January 24, 2014


Between December of 2009 and December of 2012, participants in a deep energy retrofit (DER) pilot program sponsored by National Grid and conducted in Massachusetts and Rhode Island completed 42 DER projects. Building Science Corporation (BSC) provided technical support to program participants and verification of measures for the program sponsor, National Grid. The pilot program required aggressive upgrades to building enclosure systems, implementation of ventilation and combustion safety measures and also provided incentives to upgrade mechanical systems. Thirty-seven of the projects completed through the pilot were comprehensive retrofits while five were partial DERs. The collection of 42 DER projects represents 60 units of housing.

Executive Summary

Between December of 2009 and December of 2012, participants in a deep energy retrofit (DER) pilot program sponsored by National Grid and conducted in Massachusetts and Rhode Island completed 42 DER projects. Building Science Corporation (BSC) provided technical support to program participants and verification of measures for the program sponsor, National Grid. The pilot program required aggressive upgrades to building enclosure systems, implementation of ventilation and combustion safety measures and also provided incentives to upgrade mechanical systems. Thirty-seven of the projects completed through the pilot were comprehensive retrofits while five were partial DERs, meaning that high performance retrofit was implemented for a single major enclosure component or a limited number of major enclosure components. The collection of 42 DER projects represents 60 units of housing.

Pre- and post-retrofit air leakage measurements were performed for each of the projects. Each project also reported information about project costs including identification of energy-related costs. Pilot program application forms collected pre-retrofit energy use data for 35 of the projects. BSC used energy modeling to estimate pre-retrofit energy use for 7 projects for which measured data was not available. Post-retrofit energy-use data were obtained for 29 of the DER projects. Post-retrofit energy use was analyzed based on the net energy used by the DER project regardless of whether the energy was generated on site or delivered to the site. Homeowner surveys were returned by 12 of the pilot participants.

Post retrofit energy use data is analyzed with the objective of learning what post-retrofit energy performance one can expect from implementation of the retrofit package. In other words, the focus of the study to to project where a DER project will "end up" rather than to project the savings that any one project might realize. All but two of the comprehensive DER projects achieved household source energy use below the EIA Northeast regional household average. The mean for the group is 107.2 MMBtu/year, or approximately 38% below the regional household average. In terms of site energy use intensity (EUI), all of the projects perform below the regional average, with the mean values for both the multifamily and single family DER projects below 50% of the respective Northeast region average. Two of the multifamily projects and three of the single-family projects meet the 2015 site EUI goal for the Architecture 2030 Challenge without taking any credit for on-site electricity generation.

Based on the experience of this sample of DER projects, this DER package is expected to result in yearly source energy use on the order of 110 MMBtu/year for a typical home. This is approximately 40% below the Northeast regional average for household energy use. Larger to medium sized homes that successfully implement these retrofits can be expected to achieve source EUI that is comparable to Passive House program targets for new construction.

All full DER projects achieved better than 50% reduction in total CFM 50; all partial DER projects achieved better than 40% reduction in total CFM 50. Over half of the full DER projects achieved post-retrofit ACH 50 results below 1.5 ACH50. Some variations in airtightness performance are noted to accompany variations in DER implementation approach. For example, the group of DER projects that included the basement in the air control enclosure had a better overall air tightness result than the group that excluded the basement (i.e., insulation and air control at basement ceiling). Also, the group of DER projects with unvented attics had a better overall air tightness result than the group with vented attic.

In this group of DER projects, the reported energy-related portion of project costs ranged from just over $31,500 to approximately $194,350. The reported energy-related costs averaged $34.59/sf (post-retrofit conditioned floor area) for the sample of DER projects. Noted variations in HVAC measure costs appear to relate to homeowner preferences, and do not appear to be correlated with a noticeable difference in performance (with the possible exception of one project that installed a ground-source heat pump).

Projects in this group of DER projects implemented three different approaches for attic/roof retrofit. The reported energy-related cost for a vented attic approach with insulation at the attic floor averaged $8.40/sf. The unvented attic approach with rafter cavity insulation only averaged $11.59/sf. The unvented attic with insulation both exterior to the roof sheathing and between roof framing averaged $14.21/sf. Excluding some noted outliers, the reported energy-related cost for the most typical wall retrofit approach ranged from $4.67 to $19.15 per sf with an average of $10.51/sf.

1 Introduction

The US housing stock accounts for a significant portion of national energy usage. The volume of existing housing (approximately 130 million housing units) relative to the rate of housing unit construction (between approximately 500,000 and 2 million per year in recent years ) makes energy performance retrofit absolutely essential to goals of reducing the energy use of the residential sector.

Home retrofits have been targeted as an area of great potential for significant energy savings, employment opportunities, and market growth. Typical residential retrofit activity aims at mitigating performance liabilities of existing housing (NJIT 2013). High performance retrofit techniques are aimed at improving the performance of existing building components or whole buildings to equal or surpass current high performance new construction practices.

Barriers to widespread adoption of high performance retrofit strategies remain high. Knowledge, skill, and even availability of building products represent persistent supply-side barriers to high performance retrofit. Vigorous market demand for high performance retrofit would provide the impetus for these barriers to fall. Two factors that constrain the market demand for high performance retrofit are 1) the lack of confidence in or a lack of appreciation for the benefits of high performance retrofit, and 2) perceptions relative to the high cost of a comprehensive energy retrofit.

Some in the industry have asserted that better modeling tools and methods are needed to predict savings resulting from retrofit measures.1 Clearly, a large body of evidence from actual retrofit projects is also needed to demonstrate the benefits and reveal the costs.

This project reports the measured energy performance, airtightness and costs for 42 high performance residential retrofit projects. The projects are all participants in a National Grid- sponsored deep energy retrofit (DER) pilot program. The projects implemented a consistent package of measures according to the requirements of this pilot program. Variations in the method of implementing measures as well as variations permitted in the overall package provide opportunities to investigate apparent impacts of these variations.

The evaluation of retrofit performance focuses on the level of performance (in terms of energy use and air tightness) achieved rather than reductions relative to pre-retrofit conditions. In other words, the analysis aspires to provide an idea of where a DER project implementing a similar package of measures is likely to “end up”, in terms of energy performance and airtightness. This is afforded by the comprehensive nature of the retrofits and by the measured post-retrofit performance. This approach side-steps the complication of characterizing the pre-retrofit existing conditions. Relative savings projections are hugely dependent upon accurate characterization of existing conditions. Existing conditions in residential buildings are hugely variable and difficult to define for a project that is not known. By seeking to identify and describe a relatively consistent level of performance achieved through a package of measures, the project is able to project the results of high performance retrofit in a way that is much more stable and more widely applicable than savings projections. In other words, an understanding of the level of performance attained hrough application of a package of measures allows one to project the savings achieved for a particular home with better certainty. One has only to compare the measured performance (or history) of the home in it its pre-retrofit state to the expected performance. Quantifying savings achieved for a for a package of retrofit measures is of limited value as the savings would only be repeatable for homes that, not only implement the same package of measures, but also have a similar pre-retrofit situation.

As participants in the DER pilot program, these retrofit projects all used the same set of enclosure performance targets, taken here as a “package of measures”. This report assesses the effectiveness of the overall package of measures as well as the relationship between different implementation strategies used and measures of performance. This is accomplished by analyzing the full set of performance data for the group rather than looking at individual case studies. This approach results in post-retrofit energy use and cost ranges based on the total community data that can be reasonably projected to other implementations of the DER package. The resulting energy use and airtightness projects as well as cost ranges constitute concrete evidence that can be used by homeowners to assess the potential benefit and cost of a DER.

This study does not include an analysis of retrofit package and measure costs relative to various effects sought from the measures. An analysis of cost and effect for some of the early completed projects in this pilot is included in a previous study (Gates and Neuhauser 2013). This earlier study found that non-energy benefits where either primary or significant motivations for a substantial portion of DER project expenditures. This finding highlights the importance of defining the “effect” whenever “cost effectiveness” is evaluated or discussed. The study also points to the need to acknowledge and value the range of desired effects obtained through a measure.

While the relative site energy savings from pre-retrofit to post-retrofit conditions ranges widely for this project – from 28% to 90% – the level of energy performance achieved is much more consistent. For the 27 comprehensive retrofit projects for which sufficient post-retrofit energy use data is available, the median and mean post retrofit annual site energy use per household is just below 50% of the regional average. The measured site energy use is within 20% of the mean for just under half of the projects.

The results of the pilot demonstrate that a relatively consistent level of performance can be achieved. But perhaps more importantly, the results demonstrate that the DER retrofits can meet energy performance goals and benchmarks representative of best-in-class new home construction. 

2 Background

2.1 New Construction, Retrofits, and Deep Energy Retrofits

There are a substantial number of existing homes in the US. The US Census Bureau estimates that there were over 130 million housing units in the United States in 2011 (US Census Bureau 2013). This compares to typical new home construction rate of between approximately 500,000 and 2 million per year. In the years between 2007 and 2011, the construction industry added 3.1 million homes. The rate of new home construction relative to existing housing stock tells us that the majority of houses are likely to remain more than three decades old for some time to come. The rate also indicates that even super-efficient new construction will have a very limited impact on the overall energy use of the housing sector.

Figure 1. US housing units by decade of construction (US Census Bureau, Annual Housing Survey data)

Until recent years, the primary focus of the Building America program has been research and development of techniques for new construction. The near-term goal of the Building America program for new construction homes is to reduce energy use in new construction homes by 30% relative to a baseline established by the 2009 IRC. The program has already succeeded in demonstrating performance packages achieving savings of 30-50% relative to the baseline. Despite the impressive level of savings demonstrated for these packages, the new home packages represent a modest potential impact on national energy use due to the small percentage of new homes added to the aggregate national housing stock every year.

Retrofit packages can be applied to a substantially greater portion of the U.S. housing stock than new construction packages. The Building America program near-term goal for the existing homes is to reduce energy consumption by 30% relative to the current condition of the existing home. For many existing homes, a 30% reduction in energy consumption will not be enough to elevate the performance to a level comparable to current standard practice (as defined by 2009 IRC, for example). But at current rates of construction/replacement, it will take an extremely long time to replace the current housing stock with housing built to modern performance standards. Even if rapid replacement were possible, retiring and replacing a significant portion of existing housing with high performance housing is not a reasonable proposition. Doing so would represent an unreasonable displacement and disruption of population, abandonment of physical and cultural resources embodied in existing buildings, and astronomical financial cost.

Obviously, reducing, by any significant measure, the energy use of the residential sector will require retrofitting existing housing. The DER pilot sponsored by National Grid provides an opportunity to assess whether a repeatable advanced retrofit package can elevate performance beyond that of typical new construction. The pilot also allows evaluation of the cost of achieving this level of performance through retrofit.

2.2 Previous Work

The BA program has been working to overcome the obstacles associated with DERs. There are several other BA teams evaluating the performance effectiveness of cold climate home retrofit approaches at the community scale. They include the Consortium for Advanced Residential Buildings’ (CARB) role in the Retrofit NYC Block by Block project (Eisenberg et al. 2012) and in the recently completed retrofit of the Chamberlain Heights duplex and quad affordable housing complex (Donnelly and Mahle 2012) as well as the Partnership for Advanced Residential Retrofits (PARR) team’s work in the Chicagoland project development of energy efficiency retrofit packages for typical houses in the Chicago area (Spanier et al. 2012).

These research projects have the potential to provide a significant set of post-retrofit performance data using utility bills and other testing to evaluate the energy use level achieved (and achievable) by fairly comprehensive retrofit measures packages. However, the current reports have only limited results available (if any), and most results are presented in terms of software models rather than actual performance data. In this current research project, BSC is making use of a year of post-retrofit utility bills and performance data for retrofit projects. BSC then uses this actual performance information to project achievable performance levels for the DER retrofit measure package.

The CARB and PARR research projects adopt the approach of tailoring retrofit measure packages to particular house types—e.g., ranch house, NYC row house, or triple-decker. In contrast, BSC has found that each retrofit project has its own set of unique constraints that are based not so much on house type and age as its history and existing conditions. Therefore, tailoring retrofit measure packages to specific house types may not be necessary. In the results described in the current report, a single DER retrofit measures package has been applied to a variety of housing types, as well as significantly different ages and existing conditions.

Other previous work related to high performance retrofit has focused on individual components or measures. For example, in one recent BA project, BSC worked with a weatherization program to evaluate and develop plans for inclusion of roof or attic insulation in the weatherization program (Neuhauser 2012). The current study evaluates the impact of a comprehensive measures package.

2.3 BSC and the National Grid DER Pilot Program

BSC has conducted several research projects using information, data and experiences from retrofit projects participating in the National Grid DER pilot program. In one project, BSC performed a case by case evaluation of the implementation of the DER measures for five of the DER pilot program participants (Neuhauser 2011). In a second project, BSC looked at the pre- and post-retrofit performance data for seven DER projects, four of which were early participants in the DER pilot program (Osser et al. 2012).

The thrust of these earlier research projects dealt with the individual projects—either in terms of how the DER measures were implemented or the post-retrofit performance that each one achieved. None of them compared and analyzed the performance data as a group. Now that additional projects have been completed in the National Grid DER pilot program, there are enough data available to warrant the analysis of all of these projects as a community of retrofits rather than as individual cases. The number of completed projects is large enough that the impact of the retrofit measures as a package can be analyzed, and trends from the available data about these projects begin to emerge. Using this approach, the emphasis is shifted from the post-retrofit performance for the individual case to the post-retrofit performance achievable by using the DER package.

As the designated technical support provider to the National Grid DER Pilot program since its inception in 2009, BSC has had the opportunity to learn from over 40 residential deep energy retrofit projects. BSC, in its role as the technical support team, provided technical review of project plans for all projects participating in the pilot program, and conducted in-field review and verification of measures for most of the projects. BSC also contributed to pilot program design and implementation. A significant portion of BSC’s involvement with the National Grid DER Pilot program was supported through the Building America program.

Four DER projects were completed through the Pilot program in 2010. Eleven DER projects reached completion in 2011. By the close of the Pilot program in December, 2012, 42 projects had been successfully implemented through the program.

The individual projects participating in the Pilot program have adhered to a common basic outline of target building enclosure performance. Project teams devised a variety of approaches to meet these targets. Particular conditions and configurations of the existing buildings resulted in a variety of implemented methods, and ways of addressing challenges.

Under a previous task order, BSC produced a review of methods employed and challenges faced by the early program participants (Neuhauser 2011). Also under a previous task order, BSC analyzed performance data for a sample of homes for which the DER project was completed or substantially completed prior to the last year of the program (Gates and Neuhauser 2013).

Now, with 42 DER projects representing approximately 60 dwelling units complete at the December 2012 close of the pilot, BSC has seized upon a unique opportunity to analyze performance data for a large population of DER projects. Studying this population of DER projects also reveals successful approaches for common retrofit challenges. . .

Download the complete report here.


  1. McIlvaine, et. al. 2013 and Neymark and Roberts as discussed in Aspen Publishing 2013.