Common Building Science Mistakes to Avoid

This blog post first appeared in two issues of Fine HomeBuilding Magazine: Issues 321 and 322.  Part of a series that included 17 short articles written by 7 different authors.  My first contribution appeared in the letters and reader feedback section of the February/March issue, titled “The biggest mistake of all?”  You can read that post here. 

Common mistake: Ductwork in unconditioned spaces.

The BS:  The building envelope of a home is designed to be a separator, keeping the outside, out and the inside, in.  We want the heating and/or cooling we pay for to remain inside for as long as possible and not have losses when it moves from the place where it is produced to the area where it’s supplying comfort.  When we place ductwork outside the building envelope, either in an unconditioned crawlspace, attic or other exterior spaces, there is the possibility of creating inefficiencies with these systems.  This can happen in a few different ways.  If there is an air leak in the supply air ductwork that is outside the building envelope, that air that is leaking to the outside originated from inside the home.  The lost air will be replaced by outside air leaking into the building somewhere.  Not only did you lose the air that was just conditioned, but unconditioned outside air has leaked back into the home, further increasing the heating or cooling load on the home.  Now if air leaks into the return duct from outside, the home becomes pressurized and conditioned air will be forced out of the home.  The origins of the outside air that leaks into a return duct may be of poor quality that may affect human health or a building’s durability.  Ducts outside the thermal envelope also need to be well insulated.  Not only will heat loss or gain contribute to cost and comfort issues, but condensation can form on either the inside or outside of the duct.  This can create issues with a build’s durability and indoor air quality.

Get it right:  When ducts are located outside the building envelope, codes specify insulation and air tightness levels.  These codes are found in chapter 11, Energy Efficiency, Section N1103 (R403) Systems.  The codes are very specific to levels of both insulation and the air tightness of the ducts.  Duct tightness testing is required when ducts are outside the building envelope.  Bringing the ducts into the conditioned space greatly simplifies these requirements.  In crawlspace applications, simply changing the crawlspace from vented to a conditioned space may be the best choice.  In attics, you can choose to condition the space by insulating and air sealing the roof deck instead of the attic floor.  Another option is to use a plenum truss (a notch in the truss that is created by the truss manufacturer where ducts and other mechanical systems can be installed) or create soffits inside the home where ductwork can be hidden.  Duct tightness testing has been around for several code cycles, but new to the 2021 energy code is the requirement that all ducts, whether inside or out, be tested.  The requirements are less stringent when the ductwork is inside the building envelope, a test result of 8 CFM per 100 square feet of conditioned floor area is needed.  Testing when ductwork is outside the thermal envelope requires a test of less than either 3 or 4 CFM per 100 square feet of conditioned floor area depending on whether the air hander is installed before testing is conducted.  Refer to Chapter 11 of the IRC, section N1103 (R403) for specific requirements.

Common mistake: Not commissioning a new home.

The BS: A house is a complex assembly of systems that need to work together to provide for the shelter, safety, health and comfort of the occupants.  Every home is unique and is required to be set up in a way so that the building and its equipment has the best possible opportunity to provide comfort for the people living in the home.  This is accomplished by inspecting, testing and adjusting systems, whether that be the air barrier, heating and cooling system, or the Wi-Fi system, all the systems need to be verified for proper operation.  When systems are not tested, we have to hope that they are working at the designed specifications.  As an example, an exhaust fan is installed to remove moisture and odors from a specific space inside the home.  Someone on the construction team specified the needed exhaust rate for the application and the equipment chosen based on that design.  Simply matching equipment size to the designed need doesn’t guarantee that the rate of exhaust is achieved.  The final result can be influenced by duct type and sizing along with installation variables.  We can guess that the proper exhaust rate has been met, or we can test to verify.  This air flow verification test takes seconds with the right test equipment.  Systems that do not meet the designed specifications can result in comfort and operational cost expectations not being met, humidity and indoor air quality issues that can affect human health or the durability of the structure along with warrantee issues and customer complaints.

Get it right: Commissioning a new home should not start at the end of construction, but at the beginning, during the planning stage and include inspections during the construction phase.  Assuring proper sizing and system design needs to be completed before construction gets started.  Confirming the quality of the installation of systems needs to happen as construction progresses, when making changes is easiest.  Final commissioning may be provided by several different trades, or by one person.  Some jurisdictions have code enforcement that require final testing to be conducted, but many do not.  Homes that receive a HERS score, Energy Star, Zero Energy, or Passive House Certification are typically commissioned but having a home rated or certified is not common everywhere.

At a minimum, the final commissioning procedure should include a blower door test and HVAC system testing (including all heating, cooling, and ventilation equipment) along with the associated ductwork testing to assure they are all meeting the designed specifications.  A great free resource for simplifying an inspection and commissioning process is called the National Rater Field Checklist and is available through the Department of Energy’s Energy Star program.