characterization of air leakage in residential structures— part … b12... · 2013-10-24 · 2...

Characterization of Air Leakagein Residential Structures—Part 2: Whole House Leakage

David Wolf, PhD Frank Tyler

ABSTRACT

Air will leak through a building envelope that is not well sealed. This leakage of air decreases the comfort of a residenceby allowing moisture, cold drafts, and unwanted noise to enter, and air leakage can account for up to 40% of the energyused for heating and cooling in a typical residence. With nearly a mile of exterior joints that can leak air in a typical resi-dence, knowing which joints leak the largest quantity of air allows for the most strategic placement of sealant. This two-part paper describes an extensive investigation to quantify the leakage characteristics of various types of joints and openingsin a residential structure. All-in-all, 17 different joints/openings were characterized through both laboratory and real-housemeasurements using fan pressurization. Part 1 presents the methods and results associated with the air leakage of the indi-vidual joints/openings. Part 2 adapts these individual results to the whole house, including an examination of the joint leak-age interdependence in the wall cavity.

INTRODUCTION

To reduce air infiltration and achieve an energy-efficientbuilding, the builder or building owner must seal gaps in thebuilding’s thermal enclosure. Installing high-quality,tightly sealed windows and doors is a good start, but it’salso important to seal joints and openings in the walls, ceil-ing, and flooring/foundation. To properly address the nega-tive effects of air leakage, such as wasted energy, occupantdiscomfort, condensation, and so on, all of the joints andopenings in the building enclosure should be air sealed.However, some builders or building owners may only havelimited funds available to devote to air sealing. With nearlya mile of exterior joints that can leak air in a typical resi-dence, knowing which joints leak the largest quantity of airallows for the most strategic placement of sealant. Thisstudy is an investigation to quantify the leakage character-istics of various types of joints and openings in a residentialstructure (Part 1) and to prioritize the joints/openings interms of the amount of air leakage per unit cost to seal it—a kind of air leakage “bang-for-your-buck” ranking of the

joints/openings (Part 2). The air leakage results are primar-ily reported at a pressure difference of 50 Pa due to its prev-alence in the pre-commission testing of residentialstructures in the United States.

RESULTS AND DISCUSSION

The extensive amount of data obtained in Part 1 isonly of value if it can be applied to assessing the impactof joint leakage on actual houses. Applying these resultsto actual houses requires several steps, which include:

• Establishing a means by which to scale the lab/test-house results so that they could be applied to anyhouse;

• Comprehending the effect of upstream and/or down-stream restrictions, such as drywall on the interior andcladding on the exterior;

• Projecting these results to actual houses to predict theimpact of individual joints on whole-house air leakage;

© 2013 ASHRAE

David Wolf is a research associate and Frank Tyler is a senior scientist at Owens Corning Science and Technology, LLC, Granville, OH.

• Incorporating the cost implications of sealing the vari-ous joints to establish a cost-versus-performance priori-tization.

The following sections will address these various steps.

Joint/Opening Severity and Frequency

The results presented in Part 1 were often expressed bothin terms of the total leakage for a particular joint (e.g., CFM50or L/s) as well as the normalized leakage, where thenormalization was achieved by dividing the total leakage fora joint by the relevant scaling parameter. For a joint, such asthe bottom plate-to-subfloor, the relevant scaling parameter isthe joint length (e.g., CFM50/ft or L/s-m). This normalizationwas applied to most of the leakage that was measured in thisstudy. However, some items, such as recessed lights, ductboots, and outlets, were better normalized per unit (e.g.,CFM50/unit or L/s-unit).

These normalized leakages, independent of whether thenormalization was by length or unit, could be viewed as aleakage severity, which, when multiplied by the joint/opening frequency, would constitute the total leakage asshown below.

leakage [CFM50] = severity [CFM50/ft] × frequency [ft] (1)

This simple relationship allows the scaling of lab/test-house results, cast as a severity, to be applied to any house,where the frequency of joints or openings is known.

Effect of Upstream and/or Downstream Restrictions

The leakage from outside to inside associated with thejoints/openings reported in Part 1 has not comprehended anyupstream resistance (e.g., cladding) or downstream resistance(e.g., drywall)—they are considered unconstrained leakagevalues. The one exception is the results for the top plate-to-drywall connection, which necessarily included thedownstream resistance at the drywall termination (i.e., bottomof the wall) and penetrations (e.g., electrical outlets).

Depending on the joint/opening, these upstream anddownstream resistances can be important. The leakage withsome joints/openings are exempt from this complication,such as ceiling penetrations (e.g., recessed lights, duct boots,etc.), where the pressure difference across these items is50 Pa during the testing conducted in this study as well aswhat would be experienced in an actual house undergoing ablower door test. In contrast, the leakage with some otherjoints/openings, particularly those found in the wall cavity,can be significantly affected by the upstream cladding, thedownstream drywall and trim, as well as be coupled to otherleakage paths in the wall.

Quantifying any cladding-induced upstream resistance ina wall was beyond the scope of this study. Qualitatively,cladding systems that are air permeable and/or have air spacesbetween themselves and the sheathing are expected to haveminimal impact on the results reported here, because there is

believed to be ample open area for the air to flow freely.Examples of such claddings would include vinyl siding, woodsiding, fiber cement siding, and full-thickness masonry that isoffset from the sheathing (e.g., brick and non-veneer stone).Cladding systems that are relatively air impermeable and/orhave intimate, continuous contact between themselves andthe sheathing are expected to have a significant impact on theresults reported here because of the flow resistance that theyimpart. Examples of such claddings would include stucco andstone veneer without the presence of a rainscreen.

Quantifying the drywall-induced downstream resistancewas addressed in this study. Figure 1 conceptually shows awall cavity (black rectangle), separating the interior from theexterior. The red line shows the air flow from the exterior tothe interior and draws an analogy to a series-resistiveelectrical circuit, where the airflow rate (Q) represents thecurrent, the pressures (Pe, Pw, Pi) represent the voltages, andthe resistances to air flow (Re, Ri) represent the resistors.With such a series-resistive flow condition, the wall cavitypressure (Pw) will approach the exterior pressure (Pe) forcases where the interior resistance is much greater than theexterior resistance (Ri >> Re). The test results reported inPart 1 have assumed that there is no interior resistance (Ri =0, Pw = Pi), when in fact there is.

Consider Figure 2, which shows various leakage pathsinto the wall cavity (red arrows) and out of the wall cavity(green arrows) on the first story of a two-story house (moreon why this is important later). This shows an example withfour inlets (there could be more) and three outlets. The inletsare: (2) top plate-to-sheathing, (3) between the double topplates, (4) bottom plate-to-sheathing, and (12) a verticalsheathing seam. The outlets are: bottom of drywall (B),electrical outlets (C), and drywall-to-top plate (D), whichfeeds an open floor joist cavity. Since each of the flows

Figure 1 Sketch of a series-resistive flow in a wall cavity,where P is pressure, R is flow resistance, and Qis flow rate for locations at the interior (i), exte-rior (e), and within the wall (w).

2 Thermal Performance of the Exterior Envelopes of Whole Buildings XII International Conference

depicted here has a correlated relationship to pressuredifference of the form Q = C Pn, it is possible to computethe flow associated with each of the arrows in this figure, aswell as calculate the wall cavity pressure. Before doing so,however, it is interesting to think about some possibleoutcomes.

Figure 2 shows the flow-equivalent of multiple parallelresistors on the exterior skin of the wall (red arrows) that arein series with multiple parallel resistors on the interior skin ofthe wall (green arrows). Like an electrical circuit, the relativedistribution of these resistances not only controls the amountof air that leaks through the wall assembly but also controls thedominant leakage paths. For instance, it will be shown laterthat the sealing of one individual joint on the exterior side ofthe wall could result in an increase in leakage of the otherunsealed joints on the exterior skin, even though the overallwall leakage decreases somewhat. This could happen if theresistances on the interior skin of the wall (through which thegreen arrows pass) are appreciable.

Table 1 is an example from a spreadsheet that shows sucha set of calculations. The top of the worksheet establishes thespecified length of the wall that is being calculated (8 ft[2.4 m] in this example), the specified boundary pressures (Pibeing that of the interior and Pe being that of the exterior), the

calculated wall cavity pressure (Pw), the calculated flow ratedifference between the total inlet and outlet flows (dQ, whichshould be zero), the calculated wall pressure difference withrespect to the interior (wall dP), and the calculated total flowthrough the wall per unit length of wall (Q/L). The next sectionof the worksheet is associated with the inlet joints on the exte-rior side of the wall cavity (the red arrows in Figure 2),followed by a section associated with the outlet joints on theinterior side of the wall cavity (the green arrows in Figure 2).The nomenclature section describes the other parameters.

There are several notables regarding the normalized leak-age values and exponents (originally reported in Part 1) shownin the spreadsheet table.

First, for those joints where a test house value was usedfor the normalized leakage (indicated by a “TH” in the Ref.column), the exponent came from the corresponding labora-tory results. This was because the laboratory results wereobtained under multiple pressure differences, therebyenabling the correlation of a pressure exponent. A pressureexponent couldn’t be obtained from the test house measure-ments, because they were conducted primarily at 50 Pa only.

Second, the normalized leakage for the “bottom ofdrywall” (0.651 CFM50/ft or 1.01 L/s-m) was inferred fromthe laboratory-based garage wall measurements described inPart 1. The garage wall leakage includes two drywall-to-platejoints, one on the interior and one on the exterior. An estimateof the individual joint contribution (i.e., one drywall-to-platejoint) can be made by assuming that the interior and exteriorjoints leak the same, which implies an intermediate wall cavitypressure difference of 25 Pa. The flow rate associated with theoverall pressure difference of 50 Pa, which includes the flowthrough both the interior and exterior joints, is the same as theflow through one of the joints with the pressure difference of25 Pa, all of which are predicted by the same empirical rela-tionship (Q = CPn), so C can be inferred for the one drywall-to-plate joint. Also, the worst-case garage wall leakage wasused in this calculation (see max normalized leakage inTable 1 of Part 1), rather than the average, based on the prem-ise that lab-built wall assemblies generally have tighter toler-ances than site-built construction.

Third, the electrical outlet spacing was assumed to be onefor every 8 ft (2.4 m). The electrical code requires that oneoutlet be placed a minimum of every twelve feet apart. Thecloser spacing used in this study is based on general fieldobservations, where some combination of wall layout and/oradditional penetrations (e.g., light switches, telephone junc-tions, television junctions, internet junctions, wall lightfixtures) results in more electrical-outlet-like penetrations inthe drywall.

Armed with all of this information and the requirementfor conservation of mass between the inlet and the outlet of thewall cavity, the spreadsheet computes the individual flows andthe cavity pressure, all through iteration. Such a tool is quitehandy because it then permits calculation of the leakage for avariety of permutations, where different combinations of inlet

Figure 2 Sketch showing leakage paths into and out of awall cavity.

Thermal Performance of the Exterior Envelopes of Whole Buildings XII International Conference 3

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65


and outlet joints are sealed. Note from the sample calculationshown in Table 1, which assumes that all inlet and outlet jointsare unsealed, the individual joints are leaking an amount thatis only 63% of the amount measured when the drywall wasn’tpresent (see column entitled Q/Q50).

Much emphasis is being placed on the model that gener-ated the calculations in Table 1, so a simple experiment wasdesigned in the laboratory to validate this model. It used thesame pressure chamber that generated the laboratory leakageresults discussed in Part 1. This experiment involved a wallassembly with sheathing on the exterior and drywall on theinterior. The interior drywall had five locations where the aircould leak from the wall cavity—the top plate-to-drywalljoint, the bottom plate-to-drywall joint, and up to three elec-trical outlets. While measuring the flow rate and the wallcavity pressure, various combinations of these joints weresealed and unsealed. Using the above-mentioned model, theflow rates and wall cavity pressures were also calculated.Figure 3 plots the measured results versus the correspondingpredicted results, one for flow rate and the other for wall cavitypressure. It can be seen that the agreement is quite good in bothcases.

The leakage severity for some of the joints could beapplied directly from Table 6 in Part 1, because they are notaffected in any meaningful way by the flow resistance of thedrywall/trim. These joints include:

• Band joist (top and bottom): The interior side of theband joist is typically connected to an open floor joistsystem, which is voluminous enough to impose little/norestriction on the airflow (i.e., the local pressure at theinterior side of the band joist is equivalent to the housepressure).

• Sill plate-to-foundation: Although this was not shown tobe a significant leakage path when some form of a sillgasket is used, any leakage that does occur would typi-cally have little/no resistance associated with finishesbecause it is connected to an open floor joist system, likethe band joist.

• Garage wall: The garage wall leakage was obtained withdrywall present on both sides of the test assembly, so thevalues reported here already include all of the drywalleffects. The testing did not include the effects of sealingthe bottom of the drywall on one side. However, it can beshown that such an effect has minimal influence on theleakage because there is still ample area for leakage tooccur through any electrical outlets.

• Duct boots: For those cases where duct boots penetratethe ceiling drywall from an unconditioned attic, there isno additional flow restriction to consider beyond theboot-drywall interface.

• Recessed lights: For those cases where recessed lightspenetrate the ceiling drywall from an unconditionedattic, there is no additional flow restriction to considerbeyond the recessed light-drywall interface.

Also, the leakage values in Table 6 in Part 1 that wereassociated with the top plate-to-drywall joint for a partitionwall included the constraint of the drywall. Namely, the bestestimate of 0.68 CFM50/ft (1.1 L/s-m) was obtained for thecase where the bottom of the drywall was not sealed and theoutlets were not sealed. For the case where the bottom of thedrywall was sealed, implying leakage only through the outlets,the test house results gave a leakage value of 0.29 CFM50/ft(0.45 L/s-m).

Any other joint that is a part of a wall cavity must compre-hend both the flow resistance that the drywall/trim imparts aswell as the influence of the other leakage paths. These jointsinclude:

• Plate-to-sheathing (top and bottom)• Corners (inside and outside)• Bottom plate-to-subfloor/slab• Vertical sheathing seams• Window/door framing-to-sheathing• Between top plates• Top plate-to-drywall at attic on an exterior wall

Figure 3 Comparison of measured and predicted (a) flow rate and (b) wall cavity pressure through a wall assembly.


The leakage severity of all of these joints was determinedfor two conditions—one was with maximum drywall/trimconstraint and the other was with minimum drywall/trimconstraint. The former is the condition where the bottom of thewall is sealed on the interior with caulking of the trim to thedrywall and the trim to the floor (e.g., ceramic tile), all foraesthetic reasons by the builder (see Figure 4). The latter is thecondition where the bottom of the drywall/trim is unsealed,such as when flooring treatment is hardwood or carpet,thereby precluding caulking to the floor. In either case, thespreadsheet model described earlier in this section is used topredict the overall leakage.

Figure 5 shows the effects of these constraints on the vari-ous exterior wall joints. The unconstrained condition is for thecase where no drywall is present on the interior side of the wallcavity, which is the result that was presented in Part 1. Theremaining conditions show the effects of: (1) whether thebottom of the drywall is sealed on the interior and (2) the loca-tion of the wall (upper story or lower story). This chart showsa significant reduction in leakage when any form of drywallconstraint is applied. As would be expected, the constraintbecomes greater when the bottom of the drywall is sealed,thereby allowing leakage out of the wall cavity through pene-trations only (e.g., electrical outlets). Perhaps unexpected isthe fact that the wall’s location (lower story or upper story ona 2-story house) also affects the amount of constraint.

Figure 6 shows two identical exterior wall sections, exceptone has a story above it (Figure 6a) and the other has an uncon-ditioned space/attic above it (Figure 6b). The sole difference inthe exterior wall assembly between the upper story of a house,which has attic space above it, and the lower story of a two-storyhouse, which has a floor system above it, is the leakage betweenthedrywalland the topplate. In the formercase, the leakage is intothe wall cavity (see red arrow 13 in Figure 6b), whereas for thelatter case, the leakage is out of the wall cavity (see green arrowD in Figure 6a). This results in different leakage behaviorbetween the first floor and second floor exterior walls, with thefirst floor wall enabling more leakage due to the additional outletat the top plate, which feeds the floor system, all else being equal.

The main take-away from Figure 5 is that the leakageassociated with any joint in the wall cavity will vary, depend-ing on the downstream conditions (drywall/trim treatment)and the leakage of other joints sharing the same wall cavity.This prevents the use of single leakage severity value for thesetypes of joints, and it requires the type of calculationsdescribed above to obtain a realistic estimate.

Projection to Whole-House Leakage

with Cost Impact

With the frequency of the joints/openings obtained fromthe actual house or set of plans, the whole house leakage asso-ciated with those joints/openings can be readily determinedwith Equation 1. Furthermore, knowledge of the volume of thehouse allows the results to be expressed in terms of air changesper hour at 50 Pa (ACH50), which is a common metric used toexpress whole-house airtightness.

Two different houses were analyzed—a one-story and atwo-story house—both constructed on a slab. The basic char-acteristics of these houses are listed in Table 2. The floor planassociated with each of these houses was used to obtain thevarious joint lengths (i.e., the frequency term in Equation 1).The normalized leakage rates (i.e., the severity term in Equa-tion 1) were obtained either from Table 6 of Part 1 (joints/openings not associated with a wall cavity) or Figure 5 (joints/openings constrained by drywall). Similarly, a cost can beassociated with the sealing of each joint, which enables thecalculation of a ratio of the leakage reduction achieved by seal-ing an individual joint to the corresponding incremental costto seal it. This can be interpreted as a bang-for-your-buck

Figure 4 Several photographs that show the extent of caulking that is done by a large production builder for aesthetic reasons.

Table 2. Floor Area andVolume ofTwo Houses Used

to Estimate Whole-House Leakage Values

House Type Floor Area, ft2 Volume, ft3

One-story, slab 2926 26,334

Two-story, slab 3984 35,856


metric. Clearly, joints that have a large amount of leakage and/or don’t require much material to seal will rise to the top of thelist as being the most cost effective to seal. It should be notedthat this cost assumes easy access to the joint, such as wouldbe the case with new construction. For existing houses, accessto certain joints may be obstructed (e.g., wall joints), whichwould increase the cost.

Figure 7 shows a chart of incremental performance im-provement for the one-story house on the y-axis (ACH50for a minimal drywall constraint) as a function of the in-stalled cost on the x-axis. Since the cost of sealants varieswith the type and method of application, the cost units havebeen omitted. Figure 8 shows a comparable chart of incre-mental performance improvement for the two-story house.The order with which the joints and openings are displayedin these figures is based on their individual contribution to

air leakage (i.e., the “bang,” if sealed) per unit of cost to sealthat joint (i.e., “for your buck.”) Using Figure 7 as an ex-ample, the four biggest bang-for-your-buck joints to sealwould be recessed lights, duct boots, garage wall, and top-plate-to-drywall at attic. Also note that the top-plate-to-drywall at attic joint gives the single biggest opportunity forlowering the blower door result.

Note that both of these charts display a non-linear behav-ior, where the curve is steep initially (large reduction in airleakage per unit cost), and relatively flat finally (small reduc-tion in air leakage per unit cost). Recall that the main objectiveof this study was to prioritize the joints/openings in terms ofthe amount of air leakage per unit cost to seal it—a kind of airleakage bang-for-your-buck ranking of the joints/openings. Ifevery joint in the house leaked comparably and required thesame amount of sealant to address it, then the relationship in

Figure 5 Normalized leakage associated with wall joints, where the normalizing length is the length of the wall, not neces-sarily the length of the joint (e.g., corner length ≠ wall length). The unconstrained condition is for the case whereno drywall is present on the interior side of the wall cavity. Drywall is present for the remaining conditions, whichconstrains the leakage, albeit by differing amounts depending on the location of the wall (upper story or lower story)and whether the bottom of the drywall (DW) is sealed on the interior.*Note: Inside and outside corners are assumed to be present at 8 ft per every 40 ft of wall section; a window/doorjoint is assumed to be present at every 16 ft of perimeter (one 3 ft × 5 ft window) per every 16 ft of wall section; ver-tical sheathing joint is assumed to present at 16 ft per every 8 ft of wall section.


Figures 7 and 8 would be linear, but that’s clearly not the case.Indeed, some joints/openings are far more important thanothers from a blower door perspective.

CONCLUSION

This study was an extensive investigation to quantify theair leakage characteristics of various types of joints and open-ings in a residential structure and to prioritize the joints/open-ings in terms of the amount of air leakage per unit cost to sealit—a kind of air leakage bang-for-your-buck ranking of thejoints/openings. The air leakage in all cases was assessedunder the blower door test pressure difference across the build-ing enclosure of 50 Pa. A summary of the results are providedin Table 3. These rankings were bucketed into the three cate-gories of (1) most effective joints to seal, (2) moderately effec-tive joints to seal, and (3) least effective joints to seal.

The most effective joints to seal included the following:

• Top Plate-to-Attic: This joint was shown to leak inthe range of 0.29 to 0.68 CFM50 per foot of joint.This range is driven by differences in how well thedrywall is sealed to the interior finishes. For an aver-age-sized house, there can be upwards of 500 ft ofthis joint, and the size of the gap between the drywalland framing at this location can be relatively large,due to misalignment in framing (stud-to-plate) or thepresence of top plate-to-rafter ties (hurricane ties),which can create a localized offset of 3/16 in. to 1/4in. between the drywall and plate. The cumulative airleakage for this joint is significant, resulting in nor-malized results in the range of 0.29 to 1.6 ACH50.

• Recessed Lights: Even so-called airtight recessed lights,which are mandated by many building codes, can leak anappreciable amount of air at the juncture between thelight housing and the mounting flange, as well as themounting flange and the drywall. Test results showed an

Figure 6 Schematics showing the inlet (red arrows) andoutlet (green arrows) leakage paths through theexteriors walls on the 1st floor and 2nd floor of atwo-story building.

Figure 7 Chart of cumulative reduction in air leakage asa function of installed cost (arbitrary) for a one-story house.

Figure 8 Chart of cumulative reduction in air leakage asa function of installed cost (arbitrary) for a two-story house.


average leakage of 9.1 CFM50 per light, which couldamount to upwards of 0.15 ACH50, depending on thenumber of lights penetrating the ceiling into the uncondi-tioned attic.

• Duct Boots: For cases where the HVAC ductwork islocated in the attic, there will likely be duct boots thatpenetrate the ceiling drywall to supply or return air to theliving space. The leakage at the interface between theduct boot and the drywall contributes 7.7 CFM50 perboot, which could amount to upwards of 0.13 ACH50,depending on the number of duct boots present.

• Band Joist: This joint was shown to leak an averageamount of 0.86 CFM50 per foot of band joist (includesthe leakage from both the upper and lower joints), whichresults in approximately 0.4 ACH50 for the whole-houseresult. It is notable that the band joist joints are the onlywall joints that are not meaningfully constrained by dry-wall, which makes them important contributors to airleakage. The air leakage associated with all other exteriorwall joints must negotiate the joint on the exterior skin ofthe wall (the sheathing layer) and the interior skin of the

wall (the drywall layer). Once the air passes the exteriorskin associated with the band joist, it typically encountersa large, open space in the floor system, where it travelsrelatively unimpeded.

• Garage-to-House Common Wall: This wall is unique inthat it is an exterior wall (i.e., separates the conditionedliving space from the unconditioned garage space) thatis sheathed on the exterior side with drywall, as opposedto some form of structural sheathing, like oriented-strand board or plywood. This is significant in that dry-wall has far less stiffness than OSB/plywood, whichadversely affects the airtightness of the joint that isformed between the drywall and the framing members.This joint was shown to leak an average amount of0.6 CFM50 per foot of joint, which results in approxi-mately 0.14 to 0.26 ACH50 for the whole-house result.While this is fairly high leakage on a per foot basis, itsimplication on the whole-house leakage is not as largebecause there are typically not many feet associatedwith a garage wall. However, such a joint could be veryimportant from the standpoint of indoor air quality,since it connects the living space to an area where harm-ful gases can be found (e.g., combustion products, sol-vents).

The moderately effective joints to seal included thefollowing:

• Top and Bottom Plate-to-Sheathing: This joint wasshown to leak an amount that ranged from 0.074 to0.62 CFM50 per foot of joint (includes the leakage fromboth the top and bottom joints), which results in approx-imately 0.040 to 0.38 ACH50 for the whole-houseresult. This range is driven by differences in how wellthe drywall is sealed to the interior finishes. This mattersbecause of what was stated above—namely, the air leak-age associated with most exterior wall joints must nego-tiate the joint on the exterior skin of the wall (thesheathing layer) and the interior skin of the wall (thedrywall layer). Once the air passes the exterior skinassociated with the plate-to-sheathing joint, it must alsopass interior skin through drywall penetrations (electri-cal outlets and switches, plumbing fixtures, etc.) and ter-minations (bottom of the wall, windows, doors, etc.).However, these terminations can occasionally (and unin-tentionally) be well sealed with caulk by the painter orfinish carpenter for aesthetic reasons. Such sealing,however, is of questionable durability, since such caulksare almost always low performing (i.e., low flexibility,resulting in cracking).

• Bottom Plate-to-Subfloor: Numerous independent mea-surements in this investigation showed the leakage asso-ciated with this joint to be relatively small (0.1 CFM50/fton average and up to 0.1 ACH50 for the whole house). Itis interesting to note that many builders and buildingcode officials expect/require this joint to be sealed, often

Table 3. Summary of Per Unit Leakage Rates

and Projected Whole-House Leakage for the

Various Studied in this Investigation

Joint/Opening CFM501 ACH501

Recessed light 9.1 per light 0.15 to 0.31

Duct boot 7.7 per boot 0.13 to 0.26

Top plate-to-attic 0.29 to 0.68 per foot 0.29 to 1.6

Garage-housecommon wall

0.60 per foot 0.14 to 0.26

Band joist (top andbottom)

0.86 per foot 0.37 to 0.42

Sheathing-to-plate(top and bottom)

0.074 to 0.62 per foot 0.040 to 0.38

Bottom plate-to-subfloor

0 to 0.11 per foot 0 to 0.11

Corners(interior pointing)

0.024 to 0.21 per foot 0.0021 to 0.032

Corners(exterior pointing)

0.054 to 0.45 per foot 0.0069 to 0.11

Window/doorframing-to-sheathing

0.031 to 0.11 per foot 0.020 to 0.10

Between exterior topplates

0.10 to 0.11 per foot 0.033 to 0.046

Vertical sheathingjoints

0.010 to 0.090 per foot 0.011 to 0.11

Sill plate-to-foundation2 0 to 0.030 per foot 0 to 0.025

1 Assumes all other joints in the wall cavity are sealed.2 Assumes the presence of a sill gasket.


ignoring far more important joints. It is possible that thisjoint can be quite large/leaky in some localized caseswhere the wall is constructed on the floor, tilted up intoplace, and potentially have some construction debris(wood chips, fasteners, etc.) lodged between the bottomplate and the subfloor. While this should be uncommonor very localized, the joint should obviously be sealed inthis case (i.e., “If you see daylight, seal it!”).

The least effective joints to seal included the following:

• Corners, Window/Door Framing-to-Sheathing, andBetween Exterior Top Plates: These are three very differ-ent types of joints, but the thing that they have in commonare they are not significant contributors to whole-houseleakage (ACH50) for one or more of the following rea-sons: (1) small amount of overall joint length, (2) not alarge amount of leakage, and (3) the leakage is con-strained to some extent by the drywall, which was men-tioned above. However, the leakage from any one of thesejoints could still cause thermal or acoustic comfort issues.

• Bottom Plate-to-Slab: This study did not make measure-ments for the case where the bottom plate mated with aslab, so this least-effective categorization is an extensionof the low leakage comments above for the bottom plate-to-subfloor joint as well as the reality that the bottomplate-to-slab connection typically has two advantagesrelated to airtightness. One is that the slab is often verysmooth because of the finish flooring requirements foruniformity (e.g., ceramic tile, wood laminate flooring).The other is that this joint is often addressed with an inex-pensive sill gasket, which is sandwiched between the bot-tom plate and the smooth slab for the very purpose of airsealing.

ACKNOWLEDGMENTS

The authors wish to acknowledge the important contribu-tions to this investigation by Mikael Salonvaara of OwensCorning, who consulted with them on many matters through-out the study, including experimental design, troubleshooting,and data analysis.

NOMENCLATURE

C = proportionality constant in the relation Q = CPn. It is calculated by taking the normalizedleakage and dividing by the pressuredifference of 50 Pa to the exponent n. Thepressure difference of 50 Pa is used herebecause this was the pressure differenceassociated with the normalized leakage.

Joint length = appropriate joint length associated with thelength of wall section specified above

n = measured exponent to the pressure differenceacross the joint in the relation Q = CPn. Thisalso was reported in Part 1 for a particular joint.

Normalized leakage = leakage value reported in Part 1 for aparticular joint (also referred topreviously as the leakage severity)

P = pressure

Q = leakage associated with the particularjoint, when a part of the overall wallassembly. It is the product of Q/L andjoint length.

%Q = fraction of the total flow into or out ofthe wall cavity associated with thatparticular joint

Q50 = flow associated with a particular jointif the pressure difference across thejoint were 50 Pa, which is theequivalent of having no drywallpresent for an exterior joint

Q/Q50 = ratio of the joint leakage as a part ofthe wall assembly (Q) to the jointleakage in isolation and unobstructedby a downstream resistance, such asdrywall (Q50)

Q/L = leakage per unit length associatedwith the particular joint when a part ofthe overall wall assembly. It uses therelation Q = C Pn to compute theflow, where the pressure difference iswith respect to the wall cavity.pressure (Pw).

Ref = indicates whether the leakage dataused in that row came from thelaboratory results of this study(denoted ALTA) or the test houseresults (denoted TH)

Sealed? = specifies whether the particular jointshould be considered sealed(indicated with a “y”) or not(indicated with a “n”). If it is sealed,then no flow associated with that jointis included in the calculation. If it isunsealed, then the flow is calculatedbased on the relation Q = C Pn.

Subscripts

i = interior

e = exterior

w = wall

REFERENCES

Wolf, D.H. and Tyler, F.S. 2013. Characterization of AirLeakage in Residential Structures—Part 1: Joint Leak-age. Thermal Performance of the Exterior Envelopes ofWhole Buildings XII International Conference.


characterization of air leakage in residential structures— part … b12... · 2013-10-24 · 2...

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