Saving Money On Insulation With a HERS Rating/ERI in 2021 IECC

Date Published: September 14, 2023

If you are in the building industry, you are probably no stranger to the 2021 IECC. This is the most recent iteration of the International Energy Conservation Code (at the time of this writing), and it has really tightened up certain construction requirements for newly built homes.

One of its most significant changes has been insulation requirements. In the past versions of the IECC, insulation levels have been more monetarily manageable, but the 2021 version has increased insulation requirements significantly. Most notably a call for R30 in the walls and R60 in the ceilings in Colorado’s climate zone.

If you choose to go “by the book” or take the prescriptive path when building, and adhere completely to IECC 2021 guidelines, your insulation costs are going to go up relative to prior years. However, there is an alternative route…

For the first time, the ERI performance route of the IECC codes is starting to be the more cost-effective option, and performing a HERS Rating could potentially save some serious cash on insulation!

What is HERS Rating?

First, let’s have a quick refresher. HERS, which stands for Home Energy Rating System, is a method used by architects, builders, and energy raters, to determine the overall energy use of a home. The Residential Energy Services Network (RESNET) established the HERS index, the standard that measures a home’s energy efficiency. Note that HERS is the RESnet branded term for Energy Rating Index (ERI), which is the terminology specifically used in the 2021 IECC.

HERS index. Used for HERS Rating system

In order to calculate the HERS Score of a building, a certified HERS rater must evaluate all energy-based facets of the home, including insulation, windows, heating/cooling units, etc. The rater then uses specialized software to calculate the score of the home – the lower the score, the better.

Where the HERS Rating system really stands out is that it gives you a level of flexibility through “trade-offs.” In other words, a certain aspect of the home with super high energy efficiency can compensate for another area that isn’t as energy efficient. For example, if the home is utilizing high-efficiency heating, cooling, and water heating methods, the wall and attic insulation could possibly have reduced R-values and still have a similar HERS score. With this trade-off, the overall score of the home can still be IECC-compliant, despite the lower energy efficiency of the insulation. Do you see where this is going?

To learn more about HERS Ratings, check out our 4-part article, The Ultimate HERS Rating Guide: Part 1, Part 2, Part 3 & Part 4.

HERS Rating Scores

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Prescriptive Compliance Vs. Performance Compliance

Section R401.2 of the IECC 2021 states that, in order to reach compliance, there are several options you can take. The first option is the prescriptive path. This is the “by-the-book” method, where the builder must follow the guidelines established in 2021 IECC in sections R401 through R404. This route has generally been more common. However, now that requirements are stricter, it is becoming more difficult and costly to stay within the prescriptive path.

As mentioned above, insulation standards have tightened significantly from 2018 IECC to 2021 IECC. While attic insulation previously had to be at a minimum of R49, it has been raised to R60. Similarly, wall insulation has been raised from R23 to R30 cavity (or R20 cavity + R5 exterior rigid continuous insulation). These new levels are much harder to attain (especially in a standard 2×6 wall) and it drives up insulation costs drastically.

Image sourced from:

That is where the performance path can really come to the rescue. The performance option is a less rigid route that allows builders to design the home in a fashion they see fit, as long as it meets the 2021 IECC requirements for total energy performance. For this route, design teams have the liberty to employ various combinations of design strategies, and technologies to provide the greatest energy savings for the lowest overall cost.

For Colorado, there are two viable performance options in the 2021 IECC  handbook: The Total Building Performance Option, which is explained in section R405; or The Energy Rating Index (ERI) Option, described in section R406.

The two options are similar. They both require the use of a HERS rater to do pre-reports for a permit, then do all the verification steps during construction, and produce final reports at the end of construction. The main difference is that The Total Building Performance Option produces an IECC Compliance report (further explained in R405), while the ERI Option requires a full ERI report (further explained in R406).

Saving Money with Performance Compliance

There are multiple ways to save money when taking the performance route. We’ve already mentioned it, but let’s just drive it home – doing a HERS rating can save you big bucks on insulation! If you are taking the Total Building Performance route, insulation levels must at least meet 2009 IECC levels. But anything at or above that is acceptable, as long as the home meets the 2021 IECC requirements in total performance.

On the contrary, the ERI method does not technically require an insulation minimum. As long as the home yields an overall score of 55 or less, you can take liberties with the insulation thickness. This provides you with the freedom to plan the most budget-friendly approach to your building project.

Say you do end up going with insulation that meets the 2021 IECC prescriptive standards. Using a HERS rating may give you the flexibility to save money on other components of the home.

Also keep in mind that, whether you are taking the prescriptive or performance path, you will be required to get duct leakage and blower door tests. It just so happens that our HERS service packages include both duct leakage and blower door tests. So when taking the total performance or ERI route, you will be bundling all the necessary energy tests (i.e. duct leakage and blower door) and adding that extra flexibility with trade-off options.

While every job is different and each county’s requirements vary, an analysis with a HERS Rating has the potential to unlock more freedom within your build.

HERS Rating with Scott Energy Services

At Scott Energy Services, a division of Scott Home Services, we are your one-stop shop for all your energy needs. Our energy specialists are well-versed in Colorado energy code, and can advise you on your requirements based on your municipality. We offer all the required onsite testing for a standard HERS rating, from the planning to the finished product.

We know that when it comes to energy codes, there is a lot to take in. So if you have any questions, feel free to call us. One of our energy specialists will be happy to help.

If you are in need of HERS ratings services on your next project, visit us online to learn more, or contact us to book your services now.

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About the Author: Chris Kimmel

Chris Kimmel worked as an Associate Home Inspector for two years, handling numerous services including sewer scope inspections, pest inspections, mold air sample testing, radon testing, and water quality testing. Chris now works with Scott Home Inspection as a Content Writing Specialist.

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