Whole-House Dehumidification
Building a new single story ranch with an unfinished basement in CT zone 5. I am extremely sensitive to mold. The home will be carefully designed taking many principles of moisture management into account.
My question is about the HVAC system. We will have an HVAC system with a MERV 13 filter and central A/C. Care will be taken to “right size” the A/C unit to prevent short cycling. We will not install a central humidifier but will use a standalone ultrasonic humidifier (which can be cleaned completely) in the winter if relative humidity (RH) drops below 25% not to exceed 35%. The basement will not be insulated (except for the ceiling), but it will be heated.
I’ve been advised to install a whole home Sante Fe Ultra205 dehumidifier installed as shown in the attachment. The system will be set such that there is slight positive pressure inside relative to outside. Is this good advice?
I am considering first running a standalone Sante Fe Compact dehumidifier in the basement and monitoring RH levels in the house to determine if a whole house dehumidifier is necessary.
I’ve also considered the option of a standalone unit in the installed in a centrally located closet in the conditioned space (first floor) with a louvered door as described in this article. My hesitation with this option would be noise and heat buildup.
I’m aware the Sante Fe unit is expensive but have chosen it b/c it has a MERV13 filter, a sloped drain pan (no standing water), and a back flow valve all of which will reduce the likelihood of mold growth in the unit itself.
Your advice would be greatly appreciated.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
You never insulate a basement ceiling. Ever.
If you have a basement, it is part of the house, which means it needs to be insulated and conditioned the same as the house. Having it at a different temperature, thus RH is only asking for mold and air quality problems.
Minimum is a vapor barrier under the slab and rigid insulation(not batts) on the walls.
Since you don't want to finish it, you can look at one of the rated foam products that can be left exposed such as Thermax.
This will make the basement much warmer and lower the vapor drive into the house. Warmer basement also means warmers floors above.
Zone 5 is too cold for a supply only ventilation. It will work in the summer, but will cost too much energy in the winter. What you want is an decent ERV with ECM blowers. Properly designed and installed, this will give you fresh air all year around for minimal operating cost. Plus it will allow you to eliminate 2 to 3 bathroom exhaust fans, so the extra equipment cost is minimal.
The SanteFe unit is a bit overkill if you build as suggested above. A quality stand alone dehumidifier will do the job but even that might not be needed. An ever better option for humidity control in the basement is a heat pump water heater.
Thank you for your reply.
My decision making has been based on advice I've received from:
1. an individual with a degree in building science and many years of experience as a mold inspector in climate zone 5,
2. an architect who specializes in moisture management.
I've also read extensively on this site and on the Building Science Corporation site in an effort to understand the science.
I agree that the basement should be conditioned (heated and dehumidified). In theory, it should be insulated which can be done either on the exterior or interior, however there are problems with both.
Interior: The concrete needs to dry to the interior which can take up to 2 yrs. Insulating the interior walls, even with rigid foam, will likely trap moisture resulting in mold growth beneath the foam.
Exterior: Our builder has done this in the past and stopped the practice due to termites in our area. This was a problem despite termite shields, etc.
The building science expert/mold inspector advised against use of an HRV/ERV as they eventually grow mold in the core. He owned an HRV company but got out of the business due to this problem. (A google search verifies that this does in fact occur).
Mold growth occurs in dehumidifiers, too. It's my understanding that this does not happen with the Sante Fe units b/c they have a MERV 13 filter (and a valve to prevent back flow) which prevents spores from entering the unit. The drain pan is designed to prevent standing water. Drain tubing should be changed periodically.
To comply with code, we must insulate the ceiling (because the basement is not insulated). Once we pass inspection, we will add ducts to heat the basement, remove the ceiling insulation, and dehumidify.
In light of this, what would be your recommendations?
I've not come across the suggestion to use a heat pump water heater to control humidity in the basement. Would you please explain the science behind this?
Thank you again for your advice - much appreciated.
Concrete doesn't need to dry, it cures. Heck, concrete will even cure underwater. This means you can cover it with rigid without issues. This has been part of code for a long time and it works. Important part is to install the foam directly against the concrete with an adhesive to prevent interior air from getting behind it. You can also use spray foam which is guaranteed to air seal but does come with its own issues and cost. This is exactly what BSC recommends.
Uninsulated and heated basement will bump up your energy costs by about 20%. Plus it might make your mold issues worse as the bare concrete now is exposed to moist air in the winter time and you might get condensation on the above grade section of your foundation.
You can get HRV/ERV with built in HEPA filter or even better, install an external three stage filter on the intake. HRV cores can be taken out and cleaned by soaking them in soapy water if needed.
Any time you have water condensing, you risk mold growth. Dehumidifier is no exception, the cold coil in it is no different than the heat exchanger in an HRV. If anything, the coil in a dehumidifier is much harder to clean.
I think trying to prevent mold spores from entering your house is futile, mold is anywhere. What you want to do is prevent conditions that will lead to mold growth. This means good waterproofing (foundation, slab, walls), proper insulation and air sealing. The rest, do your best to filter the air.
I don't know how it is for mold, but for pollen and pm2.5, Merv 13 doesn't do a lot. You really need a Merv8-13 followed by a HEPA filter.
Deleted
"Does Thermax meet this requirement? I thought the foil facing made it vapor impermeable"
Joe L. has changed his recommendation of semi permeable rigid, here is a detail for Thermax:
https://www.buildingscience.com/documents/enclosures-that-work/high-r-value-wall-assemblies/high-r-foundation-5-xps-foil-faced-polyisocyanurate
"Can't I just turn off the dehumidifier (and its fresh air supply) in the winter"
For a new build, you should be aiming for bellow 1.5ACH air tightness. Lot of places code min is 3ACH@50PA, even if a leakier building you will need mechanical ventilation to maintain indoor air quality. You can't (or shouldn't) simply turn off the ventilation in the winter. Providing code min mechanical ventilation without heat recovery takes a fair bit of energy. An HRV/ERV is really the best bet in your climate.
Tight houses might need dehumidification in even in the winter especially for the first year or so until construction moisture is removed. Lot of times this means running an extra stand alone dehumidifier for the first year. Once the construction moisture is removed, an HRV is the best way to maintain proper winter time humidity.
"Where can I find information on an external three stage filter"
My google foo is off, I can't quickly find it, pretty similar to a
Fantech HERO HS300 without a fan (I guess you can use the Fantech unit and not power the fan).
Designing for good indoor air quality is not a simple. Takes a lot of up front work and details to get it right.
Good points.
The article you provided was written in 2011. I'm confused b/c in 2019, Lstiburek discusses the need for interior basement walls to dry to the inside and cautions against vapor barriers on the interior of insulated finished basements. "Oh yeah, one more thing…do not install interior vapor barriers on the interior of insulated finished basements."
https://www.buildingscience.com/documents/building-science-insights-newsletters/bsi-110-keeping-water-out-basements
Isn't Thermax a vapor barrier due to the foil facing? Wouldn't using it go against his advice in the 2019 article?
Thank you for your patience.
JennL,
By interior vapour-barriers, Joe L. means those on the inside of the framed walls. He wants them to be able to dry to the inside. He still believes in using a vapour-tight materials against the concrete , as Akos said.
Thanks for staying with me on this.
Should I get an HRV or ERV in climate zone 5?
Does the HRV/ERV require separate ducting done at the time of the build, or can it be added to an existing system?
Is the configuration you suggest shown in figure 1 of this article with the external 3 stage filter be interposed between the outside air intake and the HRV/ERV?
https://www.buildingscience.com/documents/insights/bsi-022-the-perfect-hvac
Can you provide the name of the external 3 stage filter or would you recommend the Fantech HERO HS300 with the fan off?
Will the 3 stage filtration add extra load to the HVAC system causing it to wear/fail prematurely?
Can you recommend an HRV/ERV that can be easily and effectively cleaned? Looks like the core material can be paper, resin, or metal?
Thank you
This is good start for ducting HRVs:
https://www.finehomebuilding.com/2014/11/05/ducting-hrvs-and-ervs
99% of new builds would have the simplified ducting, people with high efficiency builds will tend to go for dedicated ducting which does add cost.
I think the hybrid ducting they show is really the best compromise. I would make a couple of changes to their layout though.
First you want an auto balance HRV/ERV (Panasonic Intellibalance, Broan AI or Zehnder Q series). These will automatically compensate for pressure changes caused by the furnace operation to maintain balanced flow which is important for energy efficiency.
You then want stale air pickups:
-small (10cfm) near the floor in the basement (helps with radon)
-largish (40cfm) in the kitchen kitty corner from the range about 8' away
-small (20cfm) in the main bedroom suite
-left over capacity for venting mostly used bathroom, most can do about 2 bathrooms.
This is a bit more extra ducting, saves you the cost of a couple of bath fans and associated ducting/wall caps plus it ensures air exchanges in the bedroom no matter what the air handler is doing.
You need two seperate filters.
One for the fresh air supply. If the HRV comes with a HEPA filter option, it is good to add a MERV13 pre-filter as it will make the HEPA last much longer. Fantech FB6 is good for this but should be wrapped in rigid as it is on the cold side of ducting.
If the unit doesn't have a HEPA filter option, you need an external filter box. This is the part I couldn't find.
Second you need filtering for the air handler. If you install a 5" filter frame, you can get Merv 16 filter which is not quite HEPA but close. These are not cheap but probably the simplest setup.
The other option is to use a standard Merv 11/13 filter on the air handler coupled with a stand alone HEPA setup like the Fantech HERO HS300. These connect to the side of the air handler and since they have their own blower they don't put extra strain on the air handler fan.
ERVs have the paper core. These can be washed with water but not with soap. HRVs have an aluminum core which can be washed with soap and water.
Thanks! Last question.
Is it reasonable to expect that a stand-alone dehumidifier in the basement such as the Sante Fe Compact 70 will be sufficient to manage the humidity levels in this situation? (1800 sq ft single story home with 2 occupants in climate zone 5) If not, how should a whole house dehumidifier tie into the system?
Deleted
All ventilation adds humidity in the summer time and removes humidity in the winter.
ERV will do the least, HRV in between, exhaust/supply only ventilation the most.
A venting humidifier brings in the least amount of humidity but it costs a fair bit of power to do that.
There are heat pump based ventilation systems such as Minotair that are much more efficient and will do active humidity control. This is not a budget item plus you might have a hard time getting some to install it.
"You never insulate a basement ceiling. Ever."
Would you please explain the science behind this? Unless I am mistaken, I see a number of posts on the GBA site in which experts discuss methods for insulating a basement ceiling.
I understand the importance of bringing the basement into the building envelope and heating and dehumidifying the space. I'd like to understand why that couldn't be done with ducting for heat and a dedicated dehumidifier with basement ceiling insulation in place.
I'm not disagreeing. I just want to understand why.
Thanks for your patience.
I should have been a bit more clear. You never want to the basement to be isolated from the house. Insulation in the ceiling doesn't matter as long as the basement is kept at the same temperature and humidity as the rest of the house.
Generally when people mean insulate the basement ceiling, it implies that the basement won't be heated, this is a very bad idea.
You can definitely throw enough heat and dehumidification at an uninsulated basement for it to work. It just seems very inefficient when there are better solutions.
"You never insulate a basement ceiling. Ever."
What about a retrofit? I have a 150 year old home with a full height unfinished basement. I can insulate the concrete and stone walls but certainly not the basement slab. What then?
In most cases with older rubble basements, the best solution is to insulate with spray foam against the foundation. Make sure to set the stud framing back about 2" from the foundation and carry the spray foam behind the studs.
You don't need to insulate the floor. Even in colder climate, surprisingly little heat is lost through the slab.
If you have high soil moisture content you might need to limit the moisture movement through the slab. This is best done with poly under the slab, which is impossible to retrofit. As a retrofit, the best you can do is coat the slab with a vapor barrier coating if it is needed. Before going this far, in most cases fixing the water issues outside the house (grading, drainage and downspouts) is enough to solve most problems.
Bingo. Thanks Akos. That's exactly what I was planning.
I have a 40-year-old home with a basement and there are some cracks in the walls. I have an engineer scheduled to look at it to see if it's a structural problem but that's three months away. There's insulation on some walls, and I'm going to have to remove that to see if there's damage to those walls too. So, a downside to interior insulation on the walls is that you can't check those walls for damage. However, if the home is 150 years old and there are no structural issues by now, it seems unlikely that problems will develop.
I'm inclined to remove ceiling and wall insulation in the basement anyway because I'm in a warm climate (Climate Zone 2, Austin TX) and it seems like having a cool basement will help cool the house, since we need more cooling than heating here. I'm hoping to get an expert answer on that to confirm. I'm running a dehumidifier and it doesn't have to run much to keep the RH at 45%.
Ran out of nesting, so replying here.
"Sante Fe Compact 70 will be sufficient to manage the humidity levels in this situation?"
For a new build you will probably need more dehumidification to remove construction moisture, this is best done with a couple of budget units or a high capacity commercial rental unit. Once that is out, a 70 pint unit should be good.
I'm in similar climate here, dealing mostly with 100 year+ houses with no vapor barrier bellow the slab with finished basements. Common to put a 50 pint unit into the utility room connected to the floor drain. In most cases this is enough to keep the humidity bellow 60%.
"Fantech FB6 is good for this but should be wrapped in rigid as it is on the cold side of ducting."
Trying to understand the need to wrap the pre-filter in rigid foam insulation. By the "cold side of ducting" you mean that the pre-filter is the first location at which the cold (winter) intake air contacts warm surfaces. I assume the concern is condensation. Wouldn't the same condition exist for the ERV itself when typically installed w/out a pre-filter? Is it standard to insulate intake ducts?
The FB6 should be a pre-filter on the fresh air supplied to the ERV/HRV. This fresh air is the same temperature as outdoor air, so in the winter time an uinsulated cabinet will sweat. Thus the insulation. The inside of all ERV/HRVs is lined with a layer of rigid insulation as well.
Same for the ducting going to the outdoors (stale air exhaust and fresh air intake), these need to be insulated. Once through the heat exchanger in the ERV, the air is close to house temperature, so the rest of the ducting can be uninsulated.
"Plus it might make your mold issues worse as the bare concrete now is exposed to moist air in the winter time and you might get condensation on the above grade section of your foundation."
Can this be controlled with dehumidification? Is it possible to insulate only the above grade section on the interior? It's done with rim joists. This would still allow for monitoring the foundation for cracks for the first couple of years (which I'm told inevitably occur as it settles) and to repair leaks.
Most of your heat losses are the above grade foundation and a couple of feet bellow that, so insulating just the top part (~4') will get you most of your energy savings. It is how basements here were insulated before our code was updated.
I would run this option by your building department first though.
Another option to look at is building your foundation with wood-concrete composite ICCF forms (ie Faswall). Since the wood fibers are embedded in concrete, these don't have any of the issues of exterior rigid foam and won't attract pest. This would avoid needing any extra interior or exterior insulation.
Thank you for your response.
If we chose not to insulate, could the condensation on the interior of the above grade basement wall be controlled with dehumidification?
Help me understand your 25-35% humidity goal.
I though mold went dormant at about 50% humidity why do you need to be at 35%?
I am not convinced 35% is achievable in CT at any reasonable cost. in AZ I could see it. Not having been to CT am I wrong in assuming much of the state would be a costal marsh and the outdoor humidity will exceed 75% most day May-October?
Off the shelf HVAC equipment in never going to achieved 35% humidity in the swamp I am imaging. The only way to come close will be to chill the air to nearly freezing in order to remove the moisture and then reheat it to room temperature. Wild guess but given CT electric rates I am thinking opening a door for one minute when it 80° and 80% humidity will let in moisture that will cost you $50 in electricity to remove.
Sure, when it is -20° outside you can bring in enough cold dry air and heat it to meet your goal but it will require large amounts of fuel even then.
If your health truly demands such an environment, it may be more cost affective to move you to a high desert location that to create one in CT.
If you want to avoid mold, I say skip the basement and pour an elevated slab. Even the best basement tends to be a cool damp place and is best avoided given your goals.
Note “kiln dried” lumber is far from being truly dry. Consider getting and storing your lumber under the conditions you intend on achieving for a full year. If you want to avoid spending the first year in your new home trying to remove that moisture. Most new homes will not see humidity reading under 50% until after the end of the heating season.
Walta
Let me clarify that I am on the receiving end of advice, not the giving end. I'm reading and learning from the experts on this and the BSC site.
Today in CT it was 34 deg outside and the RH is 22% inside. Typical this time of year.
If the RH in the home I hope to build were to drop below 20-25% in winter, I'd humidify to protect the wood floors and furniture but not above 35%.
Hoping I won't have to do that. Referring to whole house humidifiers, the author of the article below states, "I’m not going to say they’re never necessary, but if you have an airtight, well-insulated home, you almost certainly won’t need a humidifier."
I based the limit of 35% in winter on this article. I hope I'm interpreting it correctly.
https://www.greenbuildingadvisor.com/article/what-is-the-ideal-relative-humidity-in-winter
In the summer months target RH would be at or below 50%.
FWIW, EPA sez: "Indoor relative humidity (RH) should be kept below 60 percent -- ideally between 30 percent and 50 percent, if possible."
https://www.epa.gov/mold/mold-course-chapter-2
In my humid environment in Austin, TX, my consumer-level dehumidifier easily keeps the basement at 45%, "not-running" more of the time than "is running".
Hey Jenn! I'm also in CT in the process of renovating an old home on the CT river. While this presents an entirely different set of challenges than your circumstances, it seems to me you need an experienced, local HVAC contractor who has worked with air tight homes or at least a PH consultant. I contacted one through Eversource. Good luck with your project!
We'll be hiring an HVAC contractor when we build. Hoping to find a knowledgable one. What is a PH consultant? What services are you getting from Eversource?
Passive House consultant. If I were you, I'd be looking to hire a GC who builds these type of houses exclusively. There's a builder in Wilton, which is too far from us, but they specialize in Passive Homes.
I contacted Eversource because of Energy Rebates etc. available from them. Also there's a consulting company named McGrann Associates for residential energy consulting. You should, if you can, use an architect who specializes in this stuff too. There's a guy out of Boston who also works in CT and PH's is all he does. I came to find out about him too late. My email is [email protected] is you want to communicate off the forum.
Thanks for the info. We plan to build a house with good insulation and air sealing. It's not a passive house. I am told that newly built homes are tight enough that they need mechanical ventilation. I'm also trying to control humidity.
Understood. Don't try to reinvent the wheel. There are people in the trades who specialize in this stuff. Good luck with your project.
Not sure whether the HRV/ERV is the way to go. This is from an older post on this issue.
"I've been worried about this risk since a friend, who is one of the best mold hounds in the U.S. told me she's never tested an ERV core that didn't have mold. I think it's inherent with the design. We just cannot expect two opposing temperatures clashing in a highly conductive cell without one side or the other seeing dew point condensation. Think about it. HRV's have a drain for some reason. I don't see this problem going away until we rethink the strategy."
I have heard this from a well respected mold inspector, too.
I'm pretty sure if you test any AC coil for mold, it will also be positive. So will any kitchen sink or bathtub.
Sample of one, I have an ERV at home and we have pretty hot and muggy summers. I've looked at the core when I change filters and no sign of mold on it. I think the issue with mold buildup might be in units that don't run 24/7 and are cycled with the air handler. These might have water sitting stagnant for a period of time which can be an issue. Best is to set up the ventilation to run all the time.
If still worried, go for an HRV, all metal construction which you can disinfect if needed. If you get a Zehnder ERV, they will sell you an HRV core you can simply swap out if it becomes a problem.
I worked for a while overseas in a Passivhaus office building. One of the folks there had severe allergies, the best part of work was coming into the office where her allergies completely disappeared. Why? Well sealed building with good ventilation.
With good design and built with attention to details your home will have great IAQ.
P.S. Good design and attention to details are hard to find. They also cost a fair bit.
Note that many modern ERVs have cores made of permeable polymers, sort of like gore-tex used in waterproof/breathable jackets. At least in theory, they should be less prone to mold than paper ones. And HRV cores tend to run wet more often than ERV cores, because HRVs condense moisture out of the air stream where ERVs transfer it to the other air stream.
Understood. I'm not convinced that the HRV/ERV is the way to go.
The problem is that mold can grow on any surface with an ongoing moisture source. My guess is that condensation will support growth on the inaccessible parts of the core that cannot be cleaned by mechanical means.
This is from an older post on this issue.
"I've been worried about this risk since a friend, who is one of the best mold hounds in the U.S. told me she's never tested an ERV core that didn't have mold. I think it's inherent with the design. We just cannot expect two opposing temperatures clashing in a highly conductive cell without one side or the other seeing dew point condensation. Think about it. HRV's have a drain for some reason. I don't see this problem going away until we rethink the strategy."
I have heard this from a well respected mold inspector, too.