Finishing up our new build with spray foam insulation and the humidity level inside house has been between 65% - 71%, which is obviously way too high! HVAC contractor and GC had a J load calculation done on home when designing ac system and it was supposed to be designed to deal with tight envelope. I'm going to have to consider a whole house dehumidifier, but I also want to know if there's something wrong with the current system/design. It just should not be THAT humid. Any recs for an inspector, especially one that may have particular expertise in this area?

MAS444 said:

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Finishing up our new build with spray foam insulation and the humidity level inside house has been between 65% - 71%, which is obviously way too high! HVAC contractor and GC had a J load calculation done on home when designing ac system and it was supposed to be designed to deal with tight envelope. I'm going to have to consider a whole house dehumidifier, but I also want to know if there's something wrong with the current system/design. It just should not be THAT humid. Any recs for an inspector, especially one that may have particular expertise in this area?

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Large chance you have too big of an AC unit for your house, but I wouldn't jump down the HVAC/GC guy's throat yet. In general though, the AC does two things: cool the air but also reduce the humidity. With a big enough AC unit you can cool your house before you've cycled enough air through the unit to reduce the humidity. Tell tale signs are cold clammy rooms.

ASHRAE Formula J is this really complicated template that then gets fed into a simple answer: Do you want 2,3,4, or 5 ton units. Throw in a couple half units as well. And general contractors/HVAC guys are notorious for looking at the calculations and going, 'eh lets be safe and up the number a bit what could go wrong?'

Also, you just finished spraying the house, it could be that your house is now acclimating to your incredibly well sealed envelope for the first time. There is no place for moisture to go besides in the house.

Thanks for the response. We sprayed the house a long time ago...before the hvac was in. But it hasn't really been consistently hot enough to run the ac regularly until recently. The hvac guy did say to "let it breathe" a little and adjust before we get too worried.

I didn't think spray foam houses could breathe. That's the point. How long does the A/C run? I wonder if there is a calculator for X minutes run to cool down Y degrees with Z outside temperature. To let you know if the unit is oversized.

What's the delta on the thermostat? Some let you change it from 1 degree to 2 degrees to allow the A/C to run longer.

MAS444 said:

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Thanks for the response. We sprayed the house a long time ago...before the hvac was in. But it hasn't really been consistently hot enough to run the ac regularly until recently. The hvac guy did say to "let it breathe" a little and adjust before we get too worried.

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I think I misread your original post. I thought you had open studs and had just finished spray foaming. It looks like you've got drywall and the house has been fully closed for a while.

That points to having too big of an AC unit now. Do you have a variable speed compressor/2 stage or single stage?

By let it breathe I think he meant let it run for a few days and adjust to everything.

If you have a two stage or variable, you can adjust your thermostat settings (or equipment settings/jumpers if need be) to run the system on 1st stage/low for an extended period of time to draw out the moisture. Also, RH is also higher at a lower temperature, all else the same.

Often times, the thermostat settings are set to kick the system into 2nd stage/high when there is a temperature differential of only a few degrees above the set point. So 5pm rolls around, and the program temperature goes from 75 degrees to 70 degrees to welcome the family home, 2nd stage kicks in and quickly cools the house, leaving all the humidity inside.

Was the HVAC designed knowing it would be a foam house? What is your square footage and total AC tons?

Typically I see foamed homes with what on the surface appears to be ridiculously small AC size. The idea is to force to AC to run for long periods to help control the humidity. I have Al's seen houses retro fitted or badly designed where the AC short cycles so much that the hunfity is
awful and the occupants have to keep it 5 degrees lower than the desired temp to be comfortable.

Yes, HVAC was designed for foam. I will post the HVAC specs tomorrow. House is 3400 ft2.

What do y'all think is the max acceptable humidity level? Our current home is about 45%. The new One has been 64 - 71. The 71% was this morning after no one had been there all night, which was very concerning.

40 to 60 is the REALLY general range.. More than 50 is probably uncomfortable.

Take a picture of the serial numbers on the outside units and I can probably figure out the size.

MAS444 said:

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What do y'all think is the max acceptable humidity level? Our current home is about 45%. The new One has been 64 - 71. The 71% was this morning after no one had been there all night, which was very concerning.

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Just as a point of reference, I work for an MEP engineer and our AC systems are designed to maintain a target RH of 55%.

I live in an old home with traditional style insulation, and my ACs have no problem keeping the RH under 60%, and on some lower humidity days it will easily drop under 50%.

So like the poster above said, 40-60% is generally the target range.

The system is an American Standard (17 Seer) 2 stage 7 ton unit with zoning. Again, the house is 3400 ft2 2 story.

The humidity was 67/68% this morning when I stopped by again. It flat out doesn't feel "cool," which is unacceptable in my opinion.

Any other thoughts/input is greatly appreciated!

Edit: It's a 5 ton unit.

MAS444 said:

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The system is an American Standard (17 Seer) 2 stage 7 ton unit with zoning. Again, the house is 3400 ft2 2 story.

The humidity was 67/68% this morning when I stopped by again. It flat out doesn't feel "cool," which is unacceptable in my opinion.

Any other thoughts/input is greatly appreciated!

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Man, shooting from the hip, 7 tons seems huge for a foam insulated house- zoned or not.

I would ask to see their manual J calculations.

one MEEN Ag is right, your system is oversized. They may have done manual J calculations, but it's more likely that the HVAC subcontractor used an X tons for Y square feet and then round up rule of thumb which consistently oversizes A/C systems.

You have only two solutions here - add standalone dehumidifiers (for example Honeywell DR90) or have actual calculations done that take into account the substantially lower need for cooling due to the spray foam and remove and replace HVAC components with those which are appropriately sized. Even with that you may still need standalone dehumidification.

Personal experience talking, my relative humidity also used to be in the high 60s in late summer. Now it's closer to 50%.

edit: holy crap, 7 tons? Do you have 20 foot ceilings or something?!

I have a similar issue with my house. It's 3000sqft with a 5 ton unit. This time of year is the worst! The temp outside is not hot enough to run the ac so the humidity builds up. It's a 2 stage unit so it can run on low stage for the majority of the time. If it's zoned it can be adjusted so the low stage runs longer.
My fix to this issue was installing a whole house dehumidifier for what I call the "in between" months.
Any other time of the year it's fine. I feel the unit was not sized correctly, but I'm not a hvac professional.

Your instinct is correct. I have yet to have run calculations on a house where it was sized correctly except for a century old 900 sq ft bungalow with no insulation in most of the walls.

Thanks for the responses everyone. I will ask for the actual calculations. And no, we don't have especially tall ceilings. Basically 10' down and 9' up.

MAS444 said:

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Thanks for the responses everyone. I will ask for the actual calculations. And no, we don't have especially tall ceilings. Basically 10' down and 9' up.

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Sorry this is happening to you. Just a heads up - brace yourself for pushback. The AC guy and the GC are now going to fight you on everything because this is a 14-20k problem. Also, the AC guy is most likely not going to have any real calculations to show you and they will accuse you of not knowing what you're talking about.
I would recommend you stand your ground and reiterate that the unit was not spec'd properly. The internal humidity speaks for itself. Usually what happens is the AC guy bumps up his rule of thumb calculations and its now officially oversized. A properly sized AC unit should run all day on the hottest day of the year.

And I misspoke earlier, ACCA owns Manual J not ASHRAE. Also, there's a web app called coolcalcac.com that can help you make your own determination for AC size. I have never used it, but just a quick google search makes it look like its simple enough for laymen to use and its geared for AC professionals.

As a side note, a general rule when getting into 'model fights' is that all models are wrong, some are useful. You're trying to figure out not only what your house should be, but also why you've got a 7 ton AC set up.

7 tons seems large. I had a 3000 sq ft house with a 5 ton that worked well, but it was built in the 50s with little insulation.

Correction: It's actually a 5 ton...I was looking at a prior bid.

GC sent me the J load calc info/report (I haven't looked at it yet) and the HVAC guy is saying he followed it exactly. We also have a Nest thermostat and they're saying it may need to be programmed differently.

MAS, I was actually wondering how your build is coming.

I would have another HVAC contractor review the manual j and equipment.

http://www.aardvarkcares.com/load-calculation.php

MAS444 said:

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Correction: It's actually a 5 ton...I was looking at a prior bid.

GC sent me the J load calc info/report (I haven't looked at it yet) and the HVAC guy is saying he followed it exactly. We also have a Nest thermostat and they're saying it may need to be programmed differently.

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I think we're on to something now. You said previously you had a two stage compressor. Nest is notorious for not working with two stage compressors at all or if they do get hooked up, screwing up the set points.

I would recommend putting the original thermostat back on it for a few days, set it to the lower stage setting and see if you can dry out your house.

As said above, the Nest may be your problem here. You need a Tstat that is specifically made to control multi stage units. I recommend Honeywell.

Also, the the Nest will allow temps to get about 3* outside of the set point as part of its "energy efficiency" programming, to help with the savings. Thus you would need to adjust your set point lower than you think to achieve your desired interior temp.

Because of the multitude of issues like this with Nest, I am not a fan of them.

We got the upgraded Nest that is supposed to work with 2 stage system.

MAS444 said:

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We got the upgraded Nest that is supposed to work with 2 stage system.

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Before firing off against the GC/AC guy I still would recommend using their thermostat. That is their first retreat, blaming the Nest. Prove to them it doesn't matter. If it solves you're problem - awesome, if not then they've got to come up with another reason.

As previously stated, this time of year (shoulder seasons) is the worst for indoor humidity. I've had homeowners complain about humidity levels with these new houses that are really tightly sealed up... and these houses have single speed systems...the only thing I can tell them is to initially adjust their temperature setting downward to make the AC run longer and have the sub look at the thermostat installer settings if they still have issues.

A variable speed system would be best, but at least you have a 2 stage system...with some adjustments to some of the advanced/installer thermostat controls (my experience is only with Honeywell, not sure how good the Nest is) the HVAC sub with their best technician should be able to improve the situation. Also, how long has the AC been on? IMO a new home will soak up moisture while being built and needs a bit of time with AC on to "dry out".

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Before firing off against the GC/AC guy I still would recommend using their thermostat. That is their first retreat, blaming the Nest. Prove to them it doesn't matter. If it solves you're problem - awesome, if not then they've got to come up with another reason.

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Definitely not necessarily "firing off" yet...I've got a good working relationship with both. I just want it resolved and we're all working on that.

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A variable speed system would be best, but at least you have a 2 stage system...with some adjustments to some of the advanced/installer thermostat controls (my experience is only with Honeywell, not sure how good the Nest is) the HVAC sub with their best technician should be able to improve the situation. Also, how long has the AC been on? IMO a new home will soak up moisture while being built and needs a bit of time with AC on to "dry out".

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Ac is really only been running consistently for a few days...maybe a week.

GC also says the Nest has some kind of humidity lowering specific settings but they can only be accesses via the App and we don't have that set up yet.

I appreciate everyone's input very much!

I agree with the others about putting the original tstat back on. I'd bet that the AC unit is oversized. The big seller with spray foam to builders is smaller AC sizes (less cost). How cold are you setting the temp to? With high humidity in the home you can make the issue worse by turning the air down colder.

If you're looking for a good online calculator try loadcalc.net. It's what I used over here and I ended up going either a half or full ton on one of the systems when it needed replacement, which ended up being a great decision overall. The dehumidifier still runs but not as much as before.

MAS444 said:

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Correction: It's actually a 5 ton...I was looking at a prior bid.

GC sent me the J load calc info/report (I haven't looked at it yet) and the HVAC guy is saying he followed it exactly. We also have a Nest thermostat and they're saying it may need to be programmed differently.

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Maybe have Enviromax or some other HVAC company take a look at the report and double check their work.

A Manual J Load evaluation/calculation was done by Cool Science, Inc. and the HVAC contractor says he followed it exactly. GC sent me the results of the test but I don't really know how to read it.

It may be worth asking the GC and HVAC contractor to walk you through the results for your own education.

If they can't, that may be a sign they cant interpret the results either.

I'd get a second opinion. I'd hate to come home from vacation in August and not be able to get your temp and humidity back down.

Unless the compressor has some kind of break in period, I'd imagine a new unit should be able to turn over the house over night, especially with no furniture. It's gotta be short cycling.

MAS444 said:

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A Manual J Load evaluation/calculation was done by Cool Science, Inc. and the HVAC contractor says he followed it exactly. GC sent me the results of the test but I don't really know how to read it.

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Can you post em up?

I can email them. Can you give me an email?

And/or...how do post or link documents? There are 3 different documents - each one several pages.