Ventilation question

Discussion in 'HVAC Heating & Cooling' started by jacobsond, Aug 13, 2012.

  1. jacobsond

    jacobsond DIY Junior Member

    Messages:
    58
    Location:
    ND
    I need better ventilation in my bedroom. Here is the setup.Typical 12x18 bedroom with a master bath.Hot water heat. AC is a wall AC.When I built this addition to my house I insulated all the walls even interior and ceiling.This room is on main floor with a 2nd story above.Bathroom insulated the same.There is only 1 crank out egress type window in the room. The bathroom has no window only a vent which we always use.Here is the problem in the morning the room is always very stuffy.Even with the window open.There is a ceiling fan always running plus another fan.I cut a couple small vents one in the bathroom one in the bedroom to maybe get some airflow from the adjacent room but it didn't help much.The only thing that helps is leaving the door open a little,but that is really not desirable. No ductwork at all in the house. There is a crawlspace under this room.The humidity is usually running 65% or higher in this room. Ive got the bath vent fan on a timer so I can run it extended times.Any easy fixes?:rolleyes: Is there a small vent fan I could install in the wall to circulate the air to the adjacent room?
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,817
    Location:
    New England
    If the a/c unit is too large for the room, it won't run long enough to adequately dehumidify the room (i.e., it may be short cycling). Another thing you could do is add a fresh air recycling system. This basically has a heat exchanger (and some have a moisture exchanger) to exchange interior air with external air. If the house is tight, running the vent fan won't do much - to exhaust air, air has to be coming in from somewhere.

    Is the crawl space got any plastic on the ground, or is it open dirt? If not, ground moisture will raise the room humidity.
  3. Dana

    Dana In the trades

    Messages:
    2,785
    Location:
    01609
    As Jim correctly points out, if there is no ground vapor barrier in the crawl space it can become a significant source of humidity in very-tight construction, and is usually retrofittable with a ground vapor barrier, provided it's deep enough to work in. (6 mil polyethylene min, 10mil is better. Overlap any seams between sheets by about a foot, and seal the seams with duct-mastic. If the foundation walls aren't insulated, it's cost effective to put 2-3" of spray polyurethane sealing the edge of the vapor barrier to the concrete, all the way up & over the foundation sill & band joist.)

    To ventilate a single room efficiently, Panasonic's FV-04VE1 room-ERV set to run continously at it's lower CFM delivers good air flow at a very low heating/cooling efficiency penalty.

    The exhaust-only scheme in the bathroom may work when the outdoor humidity is low (which most of the time, in ND), but as Jim points out it WILL of-necessity pull in outdoor air somewhere else in the house in an uncontrolled fashion. ERVs are designed with balanced flow, and have both a dedicated air-source vent as well as an exhaust vent. Unlike an HRV, the ERV does a partial humidity transfer between the incoming & outgoing air streams, which lowers the latent cooling load when outdoor humidity is high, but when the outdoor dew points are lower than the indoor dew point, it will still dry the indoor air (the moisture transfer is nowhere near 100%- more like 40-50% on average.)

    [​IMG]

    It may be useful to download & read the manuals, including the installation manual before committing, but it's a good solution.

    ND is cold enough that the ERV will have frost-up issues running in winter, but your winter air is dry enough that you can (and probably should) just turn it off when it gets cold in November, then back on when it warms up in March.

    Note: The manual doesn't recommend using it as a total ventilation solution in much of ND, but if the stuffiness is primarily summertime issue, it's safe to run it whenever average daily temps are above 40F.
    Last edited: Aug 14, 2012
  4. jacobsond

    jacobsond DIY Junior Member

    Messages:
    58
    Location:
    ND
    Thanks for the reply's.The crawl is more that 3ft deep with plastic on the ground(gravel).It is not sealed as good as it could be though.The block foundation has 3in pink foam glued to it. The Panasonic air exchanger is something Im going to look at.Plastic is cheap so Ill try to seal the ground better.
  5. Dana

    Dana In the trades

    Messages:
    2,785
    Location:
    01609
    Even un-sealed the poly is a pretty effective vapor retarder. The volume of moisture moved by vapor diffusion is a function of the amount of exposed area, which is probably less than 10% of the floor area. While it's still worth sealing the poly to the XPS insulation and seams between sheets with duct-mastic, ground moisture is likely to be only a minor contributor, based on your description.

    A whole house HRV (not ERV) system might be worth considering, especially if you plan to tighten up the rest of the place. Even in fairly large houses the trunk-ducts don't tend to exceed 6", and most can be done with 4". With HRV you don't get the humidity exchange, but you can run them at colder temps without running into frost issues. But if it's primarily the tight addition, the li'l Panasonic will get you there.

    BTW: At ND deep-soil temps & frost depths it's cost effective to insulate against the crawlspace floor as well. Since the floor of the crawlspace is well above the frost depth, laying 2" of XPS (pink, blue, green, whatever, make it R10) for the 2' around the perimeter and 1-1.5" (R5-R7.5) in the middle is about right, given the R15 foundation-wall insulation. You could use cheaper (but thicker ) EPS as well, but unless you poured a 1-2" rat-slab over it it would be more susceptible to breaking up over time from repeated walking, etc. (XPS is much tougher stuff than EPS).
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