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Thread: Undercutting Doors

  1. #1
    DIY Senior Member chefwong's Avatar
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    Default Undercutting Doors

    OK. Which camp are you in.

    1/2 undercut for funtion (air return, etc)
    or
    1/4 -- nice and tight to flooring.

    Gonna do some door hanging and can't decide....

    Or maybe meet somewhere in the middle and do 3/8".

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
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    Don't do it for air, if you needed it, you'd need more than the 1/2" anyway - there should be an air return doing the job in the room. A bit more is handy to help the door go over whatever might be sitting there.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  3. #3
    Moderator & Master Plumber hj's Avatar
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    quote; there should be an air return doing the job in the room

    What universe are you living in? The majority of modern homes have only one or two central returns and depend on the gap under the door for return air when the door is closed. Make a "tight gap" and then develop muscles pulling the door open when the furnace is operating.
    Licensed residential and commercial plumber

  4. #4
    In the trades Dana's Avatar
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    In many locations where there isn't a dedicated return in the doored-off room, jump ducts or return grilles in partition walls are required by code. It would take about a 2" door cut to meet the pressure differential requirements for a duct return path. Making the door tight WITHOUT providing a low impedance return path means you're using incidental air leakage out to the Great Outdoors to provide the return path, which takes a toll on system efficiency (and capacity).

    If you want to use the door as the return path without creating a hamster & squirrel escape route, these suckers work pretty well. They're not exactly a thing of beauty- on a par with using transom grilles or partition-wall grills. Jump ducts with ceiling grilles may be a bit less visually intrusive.

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    DIYer, not in the trades LLigetfa's Avatar
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    A 4 inch duct feeding or exhausting a room is 12.5 sq inches, a 6 inch is just over 28 sq inches. Since it is a forced supply, the passive return or make-up needs be greater. A 1 inch gap under a 34 inch door is 34 sq inches, probably adequate for return or make-up IMHO.

    I say, give the hamster freedom to roam!

  6. #6
    In the trades Dana's Avatar
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    The max impedance spec Tamrack is working against for passive returns is 250cfm (min) @ 3 pascals pressure difference, which surely won't be met with a 1" door cut on a 34" door.

    Jump ducts are typically executed with 10" diameter flex duct, or 78-79 sq-in, which is roughly the same as a 2" door cut. Bigger rooms/volumes use 12" flex for the jump ducts. See the duct sizing and flow discussion, particularly the jump duct sizing in figure 3 here:

    http://www.buildingscience.com/docum...em-sizing-pro/

    See also: http://www.buildingscience.com/docum...return-systems

    While a 1" door cut would be much better than a tight fit, it would still be sufficient impedance to drive outdoor air infiltration.

  7. #7
    Moderator & Master Plumber hj's Avatar
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    You are still in an alternate universe if you really believe residential contractor install "jump ducts". Unless that "excess air" going into the room is being exhausted to the outside, it is NOT creating a suction that has to be replenished by outside air. The "pressure" in the room will "force" the air under the door, so it is NOT a true "passive return".
    Last edited by hj; 07-12-2012 at 09:31 AM.
    Licensed residential and commercial plumber

  8. #8
    DIYer, not in the trades LLigetfa's Avatar
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    Quote Originally Posted by Dana View Post
    While a 1" door cut would be much better than a tight fit, it would still be sufficient impedance to drive outdoor air infiltration.
    I'm confused about your thoughts on the direction of flow. I realize the term "infiltration" is often used in the industry regardless of the actual direction of flow. Just like you used the term "stack effect" in the context of cooling with A/C.

    As hj said, the room would be pressurized so we're talking air leaving, not infiltrating. Forcing warm humid air to leave through the envelope during the heating season can be a serious issue as it may condense on the cold side and promote rot.

    Keep in mind that there is likely some air that can get out around the other 3 sides of the door as well since the door stop seldom seals very well.

  9. #9
    In the trades Dana's Avatar
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    The alternate universe I live in is where codes regarding duct designs exist, are generally enforced, inspectors keep track, and contractors understand the requirements. I get that this not universal across the US, but it's mostly true in MA & NH. In CA under Title 24 you BET they install jump ducts or other return paths from doored-off rooms- it's cheaper to do it right ahead of time than to rectify it after the inspectors call you on it. (They seal and test duct leakage there too.) In states that have adopted IRC2009 or IRC2012 (or comparable) standards they pressure-test the house as part of the commissioning too, but I s'pose that isn't happening in AZ yet. But even where local codes may ignore the issue doesn't mean it isn't worth addressing.

    In the alternate universe I live in, air that's blown out of the house from one location creates a low pressure drawing air in elsewhere, and this is YOUR universe too! By pressurizing one room to above atmospheric, it depressurizes another below atmospheric. All exfiltration volume has an equal and opposite infiltration volume. Just because the undefined place where the air is infiltrating isn't the room with the inadequate return doesn't mean that increasing the impedance of return path isn't responsible for creating a larger pressure difference to drive infiltration- it absolutely DOES!

    Of course it's the pressure difference between the room with the supply duct(s) an the room(s) with the returns that causes the air motion, but by keeping the pressure requirements low within the pressure boundary of the house it keeps the exfiltration/infiltration path from becoming a large fraction of the return. The impedance of a jump duct or 2" door cut is still substantially higher than a path completely within a room with a dedicated return or an open door, but still sufficiently low to keep the exfiltration/infiltration paths from moving a lot of air in and out of the building.

    The leakier the building envelope, the larger the fraction of air volume taking paths outside the thermal & pressure envelope of the house. IRC 2009 calls out 7 air changes per hour @ 50 pascals (7ACH/50), which isn't super-tight, but tighter than you'd be in places where air-sealing is given little or no attention during construction. Almost all housing built in New England after 1980 would pass, or could pass with a very modest effort at air sealing. IRC 2012 calls out 3ACH/50 which still isn't super-tight, but more difficult to retrofit in houses that were built with little regard to air-tightness. But installing jump-ducts is still cost effective just on utility savings in a 3ACH/50 house. Canadian R2000 standard housing calls out 1.5ACH/50 max, which may or may not be leaky enough to warrant guaranteeing very low impedance return paths from a total heating cost point of view, but comfort might suffer in rooms that don't get the designed amount of flow.

    The leakage paths will vary- windows are the dead-obvious but not always the largest exfiltration/infiltration paths. Electrical penetrations in walls and ceilings (particularly recessed/pot lighting fixures) can often be bigger paths than the windows & doors. From the whole house perspective the foundation sills & band joists can be HUGE air leaks that become part of the infiltration/exfiltration path.

    In chefwong's D.C. location anything that reduces infiltration has a comfort benefit, both winter and summer, given the relatively high summertime dew points and not-exactly-balmy overnight low temps in winter. It's much less of a comfort (or energy use) issue in say, San Diego CA.

  10. #10
    Retired Defense Industry Engineer jadnashua's Avatar
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    It's much better to engineer this properly. A room that has the proper air flow both in AND out will be much easier to keep comfortable with the minimum energy penalty. The gap under the door is not the proper path.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  11. #11
    In the trades Dana's Avatar
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    A door cut could be a proper path if it's guaranteed to be adequate. The Tamrack door inserts give a decent path for the air while still keeping the hamsters impounded. I have a room where a 2" cut/no threshold under a pair of narrow French doors provides the return path. The only other viable option would have been a jump-duct under the floor, which had too many other issues.

    But "yes" on doing the math on duct diameters and air flows, if you have the patience and means for duct design.

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