Client Questions & Gabel Answers

At the Gabel Energy Client Seminar on September 10, 2019, we collected lots of your questions. They covered topics including building envelope, mechanical systems and solar PV, plus some overarching issues. Thank you for asking! Here is our first pass at giving answers to some of your questions, plus other resources for more detailed information. We’ll follow up with more answers in the future, but please feel free to contact us if you need help sooner rather than later!

Building Envelope and QII

Q1 (Question 1): Can you provide cheat sheets for contractors addressing exterior insulation, air sealing and PV?

A1 (Answer 1): CalCERTS has a detailed “QII Handbook” for installers and HERS raters for the 2019 code that you can download at their website under the “Resources” tab. It includes details on exterior insulation and air sealing, but not PV.

Q2: What exactly is the “Air Barrier”? Where is it defined? What code? What assemblies? What to avoid?

A2: The “Air Barrier” is defined in the 2019 Building Energy Efficiency Standards (Title 24, Part 6) as “… a combination of interconnected materials and assemblies joined and sealed together to provide a continuous barrier to air leakage through the building envelope that separates conditioned from unconditioned space, or that separates adjoining conditioned spaces of different occupancies or uses.” Having a tight air barrier is important to meet Energy Code requirements to limit air leakage into (infiltration) and out of (exfiltration) the conditioned building envelope. It is an important part of meeting QII (quality insulation installation) as well.

Materials have to meet ASTM tests to prove that they meet specific air permeance requirements to qualify as part of the continuous air barrier, and they must be installed per manufacturer’s instructions. Here is a list of materials that have already been approved as part of the continuous air barrier (2019 Reference Appendices, RA3.5.2 Definitions):

  • Plywood – minimum 3/8 inch
  • Oriented strand board – minimum 3/8 inches
  • Exterior or interior gypsum board – minimum 1/2 inch
  • Insulation board (sealed at seams per manufacturer’s instructions)
    • Foil-back polyisocyanurate – minimum 1/2 inch
    • Extruded polystyrene – minimum 1/2 inch
    • Foil backed urethane foam insulation – minimum 1 inch
  • Spray polyurethane foam (SPF)
    • Closed cell SPF – minimum density of 2.0 pcf and minimum thickness of 2.0 inches
    • Open cell SPF – minimum density of 0.4 to1.5 pcf and minimum thickness of 5.5 inches
  • Cement board – minimum 1/2 inch
  • Built up roofing membrane
  • Modified bituminous roof membrane
  • Particleboard – minimum 1/2 inch
  • Fully adhered single-ply roof membrane
  • Portland cement/sand parge, or gypsum plaster minimum 5/8 inch
  • Cast-in-place and precast concrete
  • Fully grouted uninsulated and insulated concrete block masonry
  • Sheet steel or aluminum

The CalCERTS QII Handbook described in Q1/A1 has sections on envelope air barriers and air sealing that will answer questions about what assemblies are involved and what to avoid. These sections include photos and descriptions of different Energy Code requirements. Here’s one illustrating what NOT to do regarding the mandatory requirement (Section 110.7) to seal all “… joints, penetrations and other opening in the building envelope that are potential sources of air leakage …”

Here’s an example from a 2016 Energy Code Ace Installation Guide showing one of the air barrier techniques required to meet QII:

Q3: Is hard backing required at light wells for skylights?

A3: Skylight light wells that separate indoor conditioned space from unconditioned attic space have to be insulated and designed to prevent air leakage like other parts of the building envelope. To meet QII, the light well walls have to meet the R-value or U-factor requirements for those surfaces per the CF1R Certificate of Compliance, and the light well insulation must be enclosed on all six sides with materials that meet the air barrier requirements per Q2/A2 above.

Here’s an illustration from the Energy Code Ace 2019 Decoding QII handout and the CalCERTS QII Handbook showing skylight light well requirements:

Q4: Do you have to meet all applicable QII construction details to comply with QII, or can some of them be traded away in the performance method?

A4: You do have to meet all applicable QII construction details to comply with QII. QII overall is a prescriptive requirement under the 2019 Energy Code, so, if using the performance method, you may be able to install some combination of energy efficiency measures and/or battery storage to eliminate the QII requirement altogether. However, you cannot trade away any part of the QII requirements and still get QII compliance credit.

Q5: How do you do an insulated header in an extended 2×4 wall?

A5: For this level of detail, we recommend that you review the CalCERTS QII Handbook or view the Energy Code Ace recording of the 2019 Decoding QII presentation ( Here is a slide on insulated headers from the 2019 Decoding QII handout that shows that R-3 is required for 2×4 framing while R-5 is required for other framing sizes:

HVAC and Water Heating Systems

Q6: If my project alters or replaces the space conditioning, is HERS required? Is this only for additions greater than 1,000 ft², or does it also include renovations regardless of area? Is this a new requirement?

A6: HERS testing is required for both altered and replacement space conditioning equipment in additions greater than 1,000 ft², for any size new dwelling unit, such as an ADU, and for alterations of any size, but it is limited to certain types of equipment. HERS testing is required if you are adding, altering or replacing ducted HVAC equipment and/or ducting, though there are some exceptions. For example, HERS duct leakage testing is not required if you are extending less than 40 linear feet of ducting to an existing HVAC system. This is not a new requirement, although some aspects of it, such as the clarification about ADUs, have evolved over time. For detailed information, please refer to the “Residential HERS Key” you received as one of your handouts at the Gabel Client Seminar on September 10th.

Q7: Does a MERV 13 air filter capture particulate matter down to PM2.5?

A7: MERV 13 air filters are a new mandatory requirement to improve indoor air quality in the 2019 Energy Standards. PM2.5 refers to very fine particulate matter that is only 2.5 microns or 0.0001 inch in size. This kind of particulate is especially unhealthy because it can go deep into your lung tissue and from there into your bloodstream (see for more information). A MERV 13 air filter is designed to capture at least 50% of particles between 0.30 and 1.0 microns and at least 85% of particles between 1.0 and 3.0 microns. So, a MERV 13 air filter does capture at least 85% of PM2.5 particles, and when properly installed, it can substantially improve indoor air quality.

Q8: What is “VRF”?

A8: VRF stands for Variable Refrigerant Flow. A VRF system is a type of multi-split heat pump or heat recovery space heating and cooling system that allows for flexible delivery of heating or cooling to multiple different zones within a building. VRF systems typically use a single outdoor condensing unit to distribute variable amounts of heated or cooled refrigerant to multiple indoor air handling units. They are similar to ductless mini-split heat pump systems in using refrigerant to deliver both heating and cooling.

Q9: Is there a list of “preferred” heat pump water heaters or heat pump water heater manufacturers?

A9: There is a substantial performance method energy compliance credit for NEEA Tier 3 heat pump water heaters, so those are ones for you to consider. These are heat pump water heaters that have been rated by the Northwest Energy Efficiency Alliance (NEEA) to meet Tier 3 of their Advanced Water Heater Specification. For California Energy Code compliance credit, you need to select a NEEA Tier 3 heat pump water heater from their Qualified Products List available at The list includes NEEA Tier 3 heat pump water heaters from a variety of manufacturers, and it gives water heater volume, maximum recommended household size and efficiency rating for each product (see list excerpt):


Article written by Rosemary Howley, Manager, Utilities & Education/Senior Energy Analyst
First published in the Gabel Energy Summer/Fall 2019 Newsletter