12" walls with one man door and one garage door in 4,000 basement. (subgrade on 2 sides). In Tennessee, and even with a few days of low temps and low humidity. They are still having trouble with high humidity levels. Any thoughts?? If anyone has had this issue before, let me know how you went about fixing it. Also, the house has been occupied for about 5 months with AC running. It has been dried in for about 1.5 years. Thanks guys and gals.

You need to air seal it well and then run a dehumidifier.

Ryan-Are there any HVAC vents and returns to this basement? Sounds like there aren't but should be.I was in the Pigeon Forge area about a month ago and the humidity was as high as at the beach here in SC. Also would like to know about waterproofing on the subgrade sides. It's been a really wet year in the South.

In high-performance buildings high humidity is often an issue- the AC may not run long enough to fully dry out the air, especially on days with low sensible loads but still high humidity in the ventilation air. (Dew points well over 60F)

TN's climate sees lots of those days, and a high-mass R20 building with optimized windows and a higher-R roof can have 1/4 the sensible cooling load of a 1970s tract house of comparable size, but the same latent load. If the AC is oversized for the load (usually is) it compounds the problem- if anything undersizing it slightly would be better, since it would yield longer run times, and better management of the latent loads.

I'm not sure where in TN this is, but the weatherspark.com dataset for Nashville only showed 3 days in July with dew point averages below 60F. The average outdoor dew point for the month was over 65F, so I'm not sure which few days of low humidity were (28 & 29 July, maybe?). Concrete can store quite a bit a moisture, and it takes more than a few days of outdoor dry air (and a high ventilation rate) to make a dramatic shift in the background humidity levels.

High water table or insufficient drainage at the footing or poor management of surface slopes, in combination with a wall built without a capillary break between the footing and wall could yield high indoor humidity levels from moisture wicking up the concrete, and diffusing through the ICF walls.

Either way, jonr's solution is probably going to be the right one. An EnergyStar standalone dehumidifier pulls 1.85 liters of moisture out of the air per kwh, turning that latent load into a sensible cooling load. If you set up the dehumidifer to drain into a sump (or condensate pump) for disposal it would be able to keep up with the latent loads if there is reasonable air circulation between the room with the dehumidifier and the rest of the house, as long as the house is reasonably air tight.

If the AC ducts are in the attic above the insulation that can contribute to the problem in a couple of ways. The large penetrations through the attic floor for the ducts can be a huge air-leakage point, and any duct imbalance or leakage would result in significant air-handler driven infiltration, adding quite a bit to the latent load whenever it's running, limiting the ability of the AC to dry out the place.

Wow, Thanks for the knowledge. There is no attic, this is a three level house owned by a couple who owns a refrigeration company. They did all the site work themselves, platon was installed on the exterior for water proofing. And drainage is not an issue, they are on a large slope and took the time to make sure drainage wasn't an issue. I think because they are "mechanical" professionals, the AC units they chose might be oversized. I have never thought of that. And yes, it was the last week when the temps and humidity levels were pleasant. I think it could be due to the shotty install of ICF and waterproofing. they did not seal the top between the platon and the ICF. Keep the info coming, thanks a lot !!!

Do not let HVAC 'professionals' do their own houses. Every one I have been around has been a mess. Bigger is always better, especially when you can buy wholesale. And you are right, shotty waterproofing equals no waterproofing.

I can back up what's been already been stated. I had humidity problems in my ICF house in northern Illinois where dew points are high throughout the summer. The house was built for low air infiltraton, and the selection of an HRV vs ERV also contributed to the problem (I wish I had read Dana1's guidance on that before making my purchase). My AC has just 1 ton of cooling capacity for 2200 sq. ft., but still can't maintain indoor humidity at 50% at 72F, so I installed a whole house dehumidifier which is tied into the air handler set for constant air circulation. I cool the house from midnight to 8:00am each day. That's when electrical rates are at their lows (we opted for real-time pricing from ComEd). We wake up to 72 degree indoor air temperature which gradually rises throughout the day when outdoor temps are high. Lowering the roll shutters on west facing windows reduces solar heat absorption late in the day. The AC will kick in if the indoor air temperature rises to 80F during the day, but that rarely happens in this well-insulated, tightly built house. The 710 watt dehumidifier will cycle on and off to maintain the humidity set point. Our indoor humidity is controlled in a range from 40% in the winter to 50% in the summer providing very comfortable living space year round. BTW, normal living activities generate enough moisture in winter so that we do not need to mechanically humidify the air to maintain 40% RH. --Clark

You have thermal storage, now you need some type of desiccant to also shift the dehumidification to off peak hours. But generally, both are better done with a hydronic system and tank of cold antifreeze or ice water. Hot water in the winter.

At 1/2 the price for off-peak electricity (depends on area), it may have a very good ROI. Especially if more inverter driven air to water heat pumps come to the US market.

Posted By jonr on 04 Aug 2013 11:32 AM
You have thermal storage, now you need some type of desiccant to also shift the dehumidification to off peak hours. But generally, such things are better done with a hydronic system and tank of cold antifreeze or ice water.

Are there any residential add-on desiccant solutions available?  Maybe some kind of a device incorporating a moisture absorbing chemical (calcium chloride) that can be dried out over night?

Clark: Maintaining 40% RH throughout a N-IL winter can create moisture accumulation in the walls with potential springtime mold-spore issues in many stick-built homes. It depends on the stackup & air tightness toward the interior of the wall assemblies. Bumping the ventilation to keep it between 30-35% RH in mid-winter is probably prudent, and still comfortable, and within the healthy range recommended by respiratory health professionals.

Posted By Dana1 on 08 Aug 2013 11:23 AM
Clark: Maintaining 40% RH throughout a N-IL winter can create moisture accumulation in the walls with potential springtime mold-spore issues in many stick-built homes. It depends on the stackup & air tightness toward the interior of the wall assemblies. Bumping the ventilation to keep it between 30-35% RH in mid-winter is probably prudent, and still comfortable, and within the healthy range recommended by respiratory health professionals.

Being an ICF house, am I right in assuming that condensation is not a problem in the walls here in northern Illinois when indoor RH is at 40%?  I was concerned about the ceiling, however, which is 3" of closed cell polyurethane foam over 9" of fiberglass and no interior vapor barrier.  The past two winters I've checked for condensate on the foam and found none.  When it gets really cold outside (e.g., sub-zero F) the indoor RH does drop into the 30s. 

 The manufacturer of my hardwood floor recommends maintaining indoor RH at 45% + or - 10% throughout the year.  Low RH in winter causes solid hardwood floors to shrink, creating gaps between boards.  The wider the plank the worse the shrinkage.  If the boards separate too much, the spaces fill with dirt and won't close back entirely when humidity returns in the spring.  Eventually, the gaps between boards become permanent.  I'd like to avoid that, if possible.

Dana, part of the problem is finding an accurate hygrometer.  I have a wet-bulb sling-type instrument which should be accurate, right?

An ICF wall is extremely tolerant of moisture. Concrete can take on a LOT of moisture, and redistributes adsorbed moisture freely even at saturation. Assuming 2.5-3" of Type-II EPS with a vapor permeable siding type, it can dry toward the exterior even in winter.

With 3" of closed cell foam (~R18-19), and 9" of fiberglass (~R32) you are slightly shy of the IRC prescriptive levels for R-ratio in a roof assembly in a zone 5 climate, but not by much. With composite shingles you're very unlikely to end up with a mold hazard from moisture at the foam/fiberglass boundary, it would be higher if you have light metal roofing. But if the interior side of the fiberglass isn't air-tight there is a risk for moisture accumulation in the roof deck on north-facing pitches, a risk that increases if you keep it 40% RH. See the upper-right corner of Table 4 in this document:

http://www.buildingscience.com/documents/reports/rr-1001-moisture-safe-unvented-wood-roof-systems

They didn't simulate the 3" cc foam stackup in your climate zone (US zone 5) but if they did show a small risk in two the zone-4 locations when the fiber leaked air. The risk will be higher in zone 5.

The high humidity profile in the WUFI simulations was 40%RH in winter, the "normal humidity" profile was 30%, as depicted in Figure 4.

The material of the roof deck matters too- the WUFI simulations were for an OSB roof deck, where the risk is highest. With plywood the risk is somewhat lower, and with plank, much lower.

A sling psychrometer is pretty much the gold standard for this level of humidity measurement. But for monitoring indoor humidity in a mid-range the ~$10 battery operated AcuRite humidity monitors are good enough- you don't really need to 1% RH accuracy- 2% is plenty, and they will deliver that inside a reasonably temperature & humidity-stable conditioned space (but not a greenhouse that hits the dew point temp every night, then blasts to 110F in the afternoon.)

Thanks, Dana1. Although there doesn't seem to be any moisture condensing in the ceiling at the foam-fiber interface, in the future, I'll force the RH down to 35% in mid-winter as you suggest. By the way, the foam is sprayed onto a 5/8" OSB attic deck with plumbing stacks well sealed and all other piping, ducts, and electrical wiring installed below the foam. There is very little, if any interior air leakage into the attic space and no sign of condensation on the underside of the roof, frost on nail tips, etc. The attic is well-ventilated (with raised plate), and I've measured the attic air temperature as being within a degree or two of the ambient outside air. From what you stated, I conclude that that would increase the risk of condensation in the ceiling, right? That is, the asphalt shingles don't help to insulate the ceiling. --Clark

Posted By Clark on 08 Aug 2013 04:35 PM
Thanks, Dana1. Although there doesn't seem to be any moisture condensing in the ceiling at the foam-fiber interface, in the future, I'll force the RH down to 35% in mid-winter as you suggest. By the way, the foam is sprayed onto a 5/8" OSB attic deck with plumbing stacks well sealed and all other piping, ducts, and electrical wiring installed below the foam. There is very little, if any interior air leakage into the attic space and no sign of condensation on the underside of the roof, frost on nail tips, etc. The attic is well-ventilated (with raised plate), and I've measured the attic air temperature as being within a degree or two of the ambient outside air. From what you stated, I conclude that that would increase the risk of condensation in the ceiling, right? That is, the asphalt shingles don't help to insulate the ceiling. --Clark

How would you even know if it's condensing there where the fiber meets the foam?

If the foam is on the exterior side, simple math says that it is indeed condensing, during the peak winter cold, it's just not happening a sufficient number of hours to accumulate visible or dripping moisture there when you peek at it. (Which you probably do when its NOT below 10F outside.) But with air leaks to the interior the RH at that boundary can max out, and there are definitely going to be periods where it'll haze over with condensation or frost, to be re-evaporated when the temps are warmer.

The dew point of 35% RH 70F air is about 40F, and surely that boundary is dropping below 40F during the colder weather. But if the AVERAGE monthly temperature is above 40F you won't get accumulations of water in there big enough to be visible-liquid.  IRC code spells out R20 of exterior R minimum to avoid moisture issues in your climate, but that's predicated on an air-tight interior, and an interior RH of 30%, not 40%.

But maybe I'm not understanding the stackup & assembly.  In my previous coments I was assuming the cc foam was applied directly to the roof deck, and that it was an unvented assembly.  Now it looks like you are saying this is a vented attic, with 3" of closed cell foam on the underside of the attic floor boards or something?  With the venting above  the foam the roof deck is safe, and so is the attic floor (assuming that's what the foam is sticking to), in which case there may be minor condensation events happening at the foam/fiber interface, but if air tight on the interior it'll never be enough to cause a problem.

But clarify it a bit for me- what's the stackup from the interior paint out to the vented attic space, and from the attic space to the exterior of the shingles?

Posted By Dana1 on 08 Aug 2013 06:07 PM

Posted By Clark on 08 Aug 2013 04:35 PM
Thanks, Dana1. Although there doesn't seem to be any moisture condensing in the ceiling at the foam-fiber interface, in the future, I'll force the RH down to 35% in mid-winter as you suggest. By the way, the foam is sprayed onto a 5/8" OSB attic deck with plumbing stacks well sealed and all other piping, ducts, and electrical wiring installed below the foam. There is very little, if any interior air leakage into the attic space and no sign of condensation on the underside of the roof, frost on nail tips, etc. The attic is well-ventilated (with raised plate), and I've measured the attic air temperature as being within a degree or two of the ambient outside air. From what you stated, I conclude that that would increase the risk of condensation in the ceiling, right? That is, the asphalt shingles don't help to insulate the ceiling. --Clark

How would you even know if it's condensing there where the fiber meets the foam?

If the foam is on the exterior side, simple math says that it is indeed condensing, during the peak winter cold, it's just not happening a sufficient number of hours to accumulate visible or dripping moisture there when you peek at it. (Which you probably do when its NOT below 10F outside.) But with air leaks to the interior the RH at that boundary can max out, and there are definitely going to be periods where it'll haze over with condensation or frost, to be re-evaporated when the temps are warmer.

The dew point of 35% RH 70F air is about 40F, and surely that boundary is dropping below 40F during the colder weather. But if the AVERAGE monthly temperature is above 40F you won't get accumulations of water in there big enough to be visible-liquid.  IRC code spells out R20 of exterior R minimum to avoid moisture issues in your climate, but that's predicated on an air-tight interior, and an interior RH of 30%, not 40%.

But maybe I'm not understanding the stackup & assembly.  In my previous coments I was assuming the cc foam was applied directly to the roof deck, and that it was an unvented assembly.  Now it looks like you are saying this is a vented attic, with 3" of closed cell foam on the underside of the attic floor boards or something?  With the venting above  the foam the roof deck is safe, and so is the attic floor (assuming that's what the foam is sticking to), in which case there may be minor condensation events happening at the foam/fiber interface, but if air tight on the interior it'll never be enough to cause a problem.

But clarify it a bit for me- what's the stackup from the interior paint out to the vented attic space, and from the attic space to the exterior of the shingles?

I have recessed speakers in the ceiling at various locations which I have removed and felt the foam for moisture.  So far it's always been dry to the touch after some very cold weather.  Keep in mind that on days at or below zero, the indoor RH does drop into the 30s.

The ceiling build up from inside out is as follows:

1. Latex paint - primer and finish coat
2. Gypsum board - 1/2"
3. Unfaced fiberglass - 9" thick
4. Closed cell polyurethane sprayed-on foam - 3" between wood I-joists
5. OSB - 5/8" (serves as attic floor; foam is adhering well to the OSB)
6. Well-ventilated attic space (stays within a couple degrees of ambient outdoor air temp on cloudy winter days - no hot spots on roof; rafter locations do not show through roofing)
7. CDX plywood - 1/2" nailed to 9/12 slope wood rafters
8. 20 lb felt paper (with ice shield membrane at eves)
9. Composite (asphalt/fiberglass) roofing shingles

The only ceiling penetrations from interior living space are:  (all carefully sealed)

1.  Three 3" PVC DWV stacks
2.  One 4" PVC radon vent
3.  One 6" round galvanized range hood vent
4.  Boiler exhaust and intake air pipes
5.  Two electrical conduit pipes for attic electric lighting and fire/security wiring


Except for the above, all plumbing, electrical, and duct work in the ceiling is located below the foam layer.  Air infiltration/leakage through the ceiling is nearly zero.

Based on this profile, at what range do you recommend that I maintain the indoor RH during the months of January and February?  I appreciate your help on this, Dana.

Clark

Clark, Dana has this subject well in hand as always. You can use our psychrometrics software to determine the dew point temps given your dry bulb indoor temp and either your wet bulb temp or your RH as Dana described:

Borst Psychrometrics Software

With that stackup the wood in the attic is safe, but the dew point of 40% RH 70F air is ~45F. Assuming you have R30 fiberglass and R18 foam, the foam/fiberglass interface reaches that 45F dewpoint when the attic temp is +28F. If you keep it between 30-35% @70F you'd have a dew point of about 38-41F.

The average January temp in say, Rockford IL is about 22F, and the Dec-Feb average is about 26F, according to Weatherspark datasets:

http://weatherspark.com/#!dashboard;a=IL/Rockford

Assuming the attic stays 2F warmer than the outdoors that means the attic will average around 24F over the mid-winter, and you're dead-on 28F for the whole winter average. The paint has a vapor permeance of about 2-3 perms, and as long as the ceiling is perfectly air tight it's not much of an issue- the fiberglass will take on some moisture over the winter, but it won't get soaked, and still perform, and will dry out with the warmer tempertures of spring. But with any air leakage at the gypsum layer you could end up with wetter under-performing and potentially moldy spots near the air leak points.

Keeping it at 35% or lower would mitigate that risk by quite a bit. Assuming a dew point of 40F, (RH=33-34% @ 70F) that isn't reached at the fiber/foam boundary until the attic temp is about 22F. Another solution would be to add another R8-R10 above the OSB in the attic floor for dew point control at your preferred higher humidity. (Even crummy R11 batts rolled out would do it, as long as they were tightly butted up at the edges.)

Thanks, Dana, that helps me a lot. Actually, the fiber glass layer is R25 and the foam R18. I just remembered that the question of condensation came here up back in 2009 when I was building the house. I was planning to go with R30 batts, but you pointed out the mold problems I might encounter, so I downgraded to R25. Still, on the edge, but passable if I keep winter indoor humidity down.

I've thought about adding insulation over the attic floor, but since I use the entire attic floor area for storage, I'd have to put down R10 rigid foam board instead of fiberglass, and cover it with a layer of OSB for protection at a materials cost of about $3K. In the future, I will program the HRV to vent whenever winter indoor RH exceeds 35% and even 30% during January and February. I think that will work out, though I will miss the advantages of a higher humidity environment.

If you keep an eye out for deals on reclaimed roofing foam adding 2" of fiber-faced polyiso to the attic deck can be cheaper than R11 batts (!). I have multiple vendors in my area dealing in those goods at about 1/4-1/3 the cost of virgin stock, some of whom advertise primarily on craigslist, eg:

http://hartford.craigslist.org/mat/3977170506.html

Roofing iso is typically 2lb density and has a somewhat higher compressive strength that the foil faced goods, and unlike the foil-faced goods, it's usually between 0.5-1 perms- it'll have at least SOME drying capacity. It's slightly lower-R than the lower density foil faced stuff, but still over R5.5/inch. The OSB deck you'd put above the foam can be the cheap 7/16 sheathing rather than subfloor, since it doesn't span any joists, and is fully supported by the foam. Staggering the seams of the sheathing with the foam avoids any localized foam compression issues that might crop up if they were all aligned.

I put the cheapest EPS and 1/4" OSB over my existing attic floor. While I expected it to be soft, it has been fine.