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Installed 3′ polyiso in my basement (Zone 5a) and humidity is now at 50%

user-1107451 | Posted in Green Building Techniques on

Hi all,
So the background, we have a 1400sq ft basement , I have used XPS 3′ to seal the rim joists with spray foam for the edges
The rest of the basement is straight concrete, it is 8ft tall with 6ft under grade, the concrete has been given a water proof membrane when the house was built.
I am 3/4 of the way through installing the poly iso 3″ gluing them to the walls, sealing the seams with 3″ tape and sealing the edges with a bead of spray foam to seal everything air tight
The issue is my humidity is now gone up to 49% and it feels humid when you descend the stairs
The question I have is when I finish, sealing the sill etc as per the GBA cad drawings of basement finishing will this be even worse? and do I need to install a humidifier?
The only things in the basement is a Weil McLain series 3 bolier (direct vent) and a fridge (about 10 years old.
So I cant for the life of me figure out where the humidity is coming from?
Any ideas or will this go away when I finish sealing the poly iso up?
Thanks
Darren

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Darren,
    First of all: whatever you do, don't install a humidifier. (Perhaps you meant to write "dehumidifier.")

    Installing polyiso on your walls is not contributing to high indoor humidity.

    The three most probable suspects when it comes to determining the source of your moisture are: (a) moisture of construction (in other words, the concrete is still curing); (b) outdoor air that is entering through an open window or air leaks; and (c) soil moisture entering through your walls or slab, due to insufficient waterproofing or lack of a 6 mil vapor barrier under your slab.

    If you do a little investigation, you may be able to whittle this list of suspects down to 2 or even 1.

  2. user-1107451 | | #2

    sorry Martin yes i did mean a DE-humidifier ;)
    the house was built in 2007 so i think no 1 is out, as i said sealed the rim joists and sill plate but with no 3 we havent had rain in over 2 months so I have to think that No 2 may be the issue

    The only other aspect is the direct vent boiler is in the open so whether im getting outside air when the boiler is not in use, could that be it? I was planning to close off the boiler and its tank in a "room" by itself and add a outside vent to give the boiler the air it needs for proper combustion

    Any suggestions/observations are appreciated

  3. Expert Member
    Dana Dorsett | | #3

    Punch your zip code into weatherspark.com dashboard and pull up a graph of the dew point history. See if the ups & downs of the basement RH tracks the outdoor dew points at all.

    https://weatherspark.com/#!dashboard;q=worcester ma

    In my part of zone 5a, it's been a relatively dry (dew-point speaking) summer until recently, but the past week or so has been on the higher side. But only relatively few days of this summer have been so dry as to be under 50% @ 70F (dew-point = 51F or less), and mechanical dehumidification (even of a reasonably tight basement) has been necessary. With 3" of polyiso on the walls my basement never exceeds 69-70F even on 90F days, and it averages in the mid 60s year round. The deep subsoil temp under the slab is about 52F, and the slab temp never exceeds 55F. With no slab insulation there's a steady heat flux from the house into the ground, so even when it's 78F on the first floor, the temp at head-height in the basement is still in the high 60s.

    It's possible that by insulating the walls against the summer heat the air temperature in YOUR basement is now being pulled toward slab temperature, which would raise the relative humidity without changing the moisture content. The temperature is what the humidity is "relative" to. If you take 50% RH / 70F air and cool it to 51F it's relative humidity becomes 100%.

    If you have an open outdoor vent to the boiler, I'd put strong odds that your basement's dew point temp tracks the outdoor temp a bit, and that by insulating the foundation walls you have lowered the average temperature several degrees, raising the RH.

  4. user-1107451 | | #4

    So Dana I should look at buying a dehumidifier but once Ive sealed everything up tight I should only really have to use it every summer when outside temps and dew point are high

    The direct vent I have no option to close that when not in use i.e a louver system on the outside that opens when the blower motor starts Or just leave it and when I seal the boiler into a utility room by itself I wont have to worry about that humidity leaking into the basement?

  5. charlie_sullivan | | #5

    I think Dana figured out the right explanation. If you insulate the floor, that might bring the temperature back up, and then the humidity would go down.

    If you have things sealed up well, it will be beneficial to dehumidify, but you shouldn't need to run the dehumidifier very much, just as you say.

    But 50% doesn't sound like a problem to me. That would be my setpoint for the humidistat on my basement dehumidifer (except that the humidistat broke earlier this year).

  6. Expert Member
    Dana Dorsett | | #6

    Most direct-vent boilers do not connect the conditioned space air to the intake or exhaust venting, so it's probably not big infiltration point. Clothes dryer vents are a common large air leak into basements.

    If you haven't already, sealing up all unused flue ports into any chimneys, chimney chases or plumbing stacks / chases that go from basement to the attic can suppress major stack-effect drivers of outdoor infiltration. Sealing the attic floor plane from the condition space counts too.

    I live in a comparable climate (zone 5A, central Massachusetts) and only need mechanical dehumidification in my sealed & insulated basement from late-May to late September, holding the line at ~60% RH. My antique ~1500 square foot slab (sans vapor barrier) is less than 2' from the local water table even during the dry season, but the dehumidifier doesn't pull 10 pints even on the worst days. The outdoor dew points are clearly the larger moisture source in the basement for me, despite a history of flooding. Four sump pumps keep the water from topping the slab during the spring thaw or after major tropical storm downpours, but the duty cycle on the dehumidifier tracks outdoor dew points.

  7. user-1107451 | | #7

    Im in Rockland county just north of NYC
    I havent buttoned everything up but wasnt expecting the rise of RH
    As it stands to run a dehumidifier for a few weeks/months doesnt worry me, as at this stage Ive have no concrete date to get the basement finished so I wont be insulating the floor any time soon as I said the only things in the basement is the boiler and fridge, the vent s exit has sealed around the AC handler is in the attic so there is nothing else, all plumbing and electric holes have been sealed when I did the rim joist
    The final buttonng up will allow me to figure out where Im going to be with a dehumidifier, as Ive sealed the sump cover and the wall/floor joint Ive sealed with silicone caulk to lessen radon and any air infiltration through the earth
    Thanks for setting me straight as I had to figure out what/if I was doing anything wrong and it doesnt look like I was

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