The principles are easy. Slope the top of them inward so they don’t stain the building façade. Make sure that there is a waterproof membrane under the coping. Always. Metal and stone copings leak at joints. And always have drip edges—front and back—so that they don’t stain the building façade. Did I mention the staining of the building façade? Check out Figure 4 and Photograph 1 to see it done right. If you want to get depressed, look at Photograph 2. Figure 4: Parapet Water Management—Keep rainwater from getting into the top of them. Slope the top of them inward so they don’t stain the building façade. Make sure there is a waterproof membrane under the coping. And always have drip edges—front and back—so they don’t stain the building façade. Are we done yet? Nope, not by a long shot. Now it gets weird, not the physics, but why so many buildings get the physics wrong. For the physics we go to another one of those legendary old guys who got it right and made it simple for the rest of us to understand—Max Baker. Check out Figure 5, adapted from his book “Roofs.” Connect the water control element/layer of the roof to the wall, the air control element/layer of the roof to the wall, the vapor control element/layer of the roof to the wall and finally the thermal control element/layer of the roof to the wall. Sound familiar? Coming from me it should by now.2 I call them the “Baker Principles.” Figure 5: Parapet Physics—The “Baker Principles.” Adapted from the master, Max Baker.(1) Adapted how? I just updated the words, just the words, not the principles. Everyone relax. This is probably the most influential graphic in my building science education. When I first saw it, the lightbulb went off. Continuity of the control layers between roofs and walls is the whole enchilada. Figure 11: Perfect Compact Roof—The roof I would build if I were in charge. A continuous fully adhered air control layer supported by gypsum sheathing on the top of a metal deck. The gypsum sheathing is screwed to the metal deck. Rigid thermal insulation on the top of this air control layer in two layers at least with the joints off-set horizontally and vertically. This insulation should be screwed down to the metal deck. Then on top of the rigid thermal insulation there should be a coverboard. This coverboard is also screwed down to the metal deck. Finally, a roof membrane is fully adhered to the coverboard.
The function of the coverboard is two-fold. First, it is a hygric buffer that reduces roof membrane blistering. A discussion of this has to wait for some other time. Second, and most important to our story, is that its function is to transfer the stresses of the primary roof membrane to the metal deck. Stresses from the roof membrane are transferred to the coverboard, and the coverboard does the heavy lifting and handles these stresses finally getting them down to the metal deck.
Next we have to deal with the potential for concentrated roof stresses at parapets. Figure 12a shows how the “old timers” did it—wood blocking and a cant anchored to the structural deck. Figure 12b shows how the “new pups” do it—a large backer rod supporting a bunch of extra membrane that lets things move when they have to move. As much as it pains this “old timer” to say this, with the newer more dimensionally stable membranes the “new pups” have it more right. Figure 6: Problem Parapet—This is what I see on a regular basis. Everything is wrong. Air leakage into and out of everything and everywhere. No membrane under the parapet flashing. No air control in either the roof assembly or the wall assembly. No vapor control layer and thermal bridging everywhere. Figure 12a: Steel Stud Parapet “Old Timer.”—Wood blocking and a cant anchored to the structural deck restrain membrane shrinkage at parapet. Notice the continuity of the control layers. Figure 12b: Steel Stud Parapet “New Pups.”— Large backer rod supporting a bunch of extra membrane that lets things move when they have to move. The “Zen” approach to membrane movement. Use a more dimensionally stable membrane and then let things move when they have to. Again, notice the continuity of the control layers. Figure 13: The Masonry Parapet—The thing to note here is that the concrete deck is the air control layer so an additional one is not necessary. However, joints in the concrete deck need to be addressed for air control layer continuity. Figure 14: The Balloon Framed Steel Stud Parapet— This is the ugliest parapet to get right. Notice the use of spray polyurethane foam, the high density stuff, to provide air control layer continuity across the balloon framed exterior steel stud wall. The spray foam is supported by horizontal bridging or metal blocking. This is a tricky thing to execute and, as such, we design into the upper parapet assembly a pathway for drying via diffusion to provide some performance redundancy. Figure 15: The Cantilevered Mini Parapet—Notice that air control layer continuity is achieved by wrapping the membrane over the building corner and then constructing the cantilevered portion of the parapet over the top of this air seal. •Water control layer continuity: membranes continuous under the parapet flashing; 1.The Italians have claim to the word “parapetto,” which comes from “parare,” which means “to defend,” and “petto,” which means “breast.” The military calls “parapet fortifications”—defensive stonewalls—“breastwork.” The dictionary meaning of “breasted” means “to confront boldly.” So, low stonewalls historically are called parapets and are military in origin. “Stonewall Jackson” was also called “Parapet Jackson.” OK, so that’s not true, but with the way textbooks seem to be written today I bet I could get away with it if I decided to write one. So how did they come to be located on the edge of roofs? Ah, we can thank the English for that. In the old days, London tended to keep burning down and that tended to irritate the folks who lived in London. So, projecting wooden eaves were banned in the Building Act of 1707 as a fire risk. Instead an 18 in. brick or stone parapet was required, with the roof set behind, as fire protection (http://en.wikipedia.org/wiki/Parapet). A Little History BSI-050: Parapets—Where Roofs Meet Walls
By Joseph Lstiburek buildingscience.com CONTINUITY OF ALL SYSTEMS IS KEY "Max Baker" Principles •Air control layer continuity: an air control layer in the roof assembly is connected to the air control layer in the wall assembly; •Vapor control layer continuity: a vapor control layer in the roof assembly is connected to the vapor control layer in the wall assembly; •Thermal control layer continuity: the thermal control layer of the roof assembly is connected to an effective thermal control layer in the wall assembly. The thermal control layer in the wall assembly is exterior to the structure—just as in the roof assembly. •The roof membrane is fully adhered to a coverboard that is mechanically attached to the structural deck in the field of the roof and an allowance for membrane movement is provided at the perimeter of the roof assembly. "Old School" Problems That's all folks -concealed fasteners
-360 deqree field seamed system
-manufacturer installed sealant
-thermal blocks (25% system improvement)
-minor ridges to prevent oil canning
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