What’s the deal with pressure equalization versus open-cladding?

You would think that after a half century the physics of wall design would be obvious, straightforward and non-controversial. Apparently not. Folks latch onto concepts that are partially true…true in a limited sense…without appreciation of the range of applicability. As Dirty Harry Callahan would say…a wall has got to know its limitations.

There has been an ongoing food fight within the “rain screen” community for as long as I can remember. “Pressure equalization” versus “open joints”. This should have been settled by the time my beloved Toronto Maple Leafs won their last Stanley Cup…fifty years ago. Ouch.

The father(s) of “pressure equalization” Kirby Garden, Gus Handegord and Grant Wilson taught me and others that the concept of pressure equalization only works for small volume airspaces as air is compressible. In large volume airspaces there is an air pressure phase shift and pressure equalization does not occur.

The basis of pressure equalization is that air is allowed to enter a small volume pressurizing it in such a manner that the “back” of the volume takes all the pressure and the “front” of the volume sees no pressure.  The approach is shown in Figure 1 for a window extrusion. If there is a “hole” or opening at the face of the extrusion the “extrusion” chamber or “compartment” is “pressurized” where no air pressure difference is “seen” on the “front” of the extrusion. The air pressure is for all practical purposes “equalized” across the front of the extrusion. The key element of performance being the outer seal which “sees” no pressure.  This “kinda” works. Not perfectly, but reasonably well…the pressure across the outer seal is reduced or “moderated”. The air pressure is not “equalized” but is reduced.  Note the word “moderated”.

Figure 1 Diagram of air pressure within a window extrusion
Figure 1

The point of this is the appreciation that the outer seal “ages” more rapidly than the inner seal.  It ages more rapidly because it is on the outside of the building.  Duh.  The outer seal is exposed to the three, principle damage functions…water, heat and ultra violet light.  Not so the inner seal.

We use this approach successfully in glazing systems and in two-stage joints in precast assemblies (Figure 2 and Figure 3).

The approach does not work “in the field of the wall” because in a large volume – the “rain screen” air space behind the cladding – the wind pressures on the face of the cladding changes/fluctuates too rapidly for the pressure within the wall to react quickly enough.  Remember the air being “compressible” part noted earlier.

For decades architects and builders have been told that wall assemblies “must be compartmentalized”.  Alas, this does not work…and has never worked…but it sounds “cool”. And sounding “cool” apparently makes it easy to sell stuff.

Figure 2
Figure 3

So how is this “compartmentalization” supposed to work?  Imagine a large cladding panel with a “rain screen” air space behind it that is sealed on the top and sides with weep openings at the bottom that also act as air inlet openings facilitating pressurization of the “rain screen” air space.  The pressurization of the air space is intended to match the pressure on the face of the cladding.  But it does not happen.  Yes, less rain enters, but not because of pressure equalization, less rain enters because there are fewer openings.  That is the good news part.  The bad news part is there is no air circulation or back ventilation of the cladding and enhanced drying of the wall assembly the cladding is covering.

The beauty of open joint cladding systems over a rain screen air space is that there is a great deal of air flow that significantly dries both the wall assembly and the back side of the cladding itself.  Yes, more rain enters open joint cladding systems, but more drying also occurs. The key is the balance between the wetting and the drying. Actually, balance is the wrong term. An open joint cladding system is “unbalanced” towards the drying side rather than the wetting side. It is not even close. There is an order of magnitude difference between the wetting and the drying. Drying wins big time.

With open joint cladding systems what is installed behind them matters. Is there an insulation layer?  I sure hope so. Is there a water control layer and an air control layer? I sure hope so. Are these layers protected from ultra violet (UV) radiation? I sure hope so. Is the thing protecting things protected? I sure hope so…A good example of a barrier that will provide protection by being both water-resistive and UV stabilized for permanent exposure is the DELTA-FASSADE S.

Oh by the way lots of folks think that the open joint systems look better. They tend to work better and there are fewer seals to degrade. To borrow from Miller Lite…”if you can combine great performance with less sealant you can combine anything”…“looks great, less filling”.

Headshot photo of Joe Lstiburek

About Joe Lstiburek, Ph.D., P.Eng., ASHRAE Fellow, Principal, Building Science Corporation:

Joe Lstiburek is the founding principal of Building Science Corporation, one of the most influential, innovative, and respected building science firms in North America. Dr. Lstiburek’s work ranges widely, from providing expert witness testimony to overseeing research and development projects, to writing for the ASHRAE Journal. His commitment to advancing the building industry has had a lasting impact on building codes and practices throughout the world, particularly in the areas of air barriers, vapor barriers, and vented and unvented roof assemblies.

Dr. Lstiburek is also an acclaimed educator who has taught thousands of professionals over the past three decades and written countless papers as well as the best-selling Builder Guides. His commitment to education earned him the hailing, “the dean of North American building science” by the Wall Street Journal. You can find additional details on Dr. Lstiburek on our About the Blog page.