Humidity control
cjones@mvhpc
| Posted in GBA Pro Help on
I operate a facility where humidity control is very hard. The building is 6 yrs old, built well, but cant control the humidity. We have resealed return duct, replaced tstats, added attic exhaust fans, etc. But still the building humidity matches the outdoors as fast as it changes. We try to keep the building 72f and it feels like you can’t breath and hot. Mildew forms. We keep the building 68f you can breath, but freeze and smells like a basement, and mildew forms. All exhaust fans are working, and ceiling fans are on. We are required to have fresh air returnes, but is this our friend or enemy.
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that is pretty vague, What type of building, Give construction features What is is used for. How many people are in it, What are the sources of humidity, What do you know about air sealing and has a blower door test been done,
I think you need to identify sources of humidity, Air leakage rates would be needed,
Clark,
If you are concerned about high indoor humidity levels in the summer, and if you live in a hot, humid climate, then a high rate of ventilation is probably making things worse.
I agree with Robert that we need to know more about the builiding and your climate.
In general, to control the indoor relative humidity:
1. You need to know what the sources of humidity are (exterior summer air entering the building through cracks in the envelope, a wet basement, an industrial process, mopping floors, showers, drying firewood, etc.)
2. You need to control air leaks in the building envelope (make the building as airtight as possible).
3. In the summer, you probably need an air conditioner to lower the indoor temperature and provide some dehumidification.
4. In some climates, you may also have to install a dehumidifier.
5. During the summer, the use of ventilation fans should be reduced to a minimum.
More information from Clark Jones:
22,000sf. 1 story, brick, concrete slab, 20 patient rooms-430sqft per room. 1.5 ton York HP, 3 vents per room 1 return- all on the ceiling. 2 main halls 3 ton York HP each, 6 vents,2 returns each. 2 large common areas, 5 ton York HP each, 10 vents 5 returns each. This is some of the details. Most of our growth is around any hole cut in our sheetrock. Hvac vents, light switches, plugins, anywhere the caulk is split between the baseboard and wall. Doors, windows meet the sheetrock. Never forms on open walls or ceilings, behind furniture-- blank spaces.??? The theory is we have to much HVAC and we are "Supercooling" the building surfaces and forming moisture. Also with efficiency lighting we have no natural heating. So the HVAC does not run long, then stays off too long. The building is tight as a drum and heavy insulated. We are located in North Carolina, near the Virginia border. Summers here stay in the upper 80's to mid 90's. Humidity 75% +. Winters low 30's to upper 50's humidity 40%-.
Clark,
Minimize the ventilation rate.
Perform blower-door directed air sealing work.
Install a whole-building dehumidification system if necessary.
Clark,
Nate's analysis makes sense. You wrote that you "added attic exhaust fans" -- and that's usually a big mistake, because attic exhaust fans can easily depressurize your building.
You also wrote, "All exhaust fans are working" -- implying that you may have several exhaust fans operating at once.
If Nate's hunch is right, all of these exhaust fans are depressurizing the building and making the situation worse. They are sucking hot, humid air into your building.
There is no way to confirm this hunch without a site visit. You should hire an energy rater equipped with a blower door -- someone familiar with pressure diagnostics.
Looks like a couple of issues.
As for why mold appears in your drywall holes but not behind the drywall or in open walls, it almost sounds like fresh air (which is full of moisture) is being sucked through gaps and holes in the walls rather than through your fresh air intakes. This would make it condense on the frigid, overcooled drywall and grow mold. Where is the air barrier? Sounds like it may be the drywall, in which case it's full of holes that are admitting outside air rather than your fresh air intakes.
In this environment, you'll probably want ERVs to run the ventilation and minimize the moisture in incoming air once you have the building tightened up enough that fresh air isn't being pulled through the walls. If the makeup air is at the exact same level of high humidity as the outside air, then you're increasing the building's moisture load and making more work for the dehumidification half of the cooling system's job.
Speaking of that, the building definitely appears to have an oversized cooling system that doesn't run long enough to do enough dehumidification. This is why setting the temperature lower helps; the system runs longer and removes more water from the air. If I'm understanding you correctly and doing the math right, then the building has 46 tons of cooling. If it is truly "tight as a drum and heavy insulated" then that's excessive, and its excessiveness is causing it to fail in the dehumidification part of its job.
Clark
I think Martin is right you need blower door directed air sealing and a dehumidifier. That was my original thought but your first post did not contain enough information.
Do you have a drop grid ceiling, If you do how are the return connected, There are ducted returns and wild returns. A wild return has a grid at the ceiling that lets the air above the grid, the unit sucks air from above the grid. Not a good way of doing it. This will depressurize the attic pulling more air through any leaks. The other is ducted returns, In this setup a duct runs from the ceiling to a trunk line or plenum. It pulls the air from the room.
If you have a grid system and to many units it would be easy to add more supplies and returns to some units and turning others off. This would increase run time and keep a better pattern of air circulation.
The need for separate dehumidifiers to supplement the AC is becoming more common, Newer efficient AC units are not as good at removing humidity, Combine that with a better insulated shell and a lot of people in the building. Sound like the need for a dehumidifier,
Clark,
I can help probably diagnose the problem. Look me up if you'd like to have a brief chat. I'm in Charlottesville, VA...probably not too far away from the site. From afar, I agree with everyone else...it sounds like you have a ventilation issue...possibly mixed with an oversized cooling systems issue.
http://www.think-little.com
Thanks for the input. Most of the data you all have sent, has confirmed most of my findings. But convincing our company to spend more money on this takes input and this will help my data collection. Your input has been very helpful. As a foot note. No, our ceilings are 2 layer 5/8 fire rock, The existing exhaust fans are 23 bathrooms, exhaust fans. The 8 attic fans are to relieve 125f attic heat and humidity that is hurting our data wires and baking the wiring, lumber and shingles.
Clark,
Installing attic exhaust fans is a very poor method of lowering attic temperatures. For more information on why your 8 attic fans are probably the root of all your problems, see Fans in the Attic: Do They Help or Do They Hurt?
The best way to lower your attic temperatures is to install insulation along the roof slope, creating an unvented conditioned attic. For more information on this approach, see Creating a Conditioned Attic
Considering the symptoms you have described, there is a high likelihood that this building has a very leaky ceiling that allows indoor air to escape into the attic through ceiling cracks.
If you can fix some of the problems we're discussing here, there is a very good chance that the owner's utility bills will be sharply decreased.
You previously mentioned that your ducts are all above the ceiling, and apparently "above the ceiling" now means "an unconditioned vented attic." That's bad. Ducts are likely not airtight and may be sweating, contributing to the dampness and mold. I sure hope your air handlers aren't in the attic as well.
Attic venting is not going to work here; replacing high-humidity air in the attic with high-humidity air from outside is not going to succeed in lowering the attic's humidity. Furthermore, with HVAC stuff in the attic, your attic fans have a million and one ways to pull air from the building instead of the soffits. This may be what's driving all the air movement through the poorly-sealed mold-vulnerable walls. And I hope the bath exhaust fans vent to the air and not into the attic...
In such a wet climate, is an unconditioned attic really a good idea? All the wiring and stuff up there is also exposed to some pretty wet conditions, not to mention the heat. I second Martin's recommendation to bring it into the conditioned space. It's not gonna be cheap, though. 22,000 square feet of spray foam a few inches thick could be a half million dollar job.
This kind of mess reinforces my sense that building extremely complicated, multi-layered building assemblies with numerous moisture-sensitive materials, open cavities, and unclear thermal and hygric boundaries is inviting disaster with the typical quality of construction we see in this county. This story is not in any way unique, sadly. It's disappointing that this commercial facility (even a low-rise commercial facility) was not built more to commercial standards, which would have given you near-airtight masonry walls, a flat roof with an obvious inside/outside boundary, and a conditioned dropped ceiling to put your ducts and wiring in. You probably wouldn't have any of these problems, even given the same HVAC mistakes. It sounds like the people who commissioned the construction of this structure were trying to save a buck by having it built to typical shoddy residential standards and are now paying for that foolishness, like so many homeowners do on a smaller scale.
A footnote: our 8 exhaust fans were just put in 2 months ago. Previous years we would get mildew around the sprinkler escutcheons and can lights. But since the fans were installed, we have had no mildew around the lights or sprinklers. ??
Thanks for the input. This stuff really helps some of my ideas and will help solve our issues.
Clark,
You have provided more evidence that the 8 attic fans are the cause of your problems.
Before you installed the fans, humid air from your attic was condensing on your cool sprinkler heads and can lights. This hot, humid attic air was being drawn indoors (downward) by the operation of your bath exhaust fans. The mold was a sign that your ceiling is leaky.
Then you installed 8 attic fans, strongly depressurizing your attic. So the attic air was no longer being drawn downward through ceiling leaks. Instead, the 8 attic fans were pulling cool conditioned air out of your building into your attic -- through the same leaky ceiling. Now hot, humid outdoor air is rushing through your walls -- it's makeup air for those 8 attic fans. As a result, you are getting the symptoms you describe: "Most of our [mold] growth is around any hole cut in our sheetrock. Hvac vents, light switches, plugins, anywhere the caulk is split between the baseboard and wall. Doors, windows meet the sheetrock."
If my analysis is correct, you need to do the following:
1. Disable the 8 attic fans.
2. Seal the air leaks in your ceiling.
3. Seal the air leaks in your walls.
4. Convert your vented unconditioned attic into an unvented conditioned attic by sealing the soffit vents, ridge vents, and gable vents, and installing insulation along the sloped roof line.
Please suggest the best way to condition the "unvented sealed attic"once we do this. Would dumping 1 vent from each of the 20 oversized 1.5 ton patient room HVAC do it? It would take some of the excess volume of super cold air from the rooms. It would allow for longer run times, with less super cold air. Then maybe add a small return from the attic air to each 1.5 unit, 20?? We can easily seal the vent gaps and baseboards, windows and such with mildew resistant caulk. Please give best suggestions on light switches, outlets, and wall sconces.We need to access them without tearing paint and sheetrock. Thanks
Clark,
The work we are suggesting is fairly standard energy retrofit work, and should be well understood by any qualified home performance contractor. If you aren't sure of where you can find a home performance contractor, find an energy rater certified by RESNET or the Building Performance Institute. The RESNET web site and the BPI web site have tools to help you locate a certified energy rater in your area.
If you want to condition your attic to the same level as the conditioned areas below, just perform a heating load and cooling load calculation for your attic, as you would for any other room. A responsible HVAC contractor uses Manual J to perform these calculations. Once you know your heating and cooling loads, you can calculate the size of the ducts and registers required to meet the needs of the attic.
When it comes to the air sealing work, I don't suggest that you reinvent the wheel or learn a new trade on the fly. Hire an experienced contractor -- one who specializes in air sealing work (blower-door directed air sealing). Running around the facility with a caulk gun won't work.