Sizing WH dehumidifier while taking attic into account

[NathanJK]

This seems to be the only place on the internet to find good info on whole house dehumidifiers. I'm having a bit of a humidity problem. It's a new house in a suburb just north of Dallas, TX, with an encapsulated spray foam attic (open cell). The walls are also all spray foamed, and the ceiling is uninsulated. As soon as we had the ac up and running I could tell it was humid in the attic.

Some pertinent information, the house is 2100 sq ft of encapsulated space, with a tall roof and a good bit of attic space volume, 10ft ceilings in main living areas and master, probably about 1500 sq ft worth, and a 6/12 pitch roof. The AC is set to 70 because my wife is pregnant and wants it freezing all the time, and it's running a decent amount. There isn't currently a supply or return in the attic. There's a 6" insulated flexible makeup air duct that has to make a 50ft run from the ceiling of our porch to the air handler, I'm not sure the damper on it ever closes...every time I've checked it it's showing open. It's wired to the air handler. The AC system is all american standard, 16 seer.

I'm now running two different humidity and temp sensors with smartthings and I am tracking the levels. In the attic I run on average 60% RH with peaks around 70% near 11am when it's around 85 degrees in the attic. If it rains a good bit I've seen humidity near 90 in the attic. By late afternoon when it's 92 in the attic it drops down sometimes as low as 55. It was a bit higher all the way around before real summer hit, now that it's truly hot it's kinda sorta okish. Looking at the humidity data from a nearby weather station, I would say we are averaging about the same as outdoor air.

Indoors we stay right at 55% humidity except in the master bedroom when we shower, it climbs to 63% then until the vent fan can pull it out, which takes about an hour. That's a 150cfm fan ducted with rigid 6" duct.

I'm trying to figure out what the magic is on the WH dehumidifier. I'm wondering if I can get away with a 70 pt unit since it's just augmenting the ac, or if we need to step up a notch, specifically to the ultra aire 98h. Are they rated accounting for the already present ac or not? When you add my attic volume in, it's a significant amount of air to move.

My current thought is to put the dehumidifier in the attic with a return that's wyed to one of the main house returns and to the attic space with an adjustable register on the end of it so I can dial in the percent it's pulling from the attic vs the house using the humidity sensors to get it right. Then on the supply side basically do the same. There's space to add connections to the air handler itself, so I can dump the return right to the duct box on the air handler, with a backdraft preventer right there.

I've also considered the cheap option, and just throw a standalone santa fe advance 2 90 pt model up there and hope that dropping the attic humidity does what I need for the whole house since we're definitely exchanging air between the two spaces. I can add a small return and supply to the ac system easy enough to further that.

Any thoughts? I've read a lot of teddy bear posts, hopefully he can weigh in on this. Thanks!

Nathan

[teddy bear]

Thanks for the opportunity to wade in a complicated problem. These foamed attics are special. Tell me about your roof material and the type of vapor barrier under the roofing.
Could be that moisture is being driven into the attic by the sun during the heat of the day. The moisture near the ridge can be 70-90% during the peak radient heat of the day.
With out the possible moisture from the attic, a Ultra-Aire 70H would do a nice job.
Lets discuss the attic first.
If you can measure the temp/%RH at the peak a couple times during typical weather with sun heating the roof material.
Does your a/c get the home below 50%RH during the highest cooling loads. Temp/%RH on the space and a/c supply will give us a clue on how your a/c is setup.
Looking to hear from you.
Regards Teddy Bear

[NathanJK]

The roof is architectural composite shingles with a synthetic underlayment. I’m not sure exactly what synthetic underlayment that is. That’s on top of plain old osb decking, no radiant barrier, not zip system or anything like that. I’m 95% sure it’s 1/2 or really 7/16”.

All the temp and humidity readings I’ve taken in the attic have been at the peak, so it’s consistently oscillating from 60-70% each day. That is of course with exceptions for rainy days when it gets hammered and climbs as high as 90.

The house never gets below 55% with any consistency, I’ve seen an occasional 53 but that’s the lowest. Currently at 11:50 am the attic peak is 86 degrees and 65% rh. The living space inside is 71.5 and 55%, that inside sensor is centrally located about 3’ off the ground.

The lowest the attic has gotten on rh is 55% at 4:07am when it was 75 degrees. The sun comes up and it starts climbing. I think the humidity is getting trapped in the foam overnight, and then once the roof deck heats up it drives it into the attic space. I’m concerned long term about mold in the foam and damage to the osb from that moisture.

Thanks for taking the time to look over all this!

[teddy bear]

This is a real science project.
Looks like the roof materials are passing a small amount of moisture through to the attic when the sun heats the outside material. As the roof deck cools, the moisture is drawn back to the cool deck. Because of the air tight construction, the moisture accumulates. Is the vapor barrier tar paper or a rubber materual like Ice N Snow? Dehumidify the attic with a dehumidifier or exhaust a small amount of damp air from the attic peak? If it is only a small amount of moisture, the dehumidifier will work. If a large amount of moisture, exhausting a small amount of moisture air when the air is damp may be best.
The peak of the attic is 86^F, 65%RH, a 73^F dew point, may be as high as outside, not very high if the deck is air tight. If it dries everyday, rotting is less of a problem.
Let us go back to the home. We need to know if the a/c provides <47^F dew point air. Measure the a/c supply and room air temperature/%RH to determine the actual supply dew point verses the room air dew point. My guess is that the coil is to warm and must have the air flow slowed to remove more moisture.
After this adjustment is made and we can calculate the amount of supplemental dehumidification we need to get the %RH down to where you want it. The attic may not have much to do with the lower home moisture levels unless the air is circulated.

Room and a/c supply temp/%RH is needed assure a/c setup and to size the dehumidifier. A small supply of dry air to the attic may be all that is needed for the attic. An Ultra-Aire 70H may be able to do it all if we get the a/c optimized.
Regards Teddy Bear

[servicefitter]

The attic being insulated is part of the living area with no ventilation ,humidity will run high plus with a 6" outside air a balancer would have to measure how much air is coming in. Humidity flows from high to low so with lower RH is in your living area humidity will be pulled through the attic. Is the furnace fan running all or most of the time? I would close the outside damper for a week see what happens & maybe a breeze box fan in the stairwell to the attic. A energy recovery unit should be looked at for your 6" OA duct.

[NathanJK]

So, remember that pregnant wife? We’re baby birthing today. So I’ll get back to y’all next week with more data! I thought I’d have two more weeks and would have this project knocked out

[servicefitter]

Best of luck with your new family

[NathanJK]

Alright, after learning I get MUCH less done now...I got up in the attic and took some measurements.

The room air temp was 71.8 degrees with humidity at 58% in the rooms when I took these. At the air handler, the temp on the return side was 73 and the supply side was 57.5. I believe this means supply dew point was 43, but is that accurate using the humidity level in the living space? The humidity at the first supply register was 75% and temp was 61. I tried a different sensor and got an even higher humidity level. I then moved to the furthest register and found humidity to be 76.7% and temp basically the same.

If I use the humidity at the register's it puts the dew point at 50. I'm really not sure why the air blowing out of the vents themselves is so humid.

Thoughts?

Thanks for the well wishes on the family servicefitter! I'm not sure how to figure out how much the fan is running. My clamp meter can't data log, so I can't keep track of it based on amp draw. My gut feeling is it's running more than half the time, but it's impossible to hear in the vast majority of the house if the ac is running. I tried to close the OA duct, but it I can't get the door to move, it felt like it was about to break. I'm going to have to read the manual on it or something...I'll report back when I figure that out.

[teddy bear]

We need the return temperature/%RH at the a/c return and at the supply to figure the dew point reduction.
The humidity at the first supply register was 75% and temp was 61. I about a 53^ F dew point. The dew point should be <49^F at the supply to get to 75^F, 50%RH.You should slow the air flow to reduce the supply temperature and dew point. This will slow the cooling rate and extend the run time.
Evenings and rainy days will require supplemental dehumidification. A house you size should be able to get buy with a Ultra-Aire 70H. You attic is probably a small load but builds up with a small amount of moisture being sun driven through the roof assemble. A small supply from the dehumidifier like a 4" with a damper should stop the extreme buildup. These open foamed attics are a mystry to most.
Regarding 6" X 60 ft. fresh air flex duct, you are not getting a lot of fresh air through that duct because of it's length. Fresh air when occupied is important in this air tight home.
Get the a/c to remove moisture and maintain 50%RH during hot weather. Slow the air flow as much as need without freezing the cooling coil. Add the Ultra-Aire 70 with a return from the open part of the home to the dehumdifier and a supply to a/c supply with a dampered 4" duct teed off the dehu supply. Locate the dehumidistat in the living space.
Keep us posted.
Regards Teddy Bear

[servicefitter]

RH coming off a evap coil is 100% RH it's only when the temperature of the air rises RH drops. Your td is at 16* either slow the blower up to increase td and make sure the whole coil is cold.