Understanding Our Title 24 Performance

Given the wet weather and limited on-site work underway, we scheduled a meeting today with our LEED evaluator, Darrel Kelly, so we could understand our Title 24 report.  We engaged Davis Energy Group to certify our house under the LEED for Homes program and they assigned Darrel to our project.  He is with Builder’s Energy Services, Inc.

Our Title 24 Report was prepared by Meline Engineering.  They modeled our house using Energy Pro software (by EnergySoft) and, with that model, evaluated the projected performance of our house.  Based on the model that Meline Engineering developed, our house came in at an amazing 72.9% better than the standard, or baseline, design.  In California, if a house is 60% better than standard then it will receive the maximum points available under the LEED-H program.

Graph showing how a maximum of 19 points can be earned by exceeding Title 24 by 60%.

Graph showing how a maximum of 19 points can be earned by exceeding Title 24 by 60%.

For additional context, the first LEED-H Platinum house in the Bay Area, the Margarido House, is 55% better than standard.

The City of Monte Sereno was skeptical of how our geothermal heat exchange was treated in our Title 24 report.  They questioned how the auxiliary backup water heater was treated as an energy load and would not issue our building permit until this issue was resolved to their satisfaction.  Verifying the correct treatment required involving the California Energy Commission.

Robert Hudler of the California Energy Commission requested the Energy Pro model, which Lisa Meline provided to him on November 24, 2008.  Rob reviewed the Energy Pro model in detail and informed us on December 22 that ‘Everything looks good.’

Inquiring Minds Need an Answer

Why is our house 72.9% better than standard?

As an extremely experienced modeller and analyst of Title 24 performance, Darrel was very interested in understanding why our house was 72.9% better than standard.  Also, he was interested in how our Title 24 report did not require HERS verification (Home Energy Rating System).  Selfishly, our immediate interest in understanding our Title 24 performance was so we could specify our window and sliding glass door requirements (and lower the cost of these items) while not dropping below the 60% threshold for LEED-H Platinum certification.

Our Title 24 Report cover page, showing that we exceed the requirements by 72.9%.

Our Title 24 Report cover page, showing that our design is better than standard by 72.9%.

Leveraging Technology for Remote Collaboration

In order to analyze the Title 24 report, Bryan suggested that he meet with Darrel and they sit in front of Darrel’s PC and make some changes to the elements in the model and see how the projected performance changed.  Darrel responded and said let’s collaborate over the internet since he could share his PC screen in a secure session and the discussion could be over the telephone in real time.  Perfect!  This is the way to collaborate quickly and in a cost-effective manner.

The Analysis

After Bryan logged in remotely to the collaboration session on his PC, Darrel explained briefly how the software computes the expected performance of a building.  As he was doing so, Darrel was clicking through the model and commented on how Meline Engineering had created one of the best and most robust models of a house that Darrel had evaluated (Josh Pirkl, who developed the model, should be smiling!).  Oh, the first thing Darrel did was to use a copy of the model so we could make changes and not violate the integrity of the original model.

There were a couple changes that Bryan wanted Darrel to make in the model.  These changes related to changing the characteristics of certain windows and sliding glass doors in the house.  Darrel made the changes and then calculated the performance of the house.  These changes decreased the performance marginally.

Darrel then showed Bryan how to change the global parameters of the variables that were used in the model.  Sort of like changing the key variables in a financial model, such as the interest rate on long-term debt.  Darrel changed the performance of all the windows to a basic low cost window (i.e., single pane without a low emissivity coating in a non-thermally broken frame).  He recalculated the performance of the house and it dropped a few points.  Hmmm …

Bryan suggested that the extensive thermal mass inside the house could be a major contributor to the high performance of the house.  Darrel looked at the elements in the model for each room and noted that the floor material in each room was modeled (at least nine inches of concrete).  He quickly deleted ALL of the floor components and then recalculated the model.  This dropped the performance a bit more.  It was still high.

We looked at the positioning of house relative to the sun and it looked like it could be off by a bit.  Using Google Earth, we verified the north/south and east/west positioning of the house and made a few tweaks.  No change.  Darrel didn’t want to waste time on the small stuff, so he changed the orientation of the house by a full 15 degrees.  Very little change.

This was puzzling.  Darrel scanned the model again and then decided to change the type of heating in the house to a conventional forced air natural gas furnace.  The house was modeled using radiant heating.  Darrel changed the heating system to forced air, using typical leakages for ducting and other performance metrics.

Combined with the other factors, this change now dropped the performance dramatically, to approximately 22% over standard.  We found the key variable!

Then, Darrel increased the performance metrics of the forced air heating and recalculated the model again.  We were now at 40% better than standard but HERS verification was required.  With the increased performance of the system, the building envelope required a blower door test and the ducting needed a leakage test.

Bryan asked Darrel to verify the performance of the floor under the lower level.  Darrel went to the relevant area of the model and it showed an R-5 value.  Since R-10 rigid insulation was used under the foundation, this needed to be changed.  Darrel recalculated the model and the performance increased by a fraction of a percent.  Darrel then dropped the R value to zero and recalculated the model.  Performance dropped by less than one percent.

Ah, according to the software, Bill Brown was correct.  Bill had said that insulating the foundation under the slab with expensive, high compression rigid foam would do little to reduce the energy load on the house.

At this point, we’re sure, Darrel was sitting back and feeling good as he summarized why the house has such a high performance under Title 24.  The key design elements that contribute to the high performance of the house are:

  • The climate where the house is located (Monte Sereno/Los Gatos) requires more heating than cooling;
  • Radiant heating is much more efficient than forced air; and
  • The high thermal mass requires less energy to maintain a comfortable temperature.

Overall, combining radiant heating with high thermal mass results in the exceptional performance.  This performance is then increased even more with high performance windows and sliding glass doors, and increased roof insulation (R-45).  Due to the low cooling requirements established in the calculations because of the climate where the house is located, the large overhangs and orientation of the house do not affect the projected performance by much at all.

The elements that do result in high performance, radiant heating and high thermal mass, are basic to the design so no HERS verification (i.e., special testing) is required.

Findings

Analyzing the model yielded some interesting insight.  First, we know that changing the characteristics of the materials used in the house will not jeopardize our LEED points relating to energy usage under Title 24.  This is good.  Second, after living in the existing house for over 11 years, we know that there is a higher cooling load required than is calculated in the software.  The microclimate in Los Gatos has large swings in temperature and there are brief periods with very high heat during the day and limited times when the night temperatures remain high.  Finally, we feel great about investing in radiant heat for our house.

Working together in two remote locations for almost 60 minutes was valuable to help make some tradeoff decisions and, importantly, we all learned something and it was fun.

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