Undercut Doors

Scope

Door undercuts are commonly used to provide a return air pathway from rooms with closed doors
Door undercuts are commonly used to provide a return air pathway from rooms with closed doors

Provide for pressure balancing between bedrooms and the rest of the house.

  • Install ducted returns or a combination of ducted returns, transfer grilles, jump ducts, and/or door undercuts in bedrooms to allow pressure balancing between bedrooms and the rest of the house in homes with ducted heating and cooling systems by providing a path for stale air to return to the return side of a central air handler.
  • ENERGY STAR Certified Homes requires that the dedicated return ducts, transfer grilles, jump ducts, and/or door undercuts together achieve a rater-measured pressure differential of ≤3 Pascals (0.012 inch water column) with respect to the main body of the house when bedroom doors are closed and the air handler is operating on the highest design fan speed. A rater-measured pressure differential of ≤5 Pascals (0.020 inch water column) is acceptable for rooms with a design airflow ≥150 cfm.
  • If dedicated return ducts are installed in each bedroom, contractors must seal all seams, gaps, and holes of the return duct system with mastic and seal the return box to the floor, wall, or ceiling with mastic, caulk, and/or foam.
  • If transfer grilles or jump ducts are used, refer to the balancing report provided by the HVAC contractor for the bedroom air flows to size the grilles or ducts. Ensure that both openings have the required free area. Seal all seams, gaps, and holes in the ducts and connections.
  • If door undercuts are used, pay attention to the door material. For hollow doors, you may not want to undercut or you might need to provide a bottom support after making the undercut, as the typical bottom supports are roughly 1 inch tall.
  • Test the pressure differential with the bedroom doors closed.

See the Compliance Tab for related codes and standards, and criteria to meet national programs such as ENERGY STAR Certified Homes, DOE’s Zero Energy Ready Home program, and EPA’s Indoor airPLUS.

Description

Door undercuts are a commonly used method for pressure balancing between rooms (Figure 1) but they are not a recommended approach as they do always provide an adequate return air pathway for HVAC systems. Seasonal changes in wood can result in variations in the height of the door undercut opening. Changes to the flooring material, such as adding carpet, can obstruct the return pathway. The ENERGY STAR pressure-balancing thresholds are difficult to achieve with just a door undercut for most applications, except for rooms with minimal supply airflow (<20 cfm). In most applications, the door undercut would need to be 1.5 inches or greater to allow enough air to pass. There are some through-the-door alternatives that can be effective. Consider installing a transfer grille in the door or employing another return air pathway method, such as transfer grilles or jump ducts. The video on the Training tab shows one of these options, transfer grilles.


Door undercuts are commonly used to provide a return air pathway from rooms with closed doors. (Photo courtesy of SWA).

 

Ensuring Success

To determine if an adequate pathway exists for air to return to centrally located returns, the following room-to-room pressure measurement can be used:

  1. Turn on the air handler to high.
  2. Close all interior doors.
  3. Using a manometer, connect tubing to the input port. The reference port for the differential pressure measurement can remain open.
  4. While standing in the center of the house or hallway, place the tubing from the manometer under each door and record the pressure difference from each room with respect to the main body of the house (note the presence of a negative or positive sign). The bedroom will typically be pressurized (positive) when the doors are closed.
  5. ENERGY STAR requires that rooms should not be pressurized or depressurized by more than 3 Pascals for any room being supplied with less than 150 cfm of conditioned air. If the supplied airflow to a room exceeds 150 cfm, a threshold of ≤5 Pascals is required. These are good metrics to strive for regardless of whether or not pursuing ENERGY STAR certification.  

Climate

No climate specific information applies.

Training

Right and Wrong Images

Presentations

None Available

Videos

  1. Transfer Grilles (2)
    Publication Date: July, 2015
    Courtesy Of: Tamarak

    Video describing how to install transfer grilles.

CAD Images

None Available

Compliance

The Compliance tab contains both program and code information. Code language is excerpted and summarized below. For exact code language, refer to the applicable code, which may require purchase from the publisher. While we continually update our database, links may have changed since posting. Please contact our webmaster if you find broken links.

ENERGY STAR Certified Homes

ENERGY STAR Certified Homes (Version 3/3.1, Revision 08), Rater Field Checklist

6. Duct Quality Installation 

6.2 Bedrooms pressure-balanced using any combination of transfer grills, jump ducts, dedicated return ducts, and / or undercut doors to achieve a Rater-measured pressure differential ≤ 3 Pa with respect to the main body of the house when all bedroom doors are closed and all air handlers are operating. See Footnote 34 for alternative.34

Footnotes:

(34) Item 6.2 does not apply to ventilation or exhaust ducts. For an HVAC system with a multi-speed fan, the highest design fan speed shall be used when verifying this requirement. As an alternative to the 3 Pa limit, a Rater-measured pressure differential ≤ 5 Pa is permitted to be used for bedrooms with a design airflow ≥ 150 CFM. The Rater-measured pressure shall be rounded to the nearest whole number to assess compliance.

ENERGY STAR Revision 08 requirements are required for homes permitted starting 07/01/2016.

Associated Air Barrier Council

Associated Air Barrier Council 2002. AABC National Standards for Total System Balance 2002. The manual details the minimum standards for total system balance.

National Environmental Balancing Bureau

National Environmental Balancing Bureau (NEBB) Section 15990 – Testing, Adjusting, and Balancing. NEBB is a certification association whose members perform testing, adjusting and balancing (TAB) of heating, ventilating, and air-conditioning systems and commission and retro-commission building systems. This document is the TAB procedural standards.

This Retrofit tab provides information that helps installers apply this “new home” guide to improvement projects for existing homes. This tab is organized with headings that mirror the new home tabs, such as “Scope,” “Description,” “Success,” etc. If there is no retrofit-specific information for a section, that heading is not included.

Door undercuts are not a recommended approach to provide an adequate return air pathway for HVAC systems. An alternative is to install a transfer grille in the door or consider other return air pathways methods, such as installing transfer grilles through the wall or jump ducts through the ceiling. 

More Info.

Access to some references may require purchase from the publisher. While we continually update our database, links may have changed since posting. Please contact our webmaster if you find broken links.

Case Studies

None Available

References and Resources*

  1. Author(s): Associated Air Barrier Council
    Organization(s): Associated Air Barrier Council
    Publication Date: January, 2002
    Standards book discussing changes, additions and enhancements over the 5th edition of the AABC National Standards for Total System Balance.
  2. Author(s): DOE
    Organization(s): DOE
    Publication Date: April, 2017

    Standard requirements for DOE's Zero Energy Ready Home national program certification.

  3. Author(s): EPA
    Organization(s): EPA
    Publication Date: December, 2015

    Document outlining the program requirements for ENERGY STAR Certified Homes, Version 3 (Rev. 08).

  4. Author(s): Guertin
    Organization(s): Fine Homebuilding Magazine
    Publication Date: July, 2011

    Central return designs require an open pathway between the room and the heart of the house where the return grill is located.  

  5. Author(s): National Environmental Balancing Bureau
    Organization(s): National Environmental Balancing Bureau
    Publication Date: January, 2005
    Standard that includes testing, adjusting and balancing (TAB) to produce design flows for air and hydronic systems.
  6. Author(s): Building Science Corporation
    Organization(s): Building Science Corporation
    Publication Date: January, 2011

    This article addresses the issue of undercutting bedroom doors to provide return airflow from bedrooms resulting in risks such as insufficient airflow, pressure imbalance, energy-inefficient loss of conditioned air, condensation within the wall, and moisture durability issues.

Contributors to this Guide

The following authors and organizations contributed to the content in this Guide.

 Steven Winter Associates, Inc., lead for the Consortium for Advanced Residential Buildings (CARB), a DOE Building America Research Team. And the Pacific Northwest National Laboratory.

Last Updated: 04/14/2017

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