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Spray Foam Insulation for Cavities of Existing Exterior Walls

Scope

Insulate the walls of an existing home with spray foam insulation to increase thermal performance. Remove interior finishes (e.g., gypsum board) from exterior walls and fill the wall cavities with spray foam insulation, while keeping wall sheathing, house wrap, and cladding intact as follows:

  • Inspect the exterior walls and make any necessary repairs to the water control layer/drainage plane prior to insulating the wall cavities.
  • Retain the existing cladding and trim.
  • Remove interior finishes such as lath and plaster or gypsum board, and existing insulation in the wall cavity.
  • Inspect open wall cavities for evidence of bulk water leaks, and rebuild or improve exterior cladding or flashing detail as indicated by the leak evidence.
  • Remove windows and doors as needed to allow flashing of rough openings, and air control transitions into openings.
  • Install flashings and air control transitions. Re-install windows and doors or install new windows and doors in properly flashed openings. For guidance on water management of existing windows, see Window Rehabilitation and Window Replacement.
  • Install cavity insulation per project-specific details and manufacturer’s recommendations.
  • Install new gypsum board.

For more on exterior wall spray foam, see the U.S. Department of Energy’s Standard Work Specifications.

See the Compliance Tab for related codes and standards requirements, and criteria to meet national programs such as DOE’s Zero Energy Ready Home program, ENERGY STAR Certified Homes, and Indoor airPLUS.

Description

Older homes often have poorly insulated exterior walls that have numerous air leaks around piping, wiring holes, windows, etc. One way to insulate and air seal the exterior walls at the same time is to remove the interior finishes and fill the wall cavities with spray foam insulation.

Another approach that can greatly increase the walls’ R-value is to construct a second stud wall a few inches inboard of the existing stud wall, to create an extra-deep wall cavity (i.e., converting the existing wall to a “double-stud wall”).  This cavity can be insulated with spray foam insulation alone, spray foam insulation plus fibrous insulation (blown cellulose, blown fiberglass or batt), or only fibrous insulation. These options are described in this guide.

One consequence of insulating existing walls is that less heat will flow through the assembly: therefore, less drying of the wall assembly will occur. If any water has been getting into the wall, adding insulation without first fixing the leaks increases the risk of moisture damage to the walls. The exterior walls should be inspected for the presence and condition of the water control layer/drainage plane. This inspection could include demolition and inspection from the interior, to find existing water leakage issues, or selective disassembly of the exterior of the wall.

In new construction, the recommended method for making up for the reduced drying potential is to put an air gap between the home’s exterior siding and the water control layer (e.g., house wrap) to allow for drainage, water redistribution, and drying (see BSI-038: Mind the Gap, Eh!).In older homes, it is unlikely that an air gap was installed behind the cladding, and adding a gap would require removing the exterior siding.

The reduced drying associated with increased wall insulation levels has varying impacts on various cladding systems and sheathing. These potential impacts and recommended approaches for dealing with them are as follows:

  • Wood clapboards - paint durability on the wood siding may be affected. If paint appears to be peeling, sliding wedges between the clapboards may improve ventilation between the cladding and the water control layer. If wedges do not improve the back-venting, the cladding should be replaced and installed over spacers, i.e., furring strips (see BSI-028: Energy Flow Across Enclosures).
  • Cedar shingles – this type of cladding is less likely to experience issues. If it does, however, the shingles should be replaced and installed over a polypropylene spacer mesh that provides an air gap (see BSI-038: Mind the Gap, Eh! and BSI-057: Hockey Pucks and Hydrostatic Pressure).
  • Stucco – this is a high-risk cladding, as it is likely an airtight cladding system with low vapor permeance, and thus low drying to the exterior (see BSI-029: Stucco Woes—The Perfect Storm). At a minimum window and door openings (and any wall penetrations) should be flashed to prevent water leakage issues.  Retrofitting a stucco-clad wood frame wall by installing cavity spray foam is not recommended because of the risk prolonged dampness should any water enter the walls.
  • Aluminum or vinyl – these cladding types are relatively safe for this retrofit approach, as these systems have built-in air gaps and are effectively “self-ventilated” claddings.
  • Brick – brick is relatively safe for this retrofit approach because properly constructed brick walls have an air gap behind the brick these systems typically include a built-in air gap, allowing back-ventilation.  This assumes that the brick veneer is properly ventilated (i.e., cavity is not filled with mortar) and that the brick is correctly detailed with flashing and weep holes.  Rope weeps, if present, should be removed and replaced with open or screened head joints.
  • OSB Sheathing – retrofitting closed-cell spray foam (ccSPF) into a wall cavity with OSB sheathing and no cladding ventilation raises the risks to the sheathing, as ccSPF will limit inward drying.  Risk can be gauged based on exposure and cladding type, but the safest approach is to remove the cladding and add ventilation ventilating air gap (see BSI-038: Mind the Gap, Eh!) Plywood and board sheathing are less vulnerable than OSB.

If the project requires removal of cladding, options for adding wall insulation from the exterior, such as installing rigid foam insulation, should be considered. This will create a new water control system on the exterior of the wall, addressing the moisture risks described above.  When adding exterior rigid foam insulation, new cavity insulation can also be added, but lower-cost fibrous insulation may be more cost-effective than spray foam to fill the cavities.

Filling Existing Exterior Walls with Spray Foam Insulation

This retrofit assembly retains the existing stud wall and its cladding. The interior finish (e.g., drywall) and any existing insulation are removed. The windows, doors, and trim are removed and the existing water and air control layer (the house wrap or building paper) is connected to the window and door rough openings (and wall penetrations) with self-adhered flexible flashing. The existing stud wall is insulated with open-cell spray foam (ocSPF) (Figure 1), closed-cell spray foam (ccSPF) (Figure 2), or with a hybrid approach using ccSPF and fibrous insulation (loose-fill fiberglass or cellulose or batt) (not shown).

Spray foam is an excellent air barrier material, but spray foam insulated walls can still have air leaks at wood-to-wood framing connections where there is no spray foam, such as doubled/ganged studs or framed corners. In Figure 3 infrared thermography during depressurization testing reveals air leakage at the corner of the wall. This problem is solved by applying sealant at all wood-to-wood interfaces from the interior (e.g., double studs, corners, headers, sill plate-to-floor).

Existing stud wall filled with open cell spray foam cavity insulation
Figure 1. Existing stud wall filled with open cell spray foam cavity insulation. (Image courtesy of Building Science Corporation).

Existing stud wall filled with closed-cell spray foam cavity insulation
Figure 2. Existing stud wall filled with closed-cell spray foam cavity insulation. (Image courtesy of Building Science Corporation).

Infrared thermography during depressurization testing reveals air leakage at corner of spray foam-insulated room where wood-to-wood seams in framing were not air sealed
Figure 3. Infrared thermography during depressurization testing reveals air leakage at corner of spray foam-insulated room where wood-to-wood seams in framing were not air sealed. (Image courtesy of Building Science Corporation).

Converting Existing Exterior Wall to a Double Wall and Insulating with Spray Foam Insulation

Another option that can greatly increase the insulation value of the wall is to construct a new stud wall inboard of the existing stud wall to create a deeper cavity for insulation.  With this approach, the drywall and original insulation are removed, then a second stud wall is built inboard of the original wall. Then the cavity is filled with ccSPF (Figure 4) or ocSPF (Figure 5) or a hybrid approach is employed using ccSPF plus loose-fill fibrous insulation (Figure 6).

The ccSPF (Figure 4) only partially fills the cavity with insulation, due to the higher cost of ccSPF, higher R-value per inch, and time required to “shave” the foam flush with the studs.  The ocSPF (Figure 5) insulation fills the cavity to full depth then is trimmed flush with the framing.

With the hybrid approach (ccSPF plus loose-fill fibrous insulation, Figure 6), the ccSPF prevents condensation (from interior sources) by warming the temperature of the potential condensing surface.  In colder climates, it is important to maintain a sufficient ratio of vapor impermeable insulation (ccSPF) to total wall assembly insulation.  As outdoor temperatures get colder, the amount of insulation needed to maintain the sheathing temperature increases.

Guidance for the ratios of ccSPF to loose-fill fibrous insulation can be found in the fourth column of Table 1. The table presents information taken from Table R601.3.1 Class III Vapor Retarders of the 2009 IRC (ICC 2009a) and Table R702.7.1 Class III Vapor Retarders of the 2012 IRC (ICC 2009b). It shows the minimum thermal resistance values to control condensation using rigid foam installed over or as the insulating sheathing, for Climate Zones 5, 6, 7, 8 and Marine 4. These values can also be used for ccSPF installed on the interior side of the sheathing. Column 4 of the table lists the percentage of the total wall insulation that is exterior insulation that should be vapor-impermeable insulation rather than versus fibrous insulation. This percentage applies to exterior rigid foam and can also apply to ccSPF installed on the interior side of the sheathing. 

Thermal resistance values to control condensation using vapor-impermeable exterior insulating sheathing (or closed-cell spray foam installed on interior side of sheathing),, for climate zones 5, 6, 7, 8 and marine 4 from (2009 IRC and 2012 IRC).
Table 1. Thermal resistance values to control condensation using vapor-impermeable exterior insulating sheathing (or closed-cell spray foam installed on interior side of sheathing),, for climate zones 5, 6, 7, 8 and marine 4 from (2009 IRC and 2012 IRC).

The assembly is finished with new gypsum board. The ccSPF (Figure 4) and hybrid (Figure 6) walls have sufficient vapor control that only latex paint (Class III vapor retarder) is required on the interior. In the ocSPF (Figure 5) wall, a Class II vapor retarder (e.g., vapor retarder paint) is recommended; a Class I vapor barrier (e.g., polyethylene) is not recommended.  See BA-1501: Monitoring Double-Stud Wall Moisture Conditions in the Northeast.

Single framed wall converted to double wall and insulated with closed-cell spray foam
Figure 4. Single framed wall converted to double wall and insulated with closed-cell spray foam.

Single framed wall converted to double wall and insulated with open-cell spray foam
Figure 5. Single framed wall converted to double wall and insulated with open-cell spray foam.

Single framed wall converted to double wall and insulated with closed-cell spray foam and loose-fill fibrous insulation
Figure 6. Single framed wall converted to double wall and insulated with closed-cell spray foam and loose-fill fibrous insulation.

How to Insulate Existing Walls with Spray Foam Insulation

  1. Retain existing wall cladding and trim. Repair the cladding as needed. See information above for potential moisture impacts to claddings systems when insulation is added to the wall.
  2. Remove the existing interior finishes such as lath and plaster or gypsum board. Remove existing wall cavity insulation. “Tee” intersection interior walls should have finishes removed to at least the depth of the exterior wall insulated cavity. Gaps or voids in the sheathing layer may need to be filled. The cavity should be free of dust and debris.
  3. Remove ceiling drywall or interior finish near exterior walls to allow access for insulation and air sealing to exterior wall section corresponding to floor-above section.
  4. Inspect open wall cavities for evidence of bulk water leaks and redo or improve exterior cladding or flashing detail as indicated by the leak evidence.
  5. Remove windows and doors as needed to allow flashing of rough openings, and water and air control transitions into openings.
  6. Transition the air control layer into window and door rough openings and air seal all wall penetrations. Flash window and door rough openings and all wall penetrations. Re-install windows and doors (or install new windows and doors) in properly flashed openings. Air seal window and door units to the air control transition membranes at the interior perimeter of window and door units.
  7. Install insulation in the wall cavity per project specific details and manufacturer’s recommendations. Locate the electrical and plumbing services prior to installing insulation in the cavity.  Avoid plumbing pipes in exterior walls in cold climates. If pipes cannot be moved, increase insulation value on the exterior side of the pipe with spray foam or rigid foam. Insulation outboard of the pipes makes them warmer in winter; insulation inboard of the pipes makes them colder. ALTERNATELY, convert the exterior wall to a double wall by constructing a new stud wall inboard of the existing wall. Build the deeper window and door openings ensuring a continuous air barrier and proper water flashing details. (See Air Control Upgrade for Existing Walls with Interior Retrofit for more details.) Remove or build around obstructions such as fireplaces and built-in cabinets.
  8. Install new gypsum on the walls and new gypsum and/or crown molding on the ceiling to replace ceiling surfaces cut away to access the wall.

Ensuring Success

Remediate any hazardous conditions that will be affected (e.g., exposed or aggravated) by the planned work. Examples of hazardous materials that may be found in wall assemblies of existing structures include (but are not limited to) lead, asbestos, mold, animal dropping/remains, etc.  Follow applicable laws and industry procedures for mitigation of hazardous materials.  Engage the services of a qualified professional when needed.

Inspect the existing cladding, water control layer, windows, and doors for active water leaks. Install new flashing as needed and connect it to the existing water control layer.

Spray foam is a material that is essentially “manufactured” when applied at the building site.  Given the importance of this material’s performance, quality control measures should be set in place.  Some key issues include moisture content and temperature of the substrate, applied spray foam layer or “lift” thickness, ratios of the two spray foam components during application, and storage/handling of spray foam components.  Further information is available at the Spray Polyurethane Foam Alliance.

Refer to the current adopted building and energy codes for information on appropriate levels of insulation for the different climate zones as well as the proper ratios of vapor and air impermeable and permeable insulation.

Given the increased airtightness associated with this retrofit, combustion safety and controlled mechanical ventilation upgrades are required to maintain acceptable indoor air quality.

Climate

The exterior wall assembly should be designed for a specific hygrothermal region, rain exposure zone, and interior climate.  The climate zones are shown on the map below, which is taken from Figure C301.1 of the 2012 IECC.

IECC climate zone map
IECC Climate Zone Map

The insulation levels should be based on the minimum requirements for vapor control in the current adopted building code and the minimum requirements for thermal control in the current energy code.  Additional insulation can be added above these minimums to create high R-Value exterior wall assemblies. The table below provides the minimum thermal resistance (R-value) requirements for exterior walls specified in the 2009 IECC (ICC 2009b) and the 2012 IECC (ICC 2012b), based on climate zone.

Wall Insulation Requirements per the 2009 and 2012 IECC
Table 1. Wall Insulation Requirements per the 2009 and 2012 IECC

Training

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Compliance

The Compliance tab contains both program and code information. Code language is excerpted and summarized below. For exact code language, refer to the applicable code, which may require purchase from the publisher. While we continually update our database, links may have changed since posting. Please contact our webmaster if you find broken links.

ENERGY STAR Certified Homes  

[Guidance for Version 3.0, Rev 08 is coming soon.]

ENERGY STAR Certified Homes is a voluntary high-performance home labeling program for new homes operated by the U.S. Department of Energy and the U.S. Environmental Protection Agency. Builders and remodelers who are conducting retrofits are welcome to seek certification for existing homes through this voluntary program.

ENERGY STAR Certified Homes (Version 3, Rev. 07) requires that ceiling, wall, floor, and slab insulation levels meet or exceed those specified in the 2009 International Energy Conservation Code (IECC).

If the state or local residential building energy code requires higher insulation levels than those specified in the 2009 IECC, you must meet or exceed the locally mandated requirements. Some states have adopted the 2012 IECC. Visit the U.S. DOE Building Energy Codes Program to see what code has been adopted in each state. For states that have adopted the 2012 IECC or an equivalent code, EPA intends to implement the ENERGY STAR Certified Homes Version 3.1 National Program Requirements for homes permitted starting one year after state-level implementation of the 2012 IECC or an equivalent code. However, EPA will make a final determination of the implementation timeline on a state-by-state basis. Some states and regions of the country have ENERGY STAR requirements that differ from the national requirements. Visit ENERGY STAR’s Regional Specifications page for more information on those region-specific requirements.

The ENERGY STAR Thermal Enclosure System Rater Checklist (Ver 3, Rev 07) specifies:

2.1 Ceiling, wall, floor and slab insulation levels shall comply with one of the following options:

2.1.1 Meet or exceed 2009 IECC levels, OR

2.1.2 Achieve <= 133% of the total UA resulting from the U-factors in 2009 IECC Table 402.1.3, excluding fenestration and per guidance in note “d” below, AND home shall achieve <= 50% of the infiltration rate in Exhibit 1 of the National Program Requirements.

2.2 All ceiling, wall, floor, and slab insulation shall achieve RESNET-defined Grade I installation or, alternatively, Grade II for surfaces that contain a layer of continuous, air impermeable insulation ≥ R-3 in Climate Zones 1 to 4, ≥ R-5 in Climate Zones 5 to 8

3 Fully-Aligned Air Barriers. At each insulated location noted below, a complete air barrier shall be provided that is fully aligned with the insulation as follows:

  • At interior or exterior surface of ceilings in Climate Zones 1-3; at interior surface of ceilings in Climate Zones 4-8. Also, include barrier at interior edge of attic eave in all climate zones using a wind baffle that extends to the full height of the insulation. Include a baffle in every bay or a tabbed baffle in each bay with a soffit vent that will also prevent wind washing of insulation in adjacent bays
  • At exterior surface of walls in all climate zones; and also at interior surface of walls for Climate Zones 4-8 7
  • At interior surface of floors in all climate zones, including supports to ensure permanent contact and blocking at exposed edge

3.1 Walls

(10) All insulated vertical surfaces are considered walls (e.g., above and below grade exterior walls, knee walls) and must meet the air barrier requirements for walls, with the exception of adiabatic walls in multifamily dwellings.  All insulated ceiling surfaces, regardless of slope (e.g., cathedral ceilings, tray ceilings, conditioned attic roof decks, flat ceilings, sloped ceilings), must meet the requirements for ceilings.

4.4 Reduced thermal bridging at above-grade walls separating conditioned from unconditioned space (rim / band joists exempted) using one of the following options:

4.4.1 Continuous rigid insulation, insulated siding, or combination of the two; ≥ R-3 in Climate Zones 1 to 4, ≥ R-5 in Climate Zones 5 to 8, OR;

4.4.2 Structural Insulated Panels (SIPs), OR;

4.4.3 Insulated Concrete Forms (ICFs), OR;

4.4.4 Double-wall framing 16, OR;

4.4.5 Advanced framing, including all of the items below:

4.4.5a All corners insulated ≥ R-6 to edge 17, AND;

4.4.5b All headers above windows & doors insulated 18, AND;

4.4.5c Framing limited at all windows & doors 19, AND;

4.4.5d All interior / exterior wall intersections insulated to the same R-value as the rest of the exterior wall 20, AND;

4.4.5e Minimum stud spacing of 16 in. o.c. for 2x4 framing in all Climate Zones and, in Climate Zones 5 through 8, 24 in. o.c. for 2x6 framing

5.2 Cracks in the building envelope fully sealed

DOE Zero Energy Ready Homes

The DOE Zero Energy Ready Home Program is a voluntary high-performance home labeling program for new homes operated by the U.S. Department of Energy. Builders and remodelers who are performing retrofits on existing homes are welcome to seek certification for those homes through this voluntary program.

The U.S. Department of Energy Zero Energy Ready Home National Program Requirements specify as a mandatory requirement (Exhibit 1, #2.2) that, for all labeled homes, whether prescriptive or performance path, ceiling, wall, floor, and slab insulation shall meet or exceed 2012 IECC levels. See the guide 2012 IECC Code Level Insulation – DOE Zero Energy Ready Home Requirements for more details.

2009 - 2018 IECC and IRC Minimum Insulation Requirements: The minimum insulation requirements for ceilings, walls, floors, and foundations in new homes, as listed in the 2009, 2012, 2015, and 2018 IECC and IRC, can be found in this table.

2009 IECC

Section 401.3 Certificate

Section 402.1.1 Insulation and fenestration criteria

Table 402.1.1 Insulation and fenestration requirements by component

Table 402.1.3 Equivalent U-factors

Section 402.4 Air leakage (Mandatory)

Table 402.4.2 Air barrier and insulation inspection component criteria

2012 IECC

Section R401.3 Certificate

Section R402.1.1 Insulation and fenestration criteria

Table R402.1.1 Insulation and fenestration requirements by component

Table R402.1.3 Equivalent U-factors

Section R402.4 Air leakage (Mandatory)

Table R402.4.1.1 Air barrier and insulation installation

2015 and 2018 IECC

Section R401.3 Certificate

Section R402.1.2 Insulation and fenestration criteria

Table R402.1.2 Insulation and fenestration requirements by component

Table R402.1.4 Equivalent U-factors

Section R402.4 Air leakage (Mandatory)

Table R402.4.1.1 Air barrier and insulation installation

Retrofit: 200920122015, and 2018 IECC

Section R101.4.3 (Section R501.1.1 in 2015 and 2018 IECC). Additions, alterations, renovations, or repairs shall conform to the provisions of this code, without requiring the unaltered portions of the existing building to comply with this code. (See code for additional requirements and exceptions.)

2009 IRC

Section R302.10 Flame spread index and smoke developed index for insulation

Section N1101.4 Building thermal envelope insulation

Section N1101.6 Insulation product rating

Section N1101.9 Certificate

Section N1102.1 Insulation and fenestration criteria

Table N1102.1 Insulation and fenestration requirements by component

Table N1102.1.2 Equivalent U-factors

Section N1102.4 Air leakage

Table N1102.4.2 Air barrier and insulation inspection

2012 IRC

Section R302.10 Flame spread index and smoke developed index for insulation

Section N1101.12.1 (R303.1.1) Building thermal envelope insulation

Section N1101.12.4 (R303.1.4) Insulation product rating

Section N1101.16 (R401.3) Certificate (Mandatory)

Section N1102.1.1 (R402.1.1) Insulation and fenestration criteria

Table N1102.1.1 (R402.1.1) Insulation and fenestration requirements by component

Table N1102.1.3 (R402.1.3) Equivalent U-factors

Section N1102.4 (R402.4) Air leakage (Mandatory)

Table N1102.4.1.1 (R402.4.1.1) Air barrier and insulation installation

2015 and 2018 IRC

Section R302.10 Flame spread index and smoke-developed index for insulation

Section N1101.10.1 (R303.1.1) Building thermal envelope insulation

Section N1101.10.4 (R303.1.4) Insulation product rating

Section N1101.14 (R401.3) Certificate (Mandatory)

Section N1102.1.2 (R402.1.2) Insulation and fenestration criteria

Table N1102.1.2 (R402.1.2) Insulation and fenestration requirements by component

Table N1102.1.4 (R402.1.4) Equivalent U-factors

Section N1102.4 (R402.4) Air leakage (Mandatory)

Table N1102.4.1.1 (R402.4.1.1) Air barrier and insulation installation

Retrofit: 200920122015, and 2018 IRC

Section N1101.3 (Section N1107.1.1 in 2015 and 2018 IRC). Additions, alterations, renovations, or repairs shall conform to the provisions of this code, without requiring the unaltered portions of the existing building to comply with this code. (See code for additional requirements and exceptions.)

Appendix J regulates the repair, renovation, alteration, and reconstruction of existing buildings and is intended to encourage their continued safe use.

More Info.

Access to some references may require purchase from the publisher. While we continually update our database, links may have changed since posting. Please contact our webmaster if you find broken links.

Case Studies

  1. Author(s): BSC
    Organization(s): BSC
    Publication Date: March, 2010

    Case study describing a retrofit project in the cold and very-cold climate zones.

References and Resources*

  1. Author(s): International Code Council
    Organization(s): ICC
    Publication Date: January, 2009

    Code establishing a baseline for energy efficiency by setting performance standards for the building envelope (defined as the boundary that separates heated/cooled air from unconditioned, outside air), mechanical systems, lighting systems and service water heating systems in homes and commercial businesses.

  2. Author(s): International Code Council
    Organization(s): ICC
    Publication Date: January, 2009

    Code for residential buildings that creates minimum regulations for one- and two-family dwellings of three stories or less. It brings together all building, plumbing, mechanical, fuel gas, energy and electrical provisions for one- and two-family residences.

  3. Author(s): International Code Council
    Organization(s): ICC
    Publication Date: January, 2012

    Code establishing a baseline for energy efficiency by setting performance standards for the building envelope (defined as the boundary that separates heated/cooled air from unconditioned, outside air), mechanical systems, lighting systems and service water heating systems in homes and commercial businesses.

  4. Author(s): International Code Council
    Organization(s): ICC
    Publication Date: January, 2012

    Code for residential buildings that creates minimum regulations for one- and two-family dwellings of three stories or less. It brings together all building, plumbing, mechanical, fuel gas, energy and electrical provisions for one- and two-family residences.

  5. Author(s): Loomis, Pettit
    Organization(s): Building Science Corporation
    Publication Date: May, 2015

    This Measure Guideline provides design and construction information for a deep energy enclosure retrofit solution of a flat roof assembly.

Contributors to this Guide

The following authors and organizations contributed to the content in this Guide.

Building Science Corporation, lead for the Building Science Consortium (BSC), a DOE Building America Research Team

Last Updated: 11/30/2015