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A properly built wall makes a difference. The performance of a wall system does not depend on how much insulation you have stuffed into it. From an energy point of view it depends on the whole wall R value (thermal bridging), thermal mass benefits, air tightness, internal convection, and moisture content. From a durability point of view, dew point condensation, vapor in the wall assembly, pressure differentials, forces that cause deflection, water drainage and other issues have to be designed an engineered to protect against failure. Minimizng materials for environmental reasons and reducing both material and labor costs are also considerations. Ceiling and floors are just walls rotated 90 degrees.

The more windows and doors or the more complex the design with corners, the less energy efficient due to thermal bridging in the connections.

Airtight construction. Reduces air leaks which cause convective heat transfer and controls excessive moisture leaking into the wall. An air barrier can also be a vapor barrier but air barriers should prevent high velocity air flow into or through a wall assembly. High velocity air flows can be carrying significant amounts of moisture. Air leakage through wall assemblies should be measured with simulated wind conditions from 0 to 15 mph, and pressures from 0 to 25 - 50 Pa. This can be done with a blower door test. Many of the air leakage paths are through the connections not through the typical clear wall assembly. Energy loss in a building can increase 40% due to infiltration. Air leakage primarily occurs around windows and doors, duct work, recessed lighting, electrical outlets, plumbing in exterior walls, fireplaces and perimeter joints. According to a study by Anton Teu Wolde, Charles G Carll and Vyto Malinauskas titled “Air Pressures in Wood Frame Walls”, wind induced air pressures across exterior walls were predominately exfiltrative (from the inside to the outside) even on the windward side of the building. The study was done in Florida. Infiltrative (from the outside to the inside) only occured near windward corners during short period of time. Significant air leakage was identified past the top plate into the wall cavity. Air flow retarder design needs to be correlated with vapor diffusion control (vapor barriers). The calculations they did show that a vapor barrier of 1 perm should be complemented with an air barrier system (ABS) with an ELA of 0.003 square inches/square foot (2x10E-5 m^2/m^2) or less. Lower perm (0.1 perm) vapor retarders require an extraordinary level of tightness. In the study, exfiltrative air pressures were typically 0 to 1 Pa. Air pressures due to multiple stories, stack effect or mechanical ventilation were not considered.

Air sealing is done from the inside to the outside. This reduces heat flow by convection from air movement into the insulation or wall assembly.

Vapor barrier. Should be on the warm side but this can vary between night and day or seasonally. In a 100 SF wall, one cup of water can diffuse through drywall without a vapor barrier in a year, but 50 cups can enter through a 1/2" diameter round hole. Installing a vapor barrier and not sealing it is harmful. A vapor barrier only retards moisture movement by diffusion, it does not stop it. It should have a perm rating of less than 1. It may temporarily retard the flow of water vapor but the system will try to reach equilibrium with an equal amount of water vapor in the air on both sides of the film.

A vapor barrier is not an air barrier. A vapor retarder is only effecitve if the equivalent vapor permeance due to air flow is less than the vapor permeance of the vapor retarder. If the perms of the air barrier is larger than the vapor barrier then air flow dominates and the vapor retarder is not effective.

Moisture control. Accomplish moisture control or excess water in the wall system with a rain drainage system, an air barrier and a vapor barrier. Water can enter walls by capillary action. Driving rain can move water into the walls through improper installation of siding, lack of, or improperly installed flashing, weatherstripping, and caulking. To prevent water from wicking up the basement concrete walls, into the framing members on top of it, install a termite shield, sill gaskets or a vapor impermeable membrane on top of the basement wall. Moisture in the insulation can drastically reduce its effectiveness because it creates a thermal bridge. Small amounts of water vapor can reduce the performance of fiberglass batts by 50% or more. Excess water vapor may come from indoor activities such as cooking, showering, cleaning and doing laundry. Fiberglass batt must be vented. Typically the exterior side of the wall is 5 - 10 times more permeable than the inside of the wall.

Rain Screens. Water can penetrate the wall assembly due to momentum, capiilary action, gravity and air pressure differences. A rain screen is designed so that the air pressure difference across the exterior rain screen is at nearly 0 at all times. Usually it contains an air space and an air barrier. After rain penetration, air leakage is the second most important factor in moisture movement in walls.

Condensation in walls is dependent on the dew point in the wall and the amount of moisture or water vapor present, or, the relative humidity in the wall assembly at any given point. There is always water vapor in the atmosphere or in the ambient air. Like any gas, it moves to fill its container equally. Water vapor will move into a dry wall assembly by simple diffusion. Usual water vapor migration is from higher temperatures to lower temperatures. During heat dominated months, the vapor drive is usually from the inside of the building toward the outside. In cooling dominated months, it reverses.

The saturated vapor pressure is the total amount of water that the air can hold at a given temperature. The higher the temperature, the more water vapor is present. The partial vapor pressure is the actual amount of moisture in the air and its usually a percentage of the saturated vapor pressure. This is also known as the relative humidity. The dew point occurs when the partial vapor pressure equals the saturated vapor pressure. This is temperature dependent. At this point, condensation occurs or relative humidity is 100% and it rains. Wall assemblies should be checked for condensation points mathematically.

Storage Capacity. Wood framed walls are considered to have a certain amount of water storage capacity which will inundate during rain fall events and dry out during dry events.

The drying algorithm. A wall assembly should have a drying algorithm should it get wet or pick up moisture due to a leak or other circumstances. Does it dry to the inside or the outside. Are there wall components preventing the movement of moisture to the drying plane?

Insulation. Don't leave gaps or compress insulation. Note that fiberglass batt comes in 6.25" thickness for a 5.5" wall. To install it, you must compress it, which automatically reduces its R value because it relys on air spaces to achieve its R value. See the Manual J page for a chart relative to the amount of reduction per inch of compression.

There is a temperature gradient across a wall if the external and internal temperatures are different. The greatest temperature drops occur across wall components that have the best insulating properties. From Canadian Building Digest-36. (CBD-36) Temperature Gradients Across Walls.

Take the R value of the wall component and divide it by the total R value of the wall assembly. If you multiply the result by 100 it will be the percentage contribution to the R value of that particular component. Don't do the multiplication. Multiply the value obtained by the temperature difference. (Tin – Tout). That will tell you how many degrees the temperature changes from one face of the component to the other.


Component	Thickness, n, in.	Conductivity, k	Conductancr, C = k/n	Resistance, R = 1/C	Temperature Drop, deg F	Interface Temperature, deg F

Internal Air Film (still air)			1.46	0.68	4	70
Gypsum Plaster (sand aggregate)	5/8		9.10	0.11	1	66
Concrete Block	8		0.50	2.00	11	54
Cement Mortar	¼	5.0	20.00	0.05	0	54
Foamed Plastic Insulation	2	0.29	0.145	6.90	36	18
Air Space	1		0.13	0.97	5	13
Face Brick	4	9.0	2.25	0.44	2
External Air Film (15 mph wind)			6.00	0.17	1	11

TOTAL				11.32	60

The external temperature on a wall isn't necessarily ambient temperature. Heat storage and clear sky radiation at night can effect the real temperature of the wall surface. With respect to solar radiation the color, texture, thickness, orientation and weight all have an effect. If the wall was made out of black chrome and subject to direct solar insolation in December, it probably won't be the same temperature as the ambient air.

The following information is from Canadian Building Digest 126 (CBD-126). Temperatures are in Degrees F. The figures in the tables give the maximum rise above ambient temperature that a vertical surface attains as a result of solar absoprtion. The day is cloudless, no wind, clear atmosphere, the surface of the wall is black and the wall is lightweight and well insulated. Light colored walls can reduce the temperature increase by 50%. If the walls were massive constructed out of concrete or masonary the values would be lower due to conduction into the walls and the high heat storage capacity.

Freeze Thaw Cycles. If the ambient air temperatures are at freezing or below, then when the wall drops in temperature back to ambient it will cause a freeze cycle, the next time solar absorption is available it will cause a thaw cycle. Repeated freeze/thaw cycles can do damage to the wall assembly.

There is also a vapor pressure gradient across the wall. Since vapor pressures are directly related to temperatures, the vapor pressures will reflect the changes in temperature across the wall. If the partial vapor pressure meets the saturated vapor pressure in the wall at any point, then condensation will form at that point. This can occur in the exterior sheathing, in the batt insulation or up against an internal vapor barrier if one is present. The dew point within a wall can change position dependent on exterior and interior conditions. Each component of the wall assembly has a water vapor transmission coefficient which describes its ability to allow water vapor to pass through it. This figure may be described with terms such as US perms, permeance, permeability and the water vapor transmission rate. Given a perm value and a set of physical conditions, the partial vapor pressure on each side of a wall component can be computed. Since the temperature is also known, the dew point can also be determined and the probability of condensation occuring under those conditions.

If a dew point analysis shows that condensation will never occur or that it might occur in a wall assembly component that is unaffected by moisture, it may be possible to omit the vapor retarder. Insulating the exterior of the wall may help shift the dewpoint more toward the exterior and into a material that isn't effected much, but it takes a lot of insulation to shift the dew point a short distance.

Intermittent periods of condensation may not present themselves as a mold or rot hazard if they are allowed to dry out. In the meantime, any insulation that is effected will have a greatly reduced R value.

Advanced Wall Framing also known as OVE for optimum value engineering framing system reduces the amount of lumber used to frame a home. It simultaneously increases the R value of wall systems compared to conventional methods because it reduces thermal bridging. The basic principles are to design the building to be based on 2' modules and framing is at 24" o.c. rather than 16" o.c. , using 2x6's instead of 2x4's. This also allows for more insulation thickness. Headers are designed for actual loading conditions and insulated. Select windows and doors that fit in between framing members to the fullest extent that is possible. Make corners out of 2 studs instead of 3 and use ladder blocking where interior partitions intersect exterior walls. To allow insulation in corners, use let in bracing. Eliminate cripples or curtailed studs under windows. Align the roof, wall and floor framing members vertically so that a single top plate can be used.

OVL is effective if the fenestrations are about 10% or less than the total wall space. If greater than that, then the extra studs required begin to rival 2x4 framing at 16" o.c. It also increases costs because jamb extenders must be used. This can be on the order of $12 - $15 per opening. With fenestrations that exceed the 10% figure, it may make more sense to go with the 2x4 framing and use a thick exterior insulating sheathing.

Center of Cavity R value. A measure through the wall where there is insulation and no framing

Connections. The intersections between walls and floors, ceilings, windows, doors and other walls.

Clear Wall R value. Insulation and necessary framing with no fenestrations, corners or connections. A span of studs and insulation.

Whole Wall R Value - an estimate of the opaque wall that includes typical interface details like wall to wall or corners, wall to roof, floor, door and window connections.

Framing members can account for 15% to 40% of the total wall area. If framing members have a lower R and thermal mass value than the insulation in the wall cavity then the actual R value of the completed wall is reduced from the value of just the insulation alone.

If just the studs are considered in a typical 2x4 wall, with studs at 16" o.c., with no connections and no fenestrations then the "framing factor" which is a measure of the number of framing components in a wall system, will come out to be in the vicinity of 9%. In reality, the framing factor is more on the order of 23% - 25%. For a smart framed 2x6 wall assembly with studs 24" o.c., the actual framing factor is on the order of 16%.

There are many connections in a wall assembly that can effect the actual R value of the completed walls. These include fenestrations, corners, connections between the wall and roofs, foundations and other walls.

When these inteface details are not properly implemented such as paying attention to insulation and sealing the results can be excessive moisture, condensation, stains and dust markings and the propogation of molds and mildews leading to poor indoor air quality and potentially serious health effects.

The thermal mass of a wall component is an important parameter to consider. Materials are usually rated according to their R value which is the resistance to heat flow. Materials of high thermal mass also have high resistance to heat flow.

Convective flows in very light density attic insulation can reduce performance by 40% under winter conditions. Air warmed by the attic insulation rises and escapes through attic venting and is quickly replaced by more warm air. The heat to warm the air may be coming from the ceiling directly below the insulation.

Increasing the amount of insulation in a wall may induce moisture related issues. There is potential for condensation, freeze/thaw cycles and decay. This is due to the fact that the wall is potentially colder, especially toward the ambient conditions side in the winter.

The True R value of a wall includes insulation, thermal bridging, air leakage, wind washing, convective loops, radiation enhancements, thermal and hygric mass and installation defects.

Convection can play a major role in the true R value of a wall assembly. On the outside of the wall convective currents can carry heat energy away from the wall, where more hot air rushes in to take its place from a more inside location in the wall. The same thing happens on the inside of the wall. Since the wall is colder than the indoor air, the indoor air gives up heat energy to the wall assembly, the cooler air falls to the floor and warmer air quickly rushes in to take its place, resulting in a convective loop. Regular infiltration from poor air sealing results in air flow across the wall. Inside the wall, as warm air gives up its heat to the cooler materials it is adjacent to, it wants to fall, making room for more warm air. This forms a convective loop right inside the wall insulation. Gaps in the insulation open the doors for pressure differences and forced air leakage. These gaps also form an environment for increased condensation.

Poorly installed insulation at the rim and band joists is a common place to find forced air leakage.

Fiberglass batt insulation won't typically settle. Blown in loose fill cellulose can settle as much as 20% leaving gaps in the insulation.

A study was done by Oak Ridge National Laboratory on varous wall systems to experimentally determine Whole Wall R Values. The results are heavily published. They also provide an R value calculator and a thermal mass calculator on their web site. They studied 18 different wall assemblies in detail.

For a typical 2x4 framed wall, defined at 16" o.c. studds, R-11 batts, exterior plywood and gypsum interior, the clear wall R value is 10.6 and the whole wall R value is 9.6

A 2x4 wall with 24" o.c. stud spacing, R-11 batts, plywood exterior and gypsum interior is 10.8 clear wall R value and 9.9 whole wall R value.

A 2x6 stud wall with 24" o.c. spacing, R -19 batts, plywood exterior and gypsum interior is 16.4 clear wall and 13.7 whole wall R value.

A larsen Truss wall which is R -11 batts + 8" insulation, spaced 16" o.c. , a plywood exterior and gypsum interior has a clear wall R value of 40.4 and a whole wall R value of 38.5. This system removes a lot of the thermal bridging and so the whole wall R value is closer to 5% within the clear wall R value. For the 2x4 framed walls the deviation is on the order of 10% and for the 2x6 framed walls the deviation is more like 17%.

Another case study analysis was done as a Building America Special Research Project, on High R walls and published in July of 2009 by Building Science Corporation. The data was not acquired experimentally rather by using a software package called Therm.

To model a wall assembly, use Therm 5.2. Therm was develolped by Lawrence Berkel National Laboratory at the University of California.

1) 2x6 advanced framing with 24" oc, R-19 batt and osb, yields a whole wall R value of 15.2, rim and band joist R value of 12.3, Clear wall R value of 16.1 and top plate R value of 12.5

2) 2x6 with 16" o.c. instead, yields 13.7 whole wall R value, no change in the rim band joist, a clear wall value of 14.1 and no change in the top plate

3) 2x4 advanced framing with 24" o.c., 13" of batt and osb, yields whole wall R value of 11.1, rim and band joist R value of 9.8, clear wall R value of 11.5, and a top plate R value of 9.8

4) 2x4 framing 16" oc. otherwise, same as above, yields 10.0 R value for the whole wall, the rim and band joist doesn't change, 10.1 for the clear wall R value and the top plate doesn't change.

A double stud wall was also modelled using therm. The exterior wall was 16" o.c. 2x4 and the interior wall was 24" o.c. but otherwise the same. The gap between the 2 walls can be varied. In the model case, the total wall thickness was 9.5" and cellulose insulation was used. Given the thickness of cellulose, the manufacturer declared R value should be around R34. The whole wall value came out to about R30. Due to the high R value the exterior sheathing is colder than in the other methods already mentioned. This leads to increased probability of potential wintertime condensation. If such a wall was built in Minneapolis, the there are approximately 4600 winter hours where this potential exists. For this implementation window bucks or plywood boxes are required.

There are several spreadsheets that go with this file, with full explanations below:

Once the temperature is known at each component in the wall system, that information can be combined with relative humidity and dew points at particular temperatures. When the correct conditions are met, condensation may occur in the wall.

Relative Humidty and windspeed Data can be found in “redbook”. For now, refer to the windows file to get information concerning Redbook data.

The Dew Point Method is based on the following diffusion equation and definitions:

The water vapor permeabillity of a material is the permeance of 1 in or 1 m of that material. Water vapor resistance Z is the inverse of permeance and is expressed in reps (1/perm) or m/s

Step 2. Consult Table in spreadsheet (Sheet 2 has this data) to find the saturation vapor pressures corresponding with the surface temperatures

Step 3. Compute the vapor pressure drop across each element of the assembly. Take the RH for the interior and divide by 100, then multiply by the saturation vapor pressure for the component closest to the interior space. Subtract the following quantity from it. Take the relative humidity of the outdoor air and divide by 100. Multiply the result by the saturation vapor pressure of the component closest to ambient air. The result will be the total vapor pressure drop. Multiply the result by the following: Take the diffusion resistance for the particular component and divide by the total diffusion resistance.

From :: Delta Pmaterial / Delta Pwall = Zmaterial / Zwall. Delta Pmaterial = (Zmaterial/Zwall)*Delta Pwall

Take the indoor saturation vapor pressure and start there. Deduct the pressure drop of the first component and compute a new pressure.

Compare the pressure drop at the surface of the building component to the saturated vapor pressure at that surface if the surface of the component is less then condensation does not occur.

The method only helps to predict condensation. Only diffusion is recognized and the algorithm assumes no air leakage. Wetting by driving rain or heating by solar insolation driving moisture into a wall are not accounted for.

If the air conditioning thermostat setpoint is lower than the average daily low, then use the thermostat set point instead.

Sheet 1 has the dew point method for the insulation part of a standard 2x6 framed wall with fiberglass batts. Sheet 2 has the dew point information for both water and ice. Sheet 3 has the dew point computation for the stud portion of a standard 2x6 wall and Sheet 4 has the dew point information for a 9” wall built out of 2 separate 2x4 framed walls with fiberglass insulation. There is no stud thermal bridge in this wall assembly.

There is a spread sheet available that analyzes payback analysis of various thicknesses of insulation.

The spreadsheet starts off with a small data base reflecting the change in oil prices over time. The goal is to find out the average annual increase in the cost of this fuel. Oil prices do not fluctuate with the normal rate of inflation. They change due to a variety of parameters. To find out more about this fact, there is information online.

Fiberglass batts are used for the insulation material and the cost per SF per inch is determined for this insulation choice.

As a matter of principles, you can substitute your own fuel choice and own insulation choice into the spreadsheet, just follow the same logic and mathematical formulas to find out the payback periods.

The spreadsheet models wood framed walls with various levels of insulation. Once 2x6 walls are surpassed in thickness, it assumes the wall is double framed and thermal bridging is reduced. The real wall R values are used from the documentation cited above. The temperature data is taken from Redbook. To find out more about Redbook, please refer to the windows file.

The amount of energy lost with each insulation thickness is computed using the simple conduction heat transfer equation. If we know how many Btu's of energy is lost, we can figure out the cost of that energy because we also know how many Btu's are embodied in a gallon of home heating oil (diesel fuel).

The entire spreadsheet is done for a single square foot (SF) of wall space for each condition and temperature. The results may look like miniscule amounts of energy are being lost or gained at an insignificant cost. When you multiply the numbers in the spreadsheet by your actual wall, ceiling and floor space, that's where you see that these results are not as small as they appear.

After all the wall assemblies are modelled for each month of the year, the spreadsheet then compares how much savings you gain in today's dollars if you upgrade from a 2x4 wall system to each of the other wall system presented. The only expense addressed is the insulation expense. Wood prices are like oil prices. They fluctuate all over the place. Then there is the labor charge, which can vary between contractors, year to year or even month to month.

The final section of the spreadsheet assumes a typical colonial with a footprint of 24'x36' feet and 10% wall space open as windows or doors. The various wall assemblies are multiplied by the savings of upgrades to give an idea of the impact on a typical home.

The spreadsheet takes the daily min temperature and applies that to 12 hours out of the 24 hour period. It does the same for the daily max. To get a more precise result, take the absolute value of the difference between the daily max and daily min. Divide the result by 12. Successively add that increment to each hour and redo the analysis, starting with the average daily min. When the value of the average daily max is reached, deduct the increment successively for the next 12 hours.