Steel building home
We built a red iron building and framed the interior walls with 2×4 wood. The steel beams provide an 8” gap between the exterior steel and the interior wood. The building has a vapor wrapped insulation for walls and roof. We insulated the wood walls with the recommended fiberglass with the vapor barrier. The ceiling has blown in insulation. We put in the recommended HVAC 2 stage 3 ton 16 seer unit. We live in zone 8 Georgia. Our problem is the air conditioner short cycles so our humidity reads around 60+ % during the hot humid days. It will drop in the low 50 percent range at night when the thermostat is dropped to 70. It stays so cool in the house even on extremely hot days that he air just doesn’t run long enough. The attic is unvented and completely enclosed. I’ve been told we should not have insulated the attic since the roof was insulated because we already had a “conditioned” attic with the vapor barrier wrapped insulation that came with the building package. My question is if we remove the blown in attic insulation, will this help the humidity issue inside the house and will this affect how long the ac runs? I would like to raise the thermostat temperature because it stays really cold in the house but can’t because the humidity rises due to the lack of the Hvac not running long enough. We’ve tried different recommendations by the hvac contractor but nothing has solved the problem. Any feedback would be appreciated.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
I would check what your humidity source is first. No amount of dehumidification helps if you have big air leaks in the house.
Removing the attic insulation might help a bit as it increase the amount of heat gain, thus your AC will run longer, it doesn't feel like the right solution though.
One thing to check is that your AC is set up right. I've seen 2 stage units where the thermostat is only hooked up to one stage or the blower fan is not set right and runs on high speed for both stages. If you have a decent digital meat thermometer, check the outlet air temperature of the coil. With a 70F room, you should see around 50F exit. If the temp is too high, the blower is pushing too much air.
With an over sized AC, important not to run the fan continually. This would re-evaporate any condensate on the coil after the compressor stops causing high humidity.
If you don't have air leaks and the AC is running well, you might need an additional de-humidifier. This is the case often with well insulated homes in humid areas.
+1 on checking for a two-stage thermostat. It's very common for a two-stage unit not to be set up to run as a two-stage unit.
>"My question is if we remove the blown in attic insulation, will this help the humidity issue inside the house and will this affect how long the ac runs? "
Basically no, the insulation isn't making the humidity in the downstairs space higher. The duty cycle on the AC may pick up a bit, but it's unlikely that it's going to affect the indoor humidity significantly.
The most likely major humidity source in GA is outdoor air infiltration. In mid-summer the outdoor dew point averages are north of 65F even up in Atlanta, and over 70F on the coast, whereas you're shooting for an indoor dew point about 50F (the dew point of 70F, 50% relative humidity air) or lower.
https://weatherspark.com/m/15598/7/Average-Weather-in-July-in-Atlanta-Georgia-United-States#Sections-Humidity
https://weatherspark.com/m/17828/7/Average-Weather-in-July-in-Savannah-Georgia-United-States#Sections-Humidity
Plant hardiness zones has no correlation with DOE climate zones. Georgia is predominantly DOE zone 3 , with some of the northernmost counties in zone 4, and most of the southern counties zone 2. See:
https://s3.amazonaws.com/greenbuildingadvisor.s3.tauntoncloud.com/app/uploads/2018/08/08044117/iecc-climate-zone-map-energy-code-warm-moist-line-800_0-main-700x487.jpg
I suspect the exterior shell is very leaky especially where the exterior wall meets the slab.. Is this a commercial retrofit? IMO I would start with a blower door test of the exterior shell.
I'm also assuming when you say "vapor wrap" that you mean a weather resistant barrier and not a true vapor barrier (ex, plastic sheeting). If it's plastic sheeting and your structure is fairly air tight you might have to consider de-humidification due to interior generated humidity (ex, showering, cleaning, cooking, breathing, etc).
The house has no big air leaks that we know of. We sealed everything up tight when building. In the process of elimination to address the problem, the HVAC contractor swapped out the unit for a 2 ton 2 stage (his suggestion) to see if unit would run longer. It did, however, our comfort level was significantly affected by not cooling sufficiently. They installed a whole house dehumidifier ($1400) but after monitoring it for a number of weeks, the humidity could never get to 50% (Ulta Aire). I got frustrated and turned it off after seeing no significant changes and the ac brought the humidity down just as well even though it stayed cooler in the house. HVAC Company monitored our air quality and seeing that the carbon dioxide was up they suggested an ERV back in February (another $1860). The building manufacturer suggested removing the insulation but the insulation guy thinks that’s not the answer either. The 2 stage does kick in when there is a 3 degree difference when programmable thermostat drops at night for sleeping. The humidity also drops to 50% or below because we sleep with thermostat on 70. Building company said because the wrapped ceiling/walls on the metal mimic spray foam their recommendation would be to remove insulation. I don’t mind spending the money to remove insulation if it’s a guarantee fix but no one seems to give a definitive fix. I appreciate all of your input. Don’t know where else to turn for answers.
This sounds like a leaky building but you said you’re pretty sure it’s sealed up well. A blower door test would be a good way to check this to see if anything was missed.
The other possibility is you have a large area of something that is a vapor permeable air barrier. Basically a way for moisture to get in even though you don’t have air leaks. This can be something like a non-encapsulated crawl space or a concrete slab without a polyethylene film underneath. The moisture is getting in from somewhere, and your AC isn’t able to keep up for dehumidification purposes. If a smaller AC was running continuously (since you said it couldn’t get the temperature down), and you still had humidity problems, then there is pretty much for sure a way for moisture to get into the house somewhere.
Bill
It is a commercial red iron and we basically built a house inside the steel building. We have not had the blower test and this probably needs to be done. The building came with rolls of insulation with the white plastic vapor barrier. We are on a concrete slab that had a polyethylene film beneath the slab. Thanks for all the input and advice. Blower test may need to be done before we have ceiling insulation removed as last option.
How many square feet do you have in the cushioned space?
How many people live in the space?
You say the 3 ton unit was short cycling what was your average run time per cycle?
Generally if you can get the run times over 20 minutes the humidity should be ok.
What are your supply and return temps with the 3 ton and 2 ton unit when in high and low cool modes?
Did you say the 2 ton unit failed to keep the house at the set temp?
Does your unit sound different in high cool mode than low cool mode?
The only definitive fix is to locate and illuminate to source of the moisture.
Please do not get offended but I have to ask. Do you have or do anything silly like an indoor pool, hot tub, aquarium, more than a dozen house plants, more than 15 minutes of shower run time per day, hang dry your laundry indoors, standing water in your crawlspace? If not most likely you have ground moisture coming up thru the floors.
Walta
Walter: To answer your questions,
1600 sq. Ft
2 people living in house
Average run time - 8 to 9 minutes in heat of day, 4 minutes in the mornings
Not sure about the supply and return temps, but I can check that.
You can definitely tell the difference when in high mode vs. low.
We have a couple of houseplants, and occasionally a shirt or 2 hangs to dry.
We do take 2 (minimum) Showers a day but I would say they average 10 minutes maybe. We have bathroom vents that run when we shower.
The building is on a slab with polyethylene underlay before we poured.
And no, I’m not offended at all! I welcome any inquiries that may help us solve the problem. I might add that we do not have any condensation on the inside of the windows ever. I’ve checked the attic multiple times even when raining or high humidity outside and it’s dry. There is no signs of moisture or mold anywhere.
That seems unusual that your 3 ton unit is only running 8 or 9 minutes, which isn’t long enough, but your HVAC contractor tried a 2 ton unit and it couldn’t keep the temperature at set-point? 3 tons with 1600 ft2 is 533 per ton, that seems oversized by the description of your home. 2 tons would be 800 ft2 per ton, that still isn’t anything great and seems like it shouldn’t have a problem maintaining temp. I wonder if something wasn’t correct on the install or Freon charge when he tried the 2 ton unit? If you do get the blower door test, you might also ask the auditor to check duct leakage, static pressure of the duct system, and total system airflow. For good humidity control you want about 300-350 cfm per ton. Higher airflow will remove Less moisture, but too low and you lose efficiency and cause other problems.
If it is a new build, there is a fair bit of moisture in the materials, it takes a while to remove that. With the short run-times, your AC will not help much on that front. A couple of stand alone de-humidifiers should do it. It takes about a year to year and half for this.
+1 on all the comments about outside humid air infiltration. The blower door test is the only way to measure your leakage so you can make informed decisions.
However, the high CO2 suggests that the house is probably pretty airtight. If you've got a tight house with short A/C duty cycles and high humidity, it is definitely not an either/or sort of issue. You want to run the A/C and a dehumidifier. The A/C drops the temperature and the dehumidifier drops the humidity.
If building material moisture is the cause, you will find the dehumidifer runs less over time. After a year or so, you might be able to turn it off completely. Don't just look at the humidity levels either. Measure the amount of moisture removed per day. Use a 5 gallon bucket and chart the moisture removal vs. weather. If you can chart daily moisture removal vs. outdoor dewpoints, you might see a pattern. If the two track each other, your moisture source is most likely outdoor air. If they don't, then internal sources are more likely. The A/C will mess this up some, but you might still see some patterns that help you narrow down the possibilities.
I will try tracking the moisture removal. Not sure I understand about the dewpoint and what you mean if the two track each other but I will read up on it. Thanks so much for the suggestions and information.
What I meant by tracking each other was to watch if there is a relationship. If you collect way more water on days with high dewpoints (humid days) than you do on days with similar temperatures but low dewpoints (clear and dry days), then outdoor air infiltration is your culprit. If there's no relationship between the amount of water collected and dewpoint, then indoor sources are more likely.
You can track temperature and dewpoint by using an online source like the Weather Channel, or you can get a reasonably cheap outdoor weather station. Some will even record it for you. Then you know the conditions in your own back yard.
After I posted, I thought to tell you that you can also collect the water coming from your A/C system by putting a bucket under it's condensate drain, too. By tracking the total water taken out by the a/C and dehumidifier, you'll get a better idea of the total moisture load on the building.
Thanks for explaining dewpoint (which was way more understandable than what I read online) and I'm going to track the moisture.
I thought of something that doesn’t make sense about your A/C system and sizing. If you have a 3 Ton 2 stage system, and it only runs 8-9 minutes during the hottest part of the day, it Should be running in low stage during these 8-9 minute cycles. Low stage is typically about 70% of High stage, so that’s roughly running as a 2 Ton! And you say it’s too cold inside with that system. So your contractor tried a 2 stage 2 Ton system, but it couldn’t keep up and maintain your set temp? Depending on the Thermostat and on how close to the set temp it is, it might run in low stage (maybe 1.4 Ton) then jump to high stage (2 Ton) after a predetermined time (maybe 15 minutes). Or go straight to high stage if it’s further from the set temp.
So... how is it so oversized as a 3 Ton running at low (2 Tons) but so undersized that it can’t cool when he tried a 2 Ton unit?
That’s something I would want to know.
There’s obviously no way it needs 3 Tons for 1600 ft2, and you’ve seen humidity go down when you force it to run longer cycles by setting a temp colder than you like. I would really look into something being wrong with the unit or install when he tried the 2 Ton
Dave
I’ve questioned the sizing repeatedly to the HVAC contractor but they insisted the sizing was correct. My thoughts all along have been that we didn’t need the 2 stage 3 ton but that a single stage 2.5 ton would have worked (and I’m certainly not a HVAC specialist). It may not be as energy efficient as the 2 stage but this is not the normal stick built house. They are the ones that suggested we needed to remove the ceiling insulation. I’ve been reluctant to remove the insulation because if that didn’t help any, then that’s more money wasted. They said that if we were still having the humidity problem after removing the insulation, they would address the AC again. I may be wrong but sounds like if AC was cycling properly, we wouldn’t have to worry about humidity. Thanks for your input!
What thermostat are you using? If it's an Ecobee there are a bunch of controls in there for run times, overcooling to control humidity etc. It might make sense to swap to a basic one to see if that helps at all.
Using a Honeywell VisionPRO Series with RedLINK. The temperature is off a little; about 2-3* cooler than what the thermostat reads.
Despite a common assumption, humidity has little to do with equipment over-sizing and lots to do with proper CFM/ton and infiltration.
Did you say the 2 ton unit failed to keep the house at the set temp? Did you say the 2 ton unit failed to keep the house at the set temp?
I need to say part of the problem could be your expiations.
A correctly sized will not be able to maintain the house temp 1% of the time.
You need to expect a 2 stage unit to be running 80% of the time and it will be running 100% time and on the hottest days expecting it will get a degree or 2 above the set point for a few hours.
It is reasonable to have a dehumidifier running 24/7 for up to a year during and after construction depending on how wet things got before the roof was installed.
I think you should turn the dehumidifier back on expecting it to run 24/7 for several months and log its water output over time.
When the contractor installed the 2 ton unit I assume he did not replace the indoor unit. If that is correct is your indoor unit on the list of units approved for use with both the 2 and the 3 ton units? If not the combination may not perform well together.
It sounds like your contractor has put a lot of work into trying to make you happy.
Walta
The indoor was not replaced but he did tell me he researched to make sure everything was compatible.
The contractor has gone above and beyond what we could have expected...he's been great working with us to resolve.
As suggested, I'm going to turn the DH back on and track the water. We never thought about doing this before.
Thank all of you for the feedback and suggestions!