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Odd Moisture Issue Caused by the Hydro Radiant

BenR Member Posts: 3
Looking for help figuring out how to resolve a moisture issue before it comes a bigger problem. Not sure what avenues are the best to tackle the issue.

I have a 2000 sq foot ranch with 3 zone hydro radiant on the main level with 1/2 pex loops stapled within the 16" subfloor cavities with bubble barrier stapled under that. This is my second year in the house and the heat runs fine. I have noticed moisture on windows on the ranch level when it gets really cold out and it wells up considerably if its very cold and the heat runs a long time. I can manage this by wiping a few of the rooms windows every few days.

The Relative Humidity on the main floor and the basement stay very similar to the same number and are 30-40% range in the winter.

I am finishing my basement and recently installed 3.5" rockwool insulation in the joists (under the pex, under the bubble insulation.) This had an interesting effect which is the problem i'm looking for help with. I noticed that some condensation is building up on the rockwool insulation and where they meet the joists. Also, since installing, the basement temp dropped from an average of 66 degrees to 62 degrees (the open cavities were allowing the radiant to also throw some heat into the basement. We heat the upstairs to 68-70 range in the cold months. This went from a 2-4 degree temperature difference to a 6-10 degree difference.

What is puzzling me is that condensation usually forms when warm moist air hits cold dry air. Best I can tell in this situation from all my moisture readings is that warm dry air is encountering not-much-colder dry air. There's no moisture spikes... I can't figure out how this condensation is forming.

I'll note I did put a sump pit in recently with the basement Reno and ensured I sealed it tight thinking it may be releasing moisture. If I put the moisture meter on it and its same RH as the rest of the space (again 30-40 range.)

I'll also note I had closed cell spray foam installed last spring on 3 sides of the basement. The 4th side is a walkout and has a non-vapor-barrier-protected stud wall with fiberglass insulation.

I will also mention I have central AC set up and vented in the attic. I have all vents closed and sealed during the winter.

Thanks for reading!


  • Jamie Hall
    Jamie Hall Member Posts: 17,039
    The condensation is forming on a cold surface, not because warm air is hitting. cool air. For instance... at 70 F with a relative humidity of 40%, any surface cooler than 45 F will have moisture condense on it. I would be rather surprised if the surface of the windows was more than that on a cold day. The joists may similarly be cool enough.

    Not much you can do about it.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • GroundUp
    GroundUp Member Posts: 1,281
    35-40% is quite high and will condense on most windows when the outside temps are arctic. Is this condensation forming on the rockwool only near the rim joist or everywhere? I keep a 17% RH in my house and my low-E Thermotech windows will still condense a bit when it gets down below zero, nothing much that can be done
  • Gordy
    Gordy Member Posts: 9,514
    edited February 2019
    Some observations.

    One is how little good bubble foil does, and good old rock wool insulation works in the joist bays

    When you had the spray foam done in the basement did they do the rim joist area? That is a big heat loss, and air leakage detaiL which should be addressed before insulation, and finished ceiling is installed.

    30-40% RH is not uncommon in a radiant heated home. With out an hrv.

    Dew point is the number you are looking for as to what temperature a surface must be at with a given j door temp, and RH.

    Since this is your second winter in this home, was last year the same, better, or worse for condensation.

    Was this new construction last year?

    If this is newer construction the window class will most certainly be above 45* depending on outdoor temps. Heck my old house built in 52 with storms had a glass temp of 54-60*

    Concrete is porous, and absorbs moisture either inwardly, or outwardly depending on if the foundation is water proofed, and outer soil moisture content. However since you used closed cell spray foam that path for moisture to be absorbed is gone now.

    In order for windows not to condense the RH needs to reflect the surface temp in which it does not reach dew point.

    Do you notice if humidity drops as winter progresses?
  • Gordy
    Gordy Member Posts: 9,514
    JEre is a chart which gives indoor humidity parameters for an outdoor air temp at 70* indoor temp.

  • Gordy
    Gordy Member Posts: 9,514
    I would deffinetly be sure the boiler is holding fill pressure with the main fill valve off before buttoning up that ceiling. I have concerns with batts condensing. Plus with bubble wrap hiding everything from plain site.

    Is this staple up using heat transfer plates?
  • RxRoy
    RxRoy Member Posts: 20
    I think Gordy has it right. We had moisture on the windows of our house the first ten years after we built it. Turns out the house was too tight. I installed an HRV and within two days of running it the moisture problems were gone. It's wonderful not mopping the windows off all winter long.

    This is what I put in. I wish I had put it in from day one. The brand is Fantech. Nothing fancy, but has been rock solid. Hasnt needed anything except to wash the filters and hose off the heat exchanger in the summer.
  • BenR
    BenR Member Posts: 3
    Thanks for all the comments. Answering questions in the order received.

    Condensation is most apparent where the rockwool meets the joist, and worst yet in the cavities above the sump pit. If I go around and feel the bottom of the batts maybe only 10% of them have any noticeable wetness.

    Rim Joist area was closed cell spray foamed.

    Condensation was same to worse this year than last.

    House is about 15 years old -- previous owner ignored dozens of problems i've fixed... Active water leaks into basement, dryer venting into basement, no rain gutters, busted basement door. I would describe it as he started building the house and never finished.

    Humidity drops considerably in winter. Basement/Main Floor hover around 50% RH in summer. I run a dehumidifier in the basement to keep it at 50%. Before the spray foam and water leak fixes the basement RH would go up above 70% and had a moldy musty smell.

    I think the boiler is ok. Basement is about 75 feet long. Boiler is all the way in a back corner in it's own utility room I framed and rockwooled out. The batts there are dry in and close to that room. The room is a few degrees warmer (pretty much matches the upstairs temp.) Condensation is more prevalent on the other side of the basement where the sump is and the walk out is.

    There are no heat transfer plates. Pex is stapled to the sub floor with the radiant bubble rap stapled an inch or two under that. Then a hollow gap of a few inches. Then the rockwool.

    I am going to read up on HRVs.

    The more we talk about this the more i'm leaning towards the temp difference causing the issues. The slab at ground level is 59 degrees average. The subfloor 9 feet above it is probably in the 90s or hotter with the radiant going. That's a big temperature swing in that 9 feet.

    I am reading up on and planning to add a 4th radiant zone into my basement walls to add substantial heating. I have the spray foam, then a 4" cavity, then the framed walls. I'd be able to put radiant loops in the cavity easy peasy.

    Thanks again!
    - Ben
  • Gordy
    Gordy Member Posts: 9,514
    edited February 2019
    I’d put the dehumidifier over by the sump pit. Let it run continuously. The heat rejection will help that area also.

    Sounds like a lot of previous moisture issues getting removed slowly since you have removed paths for natural removal by tightening the envelope.

    I would rule out construction material moisture at 15 years old.

    Yes I’m starting to get the picture in my head. With the issues being by the walk out portion, and the temp difference in that area.

    I really don’t want to derail your initial question, but does the radiant as is heat the house well enough?

    What type of boiler, and what water temps go out to the radiant, and return back to the boiler?

  • BenR
    BenR Member Posts: 3
    I'll experiment with the dehumidifier in that area.

    I think the boiler is a Dunkirk XEB Series II but I am unsure on it's exact size/MBH. it's propane gas fired.

    Radiant seems to heat the house well.

    Not sure on the temps either, but the main lines are at about the threshold of too hot to touch and the loops are generally warm enough touch. Again, with the touch test, the return line honestly doesn't seem too much cooler than the feed.

    I can double check specifics later. I need to do the btu calcs and math anyways to determine if I can do a simple hook in to the existing system to run a 4th zone in the basement walls. I still have quite a bit of research to do there.