January 15, 2015

Abstract: 

This Measure Guideline describes a deep energy enclosure retrofit (DEER) solution that provides insulation to the interior of the wall assembly with the use of a double stud wall. The guide describes two approaches to retrofitting the existing walls: one involving replacement of the existing cladding, and the other that leaves the existing cladding in place. It discusses the design principles related to the use of various insulation types, and provides strategies and procedures for implementing the double stud wall retrofit. It also evaluates important moisture-related and indoor air quality measures that need to be implemented to achieve a durable, high performance wall.

 1 Introduction

This Measure Guideline describes a deep energy enclosure retrofit (DEER) solution that involves insulation to the interior of the wall assembly, with the use of a double stud wall. The guide describes two approaches to retrofitting the existing the walls: one involving replacement of the existing cladding, and the other that leaves the existing cladding in place. It discusses the design principles related to the use of various insulation types and provides strategies and procedures for implementing the double stud wall retrofit. It also evaluates important moisture-related and indoor air quality measures that need to be implemented to achieve a durable, high performance wall.

An interior retrofit is typically less desirable than an exterior retrofit; exterior retrofits allow complete inspection and renovation of exterior moisture control detailing and layers. Interior retrofits are disruptive to the living space and its occupants, and the interior double stud wall reduces usable space. However, despite the advantages of exterior retrofits, many buildings must be retrofitted on the interior, for reasons such as historic preservation, zoning or space restrictions, or aesthetics. The double-stud wall approach is also suitable if a full replacement of services within the exterior wall is planned.

An interior retrofit may or may not be less expensive, depending on such factors as access and whether a gut rehabilitation is planned. However, once an interior retrofit has been chosen, double stud walls can be more cost effective than using exterior insulating sheathing. Double stud walls insulated with cellulose or low-density open cell spray foam can have R-values of 40 or higher. They have been used in high performance housing since the 1970s: their advantages include trade familiarity with construction detailing (especially at the exterior), and the use of commonly available construction materials (Ueno 2014).

However, double stud walls have a higher risk of interior-sourced condensation moisture damage, when compared with high-R approaches using exterior insulating sheathing. There are specific moisture control principles that must be followed for a successful double stud wall insulated retrofit. Controlling bulk water entry into the wall and maintaining certain levels of relative humidity (RH) are factors of vital importance.

Loose-fill fibrous insulation, such as cellulose, is a common choice for double-stud walls due to its lower cost (in most markets). However, cellulose is an air-permeable insulation, unlike spray foams, raising moisture risks (Ueno 2014).

A lower risk approach involves spraying insulating foam in the entire double stud wall cavity. Low-density, semi-permeable open cell spray polyurethane foam (ocSPF) is a good choice; in monitored data, sheathing moisture contents are lower than those in cellulose walls.

Another assembly option is to combine spray foam with fibrous, air-permeable insulation (fiberglass or cellulose) to create a lower cost "hybrid" high-R wall assembly. The common name for this assembly is "flash and batt" (Maines 2011). High-density, closed cell polyurethane spray foam (ccSPF) is generally recommended for this application. The relative thickness of the foam layer is a function of wintertime condensation control, and thus local climate.

This Measure Guideline is important to the high performance retrofit industry because it demonstrates techniques for implementing interior double stud walls in high performance enclosure retrofits.  The information included in the guide is based on the latest research performed for the Department of Energy (DOE) Building America program.

This Measure Guideline is intended to support contractors implementing an interior insulation high performance enclosure retrofit as well as designers looking to design such retrofits. The Measure Guideline may also be helpful to building owners wishing to learn more about strategies available for deep energy enclosure retrofit of wood-framed residential buildings.

2 Decision Making Criteria

 

This section discusses the major decision making criteria once an interior retrofit has been chosen, after considering issues such as aesthetics, historic significance, improved comfort, and the lifespan of the project.

Cost and Performance

Cost and performance are intricately linked and have to be studied in combination, to determine the best choice, per the decision-maker’s goals and objectives. The decision on the thermal performance depends on the specific requirements of the project.  For projects that would like to meet low energy use targets, a higher level of insulation must be provided.

The values provided in Table 1 illustrate costs for a 12” deep double stud wall; however, the depth will vary depending on the project’s thermal performance goals, budget, and acceptable square footage loss. Whether the wall is 12” deep or 8” deep, the cost of the new 2x4 stud wall @ 16” o.c. is fixed; however, benefit of higher levels of insulation must be evaluated. The depth of the wall will also have an impact on the size of the living space; therefore, homeowners need to assess the trade off between providing a higher R-value in the wall and decreasing the size of the floor area. Another aspect is the cost of the interior finishes at the window wells; deeper walls will have higher trim costs. One way to reduce the cost of deeper window wells is to use gypsum board jamb and head returns, as demonstrated by a New England-area high performance builder (Ueno et al. 2013).

Installation costs for the retrofit solutions in this Measure Guideline can vary widely, depending on factors such as contractor experience, prevalent region practices, material costs, and the particular circumstances of the project. The majority of the cost values for the various wall components were obtained from RSMeans Reed Construction Data 2013 (Reed 2013), a cost-estimating tool, which provides the cost of materials, installations as well as overhead and profit.  The values shown are for a project based in Boston, MA.

Table 1. RS Means Unit Costs for Double Stud Wall Retrofit Components ($/sf)

Material DescriptionNominal R-ValueCost Range ($/sf)
Housewrap-0.33
2x4 wood stud wall @ 16" o.c.-3.83
12" dense pack cellulose422.78
12" blown fiberglass421.80
12" open cell spray foam (ocSPF)465.28
4.5" closed cell spray foam (ccSPF)273.97
7.5" dense pack cellulose261.52
7.5" blown fiberglass261.07
1/2" gypsum board (level 4 finish)-1.60

Other items such as cladding, cladding fasteners, spacers, self-adhered membrane flashings, and metal flashings, are omitted from the table as the material use will vary based on individual projects.

Water Leakage Management

Replacing (or not replacing) the cladding and the water membrane can have a significant effect on the durability of the wall. Adding additional interior insulation (in the form of a double stud wall) reduces heat flow through the wall, and thus ability to dry. Therefore, previously survivable water leaks might cause long-term damage after the retrofit of insulation (Lstiburek 2008b).

Deciding whether to replace the cladding should be evaluated based on the existing conditions and available project budget. One key assessment is to partially disassemble the wall, to determine the presence and condition of the water control membrane (“drainage plane”). If the drainage plane is not present or deteriorated, the cladding should be replaced.  In addition, the existing cladding should be carefully inspected to ensure that are no active water leaks into the wall; this might be done by inspection of the exterior sheathing from the interior (part of the interior retrofit project).

Replacing the water control membrane (“drainage plane”) and installing new cladding with an air gap (“drained and ventilated cavity”) will greatly enhance the durability of the wall due to greater drainage and ventilation drying (see Lstiburek 2013 for discussion and details). It would generally be a prudent step in any double stud insulation retrofit.

Air Leakage Management

It is highly risky to design a retrofit that allows significant air leakage; therefore, the air leakage performance of the retrofit strategies must be evaluated before making a decision whether or not to replace the cladding the water and air control membrane. Experience has shown that air barrier systems formed by careful taping, caulking, use of appropriate air sealing materials like spray polyurethane products and fully-adhered membranes are quite likely to achieve airtightness when properly installed using standard quality control measures.

Constructibility

The ease of construction might be a consideration depending on the specific requirement of the decision-maker. For projects where the homeowners wish to perform the retrofit themselves, it might be worthwhile to consider options that involve low-tech construction techniques and are easier to implement.

How easy a measure is to implement can greatly impact the success of a project. Difficult details and construction sequences often lead to increased cost and reduced performance. Efficiency in construction is driven by simple repeatable details and common construction practice. The more that a measure deviates from common construction techniques (or requires overly complicated sequences involving multiple trades), the more likely that the work will not be completed with the intended result.

A key benefit to the use of an interior double stud wall with cavity insulation is that it does not significantly change standard construction practices and details (e.g., compared to an exterior rigid insulation wall). The installation of the water and air control membrane and cladding remains the same, with only a slight modification of adding ventilation spacers behind the cladding. The new non-load bearing 2x4 stud wall inboard of the existing structural wall will have electrical and plumbing services installed per standard practice. The only difference from standard practice is the interior finishes at the window wells.

3 Technical Description

The Measure Guideline illustrates two approaches to retrofitting wood-framed buildings from the interior side with a double stud wall.  The first one involves the replacement of the existing cladding and the air and water control layer.  The second one illustrates retaining the existing cladding, and whatever water control layer has been previously installed.

Double Stud Wall Assembly: Replace Cladding

The retrofit assembly for the first approach consists of new cladding installed over a spacer to provide an air gap between the cladding and the water and air control layer. Spacer options include 1x wood furring strips, polypropylene mesh/drainage mat, vertical strips of insulating sheathing, plastic manufactured spacer products, and others (see Lstiburek 2010, 2012b for examples). Relative low-profile (~¼ in.) depth cladding spacers are commercially available, which will reduce the impact of cladding connections to trim details.

A new water and air control membrane is installed over the existing wall sheathing in a shingle fashion. The membrane is extended into the window and door rough openings, and wall penetrations are flashed into the membrane.

A new stud wall is installed inboard of the existing stud wall at the desired depth with new cavity insulation. The cavity insulation types appropriate for the retrofit include loose-fill fibrous insulation such as cellulose or fiberglass (Figure 1), ocSPF (Figure 2), and a hybrid approach using ccSPF with loose-fill fibrous insulation (“flash and batt” or “flash and fill,” Figure 3).

 

Figure 1: New cladding approach - double stud wall with cellulose insulation

 

Figure 2: New cladding approach - double stud wall with ocSPF insulation

 

Figure 3: New cladding approach – double stud wall with ccSPF and fibrous insulation

At the new (interior) wall top plate, a piece of plywood is installed, separating the wall cavity from the rim joist cavity. This is one way to provide a closed cavity for retrofit of blown-in loose fill insulation.  This detail may be omitted at the ocSPF wall.

The rim joist is shown insulated and air sealed with open- or closed cell spray foam. This is done for wintertime condensation control, and for improving airtightness at this detail composed of multiple framing members (see BSC 2009). If this detail is used, the "flash and batt" option might be more feasible, because a spray foam contractor may be on-site. The assembly is finished with new interior gypsum board.

Double Stud Wall Assembly: Retain Cladding

The retrofit assembly for the second approach retains the existing cladding, and connects the existing water and air control layer to the window and door rough openings (and wall penetrations) with self-adhered flashing. . .

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