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Community and Q&A

Make up air in tight house

BrainMaine | Posted in Energy Efficiency and Durability on

We are in our new tightly built house in Maine and the heating season has started. Our house is sealed very well (AreoBarrier). We have an ERV ventilator (Panasonic FV-10VEC1). We have a range hood, 3 bathroom fans and a basement dryer that exhaust directly to the exterior.
We have noticed moisture on the interior panels of our wood front door- especially with colder outdoor temperatures when we are running the dryer or bathroom fans. We suspect the door panels are the weak link in our tight house and make up air drawn through the panels and is condensing on the inside and running down the door face.
What do we need to do (besides constantly mopping up the water running down the panels)? Do we need additional make up air sources? Is there an adjustment we should make to our ERV? Should we just release the multipoint lock on one of our other doors?
Is more information needed and air flow rates of fans and appliances? 
Thanks for your help!

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  1. Expert Member


    It isn't the cold air condensing on the inside of your door (cold air is much dryer that the air inside your house), it is warm inside air which is condensing when it hits a surface below it's dew point. The temperature of the door itself may be made colder by air being drawn through it, but more likely it is a simple function of the very low R value of the door compared to all the rest of assemblies in your house. So it's doubtful adjusting the relative interior and exterior pressure will do much to stop the problem.

    This sounds a lot like an excessive humidity issue. Lowering the interior humidity by boosting the ERV will probably solve things. If not the (perhaps unpalatable) next step would be to get a better insulated door.

  2. Jon_R | | #2

    > This sounds a lot like an excessive humidity issue

    Everyone should measure/monitor indoor humidity.

  3. ohioandy | | #3

    Brain, your ERV has its own balanced intake and exhaust, so everything else is unbalanced: you've got three small exhausts (bath vents) and two large ones (hood, dryer) with no corresponding intake. I bet that in addition to the panels of your front door, they're using each other's vents to draw air in, plus maybe other accidental holes you can't see. It might be accidentally functional, but it's not optimal for a tight house. Now's a perfect chance to hire in an energy auditor with an IR camera to find any other leaks, then consult on makeup air retrofits. The main benefit of proper wintertime ventilation is reducing interior humidity. Mops not necessary!

  4. walta100 | | #4

    The real questions are.
    1 What is your indoor humidity and temperature?
    2 What was the result of your blower door test measured in ACH50?
    3 What is the temperature of the door?
    4 Are you setting back your thermostat and how far?

    If your winter indoor humidity is over 55, I say that is a problem that needs to be addressed.

    If the temperature of the door is below 50° I say that is a problem that needs to be addressed.

    If you set your thermostat back to far the weak points of your house will cool below the dew point and get wet and can grow mold.


    1. Jon_R | | #5

      In Maine during cold weather, 55% interior relative humidity is way too high. I'd expect walls (eg, exterior sheathing) to suffer moisture damage.

      You may need to run some exhaust only ventilation.

  5. Andrew_C | | #6

    Part of the humidity could be due to that fact that many building materials dry out a lot during the first year. There's a lot of water in wood and concrete. Running a dehumidifier might help during this period if you don't want to adjust ventilation rates.

  6. Expert Member

    This discussion really does represent what I like about GBA. if you combine the six responses you end up with a pretty good, comprehensive answer.

  7. walta100 | | #8

    I said anything over 55% is sure to be a problem. I agree that 55% may not ideal in Maine.

    I also agree Andrew running a dehumidifier for the first year would be a good idea.


  8. Jon_R | | #9

    Unfortunately, dehumidifiers don't work well at the low (say 30% ) RH that is needed in Winter. So one is left with exhaust ventilation (or HRV or source reduction).

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