Generally, only one is needed. Humidity spreads easily but you didn't specify square footage or air infiltration. These are the key factors along with person, pet and plant count. Showering habits matter also.

But thanks for giving the location - lots of people ask questions without that basic info.

The need for supplemental humidification depends a lot on these factors. Basement humidity load in the first year after construction should be enough assuming it is connected - ie insulated and part of the conditioned space not necessarily finished.

A new house will typically have low air infiltration and thus a lower need for humidification. But this isn't always true and what I gather from Illinois in general is that codes lag/enforcement/building techniques lag behind other colder climates. In other words, from what I gather here, your typical construction isn't very good.

3 recent build examples.

# 1- humidifier in basement unit - ACH of 5. 6000 total sqft with 3 occupants. Aprilaire 650 I think. No issues except mold on those darn filters because I never replaced them.

#2 - Townhome 2018 build with Energy Star. Someone goofed because the infiltration was terrible. With the ventilation fan running, the humidity fell in the winter to where static was really annoying. 1800 sqft with same 3 occupants (and 3 pets). Built on slab.

#3 - 2019 Energy star on slab (our current house). No humidifier. No humidity issues at all. ACH 1.9. 3800 sqft (one less pet).

Housing stock in Chicago is old with high ACH which leads to dry winter interiors. Don't assume a new house will need humidification. Get a humidistat and watch it. Every flooring person will recommend a humidifier but they are used to old houses. .

Humidifiers require a lot of maintenance to keep clean and should only be installed if necessary. I also recommend you should wait until the winter to see if you have a low humidity problem. Low humidity is caused by cold dry air infiltration. New construction should be tight and not have this problem. If you do have the builder come back and plug up any air leaks first.

Thanks, David!

Our new build is about 3000 sqft--about 1000 sqft per floor, including a finished basement below grade. We have spray foam insulation throughout (I'm not sure what you mean by air inflltration). We have 4 people living here. Our humidity levels are currently 57% for the main floor and 58% for upstairs, but then again, the summers aren't a great indicator for winter humidity levels in Chicago. There are already some floor boards that are warped, but this is because the GC installed the planks right when they came on site in the winter and didn't allow them to come up to the humidity levels of the environment (what my architect suspects). According to the plans, our home is sealed pretty well.

Also in Chicago area, also have 2 zone HVAC, I have Aprilaire 2400. It's not a new build but just had new hardwood floors put in and the rest refinished in March. They let the boards acclimate for about a week inside before installing. The flooring folks said to keep the humidity around 35-45%. Most important, he said that if I adjust it, only do so by a few points at a time, over a few weeks. He said not to adjust dramatically all at once. Just sharing as an FYI (after I spent a bunch on new floors, I am babying them. :) ) I'd ask a flooring pro what humidity levels they recommend.

AIr infiltration is a measure of leakiness in a house. It is commonly measured in ACH50 - which is air changes per hour at 50 Pascals.

Typically spray foam leads to low ACH. No guarantee.

You can't really compare construction humidity to real life humidity. And yes - no one should have installed floor boards like that. That doesn't mean anything when it comes to next winter.

The post from chicagoans is exactly what I suspect - old houses in Chicago will have a humidifier. That doesn't mean your spray foam house should.

That many people in that square footage should not have a humidity problem in a tight house.

We live in Indianapolis, the house is tight (0.4 ACH), have 4" solid oak flooring throughout, and stopped running the Honeywell HE365 humidifier a couple years ago. Humidity in the winter stays around 35-40%. Flooring is just fine.

chicagoans,

An Aprilaire 2400 is an air cleaner which is a fancy term for a rectangular metal box which holds a filter. It has no affect on the humidity level. Do you also have an Aprilaire humdifier and dehumidifier?

All,

Thanks! We have 8" planks, so the warping is pretty noticeable in the few spots. We thought they would cut costs by giving us thinner planks, but we lucked out. We moved in late May, so this coming winter will be the first one (second if you include the construction). I thought that I would install this during the summer thinking that prices and demand would be a little lower.

formulaross - 0.4 ACH is sort of unbelievable. Under 1 requires truly heroic efforts. And you better have mechanical ventilation. You sure about that number?

OP - do you have supplemental mechanical ventilation?

David - Yes, 0.4 ACH, I saw the numbers on the test rig myself and just recalculated it again. During the build, we acted as the general contractor so we did a fair amount of work ourselves. To put it mildly, we got anal about air sealing. Really, really anal. Yes, we have outside air being brought in under the command of a Honeywell controller.

David,

Honestly, I don't know if I have supplemental mechanical ventilation. What would it be labeled as in the architectural plans?

formula - I am surprised that doors and windows usually sourced in the US could hit that number. Now, if you have a large interior volume with relatively few penetrations, it would be easier. Like only 1 door and few windows. The door would typically be triple point locking on a Passive House where the goal is 0.6. Even high performance builders have trouble hitting that number and it usually isn't cost effective to try and get below 1.5 or so in the US (or almost anywhere now that solar is so cheap). So color me skeptical or impressed.

OP - Architectural plans vary by region. Mine don't have anything electrical/plumbing/HVAC - and it isn't customary in my area. Other areas require it or it is customary.

If you have an HVAC plan, it might be there.

The most common thing in my area is a bath fan that runs intermittently. Common because it is cheap not because it is ideal. Formula has a good system - and a controller somewhere. If you had that, you would know about it. And the bath fan you would hear.

Energy Star (in my area) requires it and it is the reason we had it added. Turns out we put the cat litter box in that bathroom so it was very helpful.

Generally, a spray foam house is tight enough that IAQ (interior air quality) standards require ventilation. You can build a tight house and not ventiliate it. Then your VOCs can be quite high especially with a new house - all the new wood off gassing. Humidity could be the least of your concerns. Especially in the first summer, you should have a window or two cracked all the time.

Do you have a proper vent for cooking? What happens when you turn it on? Despite having some make up air, mine will create a huge breeze through a cracked window - like a loud wind noise. This to me is a good poor man's test of how tight your house is.

David - 4 exterior doors, 3 Thermatru and 1 Andersen, 48 windows, Andersen casements. As the general contractor, we were anal in paying attention to sealing things up, that made it cost effective for us. Like caulking the leaky 'air-tite' Juno can lights before install, spec'ing only one penetration of the ceiling with HVAC ductwork and then foaming that whole area in the attic, gasketing the drywall around the fireplace structure, caulking the inside perimeter of all the wall cavities, on and on ....

We have a newer build in MN (5 years), about 2800 SF. House is so tight that in winter we get condensation on the windows overnight when our blinds are closed. We didn't realize we had this problem and ran the humidifier our first winter, and ended up with water damage on the trim of our small (24") windows because water collected behind the shutters. Our humidifier doesn't work now - someone cut the line when doing water heater maintenance, we think - but we don't miss it. Our house usually stays above the levels of humidity recommended anyway.

I would think to get to 0.4 ACH would mean no cans into unconditioned space. Or boxes built around cans. Caulking an air tight can is a given - going way beyond that is usually needed.

I have 400 series Andersen casements. A fine window but not that great for air infiltration. They meet standards for good basic house construction and casements are better than double hung.

Builders going for passive house standards (0.6 ACH) typically use European windows. Zola is an American maker shooting for the European standard. I have not seen anyone use Andersen for a passive house - and that is just shooting for 0.6.

Given your number of doors and windows (and type), I have to think your air infiltration numbers measurements or calculations are incorrect.

Getting to 0.6 is nearly heroic stuff. Something experienced contractors struggle with. And honestly, done mostly as a science project. You don't need to be that tight. And the cost issue is with doors and windows. Caulking the world is relatively cheap - and even cheaper DIY.

When you mention HVAC vents, that would seem to mean you have ductwork in unconditioned space. If that is true, it is one more source of infiltration that would never be done in a passive house. And do you not have returns in the bedrooms? I say that because you say you speced one HVAC penetration per room. Do you have grills over the doors? Because modern best practice is a return per room or a jump duct. Either way, another penetration. I think our Energy Star requires it.

If you have your plans, look for a HRV/ERV.

0.6 ACH 50 is quite possible. We built two Energy Star demonstration homes back in 2001 that had infiltration rates between 0.25 and 0.3 ACH 50. Each was insulated with spray foam and had conditioned crawl spaces and conditioned attics. Nothing heroic required to achieve the numbers, just a really, really large attic volume (that is included when making the ACH calculation) and lots of cash, of course.

The insulation and the technique were new in our area back in 2001. I remember touring one of the homes with a group of architects. We ended the tour in the walk-in attic space and did a Q and A session. After 15 minutes, one of the architects realized that we were meeting in an attic in coastal VA on what was the hottest day of the year and nobody had broken a sweat. Yes, a conditioned attic can be a great place to store architects and other stuff.

A ventilation system is required to maintain indoor air quality in a home with spray foam insulation. The design needs to be carefully considered. My bet is that it isn't specified anywhere even if an architect produced the plans. It's amazing to me how something so important to both the longevity of the structure and the health of the occupants is so often left to chance.

David - To clarify,

Re: No cans in unconditioned space and going way beyond caulking is usually needed. Why? The concern was air leakage. If the cans are totally sealed themselves and then sealed to the drywall at install, the air sealing plane has been maintained and there will be zero leakage. All the cans in our upper level are installed this way. Every penetration of the house envelope, top, side or bottom, is sealed as best we could. Caulk and foam go a long way. Its the time/labor that we supplied that is costly for a regular build.

Re: Air infiltration measurement being incorrect. It's pretty simple, ACH = CFM x 60 / (Area x Height). The blower was putting out 330CFM at 50 Pa, the rest is easy.

Re: Ductwork in unconditioned space. No, I spec'ed one penetration of the ceiling, period. As a ranch with a walkout basement, 98% of the ductwork is in conditioned space, metal ductwork with sealed joints. Geothermal heat pump. The bedrooms have air returns high on a side wall. All exhaust venting is with spring-loaded, gasketed dampers through side walls to minimize roof penetrations. The one ceiling penetration allowed is the main air return for the upper level, placed near the highest point in the vaulted family room. The backside of that penetration (up through an adjacent ceiling and back through a side wall) was completely foamed-over in the attic. Outside air is brought in with a dedicated control and motorized damper.

We didn't build with the aim of achieving any specific number. We educated ourselves (thank you Building Science Corp!) and then paid attention to details during the build process. We knew we wanted an air tight structure, higher levels of insulation, rigid foam on the exterior walls and below grade, blown cellulose insulation, a wall design to properly handle water vapor, high quality casement windows for the better fenestration numbers, smart framing practices, etc.. We didn't know about the 'over the top' kind of stuff you appear to deal with. What we accomplished is certainly the result of A LOT of our own time spent on the build paying anal attention to those details. As the general contractor (under the U-Build It umbrella), that worked out well for us. In the real wold that kind of effort is expensive.

I did say skeptical not impossible. The added details make it seem totally doable. Honestly surprised you could hit it with Andersen casements and normal doors.

For cans into unconditional space, best practice is building a box. If you truly make it airtight, you are violating the install instructions regarding heat ventilation and some will argue it is a fire hazard. You probably use LEDs and it isn't a big deal, IMO, but some will disagree - including code inspectors. But I guess the real issue is putting insulation covering all that- that is the overheating issue. Curious how you dealt with that?

Sorry - I thought you said 1 penetration per room. And also not meaning to offend - with the added info, you show that you have done your research. I still think blower door calcs can be incorrectly done just probably not yours.

Walkout basement helps a bit of course.

Charles- I always find the conditioned attic method a bit of a cheat. Basically changing your denominator without changing house size.

The can lights were Juno IC926's, IC20's and IC1's (6'', 5" and 4"), all essentially a can light within a metal enclosure and rated for insulation contact. The inner cans are advertised as being sealed, on the 5" versions the inner can was a deep-drawn steel stamping, itself absolutely air tight, showing that Juno was not depending on air leakage for cooling. On the 6" versions, the inner can was a metal fabrication and we could not be sure they were completely air tight, so we completely sealed the exterior of all the fixtures.

@davidcary,

Compared with a vented attic, the thermal and pressure boundaries of a conditioned attic move to the underside of the roof deck. Accordingly, that volume gets included in the calculation of ACH 50. The same would be true for any living spaces with vaulted areas or volume ceilings. It's not cheating, it's simple physics.

However, if you consider it cheating, you might give the good folks at ASHRAE or EPA Energy Star or DOE a call and share your view.

@mike_home Thank you for the correction! You're right, the Aprilaire 2400 is what I buy filters for, for both systems. I do also have a humidifier but only for one system. Your correction inspired me to take a look at it; it's an Aprilaire 700M. To be honest, the only thing I ever do is change the filters every few months, and I let the twice/year HVAC service worry about everything else. Clearly not a pro :)

I responded to the post to share what my flooring guy said about humidity. I love my wood floors and would hate to have them warp or cup due to humidity.

Cheating in that you aren't actually getting that square footage but it gets calculated in your leakage. So in the extreme, a 5000 sqft attic with a 1000 sqft living space. The ACH is 1/6 of what the actual living space is. It isn't actual air changes that matter for energy costs, it is the amount of air moved. (Air changes is per volume so you can leak a lot of air but if the house is large, your ACH goes down).

I understand why they do it but the only thing that matters is energy use. And a 1000 sqft home with a ACH of 1 will have a lot lower energy cost than the same house with a 6000 sqft home+attic ACH of 1. Isn't energy costs or environmental damage what matters, not some specific ACH?

That is why it is cheating.

Just like calling R30 on a huge attic rafters the same as R30 on a attic floor. Code allows it but the area is 50% larger so the heat loss is 50% larger. It just isn't the same.

If you have vaulted space - at least you get to enjoy vaulted space. Now of course you could use the attic for conditioned storage but that isn't really equivalent.

Now - a foamed attic is a great idea in many circumstances but the ACH is just not equivalent - especially in a ranch with a tall attic.

Hitting a low ACH is easier with a conditioned, foamed attic - it just isn't equivalent. Despite what the EPA or ASHRAE says. Do you really feel those bodies are perfect? I doubt you do.

@davidcary,

"Cheating in that you aren't actually getting that square footage but it gets calculated in your leakage. So in the extreme, a 5000 sqft attic with a 1000 sqft living space. The ACH is 1/6 of what the actual living space is. It isn't actual air changes that matter for energy costs, it is the amount of air moved."

ACH is dimensionless. It's the number of times per hour that a volume of air turns over. The variables are volumes, not square feet. To the extent that conditioning the indoor environment includes conditioning infiltrated air, ACH matters.

"Just like calling R30 on a huge attic rafters the same as R30 on a attic floor. Code allows it but the area is 50% larger so the heat loss is 50% larger. It just isn't the same."

R-value is a physical property of a material. It's a measure of the thermal resistance of a material per unit area. An R-30 batt is always an R-30 batt independent of the total area. Spray foam installed to a thickness that is R-30 is R-30 whether it's in a vertical wall, a ceiling or sprayed under roof decking.

"I understand why they do it but the only thing that matters is energy use."

You are welcome to your opinion, but with regard to ACH, I suggest the implications are twofold: energy use for conditioning the indoor environment and indoor air quality. As energy codes become more demanding, the focus is shifting to indoor air quality.

"Despite what the EPA or ASHRAE says. Do you really feel those bodies are perfect? I doubt you do."

If your question is whether I believe EPA and ASHRAE standards are developed by knowledgeable professionals in their field, then the answer of this professional engineer is "yes."

I misspoke on area vs volume. I certainly understand the difference.

All your ACH points are valid esp IAQ.

I was just saying that EPA and ASHRAE aren't infallible. Of course they are developed by professionals.

I was also saying that R-30 over a larger area (got dimension right that time) means more heat loss. Of course R-30 is R-30 regardless of whether it is vertical, horizontal, foam, or batts. But a R30 rafters of 1000 sqft does not perform as well as R-30 on the 700 sqft attic floor. Because area matters.

There appears to be lingering confusion on R-value.

R-value is a measure of resistance to heat flow. It's measured in a laboratory environment. LIke most things, real-world performance differs from than in a laboratory environment. Walls insulated to the same R-value with cellulose or spray foam will perform better than fiberglass batts because they minimize the potential for convection in the wall cavities. They offer some other benefits, too, compared with fiberglass batts, albeit at a premium cost.

Your example of insulating under a roof deck versus an attic floor conflates insulation performance (heat transfer per unit area) with total heat transfer, which depends on the total area available for heat transfer. Of course a 1000 SF area will transfer more heat than a 700 SF area even if a contractor is doing the math.

This begs the question: Why construct a conditioned attic if the heat transfer area is greater than a conventional attic? To some extent the answer depends on the specific climate. I design and build homes in coastal VA (climate zone 4.) We get hurricanes and tropical storms that can produce horizontally-driven rain and even push rain vertically into soffit vents (I have observed this personally.) Conditioned attics eliminate the attic vents which allow rain to enter under such conditions. They also reduce potential moisture problems by keeping out hot, humid air which we have much of the year. Other reasons to construct a condition attic in our climate zone include homes with lots of transitions in framing that would make conventional insulation difficult or which have ducts or air handlers in the attic space. Conditioned attics are also much more forgiving of penetrations in the ceiling plane for HVAC ducts, recessed lights, etc.

Designing homes to work as a "system" isn't a new idea. While it's not rocket science, it is sufficiently complex that getting it wrong can have serious consequences for the longevity of the structure, the health of the occupants, and the homeowner's wallet. DIY home designers will be well served to consult with credentialed professionals in the design phase.

I have a big house with 3 furnaces and 3 aprilaire humidifying units. They work great, pretty accurate, and work fast. Really I just wanted 1 for our bedrooms, but figured why not do whole house. I love it and can tell the difference. Set it too high in the winter, and you risk condensation on the windows and rotting wood and/or growing mildew

Even though it works perfectly, DW still put a damn freestanding humidifier next to her side of the bed claiming it's too dry in the winter...