BA-1205: Attic or Roof? An Evaluation of Two Advanced Weatherization Packages

Effective Date

This project examines implementation of advanced retrofit measures in the context of a large-scale weatherization program and the archetypal Chicago, Illinois, brick bungalow. In response to the apparent weatherization program limitations with respect to homes with masonry bearing wall construction, this research project examines two distinct strategies for insulating and air sealing the top of houses. One strategy applies best practice air sealing methods and a standard insulation method to the attic floor. The other strategy creates an unvented roof assembly using materials and methods typically available to weatherization contractors.


Executive Summary

This project examines implementation of advanced retrofit measures in the context of a largescale weatherization program and the archetypal Chicago, Illinois, brick bungalow. In response to the apparent weatherization program limitations with respect to homes with masonry bearing wall construction, this research project examines two distinct strategies for insulating and air sealing the top of houses. One strategy applies best practice air sealing methods and a standard insulation method to the attic floor. The other strategy creates an unvented roof assembly using materials and methods typically available to weatherization contractors.

Through implementation of the retrofit strategies in a total of eight (8) test homes, the research found that the two different strategies achieve similar air leakage and energy performance improvement relative to the pre-retrofit conditions despite the fact that the unvented roof strategy encloses a larger volume and results in a larger thermal enclosure area. Average air leakage measure reductions were approximately 55% for both strategies. Energy modeling predicts source energy use reductions of roughly 18% for both strategies.

The cost for the unvented, roof rafter insulation approach was found to be about $7,000 more than air sealing and insulating at the attic floor. However, the roof rafter insulation strategy could also be viewed as a very cost-effective strategy to increase—by roughly 50%—the usable abovegrade conditioned floor area.

Through observations of the strategies implemented, the research described in this report identifies measures critical to performance as well as conditions for wider adoption. The research also identifies common factors that must be considered in determining the appropriate strategy for the top of the building.

1 Introduction

The predominant construction types for residential structures in Chicago, Illinois, involve loadbearing masonry walls. Methods to insulate these wall assemblies are beyond the reach of typical weatherization programs. Yet, the need for weatherization in these masonry buildings is clear—energy costs are an increasing burden on household incomes. Given the wall assembly of these buildings, the most promising opportunities for weatherization to achieve significant benefits for residents of these masonry-walled structures appears to be at the top of the building: either in the attic or at the roof.

A typical measure to reduce heat loss at the top of the building is to apply loose cellulose or fiberglass batt insulation at the attic floor. Reducing heat loss at the top of the building, especially in a cold climate like Chicago, is particularly dependent on controlling convective losses (air leakage). Common conditions of attic or roof assemblies make it very difficult to achieve effective air leakage reduction at the top of the building. There are also common conditions—such as occupied, finished and heated rooms in attic spaces—that make insulation at the attic floor inappropriate.

This research project evaluates two strategies for insulating and air sealing at the top of masonry bungalow houses in a sample of one-story brick bungalow homes eligible to participate in the Community and Economic Development Association of Cook County (CEDA) Weatherization program. One strategy involves recognized “best practices” applied to attic floor insulation (Lstiburek, 2010). The other involves insulating and air sealing in the plane of roof rafters. The later strategy is developed as part of the research effort. The research also identifies conditions under which roof rafter or attic deck strategies are appropriate. The strategies are further evaluated in terms of air flow control, feasibility and cost.

2 Project Context

Chicago Masonry Buildings

Masonry buildings are the dominant building type of residential structures in Chicago. While this may be expected for large older multifamily buildings, it is also true of smaller residential buildings in Chicago. According to Cook County Assessment Department data, 57% of 1-unit housing stock and 58% of 2- to 4-unit housing stock in Cook County is of masonry construction.

Illinois ranks second in the country in gas use per residential gas customer at 1,082 ccf/customer, behind only Alaska at 1,663 ccf/customer (AGA, 2010). Residential buildings in the Chicago area exhibit nearly 60% higher heating energy use than the Illinois state average (AGA, 2010).

CEDA Weatherization

The CEDA Weatherization programs serve income-eligible clients in Cook County, Illinois. CEDA Weatherization is one of over 30 community action agencies that participate in the Illinois Home Weatherization Assistance Program (IHWAP). The Illinois Department of Commerce and Economic Opportunity (DCEO) administers the program; DCEO’s Office of
Energy Assistance monitors all agencies in the IHWAP network.

The services that CEDA Weatherization provides are structured around a comprehensive energy audit performed by a CEDA Weatherization assessor. The audit assessment leads to development
of a work order that may comprise measures in the following categories:1

  • Retrofit measures: insulation, CFL lighting, refrigerator replacement, low-flow shower heads and faucet aerators, heating system replacement, window and door replacement.
  • Air-sealing measures
  • Health and safety measures (limited to $600 per unit): handrails, fire extinguishers, gutters, downspouts, and decommissioning of unvented space heaters.
  • Incidental repair measure (limited to $500 per unit)

DOE Weatherization program funding requires that the entire building/house receive a savings-to-investment ratio (SIR) of 1 or greater before it can be weatherized.2 Software tools are used to estimate the savings from various measures. Costs per work item are established in the program. The state-level administration of IHWAP sets a limit of $5,200 to the expenditure for each housing unit served. Approved contractors participating in the CEDA Weatherization programs implement the work scope. Upon completion of the work, CEDA Weatherization assessors conduct inspection of the work.

CEDA weatherizes all types of single-family and multifamily structures. In typical years, CEDA Weatherization has weatherized 3000-4000 housing units. With funding made available through the American Recovery and Reinvestment Act, budgeted volume of units has increased as follows:

2010: 7500 units,

2011: 11,000 units, and

2012: 9000 units.

The high proportion of masonry buildings within its service territory creates a persistent challenge for CEDA Weatherization programs. An analysis of the programs found that energy use reductions achieved by weatherization activity in homes of masonry construction are typically 1/3 less than what is achieved in wood-framed homes. It is certainly conceivable that this reflects the thermal performance of masonry wall assemblies and the lack of opportunity for improvement at levels of investment commensurate with typical weatherization programs. The furring cavity between the brick wall and interior finish does not provide adequate space for effective insulation, and interior wall build-out for insulation or insulation and re-cladding at the exterior would be prohibitively expensive.

Within limits of current means and methods and funding resources, the top of the building enclosure is likely to yield the best opportunities for improvement. Typical measures to reduce heat loss at the top of the building include applying loose cellulose or fiberglass batt insulation at the attic floor. It is well understood that controlling air leakage at the attic/roof plane is crucial to building energy performance and also has important impact on building durability and indoor air quality.

Implementing air flow control at the top of the building is fraught with challenges in typical Chicago building configurations. In one- to two-family structures where attics are often more accessible, flooring and storage of resident belongings would render it very difficult and expensive to implement air sealing at attic floor. There is also a high incidence of situations where establishing the thermal enclosure at the attic floor will not be appropriate. These
situations include difficult access to critical areas for air sealing and finished or semi-finished space above the attic floor. Walk-up attic stairs are another common feature in Chicago homes that would compromise the level of performance achieved through attic air sealing and insulation due to air leakage and conduction.

In order for CEDA Weatherization and other programs in the Chicago area to achieve significant performance improvement in masonry buildings, alternatives to attic floor weatherization are needed.

Relevance to Building America’s Goals

The goal of the U.S. Department of Energy's (DOE) Building America program is to reduce home energy use for existing homes by at least 15% compared to pre-retrofit energy use. For CEDA, such reductions in existing home energy use are imperative to maintaining housing affordability and quality of life for CEDA’s constituents.

While the specific target of measures implemented through this research and the dominant target of CEDA Weatherization programs is to reduce energy costs and improve comfort in a heating dominated climate, demonstrated effective strategies for treating the top of the thermal enclosure (i.e. the attic and roof) are applicable in all regions of the country. This research demonstrates the effectiveness of measures outlined in "Attic Air Sealing Guide and Details" (Lstiburek, February 2010) and presents implementation guidance developed during the research effort. The demonstrated performance potential and implementation guidance produced by this research serves as a jumping off point for further advanced measures to address masonry building enclosures.

It is proposed that subsequent research evaluate strategies to dramatically improve the thermal performance of load-bearing masonry walls and installation of high performance windows. Such advanced measures cannot reasonably be pursued before “lower hanging fruit” such as that addressed in this research is captured. The implementation guidance developed through this research will ensure that top-of-building strategies accommodate future measures to address other components of the thermal enclosure.

3 Research Project Description

CEDA Weatherization was able to secure special funding for a limited research project that allowed for a project budget of $15,000 (as compared to $5,200 in the regular weatherization program) in each of 10 homes. To implement this research project, CEDA Weatherization joined with BSC in a Building America research partnership. This allowed BSC to participate as the technical lead in the research project.

Upon establishing a Building America research partnership, CEDA Weatherization and BSC conducted planning meetings that identified many research needs. Analysis of the building stock within CEDA Weatherization service territory and of program performance led the research partnership to focus on one dominant housing type, the one-story brick bungalow, and on one component of the thermal enclosure, the attic/roof. The basic outline of this research project is to identify appropriate strategies, implement these in a sample of 10 test homes through a controlled process and evaluate the strategies by various parameters.

Test Home Description

The test homes selected for this research project are all one-story bungalow-style homes with brick masonry bearing walls, full basements, and cast concrete or concrete block foundations. The homes have between 925 and 1,700 s.f. of conditioned living area on the first floor. The median value for conditioned floor area in the sample of test homes is 1,152 s.f.; the average is 1,216 s.f.

Figure 1: Typical Chicago brick bungalows

Most of the homes have a walk-up attic while one of the test homes has a scuttle hatch access to the attic. The attics typically have some older loose-blown insulation at the attic floor. While some of the test homes have deliberate roof vents in the form of either mushroom vents, turbine vents, or ridge vents, ventilation intakes are, for the most part, incidental. All but the test home with scuttle hatch access have some kind of wood floor boards (apparently original to the house) at the attic floor.

A lightly framed rear porch that has been enclosed sometime after original construction to provide additional four-season living space is typical to the Chicago brick bungalow. In a few cases observed, the porch was operated as an unconditioned space. The porch may be supported either on posts or a foundation wall. Even where the porch is supported on posts, siding and sheathing are typically carried down to a curb or grade level. Thus, there is typically a
crawlspace or second basement beneath the rear porch space.

Another typical aspect of the test home type is an entry at the front, rear, or both that is inset relative to the footprint of the building. This creates an overhang of the roof and attic above the entry. Where there is an inset front entry, the inset occurs inside of the foundation walls such that there is a space in the basement with entry stairs and landing above. Often this space is separated from the rest of the basement as a closet or storage room.

Basements typically contain at least some finished or partially finished areas. A recreation room with a bar is not an uncommon feature. In some of the test homes, portions of the basement were clearly being used as sleeping areas. The majority of test homes have a history of periodic basement flooding.

Research Measures

3.1.1 Attic Deck Insulation and Air Sealing

A thermal measure commonly delivered through CEDA Weatherization programs is addition of insulation to the attic floor. The measure may or may not be accompanied by aggressive air sealing measures. In order to provide a reasonable basis of comparison between strategies, it was determined to enhance typical measures with “best practice” measures. The research strategy was to push this relatively familiar attic floor air sealing and insulation approach as far as reasonably feasible through the comprehensive air sealing measures such as outlined in the "Attic Air Sealing Guide and Details" (Lstiburek, February 2010) and developed in the course of the research.

3.1.2 Roof Rafter Insulation and Air Sealing

Because there are situations where proper insulation and air sealing of the attic floor is either impractical or inappropriate, the partnership determined that a solution that brought the attic space inside the thermal enclosure is needed. To devise an appropriate approach around insulating and air sealing in plane with the roof (enclosing the attic within the thermal barrier) BSC and CEDA assembled a group of recognized experts in the field of building performance
and weatherization. This team met in Chicago over a period of three days to assess opportunities and evaluate initial implementation within a subject home identified by CEDA. CEDA also arranged for one of the weatherization program contractors to be on hand to advise on measure feasibility. CEDA Weatherization technical staff and consultants also performed blower door testing to assess building performance before and after different measures were implemented.
This multiday operation resulted in a scope of work for the roof rafter strategy implemented in the research project. . .

Download the complete report here.


  1. Note that the list of allowable measures in the program has changed slightly since the period of research.
  2. Typically, the value of savings is taken as the present value of recurring savings for a period of time at a standard discount rate. The present value of savings is then compared to the initial cost or investment of the measure.