could a bad slab foundation cause high indoor humidity? oversized minisplits? or?
this question is slightly inspired by the office room humidity thread where a 12k minisplit was recommended for a 7×9 office.
sorry if its long.
im in the south with a 20 year old 1 story on grade floating slab 1500 sqft home is pretty cheap. full sized brick veneer –> 1.5″ gap, –> 1/2″ foam sheathing(osb on corners) –> 2×4 framing with blown in fiberglass –> drywall. no house wrap, tape or anything anywhere.
i had a 3 ton central AC that was original to the home and the ducts were not in great shape. air handler was in the attic, flex ducts were in the attic, suspended above insulation. if i took off a register cover and put my arm in the duct it was extremely hot in the summer. it struggled to keep the house cool when it was 90-95f outside. when it was 100-110, it really sucked. i don’t have any data for humidity with central AC.
2 years ago i installed 2 DIY minisplits, a 1 ton unit in a ~12×10 room where the door is left open, and a 1.5 ton unit in a ~15×15 room where the door is left open on the opposite side of the home.
https://imgur.com/a/q2sBTq1 — master bedroom and small bedroom have the wall mounted air handlers. its about 90f outside as of noon and 55% humidity outside. it hasn’t rained in a few weeks. laundry room is always shut.
at the start of the green line is when i set the 1 ton AC to 64 before i went to bed and around 1 am i closed the door. you can see how its short cycling, lol.
i don’t have any visible mold in the house and run a 150 cfm fan when showering and a little bit afterwards, my kitchen exhaust goes outside, and i have a small litter box closet thats decently enclosed but has an exhaust fan @ 30 cfm running 24/7 and exhausts outside.
there was a blower door test done and that came back at 1400 cfm. that was after i replaced 5 single hung windows that had no seals with tilt and turn windows, a very very poorly installed french door with a tilt and slide door, dumped a few cans of spray foam, sealant and flashing tape in the attic. i even removed the old central AC registers/returns entirely. theres still 2 single hung windows to replace and a few things to air seal i’ve been dragging my feet on. a 48×84 triple pane tilt and turn with 2x panes of laminated glass will be here next month. looking forward to getting that behemoth in the wall ;_;
the house had ceramic tile and i removed all of it, when removing the tile the house smelled like dirt/a basement. i have not tested the moisture content and i still don’t have the flooring replaced — just smoothed it out with a grinder. i do get a whiff of dirt after its been raining, so that sucks.
just wondering if the slab could be the possible cause of the humidity or what i should be looking at? kind of a loaded question i guess, though.
i know the minisplits are oversized, but even if i keep the minisplit doors open and run them in dry mode and the condensers are actually running 24/7 the humidity doesn’t improve that much. are minisplits sometimes not great at controlling high levels of humidity?
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Do your mini splits turn the fan off when they are not cooling? There's more to solving this puzzle, but seeing if you can get the fans to stop running when not actively cooling is a start. I cut the jumper on my oversized Mitsubishi, which stops the fan when not cooling, and there has been noticeable difference. Mini splits aren't known for their great dehumidification, but this step will maximize what they can do, especially when oversized.
that is not something that has ever crossed my mind to do -- and i just now only started to track the humidity levels. i always leave the fan on high.
but no, i don't believe so. its an aciq and mr cool unit, both are essentially identical. the best i can do is set the fan to auto which runs it pretty low.
right now its 98 with 45% humidity outside, the master bedroom is at 67f/56% humidity and the small bedroom is at 66/53%.
not completely terrible numbers i suppose. just was curious as to why the two rooms with the air handlers are the rooms with the highest humidity. which of course isn't going to be something that anyone will really be able to answer over the internet : )
winter humidity for the master bedroom was 20-30%.
ty for the response.
HOw are you running the units?
Try forcing the fan to low.
The idea is you turn the unit into more of a dehumidifier
I doubt the slab is causing humidity, probably the humidity causing the humidity. IOW outside humid air getting in.
i usually leave them at 67 with fan on max. i guess fan on max isn't the best.
when its around 100 degrees outside i can raise the temperature to around 70, but if the outside temp is in the 80s-90s then they begin to short cycle even with doors open. really wish the leviton smart breaker gateway wasn't so expensive so i could actually monitor them without going outside to see if the condenser is on lol
>IOW outside humid air getting in.
yeaa, i know that the 1400 cfm for a 1500 sqft home with an average of 9' ceilings isn't great. theres a bit to do from the interior side and a little bit of attic work, but i don't know how much better i can get it without tearing out all of the drywall : )
With my Mitsubishi I found that (in heating anyway) running the fan on low speed caps the percentage the compressor runs at. On my energy monitor I've seen this extend run times for more winter comfort and efficiency, though I haven't confirmed the effect for a/c yet. If you're comfortable adding power meter clamps in the breaker panel, I'd recommend an emporia Vue energy monitor. That's been very helpful in knowing what's going on with my heat pump and finding better ways to run it.
As someone with a very leaky 70s house, I found some success using a spray foam can to seal the gap between the drywall and subfloor. My metal receptacle boxes don't lend themselves to good results when caulking holes from the inside, but plastic boxes might be a place for that method to work.
i'm gonna try a lower fan speed for a few days and see how that works.. the emporia thing is cool, maybe one day ill get the leviton smart hub since i already have a leviton panel. i irrationally hate needless cables or cables that are difficult to organize. the emporia thing would be a personal nightmare lol.
>As someone with a very leaky 70s house, I found some success using a spray foam can to seal the gap between the drywall and subfloor. My metal receptacle boxes don't lend themselves to good results when caulking holes from the inside, but plastic boxes might be a place for that method to work.
yea, so that was a huge source of air leakage when tested. i used a combination of lexel and sprayfoam. it made a noticeable difference last winter when standing next to exterior walls. was a pita to cut away the excess sill gasket, fiberglass netting, vacuum, etc..
just drywall gaps in general are issues. i've been pumping every crack full of sealant as i remove and replace door frames/jambs, entryway trim, window returns, etc..
Turn the fan down until it fails to maintain setpoint
Stop changing the temperature
Actually turn the fan down to low and only raise it when it fails to maintain setpoint
See what your humidity does then
From your screenshot, the dewpoints are:
Living room: 58 °F
Master bedroom: 56 °F
Laundry room: 65 °F
Small bedroom: 56 °F
Office: 57 °F
Relative humidity is relative. When the exact same bit of air gets warmer, the absolute humidity stays the same, but the relative humidity drops. Dewpoint is a convenient measure of absolute humidity.
From the dewpoint, you see that your rooms with the mini splits actually do have the lowest absolute humidity. Their relative humidity is a bit higher because they are cold. As that air moves into the rest of the house and warms up, the relative humidity drops.
Dewpoint helps explain why you need to lower your fan speed. When you cool air below its dewpoint, humidity condenses. The slower the fan, the colder the air gets over the coil, and the lower you can drop the dewpoint.
thanks, that helps.
“The master bedroom is at 67f/56% humidity and the small bedroom is at 66/53%.”
This is about as good as one can expect for any AC unit in most climates. The air discharged from you AC ducts will be at about 99% relative humidity then it mixes with the warmer air in the room and upper 50s is about as low as one can expect unless you live in a desert.
Did you block all the old duct work from the attic?
When they did your blower door test did, they calculate your ACH50 number? The raw leakage number does not tell us much.
If you want to lower the humidity, try shutting down one of the two units this will force the other to run continuously.
Do your units have a “Dry” mode setting?
Walta
really my only comparison is coworkers with new homes and their few sensors being lower than mine :)
>Did you block all the old duct work from the attic?
i fully committed.
painfully ripped out each register boot, threw a piece of foam board over the hole, spray foamed around the board and then eventually patched the drywall. got rid of the duct and somehow found an hvac company to recapture the old r-22 and remove the air handler + furnace out of attic for a whole $200.
>When they did your blower door test did, they calculate your ACH50 number? The raw leakage number does not tell us much.
no, but the house is around 1550 sqft with around 300 sqft having 8' ceiling and the rest having 10'. so something in the 5.5-6ach50 range? or is there more to it than that?
>If you want to lower the humidity, try shutting down one of the two units this will force the other to run continuously.
so recently what also prompted me to start paying attention was when i had 1.5 ton unit set to 72 and i noticed the room smelled like a basement during this spring. normally the AC had been set to 70 and that was less of an issue, or at least i never noticed it.
https://i.imgur.com/3ujuCZL.png
size is not accurate, but the shapes are. the bedroom without the minisplit stays comfy for as long as the door stays open. but with the bottom right minisplit off, the living room really suffers in comfort.
>Do your units have a “Dry” mode setting?
yea, i don't really understand what it is functionally doing. it will make a room almost frigid in temperature -- i've gotten a room into the 50s with it, can't do that with "cool". then regardless of the temperature set, condensation visibly accumulates on the air handler. i guess its doing what it needs to, idk.
https://imgur.com/a/YEqRi4n
outside temp is around 101f, humidity is around 40%. fans have been set to mid since yesterday.
whenever one of these units dies or i have excess $ to replace premature, i will figure something else out -- especially for winter performance, the living room suffers more than in the summer. maybe a third zone without increasing the overall capacity would help along with a hyperheat model.
https://i.imgur.com/7M5PL2e.png
it has been hotter and slightly less humid so its a little hard to say, seems like an improvement, though. fan speed has been lowered since the 22nd. and my last window was finished today after a 3 month wait ;_; https://i.imgur.com/smX4sCO.jpeg
thx team.
so about 6 months ago i cut out drywall around a window in the bedroom with humidity issues so that i could measure for the new window.
there was some fiberglass stuffed around the arch but i never bothered to look behind it. today i started to prep the window for removal and found this lovely detail;
https://i.imgur.com/Fs7Ut8A.jpeg
house is brick veneer with a ~1.5" air gap between sheathing and brick, except this window has a stone arch above it. which i guess rests directly against the osb over the straight portion, with no osb at all for the radius.
i'm no expert but that doesn't seem very great.
its only 91 today but daggum, that stone was very warm on the interior side :)
i know that the humidity issues are a culmination of things, but a 48x84 window with zero flashing, foam, sealants, etc on top of being a single hung window with busted seals can't really be helping too much.
life update for all of my fans out there;
i removed a jetted tub from the main bath, which had no drywall behind the tub, just insulation. the bottom plate did have foam but it was extremely loose. certainly would explain extremely cold and hot temperatures + the crazy amount of air felt during a blower door test. got everything nice and sealed + some goboard. theres a 48x48 ~r2 window -- north facing & right above the tub area, once that goes i imagine the bathroom will be a lot more comfortable.
but i guess the mystery of the stinky earth smelling slab is also solved; i have no vapor barrier. just a slab on top of several feet of sand.