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Is a freezing lock a sign of negative pressure or just extreme weather conditions?

jeasto | Posted in General Questions on

Hi everyone!

I moved into an energy efficient house in August in climate zone 5A, and we experienced our first sub-zero temperatures around Christmas during Winter Storm Elliot.

As an aside, we were amazed at the performance of our Zehnder ERV, the app of which showed 2 degree F outdoor air becoming 65.8F before it entered the home (I forgot to look at what it was doing on the subzero days). 

What wasn’t cool was our high-performance doors had ice forming on the locks. I reached out to the manufacturer (Alpen), and they sent me a document that explained that it’s common and what I can do to solve it.

Some quick background, I think the coldest it got was -5F. And we only noticed it as the weather approached 0F. I believe the issue was resolved by 10F outdoor air temperature.

The first thing in the document was indoor humidity. It recommended 40 percent or lower to prevent ice. We have already been closely monitoring our indoor humidity, and we know we were at 35 percent or lower during the cold snap.

Second, it suggested that the mechanical system may not be balanced, which means more air than necessary could be sucked into the house through the key hole.

Our ERV is set on “balanced” ventilation, and it adjusts itself to maintain that balance. We also had all the other mechanical equipment checked / balanced as part of our ZERH certification. So that seems unlikely to be the issue, unless balance can get messed up in half a year (?).

We have a vented hood in the kitchen, but we have never had the need to use it (the “boost” feature of the ERV seems to work perfectly well for kitchen smells and humidity, and it maintains the balanced pressure when it does).

The Alpen information sheet also explained that very airtight homes (like ours) are at risk for this because there are so few ways for air to get in the house, so it beams through the keyhole, at that is made worse by pressure issues. (And, of course, that no matter how well insulated/airtight a door is, it still needs a keyhole and it is simply a weak point.) This seems like something someone should have warned me about.

Here’s my question: Is ice/condensation on the locks a sign of a problem, such as a depressurized house? Or is it simply something that may happen in extreme weather in an airtight home?

Has anyone experienced this, and is there any way to solve it? For example, it’s possible to set our ERV to slightly positive or negative pressure. We have been afraid to mess with that because we don’t understand it / didn’t want to mess up the house.

Thank you in advance!

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  1. Expert Member

    Id chalk this one up to "it happens". Even stainless steel's lower thermal conductivity would have problems when the outside temps are that low. The humidity inside your house would need to be extremely low for the dew point on the surface of the lock not to condense the vapor in the air.

    Unless we figure out how to thermally break, and air seal locks, I think it'll be one of the few remaining hard spots to figure out.

  2. Expert Member


    It probably does happen more with well air-sealed houses, as the leaky ones can find all sorts of other spots for air to flow when they try and equalize pressure. I doubt you could introduce an imbalance that would consistently stop the freezing.

    I find the lock on my truck less susceptible when I spray it regularly with WD40.

  3. maine_tyler | | #3

    Is the ice on the interior side? Is the lock still functional?

    As far as infiltration, you could take an incense stick or similar and hold it near the lock or other crack in a similar location. A house is unlikely (impossible?) to be balanced at all elevations given that there is temperature differential with the outside.
    In cold winter climates, non deliberate infiltration is usually less of a concern than excessive non deliberate exfiltration since the incoming air is less humid (absolute humidity).

    1. jeasto | | #6

      Hi maine_tyler!

      The ice was on the inside, and the locks were still functional (thank god). I'm attaching a pic (unfortunately, I didn't think to take one when there was ice on it).

      Moisture accumulated on the white turning mechanism, and eventually created ice on the exposed metal parts (what the knob attaches to) and that bottom screw. I don't think the outside part had ice accumulation.

  4. Trevor_Lambert | | #4

    I had this happen during the recent winter storm in SW Ontario. This is our fifth winter, and this was the first time. We have seen several periods of colder weather than this (around -19 to -21C), but several variables were different. One was 60km/h sustained winds, another was relatively high indoor humidity, around 45%. Lastly, the weatherstripping wasn't sealing around the door very well (it only happened on one of four exterior doors), keeping the area around the door a little cooler than it otherwise would have been. The frost only appeared on the lock knob, not the handle, I guess because the handle is a bigger heat sink.

    -Trevor Lambert

    1. jeasto | | #9

      Yes, it was the lock knob for us, too. The handles were fine (as were all the handles on all the windows, which look very similar to the doors).

      I wonder if the wind had something to do with it for us, too. We had gale warnings and otherwise very blowy/blizzard conditions during this time. Because this is our first winter in the house, I have nothing to compare it to.

      We have three of these locks (two on the French doors and one on a single door), and two of the three knobs developed frost. Separately, we noticed the single door seemed not to be sealing properly, which we had never noticed before. We could hear and feel air. When it started getting relatively warmer, that seemed to stop.

      Our builder told us vinyl (which the doors/windows are made of) has a tendency to shrink and expand, so it could be a one-time thing. But the manufacturer is visiting our house for a different reason next week, so I am having him check that the door isn't defective.

      Did your doors seal properly afterwards and it was a one-off thing?

  5. josh_in_mn | | #5

    Is the ice forming in the keyway on the outside of the lock? If so, it would have to be that the house is pressurized, and leaking warm humid indoor air out through the lock, where it freezes.

    There was an article on GBA a while back explaining this phenomenon, and the steps the author took to fix it, which was to slightly depressurize the house so that it leaked inwards.

    1. jeasto | | #10

      No it was inside, on the knob / metal surrounding the knob. The keyhole on the outside creates a pathway (I assume) to the knob, so I think the air from outside was coming in.

  6. Expert Member
    Michael Maines | | #7

    I have only seen this condition a few times, and on the one that was most pronounced, we had a known depressurization problem. I don't know of thermally-broken hardware, though it might exist.

  7. Expert Member
    BILL WICHERS | | #8

    On little used doors, you may be able to use one of the styrofoam covers normally used to protect outdoor water spigots from freezing. You could attach to a handle or latch to insulate the lock/handle on the outside to minimize the risk of freezing. For air leaks, I have often used painter's tape as a sort of temporary air seal. It would be easy to put a piece of painters tape over a keyhole if it was windy to seal out the wind, so that's something easy and cheap that you could try. Painters tape comes off clean when you remove it.

    I have also only rarely seen this particular problem, and it seems to need just the right conditions to form. A hair dryer can usually thaw things out in short order if needed.


  8. mgensler | | #11

    I'm guessing this is more thermal transfer rather than air leakage. We have an aluminum threshold that doesn't have a thermal break. It will ice up on the inside on really cold days.

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