now that we've been in our house about a month, I'm noticing slight cupping in our floors, & I'm wondering if it's normal. 

The floor is sanded on site maple.  Before it went in, they acclimated for around 2 weeks, while running a couple dehumidifiers.  Once the moisture level was measured low enough, they nailed it in.  Once I noticed the cupping, I had the hardwood guy come over.  He saw it, but said it's fairly normal.  His explanation was that even when the moisture level was down low enough to install, you will continue to have moisture come out from the drywall, etc.   Once the house drys out, maybe 6 months, the floor should smooth out.  I know that a new house will continue to give off moisture for a while, but I'm questioning if a floor can really "un-cup" as it dries out.  Our builder has used this hardwood guy for quite some time & knows him well so my first instinct is to trust what he's saying, but a second opinion is never a bad thing..

As an added note, it has been hot here the last month, so our central air has been on pretty much non-stop.  Between that and having the HRV running constant, I would think that would pull a fair amount of moisture out.  He suggested getting a dehumidifier & running it in the basement as well.

What is your indoor humidity? With your air running a lot, your indoor humidity should be comfortable. I bought indoor weather stations at Walmart for about $7 each, tells me temp and indoor humidity. I have one upstairs in the baby's room and one downstairs in the great room. Right now, our indoor humidity is 45% and temp. is 77 degrees. BTW, I'm in Michigan.

Posted By Jere on 02 Aug 2010 05:12 PM
What is your indoor humidity? With your air running a lot, your indoor humidity should be comfortable. I bought indoor weather stations at Walmart for about $7 each, tells me temp and indoor humidity. I have one upstairs in the baby's room and one downstairs in the great room. Right now, our indoor humidity is 45% and temp. is 77 degrees. BTW, I'm in Michigan.

That's a good tip, I should pick up one or two of those.  I can say, though, that it feels comfortable in the house - it doesn't feel humid/damp/etc., so I'd be surprised if the humidity was higher than normal..

Something else you could try, is running your HRV in low for part of the day.  Have you looked into a timer or programmer so the HRV runs, either on low all of the time and kicks on to high for a set amount of time, or runs on high for a set amount of time each hour?
Is the HRV dumping the moisture from the fresh air coming from outside?

The fact that the AC has been running non-stop means the RH is probably pretty low, probably under 40%, maybe even under 30%. It won't feel overly-dry until you're under 25% RH. "Typical" average RH in conditioned space over a year might be in the 40-45% range, but with seasonal variarions of more than 10%. If the wood was acclimated at 45-50% RH and you're running it 25%, there could be both apparent cupping & gaps, but those will reverse when the RH climbs back into the 40s for a few weeks.

If your outdoor dew points are over ~55F the HRV will be adding to, not subtracting humidity from the conditioned space air- it's a humidity SOURCE. (The dew point of 75F 40% RH air is 55F.) The amount of dehumidification that the central AC is doing is dependent on blower speed- it's running long duty cycles on low speed, it'll pull a lot more humidity out than if operated at higher speed/shorter cycles. Measure the RH, keep it between 30-60% by adjusting ventilation rates & AC speed (if adjustable).

Monitor the RH of both the basement air and the air in the fully conditioned space. Basements in heating-dominated climate areas are often 5-15F colder than the first-floor during the summer peaks. The same 50% RH 78F first-floor air pulled into a 65F basement has an RH of ~75%, which is conducive to mold & mildew growth. In a reasonably tight house limiting the basement RH to ~60% is usually keeps the first-floor humidity from becoming too high as well. Since it's all within the pressure boundary of the structure it's the same air, same dew point, but at different temperatures it'll have a different RH. (Pull up a psychrometric chart off the web- any horizontal line represents the same box of air at different temps. The dew point is the 100%RH curve on the left.)

Posted By Dana1 on 04 Aug 2010 11:16 AM
The fact that the AC has been running non-stop means the RH is probably pretty low, probably under 40%, maybe even under 30%. It won't feel overly-dry until you're under 25% RH. "Typical" average RH in conditioned space over a year might be in the 40-45% range, but with seasonal variarions of more than 10%. If the wood was acclimated at 45-50% RH and you're running it 25%, there could be both apparent cupping & gaps, but those will reverse when the RH climbs back into the 40s for a few weeks.

If your outdoor dew points are over ~55F the HRV will be adding to, not subtracting humidity from the conditioned space air- it's a humidity SOURCE. (The dew point of 75F 40% RH air is 55F.) The amount of dehumidification that the central AC is doing is dependent on blower speed- it's running long duty cycles on low speed, it'll pull a lot more humidity out than if operated at higher speed/shorter cycles. Measure the RH, keep it between 30-60% by adjusting ventilation rates & AC speed (if adjustable).

Monitor the RH of both the basement air and the air in the fully conditioned space. Basements in heating-dominated climate areas are often 5-15F colder than the first-floor during the summer peaks. The same 50% RH 78F first-floor air pulled into a 65F basement has an RH of ~75%, which is conducive to mold & mildew growth. In a reasonably tight house limiting the basement RH to ~60% is usually keeps the first-floor humidity from becoming too high as well. Since it's all within the pressure boundary of the structure it's the same air, same dew point, but at different temperatures it'll have a different RH. (Pull up a psychrometric chart off the web- any horizontal line represents the same box of air at different temps. The dew point is the 100%RH curve on the left.)

Dana -

Thanks for the excellent response!   It's been very hot & humid here lately, with due points in the mid-upper 60"s.  I've been running the HRV to try to vent out some of the paint/"new house" smells (I have an expecting wife that does NOT like those smells right now) but maybe I should slow that down & see what happens.   I'm going to pick up a couple of those indoor weather stations tonight so I can see where we're really at. 

Just to clarify, the AC hasn't been running "non-stop", but I haven't turned it off.  It turns on every couple hours during the day, & basically not at all at night when it cools down outside..

If your HRV is running pretty much continuously with a 65F outdoor dewpoint, if it's ~75F in the house the indoor RH will be a relatively humid ~70% at night, but will drop significantly during the day when the AC is running. If the dew point is 60F, your nighttime RH will be ~60% @ 75F indoors.

The sub-$10 Acu-Rite 00613 RH & temp monitors available at some box stores (& Wal-Mart) aren't bad. I have ~ 7-8 of them that I use for both monitoring my home & office, and as a cheap min/max data logger to sleuth out potential humidity problems in semi-conditioned spaces. Of my limited sample one has gone quite a bit out of calibration- not sure if they all drift or if there's a bad component on the one, but most of them continue to agree, some have been in use for a couple years now. I believe it's temp sense has gone off by a couple of degrees, not it's humidity sensor, but since it's calculating the RH based on both the RH reading isn't useful on that one. (IIRC the out-of-cal one had been used outdoors under a roof overhang for a few weeks, after which it's temp was off. It's not rated for that app, so I'm not surprised.) They look like this:

http://www.weatherconnection.com/product.asp?itmky=868373&model=00613&cat=&subcat=&sortby=&filtermfg=&page=

I made a typo on the previous post- the dew point of 50% (not 40%) RH 75F air is ~55F.

(Congrats on the anticipated addition to your household!)

> AC turns on every couple hours during the day, & basically not at all at night when it cools down outside.

This plus the HRV will cause higher humidity. I'd go with a dehumidifier and the monitor.

thanks for all of the good advice, guys! I picked uip one of those indoor temp/humidity sensors today & will get it going tonight.

I'm not sure if there is a solid answer on this, but I didn't see anyone address the thing that's concerning me the most. If a floor cups, once the house dries out a little bit, will the floor flatten back out? Or is it done for?

The cupping is caused by uneven moisture content between the bottom & top of the board- the concave side being lower humidity, the convex side higher.  If you have a steady flux of bottom-side moisture keeping the bottom side wetter than the top, it'll stay cupped, but if the moisture stays relatively constant  it'll usually flatten out.  Depending on the permeability of the top finish and the  slip layer below, this could take months though. It sounds like it was installed at a higher humidity level than you are keeping the place.

Keep the installer apprised of how that progresses, and track both basement and room humidity& temperature in a log (weekly, if not daily), to verify that basement & room dew points stay within several degrees of one another, and keep it between 30-50%RH in the room by adjustments of ventilation rates & dehumidifier/AC, if you can.  If it doesn't flatten out by November they may owe you a sanding & finish job, since that would indicate it wasn't dry enough when it went down. 

In winter keeping the first-floor between 30-35% RH will be protective of the structural wood by limiting wall-cavity condensation potential, while maintaining a human-comfort & health level.  In summers letting it rise to 60% RH is fine, but if anyone int the house has dust-mite allergies holding the line at 50% (which will require mechanical dehumidification much of the summer) is better.

I picked up a humidity sensor last week & have been keeping an eye on it. I didn't think to get another for the basement, but I plan to do that today. Something I'm noticing right off the bat.. When the AC turns on, within about 10 minutes the humidity comes down to right around 50%. When it turns off, the humidity starts to climb again, and always seems to settle in around 57-60%. I have turned the HRV off completely for now, so I know that the humidity is not coming from the outside air.

Should I assume that the humidity level going up 10% is just from the excess moisture in the walls & stuff?

Posted By jerkylips on 10 Aug 2010 09:44 AM
I picked up a humidity sensor last week & have been keeping an eye on it. I didn't think to get another for the basement, but I plan to do that today. Something I'm noticing right off the bat.. When the AC turns on, within about 10 minutes the humidity comes down to right around 50%. When it turns off, the humidity starts to climb again, and always seems to settle in around 57-60%. I have turned the HRV off completely for now, so I know that the humidity is not coming from the outside air.

Should I assume that the humidity level going up 10% is just from the excess moisture in the walls & stuff?

Unless your ACH/50 blower door test numbers are less than 0.5 air exchanges/hour, a significant source of the humidity may still be from outdoor air.  (If if the ACH/50 numbers ARE less than 0.5 an hour, you absolutely MUST run the HRV to stay healthy!).  Does it recover to 57-60% RH in 15-20 minutes, or over the next hour or two or..???

Showers (even with the fan on), and cooking (even with the vent-hood on) can be major sources of indoor humidity, as are all of the sweaty air-breathing inhabitants.  Some amount of moisture could be coming from poured concrete even months after the fact, as well as from any wet-sprayed cellulose applied in the past 6 months.  Some amount will be stored/released from the wallboard in fairly rapid cycles, but that's not likely going to be enough to boost the RH 10%.

See if you can correlate the settling point humidity with the outdoor dew points.  In most cases it will roughly track the dew point of the interior air, with some amount of offset on days when the air-conditioner is cycling.

But at a steady cycling between 50-60% room RH the flooring shouldn't be cupping unless it either has a source of moisture from below, and/or had too high a moisture content when it was installed.  If the underside doesn't have constant moisture supply it may still flatten out though- give it a few months.

Does your air handler run on continuous or only on auto when a/c is running? If it runs continuous, condensing moisture on the cold coil could be putting some moisture right back into the house until the coil warms and dries.

Do you have a humidifier on your HVAC? I so, make sure it is turned off during the summer.

Posted By Dana1 on 10 Aug 2010 12:08 PM

Posted By jerkylips on 10 Aug 2010 09:44 AM
I picked up a humidity sensor last week & have been keeping an eye on it. I didn't think to get another for the basement, but I plan to do that today. Something I'm noticing right off the bat.. When the AC turns on, within about 10 minutes the humidity comes down to right around 50%. When it turns off, the humidity starts to climb again, and always seems to settle in around 57-60%. I have turned the HRV off completely for now, so I know that the humidity is not coming from the outside air.

Should I assume that the humidity level going up 10% is just from the excess moisture in the walls & stuff?

Unless your ACH/50 blower door test numbers are less than 0.5 air exchanges/hour, a significant source of the humidity may still be from outdoor air.  (If if the ACH/50 numbers ARE less than 0.5 an hour, you absolutely MUST run the HRV to stay healthy!).  Does it recover to 57-60% RH in 15-20 minutes, or over the next hour or two or..???

Showers (even with the fan on), and cooking (even with the vent-hood on) can be major sources of indoor humidity, as are all of the sweaty air-breathing inhabitants.  Some amount of moisture could be coming from poured concrete even months after the fact, as well as from any wet-sprayed cellulose applied in the past 6 months.  Some amount will be stored/released from the wallboard in fairly rapid cycles, but that's not likely going to be enough to boost the RH 10%.

See if you can correlate the settling point humidity with the outdoor dew points.  In most cases it will roughly track the dew point of the interior air, with some amount of offset on days when the air-conditioner is cycling.

But at a steady cycling between 50-60% room RH the flooring shouldn't be cupping unless it either has a source of moisture from below, and/or had too high a moisture content when it was installed.  If the underside doesn't have constant moisture supply it may still flatten out though- give it a few months.

unfortunately we didn't have the blower door test done.  I mentioned that I wanted it done, but we were on an extremely tight deadline, had weather delays, etc., & I think we both forgot about it.  I can tell you that we paid very close attention to air sealing so I'm guessing that it is very low.  We sprayfoamed around the recessed cans in the ceiling, caulked all studs, sprayfoam the sills & stud caviites, and nailed & taped rigid foam board.  I still want to get it (the blower door test) done just so I know where we're at but right now I'd be guessing at a number.

I need to pay closer attention to how quickly the humidity goes back up, but when I noticed it over the weekend it was in the middle of the day - no showers & no cooking going on at the time.

Posted By Jere on 10 Aug 2010 12:43 PM
Does your air handler run on continuous or only on auto when a/c is running? If it runs continuous, condensing moisture on the cold coil could be putting some moisture right back into the house until the coil warms and dries.

Do you have a humidifier on your HVAC? I so, make sure it is turned off during the summer.

I previously had the HRV running constantly.  I have since turned it off completely, to see if the outside air had been adding to the humidity.  That's only been a couple days so I want to continue to monitor.  I don't have the blower on the furnace running constantly - only when the AC is running - and no humidifier.

thanks~

Humidifiers are an inherently bad idea (in most climates.) If your house is reasonably tight it's usually easy to adjust the humidity up to keep it at ~30% RH in winter in climates of 10,000 Fahrenheit heating degree days or less simply by lowering the ventilation rates. This is almost all of the lower 48 below 8000' of altitude, and the most populated regions of Canada. For reference, Green Bay WI is ~8000HDD, Fargo ND is ~9000HDD, Int'l Falls MN is ~10,000HDD.

Normal human activity is enough to keep the RH up at ventilation rates still high enough to purge indoor air pollution for healthy air. The PassiveHouse standard requires ACH/50 of 0.6 or lower, and winter humidity in those structures are controlled entirely by adjusting the ventilation rates, since winter air is substantially drier than 68F 30% RH air. (Any time it's below 37F outside, it's drier than that.)

If the indoor air in your house tends to drop and stay below 25% RH in winter without adding humidity it's an indication that you have significant air leakage. Tighten up the house first- adding humidity to the indoor air will create condensation spots on the exfiltration path, increasing mold & rot potential. (The wicking surfaces of air-handler type humidifiers also tend to be loaded with mold spores too, unless you change them often.)

Below 30% has an increased risk to human health from viruses, below 25% it becomes uncomfortable as well.

But above 30% increases the potential for condensation inside the wall structures when it's cold out- it's less healthy for your house and if mold conditions develop inside the wall, less healthy for you too.

In spring/summer/fall (whenever it's over 50F outside) letting the indoor humidity rise to 50-60% is OK for both you and the house. Anything above 65% has increased mold potential, but if anyone has dust-mite allergies dehumidifying to 50% is well-advised.

Posted By Dana1 on 10 Aug 2010 12:08 PM

Posted By jerkylips on 10 Aug 2010 09:44 AM
I picked up a humidity sensor last week & have been keeping an eye on it. I didn't think to get another for the basement, but I plan to do that today. Something I'm noticing right off the bat.. When the AC turns on, within about 10 minutes the humidity comes down to right around 50%. When it turns off, the humidity starts to climb again, and always seems to settle in around 57-60%. I have turned the HRV off completely for now, so I know that the humidity is not coming from the outside air.

Should I assume that the humidity level going up 10% is just from the excess moisture in the walls & stuff?

Unless your ACH/50 blower door test numbers are less than 0.5 air exchanges/hour, a significant source of the humidity may still be from outdoor air.  (If if the ACH/50 numbers ARE less than 0.5 an hour, you absolutely MUST run the HRV to stay healthy!).  Does it recover to 57-60% RH in 15-20 minutes, or over the next hour or two or..???

I was finally able to check this out last night.  The AC turned on & the humidity got down to 49%.  After about 2 hours, it was up to 54%, and after about 3 1/2 hours it was up to 60%.  Shortly after, it turned back on again. 

I think I need to have some patience & hope the floor flattens out.  I'm guessing that once we hit the heating season, it will dry out quite a bit with the furnace running.   The builder warranties the home for 2 years so I'm not at any risk of waiting too long & not having something fixed.

thanks for all of the help!!

It's not the furnace running that will flatten it out- it'sthe stabilization of the subfloor humidity with the room humidity over that much time, along with a more stable & controlled (if lower) RH. For the board tops to be concave, the bottom of the board has to have more moisture content than the upper part (wood shrinks as it dries, expands where it takes on moisture.) But if you've been keeping the house within at 20-30% RH range (such as within 15% of 45% RH) for 5-6 months, that should be adequate time for even the subfloor's moisture content to be pretty much the same as the top 1/8" of flooring, and the board should be flat. By air conditioning the place you haven't seen huge humidity cycles in the room air that might cause the wood to do something else.

The dew-point of, say, 70%RH 35F outdoor air is ~27F, which when heated up to 70F indoors results in an indoor RH under 20%, which is both unhealthy & uncomfortably dry. Outdoor dew point will fall dramatically by November, and you'll have to throttle back ventilation rates on the HRV to keep the indoor RH up around 30%, and running the HRV via dehumidistat control to maintain 30% is usually the right way to go. Keeping it ~30% will keep the wood flooring/subfloor from becoming overly dry too. When outdoor humidity picks up in spring you have to move back to some other control method (timers, etc), and the humidity will be then going into to the wood from the room air as the room-air's RH rises with the season. You might see minor shifts in board gaps between the 30% RH mid-winter and the 55-60%RH mid-summer, but the transitions don't happen faster than the boards can accommodate.

I have hickory flooring with walnut trim around the edges, and got quite a bit of cupping at one time. By keeping moisture down with AC and a BIG dehumidifier the floor leveled out pretty well. Right now there are even tiny spaces between the boards, which is how a proper installation should be. I've seen flooring mechanics use matchbook covers as spacers between courses of wood, but we didn't on my floor. Live and learn.

The most common condition causing cupping is the sub-floor having a higher moisture content than the wood itself. The moisture moves from the sub-floor into the drier wood flooring, leaving the bottom higher in moisture content than the top of the board, causing the bottom to swell more than the top of the board. The top is not swelling at the same rate as the bottom, leaving the top smaller and cupped.

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