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Building Science

When Are Door Undercuts Sufficient for Return Air?

You don’t always need to install individual returns, transfer grilles, or jumper ducts

A door undercut can act as return air pathway. But will it allow enough air to move out of the bedroom?
Image Credit: Image #1: Energy Vanguard

Most people don’t know that simply closing a door in their home can make them sick, increase their energy bills, or reduce their comfort. We live in this invisible stuff called air. We pull many pounds of it into our lungs each day. A typical air conditioner, heat pump, or furnace easily moves 20 tons of air a day. (Yes, I’m talking about 40,000 pounds! We’ll save that calculation for another day, though.) And the simple act of closing a door changes the dynamics of a house in ways that can have profound impacts on the people inside the home.

Last week I wrote about the problem of bedroom doors getting closed, the consequences of that action, and one way to alleviate the problem. In that article I mentioned the issue of undercutting the bedroom doors as the standard method many homes used as a return air pathway. The air pumped into a bedroom needs to find its way back to the air conditioner, heat pump, or furnace. Door undercuts are one such return air pathway. But, I wrote, “Door undercuts typically won’t allow enough air to get out of the bedroom unless you leave a gap bigger than most people want under their doors.”

The conventional wisdom on door undercuts

Ah, conventional wisdom. It’s often not wisdom at all and you may get different versions of it depending on which convention you believe. That’s certainly the case here. Talk to people in the HVAC industry and you’ll find quite a few who say you never need anything more than a door undercut. The one comment I’ve gotten so far on the video on bedroom pressures I made for my last article was, “Ha, ha — 7 pa is >0.03 inches w.c., so… Very little. It…

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  1. User avater GBA Editor
    Martin Holladay | | #1

    Bathroom fan makeup air
    Joe Nagan, an energy consultant in Wisconsin, recently sent me a short paper: Bathroom Fan Makeup Air Myths.

    It makes for interesting reading. In most homes, bathroom exhaust fans find plenty of makeup air, even when the door undercut is taped over with polyethylene. The likely reason: there are lots of air pathways into a bathroom, including at the door jambs and the door head, as well as through cracks in the partition walls separating the bathroom from adjacent rooms.

  2. Jon R | | #2

    I'm curious about the science
    I'm curious about the science behind "2.5-3 pascals is OK", especially in cases of continuous positive pressure (eg, supply into a closed door bedroom). For example, does it include partition moisture damage?

    I completely agree that baring a door undercut or vent per calculations, the right answer is "test it". Far too many potential variations in fan torque, air sealing, etc.

  3. User avater GBA Editor
    Martin Holladay | | #3

    Response to Jon R
    The common-sense answer to your question is simple: A pressure difference of 2.5 to 3 pascals isn't much -- wind effects routinely create bigger pressure imbalances -- and there isn't any evidence that 3 pascal pressure differences result in a significant energy penalty or any moisture problems.

  4. Joe Nagan | | #4

    Room pressures limit history
    Great discussion and I certainly understand the concerns with room pressures + or -. We regularly read this pressure limit set right around 3 pascals. We used to have that as a Standard in our New Homes program here in Wisconsin and we measured for this; but I always wondered why 3? So several years ago a group of us are at a National Conference and we sat in on a John Tooley presentation. After he was done I told the group I was going to ask John about where the 3 pascals came from; why 3? All of us who John can just picture him smiling about now. He was. He paused for several seconds while we all waited for his answer. In an arm motion similar to someone trying to catch a fly "out-of-the-air above your head, he chuckled with his fist clinched as if he'd caught the fly. We all got it, so I then again asked why he always used the 3 pascals limit in his trainings. He simply replied; "they asked me for a number and I thought 3 was reasonable"! The group all chuckled then big-time. When we got back we removed that Standard from our program. I don't think John would mind if I tell this story. I still can see him swatting at the air. Good stuff.

  5. User avater GBA Editor
    Martin Holladay | | #5

    Response to Jon R
    Jon R,
    The old rule of thumb for blower-door operators is that putting a house under 50 pascals of depressurization is equivalent (roughly) to the effects of a 20-mph wind. A house under 50 pascals of depressurization usually experiences a lot of leakage. You can often feel the effects with your hands. So a 20 mph wind is significant.

    Here's what happens with the wind, however: One side of your house is windward, and the other side of your house is leeward. The windward side is experiencing infiltration, while the leeward side is experiencing exfiltration. If you have two adjacent rooms, one of which is windward, and the other of which is leeward, you now have a significant room-to-room pressure imbalance. My gut tells me that this pressure imbalance (under a 20 mph wind) is significantly more than the 0.5 pascal you postulate. What happens instead is the kind of air pressure differences seen when a blower door is cranked up.

  6. Jon R | | #6

    20 mph (where did this come
    20 mph (where did this come from) and/or 50 pascals is significant, but what does that have to do with long term average moisture deposits? Typical ACH-average-natural values are often said to be ~1/20 of the measured @50 pascals value. (.5/50)^.65 = 1/20 (pressure vs ACH isn't linear).

    3 pascals is significant compared to the pressures a house normally experiences.

  7. Jon R | | #7

    A little math suggests to me
    A little math suggests to me (can someone verify this) that single family home average (IMO, the right value to use) pressure differential is around .5 pascals - meaning that a continuous 3 pascals would cause a large increase in in/ex-filtration. Stack effect would be a better comparison than wind - but 3 pascals over all the surfaces is also significant compared to that (a few pascals depending on temperatures and height).

    My guess - if you pressurize or depressurize to 3 pascals, you should expect at least a doubling of in/ex-filtration driven wall moisture and air change heat gain/loss. This may or may not be a problem.

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