BA-1106: Leveraging Limited Scope for Maximum Benefit in Occupied Renovation of Uninsulated Cold Climate Multifamily Housing

Effective Date

This project examines a large scale renovation project within a 500 unit, 1960’s era subsidized urban housing community. The development comprises low-rise and mid-rise structures both of which exhibit exposed concrete frames with uninsulated masonry infill walls. The renovation project has a particular focus on indoor environmental quality and energy performance. The nature of occupied rehabilitation necessarily limited the scope of work implemented within apartment units. This research focuses on the airflow control and window replacement measures implemented as part of the renovations to the low-rise apartment buildings.


Executive Summary

This project examines a large-scale renovation project in a 500-unit, 1960s era subsidized urban housing community. The development comprises low-rise and mid-rise structures, both of which exhibit exposed concrete frames with uninsulated masonry infill walls. The project has a particular focus on indoor environmental quality and energy performance. The nature of occupied rehabilitation necessarily limited the scope of work implemented within apartment units. This research focuses on the airflow control and window replacement measures implemented as part of the renovations to the low-rise apartment buildings.

The window replacement reduced the nominal conductive loss of the apartment enclosure by approximately 15%; air sealing measures reduced measured air leakage by approximately 40% on average. The full scope of renovation work, which includes mechanical system upgrades in addition to the air sealing and window replacement measures, is expected to achieve energy savings of approximately 30% relative to existing conditions.

The window replacement measure and much of the air sealing correspond to typical building upkeep and component replacement activity. The research provides specific findings relative to window details and effective air sealing strategies. It also aims to convey broader lesson in leveraging upkeep and maintenance activities to benefit durability, comfort, indoor air quality, and energy performance.

1 Introduction

This project examines performance measures in the context of a large-scale renovation project. Castle Square Apartments is a 500-unit, 1960s era subsidized urban housing community in Boston. Castle Square Apartments is owned by the Castle Square Tenants Organization (majority owner, hereinafter CSTO) and by WinnDevelopment (Winn). The development includes low-rise and mid-rise structures. The wall assemblies for both types of structures consist of exposed concrete frames with uninsulated masonry infill. Existing fenestration is nonthermally broken aluminum-framed double-pane windows.

The renovation has a particular focus on indoor environmental quality (thermal comfort, odor control, and ventilation) and energy performance. In the low-rise (two- to four-story) apartment buildings, these goals will be pursued through a renovation project involving kitchen replacement, window replacement, mechanical system upgrade, and a limited scope of remediation air sealing that is implemented mostly in the kitchen and mechanical room areas.

The research project focuses on evaluation of the air sealing measures and window replacement implemented as part of the renovation scope. The iterative nature of the measures implemented in a large number of apartment units suggests the opportunity to assess the effectiveness of various airflow control, air sealing, and air quality measures implemented at a large scale within occupied residences.

The Castle Square Apartments community represents a type of building construction and situation of building occupancy/ownership that presents acute challenges to high performance retrofit. These construction types and situations are also reasonably common, particularly in metropolitan areas across the heating dominated climates of the United States.

The limited scope of engagement within the apartments and, in particular, the limited scope applied directly to energy and indoor air quality (IAQ) measures necessitated carefully targeted measures. This project provides the opportunity to assess the effectiveness of various airflow control, air sealing, and air quality measures implemented at a large scale within occupied residences in uninsulated masonry and concrete structures.

2 Project Context

2.1 Background

2.1.1 Midcentury Subsidized Housing

Across the country, and particularly in the Northeast and upper Midwest, a multitude of uninsulated masonry structures were built to provide durable and functional subsidized housing. Many of these structures were built in an era when it was acceptable cold climate practice to provide a building enclosure with no added insulation. Although considerable expense may have been applied toward making the buildings hardened and abuse resistant, comfort did not appear to have been a priority. To say that aesthetics are often sparse would be an understatement. Approaches to IAQ have evolved considerably in the time since these buildings were built.

Despite the compromises of comfort, charm, and healthful interior environments, the need for affordable housing is so acute that vacancies in subsidized housing developments are often scarce in major metropolitan areas. The uninsulated enclosures and sometimes arcane mechanical infrastructure drive many of these buildings toward being unaffordable for the housing authorities, community development agencies, and tenants’ organizations that operate them.

Clear guidelines about effective and technically sound retrofit strategies are needed that can be implemented in occupied housing. The sustained viability of these buildings may also require strategies to significantly improve aesthetics, comfort, water management, and energy performance.

2.1.2 Castle Square Apartment Renovations

In preparation for regular and periodic refinancing, the CSTO and Winn sought to develop a plan to address ongoing performance concerns and substantially modernize the facility. Surveys of residents and frequent resident input meetings found that improving IAQ (reducing transmission of odors) and thermal comfort are top priorities (see Appendix A). The CSTO and Winn also expressed a strong desire that the renovations to the community be as “green” as possible and that energy costs be reduced as much as possible. Because of the acute need for affordable housing in the area, the high cost of relocating residents, and the extremely low vacancy rate, it was determined that the renovations must take place without displacing residents.

The CSTO-Winn joint venture hired an Architect, Elton + Hampton Associates, an engineer, Petersen Engineering, and Building Science Corporation (BSC) as building science and enclosure consultants, to assist in developing project directions. Initially, the joint venture aspired to implement a Passive House-level retrofit. Although the goals did need to adjust to financial circumstances, CSTO and Winn maintained a firm commitment to super-insulation of the mid-rise buildings.

The scope of renovations in the low-rise apartments comprises kitchen replacement, window replacement, provision of kitchen and bath exhaust, and replacement of the furnaces and water heaters. This scope is representative of what may be included in a typical “modernization” or upgrade of housing units for nonenergy reasons. The scope also replaces components that would need to be replaced multiple times over the normal service life of a concrete and masonry building. The renovation project at Castle Square Apartments seeks to leverage this rather generic scope for maximum energy and IAQ benefit.

A significant portion of the design development for this project occurred prior to the start of the present Building America (BA) project. BSC had worked with the project team under prior BA program years as well as under direct contract with the project team. During this earlier phase, BSC contributed to the development of air sealing strategies and of performance specifications. Investigations and diagnostic testing were conducted in vacant apartment units designated as mock-up/investigation units. During the present BA program period, BSC contributed to the refinement of pertinent performance details and assessment of performance specifications. After the contract for implementation of the renovation was awarded, an additional vacant apartment was made available for investigations and measurements. BSC worked with the architect and selected general contractor to refine strategies based on work conducted in this apartment unit.

2.2 Relevance to Building America’s Goals

The goal of the U.S. Department of Energy's (DOE) BA program is to reduce energy use for existing homes by 15% (compared to pre-retrofit energy use). Based on estimates of the design team, the renovations to the low-rise buildings are expected to yield energy use savings of approximately 30%.1

The measure examples and guidelines produced by this research project will be applicable to uninsulated masonry multifamily structures in heating-dominated climates. Of particular interest is that the lessons learned will be representative of the context of subsidized housing and occupied renovation. Furthermore, measures implemented in the low-rise apartment renovations address components that typically will need to be replaced or assemblies that will need to be refurbished during the service life of buildings. Measures guidance for these measures provides support for building owners/operators to capitalize on performance improvement opportunities represented by regular maintenance and replacement activities.

3 Data Sources and Methods

3.1 Review and Observation

In the capacity of a technical support consultant, BSC reviewed shop drawings pertaining to window installation and other measures affecting either water management or airflow control. BSC suggested changes through the project architect.

BSC also reviewed implementation of work in the field and participated in refinement of details with involvement of the implementing trade contractors, general contractor, architect, and owners’ representative. In-field observation of work involved visual observation and documentation by digital photographs. In some cases, BSC reviewed photographs and documentation provided by persons in the field.

3.2 Measurements

Air leakage performance was measured via multipoint blower door testing. Total apartment enclosure air leakage performance was assessed by testing apartments in a fully unguarded configuration, that is, with neighboring apartments open to the exterior. During initial stages of the renovation project, some iterations of guarded blower door testing were performed as well. During initial construction stages of project, we performed air leakage testing at a small sample of apartment units at intermediate stages of scope to assess success of implementation and help the contractor understand air sealing measures.

Total apartment enclosure air leakage performance after completion of the renovation scope was measured in a 10% sample (31 apartment units) of the renovated apartment units. Pre-renovation measurements were taken in a smaller sample of units. It was generally not practical to directly measure pre- and post-renovation air leakage within specific apartments.

3.3 Analysis

The performance specification referenced in the project specification is expressed in terms of an effective leakage area (ELA) ratio where the calculated effective leakage area is normalized to 100 ft2 of enclosure/boundary surface area. This ratio is given the shorthand ELA/100. The ELA is defined as the area of a special nozzle-shaped hole (similar to the inlet of the blower door fan) that would leak the same amount of air as the building does at a pressure of 4 Pa.2

The TECtite software was used to calculate the ELA based on multipoint air leakage tests. BSC performed area and volume takeoffs of the apartment unit plans to be able to normalize the air leakage measurements in terms of ELA/100, ACH50, and CFM50/ft2 of enclosure area.

Because it was generally not practical to directly measure pre- and post-renovation air leakage within specific apartments, reduction of air leakage performance is taken from a statistical analysis of pre- and post-retrofit air leakage measurements. Normalized measurements for the pre-retrofit sample are compared to normalized measurements of the post-retrofit sample. . .

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For additional information about the Castle Square Deep Energy Retrofit Project, click here.


  1. The design team used input from a variety of analyses to arrive at energy reduction estimates. BSC provided
    modified heat flux analysis to reduction in enclosure heating load. The mechanical engineer prepared estimates of reductions in energy use of the mechanical systems relative to loads. The estimates of relative reductions could then be applied to historical consumption data to arrive at estimates of the value of energy savings.
  2. ELA was developed by Lawrence Berkeley National Laboratory. Under less than optimal testing conditions,
    coefficients and exponents derived from multipoint testing can be relatively unstable resulting in extrapolations to airflow at lower pressures that are also unstable. The measurement of air leakage flow at 50 Pa, where 50 Pa is near the upper end of the test pressure range, is taken to provide a more stable measure of air leakage. Extrapolation using fixed coefficient and exponent values applied to the calculated cfm50 measurement is believed to provide a more repeatable and stable measure of effective leakage area.