January 15, 2015


This Measure Guideline describes a deep energy enclosure retrofit (DEER) solution for insulating mass masonry buildings from the interior. It describes the retrofit assembly, technical details, and installation sequence for retrofitting masonry walls. Interior insulation of masonry retrofits has the potential to adversely affect the durability of the wall; this document includes a review of decision criteria pertinent to retrofitting masonry walls from the interior and the possible risk of freeze-thaw damage.

 1 Introduction

This Measure Guideline provides design and construction information for a deep energy enclosure retrofit (DEER) solution for insulating mass masonry buildings on the interior. It describes the retrofit assembly and the strategies and procedures for an interior retrofit of masonry wall with the use of rigid insulation.

An exterior retrofit is generally more favorable than an interior retrofit because it improves building durability, by reducing the likelihood of cold weather condensation within the structure (Straube et al. 2012). Exterior retrofits are also less disruptive to the living space, and typically allows a structure to remain occupied during the project.

Despite the advantages of exterior insulation, many buildings must be retrofitted on the interior, for reasons such as historic preservation, zoning or space restrictions, or aesthetics (Figure 1). Load-bearing masonry buildings often (not always) have historic significance and highly valued aesthetics that preclude exterior retrofits.

Figure 1: Historic mass masonry buildings

Interior retrofits of load-bearing masonry are often desired to preserve the exterior appearance. There are many possible interior insulation approaches that are, by and large, reasonably well understood. Adding insulation, increasing airtightness, replacing windows, and improving rain control constitute a normal retrofit package. Adding insulation to the walls of such masonry buildings in cold (and particularly cold and wet) climates may cause performance and durability problems, particularly rot and freeze-thaw damage. There are specific moisture control principles that must be followed for a successful interior insulation retrofit of a solid load- bearing masonry wall (Straube et al. 2012). Increasing the building airtightness as a result of the interior insulation retrofit can cause indoor air quality problems: mechanical ventilation, pollution source control, and combustion safety measures must be implemented to manage the risk.

Numerous obstacles to more wide-scale deployment of interior retrofits include concerns about freeze-thaw damage caused by reduced outward heat flow and reduced inward drying, and the potential for decay of wood structural framing members (typically floor joists) that are embedded in mass assemblies. The problems and some case studies of interior retrofits are outlined by practitioners such as Gonçalves (2003), Maurenbrecher et al. (1998), Straube and Schumacher (2002, 2004), and Straube et al. 2012.

This Measure Guideline includes a review of decision criteria pertinent to retrofitting masonry walls from the interior and the possible risk of freeze-thaw damage. These criteria include cost and performance, durability, constructability, freeze-thaw degradation risk, air leakage performance and thermal performance. It also discusses fundamental building science and design principles for the use of interior insulation in masonry buildings, and construction detailing and procedures developed to provide understanding of how the various elements of the design are implemented.

This Measure Guideline is intended to support contractors implementing an interior insulation- based high performance enclosure retrofit for masonry buildings, as well as designers looking to design such retrofits. It may also be helpful to building owners wishing to learn more about strategies available for deep energy enclosure retrofit of masonry residential buildings on the interior. The document could also be used by owners to implement the retrofit strategy themselves, as it is a low-tech application and does not involve many safety measures.

2 Decision Making Criteria

This section discusses the major decision-making criteria once an interior retrofit has been decided on, after considering issues such as aesthetics, historic significance, improved comfort, and the lifespan of the project, which tend to dominate the decision-making process to determine the type of retrofit to be undertaken.

Cost and Performance

Cost and performance are intricately linked, and must be studied in combination to determine the best choice, per the decision-maker’s goals and objectives. Installation costs for the retrofit solution described in this Measure Guideline can vary widely from estimates in the referred sources, depending on such factors as contractor experience, prevalent region practices, material costs, and the particular circumstances of the project.  It is worth noting that the range is sometimes a factor of 5 to 10.

A number of suitable interior insulation options for the masonry building include XPS (extruded polystyrene) rigid insulation, 2 pound/cubic foot closed cell spray polyurethane foam (ccSPF) or a hybrid approach of ccSPF or XPS with fiberglass batt or cellulose insulation. These options can be compared in terms of the cost of materials and labor, constructability of the system, as well as the performance aspects of each option, to determine the best retrofit option.  In many cases of insulation retrofits of load-bearing masonry buildings, the energy savings are not that important to the owner as much as increased thermal comfort, controlling rain penetration, and ensuring good indoor air quality. 

The following options were considered and evaluated in terms of cost and performance for the wall retrofit; R-values are summarized in Table 1. Similar analysis could be done for equivalent options such as expanded polystyrene or polyisocyanurate rigid insulation.

  • Three layers of 2” XPS rigid insulation (2x4 stud wall inboard of insulation)
  • Two layers of 2” XPS rigid insulation (2x4 stud wall inboard of insulation)
  • 5” of ccSPF (2x4 stud wall inboard of insulation)
  • 2” of ccSPF with 5.5” fiberglass batt (fiberglass in 2x6 stud wall inboard of ccSPF)

There are advantages and disadvantages associated with each wall system. The disadvantage of using rigid sheet goods (such as XPS) compared to a monolithic material (such as spray foam) is the challenge of establishing a continuous air barrier, as it requires ensuring that the board is firmly in contact with the masonry without any gaps, as well as the implementation of the drawn details. The advantage, however, is that the installation can be performed by a homeowner, providing a significant cost savings.

Spraying 5” of ccSPF onto the masonry walls will contribute to the air tightness of the assembly and will result in excellent condensation and vapor diffusion control. The work has to be performed by an industry professional, and therefore can be completed in a shorter amount of time. The drawback, however, is the cost of material and labor (Ueno et al. 2013). Also, ccSPF installations beyond 2”-3” thicknesses must be done in layers, further increasing the installation time and labor cost.

The option of spraying 2” of ccSPF and installing batt insulation in the wall cavities creates a “hybrid” assembly that uses each material to its best advantage. The spray foam creates a robust air barrier and controls interstitial condensation risks, while the batt insulation raises the R-value of the assembly. The fiberglass batt insulation is a more affordable product and can be installed by a homeowner; however, the ccSPF has a higher cost and the installation must be carried out by a professional.

Table 1 lists R-values and costs for the four compared exterior wall insulation options: 6” of XPS rigid insulation, 4” of XPS rigid insulation, 5” of ccSPF and 2” of ccSPF with fiberglass batt insulation. The cost values were obtained from RSMeans Reed Construction Data 2012 (Reed 2012), a cost-estimating tool, which provides the cost of materials, installation, and  overhead and profit. The R-values listed in Table 1 were derived from earlier BEopt models (Christensen et al. 2006).

Table 1. Cost Values for Wall Retrofit Options

PointWall Retrofit OptionsR-valueRSMeans Cost Values
16" XPS with 2x4 stud wall31.5$9.46/sf
24" XPS with 2x4 stud wall22.6$7.47/sf
55" ccSPF with 2x4 stud wall31.1$7.53/sf
62" ccSPF+5.5" fiberglass batt
with 2x6 stud wall

The cost optimized wall system based on market rates for labor and materials would be 2” ccSPF to the interior face of the masonry wall, with 5.5” fiberglass batt insulation in a 2x6 framed wall. The 6” XPS with 2x4 stud wall is the one with best thermal performance (by a small margin); it also involves low-tech construction techniques (making it relatively easier to implement), and was thus chosen for discussion in this Measure Guide.


Solid load-bearing masonry assemblies are by their nature durable. However, the manner in which they manage moisture is quite different than modern, framed, multilayer assemblies. It is important to understand the difference in behavior to support decision-making during retrofits.

The primary concern with insulating older load bearing masonry buildings in cold climates is the possibility of causing freeze-thaw damage of the brickwork and decay in any embedded wood structure, both of which are caused by excess moisture content. Other durability concerns of interior insulation retrofits are that the assembly will reduce drying to the interior and the amount of energy flow through the wall (and thus the drying potential) will be minimized.  An additional durability risk is that bulk water entry will not be as evident from interior inspection in the post- retrofit building.

Yet another concern is the rot/corrosion of embedded elements (e.g., wood joists or reinforcing steel). The worst issues with embedded joist durability are related to bulk water issues, such as excess deposition on the wall, cracks in the façade (allowing leakage), or proximity to grade (or worse, below grade conditions). If any of these issues are found in the site inspection, they must be addressed before considering an interior insulation retrofit.

A counteracting aspect of this issue with interior insulation retrofits is that although the assembly may have higher moisture content, it is also much colder during winter months, which slows the rate of both corrosion (chemical reactions) and rot (biological reactions) (Straube et al. 2012).


The ease of construction might be a consideration depending on the specific requirement of the decision-maker.  For larger projects with higher budgets, it might be possible to hire trained professionals to perform complicated jobs.  But for smaller projects where the building owners wish to perform the retrofit themselves, it might be worthwhile to consider options that involve low-tech construction techniques and are easier to implement. Among the options listed in Table 1, the interior retrofit options consisting of XPS with stud wall are comparatively easier to implement and do not require trained professionals.

Freeze-Thaw Degradation Risk

For existing mass masonry walls, a site assessment must be performed to examine the existing water management features of the building, and look for evidence of existing damage and/or water penetration. Existing problems will be exacerbated by interior insulation. The brick material properties may then be tested to ensure that the wall can be retrofitted without durability problems (Straube et al. 2012). This would involve material property testing (laboratory testing of sample bricks), and hygrothermal computer simulations to diagnose the cause of current issues and predict the effect of potential interior insulation retrofit.  Further information on the testing procedures can be found in Straube et al. (2012).

The purpose of the brick analysis is to ensure that the addition of high levels of interior insulation does not present a risk of freeze-thaw damage to the mass masonry walls in the building. The freeze-thaw degradation risk is assessed by predicting the masonry moisture content during incidents where the material temperature drops below 23°F (-5°C) (Ueno et al. 2013).  Individual sample bricks are collected from the interior and exterior of the original building, followed by material property testing to assess freeze-thaw risk. Testing includes dry density, water absorption coefficient (A-value), free water saturation, vacuum saturation, and determination of critical degree of saturation (Scrit). Scrit which reflects a brick’s resistance to freeze-thaw damage; relatively high Scrit values (~0.75-0.80),  indicate good resistance to damage. The measured values are then used in WUFI simulations to predict the brick moisture content during freezing conditions for the existing and proposed retrofit wall assemblies under varying rain exposures.

Air Leakage Performance

It is highly risky to design a retrofit assembly that allows significant air leakage; therefore, the air leakage performance of the retrofit strategies must be evaluated before making a decision. However, experience has shown that air barrier systems formed by careful taping, caulking, use of spray polyurethane products and fully-adhered membranes are quite likely to achieve airtightness when properly installed using standard quality control measures. In general, it is assumed that air leakage across the enclosure has been essentially controlled using appropriate air sealing materials and techniques, such as taping the joints of the rigid insulation’s innermost and outermost layers, and using interior spray foam insulation.

Thermal Performance

The decision on the thermal performance depends on the specific requirements of the project. For projects that need to meet stringent energy performance goals, a higher level of insulation must be provided.

Thermal insulation follows the law of diminishing returns: return on investment decreases with increasing insulation thickness. Given that the wall assemblies are being changed from uninsulated to insulated assemblies, it is likely that the initial inch or two of insulation should be highly cost effective. Optimization would be a function of insulation cost, climate zone, and energy costs.

3 Technical Description

Masonry Wall Interior Insulation Retrofit Assembly

The retrofit assembly consists of three 2” layers of rigid foam insulation (with staggered seams), adhered to the masonry and between layers with a single-component polyurethane adhesive. The innermost layer of rigid insulation (closest to the interior) has joints taped to create an air barrier. Wood 2x4 framing is installed inboard of these layers, with no insulation in the stud bay cavities, for installation of the interior finishes and to provide space for running services. (Figure 2)

Figure 2: Bearing Masonry Interior Insulation Retrofit Approach

System Interaction

If an interior retrofit improves both the insulation value and airtightness, numerous risks must be assessed.  Both improvements may reduce the durability of the masonry, because the masonry will be colder for longer periods of time than before, and will have less drying capacity, both because it is colder and also because the interior layers added by the retrofit will restrict vapor diffusion. The colder post-retrofit masonry will also make air leakage condensation much more likely in cold weather. The retrofit solution outlined in this Measure Guide includes robust air control measures in addition to high R-value thermal performance.  The performance of the bearing masonry interior insulation retrofit solution relative to critical control functions is described below.

Water Control

Controlling bulk water entry into the wall when executing interior retrofits is of vital importance, especially as water leakage will no longer be visible from inside until damage occurs to interior finishes. If rain control cannot be addressed and upgraded, interior insulation should not be implemented (Straube et al. 2012).

In most walls, a water control layer protects the structure. Water control layers are water repellent materials (building paper, housewrap, sheet membranes, liquid applied coatings, or taped and sealed rigid insulation boards) that are located behind the cladding and are designed and constructed to drain water that passes through the cladding. They are interconnected with flashings, window and door openings, and other penetrations of the building enclosure to provide drainage of water to the exterior of the building. The materials that form the water control layer, in this case the innermost rigid insulation board behind the masonry wall, overlap each other shingle fashion or are sealed so that water drains down and out of the wall (see Figure 3). The water control layer is often referred to as the “drainage plane” or “water resistant barrier” or “water control layer”.


Figure 3: The "down" and "out" approach to flashing

The manner in which load-bearing masonry wall manages water is quite different than modern, framed, multilayer assemblies. These mass masonry walls absorb and safely store water during precipitation, and later dry during more advantageous conditions. Solid masonry walls may contain many hollow spaces or voids (Figure 4), which act as capillary breaks, and may allow water to accumulate or concentrate, as they are invariably not intentionally drained.  Guidance for key details and conditions to address are covered in Straube et al. 2012 .

Figure 4: Examples of mass masonry walls and rain control

In order for the water control layer to be effective, all windows must be installed in a pan flashed and drained opening, with the jambs and head of the frame taped or sealed to the wall water control layer. Flashings are the most under-rated building enclosure component and arguably the most important. Drainage and shedding are accomplished by a sloped sill detail with end dams, and a sufficient drip edge beyond the wall below.

Air Control

Effective air barriers are an important component for good energy performance, good indoor air quality, and control of interstitial condensation. In addition, an effective air barrier between units of multifamily housing reduces transmission of sound, odors, and smoke, lowers fire spread risk, and helps control stack-driven airflows (Lstiburek 2005).

Load-bearing masonry walls pose a condensation risk at the masonry-to-insulation interface. Air leakage could bypass imperfectly installed air barriers, resulting in condensation problems. This condensation-based wetting would occur in a layer this is cold enough to precipitate condensation, and often would drop below freezing. To avoid this problem, excellent airtightness on the interior is essential.

Options for retrofitting an air barrier at a mass masonry wall include the application of a liquid- applied or membrane air barrier on the interior side, or the use of an insulation material that creates an air barrier. In case of interior retrofits using rigid board insulation, an interior air barrier in the form of taped and sealed joints is required to prevent interior air from contacting the cold masonry. Given the construction quality sensitivity of this taped system, air leakage (blower door) testing prior to interior finishes (gypsum board) would be a prudent step. Material compatibility must be ensured before selection of air flow retarder system component. . .

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