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Condensation on foil-faced polyiso panels?

BvilleBound | Posted in General Questions on

I stacked a bunch of 2″ thick foil-faced polyiso insulation boards in the basement of our house, to prep for a major remodeling project. I returned a week later and noticed a puddle of water under the polyiso boards. (This happened in June, in a house along the shore in Massachusetts. The weather was humid, as usual.)

“Oh, no! A water leak!”, was my first thought. I examined all of the copper pipes, looking for the leak. No luck.

Then I picked up the polyiso board to move them, and noticed that the foil surface was wet. Voila! Water was condensing on the foil, which was well-insulated from circulating air and potential warmth.

This immediately raised a concern with foil-faced polyiso in a humid environment. The foil surface is very thin and super-insulated on one side. If the dew point is close to the air temperature, condensation occurs and a significant amount of water can collect on a polyiso panel.

Has anyone else observed this problem?

Mark Hays

GBA Prime

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  1. mackstann | | #1

    This is why it's important to air seal the foam when you install it. Otherwise, warm/humid interior air will get behind it, cool down, and cause accumulating condensation on the cold concrete wall.

  2. BvilleBound | | #2

    Dear Nick: Thanks for the quick reply. The potential problem, however, is not condensation on the concrete wall -- but condensation on the foil surface. If moisture could condense on the panels standing in the basement, the same thing could occur after they are installed, and condensation could occur on both sides of the polyiso panel, facing the concrete wall and facing the room.

    This could also be a significant problem with external installations of foil-faced polyiso panels. In a humid environment, moisture could condense on the outside and inside surfaces

    Has anyone run any tests on foil-faced polyiso panels, to determine when condensation occurs?


  3. nkurz | | #3

    Hi Mark --

    The usual answer would be that moisture condenses any time the temperature of that surface is less than the dew point, regardless of what the surface is made of. But maybe I'm not understanding your situation.

    Could you explain more about the moisture you found? How were the panels positioned? Were they stacked flat on the floor foil-side down? Leaning up against the wall foil-side in? How are you planning to install the panels in your finished project? How moist is your basement, and how will you be dealing with this?

    If you haven't seen them, these links might be useful:

  4. GBA Editor
    Martin Holladay | | #4

    Condensation occurs on cold, hard surfaces, like refrigerated beer cans.

    If you place a piece of foil-faced foam on a concrete slab, the insulation prevents the slab from warming up, so the concrete is cold. The foil facing closest to the concrete is also cold. If warm, humid interior air has access to either of these cold surfaces, condensation can result.

    If you install a layer of foil-faced foam against a concrete basement wall, it's important to seal the perimeter of the rigid foam, to keep humid indoor air from getting behind the rigid foam. If you do that, the back side of the foam (the side of the foam nearest the concrete) will be cold, but there won't be any condensation (because air will have been excluded). There will be no condensation on the side of the foam facing the room, because that side of the foam will be warm.

  5. BobHr | | #5

    I would be asking myself where is the moisture coming from? Was it moisture in the air? Was is moisture passing through the concrete? A combination of both?

    Think of it this why. With a wall the interior surface was closest to room temperature. As you go deeper into the wall the temperature get colder...assuming is cold on the opposite side of the wall. If at some point the interior surface of the wall drops below the dew point you get condensation.

    Stacking up the polyiso you have created a wall with air leaks. This allowed for water vapor to either pass through the concrete or moisture laden air to a cold spot. If this had been a stack of drywall you might have had moldy drywall.

    Now when the polyiso is pressed firmly against the concrete and sealed at the edges there will be no pathways for moisture to accumulate.

    Had this been a basement wall constructed using fiberglass you would have had moisture and mold inside the wall.

    This is a valuable lesson on the importance of air sealing. Learn the proper materials used to air seal and dont think any little air leak is to small. If you had a boat you wouldnt allow water leaks.

  6. davidmeiland | | #6

    If I'm reading right, the panels were leaning against the wall, not laying flat on the slab. I can think of a couple of possibilities not mentioned above, although my money's on Robert at the moment.

    One, the panels were cold when brought into the basement and the condensation occurred very soon after you put them there, Two, the bottom edges of the panels in contact with the slab got cold and the condensation occurred very close to the slab, not uniformly across the face of the panels.

    Or... maybe the panels had high MC when brought in, and the slab itself was actually the condensing surface, with moisture leaving the foam and finding the cold slab?

  7. BvilleBound | | #7

    Thanks for all of your follow-up. Here are some additional details:

    > The polyiso boards are the DOW Super Tuff-R brand from Home Depot, 1.875" thick, with foil on both sides.

    > The boards were stacked vertically, leaning against the stud wall in the basement, with one end resting on the bare concrete floor.

    > Condensation occurred on the entire surface of the boards, on both sides, not just next to the concrete floor.

    > The humidity was roughly 75%, a typical June morning. The humidity level was probably higher in the basement, with an uncovered concrete floor.

    Overnight, the air temperature falls and the humidity level and dew point rise, until they are separated by only a few degrees. The foil surface will cool gradually overnight, then warm more slowly than surrounding air after daybreak -- with nearly 2" of polyiso attached to one side. The result: condensation. (Just like the dew on my car in the morning.)

    Here are my concerns: (a) If foil-faced polyiso boards are installed in basement walls, what would prevent daily cycles of condensation on the room facing side? (2) If foil-faced boards are installed on the exterior of a house, what would prevent the same daily cycles of condensation on both sides? (Both scenarios assume a humid environment.)

  8. BvilleBound | | #8

    One more note: In case anyone is wondering, I have no connection to the building or insulation industries and vendors -- other than as a customer.

  9. Expert Member
    Dana Dorsett | | #9

    The average subsoil temp in MA is about 50F.

    The average outdoor dew point temp in June in coastal MA is about 55-60F, so even the ventilation air won't be helping you dry out a basement very much.!dashboard;a=USA/MA/Ipswich If you have no vapor barrier in a slab poured on dirt, or even wall backfill that is dirt rather than 3/4" screenings you can have high moisture inputs from ground moisture as well, raising the dew point of the basement air to well above the outdoor dew point. If the studwall is new, made of freshly cut timbers (not kiln dried) or was rain-wetted prior to installation, the wood itself can be a big source of moisture.

    The same moisture that is condensing (harmlessly) the foil facers is being adsorbed by any exposed (and moisture sensitive) wood- but since it's inside the wood you don't get to see it. If the studwall is up against the foundation wall, that wall is colder than the room, and insulated from the room by the iso, pushing the temp of the concrete closer to the deep subsoil temp, which is well below the dew point of even the outdoor air, ergo condensation occurs. If you put the iso against the foundation wall, the foil facers effectively block the contribution of room it blocks the contribution to the basement moisture, and the side facing the room will stay dry, since it's pretty much at the same temperature as the rest of the room. This is why when insulating foundations with a foam + studwall, the rigid foam board goes between the foundation and studwall, keeping the temp at the cold edge of the studs and the foam/fiber interface well above ground temp, and (except in extreme cases), above the dew point of the room air.

    Air that is healthy for humans has an upper bound dew point of about 55F, above which susceptibility to skin fungus and high mold-spore counts begin to ramp up. In a 68F June basement that translates to about 52% relative humidity. If the basement is only used for storage and as the utility room, 60% RH @ 68F (dew point of about 54F ) would still be OK, but beyond that some amount of mechanical dehumidification of the basement is called for.

    Overnight temperature swings in insulated basements are extremely small, much smaller than the above grade rooms, due to the much lower heat losses. (Basements have very little window area.) Even in a worst- case situation you will not get condensation on the room-facing side of the polyiso due to those temperature swings. Unlike you car, which is facing a sky with a radiation temperature WELL below the ambient air temp, foil facers in a basement wall are exposed to a radiation temp approximately equal to the temp of the facer. Dew due to the radiational cooling of the surfaces radiating their heat into space. That simply isn't going to happen in a basement unless the tornado takes the rest of the house away. Roof surface temps on clear nights are often 10F or more below the ambient air temp by morning, the low temp limit being the dew point humidity of the air. At the dew point the heat of vaporization of the accumulating dew keeps the roof temperature at the dew point, which falls only very slowly as the surfaces take moisture out of the air.

  10. BvilleBound | | #10

    Dana: Thanks for your detailed response. A few key points:

    > My concern with foil-faced insulation applies to (a) the interior surface in a home, and (b) the interior and exterior surface if the insulation panels are mounted on the exterior. In a humid environment, I do not see how daily cycles of condensation could be prevented in the early morning hours. With many square feet of foil-facing, this could generate a significant amount of water. Overnight humidity levels in our area often reach 80% to 90% at 6am, for example.

    > The only solution on the interior would be to install a whole-house dehumidifier. We purchased one after I saw the puddle under the foil-face polyiso foam panels.

    > Although you noted that condensation "...simply isn't going to happen in a basement...", this definitely occurred in our basement -- probably because windows were open to the cool night air.

    I am going to follow up with DOW directly regarding this issue, to see what they say about their foil-faced TUFF-R product in humid environments. I will let everyone know if they reply.



  11. BvilleBound | | #11

    Update: Spoke to DOW Tech Support

    I called the DOW Building Solutions tech support line and spoke to one of their representatives. I described what happened in the basement with TUFF-R polyiso panels, and he said, "Yes, there is a possibility that condensation could occur." He noted that he lives in Minnesota (the home base for Building Solutions in the USA) and was not as familiar with potential issues in a more humid climate.

    I noted that I also reviewed all of the product info, technical info and MSDS documentation for TUFF-R on DOW's website, and could find nothing that mentions condensation on the foil surface.

    He said that he would review this concern with their technical team, and get back to me.


  12. Expert Member
    Dana Dorsett | | #12

    You simply don't HAVE daily cycles of humidity in an air-tight insulated basement, and your concernes are unwarranted!

    The above-grade concrete will have somewhat higher moisture content at dawn that it does at sunset, but concrete isn't damaged by those moisture cycles, and it's very vapor-permeable. The conditioned space moisture will be determined by the ventilation rates and other moisture drives- you will have blocked moisture from entering via the walls.

    Bringing cool night air into a warmer basement can't create a condensation issue, even if the cool air is at 100% RH (fog).

    A whole house dehumidifier is a waste of money unless you insist on VERY low RH combined with high ventilation rates in summer. If the only humidity issues are in the basement, and the basement is open, not doored & walled off into multiple rooms with little air exchange between them, a standalone room dehumidifier will handle normal loads. (My antique slab that has no vapor barrier is at the water table- under the water table sometimes in the spring thaw, and I can easily keep the ~1500' of area under 60% RH with a standalone dehumidifier- it never comes close to running even a 30% duty cycle, let a alone 100%.)

    The foil facers can't "generate" condensation- the air does. If you chill the polyiso to below the dew point of the room it's in, yes you can get condensation on it. If you move it from a 65F warehouse and drop it into a damp 70F basement with a 68F dew point air, yes you'll get condensation on it. It takes awhile for the polyiso to reach room temp due to it's insulating factor. But once it's up on the wall and the surface temp is tracking with the room temp, there is simply no way for condensation to occur (unless it's extremely cold on the exterior side of the insulation, pulling the surface temp on the interior side sufficiently below the dew point of the room temp, but I guarantee you that didn't happen in June.)

    It's literally impossible to develop condensation on a surface that is at same temperature as the proximate air, unless you have a way of re-inventing the well established physics of the phase change of water vapor to liquid water. It doesn't happen.

    I'll be surprised if Dow responds with any meaningful detail. This stuff has a long history of being installed in basements and under siding, where the rain-penetration puts peak humidity levels WELL over what you would see in a basement that isn't actually flooded.

    The overnight outdoor relative humidity levels (nearly) always rise, because (as the name states) it's a RELATIVE number- the humidity relative to the temperature. As the temp falls, the RH goes up, but you haven't changed the amount of moisture in the air, only the temperature. If you warm that air up, the RH goes down, but it's still the same amount of moisture. So, if you take 99% RH 55F outdoor air (which has a dew point of 54.7F) and move it into a 68F basement where it warms to 68F, the relative humidity in the basement will be about 63%, the dew point is still 54.7F.

    In mid-summer when you take 80% relative humidity 85F air into a 70F basement, you will have condensation (or adsorption), because the DEW POINT of 85F/80% RH air is 78F- any surface that is below 78F will start taking on water as condensation (or as adsorb: materials that are micro-porous will show no surface liquid, but will load up with internal moisture.)This is psychrometrics 101 stuff (a subject most people never really study), but it's the dew point, not the relative humidity that is the measure of the absolute (not relative) moisture content of the air. (Another absolute measurement would be "wet bulb temperature", which is easier to measure directly.) And the result of all that condensin' and adsorbin' is that the moisture content of the mass of air always has a dew point identical to the room temp, when steadily replenished by a steady trickle of that higher-moisture content air.

    Bottom line- whatever your concerns are here, you needn't worry about foil faced polyiso adding to the humidity in the basement. If anything it'll lower it, by blocking ground water moisture from coming through the walls as water vapor.

  13. Expert Member
    Dana Dorsett | | #13

    Looks like we cross-posted.

    Rather than point you to a psychrometric chart (which requires some education to read correctly), I dug around the web looking for a simple to use online dew-point/relative-humidity calculator. This one is probably the easiest dumb-as-a-box-'o-rocks versions out there,with fewer terms to deal with:

    Play with it a bit to get a feel for what it means, and why 80% relative humidity is an incomplete data point without the air temperature (also called the "dry-bulb" temperature, in the psychrometric measurement world.)

    For your purposes you don't really need the wet-bulb and altitude aspects to fine-tune it. The fundamental is still the same: A surface temperature at or above the dew point of the proximate air will not form condensation. If the condensation forming on your iso was from the outdoor air, it was from the warm humid daytime air, not nighttime air cooler than your basement. And the same humid daytime air was raising the moisture content of the wood at the same time, albeit in a less visible way.

  14. GBA Editor
    Martin Holladay | | #14

    I agree with Dana. If the polyiso is indoors, the foil facing will be at room temperature, and there will be no condensation.

    Of course, if the polyiso is attached to a concrete wall, you need to do a good job of air sealing between the sheets of foam and at the perimeter, to prevent any warm, humid interior air from contacting the cold concrete or the cold foil that is adjacent to the wall. As long as you do a good job of air sealing, though, you shouldn't have any condensation problems.

  15. BvilleBound | | #15

    Dear Dana and Martin: Thanks again for all of the detail on humidity, dew point etc.

    We may have a disconnect in one key area: This is an older house that is being remodeled. Although we radically upgraded insulation, it is not "air tight" with a HRV / ERV for fresh air. In short, this house is similar to many, where foil-faced polyiso panels may be retrofitted to reduce heating/cooling costs. I am concerned that many other people may run into the same problems.

    The facts and concerns are simple and straightforward:

    > This basement is a walk-out, and windows were open to let in the cool night air -- a common approach to cool a house and reduce HVAC costs. The same problem occurred on the main floor of this house, where a foil-faced polyiso board was laying flat.

    > The proof is in the puddle of water. If foil faced polyiso panels are exposed to cool air overnight in a humid environment, condensation can result. With hundreds of square feet of foil on the interior wall surface, this can produce a significant amount of moisture. The foil surface cannot be air-sealed on the interior side, e.g. with poly sheet, or moisture would be trapped within the wall assembly. The only option: keep the windows closed and make sure the humidity does not rise, e.g. with air conditioning and/or a dehumidifier. If this a requirement, customers should be aware of it.

    > As I mentioned to the DOW tech rep, I am also concerned about condensation with exterior applications where polyiso board may be installed over existing vinyl siding, shingles, OSB, old 1 x 6 etc. Polyiso panels cannot be tightly "air sealed" against most of these surfaces. If humid and cool night air migrates into the interior side, e.g. the space between the shingles/ siding / OSB and the foil-faced polyiso board, the same condensation could occur. (On the exterior side, significantly more condensation would occur -- which should trickle down the surface of the polyiso panels and evaporate.)

    > A whole house dehumidifier is a good option, by the way -- to reduce AC costs when used correctly. A dehumidifier consumes significantly less energy than the typical compressor / heat pump. The ducted dehumidifer we installed, for example, consumes 840 watts, vs the two ton AC compressor that consumes 3,500 watts. (Many AC compressors are larger and consume even more energy.) With reduced humidity, we can set the AC higher and reduce run time. During the morning and evening, and spring and fall 'shoulder seasons', a dehumidifer can sharply reduce or eliminate the need for AC in many areas. (As noted in a number of GBA articles and posts.)

    > I am waiting for the DOW Building Solutions tech team to respond, following the call yesterday. I will let you know when I hear from them.

  16. Expert Member
    Dana Dorsett | | #16

    Opening up the windows at night during humid weather reduces you sensible cooling load (temperature), but increases your latent cooling load (humidity.) My experience in an 1920s vintage house central MA is that the average latent load exceeds the average sensible load, and that the (measured with Kill-a-Watt meters) increase in energy use needed to dehumidify using a nighttime ventilation strategy exceeds the energy use need to simply air condition to manage the sensible load. Basically, whenever the dew point is over about 55F, night time ventilation you are introducing a latent load. The foil is just an impermeable condensing surface- on other surfaces that moisture is being adsorbed, then released during the day when the house temps rise. This additional humidity you are injecting into the materials of your home is not a negligible factor- it's more than half your average air conditioning load in coastal MA.

    If you pull up a dew point graph for your location on and scroll out to view a whole late-spring to early fall weather history, then eyeball it with the cursors, you'll see that in most coastal MA the mean outdoor dew point exceeds 55F from about the beginning of June to the end of September. Take Ipswich, for instance:!dashboard;a=USA/MA/Ipswich (you'll need to use the pull down menu under the "Graphs" button on the upper right to select "dew point", and use the slider-cursor at the bottom to scale and center the weather history.)

    A whole house dehumidifier is just plain overkill for the shoulder season latent loads of any MA location if you just keep the windows closed. During the summer the combination of a room-dehumidifer in the basement and the central or window AC where sensible cooling is needed will use about the same amount of power as the only slightly more efficient whole house dehumidifier, at a fraction of the upfront cost. The tighter you make the house and the lower your ventilation rates, the lower your latent cooling loads will be.

    Unless you actually instrument the house and the iso, the "proof" of a puddle is a bit on the fuzzy side. (I'll trust the actual measured building science to get it right more than the anecdotes, but then again, my views are probably biased by my physics degree and engineering profession. :-) )

  17. BvilleBound | | #17

    Dear Dana: Thanks for your reply. I also found Weatherspark's website a week ago when I was researching dew point and temperature data for our area. I was preparing a document highlighting the issue; a copy is attached to this post with examples from May through August of 2013.

    I agree that keeping the windows closed at night will be necessary with foil-faced polyiso insulation, to keep condensation from occurring on the impermeable foil surface. This is one of the points I raised with DOW's technical rep; if condensation is a concern in humid areas, this should be clearly noted in their TUFF-R sales literature -- and definitely in more technical documents provided to builders / contractors. Many people like to leave their windows open in the evening for a fresh breeze and the cool night air, and they should be aware of this potential issue.

    I have spent 25 years in the IT industry and appreciate the value of precise quantitative analysis. Sometimes real-world evidence is more enlightening, however, than a pile of data -- and sparks important questions, as happened here. The puddle under the polyiso foam panels was surprising, large and wet -- not "fuzzy".

    A ducted whole-house dehumidifier made more sense as part of the new, high efficiency HVAC system we installed. Lower humidity air will be distributed throughout the house, for ~30% of the energy cost compared to the air conditioning system. Plus this central dehumification system with larger coils is more efficient than portable models.

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