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Community and Q&A

Where is the excess humidity coming from? No obvious sources

jeffwatson | Posted in General Questions on

I always read about optimal (relative) humidity being somewhere within the 30%-50% range.

I started monitoring the humidity in my single story brick house over winter it appears it’s always in the upper 40% range. I am curious why it is so high. This is an old 2-wydthe brick house in Zone 5.

Sample readings:
– Basement: 52% @ 59deg, DP: 41deg
– Main floor: 47% @ 68deg, DP: 47deg
– Outside: 60% @ 33deg, DP: 22deg

Usual suspects:
– 2 people, gone during business hours
– bathroom fan vented outside through the roof, always in use during showers
– furnace sealed combustion (nat. gas)
– water heater sealed combustion (nat. gas)
– exhaust fan over the (nat. gas) stove always in use while cooking, vented outside through the roof
– (nat. gas) clothes dryer vented outside
– sealed sump pit
– no obvious foundation cracks
– no house plants
– no obvious plumbing leaks
– no carpet in the house, no firewood or fireplace

1) What could be causing this high humidity?

2) Am I correct that if the dew point outside is lower than what it is inside, allowing outside air to come in is more efficient than running the dehumidifier? I bet it will cause frosting at the windows, but is this natural dehumidification better from an energy perspective?

Thank you.

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  1. davidmeiland | | #1

    47% at 68F is not ridiculously high, especially in a small-ish house with a few occupants. Questions I would ask are, what instrument(s) are you using to measure and how do you know they're accurate, and what happens if you ventilate more, perhaps running the bath fan for 15 minutes of each hour when you are home? Does that reduce the reading?

  2. GBA Editor
    Martin Holladay | | #2

    David's answer is a good one. Many hygrometers are inaccurate, so it might make sense to verify your readings with additional instruments.

    In an older home, large amounts of moisture can enter through basement slabs and basement walls, even when these surfaces appear dry to the touch. You may want to tape an 8"x8" square of polyethylene to these surfaces. After 24 hours, look for beads of moisture between the concrete and the polyethylene. If you see any, that's a sign that the foundation is contributing moisture to your indoor air.

  3. charlie_sullivan | | #3

    Running a dehumidifier in the winter is not a bad thing to do. The electricity it consumes gets turned into heat, plus the latent heat associated with the moisture it removes gets turned into sensible heat. An energy star dehumidifier operating at its rated performance provides about double the space heating you would get from a space heater using the same amount of electricity ... it is the equivalent of a cop = 2 heat pump. It will get less efficient at lower humidity, however. There are a few dehumidifiers that beat the energy star spec and would do even better.

    But I agree that 47% is not a problem.

    As far as sources, there are people, pets and plants. There's also the fact that a shower continues to put moisture into the air as it and the towel dry. Given those sources, the humidity might primarily be an indication that your house is pretty tight. You might then want more ventilation for other reasons too. There are starting to be $200 air quality meters available that measure lots of things. That technology is brand new, so it is hard to give advice about what to buy or what to make of the numbers.

    Given that your basement dew point is lower than the upstairs, that doesn't sound likely to be a primary source. Brick walls can get wet from rain. Normally in the winter they would dty to the outside, but without knowing much about your wall construction, I will leave open the possibility that some moisture is coming inside from rain wetted walls.

  4. jeffwatson | | #4

    Sorry, forgot to mention that, this is a 1000sqft home with a poured concrete basement over the full square footage.

    My concern is primarily that the inside humidity @ 68deg (47%) is already near the top of the scale and it's still cold out with low moisture content (22deg outside dew point). Not a problem now, but figuring it would be a lot worse when temps warm up (and not using A/C).

    I'm using those small $10 Accurite temp/humidity gauges, which won't have scientific accuracy - but I don't think that's required for my loose monitoring, so their margin of error is acceptable. In addition, they are accurate amongst themselves when placed in the same area.

    Yes, running the bath fan or the over-stove fan does bring down the humidity (and temp slightly). Those are our ventilation mechanisms. Showers in the morning, airing out during the day with windows closed, and cooking in the evening. Our CO2 does run high overnight (1500ppm+) but after the morning showers & going to work, coming back home it'll have dropped back down to 47%, 59deg -> 57deg, which yields about a 2 degree drop in dew point. Sure the basement got colder, but we're never down there anyway & don't have any registers open. Just feels like I'm spending 3KWh/day in winter to keep a basement at a higher humidity than what's outside - sounds backwards.

    Wall construction on the main floor: 2 layers of mortared brick, about a 1" thick kraft-faced fiberglass layer, drywall on furring strips.

    So, it sounds like "people" are my problem (no pets or plants). Where I'm confused is why the CO2 drops significantly when we're gone (1500->700) & the windows are closed, no HVAC or mechanical ventilation active. That would say the house is not very air tight. But then there's the elevated moisture content, which would say the house might be tight. Which one is it if we're not purposely generating moisture except by breathing?

  5. PaKettle | | #5

    Perhaps the problem is your dealing with relative humidity.

    From wiki....
    The relative humidity (RH or phi) of an air-water mixture is defined as the ratio of the partial pressure of water vapor (p_{H_2O}) in the mixture to the equilibrium vapor pressure of water (p^*_{H_2O}) at a given temperature:[1][2]
    phi = {p_{H_2O} over p^*_{H_2O}}
    Relative humidity is normally expressed as a percentage; a higher percentage means that the air-water mixture is more humid.

    Because the outdoor air is much colder it can hold a lot less actual water so it will have a higher relative humidity then the same air would have indoors... I haven't done the math but the figures you give all appear to have roughly the same water content....

    Your CO2 issue may partially be the same one the biodome projects ran into - Concrete outgassing

    I would suggest doing a door test to find out how tight your house actually is.....

  6. charlie_sullivan | | #6

    Oh, I didn't realize you were already running a dehumidifier in the basement. That is unusual to need to do that in the winter, and it's not likely that that moisture is coming from people breathing or showers. That then does sound like moisture permeating the walls and slab. How high does the basement humidity get if you leave the dehumidifier off for say a week?

  7. Expert Member
    Dana Dorsett | | #7

    If you have to run dehumidifiers in the basement in winter it's almost always a ground moisture issue. Surface grading, surface drains, and footing drains could be needed.

    With a double wythe brick, no-cavity wall and thin roof overhangs, direct wetting of the brick wicking dew & rain toward the interior can also be a strong factor. If you haven't already, using the appropriate masonry sealers on the exterior can take a big chunk out of the wicking issue.

    Ventilating at a high rate with a heat recovery ventilation (HRV) system can bring the interior moisture down in winter and well into the shoulder seasons more efficiently than running a dehumidifier. Using an exhaust-only at a high rate on the first floor could actually pull nearly saturated soil-gases /air into the basement. The bigger your needed ventilation rates, the more important balanced ventilation becomes.

  8. jeffwatson | | #8

    Brian - I thought normalizing RH/Temp to dew points would give a decent comparison among the different numbers. Is this not correct/ideal? Can you give more information about the biodome / concrete gassing issue you mention? This is a house that is probably at least 50 years old with no major renovations/construction.

    Charlie - before I started closely monitoring the humidity in the basement, I had seen some 60-65% numbers around 56-58deg. That's when I started using the dehumidifier set at 55%.

    Starting from a point where the windows were open in the basement & dehumidifier off and then closing the window to see the reaction: in 24 hours it's climbed from 43% to 50% RH at 59deg (36->40deg dew points). However, the outside dew point has also grew from 17 to 35deg (temps 31 -> 43deg).

    Dana - are you saying elevated moisture in the concrete is primarily an improper use of gravity of the ground surface? FWIW, there is a sump with drain tile that is working properly. Also a little unclear with your masonry sealer comment - are you saying there's something I can apply to the exterior brick that would make it less permeable to vapor?

    Also, my last blower door was 800cfm into 12K cuFt at -50 Pascal - I can't keep track of what the current units are for interpretation of those numbers though.

  9. davidmeiland | | #9

    You're right at 4.0 ACH50. That's a lot leakier than we would be shooting for in new construction or a major remodel/retrofit, but probably around average for an existing house (and better than many).

  10. STEPHEN SHEEHY | | #10

    Jeff: Be very careful if you apply a masonry sealer. You may want something that keeps out bulk water, but allows water vapor to pass through. You want Goretex, not a rubber coat. Moisture that gets in the brick needs to be able to get out. Otherwise, trapped moisture will freeze and spall the brick.

    Before I retired, I represented a condo association with a very expensive problem caused by use of the wrong sealer.

  11. Expert Member
    Dana Dorsett | | #11

    Jeff: Stephen has it right- the masonry sealer wouldn't/shouldn't be un-permeable to water vapor. The sealer is intended to make the surface less wickable for LIQUID water. Most masonry sealers are not vapor barriers, but some are. (I should have made that clear.) With vapor-barrier masonry sealers the risk of freeze/thaw spalling is small but not impossible in a zone 5 climate.

    Pumping water out of the sump and footing drain is necessary, but not sufficient. If the below grade walls aren't waterproof and surface water runs down the foundation a lot of that moisture can wick into the foundation, and evaporate into the basement air. If there's no vapor barrier in the slab, water vapor can and will come in from nearly saturated soils below, and into the basement. When you depressurize the house with exhaust-only venting, the soil gases come in even faster, and it's a race between the dilution factor of other air paths and the gases & moisture coming through the slab & below-grade walls.

    For below grade masonry walls and slabs a vapor-barrier masonry sealer will sometimes help.

  12. PaKettle | | #12

    Dewpoint is the maximum amount of water vapor the air can adsorb which changes based on temperature. You want to know how much extra water is being adsorbed/released with the outside air serving as your reference point.
    Here is a convertor from relative humidity to absolute.

    One other measure that can help is a surface moisture meter/tester. It will give you a good indication of how much moisture is in the walls and slab.

    I was backwards on the co2 issue for Biodome - the concrete was removing CO2 and converting it to calcium carbonate. Not the sole cause of the failure but a heavy contributor.

    1500 is on the high side for normal indoor co2 levels and considering the amount of air leakage you are indicating I would suggest locating the cause(s) before you try to tighten up your envelope.

  13. jeffwatson | | #13

    I'm attaching the graph of the absolute humidity values. Blue line is absolute humidity in the kitchen, green is absolute humidity in the basement laundry area, and red is the absolute humidity outside in g/m^3 units. The dashed purple line is the CO2 in ppm (uses the 2nd Y axis).

    The data starts where we were dehumidifying the basement to 55%, then we turned off the dehumidifier & opened the basement window, then left the dehumidifier off with the basement window closed. Only 4/8 8pm was the stove on, where the windows are open & the stove fan running.

    Is there anything stand-out or odd about this data? Note that the data collection is manual & not at any regular intervals. But I usually do a wake-up reading, late morning reading, and an evening reading.

    I've used a surface moisture meter with no indications of excess values & I've done the plastic taped-to-the-wall test a couple times. Nothing stand-out.

  14. Dana1 | | #14

    So, when the window was open, both the CO2 & H2O levels dropped, tracking with (but not quite reaching) the outdoor H2O level. This is not surprising, but it's also an indication that you'd probably be able to control winter & shoulder season humidity levels with higher rates of (preferably heat recovery) ventilation.

    Assuming the laundry in the basement, the slower response to the windows being open is probably an indication of the window size (ergo lower ventilation rate) &/or the basement is a source of the moisture, which is common with basements that have no ground moisture vapor barriers or indequate waterproofing & drainage at the foundation walls & footings.

  15. PaKettle | | #15

    Good job on the graph.

    I am a bit concerned that you may be leaking combustion products (water and co2) from your flue(s). I would suggest you go over all your connections and make sure they are "gas-tight". All the joints should be fixed with screws, All the seams should be taped or caulked shut and all the pipes are heavy gauge steel and not aluminum. Everything should be wiggle free as well. I would also check if there is any back drafting when both the bath and stove vents are running.

    Aside from that I agree that this is most likely a damp basement type of issue. Assuming your walls and floor are staying dry I would consider trying a drylok type sealer and making sure your sump is sealed (prevents radon gas as well). Some HRV as Dorsett suggested would help equalize your basement with the outside air.

    Overall I am not convinced there really is a problem though. I would really suggest that you monitor the situation for a full year before going nuts on this. None of the numbers your showing really indicate a dire need to take action.

  16. jeffwatson | | #16

    When we have the kitchen window open, we're talking maybe a 3 by 34 inch opening and the stove is about 8 feet from that window. The stove fan claims 150cfm (never measured). So both the temperature & RH drop (especially when the starting point is 68 in & 30-something out).

    The basement window is a glass block with a vent, so it does have a smaller opening, although being in the basement, you crack the vent & you can feel the air rushing in, sometimes with a low rumble or howl sound.

    I understand that it's likely an old house like mine will just be more damp in the basement - I just didn't think it'd be this way in the middle of winter when it's less than 30 degrees out if my house is not as air-tight as newer houses are. From my numbers, the laundry (in the basement) does drop in (absolute) humidity with (natural/non-mechanical) ventilation, but once that's stopped, it slowly climbs back up to its seemingly steady-state value of around 6.7 g/m^3. The two dips at 4/5 and 4/9 have both leveled out to 6.7 g/m^3. When it rains, it seems to only have a tiny impact on the basement moisture content.

    I wish I had a better way of tracking this because I don't have historical data from my meters & the values are only retrieved when I physically look at them & put them in Excel. My CO2 meter does give historical, which is the only way I know that our CO2 steadily climbs into 1500 and 1600 territory overnight, and once the furnace comes on from its setback around 5am, is when the CO2 either drops or evens out. Not sure if that means my furnace is bringing in outside air. I did notice that the absolute humidity does seem to rise slightly after the furnace turns on, except I need to make sure I look at the meter before we start taking showers & the bath fan is active.

    Brian, my water heater & furnace both vent with PVC (sealed combustion). The stove does not have an externally vented flue. The dryer is vented with rigid flex duct, but it sees usage maybe every other week. I didn't want to mention it, but I have been trying to track down health issues which seem to correspond with poor IAQ for the last 2 years now. Water heater & furnace used to be naturally drafted (e.g., they're brand new within the past 2 years). Problem is, I haven't "tracked" stuff - it's all been visual observations every now & then.

  17. jeffwatson | | #17

    Let me pose this question -

    Is it possible that furnace combustion gases are mixing with the air in the blower chamber, even if the intake/exhaust PVC pipes are all "tight" & routed outside?

    Looking at the historical data on my CO2 meter in the kitchen, it seems like the humidity in the kitchen kicks up when the furnace comes on both in the morning as well as in the evening after its 5 degree setback. Few possibilities that make me doubt this:

    1) CO2 meter has separate temperature & humidity sensors with different response times
    2) Air is just being re-circulated & humid air is just balancing out

    Is there a test I could do to really see if combustion gases are leaking into the blower? I can clearly see the steam outside from the exhaust and there's always a constant drip into the condensation pump when the furnace is on - makes me think the system is working. I don't want to stick a temp/humidity meter right above the heat exchanger which produces about 120degree air.

  18. jeffwatson | | #18

    Just to add one more thing to the mix, just did a visual of the furnace and there's a lot of orange/brownish dust on the outside of the ductwork right above the heat exchanger where the coil would be. Is this some type of combustion dust?

  19. PaKettle | | #19

    Likely its just rust....

    Any chance Your exhaust gases from outside are finding their way back into the house? Open window or a vent anywhere nearby? Loose siding in the area?

  20. jeffwatson | | #20

    Closer inspection, it's not orange, but more like a pale brown or beige color & is extremely fine. Hard to believe it's rust on a 2 year old furnace, but I'll have to look at the A-coil.

    Rubbing my finger across this dust on the ductwork, it still feels like I'm touching the ductwork, but then I can see the chalky residue left behind. It appears to be more fine than drywall dust. I think I've seen this same kind of dust cleaning out my bathroom fan motor (not the grille). Beige, really fine, gives no distinct feeling when touching.

    As far as gases finding their way back in - all windows are closed and this is a brick house (no siding). The only purposeful holes are for the bathroom fan (which does have a backdraft damper), the chimney, and soffit vents for the attic. The chimney does not have anything connected (capped off from the inside). The venting for the furnace & HWH exit through the wall a foot or so above grade & the roof starts at about 15 feet off the ground.

    But if we assume that somehow the gases are finding their way back in, does the description of this dust match something that could be produced by combustion?

  21. PaKettle | | #21

    Gas burners usually only produce gases. Sounds like it may just be common dust.

    Best I can tell you at this point would be to try and pinpoint where the co2 is highest and go from there.

    As to your moisture issue... have you looked into getting an airborne mold test done?

  22. jeffwatson | | #22

    CO2 is a by-product of combustion and if the furnace was not fully exhausting it & circulating it around the house, the levels would rise. We see the opposite which kind of just points to needing more ventilation; there's a shoot from ~1000ppm towards 1600ppm overnight while we sleep, and as soon as the furnace comes on from it's setback, it drops significantly to around 1200ppm.

    This distinct rise & fall happens every night/morning - the furnace seems to be diluting the air.

    And when we leave in the morning, the CO2 falls slowly until we get home. It then starts climbing again, which leads me to believe it's just CO2 from human respiration.

    I would assume that moisture should see a corresponding fall when CO2 drops, because based on my monitoring, the absolute humidity outside has always been below the inside value & the outside temperature has not reached inside temperatures (not even the basement's 50degree-ish values). This doesn't seem to be the case though.

    How can simply running the furnace after a night-time buildup of CO2 drop the CO2 levels but not the humidity? My guess is the furnace is probably pulling up basement air where the CO2 levels are probably significantly less than the 1500ppm levels upstairs from 2 people sleeping 7+ hours.

    The moisture upstairs DOES drop when we use exhaust-only ventilators. However, once we turn those off, it's like it pops right back up to where it was. It's as if moisture is stored & is being released inside the house. Do people run dehumidifiers in their conditioned living space in the middle of winter? Seems odd to have to do that.

    Brian, I have seen 2 environmental specialists. The first one did a quick visual inspection, told me to get my ducts cleaned, and then wanted an additional X amount to collect samples. The other actually walked around with some device measuring VOCs, formaldehyde, CO2, CO, NO2, humidity & basically said "stop air sealing NOW!" due to high CO2 and humidity. I have not had anything lab-tested because I'm unsure how it actually helps if the specialists cannot find root causes.

    I actually contacted Building Science to see if they would come out & investigate but they said my case wouldn't be cost effective for them based on their minimums. Funny though because if I add up all the local people I've had look at my case, the price I've paid approaches BSC's minimum value!

    I guess at the end of the day, it may just be as simple as - house is too tight, you need more ventilation, and your (unsealed) brick/old-foundation probably makes the situation worse. Then when I contact people about installing an HRV, they look at me like I'm crazy when they see my old tiny Chicago brick house & feel like it's not worth their time! Given the cost of one, I still haven't gotten a sense that increased ventilation is really a fix as opposed to masking another issue.

  23. davidmeiland | | #23

    Your HVAC may be pressurizing or depressurizing your house due to imbalanced supply/return pressures and duct leakage. It is fairly easy to check--you get a manometer, connect a tube to it and put it under a door, make sure everything is closed, get a baseline pressure, and then turn on the HVAC and see what the change is.

    I don't know that you need BSC to come out and investigate, but your house sounds like a great puzzle for an energy auditor, someone with blower door, duct blaster, IR, and other gear. You might start with a blower door test, check your exhaust fan flow rates in bathroom and laundry room, and then implement the ventilation rate recommended by the blower door test... and see if that changes things.

  24. PaKettle | | #24

    When the hvac runs it circulates CO2 laden air from one room and dilutes it with air from another room.

  25. jeffwatson | | #25

    David, can you elaborate on the manometer method or provide a good link that describes this test? Only reason I ask is because I've done this test myself before & my results were inconclusive (no change in numbers). I do have a differential manometer which I've used to measure static pressure & ensure it's in-range.

    I've had 3 audits so far. My bathroom fan was measured as pulling 70cfm. I know I can use that as ventilation, but the fact that this depressurizes the house & could pull in soil gas is the reason I don't use it constantly. We use it a lot, just not constantly.

    I've had a duct leakage test with a duct blaster - all that yielded was a number where the interpretation was just that "it's high." What I was looking for was where is the leakage & didn't get that. My exposed ducts are already sealed with mastic as well as the duct boots/terminations.

    Things have been tested & measured, but interpretation & correlation is what's lacking when I've hired someone.

    I like making things generic so that I'm not the only one receiving information, so if I was a random client & just stepped back and said, "I think the humidity in my house is high or I think I have an IAQ issue" - what things would you guys test for to either prove or disprove that? And let's assume the basics like long, unventilated showers or cooking & obvious plumbing leaks are not an issue.

  26. PaKettle | | #26

    You can use a fog machine to locate your duct leaks. You can also check the output of each register with an Anemometer and calculate roughly how much and where your losses are.

    Hvac is allowed to leak/lose up to 15 percent of the air. Unless there is a distribution problem such as a cold or hot room it may not be worth the cost.

  27. GBA Editor
    Martin Holladay | | #27

    You wrote, "The fact that the bathroom exhaust fan depressurizes the house & could pull in soil gas is the reason I don't use it constantly."

    Your concern is misplaced. Here is a link to an article with more information on the topic: Exhaust-Only Ventilation Systems and Radon.

  28. charlie_sullivan | | #28

    Jeff, if you have a differential manometer, that's the ideal tool for determining how much the house is depressurized when you run the exhaust fan. You put one tube (or an open connection) in the indoor space of interest, and run another outside through a door or window or other little crack. Then you can do experiments to see what it reads during different weather conditions, and with the HVAC on and off and the exhaust fan on and off. You can repeat those experiments in different spaces.

    Those results are only a piece of the overall puzzle, but if you were to post those numbers we could help you sort through what they mean and whether they help you decide how to proceed.

  29. davidmeiland | | #29

    In this photo there is a thin piece of stainless tubing at the end of the clear tubing, and it's pushed under an exterior door so the end is outside. There is a rubber door shoe on the bottom of most doors and this method works well. With the HVAC running in this building there was reliably -3 pascals inside. There was very little return duct and most of it was inside; there was quite a bit of supply duct in the crawl space.

  30. jeffwatson | | #30

    Charlie/David/Brian - thank you for explaining. Now that you guys have mentioned some units, that was probably my issue - the manometer isn't sensitive enough. Its resolution is 0.01 inh20 which translates to lowest readings of 0, 2.4, 4.9, ..., Pascal. Any readings at those levels are probably in the noise as well.

    Brian - I took you up on seeing where CO2 is highest. The difficulty is that I have a single meter & it takes 10 minutes or so for the values to stabilize. All readings below are after at least a 10 minute waiting period. So, after a night where the furnace stayed off, bathroom fan never run, and windows all closed, I measured in the morning:
    - Kitchen CO2: 1600ppm
    - Other rooms on main floor: Kitchen +/- 50ppm
    - Various areas in basement: 670ppm +/- 20ppm

    So, you could say main_floor = 1600ppm, basement = 670ppm. That's a pretty good separation, would never have guessed it would be more than twice as much. Is it common to see such a high separation? What could this mean?

    I then ran the fan on the furnace for 2 hours without heat, and the result matches the thinking that basement air is diluting main floor air:

    - main_floor after 2hrs: 1600ppm ---> 1450ppm
    - basement after 2hrs: 670ppm ---> 1055ppm

    Based on some probably far-reaching calculations, the total volume of CO2 in the house rose 1.17cuft in 2 hours (0.016m^3/hr), which matches what says is a little bit above the CO2 emission of a person sleeping (0.013). I was the only one present & only watching TV/internet.

    The fact that the total CO2 volume increased consistent with CO2 emission by a human sleeping or doing very little work, I'm inclined to think that turning on the furnace fan doesn't pull-in/push-out (very much?) outside air or depressurize the house that much. Is my thinking flawed? But I have learned that my basement (which has no registers) does suck in air from the main floor when the furnace fan is on. I did not have the heat on; I bet if I did, I would have seen a smaller increase in the total CO2 volume due to stack effect providing ventilation, especially from hot ductwork in a cold basement. However, it's 60+ degrees outside (inside is 67, basement about 59).

    From measuring absolute humidities, the main floor humidity has always been higher than the basement humidity. This tells me when the furnace fan is on, I'm dumping humidity into my basement (or vice versa if roles are swapped). Excel shows a trend line with a positive slope for main floor absolute humidity.

    Martin - thanks for that article. I will take that into account - guess I should just measure radon instead of making assumptions. There was a time where my dual-function sump/ejector pit was not sealed or vented which started that concern.

  31. PaKettle | | #31

    Interesting , most of your CO2 and humidity are coming from your living areas - It sounds like your main floor is not exchanging much air with the outside inspite of what your air leakage test indicated.

    At this point I would suggest that if you feel the air quality is having an impact on your health then you should consider installing some form of HRV system with an upstairs intake and a basement exhaust if possible. Weather permitting, you may want to vent using windows for a month or so and see if your issues improve first.

  32. jeffwatson | | #32

    All my blower door testing has been with the basement/mainfloor door open. Though it's an interior door, I've treated like an exterior by adding weather stripping. I'm curious how my numbers would change if the test was executed with that door closed.

    This has always been in the back of my head - is the basement considered outside the envelope if it is not conditioned (e.g., no HVAC registers)? There is no insulation between basement/main floor, but on the main floor, I have done a lot of caulking to baseboards. Was this a bad thing to do?

    I started this thread knowing that increasing my ventilation would likely help. I wanted to make sure there was no other phenomenon that was happening that increased ventilation would be masking.

    You guys have brought up:
    - basement humidity from the slab/foundation walls of an old house which likely doesn't have the waterproofing protections that you'd see today
    - bricks possibly wicking up moisture if no masonry sealer has been applied

    The hesitation with applying masonry sealant is that a moisture meter does not yield any "wet" readings on the interior walls. The past week we've had no precipitation but yet the humidity of the main floor house slowly rises, even after the CO2 rises & falls with ventilation cycles from morning showers & evening cooking.

    As far as opening the windows - last summer we did that when it wasn't as hot & yes we didn't get irritated. Last summer, anytime the A/C came on was an immediate onsought of itchy eyes, stuffy noses & sneezing for two people who've never had any allergies. Those same symptoms persisted into winter but at a lesser degree - mainly itchy eyes & the occasional cough in the morning.

    The HRV setup - I'm curious why you say upstairs intake & basement exhaust? If our humidity issue is primarily upstairs, wouldn't we want to locate the intake there? Also, won't an HRV/ERV introduce humidity if it's more humid outside?

  33. PaKettle | | #33

    You want to draw the fresh air into the upstairs living area where its "stale" and exhaust it from the basement. I really don't think you have a humidity problem but if the local climate does have severe humidity issues I am pretty sure they make HRV units that can deal with it.

    Last summer, anytime the A/C came on was an immediate onsought of itchy eyes, stuffy noses & sneezing for two people who've never had any allergies. Those same symptoms persisted into winter but at a lesser degree - mainly itchy eyes & the occasional cough in the morning.

    A/C units are well known for hosting mold colonies....
    I hope you did get your duct work professionally cleaned as was suggested along with the coils and filters.

  34. Dana1 | | #34

    Q: "This has always been in the back of my head - is the basement considered outside the envelope if it is not conditioned (e.g., no HVAC registers)? "

    A: The basement is normally considered to be INSIDE the conditioned space, even if not actively heated & cooled, since it is inside the pressure boundary of the building envelope, and not air-sealed from the rest of the house.

    Q:" There is no insulation between basement/main floor, but on the main floor, I have done a lot of caulking to baseboards. Was this a bad thing to do?"

    A: Air sealing between floors is a GOOD thing to do, since it impedes stack-effect infiltration forces. Insulating and air sealing the basement walls is almost always more effective than insulating and air sealing the basement ceiling, but the latter still has some benefit.

    Even if the basement walls show low moisture content as adsorb or liquid doesn't mean they aren't passing copious amounts of water as water vapor. The more porous the material, the more quickly liquids & adsorb are released to the interior. Taping a sheet of polyethylene at least a foot square to the wall for a week might reveal just how much is coming through the walls. The dew point of the entrained air in the soil tends to be roughly the soil temperature. There is no real down side to sealing the walls. Some masonry sealers are also vapor barriers, which would be a good thing in your situation, reducing a potential moisture intrusion path.

    An ERV/HRV will definitely add humidity to the house when the outdoor dew points are higher than the interior. When the basement temperature is below the outdoor dew point that becomes a problem. In my area it's usually advisable to lower the ventilation rates during sticky high dew point days in summer, unless there is sufficient mechanical dehumidification in place to manage it. If you put your ZIP code into a and pull up a dew point graph, then zoom out to see the averages over the warmer 6 months you'll be able to estimate how often that will occur in your area.

  35. jeffwatson | | #35

    Thank you for clearing it up Brian - we were talking about the same thing - pull outside air into the main floor, push old air out of the basement.

    We've had two duct cleanings in the past two years since we've lived at this house. What I was told was if I wasn't spending at least a thousand dollars on a duct cleaning, it wasn't good enough. The cleanings I have had were in the 500 dollar range as it is. Same thing with the vacuum & air purifiers - that I needed to be spending upwards of 6 to 7 hundred dollars on these things. We're not chronic allergy sufferers who've had to be shielded from stuff our whole lives - felt like overkill. And at this point, I've spent so much money on recommendations with no positive outcome which just inspired me to monitor temp/humidity myself.

    I have used those no-rinse evaporator coil cleansing sprays on my A-coil last year when we started showing symptoms. Visual inspection didn't show any thick dust or dirt buildup. At the same time, I have no idea if it made things better or worse.

    Thank you Dana, my hypothesis matches what you describe - the HRV doesn't serve as a dehumidifier (you're still at the mercy of the outside dew point). This looks to be the case in my area. So I'd still need a dehumidifier & an HRV to maintain the air.

    Dana, I've done the plastic-taped-to-the-wall test before with no condensation resulting. Is there a chance vapor is still getting through but something else has to happen to enable the plastic/condensation test to visually indicate a response?

    Edit: attached last two weeks of data. Observation: kitchen humidity (blue) steadily climbs regardless of outside humidity (red). Average kitchen temp/RH is 68deg/47% -> 8.1g/m^3 average abs humidity. Average outside temp/RH 43/63% -> 4.7g/m^3 average abs humidity.

  36. Expert Member
    Dana Dorsett | | #36

    There doesn't need to be condensation on the plastic for there to be a lot of moisture moving through (though if there were condensation it would be a MASSIVE source.) If you have a moisture meter, measure it the area under the plastic, and compare it to other locations.

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