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Community and Q&A

Reducing humidity in tight house

cjohnsen89 | Posted in General Questions on

I am in the process of finishing my house and have higher than ideal humidity levels and looking for advise as to the best way to overcome the humidity. I live in Massachusetts. 


The house:


It is 700sqft single story with a full finished basement. On the ground level about 400sq of it is cathedral ceilings and the rest is normal 8ft ceilings. I have a semi finished loft over the 8ft ceilings with a 4’x4’ opening towards the cathedral ceiling. The ceiling is R42 closed cell spray foam and the walls are R36 open cell spray foam. I have radiant floor heat in the basement set at 68 degrees. On the ground floor I have mini splits set at 70 degrees.




I turned the mini splits on in July to dry the house out so I could instal the hardwood floors. I wasn’t living in the house throughout the summer so I don’t know the exact humidity levels but it was comfortable. Maybe a little on the humid side but nothing terrible it “felt” good inside. I moved in to the house in November and started monitoring the humidity levels then. I’m constantly around 50% humidity. If I crack a few windows and turn on the shower fan and the stove hood for a few hours the humidity will drop to around 42-45% and even that feels a ton better than the 50+




I would like to get the humidity down to 40% or even high 30’s. Since I don’t have any make up air for the house and I don’t have a central HVAC system I was thinking a dehumidifier. I don’t want the portable ones that I have to empty out every day. I was wondering if there was a permanent type of dehumidifier that I could either:


Install a permanent dehumidifier in the semi finished loft pulling air and venting into the cathedral space and finding a way to drain it




Install a permanent dehumidifier in the basement and run ductwork to pull air and return air to the floor below the cathedral portion of the first floor



Sorry for my lack of knowledge on the subject I don’t even know if such a dehumidifier exists or if it’s possible to hook it into ductwork I’m just looking for some input from people on the best way to deal with this higher than ideal humidity permanently. Thanks in advance 

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  1. erik_brewster | | #1

    If cracking the windows and turning on the bathroom fan does the trick, an HRV will work for you. I have a 1000 sq. ft house with a mini split for cooling and heat. I had humidity problems in the winter until I installed a Broan HRV80S. It's not fancy, but it does the job. I needed to duct it, but was able to hide the ducts pretty well. Sweating windows completely stopped after this. I don't think I ever got down to 30% humidity, but I'm in California, so it isn't super dry here in the winter. The energy usage was a lot less than the dehumidifier it replaced. It also makes sure we have regular air replacement, which makes the house feel noticeably fresher.

  2. Expert Member
    BILL WICHERS | | #2

    +1 for an HRV, but it’s probably worth checking to see where all that extra moisture is coming from. Do you have a lot of plants or fish tanks? Things like those can add moisture to the air. The other possibility is something coming in the basement, but that’s much less likely assuming you’re basement was prepared properly prior to being finished.

    Note that many of the portable dehumidifiers have garden hose connectors inside so that a drain hose can be connected so that you don’t have to constantly empty them. I have a unit like this in my own basement, with the hose going to a nearby floor drain.


  3. Expert Member
    Dana Dorsett | | #3

    The outdoor air in MA in winter is usually pretty dry, with dew points averaging in the 20s or lower. This winter it's more like the 30s, which is still dry enough to dehumidify by active ventilation. Heat recovery ventilation allows you to run fairly high ventilation rates with only a minimum heating energy use penalty.

    I have a single pair of Lunos E2 ductless HRV (22 cfm at max speed) running constantly in my insulated but not fully finished 1600' basement in Worcester, which is enough to keep the basement humidity in the 30% RH @ 68F range in winter, but adds a bit to the mechanical dehumidification load in summer. Unless you have a major humidity source (like a clothes dryer venting into the basement) that would probably be sufficient for your basement too.

    If you have an electric water heater, swapping it out for a heat pump water heater will also provide a lot of "free" dehumidification, turning the latent heat of the moisture it's pulling from the air into sensible heat in the water inside the insulated tank. Heat pump water heaters are also subsidized with a significant rebate in a wide swath of MA under the MassSave program.

  4. cjohnsen89 | | #4

    Thank you for the responses.

    I do not have any sources of humility. The washer/dryer are in the basement and correctly vented. The basement is very dry and is very well insulated both below and on the sides.

    I recently visited a friend in Vermont who just built a house and he was talking about his moisture issues with a tight house. He installed a HRV in the attic and it made it like a completely new house.

    I took a few pictures of his setup and I think I will be going that route. I plan on installing it in the lofted are to pull air from the bathroom, bedroom, and cathedral ceiling portion of the house. I will return air to the cathedral portion of the house. It will be an easy hook up to get fresh air and to vent to the outside up there. Thanks again for the comments

  5. maine_tyler | | #5

    This might help a bit too:

    I don't say this to be pedantic, but you DO have humidity sources if you're humidity is higher than environmental. But, seeing as your house is small and tight, these sources may be as commonplace and unavoidable as cooking, showering, and breathing. But it's still worth considering what these sources are since running your specific systems to deal with them will be the most effective (i.e. bath fan when showering, stove fan when cooking, water recovery respirator when breathing). Alright the last one was a joke. Any Frank Herbert 'Dune' fans out there familiar with the stillsuit? (sweat recovery).

  6. walta100 | | #6

    It would not be unexpected for newly constructed home to have high humidity thru the first year. All the wood concrete and drywall all are full of water that will be release into your house in the first year.

    Running a dehumidifier in the first year is not a bad idea. It is wasteful to buy one knowing you will only use it for a few months.

    How tight is your house? Until you measure with a blower door test you are only guessing at how tight any house is and builder are often surprised. Rarely pleasantly and often no so much.

    In my opinion if your house tests very tight (less than 1 ACH occupant) you need to add ventilation.


  7. cjohnsen89 | | #7

    I know there has to be humidity coming from somewhere because like what you said it’s higher than outside. I don’t have a washer/dryer installed in the house yet. My shower fan does an amazing job getting the steam out and the bathroom seems dryer than the living room when I come out. I have no plants or aquariums. I think it’s mostly from breathing. When I cook I crack the window next to the stove and turn the hood on. Which actually helps the humidity issue.

    I’ve been wondering if it’s from new construction but I got the Sheetrock and plaster put up last June. I had the air conditioning running since late July until the colder months started.

    After spending hours reading about air exchange even if I didn’t have the humidity issues I think I need an HRV. When I come hold from work it smells like stale air.

    I haven’t gotten a blower test done. I live on an island and it’s mostly tendered towards rich people building huge houses. Sub contractors are booked longer than you would think and I’ve called around a bunch and there is so much money building for the rich people that the sub contractors just won’t call me back.

    In Massachusetts you need a licenced HVAC installer to do any duct work so you can get your CO. I called every company I could find and only 1 called me back and he was straight forward and said he was too busy to take on a house as small as mine. Therefor I was forced to do my ducting for my stove hood, bathroom venting, and the duct work for my eventual dryer. I’m just hoping I can BS my way through getting my CO when that time comes.

    Luckily there are forums like this where I can ask questions and get some good advise

    Edit: I also did run a humidifier full time (Along side the mini splits) from late July until mid September to try and get some moisture out from the new plaster and whatever moisture was still in the concrete slab in the basement floor. Maybe I didn’t run it long enough

    1. CMObuilds | | #8

      Setup the dehumid up to go directly into a drain, set it at 40 and let it go. It can take a month, it can take a year and a half.

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