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Community and Q&A

Air exchange / ventilation / dehumidifier options for a sealed, 640 s.f. structure

njhiscock | Posted in Mechanicals on

I am building a 640 s.f. structure and insulating i twith closed cell spray foam. Heat is provided by a fireplace with an outside air supply and baseboard heat.

I need an energy-efficient air conditioning / ventilation / dehmidifier / air exchange solution, mainly to keep it dry, but also to keep it cool.

It is located in a typical, four season climate where we get hot, humid summers and cold, snowy winters.

I’ve looked at the PTAC / PTHC units, but they are rather loud and ugly. Mini split is more than I want to spend, plus I need the air exchange. Traditional HVAC is likewise overkill.

My sense is that for such a small, controlled space there is a simple solution – even if that solution is as simple as an in-wall exhaust fan with humidity-activated swith, plus a couple quite, portable A/C units.

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  1. Expert Member
    Dana Dorsett | | #1

    Getting to the right solution requires a much more information to work with than what's given. An actual location (for design temperature requirements) and a better description of the construction & R-values would be a good start. There is no universal definition for ", humid summers and cold, snowy winters...", and the particulars matter. Without an estimate of the true loads there's no way to specify a solution.

    Then there are the finer points, such as how big are the windows, and what are their U-factor & SGHC ratings, and which direction are they facing?

    Would there be a water heater? (Probably yes- looks like there's a kitchen.) If yes, what fuel? Is the baseboard heating electric, or hydronic?

    The amount of ventilation required also depends on what the building is being used for, maximum number of occupants, etc.

  2. njhiscock | | #2

    Thanks so much for the prompt reply and attention to detail!

    The structure is located in Bloomington, Indiana in the woods. It is a cabin on some timber land for storage and occasional recreational use for my family and close friends.

    Yes, there is a kitchen. Everything is electric. Water heat is on demand hot water, for the two sinks and the tub shower. The bathroom has a vent to code. The baseboard heating is electric. The fireplace has a squirrel cage fan and has outside air as the source.

    The entire envelope is to be spray foamed at or exceeding code. Very tight.

    The walls are 10' tall, and the ceiling is a cathedral ceiling throughout.

    There is a 20' by 40' loft above the kitchen and bath area if you can't tell from that diagram.

    There are two south-facing 21" X 61" windows
    There are 4 24" X 42" windows facing west
    There are 2 24 X 42", 2 21" X 61", 1 20" X 16", and 1 32" X 42" windows facing east
    There are 1 21" X 61" and 1 24" X 24" window facing north (the latter being in the loft)

    All windows are single hung vinyl with U-Factor of 0.31 and SHGC of 0.35.

    There are two thick steel doors of 36" width.

    There are also 7 boxes 12" X 16" with < 1" of spray foam insulation due to an in-wall speaker system.

    The fireplace rough-in size is 37 5/8" X 44".

  3. Expert Member

    Can you clear up the size of the cabin for us? In the title it is 640 sf, but your follow up post includes an 800 sf loft.

  4. njhiscock | | #4

    Yes thank you. The loft is 16 ft X 20 ft. 320 s.f.

    It is open to the other are. 2 X 10 framing. 12 in o.c.

  5. GBA Editor
    Martin Holladay | | #5

    This is a very small building. It should be easy to ventilate. If I were you, I would make sure that the bathroom exhaust fan is a high quality model -- for example, a Panasonic WhisperGreen -- and I would control it with a 24-hour programmable timer. That's all the ventilation you need. Experiment with different air flow rates and operational modes -- maybe 20 minutes per hour, for example -- until you achieve the ventilation rates you like.

    If you want an air conditioner, then install an air conditioner. A through-the-wall (or window-mounted) air conditioner is the cheapest. A ductless minisplit costs more, but will be quieter and somewhat more efficient.

    The ductless minisplit has a major advantage over a window-mounted air conditioner: it can also provide space heating. If you install one, you can eliminate the fireplace and the electric baseboard heaters.

  6. njhiscock | | #6

    Thanks for the feedback!

    I definitely am interested in a ductless mini split long term. There are higher budget priorities for this project now, and I already have the baseboard and will always keep a fireplace anyways for ambiance and all the dead wood I have available to me from the surrounding woods I manage.

    It will be easy to install one as the structure is on a post and pier foundation, which will make it easy to install floor registers.

    One thing I should have mentioned is that I will winterize this structure for weeks at a time during colder months. All the plumbing is on the bathroom walls, so the plan is to close the door on the bathroom and run the small in-wall heater I have installed there to keep it from freezing and minimize electrical costs. Ergo, it may be wiser for me to purchase an additional venting fan (such as the Panasonic Whisperlite) and install it elsewhere where it will not exhaust the heat along with the air.

    If I were to do so, is it better to locate the exhaust fan in the loft area? In the loft floor? The reason I ask is because i the summer, the loft area is significantly warmer and more humid.

  7. GBA Editor
    Martin Holladay | | #7

    If you want to keep the pipes from freezing when the house is unoccupied during the winter, just make sure that you turn off the bathroom exhaust fan when you leave the house. Problem solved.

    If your loft is hot during the summer, that's where you need to install your air conditioner.

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