The Heart of HVAC Design

the heart of hvac design image

Many in the building industry believe that the heart of a home is the HVAC system, but what is the heart of the HVAC system?

Just as a heart has four chambers that work together to pump blood around the body, the HVAC system has four components that, when integrated, create a system that works in unison to create comfort in the home.

  1. Manual J evaluates and describes the tight well-defined and constructed thermal envelope.
  2. Manual S provides guidance for how to properly size the heating and cooling equipment.
  3. Manual D directs the design of the duct system that will deliver conditioned air around the home.
  4. The ASHRAE 62.2 standard ensures that we have the proper balance between air tightness, efficiency, durability and comfort with ventilation needed for the occupants and background moisture control.

In the heart of the HVAC System: Manual J

Manual J is used to determine the heating and cooling load for the house and directs us toward the heart of the HVAC design. The objective of the heating and cooling load is to tell the designer how much energy is needed to achieve the design goals with a specific thermal envelope and piece of mechanical equipment. Heat moves from warm to cold and the thermal envelope retards or slows that flow. The International Energy Conservation Code (IECC) and the International Building code (IRC) both lay the ground work for the HVAC design.  These codes give us design temperatures.  In other words, they tell us the temperature difference (or Delta T) between inside and outside in the winter and summer seasons.

R-Values & U-Values, the Thermal Envelope

The R-values and U-values of the thermal envelope resist or slow the movement of energy from the inside to the outside in the winter and from the outside to the inside in the summer through the building envelope. The thermal envelope happens to be one of the most important components in the HVAC heart because its execution determines the success of most of the other interactions that take place to allow the home to be efficient, durable, safe, and comfortable. The IECC lays out climate zone-specific minimum R-values and U-values and construction installation techniques that have been deemed to create comfort in each area of the country.

When a designer inputs the specification for the insulation, windows, air barriers, and assemblies into the software to create the heating and cooling load for the house at the summer and winter design temperatures, the assumption is that the installation of the specification is perfect. The installed R-value is achieved, the house is airtight, and there is complete alignment between the thermal barrier and the air barrier of the home, as required by the IECC to ensure that the energy generated by the furnace and AC system will heat or cool the house during worst case conditions. A well-designed and constructed thermal envelope better resists the flow of energy through its assemblies. If the insulation, airtightness, and alignment of the air barrier and thermal barriers of the home are not executed well, modern HVAC systems that are not correctly sized could have difficulty maintaining comfort in the home. The ultimate objective is for the systems to run continuously to maintain the indoor design temperature when the worst case outdoor temperature has been reached or exceeded for those few hours of a day.  It turns out that the HVAC design process already has a built in safety net. If you thought that oversizing the system is a technique to ensure comfort, you would be wrong.  In our modern homes oversizing leads to equipment that will not perform properly – freezing coils, short cycles, etc.

Our homes are integrated systems that rely on each component of the system to work in unison with the other components. This is why EnergyLogic begins our HVAC design process with an understanding that we will evaluate the construction of the thermal envelope and its compliance with the requirements of codes and programs that aim to ensure that high-performance homes perform. To learn more about EnergyLogic’s HVAC design process, visit our New Residential HVAC Design page.


Robby Schwarz (faked)

Who to Contact:

Robby Schwarz
Principal, Director of Builder Relations

Email Robby

EnergyLogic Tech Bulletins

EnergyLogic crafts many Tech Bulletins on a variety of subjects to help educate our clients while ensuring that we are all on the same page regarding what is required by a program or a code.

Our Tech Bulletin’s topics range from ventilation to insulation, from fan ducting installation to warm/conditioned crawl space design, and more. Most topics we cover come from questions you ask that need added research and understanding. If necessary, plan sets or scope-of-work documents may be created to address further details for each topic.

Our tech bulletin on bath fan installation is a good example of a topic that initially came from a question that needed a better building science perspective. In consideration of ENERGY STAR® best practices, as well as specific building codes, we are measuring the flow through bath fans more often.

Other Tech Bulletin topics that might be of interest to you can be found on the EnergyLogic Blog. Below you will find a few relevant post links for reference:

Whole-House Ventilation Strategy

Vent caps and fan flow

Training resources to help reduce re-inspections

If you have a topic or situation that you think needs a better-applied building science perspective, please let us know and we will do our best to research your question and get a new Tech Bulletin on the subject.


Robby Schwarz (faked)

Who to Contact:

Robby Schwarz
Principal, Director of Builder Relations

Email Robby


Energy Logic – All Homes Will Be Energy Efficient


EnergyLogic is in the news! Check out this interview with Robby Schwarz, one of the founders and continuing principals here at EnergyLogic. You’ll find the article supporting how EnergyLogic continues to help builders, salespeople, and consumers in our industry better understand the long-term benefits that energy efficiency will bring to their lives.

To access the article, please click on the link below:

Northern Colorado ENERGY STAR® Article


Robby Schwarz


Who to Contact:
Robby Schwarz
Principal, Director of Builder Relations

Email Robby

Field Fusion Recap: The Disconnect Between Fire & Energy Codes

The Q3 Field Fusion event delved into air-sealing and sound transmission challenges in multi-family units through a guided discussion that included perspectives from Code Officials, Insulators, and Raters. Read more here.

There are many complexities that accompany building townhomes and duplexes.  For example, townhomes and duplexes built with common fire separation walls (party walls) are twice as leaky as single family houses that are twice their size.


The shaft wall, which we see most often in Colorado, is open directly to the outside through the designed gap between the shaft liner and the framing, thus creating a leaky assembly.  An additional complexity arises when the reduction of unit-to-unit sound transmission is taken into account, which requires correctly installed insulation.

EnergyLogic’s August 31st Field Fusion delved into the details of these assemblies through a guided discussion that included perspectives from Code Officials, Insulators, and Raters.

Clarification: What Part of the Shaft Wall Assembly is Fire-rated?  recap-of-ff-image_1

We must first define what part of the shaft wall assembly is fire-rated, as the entire assembly is not.  This is an important distinction that allows for more air sealing options once understood.

In chapter 3 of the IRC, Section R302 “Fire Resistant Construction” and Section R302.2 “Townhouses” states, “The common wall shared by two townhouses shall be constructed without plumbing or mechanical equipment, ducts or vents in the cavity of the common wall. The wall shall be rated for fire exposure from both sides and shall extend to and be tight against exterior walls and the underside of the roof sheathing.”

This statement in the IRC is our first indication that the two layers of sheetrock in the shaft liner wall are the fire-rated two-hour wall, designed to slow the spread of fire from unit to unit.  Thus, the two layers of 1” drywall cannot be penetrated with ducts.

The framing (which is held off the fire-rated assembly by a clip) often has ducts or plumbing in it and is specifically designed to burn and separate from the two-hour assembly when the clip melts.  This allows one unit to burn and fall before fire is able to pass through to the adjacent unit.  The UL (Underwriters Laboratory) listing for many of these assemblies’ references section 705 of the International Building Code (IBC) which states in Section 705.2 “Structural Stability”, “Fire walls shall have sufficient structural stability under fire conditions to allow collapse of construction on either side without collapse of the wall for the duration of time indicated by the required fire-resistance rating.”  This is another indication that UL listings and the code are in agreement that the fire assembly is the two layers of 1” drywall and not the framing adjacent to the drywall.


It is important to point this out because throughout Colorado there is not a common understanding of what constitutes a shaft liner fire-rated party wall assembly.  Some jurisdictions still hold that the assembly is the drywall, air gap, clip, framing and interior drywall while others hold that it is as explained above.   What is consistent is the understanding that the assembly must be built continuously from the foundation to the roof deck.

EnergyLogic suggests having a discussion with jurisdictions, in an effort to:

  • Ensure a common understanding of this assembly
  • Determine how the assembly will be air sealed to control airflow to meet the air leakage requirements of the energy code.

One thing to note: jurisdictions throughout the state require that the two layers of sheetrock run continuously from the foundation to the roof deck, but not the entirety of the rest of the assembly.  The most conspicuous example is the interior drywall, which is always missing on the ventilated attic side of the party wall.

Challenge: How to Achieve 3 ACH50?

Now that a common understanding of the assembly has been achieved, it is time to determine how the assembly can be air sealed in order to meet the air leakage target of 3 ACH50 for the 2012 and 2015 IECC.  Most jurisdictions have not amended the requirement to meet this airtightness level, so pre-planning is crucial in order to be successful.

The clip that holds the framing off the two-hour party wall assembly creates a 1” gap that is connected directly to the outside at the front and back of the unit, as well as to the attic.  This is where the UL listing of the assembly comes into play. UL is an American safety consulting and certification company that provides the one or two-hour rating for fire-rated assemblies by testing them in a laboratory environment. The UL listing for these assemblies is often mixed up with code’s definition of the assembly, which creates confusion regarding what materials are allowed to be used to seal them.

UL often refers to fire-blocking materials.  Fire blocking materials are usually defined within the UL assembly and can be any one of the following:

  • 2” nominal lumber
  • Two thicknesses of 1” nominal lumber with broken lap joints
  • One thickness of 0.719” wood structural panel with joints backed by 0.719” wood structural panel
  • One thickness of 0.75” particleboard with joints backed by 0.75” particleboard
  • Gypsum board, including 1” DensGlass Ultra® Shaftliner and 5/8” DensArmor Plus drywall
  • Batts or blankets of mineral wool or fiberglass
  • Other approved materials installed in such a manner as to be securely retained in place shall be permitted as an acceptable fire block (Section 717.2.1, 2006 IBC). recap-of-ff-image_3

As some fire blocking materials are air barriers and some are not it important to choose a material that can stop the flow of air.  EnergyLogic has seen the most success when builders tackle fire blocking on each individual floor.

Application: The Picture Frame Method

When looking at the party wall assembly, envision a picture frame around the perimeter of the wall. All four sides need to be fire blocked.  The material of choice right now is the same 1” gypsum board used in the 2-hour rated assembly.  Install the 1” gypsum board in the 1”  gap between the interior of the unit and the outside at the front and back of the units, between floors, and to the attic.  Again, picture framing the party wall.


Depending on your foundation type, if you are standing on the first floor you will need to air-seal the two layers of gypsum and the bottom plate to the slab in the shaft wall, or address the rim joist connection in the basement or crawl space to the shaft wall. At the rim joist be sure sill seal has been installed between foundation and sill plate as it is your primary capillary break, then seal the sill plate to foundation, seal rim board to the sill plate, and seal the rim board to sub floor.  Pay special attention to any knockouts for foundation bolts.

Once the large 1” gap has been fire blocked with an air-impermeable material such as gypsum, seal the smaller gaps between the fire block and the shaft wall and the fire block and the framing.  A fire-rated caulk or expanding foam works for this. Following these steps, with careful attention to detail, should enable you to successfully achieve 3 ACH50.recap-of-ff-image_5

A few words of caution:

  • Ensure that the drywall lid is air-tight: duct boots and other penetrations need to be sealed. In addition, as required by ENERGY STAR, the drywall to top-plate should be sealed. (This is a requirement of code that is generally not enforced.)
  • Mechanicals can derail all good air-sealing intentions. Undampered ducts run to the exterior for combustion or ventilation air as well as atmospherically vented appliances. These combustion air ducts can ruin one’s ability to build a tight home that gains control and predictability of the airflow in the building.

Don’t Forget: Sound Reduction

Lastly, these assemblies should reduce sound transmission from dwelling unit to dwelling unit.  The party wall is assumed to be an adiabatic wall, i.e. there is no heat loss or gain through the wall between two conditioned spaces as the temperature is the same on each side. Therefore, the insulation is primarily installed to lower sound transmission.  The principles of sound reduction and heat flow are the same, so proper installation of the insulation in the framed cavity of the party wall is imperative.

NAIMA, the North American Insulation Manufacture Association, states that the installation of insulation in a party wall application should “comply with the manufacturers’ instructions including filling the entire stud cavity and cut to fit around outlets, junction boxes, and other irregularities in the cavity.”  In other words, the insulation in a common party wall should be installed to a RESNET, Grade 1.

To learn more please see EnergyLogic’s Tech Bulletin on “Fire-rated Party Walls”  which includes an article by Building Science Corporation.

Have a technical question? Contact Robby Schwarz.

Our next event will take place on November 16th. It is focused on Selling High-Performance Homes.  Our guest speaker, Todd Gamboa, President of Building Trust LLC., has a wealth of experience and perspectives to share.  Please see details here.

If you have suggestions for topic you would like to see discussed in depth, please let us know. We will be releasing our Q1 2017 event topic and date soon.

Robby Schwarz (faked)


Robby Schwarz

Principal / Director of Builder Relations

EnergyLogic, Inc.


Contact Robby Schwarz

Introducing our new HVAC Designer, Scott Olson


Introducing our new HVAC Designer, Scott Olson

Q&A: Learn more about Scott, his design philosophy, background, and credentials!

What was your first job in residential construction industry?

Construction Superintendent for a National Home Builder in Denver. The Job consisted of front and back‐end scheduling, and warranty work.

How and when did you first become interested in high performance homes and energy efficiency?

While working for a local home builder, I helped create their High Performance testing on all homes. I received my Residential Energy Services Network (RESNET) Certification and tested homes for energy code compliance and made sure homes were ENERGY STAR compliant. I balanced the needs of a production builder with ENERGY STAR best practices.

How did you get into HVAC Design?

I was challenged by my boss, while working for a large production builder, to take on a whole new role as an HVAC Designer. My construction experience as a superintendent was a huge advantage and I strived to make my designs be trade friendly and cost-effective.

What insights did you gain on HVAC designs when you worked directly for a large production builder?

Designed cost effective, construction friendly and functional HVAC systems

What elements of EnergyLogic’s HVAC design philosophy resonated with you and attracted you to the job?

EnergyLogic’s HVAC design services, with a goal of ensuring comfort and efficiency with properly built envelopes integrated with properly-sized and designed HVAC systems.

What are some of the common design mistakes or misconceptions builders should watch out for?

Sizing HVAC with a rule-of-thumb calculation. With today’s tighter building envelopes, rule of thumb often results in oversized systems that cost more upfront and cost more to operate over time. Oversized systems also don’t dehumidify as well as properly sized systems because the run time is shorter, so occupants are actually paying more for discomfort. 

What do you like to do in your free time?

In my free time, I enjoy grilling, working on projects around the house, and camping.