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ARCH 2614/5614 Lecture notes

Jonathan Ochshorn

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Control layers

The building enclosure consists of four "environmental" control layers, not always articulated as separate layers, and not always present to the same degree.

These layers can be characterized as:


Watch only the first two minutes of this video...

I. Thermal control layer

Insulation and heat flow

Purpose of insulation

  1. controls heat flow
  2. prevents condensation
  3. improves comfort (by eliminating hot or cold surfaces and consequential radiant heat flow between the human body and those surfaces)

How heat flows

1. Conductivity (conduction)

Resistance, R = 1/C or l/k; i.e, it is the inverse or reciprocal of U-value.

Rt = R1 + R2 + ... + Rn

To find U: U = 1 / Rt. Note that U is not equal to C1 + C2 +...+ Cn

Heat flow, Q = U x A x (temp. differential) expressed in BTU/hr. units.

Example of U-value calculation for a 10' x 10' wall consisting of three layers as follows:

Note that these numbers are adjusted to make the calculations easier, but they are close to reality.

Rt = 0.5 + 19 + 0.5 = 20.

U = 1 / Rt = 1/20 = 0.05.

Calculate heat loss assuming an outside temperature of 20 deg F and an inside temperature of 70 deg F (so the temperature differential is 70 - 20 = 50 deg F).

Q = (0.05)(100)(50) = 250 BTU/hr.

Separate heat flow calculations could be made for all parts of the enclosure, including walls, roofs, windows, etc., and the resulting partial heat flows can simply be added together to obtain the heat flow for the entire building, or any given space. It is this number that then determines the size of a heating or cooling unit.


Thermal bridges are elements of construction that violate the continuity of the thermal control layer. They should be avoided, or at least minimized. Cladding systems, or elements that extend beyond the building's cladding, are often the primary cause of thermal bridging, since they need to penetrate the thermal control layer in order to be fastened to the thermally-protected structural system.

thermal bridges at 201 College Avenue, Ithaca
Thermal bridges at 201 College Avenue, Ithaca, NY: both the projecting metal overhangs, as well as the screws fastening the steel studs holding the cladding itself, create thermal bridges (photos by Jonathan Ochshorn, Oct. 2017).

2. Radiation


3. Convection

Note that air is a good insulator if convection is prevented:

Insulation

continuity of insulation
Continuity of insulation (based on image in Architectural Graphic Standards)

Insulation takes many forms, including the following:

Radiant insulation is essentially any light colored or shiny surface. Note that such a surface must have air (or vacuum) in from of it to work. A light-colored roof (e.g., a white metal roof) is an effective example of a radiant insulative material, as it reflects the sun's energy before it gets a chance to enter the building. Note that this is useful only for buildings where cooling is the major energy issue.


II. Rain (water) control layer

traditional masonry building
This layer usually is what the "architecture" is made out of: including the facade and the roof. In traditional masonry buildings, rain (water) is controlled by the masonry itself, which deflects most water, but is thick enough to absorb some water without damage to interior finishes, after which the water evaporates, either to the inside or outside of the building.

barrier wall building

In other schemes, the outside surface of the building is designed to repel water; in such cases, the surface must be perfect, with no gaps or holes, since there may be no means of absorbing water that penetrates the outer surface.

rain-screen building

In modern construction, an air-water barrier may be placed behind the "cladding" or rain-screen, so that the visible surface of the building is designed to deflect most, but not all, wind-driven rain. More on these approaches later...


III. Vapor control layer

Condensation


video link

A. dew point = temperature at which water vapor in air becomes liquid (i.e., condenses).

Hotter air can hold more vapor than cold air; as temperature is lowered, dew point temperature will be reached.

Dew point temperature corresponds to the amount of moisture (humidity, not "relative humidity") in the air: the dew point will be higher for air containing more moisture than for air containing little moisture.

psychometric chart
In these psychometric charts, one can see that as the relative humidity increases (i.e., as the heavy blue line moves upwards), the dew point temperature also increases; at the limit of a relative humidity of 100%, the dew point temperature equals the ambient temperature of the space.

B. Permeability: measures movement of vapor through material (in units of "perms"). See also more detailed permeance example.

Note that 1.0 perm or less is considered adequately impermeable to act as a vapor retarder; while greater than about 5 perms constitutes a permeable material.

Relationship between insulation, vapor retarders, and condensation is illustrated in the image below, with cold exterior conditions:

condensation, dew point, and vapor retarders

Both the actual temperature and the dew point temperature increase from left to right (exterior to interior conditions) along gradients determined by the thermal resistance of the various wall components (in the case of actual temperature); and by the permeability of the various wall components (in the case of dew point temperature). For example, if we assume that the sheathing and finish materials have R = 0.5 and the insulation has R = 19 in the middle diagram, then the temperature change within the insulation is 19/20 or 95% of the total change. Similarly, if we assume that the two surface materials (sheathing and drywall, for example) have equal permeability, then the dew point temperature curve would increase equally within those two materials, and would not increase at all within the insulation, which is assumed to be completely permeable.

The use of a vapor retarder in cold climates is fairly straightforward: it should be applied on the inside (warm) surface of the wall, typically immediately between the insulated wall studs and the interior finish material. Similarly, a vapor retarder in a warm, humid climate in which interior spaces are air conditioned (cooled) does not present conceptual problems: it would be placed on the outside (also the warm side) of the wall. Where building walls are subjected to both hot-humid and cold climates during different seasons, the problem is more complex. The 2006 IBC identifies climate zones where the use of vapor retarders is not required (generally, the line separating cold-weather vapor barriers on the inside of the wall, and moderate to warm climates where the use of a vapor retarder is not required, is located east-west through the middle of the U.S., turning down towards Texas, and heading northward up the west coast, including parts of Washington State in the "optional" category: see map below).

See this concise article on where to put vapor control layers.

And see this interesting article on vapor pressure, explaining the causes of paint bubbling on the surface of a building and tea bags expanding in hot water.


IV. Air control layer

Air barriers and infiltration

A. Basic air barrier theory: Moisture moves through material either by diffusion (based on permeability of material) or leakage. Leakage can be the more serious mechanism, with up to 400x more water conveyed.

B. Why vapor barriers are different: Vapor retarders work to prevent diffusion of moisture, but not necessarily leakage or infiltration of air.

C. Air barrier composition: Air barriers are designed to prevent infiltration, and consist of different materials within the building envelope with the following characteristics:

  1. low permeability to air
  2. continuity
  3. durability under dynamic pressure
  4. need not be on "warm side," but must account for build-up of moisture is placed on cold side (by using permeable air barriers such as "Tyvek").

Infiltration

Infiltration (leaking) of air into and out of buildings can cause serious building envelope problems including:

  1. energy loss
  2. contamination of indoor air (where the outside air quality is poor)
  3. drafts, heating/cooling issues
  4. HVAC malfunction or under-functioning
  5. freezing pipes
  6. damage to plaster, gyp. bd., paints, etc.
  7. stains, rot, frost damage, mold

Generally caused by various mechanisms that create differential pressure between inside and outside; requires "hole" in building, but such holes are common (gaps, tolerances, cracks, expansion, contraction, and so on, all can lead to holes). The differential pressures result from:

Five forces can drive water into a building:

  1. gravity
  2. kinetic energy of raindrops
  3. capillarity
  4. air pressure (i.e., wind or the other sources listed above)
  5. surface tension

Air pressure differentials are the most important; in addition to the force that results, two other things are necessary for the water to be driven into the wall: an opening ("hole"), and the presence of water on the surface of the wall. Overhangs on short buildings are effective means of keeping water off of walls, but generally are not effective on tall buildings. Eliminating all openings ("holes") is difficult to accomplish, as buildings consist of so many discrete parts, are subjected to extreme environmental conditions and structural movement, are built by contractors who are in a hurry to complete the job at a profit, and are designed by architects who may not have continuity of the building envelope surface as their primary design concept. On the other hand, air pressure differentials can be addressed through rain-screen or pressure-equalization theory. The idea, illustrated in the diagram below, is that a pressure-equalization chamber (cavity) can be vented to the outside, and sealed on the inside with an air barrier; wind pressure causes air to enter the chamber until the pressure inside the chamber is in equilibrium with the pressure outside; at that point, rain will no longer enter the wall: problem solved.

pressure-equalization theory diagram

Of course, this all assumes a steady-state wind condition. In reality, the wind pressure is constantly shifting, even changing from positive to negative, and the internal "pec" pressure is constantly seeking to reach equilibrium with the outside pressure. This leads to periods of time when rain does enter, and other periods when rain is "sucked" out. In any case, it is necessary to drain the cavity so that rain that does enter can find a way out. Ideally, one would want a rain-screen wall that minimizes the so-called "time to equalization" under conditions of positive pressure, but maximizes the time to equalization during periods of negative pressure (so as to force water out of the "pec"). If pressure on the rain-screen is minimized (it can never be eliminated), one can also reduce the size and strength of the rain-screen cladding, saving weight and money.

Other applications of pressure-equalization theory: So-called "2-stage joints are an application of rain-screen theory to the design of small joints, e.g., between metal or precast panels, or in curtain wall mullions. In the diagrams below, the joint (or mullion) is sealed at its back face, leaving the front open, vented, and drained. This creates a mini-"pec" within the joint that pressurizes in response to wind-driven rain, thereby keeping the rain out of the joint. It may, on the other hand, be difficult to install and maintain the seal, since it is not accessible from the exterior.

2-stage joints in precast and mullions

Related notes

Some notes on stucco

(from ASHRAE Journal, Feb. 2008, Joseph Lstiburek, "The Perfect Storm Over Stucco")

Traditional stucco construction (3-coat) performed well. Modern systems have experienced problems.

  1. Changes in building papers and water-resistant barriers (WRBs): Older building papers were heavier, and bond didn't form with stucco. This permitted some drainage behind the stucco. Plastic WRBs (e.g. tyvek) develop strong bond preventing drainage. Solution: use bond break in the form of a second layer of building paper. Vapor-permeable WRBs also cause problems when they don't have just the right degree of permeance. Too much vapor transmission allows water in stucco to move inwards to sheathing; while too little vapor permeance can cause problems with internal moisture control (it can't move out easily). Solution: need just the right amount of vapor permeance (low perm materials seem to do better in this context); or, create ventilated drainage cavity.

  2. Change from plywood to OSB: OSB does not become more vapor permeable as it gets wet (as plywood does); moisture becomes concentrated in the OSB which can cause more problems.

    Solution: use a ventilated air gap between the stucco and building paper/OSB (3/8" will do). For example, a drainage mat can be used. This also helps in the other direction; allowing humidity from inside to escape without condensing within the outer layers of the wall assembly.

  3. Higher levels of insulation (thermal resistance): These make the claddings (and sheathings) colder during winter months, so there is less drying of the cladding that might have been helped by heat loss through the wall assembly. As many of these materials are hygroscopic, they pick up more moisture when they are colder (since they are sensitive to relative, rather than absolute, humidity levels, and these are higher as the temperature decreases). Solution: same as before: use an air gap to facilitate evaporative drying to the back of the cladding.

  4. Use of interior plastic vapor barriers: This prevents inward drying during cool weather. Should not be used except in regions with cold weather (see map below based on 2006 IBC). Per Lstiburek (Nov. 2013), "...interior vapor barriers in the U.S. should be limited to IECC Climate Zones 6 and 7—or higher. In Canada, that means in zones where the Celsius heating degree days above 18°C are 4,000 or higher. So no plastic in Toronto—leave plastic to the folks in Ottowa and Montreal."

    US climate zones map

  5. Changed stucco properties: Older mixes were more vapor-permeable due to increased lime content. Modern polymer-based mixes can be impermeable (so that internal moisture cannot get out).

    Solution: Recommends use of water-managed EIFS, with a drainage space and without an interior vapor barrier.


Wall strategies for the building envelope

face-sealed barrier wall

1. Face-sealed barrier wall: no redundancy; single surface intended to prevent air-water-vapor penetration; thermal control may or may not be within this single surface; includes systems like EIFS, stucco, concrete, and CMU.


internal drainage plane wall

2. Internal drainage plane wall: contains at least a layer of building paper or "house wrap" (e.g., Tyvek) under the outside finish material; therefore has some redundancy, since it can block air-water that gets by the first line of defense; includes systems like some stucco and EIFS, as well as wood, fiber-cement, and vinyl siding systems. Vapor control is typically on the inside face of the thermal control layer; vapor control as the "house wrap."


drainage cavity wall

3. Drainage cavity wall: Contains an explicit cavity within the wall to intercept water and divert it back out, and , more importantly, to let any "wetness" in the cladding or interior wall elements evaporate into the cavity; the classic example is the brick-CMU (or steel-stud) wall. In the diagram, A = cladding; B = cavity; and C = back-up wall.


pressure-equalized rain-screen wall

4. Pressure-equalized rain-screen wall: A variation on the cavity wall that explicitly provides openings (vents) in the outer cladding or rain-screen; to do this right, one really needs to compartmentalize the cavity or pressure-equalization chamber (pec), and to install a continuous air barrier to seal the inner surface. See the recommendations of Ronald Brand below.

Wall strategy diagrams adapted from Brock, Designing the Exterior Wall, Wiley (2005)

For an excellent summary of wall strategies, see The Perfect Wall


Ronald Brand's recommendations for insulated building envelopes

See my paper on "Disembodied Technology and Design" for a discussion of Prof. Brand's eight rules for designing building envelopes subject to cold weather conditions.

The key diagram is reproduced below, with additional annotations as follows: A = the structure and back-up wall structure (notice the space between the top of the back-up wall and the structural slab to allow for movement); B = the air barrier (notice the allowance for movement where the air barrier spans the "void" at the intersection of wall and slab) including flashing to drain the cavity; C = insulation tight against the air barrier; D = a drainage cavity (or pressure-equalization chamber if detailed rigorously); E = the rainscreen/sunscreen.

Brand schematic diagram of wall section with air barrier

Roof ventilation

Ideally, one would "ventilate" the building envelope on the side opposite the vapor barrier, so that any stray humidity would not build up within the wall. In traditional wood-frame construction, a shingled wall provides such a ventilated space, since humidity can work its way through the relatively permeable outer layers of the wall. In sloping rafters or unheated attic spaces, the ventilation should be made more explicit, using soffit and ridge vents to promote the flow of air below the roof surface.

Ice dams may form over the eaves of roofs that are not properly insulated and ventilated: heat rising from occupied spaces melts snow on the roof; the melted snow freezes over the unheated eaves, forming an ice dam that blocks the flow of melted water. This pool of water works it's way through the roof shingles and into the wall, causing damage (see diagrams and image of the Foundry below).

ice dam theory

Foundry at Cornell: ice dams and icicles
The Foundry at Cornell, Ithaca, NY (erected 1890 as a blacksmith shop and foundry for the Sibley College of Mechanical Engineering, from plans by Assistant Professor Alfred B. Canaga)
Photo by J. Ochshorn, 2007


The basic rule of thumb for attic ventilation is to supply vent areas equal to 1/300 of the horizontal attic area.

attic ventilation rule-of-thumb


See Architecture's Dysfunctional Couple: Design and Technology at the Crossroads for my discussion of control layer theory and architectural pedagogy.