ASHRAE Journal - July 2011 - 55

Inner, Protected Seal

Outer, Exposed Seal

Precast Panel (Installed First) Outer Sealant on Backer Rod Drained at Vertical Joints Inner Sealant on Backer Rod Continuous for Water and Air Control Continuity Note: Precast Concrete is the Water and Air Control Layer Panel Connection Cast Into Panel C/W Leveling Shims; Fill With Spray Foam Corrosion-Resistant Metal Pin To Connect Panels Without Penetrating Air and Water Layers Gypsum Board Steel Stud Air-Impermeable Spray Or Board Insulation Cast in Place Anchor

Drain and Vent Opening

Sealant Backer Rod

Drainage Space Backer Rod Sealant

Smoke Seal (Air Seal) and Firestop Fill Space Between Slab Edge And Back of Panel With Mineral Fiber Firestop Line of Outer Sealant At Panel Joints Line of Inner Sealant at Panel Joints Structural Columns and Walls Should be Held Back from Slab Edge to Allow for Installation of Air and Thermal Control Layers

Drainage Opening

Exterior Face of Cladding

Figure 5: Low Risk Wall. Impermeable face-sealed cladding (“perfect barrier”) that is internally insulated with air impermeable insulation. Typically spray applied 2 lb/ft3 (0.91 kg/m3) polyurethane foam or rigid foam board directly adhered to the back side of the exterior cladding with all joints sealed.
Steel Stud Wall Air-Impermeable Spray or Board Insulation Gypsum Board

Figure 4a (top): Draining Joint (2-D). Clever use of sealant to direct water that may enter joint cavity to the exterior. Figure 4b (bottom): Draining Joint (3-D). Need to draw it up three dimensionally in order to “get it.” Brilliant.

Spray Foam Air Seal Flexible Transition Membrane for Air and Water Control Continuity Sealant on Backer Rod As Rain Screen

(Photo 5). But will it work? Ah, the high risk versus low risk question? Curtain Wall Frame The moisture in the air part can be Solid Blocking controlled by not humidifying and by ventilating buildings in cold climates Precast Panel (Installed First) to keep interior relative humidities durLine of Outer Sealant ing the coldest part of the winter very Seal/Rain Screen Joint Line of Inner Seal Air Control And low. How low? Uncomfortably low. The Drainage Plane Water Control colder the climate, the lower the interior humidity necessary. How do you figure Figure 6: Window-to-Wall Interface. Note the use of two-stage joints around the this out? Well, when the bad things stop perimeter of the window opening. happening you know what that humidity is. Aw, come on. Can’t you calculate it? Not really. Have you There are only a few guys I would trust with this, and even then, any idea how complicated a problem it actually is to calculate it is nice to know what the answer is beforehand. if you wanted to do it correctly? It is a three-dimensional fluid So, what do I recommend? Experience tells us that in most flow problem with a whole bunch of heat transfer thrown in to cases “that it can’t be worse than this” and “it can’t be better than make things interesting. And the hard part is that you have to this.” You bound the problem, and use judgment (a real good guess at the boundary conditions such as what the actual leakage guess based on experience) as to where things lie between the pathways are and what the actual air pressure differences are.8 best and the worst case. Here is where the calculations become
8. Simulations without real-world benchmarking are a huge unknown. You can get almost any answer you want. You have to be really good and really careful about your assumptions. It is not that it can’t be done; it is just that it is often easy to get it very wrong.

July 2011

ASHRAE Journal

ASHRAE Journal - July 2011

Table of Contents for the Digital Edition of ASHRAE Journal - July 2011