1:5:10:365 EcoTip Blog

April 21, 2008

:112 Door Undercuts

Filed under: :112 Door Undercuts — Tags: , , , , , , , , , — John Banta @ 12:26 am

Welcome to today’s 1:5:10:365 Tip for becoming a better steward for our home and planet.

1:5:10:112 EcoTip: Doors are deliberately undercut to provide a path for airflow created by central heat and air systems. If the room has both a supply and return register – and is balanced as described in EcoTip:111 The undercut isn’t necessary. If you only have supplies registers in a room – but no returns – then the air must have a path to follow back to the central system to provide proper ventilation. This is provided by the space under the door. The door undercut may not be enough in which case you need to undercut it more or provide an RAP.


Tamarak Technologies Return Air Pathway


 Additional Information

Suggested Review – :111

The amount of gap under the door is important to allow good air exchange and prevent pressure differences from developing in the house. Many builders cut a standard half-inch to one inch gap without understanding that size does matter. If carpet is installed, it may close the gap as well.

A 30 inch door with a half-inch gap can handle up to 25 cubic feet per minute (cfm) of air coming from the supply register in that room when the door is closed. To handle 75 cfm it would be necessary to have a 1.5 inch gap. If the gap isn’t big enough then back-drafting can occur (more on this in tomorrow’s tip).

Tamarack Technologies has informtion about the necessary door way undercuts or as an alternative you can use their Return Air Pathways (RAPs) instead of needing to undercut a huge gap under the door.

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1 Comment »

  1. Section 601.4 of the New Florida Building Code: “Restricted return air occurs in buildings when returns are located in central zones and closed interior doors impede air flow to the return grill or when ceiling spaces are used as return plenums and fire walls restrict air movement from one portion of the return plenum to another. Provisions shall be made in both residential and commercial buildings to avoid unbalanced air flows and pressure differentials caused by restricted return air. Pressure differentials across closed doors where returns are centrally located shall be limited to .01 inch WC (2.5 Pascals) or less. Pressure differentials across fire walls in ceiling space plenums shall be limited to .01 inch WC (2.5 Pascals) by providing air duct pathways or air transfer pathways from the high pressure zone to the low zone.”

    Comment by John Banta — April 20, 2008 @ 9:51 am

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