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Ventilation needed? Humidity control, comfort, and efficiency

raul4817 | Posted in General Questions on

So as I get closer to finishing my insulation and contemplate my airtightness I have begun to read more into the importance of ventilation. My newly constructed second floor is r23 wool with possibly another r3 to r6 of interior foam board that will act as my air barrier and thermal break at the studs. I am paying very close attention to airtightness upstairs.

So as I read through the site and start gather more info it is a lot to digest just on ventilation alone. I live near chicago climate zone 5 and just as our winters can be brutally cold our summers can be brutally humid. So I begin to think about dehumidification and the comfort aspect of things.

So through process of elimination I think I’m getting closer to the right answer and hopefully most economically feasible.

I started with looking at hrv and erv equipment or possibly a ventilating dehumidifier such as the ultra aire 120h. Neither of these are cheap so I began digging deeper.

I then read up on a central integrated fan supply and this seems to fit the budget better than option 1 and 1a.  I Will have 3 bathroom exhaust fans on the 2nd floor and gas dryer exhaust. 1st floor will have another bathroom exhaust with a range hood in the kitchen. I figured this should be enough to exhaust stale air. But the cfis will not handle the humidity I figure it will actually add more humid air to the house.

But then I begin to ponder. Do I really need ventilation? My first floor is still the original multi wythe brick walls from 1952. Even though I do plan on pointing the mortar and replacing and air sealing the 1st floor windows sooner than later, I ask myself is this really considered a “tight” house.  Does my brick exterior constitute airtightness? And is this brick 1st floor wall actually bringing in enough  fresh outside air that I can omit the cfis system entirely?

So I’m undecided and truthfully unsure wether I install a CFIS on both of my hvac units(one located in the Basement and the second in a vented conditioned attic) or just one of the hvac systems(but which one?

But after all this I still haven’t addressed the humidity. A whole house dehumidifier does not agree with my pocket book. The portable dehumidifiers are looking real attractive. I could easily place one in my basement near a floor drain So the condensate can drain. For the price I’m thinking of buying a second portable unit to be placed in the 2nd floor laundry room that will also be equipped with a floor drain.

And lastly will my interior foam board help with the humidity? I’m planning on using a 3/4″ low density eps. I chose this because it is cheap, readily available near me. I do like the idea that it is more vapor open to allow my walls to dry out both ways if needed. I do not have exterior foam board just plywood, tyvek, and vinyl siding. And as Dana advised me in an earlier post, the vinyl cladding is somewhat forgiving. So given this I thought I could possibly use a 1/2 ” r3 polyiso with a reflective foil face on one side and a matte white foil face on the other.  This will not allow drying to the interior but do i really need it too with vinyl siding? Will the polyiso do anything for humidy control in the summer? I am planning for a service cavity of 2×3 studs in front of my interior foam board. I believe I read somewhere that the air space in front of the reflective foil face with add some extra r value but how much does that amount to with 2.5″ between the foil face and drywall. Does my drywall then have to be airtight as well to actually reap the r value benefits of the space?  

Comfort is important but cost is as well.  I think I’ve gone through most options but I may have missed something along the way. 

Raul

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Raul,
    You wrote, "But the central-fan-integrated supply ventilation system will not handle the humidity; I figure it will actually add more humid air to the house."

    The supply ventilation system will only add humid air to the house during hot, humid weather in summer. When that happens, the way you control your indoor humidity levels is with your air conditioner. (Remember, too, that you should minimize the ventilation rate during the summer, especially if you have humidity concerns.)

    During the winter, operating your ventilation system will tend to lower the indoor humidity level, not raise it.

  2. this_page_left_blank | | #2

    You don't mention whether you have or will have a/c. If you will, then that makes de-humidifiers unnecessary. If you don't, then it makes portable de-humidifiers of questionable value, as they will add a lot of heat to your already hot house in exchange for humidity reduction.

    The humidity level in the summer is mainly going to be determined by the outdoor humidity, and the air exchange rate of your house. The particulars of how that is determined probably don't matter too much. The best way of controlling it is making the house as air tight as possible, and mechanically ventilating. Using an ERV would help in this case. You really need to know what the air exchange rate is going to be in order to make good decisions. If you're going to have four bathroom fans, range hood and a gas dryer, you've already kind of committed to having a leaky house. All of those things need a lot of make-up air.

    I think the radiant barrier would still have an effect without an airtight seal, however if you have air flow in that gap you might end up with dust on the foil surface. Even a tiny amount of dust will affect its performance, and a surprisingly modest amount will reduce its performance to zero.

  3. raul4817 | | #3

    Trevor, yes the house will have 2 a/c units. One for each floor. I understand the dehumidification effect of the a/c. This works fine when the a/c is needed for long stretches of time but I still find the air to be a bit uncomfortable. This is where I thought the dehumidifiers could come into play. Is the added heat and energy consumption of these units a wash?

    In regards to exhaust fans. Yes I guess I have committed to a leaky house. I have no issues about omitting the bathroom fans all together. The laundry is not negotiable and the range hood has had a lot of good use, so I'd rather leave it. But as bathrooms go I am in rough in stages now so I could choose to just not exhaust my bathrooms. Each bathroom has an operable window so I think I would still adhere to the code. In this scenario does it make sense to add an erv. I still not sure of airtightness with the masonry half of my structure.

    As far as the radiant barrier goes I can try to reduce airflow of the cavity but it is inevitable that I will accumulate some dust on the reflective surface. So if this dust negates the extra r value is spending the extra dollars on polyiso even worth it? I could easily us eps for about half the cost of the polyiso and still create a reasonably detailed air barrier

  4. Jon_R | | #4

    You are correct. Most climates have some humid, mild temperature weather where AC doesn't create a comfortable humidity/temperature.

    A leaky house is an unreliable way to get ventilation air. For example, there is little fresh air when there is no wind and no stack effect. And there is way too much air when it is windy.

  5. this_page_left_blank | | #5

    I wouldn't recommend omitting the bathroom fans unless you were going to install an ERV. But by the sounds of it, an ERV wouldn't really be justified.

    If you have a/c and find that it doesn't reduce the humidity enough, then you can add the de-humidifiers. The extra heat can be removed by the a/c. I think normally, the a/c is enough.

    My thoughts on the radiant barrier are theoretical only. I don't know whether it makes economic sense or not. At double the price, I kind of doubt it. Wait until some others chime in.

  6. Expert Member
    Akos | | #6

    Tevor,

    If you find that the humidty with the AC running is too high, you have either:
    -AC unit that is way oversized for the load and short cycling
    -Over sized or incorrectly adjusted blower on your HVAC (the air leaving the evaporator is not cooled enough to take the humidity out)
    -AC unit low on refrigerant

    With the aircon running properly, there should be no humidity or comfort issues with a reasonably sealed up house.

    1. Jon_R | | #7

      Of course when it is 72F and 90% humidity outside, the AC doesn't run. So one will be uncomfortable. This is also true at low loads, even when everything is correct.

      Tight houses and variable speed AC help with humidity control.

  7. raul4817 | | #8

    So if an erv doesn't make a lot of sense then would the Cfis be viable. I figure I could really take advantage of this in spring and fall when open windows with no breeze isn't quite cutting it. This would be simple and cost effective. Would I really need one for both sets of hvac equipment or will one on the basement hvac do the trick? I have 2200 above grade sf and 800sf in the basement.

    I have not got as far as hvac equipment just yet. I did have one quote that called for a 3 ton ac unit just for the 1100 sf 2nd floor. I knew before he even left this was way oversized. I will be r26 walls and r50 roof. Without doing a full on manual j my gut is telling me I should be closer to a 2 ton unit.

  8. Expert Member
    Akos | | #9

    Raul,

    If you are unsure about the amount of ventilation you need and you have the option of adding in bathroom ventilation, I think the compromise might be to use one of the either a WhisperComfor ERV or a pair of Lunos fans in your bathroom. This would give you the option to ventilate if the air is stale without bringing in as much moisture in the summer as a Cfis system. Cost would be comperable to Cfis plus standard bath fan.

    Handling about 3000 sqft of conditioned space with a single HVAC system is not a problem (common around here in Toronto but we also never run ducts in attic) provided the ducting is properly designed. Important detail not to skimp on is to have a large enough return on the 2nd floor near ceiling height. Doing the load calcs for all your rooms is a must as your heating loads on your main floor will be significantly higher than upstairs, getting those wrong will make the house uncomfortable. With a single HVAC system, the basement will probably need extra de-humidification, stand alone dehumidifier is the cheapest/easiest.

    Interior humidity comes from air leaks, from occupants (showers/cooking etc) and damp basement slab/foundation. Interior rigid insulation will only help if it reduces the air leakage of the house or sealing up the fundation. It would make a big difference in winter comfort and heating cost with your multi wythe brick walls, might be worth it just for that.

    1. raul4817 | | #10

      Akos,
      single hvac sytems for 3000sf is also very common around here as well. I opted to add a second unit for a number of reasons. One being my 1st floor is finished and not needing to make way for a trunk through it made sense to me. I also like the idea of controlling temperatures on different levels of my home at different times. I currentley have an ecobee thermostat installed and it really makes programming temps based on your schedule almost effortless. I will have a conditioned attic so there is not a concern with the attic duct installation and i like being able to access my mechanicals without ripping drywall. Im the guy that will finish my basement and leave the ceiling exposed so i can access everything. The wife is not thrilled about it but i like access. I could have opted to bring the duct downs in a soffit or exposed loft style on the 2nd floor but not really aesthetically pleasing.
      If i opted for the erv option maybe spending a few more dollars on say the panasonic comfort or even a traditional panasonic or broan erv would i be ok to not install bath exhaust fans? I could exhuast air from one of the bathrooms ot locate it just outside the centrally located laundry room. This could work well considering i have an entire attic to install and run ducts with relative ease.
      Foam board in my basement is in the pipeline but i will need to read a bit more on the risks and rewards of insulating my brick safely. If my 1st floor plaster walls where in worse shape i would probably be gutting and insulating them as well, however with everything being what it is ive decided to leave them uninsulated and opted to insulate the basement, 2nd floor, and vented attic rafters and leave the 1st floor as is. I do have approximately 1” of space between the plaster and cmu that i could insulate from above with access from 2nd floor while in the rough-in stage. But i would be limited to a blown in insulation material and as others have advised on this site that it could be risky with the brick and may not really be worth it given there is only about an inch of cavity space to fill. Thanks for the suggestions i will continue to read on.
      Raul

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