HPA and HPW Cost Analysis

By: William Allen, PhD


In California, energy efficiency requirements for new buildings are governed by the Building Energy Efficiency Standards, which form part 6 of Title 24 of the California Code of Regulations. The requirements, commonly referred to as Title 24, are updated on a three-year cycle. The current version is the 2016 code, which came into effect on January 1st, 2017. The requirements of the 2019 code were approved on May 9th, 2018 and will come into effect on January 1st, 2020. Any buildings for which the permit is pulled after January 1st, 2020 will be governed by the 2019 code.

Title 24 allows for either prescriptive or performance-based compliance with energy efficiency requirements.  Nearly every residential building constructed in California follows the performance path, which allows for tradeoffs between different building components to meet an energy budget.  Prescriptive compliance requires each element of a building design to meet a specific standard (such as R-value of wall insulation) defined in the energy code. This is a clear and straightforward approach that requires no calculations. The disadvantage of the prescriptive method is that it allows no flexibility for relaxing the code for one element of the building in return for exceeding it on another, which limits the options available to the architect or builder.

The performance method for compliance resolves this issue by considering the energy efficiency of the building as a whole. This allows builders the flexibility to meet Title 24 requirements in different ways that may depend on criteria such as siting, customer demands, aesthetic considerations—or perhaps most importantly, cost. To determine compliance using the performance method, the building is modeled using an approved software package and the energy use of the building is compared to a ‘standard’ building. The standard building has the same floor area and window area as the proposed building but follows the prescriptive standards for each component. The proposed building complies if the energy it uses is less than or equal to the energy used by the standard building. Under the 2016 code, the standard building is therefore modeled using a high performance attic (HPA) and high performance walls (HPW).

The energy efficiency of the building is measured using the Energy Design Rating (EDR). This is a number based on the energy use of the building, where an EDR of 100 matches the efficiency of a building that would meet the 2006 building code, and 0 would be a building that uses no net energy (to achieve a rating of 0, the building would need to be equipped with renewable generation capacity to match the energy it uses).

Under 2016 Title 24 rules, solar PV can be used to boost the efficiency rating of a house for compliance purposes. However, in the upcoming 2019 Title 24 code this will no longer be possible. To ensure that overall energy consumption is minimized (i.e. to avoid inefficient buildings using large PV arrays to compensate for high energy use, compliance under the 2019 code will have two components, an Efficiency EDR rating and a Final EDR rating.  The efficiency rating is based on the performance of the building before any renewable generation is considered, and the final rating takes the renewable generation into account.

The table below (taken from the CBECC-Res compliance software) shows an example of how compliance will be determined. Both the Efficiency EDR (i.e. just the building) and the Final EDR (i.e. building + renewables) of the proposed design must be equal to or less than the ratings of the standard design for the building to comply.

The efficiency requirements under the 2019 code are more stringent than the 2016 code, which were more stringent than the 2013. The ongoing drive for more efficient buildings will require changes in the way houses are constructed. Buildings can be made more efficient through changes to the envelope (e.g. lower air leakage, High Performance Attics (HPA), High Performance Walls(HPW), upgraded windows) and/or changes to the equipment (e.g. more efficient water heaters, lighting, HVAC systems, etc.). One of the key questions to be answered is: what is the most cost-effective way to build in order to comply with the new standards?

To calculate the effectiveness of different energy efficiency improvements, a 2700sqft 2-story single-family home and an 13000sqft 3-story 7-unit multifamily building, both with basic features, were modeled in CBEEC-Res. The features of the modeled building were typical of those used in current construction. Both buildings were modeled in climate zone 3. A series of individual energy efficiency measures were added to the models to determine the improvement in building EDR that results from each individual measure.


The measures included are listed in Table 1 below

Table 1

The cost of a change to established building practices can be hard to determine precisely. The cost of, for example, upgrading a domestic water heater from a 0.82 energy factor to a 0.95 energy can be easily determined – the labor cost will not change, so the only change is in the unit cost of the heater itself[2].  Calculating the cost of less straightforward measures is complicated. For example, moving from a wall construction that uses 2×4 studs with R13 fiberglass batt insulation in the cavity to using 2×6 studs with R-19 batts and 1” of continuous XPS insulation on the outside involves not only a change in material costs, but also a change in labor costs. The labor costs can change both because of additional assembly steps (adding exterior insulation in this example) and because of unfamiliarity with the new construction methods.

The modeled cost will also be affected by the assumptions made about how walls were built before the change to high performance walls. If the ’before‘ wall construction used a high density mineral wool blanket rather than a cheaper fiberglass batt, the extra cost for the ‘after’ wall construction would be less. If the ‘after’ wall uses an R-19 batt in the cavity and R-4 continuous insulation on the wall exterior, this will have a different cost than using an R-21 batt and R-2 on the exterior, despite the two walls having the same R-value. For this work the comparisons made were, as far as possible, like for like. This gives the most realistic comparison of the cost, and disruption to working practices, arising from the changes.

In order to get as accurate an estimate of costs as possible for the calculations used here, each cost component was taken from three sources. For material costs, this was a combination of supplier’s price lists, internet pricing, and information from builders. For labor costs, a variety of sources were used, including RS Means, manufacturers associations, suppliers, sub-contractors and builders.

For each measure the change in EDR was then divided by the cost, giving the cost effectiveness of each individual measure.

Figure 1 shows the cost effectiveness ranking of various measures for the single-family home and the 7-plex in climate zone 3. The cost-effectiveness is measured in EDR points per $100. Visually, the longer the line, the more cost-effective the measure is.

Figure 1: Cost effectiveness of individual measures for a single-family home and a 7-plex in CZ-3

The difference in building type results in different rankings for the measures. Several measures, such as upgraded furnace and ducts in conditioned space, are among the most cost-effective measures for both buildings.

Packages of measures

Deciding which efficiency improvements to use is made more complicated by the fact that the effect of improvements to the building are not independent of each other: for example, the benefits of moving ducts into conditioned space will depend strongly on conditions in the attic they would otherwise be in. The energy savings that come from moving ducts into conditioned space will be lower if done in conjunction with a high-performance attic than if it is one as a stand-alone measure. As the energy savings are lower, so will the cost effectiveness. To properly determine the benefit of a measure when implemented alongside other measures, it is necessary to model packages of measures and consider the overall cost effectiveness of the combined measures.

The packages presented below represent combinations of measures that will bring the modeled building from a “bare bones” design—consisting of the lowest-performing equipment and assemblies currently used in today’s residential new construction market—to a home that would comply with the proposed efficiency requirements for Title 24 2019. The measures used in the packages were selected based on their stand alone cost effectiveness – while there is a measure of judgement involved, the choice is simplified by the fact that while the cost effectiveness of one measure may be reduced by combining it with another measure, it will not be increased, so a measure that has a low cost-effectiveness will not get more effective as part of a package. Table 2 shows the measures modeled and the costs for the single family home.

The ranking of the packages was calculated by modeling every possible combination of the measures studied and finding the total improvement in EDR due to the package. This was then divided by the total cost of the package to allow the packages to be ranked in order of cost-effectiveness.

Table 2: Measure costs for a 2-story single-family home

Table 3: Most cost-effective packages for compliance for a 2-story single family home


Because of the complexity of determining the cost of each measure, Table 3 (and Table 5) lists the change in EDR due to each package of measures as well as the cost effectiveness. This will allow readers to independently calculate their own costs should they feel the costs listed differ from their own experience. The change in EDR listed is the total due to all the measures combined.

For the 7-plex, the measure included in the packages are listed in Table 4, and the most cost-effective packages are listed in Table 5

Table 4: Measure costs for a 7-plex

Table 5: Most cost-effective packages for compliance for a 7-plex

How to read the tables

The features in the cost tables for the measures use the abbreviations from Table 1. The tables showing the total package costs either list the option number when there is more than one option (ducts and attics) or use a ✔ to show the upgraded option when there is only a choice of basic or upgraded. In both cases the basic option is shown with an x.



There are many energy efficiency upgrades possible when designing and building houses to meet the increasingly stringent energy efficiency standards. Builders can choose packages of measures that will best suit the needs of the project or customer. It is clear from the tables above that high performance attics and walls can form a useful part of package of efficiency measures that allow a building to meet the new standards. As efficiency standards continue to become more stringent, high performance walls and attics are likely to become ever more prevalent throughout the building industry in California.


[1] Attics with different level of insulation are included in the calculations. All the attics model fall into one of these three types, either with or without a radiant barrier (RB)

[2] Even here, it is possible that if all builders switch to more efficient heaters, the demand will raise prices of the heaters and make the replacement less cost effective than it seemed. So all the conclusions here should be treated with caution.