Rigid Foam Insulation for Existing Exterior Walls

Scope

Right – This foil-faced foam sheathing has taped seams and proper flashing details so it can serve as a drainage plane.
Right – This foil-faced foam sheathing has taped seams and proper flashing details so it can serve as a drainage plane.

In this retrofit wall assembly, the air control layer is applied directly over the existing wall sheathing and then covered by at least two layers of insulating sheathing held in place by vertical furring strips. The vertical furring strips also provide the means of attachment for the exterior siding.

There are two possible locations for the water control layer for this retrofit wall assembly – at the outer face of the insulating sheathing or between the insulating sheathing and the existing exterior wall.

Insulate the walls of an existing home by removing the existing cladding and installing rigid foam insulating sheathing to upgrade the major building enclosure functions of the wall – water control, air control, vapor control, and thermal control, as follows:

  • Remove existing wall cladding and trim.
  • Prepare the wall sheathing for air/water control membrane.
  • Remove windows and doors as needed to allow flashing of openings and air control transitions into openings.
  • Install a continuous air/water control layer to the original sheathing or to the insulation sheathing.
  • Transition the wall air/water control membrane to adjacent assemblies.
  • Install flashings and air control transitions.
  • Re-install windows and doors or install new windows and doors in properly flashed openings.
  • Install insulating sheathing in accordance with code to satisfy the high R-value performance target for the assembly and for adequate condensation control.
  • Install furring strips over insulating sheathing and attach to structure (structural sheathing and/or framing) through insulating sheathing. Install furring strips in a vertical orientation only.
  • Install wall cladding and trim, by attaching to the furring strips.

Description

One method for increasing the insulation level of the walls in existing homes is to remove the exterior cladding and install rigid foam insulation, possibly new house wrap, and new exterior cladding over the walls. This step not only increases the wall R-value, it can also greatly improve the performance of the walls to control the movement of air, vapor, and water through the walls.

Some advantages to this retrofit approach include minimizing the impact of the wall retrofit on the interior finishes of the existing house, supporting continuity of the water and air control layers, reducing thermal bridging or heat transfer through wall framing, and lowering the risk of water damage and condensation within the existing wall structure.

In Figures 1 and 2, the wall siding is represented as lap siding, which could be wood, vinyl, or fiber cement lap siding. Other types of siding that can be attached using the vertical furring strips may be used as well, provided the weight of the siding is less than 10 lb/sf.

Taped insulating sheathing may serve as a layer to control water, air, and heat loss.  However, house wrap or a paint-on membrane could also be applied to the original sheathing before the insulating sheathing is installed.

The insulating sheathing is held in place by vertical furring strips, along with other attachments per manufacturer’s instructions. The vertical furring strips also provide the means of attachment for the exterior siding and provide a ventilation gap under the siding.

The house wrap should be overlapped and taped at all seams and attached with fasteners per the manufacturer’s instructions. (See Figure 2.) If more than one layer of foam is used, the exterior layer of the foam is the water control layer and all seams in that layer should be taped with appropriate foam-compatible tape. The exterior foam layer should be foil-faced polyisocyanurate or XPS and the layer should be at least 1 inch thick. EPS should not be used as the outer layer but EPS Type II could be used as the inner layer. If the exterior of the rigid foam serves as the water control layer, the air control layer would still be house wrap or paint-on membrane that is installed between the existing house sheathing and the rigid foam.

The location of the water control layer has implications for how the water control function is handled at transitions and interruptions. For example, in window installations, if the water control layer is over the existing sheathing, the windows are installed within the existing wall framing. If the water control layer is at the face of the insulating sheathing, the windows are installed within the insulating sheathing layer of the wall, in order to integrate the window flashing into the water control layer.

When installing this retrofit wall assembly, it is recommended that multiple layers of insulating sheathing be applied to the exterior of the existing wall. For example, rather than installing one 2-inch layer of rigid foam, two 1-inch layers are installed. This creates a vapor impermeable layer on the exterior of the existing wall. If there is a wetting event (e.g., a plumbing leak) that causes the existing wall structure to become wet, it is important that the wall is able to dry to the inside. To allow drying to the inside, any new or existing application of closed-cell spray foam in the wall cavities should be limited to a thickness of no more than one inch. For other types of spray foam insulation, the vapor permeance of the installed thickness must be at least 1.0 perm. Open-cell spray foam insulation meets this criteria for typical framed wall cavity depths. This may not be the case for some medium-density spray foams.

It is advisable to avoid use of an interior vapor barrier and vapor impermeable wall coverings such as non-latex paint and vinyl wallpapers with this retrofit wall approach because these limit the drying potential to the interior. These types of wall coverings may contribute to problems if the interior space has high humidity, there is a history of water leakage in the wall, or the exterior wall has gotten wet during construction.

Insulating sheathing is installed on exterior of an existing framed wall with water control between existing sheathing and insulating sheathing

Figure 1. Insulating sheathing is installed on exterior of an existing framed wall with water control between existing sheathing and insulating sheathing.

Insulating sheathing is installed over existing wall sheathing in a wall retrofit to improve insulation, air, and water control. The water control layer is at face of insulating sheathing, which is foil faced and has taped seams

Figure 2. Insulating sheathing is installed over existing wall sheathing in a wall retrofit to improve insulation, air, and water control. The water control layer is at face of insulating sheathing, which is foil faced and has taped seams.

How to Install Rigid Foam Board Insulation at Exterior Wall

  1. Remove the existing wall cladding and trim, and inspect the structural integrity of the wall. Check the wall framing for any deficiencies, rot, insect damage, etc.  Based on the findings of the inspection, revise the wall assembly plans and review specific detailing as needed. Proceed only after needed repairs are performed. Meet or exceed the minimum requirements of the current adopted building and energy codes.
  2. Prepare the wall sheathing to receive the air/water control membrane or house wrap, if one is going to be used. Appropriate preparation of the wall sheathing will depend upon the nature of the existing sheathing and the air control strategy pursued. If using a sheet good (house wrap) as the air/water control layer, all protruding fasteners must be removed to avoid punctures or tears in the membrane. Gaps or voids in the sheathing layer may need to be filled in. Use a primer for self-adhered membranes if recommended by the membrane manufacturer if installing membrane.
  3. Remove windows and doors and trim to allow for proper flashing of the window and door openings, and to permit the installation of air control transition membranes.
  4. Install a continuous air control membrane (house wrap or paint-on membrane) over the existing sheathing as shown in Figure 1. Connect the air control membrane to the air control layer of adjacent assemblies in a tight and durable manner. Seal all penetrations against air and water leaks. This layer may also serve as the water control layer, or the surface of the rigid foam can serve as the water control layer if all seams are sealed, as shown in Figure 2.
  5. Install flashings and air control transitions. Transition the air control at the top and bottom margins of the exterior wall into the window and door rough openings and air seal all penetrations through the wall. Flash window and door rough openings as well as all wall penetrations.
  6. Re-install windows and doors or install new windows and doors in properly flashed openings. If the water control layer is over the existing sheathing, the windows are installed within the existing wall framing whereas if the water control layer is at the face of the insulating sheathing, the windows are installed within the insulating sheathing layer of the wall. This is to ensure the window flashing is properly integrated into the water control layer. Air seal the window and door units to the air control transition membranes at the interior perimeter of the window and door units.
  7. Install insulating sheathing over the air/water control membrane. Butt joints tight. When installing multiple layers, offset seams in two directions. If the surface of the rigid foam will serve as the water control layer, then the exterior layer of rigid foam cannot be EPS, it must be foil-faced polyisocyanurate or XPS and all seams of the exterior foam layer must be taped. Until furring strips are installed, insulating sheathing pieces can be held in place with cap nails or screws with roofing washers.
  8. Install furring strips over insulating sheathing and attach furring strips to the wall structure (structural sheathing and/or framing) through the insulating sheathing. Install furring strips in a vertical orientation only. It is important to install furring strips in a vertical, not horizontal, orientation to allow drainage behind the cladding/trim and to prevent water from dwelling within the system. 1x4 furring is recommended. The furring need not be preservative treated for moisture protection. The spacing of fasteners through the furring strips must be such that the cladding load is distributed to no more than 10 lb per fastener.
  9. Attach cladding and trim to the vertical furring strips.
  10.  

Ensuring Success

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.

Remediate any hazardous conditions that will be affected (e.g., exposed or aggravated) by the planned work. Follow applicable laws and industry procedures for mitigation of hazardous materials. Engage the services of a qualified professional when needed.

Given the increased airtightness associated with this retrofit, combustion safety testing 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 zones

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.

Table 1. Minimum Wall R-Values Required by 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

[Note: Guidance for Version 3.0, Revision 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 3.0, Revision 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.

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.

The ENERGY STAR Water Management System Builder Checklist specifies:

2 Water-Managed Wall Assembly

2.1 Flashing at bottom of exterior walls with weep holes included for masonry veneer and weep screed for stucco cladding systems, or equivalent drainage system.

2.2 Fully sealed continuous drainage plane behind exterior cladding that laps over flashing in Item 2.1. Additional bond-break drainage plane layer provided behind all stucco and non-structural masonry cladding wall assemblies.

2.3 Window and door openings fully flashed.

DOE Zero Energy Ready Home

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 conducting retrofits are welcome to seek certification for existing 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.

IECC 2009

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

IRC 2009

Section R302.1 Exterior walls

Table R302.1 Exterior walls

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

Section R316 Foam plastic

Section R403.3.4 Termite damage

Section R703 Exterior covering.

Section R703.11.2 Foam plastic sheathing

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

IECC 2012

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

IRC 2012

Section R302.1 Exterior walls

Table R302.1 Exterior walls

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

Section R316 Foam plastic

Section R403.3.4 Termite damage

Section R703 Exterior covering.

Section R703.11.2 Foam plastic sheathing

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

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): PNNL
    Organization(s): PNNL
    Publication Date: December, 2011

    Case study about a deep energy renovation of a home in the marine climate.

  2. Author(s): BSC
    Organization(s): BSC
    Publication Date: November, 2013

    Case study describing optimal design and limitations of cladding attachments over exterior sheathings.

  3. Author(s): BSC
    Organization(s): BSC
    Publication Date: October, 2013

    Case study describing research about using exterior, vapor permeable insulation of retrofit walls with vapor permeable cavity insulation.

References and Resources*

  1. Author(s): ICC
    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): ICC
    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): ICC
    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): ICC
    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): Pettit, Neuhauser, Gates
    Organization(s): BSC
    Publication Date: July, 2013
    Guidebook providing useful examples of high performance retrofit techniques for the building enclosure of wood frame residential construction in a cold and somewhat wet climate.

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: 12/11/2015

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