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Humidity in a Spray Foam Attic


Spray foam insulation can solve problems that other insulation materials can’t. One of those problems is moving the building enclosure from the drywall ceiling of a home to the roofline. By doing so, you bring the attic inside the building enclosure. But what happens to that attic space when you move it inside? Dr. Joe Lstiburek has been saying for a while now that we can’t just ignore the space. He says we shouldn’t be calling it a sealed attic or encapsulated attic or unvented attic. We should be calling it a conditioned attic. Here are some data to show why that’s true, at least in a humid climate.

A couple of years ago we got called to investigate a house with spray foam insulation on the roofline. The homeowners complained of a slight odor in the house on hot days in the summer. This wasn’t a new foam job. They had remodeled their home five years earlier, added a second floor, and had the attic encapsulated with open cell spray foam insulation.

After talking to one of the owners for a while, we took a look. Before we left that first day, we installed four data loggers to measure temperature and relative humidity. They were placed in the following locations:

  • Outdoors
  • Living area, second floor
  • Attic floor
  • Attic ridge

The graph below shows the dew points for the four locations.

The dew point temperature at four locations in a house with a spray foam insulated attic
The dew point temperature at four locations in a house with a spray foam insulated attic

Dew point is a better measure of what’s happening with the humidity because it shows whether or not the actual number of water vapor molecules changes over time. Changes in relative humidity don’t tell you if you’re getting more or less water vapor. You have to look at both the relative humidity and temperature to see that.

The graph above raises a lot of questions, but let me just say a few things here about what you see there.

  1. The outdoor dew point ranged between a low of about 64° F and a high of 74° F.
  2. The indoor dew point ranged from about 49° F to 54° F. For reference, the indoor design conditions recommended by the Air Conditioning Contractors of America (ACCA) are 75° F and 50% relative humidity. The dew point for those conditions is 55° F. Apparently, the living area is nice and dry.
  3. The attic floor dew point ranges from about 49° F to 70° F.
  4. The attic ridge dew point ranges from about 48° F to 85° F.

I’ll come back with a followup article to write more about this, but I’ll point out two important facts about the data above:

  1. The humidity at the top of the attic gets much higher than the humidity at the floor of the attic.
  2. The humidity at the top of the attic gets much higher than the outdoor humidity.

What do you think?


Read the followup article:  High Humidity in a Spray Foam Attic, Part 2


Related Articles

Will Open-Cell Spray Foam Insulation Really Rot Your Roof?

Dew Point — A More Meaningful Measure of Humidity?

3 Reasons to Remove Attic Floor Insulation in a Spray Foam Attic

How NOT to Retrofit a Furnace in a Spray Foam Insulated Attic


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This Post Has 54 Comments

    1. Hi group I need help one of
      Hi group I need help one of my clients did close cell insulation at the roof raptors and he still have the floor fiber glass and it’s very humid and he wants do flooring it’s very humid and hi doesn’t have any kind of ventilation or any like that what option I can do to do this place nice and comfortable

  1. We started monitoring sealed
    We started monitoring sealed , uh – conditioned attics (I actually don’t like that name because it implies that we are keeping the attic at set point) in about 2005 after a bas experience with a house on the beach. As with a sealed crawl, you want to know what’s going on in that space before it becomes apparent from inside the house, at which point it is likely to be a disaster. What we found is that these spaces (in our climate of coastal NC anyway) ran very high humidity levels even when everything is done really well with no defects in the foam sealing. So we started providing drying potential via a 6″ collar on the HVAC supply trunk line with a manual damper. We have also used dehumidifiers (not our first choice) and air from the house, depending on the situation. And a web-based humidity/dew point monitor with alarm to let us and the home owners know if it is out of spec – before there are any associated problems.

  2. Hi Allison,
    Hi Allison,

    I’m unable to come up with an explanation for these diurnal increases in actual humidity in the attic and thus am wondering if it might be “measurement error” or perhaps “calculation error.” As you know, whatever loggers you are using are measuring temperature and relative humidity (RH), with the absolute humidity – dew point temperature in this case – then being calculated from that data. Back before the Onset loggers that I use included the option of reporting and plotting dew point temperatures I created a spreadsheet to calculate DP. I recall it being a fairly complicated calculation. I also recall that I discovered a number of different ways of doing that calculation, each based on different assumptions. Thus my questions: 1) Are these DP temperatures being done by the logger software you are using, or by your own calculations? 2) Is there any chance that the calculations are giving incorrect results because the temperatures are outside the realm implicit in the underlying assumptions?

    Or… maybe I’m missing the source of the increased moisture here, in which case I eagerly await your explanation. I’m always learning new things from you and hope it’s no different this time.

  3. In North TX, we spray 2″ CC
    In North TX, we spray 2″ CC foam and 7.5″ OC foam, R38 min., against the roof decking, and then we install supply vents at 1cfm/150sf to move air and CONDITION the attics. It seems like a good solution.

  4. #1 No supply/return duct in
    #1 No supply/return duct in the attic space to condition the attic space.

    #2 Water vapor laden air rose from the 2nd floor bathrooms/laundry into the attic space.

  5. Thanks very much for talking
    Thanks very much for talking to a broader audience about this. It also appears from your chart that the home does not have a continuous insulation barrier on top of the roof framing – this is one more reason for doing this – stick framing will act as stick framing will act – as a significant thermal bridge introducing a whole host of unforeseen issues to an ever tighter building envelope (which is still the ideal, I contend). If you can eliminate that thermal bridging in the attic you will significantly negate this issues you are seeing in that attic. For almost 10 years we have been encouraging people to condition an attic space if they were going to seal it – for just this reason (we mostly operate in a mostly humid climate – Southeastern US). Open cell spray foam is like insulating with a sponge – it works great when it is not damp/wet. EPS foam is only a little better if under 1.7 lb density (and at that point, according to GBA, it acts as OPEN CELL FOAM) – and this is where most of the EPS industry operates, in the 0.9 to 1.5 lb density range. It’s not rocket science, I think it’s common sense – if you are going to seal it up you need to condition it. (dedicated dehumidification can work but from our experience consumes more energy – you are throwing more money after more money) I think people are just reluctant to change or don’t think it is much of a problem. I look forward to your future articles on the subject.

  6. I am lucky to live in the
    I am lucky to live in the relatively dry climate ( and relatively dry humor ) of Colorado, but I know we don’t get what we expect, we get what we inspect! I would recommend an attic humidity activated ventalation fan, which would warm up an over Air Conditioned Attic.
    In my dry climate (Denver CO) I recommend evaporative coolers, to help us keep our cool. (and our dry sense of humor) Even when homes have A/C, I recommend installing a humidistat. I recommend when it reads above above 60% use their A/C, when below 30% use their Evaporative Cooler. When between 30% and 60% they can choose their preference, and they won’t sweat it or regret it!

  7. Great post and I look forward
    Great post and I look forward to the follow-up. My critique is that the intro is missing some clear language explaining that moving the envelope from the flat to the deck effectively increases the cubic footage of space conditioning, consuming more heating and cooling resources. This aligns with JL’s point about calling it a conditioned attic. Cheers!

  8. Thanks, Bob. Regarding
    Thanks, Bob. Regarding increasing the volume, that really doesn’t matter the way a lot of people think it does. The heating & cooling loads come mainly from the building enclosure, not from the volume in the space. But yes, it definitely should be called – and operating as – a conditioned attic.

  9. Water vapor is lighter than
    Water vapor is lighter than air. In still air, it will rise to the highest point within a given space. Water vapor will rise from the living space through the ceiling into the attic space.

    On older homes, attics were vented, not to get rid of heat, but to get rid of water vapor. Bringing the attic inside the envelope doesn’t mean you can stop addressing the water vapor.

    To truly be a fully conditioned attic, it should be included in the HVAC or ventilation system. If it’s a nasty old attic, vacuum it out and add a high MERV filter on the return from the attic.

  10. Dale, although the lighter
    Dale, although the lighter-than-air idea sounds reasonable, I think there’s something else going on here. If you have a box with an isolated volume of humid air inside, diffusion should result in a uniform mixing of the water vapor and other molecules in the air. If it really were a matte of weight, then oxygen should be below nitrogen, too.

    But you’re absolutely right that we have to address the water vapor in spray foam attics.

  11. Here’s another thought re
    Here’s another thought re possible measurement error: Assume conditioned air in the living area to be at 50 deg F, with a DP of 52 deg. Those conditions yield an RH of 50%. Now assume that the attic space and the living area are in equilibrium in terms of amount of actual (absolute) humidity in the air, a reasonable assumption I believe. Now heat that attic air to 140 deg F, reasonable for the temperature at the peak of the attic. Keeping the absolute humidity constant, the RH of that air drops to 6%. That is pushing the limits of most RH loggers. (The accuracy of the RH sensor in the Onset logger I use falls off pretty dramatically at 10% and lower.)

    If the logger measured 14% instead of the correct 6% the indicated DP temperature would be 75. If it measured 17% instead of the correct 6% it would yield the DP temperatures in the 80s that your measurements indicated.

    I fear I might be grasping at straws but it’s the best explanation I can come up with.

  12. Not sure of the moisture
    Not sure of the moisture source, but I think the vapor drive may be from the attic into the upstairs; which explains the odor. (Vapor moving towards a cooler surface?)
    I agree with conditioning the space, but not before locating the source of the high humidity.

  13. Thanks for the response,
    Thanks for the response, Allison! Is your point that the ∆T isn’t as large from the attic to the outside as is from the rest of the home and that convection and conduction are lessened with proper foam application? Or is there more to consider here?

  14. Allison,
    I see very repeatable diurnal fluctuations in the attic dewpoints. I’m guessing that the dewpoint minimum, when the dewpoints at both attic locations are essentially equal to the dewpoint in the house air (meaning the density of water molecules in the air is the same for all locations)occurs sometime in the morning (and also evidence that the loggers are reading correctly). Assuming that the 7:12 time stamp means 7:12 AM. Sometime during the morning the number of water molecules in the air at both locations in the attic rises at a dramatic rate. It rises for several hours, levels off at a peak and then water molecules begin to be removed from the attic air in what appears to be an exponential decay, until they are once again equal at all locations. I can imagine two explanations:
    1) Scotty is beaming water molecules into the attic every morning and beaming them back out every afternoon at the exact rates to mimic a capacitance charge-discharge curve;
    2) or , more likely but less entertaining, the water molecules are going from the ad and absorbed state in the exposed porous materials into the attic air in morning as the sun hits the roofing materials and drives water vapor out of the sheathing, rafters, open cell foam into the attic air. There is very little ventilation in the attic, so the water molecules blown out of the porous materials by the solar heating raise the dewpoint in the attic into the 80s. I see no mold growth in the photo so I assume the air temperature in the attic is higher than dewpoint – The attic is hot during the day. When the sun goes down, the water molecules slide the energy gradient and are re-ad-absorbed by the porous materials in the attic. I guess that the 13th and the 17th (shorter peaks in the dewpoint) are cloudy days, which lowers the solar radiation reaching the roof, in turn lowering the temperature in the roof system materials. Dewpoint at the peak is higher because of thermal and molecular buoyancy. Dale got it in one.

    Last, where did the water molecules come from? They may have come from inside the house or outside the house. To figure out which I would need to know more about the roof assembly details, water vapor sources inside the house and occupancy schedules (e.g. is this a summer house, left vacant and unconditioned during the winter). I’m pretty sure they’re not beamed in by Scotty, though.

  15. It would be really helpful to
    It would be really helpful to know either the measured RH or the temp, or both to figure this out. Assuming we’re calculating Dewpoint like Td = T – ((100 – RH)/5.) and that the Attic ridge is something like 90F, then there would have to be a relative humidity of something like 70% at the ridge. Something has to be dumping a TON of humidity into the attic or there is a serious problem with the R value right at the roof and it’s super-hot right there. The Attic floor doesn’t look TOO bad. I have about 5″ of closed-cell in my attic and my min/max dewpoint numbers look like this: AtticFloor:52°/57°, AtticRafter: 52°/61°

  16. I would also like to see the
    I would also like to see the temps and relative humidity of all 4 areas graphed. Given the radical swings of both the attic floor and ridge dew points (compared to outside and lower “conditioned” areas) and looking at the photo – I would guess that the amount and application of open-cell foam is inadequate for this building. Note that you can clearly see some roof joist area, and even if it were completely filled it would not be enough to act as both a vapor barrier and proper insulation (assuming a 5.5″ joist). I’d also assume that the roof deck did not get insulated nor a secondary roof deck installed with an air gap to reduce thermal bridging and solar gain.

  17. Yes, but the atomic weight of
    Yes, but the atomic weight of molecular oxygen (O2) is around 32, that for nitrogen (N2) is 28, while that for water is only 18, so there will be a tendency for the water vapor to rise, much more so than for the O2 and N2 to separate. Same can be said for why we look for gas leaks below the pipe for propane, and above the pipe for natural gas.

  18. That’s sort of true, Walt.
    That’s sort of true, Walt. But it’s not the differences in molecular weights that make the difference. If all were uniformly distributed in the attic, I don’t think there would be much stratification. As Terry Brennan said, the water is driven out of the porous materials so the air where it enters is lighter than the surrounding air. Convection then does its job.

  19. David, I’ve got those graphs
    David, I’ve got those graphs coming in the next article.

  20. Dan, on the worst day (6/14),
    Dan, on the worst day (6/14), the temperature and RH at the ridge both got up to almost 90. I’ll show the graphs in my next article on this subject.

  21. Terry, your thinking aligns
    Terry, your thinking aligns with mine on this topic. Joe and Foster Lyons and I had some good discussions on this issue at Summer Camp a couple of years ago, and Joe’s been talking about it in his presentations lately.

    And I agree with you about the lack of plausibility in the transporter hypothesis. Such devices don’t exist…yet. Michio Kaku has said we might have something like that in a hundred years, though.

  22. Bob, yeah, there’s a bit more
    Bob, yeah, there’s a bit more to it. A vented attic gets hot in summer because of heat transfer through the roof. With a well insulated and air sealed ceiling, not much of the heat in the attic gets into the house. If you move the insulation to the roofline, now you’re limiting the amount of heat transfer into the attic. The cooling load doesn’t rise proportionally to the increase in volume. It rises a bit because there’s usually a bit more heat gain through an insulated roofline than through an insulated ceiling. I should write an article about this.

  23. Jake, but there’s more water
    Jake, but there’s more water vapor near the ridge than there is at the attic floor. Do you think the moisture is coming in through the roof?

  24. Bob, I can’t remember if I
    Bob, I can’t remember if I read it somewhere or not, but a conditioned attic acts as an interstitial space just like the space between two pieces of glass in a double paned window. If outdoors is 30 and indoors is 70, that large delta moves a fair amount of heat. If there is an interstitial space in between at 50, heat moves much more slowly. I suspect that’s part of why conditioned attics don’t add load like you would expect. They have certainly been messing with my head as I’ve started doing them in Cleveland where I still get looked at askance for doing it.

  25. I may need to get a bunch of
    I may need to get a bunch of Hobos or the like and try this in a closed cell attic… what would you expect the difference to be?

  26. Nate, since the moisture is
    Nate, since the moisture is going in and out of the open cell foam in this attic, you shouldn’t see the same kind of stratification of water vapor with closed cell foam. You may well have high humidity there if the air isn’t conditioned in some way, but it should be distributed more uniformly. We haven’t tested an attic with closed cell foam yet.

  27. My guess. The air at the
    My guess. The air at the ridge is hotter therefore has more potential to hold moisture. Moisture moves from areas of low potential to areas of high potential therefore the top of the attic is “sucking” the moisture from the house and making the living space abnormally dry which is not really a great thing health wise. I agree with encapsulating the rafters and ridge as was suggested and I would also like to see some return air drawn from the upper levels of the attic.

  28. We haven’t compared closed
    We haven’t compared closed and open cell in a scienfical sort of way, but we’ve been installing and monitoring both for years and the behaviors are similar.

  29. Lots of good comments here. I
    Lots of good comments here. I see a phenomenon going on here that no one has mentioned yet (EDIT: I see SteveM hints at it)…

    Due to thermal bridging (as Charles pointed out) and lack of direct conditioning, the attic is naturally warmer than the house and the ridge is warmer than the floor, especially late afternoon (indeed, I see Allison just posted 90F). The warmer air has more capacity to hold moisture, thus creating a significant vapor drive between attic floor and ridge. Thus the water molecules tend to pool at the ridge at mid-day. This explains how the ridge DP can be higher than the outside air DP. Then as the attic cools at night, the molecules redistribute, bringing the dew points close together.

    I’m guessing this attic isn’t very well sealed, which explains where the moisture is coming from. This could be a very leaky attic if the open cell isn’t uniformly thick enough to forum an air barrier (3.5 to 4 inches?). Leaks also increase stack effect, further exacerbating the delta-T between attic floor and ridge. This would increase vapor drive that pushes moisture up to the ridge during the day.

    What I don’t understand is why the dew point drops so low at night – even lower than the house the first day! This suggests direct conditioning in the attic at night (did you leave that detail out?). OTOH, I don’t see evidence of mechanical moisture removal in the curves.

    It would be easy to blame the logger calibration (dew point is notoriously difficult to measure accurately), but a 35 degree swing in DP without direct conditioning is hard to fathom. And the two loggers seem to be in reasonable agreement with each other at night, which seems reasonable since vapor drive disappears. Hmm…

  30. David, the attic is well
    David, the attic is well sealed. We tested it with the attic hatch open and closed and did some zonal pressure diagnostics. There was a little bit of leakage but not much. And no, there’s no direct conditioning of the attic, day or night, during the time we ran this test.

  31. Allison

    Nice article. I will keep reading as you post. We were just in Atlanta for another SPF humidity/odor issue.

    Take care and keep writing.

  32. Tired, but find it hard to
    Tired, but find it hard to not respond to a very interesting discussion, here’s my 2¢. I think that many of the suggestions are reasonable and I concur with many of the identified problems. However, I would like to share my perspective on the situation.

    Assumptions used: 1) Attic & living space have equal or close RH at some point in the day (probably early morning in summer) and 2) the attic was tightly sealed.

    As attic temperature rises, RH in that air mass would initially decrease, which would mean DP would rise and certainly not fall and would also be higher at the attic ridge than the attic floor. My coming from a cold climate, similar things occur in the winter when cold moist outside air is heated and RH of the heated living spaces drops. While the DP of the attic air might be below the DP of the outside air at some points, unless the surfaces that that the attic air contacts are below the DP, what problems could occur?

    Unless there was also a highly unlikely significant and rapid drop in outside temperature, I would expect things to normalize in the attic and living spaces every cycle, as the DP chart shows without any significant moisture issues occurring.

    Given a tightly sealed attic, the hotter attic air would also take up less volume with the result being some attic air would be forced into the living space below. This could easily explain the slight odor that is being noticed on a hot summer day.

    While instruments measured the temperature and relative humidity, the DPs shown on the graph must have been calculated separately. As the data from the instruments that supports those DPs is not presented, checking the calculations is not possible. However, I can see no reason of concern for the attic DP going up and down and varying from the living space’s DP, as there is obviously some air movement between the two given the “odor”.

    There could be other issues involved, but without seeing the home, the DPs shown on the graph are no surprise to me.

    BTW: While I believe that the attic was tightly sealed from outside air, I concur with others that the exposed and un-insulated thermally bridging rafters (in supplied picture) would certainly help to raise attic temperatures above what could be achieved with them covered with some foam insulation. That alone would help to drop the RH, DP, and odor, and could easily have been applied upon original foam application and may possibly be the simple solution.

  33. Thanks so much for starting
    Thanks so much for starting this discussion, Allison. It has been fun! And informative at so many levels. I hereby retract my hypothesis about data quality now that you’ve posted maximum temperatures at the attic peak. Note, however, that I’m not at all apologetic for suggesting the possibility that we were looking at incorrect data and not a real phenomenon. In my many years of working and teaching in this field of building science I have on more than one occasion ended up with egg on my face – coming up with explanations for observed phenomena which in the end turned out to be nothing more than errors in data acquisition. As a result, when faced with a result that is not immediately explainable, I first make every attempt to rule out data quality problems.

    And now, speaking of egg on my face, it appears I’ve done it again! I have in fact measured temperatures of 140 inside an attic at the peak, but those attics had insulation on the floor, not at all like the attic being discussed. Just another momentary brain fart.

    I’ve always been impressed with Terry Brennan’s work and I think he’s on to something here. Thanks again for starting this conversation, Allison. It’s apparent to me that there’s still much to learn about how foams work in these situations.

  34. As a secondary comment, what
    As a secondary comment, what Allison is seeing in this situation also occurs inside of EACH AND EVERY stud wall cavity of a home if there is an air gap larger than approx 5/8″. This is why open cell spray foam that does not fill – or almost fill – the cavity can be problematic over the long term and even over the short term can negatively impact the performance of the foam through convective currents set up inside of the wall cavities between the different temperature surfaces having air gaps of 5/8″ or larger. Again, what you are seeing here is also happening on a much smaller scale over a potentially much larger surface area.

  35. Root cause should be
    Root cause should be interesting. My numbers (with closed cell) vary so drastically from that, especially in RH, it would be good to know what I apparently did right 🙂
    BTW, I have about 16 months worth of data structurally very similar to this if you want some data from an Atlanta home with closed-cell.

  36. @David, thanks. My raw data
    @David, thanks. My raw data at home is based in C but I change it in my visibility tool to F for convenience. I was suspecting there was a brief period in the late morning where temp in my attic dropped below dewpoint, but now I think the trend line was due to calculation error. 🙂

  37. Bells ringing…”ridge rot”,
    Bells ringing…”ridge rot”, “vapor diffusion vent”

    Some (well, a few well-read) people in Florida are probably wishing they had some vapor diffusion vents right about now…

    It seems clear that everything inside the conditioned envelope (including sealed attics and crawlspaces) needs to be…conditioned.

  38. As builder if I’m sealing a
    As builder if I’m sealing a crawlspace or attic space I’m supplying that space with conditioned air. In the picture showing the drywall the potential for stale air from the living space to get into the attic are at every ceiling fixture, interior wall switch and bath or kitchen fan. All bad air with odor. Im surprised they don’t have ordors in the house for the same reason.

  39. Moist air is less dense than
    Moist air is less dense than dry air, and will rise. This can be verified buy looking at meteorology, as this is a significant force with weather modeling. I do not know if that is what is going on in semi-conditioned attics since the overall volume is so small, but it might be. I suspect that there is more than one simple thing going on here, and the details are quite varied since we really do not know as much about the water vapor permeability of construction materials as we would like – especially when we are talking about site-mixed spray foams. As far as solutions go, Joe’s vapor diffusion venting seems like a good approach to dig into further.

    Thanks for promoting conversation about this important topic!

    – Ron

  40. Just received this great
    Just received this great piece from Dr. Joe. I think it pretty well explains what these data were showing, just as Terry Brennan suggested. Thanks again, Allison, for presenting this problem to a wider audience. Here’s the link to Lstiburek’s take on this:

  41. Spray foam odor complaints
    Spray foam odor complaints often arise during warmer months and can have several causes. This thread is great but where is the odor coming from? Humidity? If its from the foam then there my be something else going on rather then humidity. I have oc in my attic and while I’m sure that the humidity varies most of the time the space feels very close to the temp and humidity level in the rest of the home. A foam odor complaint could be caused by off ratio or under processed foam…or some other application/product issue. Great comments. I’m curious what happened with the odor? Was it ever resolved?

  42. BSC is one of my 3 sites that
    BSC is one of my 3 sites that I visit regularly and as the article states he has spoken about this “ping pong” effect before. Hopefully he’ll get the code changed to allow conditioning in these sealed attic spaces.

  43. I have one story home, with
    I have one story home, with an attic converted to living space and a basement.

    I insulated the cathedral ceiling by:

    1) cutting and cobbling expanded polystyrene into the rafters, for six inches of eps between the rafters,
    2)another 4-6 inches of eps under the rafters, and
    3)5+ inches of mineral wool under that eps.

    I was able to remove the central air and furnace, and install a 5,000 window air conditioner. I like this because it frees up floor space and is simple.

    During summer I have found placing the air conditioner in the attic keeps the house cool and comfortable, while placing it in the first floor leaves the attic uncomfortably warmer and humid.

    I have assumed it is because the humid air rises, so having the air conditioner on the second floor is better at removing humidity.

    On another note, the attic is unventilated. There have been suggestions to install venting above the roof deck. I understand the need to monitor indoor dew point, but it seems that if the eps is tightly fit, and it is, along with overlapping layers below that, there should not be interior moisture reaching the roof. I cut the eps for a tight fit, and used a great deal of caulk and Great Stuff Pro spray foam to seal it.

  44. I have air handlier abd
    I have air handlier abd furnaace in attic.
    And have thought about have underside of roof spray foam insulated since attic is hot in summer and cold in winter. Home is rambling ranch built in the 50s.
    Also condered power roof vents.
    After reading articles here, have done neither.
    I do have 16 inches of insulation on attic joist.

  45. Steve, I agree with
    Steve, I agree with everything that you said. I tried to say what you said earlier, but with less clarity.

  46. Eldon,

    I still think a foamed attic is often a good idea. However if you still have reservations then I suggest:

    Seal the ceiling plain (wire penetrations, electrical boxes ect)
    Add insulation to R60
    Get rid of the power vents

  47. Dan Geist, I would be most
    Dan Geist, I would be most interested in your data with closed cell foam for a home in Atlanta, Ga.

  48. If I may add to this
    If I may add to this discussion, I had open cell foam sprayed under the roof decking. There’s about 7 and 1/2 in a phone between all of the rafters and at the Gable end. Prior to this there was Rockwall in the Attic floor. It’s been quite humid here in Upstate New York the past few days and most of the house stays at around 60 to 65% relative humidity. I have a whole house dehumidifier down in the basement that keeps the humidity below 59% and average is 55%. I have my central air system on the 1st and 2nd floor running at 59 degrees Fahrenheit. It feels nice and cold in most of the house. When I went up to the Attic a couple of days ago I noticed that the wood was wet, particularly along the edges of the rafters where it met the spray foam. I took my moisture meter and it was reading 54%. I have never had this problem in the Attic until the open cell foam was sprayed there. It seems to be trapping the humidity in the Attic. My humidistat was reading almost 80% humidity around the attic floor level so I assume it was even higher towards the ridge. I peeled back some of the open cell foam and the rafter was dry underneath it. So it seems to be happening where the rafter is exposed to air in the Attic. I’m trying to figure out a way to solve this before I start growing mold or having other structural issues because of this. I do have a supply vent going up to the Attic but it is very weak so not a lot of cold air is going up there. I don’t know if keeping the house so cold is contributing to the problem but I am contemplating putting an air conditioner up in the attic if that will help solve the problem. Any advice or thoughts would be appreciated.

  49. Very interesting info and
    Very interesting info and comments! I’m dealing with a similar challenge: open-cell SPF under the roof deck with a bad odor in hot, humid weather (CZ4a). I’ve had a data logger for RH&T in the attic since about a year before it was encapsulated, though having one at the floor and ceiling is better. Mine is about 18″ from the ridge. Might have to add a sensor.

    I have a slate roof and requested that baffles be installed under the roof deck. There are other defects with the installation, including SPF pulling away from the rafters pretty significantly (>1/4″), and some skippers at the ridge. Hot, humid air POURS out from those gaps at the ridge. I just touched them up with canned foam, but I doubt it’ll make much difference.

    I tried covering the SPF with plastic sheeting in desperation a few weeks ago as it started to get warm because I realized that I would have to a) reinsulate all of my ductwork (long story) and b) jettison the ERV if I were to revert to a vented attic. I wanted to see if that would lessen the odor, knowing that it couldn’t be kept due to being a vapor barrier in the wrong place. It did lessen the odor, but not enough. So I stuck in an exhaust fan to depressurize the attic as a band-aid measure. And sure enough, the plastic did have condensation all over the side facing the SPF on a hot, humid day. And, interestingly, some condensation on a small portion of the interior side near the ridge. I immediately removed it and the odor and RH both increased.

    So, I think Steve McKenna is right about the fundamentals: Hot air can hold more moisture. I’m clearly getting stratification in the attic, partly due to an air handler on the attic floor and uninsulated ductwork. But it happens on sunny days when the A/C is not being used, just to a lesser extent.

    The AccuVent baffles are channeling air heated by the roof to the ridge, which is no surprise. I’m still puzzled about the source of the moisture. If the roof heated the air in the baffles from 85º outdoor ambient to 115º in a closed system, the RH would fall. That doesn’t seem to be happening. On an 85º day (DP 71º), I’m seeing 81º (DP 80º, really) at the ridge. Does the SPF, rafter and roof deck really hold that much moisture?

  50. We spray foamed an
    We spray foamed an unconditioned/ unfinished attic space that sits above the garage. This large attic can be accessed by going through the laundry room of the house . Its a new construction home and we were told by the foam company it would get too hot in there so to go ahead and foam that room until we were ready to finish it. It was very clear we would not be finishing any time soon and already a few months in and it is a sauna in there. Humidity high. We live in Atlanta.
    What can we do now? We had plans to finish, but not for some time. There is no ductwork accessibility into the area and the soffit vents were covered up by the foam. There is no ways for this area to escape it appears. We do have windows in there. Any guidance as to next steps would be very appreciated. Thank you

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