Two condensation questions
I have a newly foamed attic with open-cell foam (average 8 inches or so with rafters covered) and a brand new AC unit with furnace. I had thought that lowering the temperature of the attic would take away the need for insulation on the condensate drain line, but it is still sweating. The upstairs temperature is in the 75 degree range in the Attic is in the 81 degree range, but the Attic humidity has gone from being in the thirties or forties when it was a vented attic to being about 70% now, so I guess the dew point is still high enough to necessitate insulating the drain line.
My first question is if that is a normal occurrence and should I just insulate the drain line and be done with it?
Second issue… As this did before I foamed the attic, if I leave the door to the attic open (it’s a full size door with stair walk-up), the two ceiling vents will both condensate a lot, just in the room with that door. No other upstairs vents will condensate. What may be the issue there?
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If it is newly framed, the lumber and other wood materials probably contain quite a bit of moisture and need to dry out. Were the sheathing intersections air sealed before the foam was installed? Any ducts running through the attic to the exterior?
As a followup question.. .there is a single window in my foamed attic. Is this likely to cause any issues with humidity?
Condensate is a noun, referring to the liquid that forms on cold surfaces. The verb you're looking for is "condense".
Your problem is likely to be related to the moisture cycling into and out of the open cell foam & roof deck. The sun bakes the moisture out during the day, but when the roof deck cools at night it is readsorbed. This a commonly reported issue in attics with open cell foamed roof decks when there is no interior side vapor retarder- it comes up here fairly regularly, eg:
https://www.greenbuildingadvisor.com/community/forum/general-questions/27408/open-cell-foam-roof-deck
The solution is to actively condition the attic space &/or install an interior side vapor retarder. The permeance of the vapor retarder does not need to be super-low- standard latex paint on wallboard is good enough. You may or may not be able to get there with "vapor barrier latex" sprayed directly onto the foam.
In the mean time, a room dehumidifier would installed in the attic should bring the total moisture in the attic down, and lower the moisture cycling that you are seeing.
Note: 70% RH @ 80F is in the exponential growth conditions territory for many mold species- you can't/shouldn't just insulate just the cold surface to limit condensation.
John,
There is increasing concern in the building science community about attics insulated with open-cell spray foam on the underside of the roof sheathing. Lots of reports are coming in about very high indoor humidity levels in this type of attic.
I have written an article on this issue. Here is a link: Open-Cell Spray Foam and Damp Roof Sheathing.
The issue is concerning enough that I will be writing a follow-up article on the issue in the next few weeks. In the meantime, here is my advice:
1. Any GBA readers (in any climate zone) who are thinking of spraying open-cell spray foam on the underside of their roof sheathing should think again. Choose a better option: either rigid foam above the roof sheathing or closed-cell spray foam on the underside of the roof sheathing.
2. If you already have this problem, it is essential that you find a way to actively condition the attic (heat it in winter and air condition it in the summer). In most cases, this means installing a register from a forced-air heating and cooling system.
3. Some building inspectors are concerned about smoke traveling from this type of attic through the forced-air ductwork in the event of a fire. Suffice it to say that the code issues are complicated -- one more reason why open-cell spray foam on the underside of the roof sheathing is a much more complicated and expensive option than it first appears.
Thanks all. This was a retrofit, not new construction to answer Steve's question.
Is a single supply usually enough? Maybe ~ 80-100CFM? No return necessary? (I would like to avoid a return anyway since I assume the foam will have at least SOME level of off gassing for a while).
John,
Joe Lstiburek (from Building Science Corp.) advises supplying 50 cfm per 1,000 square feet of attic.
In most cases, it's OK if you don't have a return grille (because not installing a return grille allows you to avoid some of the code problems connected with the smoke issue) -- as long as your plan meets local code requirements.
Oh wow. This attic is around 2700 sq feet, so that would be 1350 CFM...?
John,
Sorry! That was a typo. I meant to write 50 cfm per 1,000 square feet of attic.
And to answer your first question, yes, just insulate the condensate line and be done with it. Even if you solve the other problems, you can expect some condensation on the outside of the condensate line.
How much benefit in terms of humidity drop might I expect if I were to start trying to remove the existing insulation in the attic? It is mostly old blown in insulation (20 years old). I read your article about trying to seal areas in the house that are vapor producing from communicating with the attic, but I was thinking that getting all possible insulation from the rest of the attic floor might help humidity?
Removing the old insulation on the attic floor will not improve the high relative humidity issues. It could even make them worse by lowering the attic temp a few degrees without removing any of the humidity.
A steady purging of the humid air with dry, conditioned air whenever the air handler is running is the right solution. A conditioned air rate of 100-150cfm isn't going to add much to the whole house load, but it will displace the humid attic air with much much drier air.
Ok great thanks. I have the HVAC company coming back out here Thursday to add a supply line to the attic space. Anything specific for me to tell them?
Last related question...what is the reason that it's not recommended to just put a supply in my encapsulated crawlspace to deal with the humidity there as well as the attic? My crawl has 12 mil vapor barrier with taped joints and closed cell on walls, open cell in rim joist. So far the humidity is around 60 as long as the somewhat cheap Costco 70 pint dehumidifier is running. It would certainly be nice to get rid of the dehumidifier and just run a supply to both the crawl and attic.
John,
Q. "What is the reason that it's not recommended to just put a supply in my encapsulated crawlspace to deal with the humidity there as well as the attic?"
A. It is, in fact, recommended that you do exactly that. There are two code options for conditioning a crawl space. One requires that "the crawl space have a forced-air register delivering 1 cfm of supply air from the furnace or air handler for each 50 square feet of crawl space area" and that you install "a duct or transfer grille connecting the crawl space with the conditioned space upstairs."
Here is a link to an article with all of the details on this code requirement: Building an Unvented Crawl Space.
Ah ok....maybe I was thinking it was a previous post or answer I read in here that indicated putting a supply in the crawlspace was an option, but one of last resort. My HVAC company owner, who seems OK with putting a supply in the attic, resists putting it in the ctael, saying "that air needs to stay down there in the crawlspace". He wants me to use a dehumidifier only. It is acceptable to put a supply in the crawl and no return or transfer, as is OK for the attic?
John,
The building code requires you to choose one of the two options listed in my article on crawl spaces.
If you choose the option of introducing supply air from your HVAC system into the crawl space, then you are required to include a floor register that allows crawl space air to return upstairs. (After all, you are pressurizing your crawl space.)
If you don't like the idea of crawl space air entering your house, you should choose the other option (depressurizing your crawl space with an exhaust fan installed in the rim joist of your crawl space). If you choose this option, you still need a floor register to allow makeup air from upstairs to enter your crawl space.
When it comes to your attic, the code requirements are not yet settled. There is a huge code controversy on this issue, which will be played out over the coming weeks and months. I'll be writing about the controversy in an upcoming article.
Supply only but no return for attics? Maybe. (In that case, the extra air leaks through ceiling cracks into the room below.) Supply and return in the attic? Maybe, but that might be hazardous during a fire. Stay tuned for more information on the issue.
Removing the code issue, is there a reason it seems "OK" to pressurize the attic with supply air and rely on ceiling leaks to let the air back, but in the crawl it is not a good idea to pressurize the crawl and rely on floor leaks to relieve that pressure?
Thanks so much for your help!
John,
Providing a grille that allows air to transfer from these spaces is a good idea. Whether the grille is needed depends on the area of the leaks in the crawl space ceiling or the attic floor. Every house is different, but in all homes, it's good to install such a grille.
When it comes to attics, the smoke-spread issue (during a fire) is being discussed in regards to attics with exposed spray foam.
Of course, crawl spaces may or may not also have exposed spray foam.
Some building officials insist the spray foam be enclosed by drywall. Others don't.
There are lots of moving pieces to this puzzle.
Ok I will think of a good location for these grills. Is it a good idea to install the supply on one side of the crawl or attic and the transfer grill on an opposite side to encourage air movement across the space?
John,
Q. "Is it a good idea to install the supply on one side of the crawl or attic and the transfer grill on an opposite side to encourage air movement across the space?"
A. Yes.
Ok thanks for the help! My HVAC guys just left and I now and the proud owner of supply grills in my crawl and attic. Here's hoping that RH% starts falling!
Well//ugh..so far the only thing this has gotten me is two supply registers in the attic that are condensing like crazy. Its 74.5 degrees, 67% humidity, and 62.8 dewpoint up there.
Also, the registers they installed are directly on the supply trunks, not at the end of flex duct. Does that matter?
Installing the registers on the supply trunks is fine.
Dropping from (81F / 70%RH = ) 70F dew point to your current ~63F dew point is a good start. If the supply registers had been there since the end of the heating season the attic would have become dry enough to not be condensing at the registers by now.
Installing a room dehumidifier up there to help it along for the first week or so until the register condensation issue abates should work.
Since posting, I made the apparent mistake of putting the thermostats in the fan "on" setting instead of auto, thinking that may help to dry things out. Now humidity is up to 70% :(. I have read an article or two about that now and have put them back to auto. The crawlspace humidity is climbing as well after installing the vents... sheesh....
Would turning the A/C on lower than I normally would for this time of day help anything? Its usually around 74 in my house, but I can turn it to 71 or so if those units being on, despite the condensing, would be a benefit.
The crawlspace does have a dehumidifier, I will get one for the attic. Although the crawl humidity is rising as well now.
Measuring the temp of the air coming from the vents, I get 49 degrees. Not sure how I would ever get the dew point into that range.
Well this is very frustrating. Putting the vents in the attic and crawl has resulted in nothing except condensation on the grills and no lower humidity. It drops the humidity 2 or 3% but as soon as the air shuts off it jumps right back up. And unless I want to turn the upstairs units way down low, lower than I would even be comfortable with the temperature, there is no way to keep it running constantly to eventually dehumidify. This is running on a Trane xr17 5 ton unit. Maybe it is oversized....although every contractor who gave me a bid was worried about it being under-sized. Including the two BPI/ resnet companies that gave me a bid. So this is very very frustrating. I guess the only thing to do is get a serious dehumidifier for both the attic and the crawl. That doesn't sound very cost-effective in the short or long term. Any other thoughts or suggestions?
You have a big reservoir of moisture in the open cell spray foam. So it's not surprising that the humidity is only decreasing slowly. If you want to accelerate that process, a dehumidifier could help a lot. You should not need to run the dehumidifier on an ongoing basis, but it might take a month or two to get to that point.
Thanks again for the answers. Let me ask this in a slightly different way as well. If my goal is to just make the Attic adequate and to keep any moisture from getting into the foam and hurting my sheathing long-term, what relative humidity level should I shoot for? From the online calculator I used, it does not seem like it is feasible to get the humidity level low enough that the registers will never condensate, so is that a big issue? Or should I just shoot for whatever percent you guys tell me is low enough to keep moisture out of the foam and let that be it? The temperature in the Attic will end up being somewhere between 73 and 81 degrees because the upstairs of the house is normally about 73 to 77 degrees.
John,
You certainly want to lower the humidity enough to prevent condensation on the registers.
So based on this website's calculator ...http://www.dpcalc.org/ ... If I am at 75 degrees up there, and the register is blowing 49 degree air, I would have to be under 40 rh% to NOT condensate? How would that be doable without running a $50 or more a month high power dehumidifier bill? Or you guys are saying that once I got it that low it would stay there with minimal running of the unit? If not, it sounds like I just traded an expensive cooling bill for an expensive dehumidifying bill perpetually.
My experience was that the dehumidifier ran almost continuously for a couple of months. After that, it rarely turned on. If your unit is still running for hours at a time after a few months, I would be suspicious that the space had some sort of leak. But perhaps someone can offer a more science-based opinion.
Ok--- I will give it a try. Do I need to invest in a "serious" ($2000 -ish Santa Fe) dehumidifier for this attic space, or is it worth it to try a more Home Depot style unit, like this one?
http://www.homedepot.com/p/GE-70-Pint-Dehumidifier-with-Built-in-Pump-ENERGY-STAR-APEL70LT/205843987
I would buy the GE or some other Energy Star rated unit.
Ok I will head to get that today. I have a Aeon air 70 pint unit in the crawl that I got at costco... do you believe that unit is also adequate for my needs as much as the super expensive one would be if overall lowest cost to dehumidify...taking into account upfront cost and future ongoing cost, is my main objective?
John. Your initial post indicated you discovered a humidity problem after the new attic was completed. I'm assuming things were okay before this work was done. Perhaps you should shift the Aeon to the attic and let it run. If your humidity levels don't improve and start to trend down, I would look for a hidden leak or other source of water intrusion. You can continue to monitor conditions in the basement and buy a second dehumidifier if necessary. But I think we are hoping your new construction will dry out and your problem will go away.
Ok... but its not a new attic... its a 20 year old attic that I installed open cell foam in. I just discovered the humidity problem because before it was 120 degrees up there and 35% humidity. Now that it is foamed, the temperature dropped and the humidity increased.
Sorry. Your post said "a newly flamed attic." I assumed you meant a newly framed attic. Perhaps you are more comfortable with keeping the Aeon in the basement. If so, I would spend $200 before spending $2,000 on a new dehumidifier. If you able to dry out the attic, your home's HVAC system may be able to keep things in check with the new supply and transfer grill.
OK thanks again, - ha i never even noticed that typo - probably from typing on my phone. That should have been a newly FOAMED attic, lol.
I have that GE dehumidifier that I just bought at Home Depot running up there now... will post back how well it does. I guess I will just set it at around 45% and see if it can get it that low.
This stuff is just crazy. So I got the GE 70 pint from Home Depot and installed it yesterday. It brought the humidity down to 55% yesterday, and all the way down to 51% overnight until I guess the bucket filled up. I emptied the bucket this morning when it was 55% again, and it has been running all day. I also turned down the air so it would run much more today. Through the day, it ROSE to 55 and now 56% despite the AC and dehumidifier running much of the day. Not sure what else I can do. The dehumidifier also heated it up a great bit in the attic.. up to 86...it had not gone over about 80 recently.
John,
Don't give up -- you're gaining on it.
I certainly hope that you aren't ventilating your house very much with any exterior air. If you have an HRV, ERV, or any exhaust fans, make sure that you are ventilating as little as possible.
Nope no intentional ventilation. Its a 20 year old house with white flext duct for the exhaust fans. I can only assume that the foam contractor didnt plug the ends. But those fans aren't on right now anyway. The only intentional holes in my roof are those for the two PVC pipes for the direct vent furnace in the attic, the chimney from the fireplace, and the metal hot water heater flue.
The wood windows are 20 years old as well and I am sure dont seal 100%, but the main living area of the house stays almost exactly 50% humidity all the time. Its just the attic and crawl that want to creep up. The attic does have a single small operable window in it.
So weird that it got down to 51% so easily yesterday, and then after it was allowed to "come back" a little, it cannot get it below 56% today... and its not humid outside.... thanks for all the help... its fun to learn this stuff but then frustrating when doing the "right" things doesn't seem to fix anything!
What I think is happening is that heat is driving moisture out of the closed cell foam. That includes heat from the dehumidifier and heat from the sun on the roof. That's frustrating, but as Martin says, you are making progress. You want to get that moisture out of there. Then it will take less effort to maintain low humidity.
To prevent moisture from accumulating to potentially roof-rot levels is a matter of controlling the RH in the WINTER, to a dew point of no more than ~38-40F or so (30-35% RH @ 68-70F) for climate zones 5 or colder. That can be relaxed a bit for warmer climates, but humidifying to 50% RH in winter would be a mistake in any US climate except maybe zone 1.
The average temp at the roof deck in summer is much higher than in winter, which drives the moisture out of the roof deck and into the foam.. Open cell foam is fairly vapor permeable, which is why the attic humidity varies over the course of the day, and from day to day. Days with more daytime sun and higher daytime outdoor temps drives more moisture out of the roof deck & foam than cloudier & cooler days. But if you keep dehumidifying the attic, eventually the average moisture content of the attic air will track that of the conditioned space.
Once the moisture has been purged with summertime dehumidification efforts, a class-II vapor retarder (or smart vapor retarder) on the interior side of the foam will limit the amount of moisture that gets into the foam & roof deck over the following cycle. At 2" or more closed cell foam becomes it's own Class-II vapor retarder, which would pretty much make the roof deck moisture safe, but if there were open cell foam or fiber insulation to the interior side of the closed cell foam there is still some potential for moisture cycling. But even without the vapor retarder, having the supply registers in the attic will start the drying season much earlier in the year, and you shouldn't see 70%RH+ spikes in the attic air even on sunny mid-spring days.
Ah I see... that makes me feel better that its winter and not summer that I need to be "worried" about. I am in Nashville, TN, by the way.. so zone 4 I guess...
Its funny that right now it "seems" like having the air conditioner OFF makes the humidity drop faster in the attic while the dehumidifier is running than with it on.
A few related questions if you don't mind:
1) What would be a good choice for a class II vapor retarder if I wanted to go that route? This attic is cathedralized and has a few interlocking sections that communicate with each other, so the install may be slightly difficult depending on what it is. Also, is a "perfect" install necessary... as in if a section is "missed", does that let moisture into all of it and then not be able to dry, creating a bigger issue than there already was?
2) Why is it that it is said that once you get a space dehumidified, it is easier / cheaper to keep it that way? Why does the dehumidifier not have to run basically just as long to keep a low RH% point as it did to get it there?
In addition to those last two questions above....for right now, in the summer here in zone 4, what RH% should I shoot for in the conditioned attic and crawl? I dont want to overdo it and pay to dehumidify when I dont need to. I believe the condensing has already quit in the attic when I got to around 50%. Any reason to take it lower right now?
John,
50% or even 60% should keep you out of trouble.
That sounds doable... great! That is for both the foamed attic and the encapsulated crawl? So I should not be concerned as long as either are 60% or lower? Or does one have a slightly different goal than the other?
I see it commonly posted that after an initial drop of getting the humidity down where I want it, in this case I have the crawl set at 55 % and attic at 50%, the dehumidifier starts having to be on and work less and less. What is the cause of this? Why does it not take just as much work for it to KEEP the humidity at a level that you want it? I certainly hope this happens for me, since even though the humidity is at the levels I set it at, both dehumidifiers run continuously all day every day, with just a minute or two break here and there. What will cause them to eventually stop needing to run so frequently? (other than a seasonal change of course). Its funny, the main level and upstairs of my house has continued to ALWAYS be between 45 and 55% RH without me trying to do anything to it while I continue to fight the encapsulated crawl and foamed attic.
John,
If a space has a long-standing humidity problem, everything is damp. The contents of the room (furniture, books, items sitting in boxes) are damp. The drywall is damp. The flooring is damp. The studs, joists, and rafters are damp.
It takes a lot of dehumidifier runtime to pull all this moisture out of the air, the furniture, the objects, the drywall, and the framing lumber.
Once you achieve 50% RH, staying there is a piece of cake.
Right now, you are driving uphill, starting from a dead stop. Once your car hits 65 mph, it won't take as much gas to cruise along as it did to accelerate.
Ok great -- I will just "keep the faith" for a while then. I am glad it has no issue now staying at the humidity I set, although I will be much happier when I start going into the attic or crawl and finding the dehumidifier OFF instead of always running! :)
Now that it's dried out substantially- regarding Class-II vapor retarders, "vapor barrier latex" primer paint on gypsum board works, and would provide a thermal barrier against ignition, enhancing fire safety.
In area where that's not possible, 2-mil nylon (Certainteed MemBrain) detailed to be as air-tight as possible also works. The sheet nylon solution is really a variable permeance solution- it behaves as a Class-II vapor retarder when the RH of the attic air is low (as it would be in winter), but becomes more vapor open, Class-III or higher if the air between the foam and vapor retarder reaches mold-growth levels, which allows the roof deck to dry out rapidly as the warmer temperatures of spring arrive. If you can't find it in stock locally, the big orange & blue box stores are now carrying it online. Alternatively, Intello Plus is a bit more rugged as well as more vapor tight than MemBrain, but also has a favorable variable permeance characteristic. Don't try to substitute polyethylene sheeting here, or you'll have a moisture trap on your hands.
Ok thanks. I will look into this... although this would be a very difficult install for me. The cathedral attic ceiling reaches 15+ feet high in places, and the foam is incredibly lumpy, out well past the rafters in most places. I assume there are people around here that could do the job... although I had planned on being finished with expensive attic treatments...
Guess I am going to try the Santa Fe Advance 2 model for the attic and then for the crawl as well. THe current GE one just runs all day and that has to be sucking down a ton of power. Plus its making the attic almost 10 degrees hotter than it would otherwise be. I now understand that the box store dehumidifiers are rated at things like 70 pints a day at SATURATION, while high dollar dehumidifiers are rated at higher ratings like 90 pints a day at REAL conditions, like 80 degrees, 60% humidity... so that apparently is the huge difference in price and performance. So hopefully once I get this new one it will be a set it an forget about it affair.
As I recall, it took me a few weeks to reduce moisture levels below 50%. The Santa Fe might get you there faster, but one (much less two) seems like overkill unless you have an ongoing moisture issue, which would be a bigger issue.
No I am already there... 50% was easily achievable, but the thing almost never shuts off. All day everyday. SHuts off for about a minute every once in a while then right back on. The "two" would be one for the attic and one for the crawl.
Seriously- this is using sledgehammers to swat at flies!
Once the roof deck is dried out and a vapor retarder installed there will be no appreciable quantities of moisture to pull out, and the moisture burden would be manageable with the air conditioning.
BTW: ANSI/AHAM DH-1-2008 test conditions for dehumidifers is not at saturation, or anywhere near saturation. At 60% RH @ 80F is a wet-bulb temp of 63.9F, which results in many more pints/day capacity than test conditions, and higher liters/kwh efficiency. A higher dry bulb temp mitigates frost-up, with a lot more moisture available for removal than in a typical cold climate basement dehumidification application where 60% RH @ 65F is the goal, a wet bulb temp of 56.5F.
The very reason it's heating up the attic is the high wet bulb temp and the heat of vaporization of the water it's condensing. A bigger or higher efficiency dehumidifier won't change that by very much at all- the bulk of the heat is from the condensed water, not the compressor & blower motor.
If you're already at 50%RH, it's time to just turn the dehumidifier off and let the AC handle it.
Well...if I turn it off, the humidity will rise back into the 60s pretty quickly. If the HVAC were doing any kind of decent job handling humidity in the attic, wouldn't the dehumidifier already be shutting off for much longer periods at 50% instead of running all the time? I don't think the HVAC does much for this. Heck, the temp stays constant for a long time without the HVAC being on. What is a good way to try and troubleshoot where the humidity is coming from in the attic? My house stays between 43 and 49% humidity without me trying to treat it at all, but obviously the attic behaves much, much differently. What are possible causes? From my examination the foam job did a very good job of sealing all areas....
It looks like you've been running it for 11 days. I don't really know how long it takes to dry out the foam and the roof deck, but the usual advice about given hardwood flooring time to acclimate is 2 weeks. And that's without it being covered by foam, and without it being as wet as your materials may have been to start. So I would give it at least a month before considering upgrading the dehumidifier.
We are assuming that you don't have a hidden roof leak or something like that replenishing the moisture up there regularly. I'm not sure whether that has been examined somewhere in the 50 comments here, but if it hasn't it's worth a visual inspection from above to look for any likely trouble spots.
I wouldn't think that anything except the foam itself would have been wet to start. Like I said, this attic was at 30% humidity or so for a long, long time before I foamed it. Thats when it started rising and I got the dehumidifier pretty quickly.
I did turn off the dehumidifier last night and have kept it off. The humidity has moved only slightly since then, from a low of 49 to 53 now. Lets see how it does through the day today. It is supposed to be quite hot and muggy here in Nashville today, if outdoor humidity affects this foamed attic. It seems as though I read that indoor "people induced" humidity is supposed to be my bigger concern now, though. I also wonder why I ALWAYS find that dehumidifier in the attic running if the humidity is only barely rising without it going. Not sure why that is happening.
It's not the foam (which is NOT hygroscopie) that's wet, but rather the roof deck which far more moisture buffering capacity than the foam, and it the most likely reservoir that needs to be depleted.
When the roof deck is cooler overnight it is like re-adsorbing some of the humidity it released during the day. When the sun heats the roof (to well above the outdoor ambient temp), the moisture gets released at a fast clip, but impeded somewhat by the vapor redardency of the foam.
The dehumidistats on portable dehumidifers are not precision instruments, and the design & operation algorithms of some dehumidifiers short cycles them by reading the artificially high RH inside the case of the unit due to the wet coils as they warm up after a cycle, rather than measuring the RH of the room instead.
Mind you, the wet bulb temp of 50% RH 80F air is still 66.5F, which is still plenty high for efficient dehumidification compared to the typical cool climate basement applications.
When reporting the RH numbers it's important to report the temperature to which the humidity is relative. Rising from 49% @ 80F to be 53% at some lower temp takes a mere ~1.5F drop in temperature. The absolute humidity can be expressed either in wet bulb temp or dew point temp, but the relative humidity is always relative to a temperature.
Couple of pics
Pics are just of a small section of the attic, but the only one with a window. Total attic floor sq footage between all of the connected areas is probably in the 3500 -4000 sq ft range. Some of the areas only connect to the main middle attic area through relatively small openings. Large enough for a person to get through, but thats about it.
With the dehumidifier off, it has risen to 65% at 76.6 degrees currently. Started at 72.7 degrees and 49% at some point last overnight.
There is the single window that I mentioned in the attic. Could this be a significant source of humidity?
The window is not likely to be the source of humidity, but the fact that the RH is rising with temperature during the day is an indication that it's likely to be moisture still being cooked out of the roof deck.
At night the roof deck is several 10s of degrees cooler than during the day leading to much lower vapor pressure drive at night. Roof decks typically drop down to near the outdoor dew point at night (below the ambient air temp) due to radiating into the night sky, but during the day solar gain raises the temp of the roof deck to well above the ambient air temp, which is usually higher than at night.
Your current RH of 62% at an indoor temp of 75.6 is a wet bulb temp of 66F, a dew point of 62F. If the outdoor dew point drops below 62F at night, the roof deck will be re-adsorbing some of the moisture in the attic, or at least won't be driving much moisture out at night.
Thanks again for taking your time for this analysis... so given the large rise in RH% during the day, should I be running the dehumidifier all day, or leaving it off and let the natural cycle take place, assuming it will lower at night? I like the plan you gave of the vapor barrier, but given the extremely lumpy and uneven install of the foam, coupled with the very large space and very high areas, the installation of that material seems unlikely, at least for a while. Should I just plan on having to run a dehumidifier often up there, or something else? I have to think that a great majority of the foamed attic installs out there simply never think about this stuff and are "mostly (?)" fine... so I don't want to spend a bunch of money and monthly energy expense worrying about it if it is not truly necessary, but I don't want to leave it incorrectly done either. What is your opinion on running the dehumidifier? Since I can get it to 50% easily, but it just comes back, it is truly drying anything out? Since as you say it may just still be coming out of the roof deck, I assume the roof deck simply gets more moisture every day? I dont read a lot about other people with foamed attics having to worry this much about the humidity up there... or perhaps I am just not reading all of the articles.
If you now have ducts supplying conditioned air into the space the drying season will start much earlier in the season, and your peak RH will likely be low enough to not need seasonal remediation with a dehumidifier, (even without the vapor retarder.) In a few months the daily average temp at he roof deck will be below 65F and the AC won't be running as much and when the heating season begins for real the average temp at the roof deck will be low enough that it'll keep the attic dry (by taking up the moisture into the roof deck.)
In late winter on into spring the daytime RH will start rising, but with a steady supply of conditioned air it will be under steady dilution by drier air, and it will probably keep up, and stay well below 70% RH during the daily peaks even before the air conditioning season begins. Start checking the attic RH periodically in early spring, particularly on warmer or sunnier days.
From the pictures it doesn't look like it would be too tough to install some 1x furring through-screwed to the stud edges on which to hang a 2-mil nylon vapor retarder if the conditioned air feed doesn't seem to quite do it. Alternatively, a 1" shot of closed cell foam (about 1 perm) would be a sufficient vapor retarder, though quite a bit more expensive than 2-mil nylon.
John. Do you know how many cfm your supply vent is delivering to the attic? Just curious if it is enough ventilation. I imagine you have read Martin's new article on High Humidity in Conditioned Attics (https://www.greenbuildingadvisor.com/articles/dept/musings/high-humidity-unvented-conditioned-attics).
Just read the article... hey I am famous! :)
I am not sure what the CFM would be - its a 4x6 grill and I think the installers "guestimated" in the 100cfm range each. Who knows. There is one grill attached directly to the supply trunk on each of the two zones of HVAC ductwork in the attic. No return of course.... My opinion is that these registers dont do much at all to help the issue. The only thing they do it get condensation on them.
So I guess I may just be left to having to condition the air up there with a dehumidifier, indefinitely :( .. I would have probably bitten the bullet and installed closed cell foam had I known about these issues.. :( Even though its more expensive. Hopefully the extra efficiency of the house will more than offset the dehumidifier cost. Probably worth it to buy the expensive high efficiency Santa Fe ones if its going to be needed for the long term.
But it won't need to be conditioned with a dehumidifier next year. This year you were playing catch-up, due to the fact that the attic was allowed to stagnate. Now that you have some conditoned air exchange it's unlikely to reach anywhere near the levels that you saw this year.
And I stand by the "sledghammer" analogy regarding the Santa Fe for dealing with this moisture problem. Unless there is real source of new moisture entering the attic on an ongoing basis there's no way to rationalize something with that much capacity just to accelerate drying from wintertime moisture accumulation. With the conditioned air exchange with the attic the drying season will start no later than March, not half-past August, and peak humdity will never reach the proportions you saw this year.
You've already proven that a portable dehumidifier can keep up with the load even WITHOUT the conditioned air. It's unlikely to be needed next year, but even if that's the case it will see nothing like the duty cycle you are currently experience. You've been trying to pack 4-6 months of drying season into a couple of weeks with the portable dehumidifier, and it's actually doing the job.
Ah maybe the light is going on a bit... I guess I assumed that if I got it to 50% humidity, then that is the same 50% humidity as any time of the year. You are saying that even though I get the air to this humidity level, there is so much pent up moisture that it just comes back. But after this coming winter none of that pent up moisture will be there, and I will only have to deal with the daily moisture, which you think the air conditioner will handle. I guess I became discouraged that it would handle it since the HVAC seems to do so little about it now unless the dehumidifier is running.
It probably won't be anywhere near 75.6F in the attic in winter (unless you're heating the house to that temp or higher. At a lower temperature the absolute amount of humidity in the air is lower, and in winter you ventilation air will be MUCH drier. The attic air and all conditioned space air should be well under 50% RH in mid-winter, probably under 40% RH. If the house isn't under 40% RH @ 70F in winter you need to increase the ventilation rate in winter to bring the average under 40%, under 35% would be even better.
The higher the wintertime RH, the greater the amount of moisture build up in the roof deck will be, and the longer it will take to purge it. But with a steady ~100cfm of conditioned air whenever the air handler is running, the roof deck WILL dry out substantially, well before half-past August. You may still see some RH cycling, earlier in the season, but the peaks won't be insanely high, and it may need no other remediation. Keep checking, and you'll know if you're already done, or if it needs anything further.
The human-healthy range goes as low as 30% at the low end ( below which airborne viruses become somewhat more contagious. ASHRAE puts the comfort limit at about 25% RH at the dry end, below which chapped lips and split nails become more prevalent. High end of the human-healthy range is 50% RH for those with dust mite allergies, 60% for the rest of us, above which skin fungus and other issues begin to creep up. ASHRAE puts 65% RH at the high end for comfort, and that is still a pretty reasonable humidity limit for mold growth. Growth rates don't really go stratospheric until ~70% RH or higher.
I didnt know those upper and lower limits.... great info! I actually am, supposedly according to the tests, allergic to dust mites, so keeping it at the upper 40s or lower all the time should be a benefit for that as well. Sounds like right around the 40 mark, year round, would be a great target for crawl, attic, and main floor? That is not too low to split moulding or cause the hardwood floors to shrink, correct?
Dehumidifying to 40% RH in summer when the moisture content of the ventilation air is higher is both expensive and unnecessary:
The average temperature at the roof deck in summer is much higher than the dew point of 60% RH/ 75F air (=60F). As long as the average temp at roof deck is higher than the dew point of the attic air, it won't be accumulating moisture. If you don't have dust mite allergies 60% RH is fine.
It takes either extremely dry (arctic) air or a rapid and severe shift in humidity to split the trim or dramatically shrink the wood flooring etc. Even if you kept it at 65%RH in summer and drop to 25% in winter it's unlikely to be an issue. But 75%RH / 10% RH. might be. Variations of humidity within the human-healthy 30-50% (or 60%) range will not have a negative impact on the woodwork.
The biggest mistake you can make in this situation (and a common one) is to actively humidify to keep it a 40% RH or above in winter, since the average temp at the roof deck in winter is likely to be colder than the dew point of 40% RH/ 70F air (= 45F), and would thus be taking on moisture.
So should I be actively humidifying the first and second floor of the house at all in the winter? I have no idea what the humidity will get to here during that time yet, but the first and second floor are already in the low 40s (42 is the lowest I have seen so far). So if it is low 40s already when I am not even TRYING to get the first and second floor humidity down, I assume it will get much lower in winter. The only dehumidifying I am doing right now being the crawl and attic. Both are set to 50% on the dehumidifier's settings. I believe if I set the dehumidifiers to 55% or 60%, I will be back to getting condensation on the supply registers and HVAC drain lines in the attic and crawl.
I will be buying a new first floor HVAC unit soon and I "was" planning on adding a humidifier to it when it is installed. Is that something I should forgo?
John,
Q. "I will be buying a new first floor HVAC unit soon and I was planning on adding a humidifier to it when it is installed. Is that something I should forgo?"
A. Yes. Forgo humidifiers! They are dangerous and put your sheathing at risk of moisture accumulation.
Take the money you saved by not buying a humidifier and use the money for air sealing work. Limiting infiltration and exfiltration will help solve your wintertime "dry air" problem.
Active humidification is (almost) always a mistake! Unless prescribed for medical reasons DON'T!
In winter the outdoor air is usually dry enough to keep the indoor humidity bounded by adjusting the ventilation rate. In typical houses bumping up the ventilation isn't usually necessary to keep it under 40%, but in tight houses you may have to pay a bit more attention, or run the ventilation under dehumidistat control to keep it under 40% for the coldest 2-4 months. For now, just monitor it in winter, and adjust accordingly. Only when the average daily outdoor temps are below 45F do you need to make sure it's no more than 40%, and in a zone 4A location that might not be until the end of November or half-past December.
But track it for awhile- take the daily high/low temps add them together, divide by two. If it's under 45F, it's time to verify that the indoor RH is under 40%,
In general, adding only a forced air supply or return will pressurize or de-pressurize a room. Not a great idea for energy use or wall moisture.
Dana I sure hope you are right about it being a TON easier to keep the humidity down in the attic once I go through the upcoming drying season and into next summer. I did end up getting the Santa Fe Advance 2, and it of course does a great job getting the humidity down to wherever I tell it to, but it still runs almost all of the time. During the day especially. If I tell it to go to 45% humidity, it does, and runs all the time. If I tell it to go to only 50% humidity, yet again it does, but still runs all the time. It seems that the attic just catches up to whatever the setting is and the dehumidifier just keeps running to keep it there.