High performance passive buildings are comfortable by design: quiet, draft free, and evenly warm.

Unlike conventional buildings that rely on energy-intensive mechanical equipment to blast hot or cold air to compensate for uncomfortable design flaws – design flaws that we have all taken for granted as “normal” – high performance buildings deliver comfort through their very construction.

As the image above shows, an assessment by the US Department of Energy’s finds that the comfort delivered by Passive House buildings is dramatically superior to both ENERGY STAR and conventional buildings.

Why? First, high performance passive buildings are comfortable because they are built airtight. (Worried about indoor air quality? Read here about the fresh air ventilation strategies that provide superior air quality to high performance buildings.) By controlling the movement of air, we are also controlling the movement of heat and moisture, two fundamental determinants of human comfort. Nobody enjoys a damp, cold draft on a blustery winter day. Airtight construction eliminates the air leaks that rob structures of comfort.

Second, passive buildings are comfortable because they are thermal bridge-free. A thermal bridge is any building element that cuts across a building’s insulation, or “thermal envelope,” and facilitates the transfer of heat between inside and outside. Thermal energy is so good at finding the path of least resistance that thermal bridges can become superhighways of thermal loss from a building and make interior surfaces cold and uncomfortable. By carefully detailing building assemblies and wrapping the exterior of buildings with continuous, monolithic insulation, high performance building designs can eliminate these energy-sapping thermal bridges.

Our goal is to isolate, or “thermally break,” the building’s interior from the outside environment. A uniform thermal break with superinsulation creates even interior surfaces temperatures in a building, a key component to occupant comfort. Because the human body is such a sensitive sensor of hot and cold surfaces nearby (radiant temperature), a cold wall will make you feel chilly, even if the air temperature is a warm 72 degrees. The natural occupant response is to crank up the thermostat and burn energy blasting warm air into the building. But that mechanical response doesn’t address the actual cause of the discomfort. By providing even, warm surface temperatures, high performance envelopes address the problem at its root through passive means.

These even conditions also help establish even ambient air temperatures in a building. Because warm air rises and cold air falls, the uneven surface temperatures and leaky envelopes of conventional buildings means that air at head level can be too warm while air at your feet is too cold. Or the air on the second floor is unbearably warm while the ground floor is chilly. The natural occupant response? Crank up that thermostat, maybe open a second story window. Again, this not only wastes energy, it doesn’t address the root cause of the problem. High performance buildings and their even surface temperatures provide constant, comfortable air temperatures throughout the building.

High performance buildings are also draft-free. This is partly thanks to the airtight construction that eliminates air leaks through the building envelope. But even an airtight building can be drafty if full of cold interior surfaces. These cold interior surfaces cause convection currents. Take the inside face of a typical mediocre window, for example. Warm interior air will hit that cold surface, cool, drop, skid out along the floor, warm back up, rise, hit the windowpane again, and repeat the cycle. Behind every poor window you’ll have a nice draft established by this loop. Even if interior air temperatures are 72 degrees, if you’re sitting next to a window you’ll be uncomfortable with that draft on the back of your neck. High performance buildings with good windows and thermal bridge-free construction solve this problem.

Comfort has a powerful visual component, as well. Occupant happiness depends on access to views outside and natural daylight, especially in the Pacific NW. Contrary to the myth of the Passive House as a windowless “thermos,” high performance buildings and generous glazing are naturally complementary. In fact, when deployed well, high quality windows are often “energy positive,” meaning the daylighting benefits and passive solar gains captured by the windows more than offset any thermal loss through them.

These energy positive windows do raise the potential for overheating during certain hours of the day, in non-winter months. That’s why solar management systems (eg. operable exterior shades) should be part of most high performance building projects. That way the building can capture passive solar energy when it’s needed and shield it when it’s not.

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Healthy buildings are infused with fresh air and natural light. It’s a simple recipe, yet healthy buildings are remarkably uncommon in today’s built environment. High performance building is changing that.

Most of us have grown accustomed to stale, hypoxic (low in oxygen, high in carbon dioxide) interior air. This has a direct impact on our daily experience. That drowsy feeling you get a half-hour into a meeting around a packed conference room table may have more to do with the quality of air you’ve been breathing than with the quality of conversation. This is likely also true for kids struggling to stay focused in conventional classrooms.

Now consider your house. Its interior air probably comes through accidental air leaks in the wall, maybe from that scary crawlspace, perhaps back-drafted through a sooty chimney. Now consider the impact of off-gassing carpets, combustion by-products from the kitchen stove, and the prevalence of respiratory ailments, including asthma, should come as no surprise.

When compared to the conventional buildings that we’ve grown to accept, healthy buildings are a “breath of fresh air.” Airtight construction eliminates the leaks that allow the flow of poor air into the building. A balanced heat recovery ventilation system (HRV or ERV) delivers a generous, 24/7 supply of filtered fresh air. The HRV or ERV captures the heat energy from the stale air it exhausts from the building and recycles it into the fresh air it brings inside, without the two air streams ever mixing.

This results in a pleasant and subtle variant on the “inside/outside connection”: the thermal comfort you expect inside a building combined with the oxygen-rich air that you associate with being outside.

This speaks to the psychological benefits of well-designed high performance buildings. The visual connection to the outside that high performance windows can provide in passive buildings makes these spaces gracious to inhabit. We often focus on the energy benefits of passive solar gains and daylighting when we discuss high performance glazing, but their capacity to raise our spirits – especially in the Pacific Northwest, land of seasonal affective disorder – is important too.

Finally, by safeguarding the integrity of building assemblies and ensuring that they are free of moisture problems, high performance buildings protect occupants from molds and the illnesses they can cause. In conventional buildings, thermal bridges (any building element that cuts across and bypasses a building’s insulation or “thermal envelope”) create cold penetrations through building assemblies, increasing the likelihood of condensation, rot, and mold. Thermal bridge-free construction eliminates these dangerous thermal bridges.

Furthermore, ventilated rain screens dramatically increase the drying potential of exterior walls, so if moisture does make it into these assemblies that moisture can escape before it can create problems, like mold.

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Left to her own devices, Nature would turn buildings into mulch. Her primary tool for doing so is water, because water plus oxygen plus wood equals rot – at least in the mild temperatures of the Pacific NW – and rot means building failure: mulch.

The key to ensuring building durability is to keep building assemblies dry to prevent rot and to facilitate the drying of assemblies whenever they do get wet. This means managing bulk water (rain, mostly), airborne water vapor, vapor transmission at the molecular level (across assemblies from areas of high vapor concentrations to low vapor concentration), and condensation.

The old way of doing all this was to build walls full of air leaks, clad them well, put on a good roof, and blast the building with heat from a fireplace, boiler, or furnace throughout the winter. Even with this blast of internal heat, the leaky walls might accumulate moisture during the winter in our maritime NW climate, but they would dry out during the summer: all that air and heat passing through them did the trick. It was a resilient approach. Unfortunately it was also wasteful of energy, uncomfortable, and led to poor indoor air quality.

What’s the new way? How do high performance buildings ensure durability? A prerequisite is to execute the level of craft necessary to eliminate bulk water intrusion, of course. With that accomplished, the first priority of high performance building is airtight construction because by controlling the movement of air we also control the movement of moisture and heat. We know that one of the quickest ways to drive moisture into a wall is through an air leak. Warm interior air carries water vapor with it into the wall assembly, and if it hits a cold surface, that vapor can condense into liquid water and wreak havoc. Airtight construction stops air movement – and therefore the movement of airborne vapor – into building assemblies.

The next step in ensuring durability in high performance building is thermal bridge-free construction. Because thermal bridges (any building element that cuts across and bypasses a building’s insulation or “thermal envelope”) create cold penetrations through otherwise warm parts of building assemblies they can become focal points of condensation, moisture build-up, and rot. By eliminating or mitigating these thermal bridges, high performance building removes these condensation-inducing cold points in building assemblies.

With airtightness and thermal bridge-free detailing dialed in, we then turn our attention to managing vapor drive, or the movement of vapor molecules from areas of high concentration to areas of low concentration. As we do this, we pay close attention to the dew point (a function of heat and humidity) to ensure that condensation doesn’t occur where it can do damage. To guide our work, we harness the power of thermal and hygrothermal modeling software, charting the behavior of moisture and heat through assemblies over time. Generally speaking, in our maritime NW climate, our hygrothermal analysis leads to wall assemblies that are “vapor open” in both directions, into and out of the building. If, for some reason, moisture concentrations build up, that moisture can then readily escape the assembly.

See the Pumpkin Ridge Passive House wall assembly and Madrona Passive House wall assembly for two examples of walls that are vapor-open in the both directions.

To increase the drying capacity of high performance wall assemblies, we add ventilated rain screens. The ventilated cavity behind the cladding facilitates lots of airflow (measured in the tens to hundreds of air changes per hour) across the face of the wall assembly proper, dramatically increasing the wall’s drying capacity and, therefore, resiliency. This air movement also allows the cladding itself to dry thoroughly, increasing its lifespan and reducing the maintenance requirements of the building exterior.

This high performance approach to building durability is informed by decades of building science insight. By their very nature, high performance building envelopes limit the flow of heat across them, which, all things equal, would inhibit their ability to dry out. But by stopping airborne vapor intrusion, eliminating condensation-inducing thermal bridges, managing vapor drive, and augmenting drying capacity with ventilated rain screens, the durability of high performance buildings exceeds that of the old, leaky approach to construction. This is why the US Department of Energy rates Passive House buildings as so much more durable than conventional ones.

Nature’s desire to turn our buildings into mulch is foiled by the holistic management of air, moisture, and heat by high performance buildings.

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High performance building can revolutionize the energy efficiency of buildings. Passive buildings routinely achieve 90% reductions in heating energy use compared to conventional buildings, and similar reductions in cooling energy. When combined with day lighting, heat pumps for HVAC and domestic hot water, and superefficient appliances and fixtures, total energy use in high performance buildings can readily hit 75% less than that of conventional buildings.

For example, the Bullitt Center, the noted Living Building and location of Hammer & Hand’s Seattle office, has taken building energy efficiency to new heights. Its energy use intensity (EUI) of just 12.3 kBTU/sf/year is 83% lower than that of an average office building in Seattle (72 kBTU/sf/year). Importantly, this energy efficiency does not depend on hair shirts or other sacrifices; the Bullitt Center delivers all the comfort and health benefits that we expect of high performance buildings. H&H’s office is a gracious, light-filled space equipped with a full suite of office components. The building and its systems are simply designed for excellent energy performance.

High performance buildings achieve these levels of energy efficiency by focusing first on the envelope. Advanced building envelopes push down heating and cooling demand to very low levels. Super-efficient heat pump technology then meets those micro-loads. Heat recovery (or energy recovery) ventilation maintains thermal energy inside the envelope as it refreshes air inside.

It all starts with airtight construction. When air moves, heat moves with it, so by eliminating air leaks through walls, foundation, and roof, airtight construction eliminates a major source of energy loss.

The next step is to super-insulate building envelopes to isolate, or “thermally break,” inside from outside. This begins with a thick layer of interior insulation in the building envelope, followed by a monolithic layer of exterior insulation to create a continuous wrap around the building.

Key to the integrity of this “thermal envelope” is the elimination or mitigation of any thermal bridges, or building elements that cut across and bypass a building’s insulation. Because heat energy is so good at finding the path of least resistance between inside and outside, thermal bridges can rob buildings of lots of energy. A thermal bridge across an otherwise high performance wall will short-circuit our best-laid plans; the heat will treat that thermal bridge like a superhighway for energy transfer, bypassing the wall’s insulation. The wall’s overall capacity to insulate will plummet. Careful detailing of assemblies to avoid these energy-sucking thermal bridges allows high performance buildings to maximize thermal performance. And that final wrap of continuous exterior insulation finishes the job of thermally breaking the building from the outside environment.

High quality windows and doors are also key to high performance building. If you picture the super-insulated, thermal bridge-free envelope of a high performance building as the hull of a ship, each window and door is like a big hole in that hull. We need to carefully address these weak points to ensure they maintain the integrity of the building’s air and thermal barriers. This means using triple pane high performance windows and super-insulated, triple-gasketed high performance doors. It also means expert installation of each window and door, with careful detailing so that the airtight layer and thermal envelope both transition unbroken from wall assembly to window or door and back again.

Contrary to the myth that high performance buildings have to be windowless shoe boxes, glazing can be a Passive House building’s best friend. High quality windows can be “energy positive,” meaning that the energy gains that come in through them as day lighting and passive solar heat gain can more than offset any heat that escapes out through them. Done right, generous glazing helps passive buildings perform well.

Generous use of “energy positive” glazing does raise the possibility of overheating when outside temperatures are mild or warm. So we commonly shade windows on high performance buildings, both with exterior overhangs and with operable exterior shades. We can then manage passive solar energy effectively, capturing it when the building needs it and shielding from it when the building does not.

With the performance of the envelope (walls, foundation, roof, and fenestration) optimized, energy demand for heating drops by as much as 90%. To meet these micro heating and cooling loads we draw on super-efficient heat pump technology like air to air and air-to-water heat pumps. Heat pumps can also deliver domestic hot water super-efficiently. (Energy efficiency in other appliances is simply a matter of some smart shopping.)

The final piece of the super-efficiency puzzle is to bring in fresh air and exhaust stale air without venting the building’s precious interior thermal energy. Heat recovery ventilation (HRV) technology does this beautifully. (Note: energy recovery ventilation, or ERV, is closely related to heat recovery, but manages humidity in addition to “sensible” heat.) At the core of most of these HRVs is a honeycomb-like heat exchanger that creates a very large surface area across which thermal energy from the exhaust stream of the stale interior air transfers to the incoming stream of fresh outside air, without the two streams ever mixing. HRVs infuse buildings with a continuous supply of filtered fresh air while conserving interior thermal energy.

(Note: super-efficient appliances are an important piece of the high performance building puzzle, too. The solution is straightforward: go shopping.)

Drawing on the techniques above, high performance building has transformed what is possible in building energy efficiency today. The Passive House approach to building is at the leading edge of this movement, and charts a field-tested path not only to ultra-low energy use in buildings, but also superior comfort, health, and building durability.

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ADVANCED BUILDING ENVELOPE

The conservation-first approach to high performance building starts with the advanced building envelope. Guided by physics and building science, advanced building envelopes combine a simple suite of components to manage heat, air, and moisture and deliver superior efficiency, durability, comfort, and occupant health.

The first step is airtight construction. By controlling the movement of air across building assemblies we also control the movement of heat and moisture. To this end, we wrap a continuous, unbroken air barrier around our high performance buildings.

Next we super-insulate, combining a thick layer of interior insulation in wall cavities with a continuous, “monolithic,” layer of exterior insulation to thermally isolate the envelope and to keep sheathing warm and free of condensation.

We also detail carefully to “break” the thermal bridges that can compromise a building’s thermal layer and rob it of efficiency and comfort. These insulation-bypassing flaws in building design can also undermine durability by introducing condensation-causing cold penetrations into otherwise warm layers of an assembly.

To protect the envelope from bulk water intrusion we install a water-resistant barrier, or WRB. Often, though not always, this WRB also serves as the building’s air barrier. The WRB should not be confused with a vapor barrier. In our maritime Pacific NW climate we tend to avoid vapor barriers in our wall assemblies and therefore employ vapor-open WRBs in our assemblies. Because vapor drive can oscillate between inside-to-outside and outside-to-inside numerous times in a single day in our climate, we like to detail our walls to be vapor open in both directions to facilitate drying.

To further promote this drying capacity, high performance buildings employ a ventilated rain screen, a gap between cladding and wall assembly that not only provides a channel for bulk water to drain away, but also generates air movement across the face of the assembly to dramatically increase drying. All things equal, a highly thermally resistant wall will have less drying capacity than a conventional wall, so the air movement provided by the ventilated rain screen helps ensure the resilience and durability of high performance wall assemblies.

As a holistic system, the advance building envelope transforms building energy efficiency with its airtight and thermally resistant layers, promotes occupant comfort by creating even, warm interior surface temperatures, and ensures building durability and occupant health by minimizing the risk of the moisture accumulations that lead to rot and mold.

Example of Passive House Wall Assembly

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AIR BARRIER

Airtightness is where it all begins with high performance building, because by controlling air movement we control heat and moisture movement. If an assembly is airtight then bulk water cannot be blown into or through it, nor can a cold wind.

We use a variety of materials to create a building’s air barrier, many of which double as the water-resistant barrier (WRB).

(Note that an air barrier or water-resistant barrier need not be a vapor barrier. In fact, in most of the wall assemblies that we employ in the maritime Pacific NW, our air barriers and WRBs are intentionally vapor open to facilitate drying.)

The wall assembly at Karuna House, for example, employed a vapor open, fluid-applied membrane on plywood sheathing to create the air barrier and WRB. OSB sheathing with fluid-applied sealants made up the air barrier in the wall at Pumpkin Ridge Passive House. At the Glasswood Commercial Passive House Retrofit we used OSB with taped seams for the air barrier. While at Madrona Passive House ZIP sheathing (OSB with a vapor permeable WRB layer applied on its face) with fluid-applied sealant at seams provides the wall assembly’s airtightness.

Of course, the air barrier of a building does not stop at the walls – it must continue unbroken down under the building and up through and across its roof or attic. A key task for high performance building designers is the “red pencil test”: can you draw a continuous air barrier around each of your envelope cross sections, without ever picking up your pencil? How does the air barrier transition from wall to sill to window and back? From wall to roof and across? The designer and builder must address every condition and every transition in order to create an airtight structure.

The performance gains of airtight construction are well worth the effort.

See the Air Sealing section of our Best Practices Manual for step-by-step approaches to extending air barriers across critical junctions in building envelopes.

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CONTINUOUS INSULATION

“Super-insulate” is a rallying cry in high performance building for good reason; by wrapping a thermal layer around the building envelope we isolate the building interior from outside temperatures. Small inputs of heating or cooling can then fine tune interior temperatures to comfortable levels while dramatically reducing energy consumption.

Cavity Fill Insulation at Madrona Passive House

The key to super-insulation is to make the thermal layer continuous, with no weak spots to allow thermal energy to escape across. To this end we start with advanced framing for our interior walls to reduce the number of studs (each of which is a thermal bridge) and increase the width of stud cavities so that more of the interior wall can be dedicated to insulation. We then add a monolithic layer of exterior insulation to complete the thermal break between inside and outside; the thermal bridging caused by the interior wall’s studs are broken by this continuous exterior insulation. The exterior insulation also warms structural sheathing sandwiched behind it. By keeping this vapor-constricting layer warm we mitigate the risk of condensation and dangerous moisture build-up inside our walls.

Exterior Insulation at Madrona Passive House

See both the Insulation and Exterior Continuous Insulation of Walls sections of our Best Practices Manual for notes about installation techniques.

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THERMAL BRIDGE-FREE CONSTRUCTION

A thermal bridge is any component in a building assembly that “bridges” inside and outside thermally, allowing heat and cool to short circuit the thermal resistance built into that building assembly. They hide in plain sight: in the form of wood framing, or a junction between wall and concrete foundation, a balcony slab, or even a single metal tie penetrating a wall.  In each case they interrupt the insulation layer with a material that conducts heat, providing a direct line for the transfer of thermal energy across the building envelope.

Thermal bridges can short-circuit the best-laid plans for building envelopes. They rob buildings of comfort and efficiency.  And because their cold influence can introduce condensation into the inner recesses of assemblies, thermal bridges can cause mold, rot, and building failure.

While thermal bridges don’t have a huge effect on poorly insulated buildings, as buildings get more and more insulated, the weak link of thermal bridging becomes more pronounced, as does the impact on building performance, comfort, and durability.

The best insulation in the world won’t get you anywhere if you ignore thermal bridges. Thermal energy seeks the path of least resistance, and common building elements like concrete slabs and steel plates can serve as a virtual autobahn for escaping energy, an easy detour around even the thickest of insulation.

A common mistake in the building industry is to assume that a wall’s thermal resistance is equal to that of the insulation it contains. Consider a super-insulated example: a 15” thick wall full of cellulose insulation. 15” of cellulose insulation has an insulative value of R-50, so by definition the wall must be R-50, too. Right? No. We have to look at the thermal resistance of the wall in assembly. How is the wall constructed? Do any of its components reduce thermal resistance or create thermal bridges?

A standard wood frame wall, for instance, contains roughly 25% wood studs, leaving 75% for insulation cavity. And though wood isn’t particularly conductive, each of those studs is a thermal bridge, particularly when compared to R-50 insulation. The back-of-the-envelope analysis below, prepared by Skylar Swinford, shows that the wood in a wood frame/cellulose insulation assembly drops the insulative value from R-50 to R-32, allowing 58% more heat loss than an R-50 cellulose monolith. (Note that the calcs below do not include the slight insulative value of surface air films.)

If we move to a more conductive material, like concrete, then the impact of thermal bridging becomes more dramatic. A 6” concrete beam cutting across a 10’x10’ wall of R-50 cellulose will drop that assembly to just R-13. The concrete makes up just 5% of the assembly but leads to 293% more heat loss.

If we move to a more conductive material, like concrete, then the impact of thermal bridging becomes more dramatic. A 6” concrete beam cutting across a 10’x10’ wall of R-50 cellulose will drop that assembly to just R-13. The concrete makes up just 5% of the assembly but leads to 293% more heat loss.

So, mind those thermal bridges because they can wreak havoc with your building’s performance and durability.

We start by carefully detailing buildings to minimize thermal bridging in the first place, through advanced framing, carefully detailing junctions, no uninsulated concrete or metal penetrations of the thermal envelope, and more. We then wrap our high performance buildings with continuous insulation to break any remaining bridges.

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WRB

While a building’s cladding provides the first line of defense against bulk water intrusion, the WRB or water-resistive barrier, represents the final stand against liquid water. The WRB often, though not always, also serves as the air barrier.

Most of the WRBs that we employ in the maritime Pacific NW are vapor open; they stop water in its liquid form but allow water in its vapor form to move through, increasing the drying capacity of the assembly.

Fluid-applied WRB at Karuna House

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RAIN SCREEN

Despite their name, rain screens’ main claim to fame is not the screening of rain. That’s what siding does. In our wet climate, the most important function of a rain screen ­– more precisely, a ventilated rain screen – is to increase the drying capacity of wall assemblies.

When we build a high performance wall we are, by definition, decreasing the amount of air and heat energy that crosses that assembly. So, all things equal, we are decreasing the drying potential of that wall. Knowing this, we are careful to design the assembly to manage vapor and moisture and avoid condensation. We conduct thorough hygrothermal analysis with powerful modeling software to simulate how moisture and temperature will interact over the seasons year after year. Still, to ensure resiliency we want to increase the drying capacity of our high performance walls.

Enter the ventilated cavity of the rain screen.

Rain screens consist of three layers: the cladding, the gap, and the water-resistive barrier (WRB). The cladding’s job is to protect the WRB from the elements, such as rain and wind. The WRB’s job is to protect the rest of the building from the elements, especially wind-driven rain.

When you add a gap between the cladding and the WRB you’ve created a rain screen. All it takes is a simple batten system. But the simplicity of rain screens belies their power, because that cavity, when ventilated at top and bottom, is key to the durability of thermally resistant buildings in our damp, maritime Pacific NW climate.

Built properly, this cavity will:

1. Introduce a capillary break between the cladding and the WRB, short circuiting the wicking of moisture between those two layers.
2. Create a drainage plane between the two layers, allowing any bulk water that gets through the cladding to drain away harmlessly.
3. Generate an air-moving stack effect (when ventilated at top and bottom) that creates hundreds of air changes per hour between the cladding and the WRB.

(Note: many rain screens built in drier regions outside the maritime Pacific NW are not ventilated and therefore only serve the capillary break and drainage plane functions.)

Ventilated rain screens dramatically increase drying capacity. All that air moving across the face of a vapor-open WRB acts like a big fan, drawing moisture out of the wall assembly. This drying effect is also great for the longevity of the cladding.

It turns out that the ventilated cavity is not new. The Romans knew about the virtues of ventilated wall cavities over 2,000 years ago. As we recently learned from Mark R. Morden, Associate Principal at WJE, Vitruvius wrote about them in his “Ten Books On Architecture.” Joe Lstiburek also cited Vitruvius recently in his “Vitruvius Does Veneers.”

“…if a wall is in a state of dampness all over, construct a second thin wall a little way from it…at a distance suited to the circumstances…with vents to the open air…when the wall is brought up to the top, leave air holes there. For if the moisture has no means of getting out by vents at the bottom and at the top, it will not fail to spread all over the new wall.” (Book VII, Chapter IV)

This video shows a real-world example of a ventilated rain screen:

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