Summer mid-day Condensation in newly insulated unvented, cathedral ceiling using foil-faced polyiso


We recently placed 3.5" of RMax foil-faced polyiso foam board (air and vapor impermeable) neatly cut and placed between the rafters of our cathedralized ceiling home in the Santa Cruz Mountains of California (hot dry summer, very wet and rainy winters, never much below freezing). Home is roughly 80 years old, built originally as summer cabin (very leaky, etc.). The roof is unvented, medium color asphalt shingles in good condition, with no roof leaks after three years living in the home. Roof deck is tongue-and-groove 1x8 pine, painted from below as a finished ceiling several years ago.

3-days after installing and sealing the perimeter of each sheet with can foam, I felt water drip on me while working at home mid-day (~2pm). I started paying attention, and we got many, many more drips, right below the ridge blocking. This occurs only in mid-afternoon from roughly 1pm to 3pm on hot sunny days, and is most evident on the South-sloped roof surface. This is a handful of drops, not a deluge of water, but still clearly unexpected and not good. 

We opened up two south-facing panels yesterday at 2pm. We found a completely dry and hot roof deck, and completely soaked ridge blocking and structural metal hardware. The ridge was beaded up all over with condensation, and the drops I observed were coming from the Simpson hardware at the ridge. I count myself lucky to have seen this issue prior to sheetrocking the whole home. I've added some photos below to show context, install quality, and observed condensation (note, this shows beaded water on the hardware, but it also covered the entire ridge block, just hard to see when painted white). 

I have a clear hypothesis of what's happening in my mind, but I want to know what others experience has been with this type of assembly and insulation strategy. Is it common in your region to cut and place foam board between rafters in an unvented roof assembly? It meets the letter of the model codes and local CA codes.

Have you ever experienced or heard of this issue, specifically condensation on hot summer days in unvented roofs insulated with polyiso or other foam boards? Or moisture stained sheetrock or rotted ridges in such roofs. (Note: I'm very familiar with moisture issues in these roofs in general, but am curious about this group's experience with this specific strategy).    

I also have a sense of how to fix it, but what would this experienced crew suggest? If removed, what would you recommend replacing this insulation with, prior to sheetrocking? 

Tags: Polyiso, condensation, roof, unvented

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I assume the blocking is flush with the face of the rafters and NOT covered by any foam? Could not tell by pictures.  Also, not sure what type of AC system.  So, my guess is the blocking is getting cold from the interior AC and acting as a conduit to the outside.  Then, it looks like the tops of the rafter bays may not be airtight allowing some warm moist air from outside to get in.  This, combined with cold metal and you have a perfect condensation surface.  I'd say either insulate and seal the bottom of the blocking, or make sure the bays are really air tight.  Also, using foam on the whole seam between the foam and rafters compared to just the face will help close up the air channels.

The "blocking" is more like an undersized ridge beam running the length of the room. I'm sure it's thermally bridging and represents a hot surface, but cool enough to support condensation at likely very high vapor pressure. I think the air up there probably has a dew point >100F, and we're at that kind of temp. The hardware might be cooler, or it might just absorb less water than the painted wood, so it beads up more and drips (thankfully). A/C is not on/operating at these times, but the indoor temp is <75 F. Outside air is very dry here in CA in the summer, so I don't think its leaking from outside. My hypothesis is that water vapor is emitted from the dry wood roof deck, because it is so hot, and this water vapor is driven to the ridge by buoyancy, where it condenses even on very hot surfaces.     


I am in the process of designing a new shop and have been researching this very thing - ways to insulate a raftered ceiling. I found this article helpful: There's a section labeled, "What about the cut-and-cobble method?" which seems to fit your situation; there's also a separate article for that detail. Hopefully, this is helpful. Good luck.

Thanks Paul, do wish I had read that article earlier this year. It's frustrating that this assembly meets the letter of the model building codes, but then there's this apparent consensus that this is a sketchy or DIY practice? My insulation sub and others I've spoken to in the area do this type of install regularly, and have reported no issues/call-backs. Not sure I believe that...

The other issue is that we normally think about these moisture issues as occurring during cold weather, and being due to air leaks in the ceiling/living space. I'm feeling very confident that this is moisture emitted from the "dry" roof deck into a small space, concentrating at the ridge, and condensing on hot surfaces. Very counterintuitive for all of us trained in thinking this is a cold weather issue driven by interior moisture loading. And in thinking that condensation only occurs on cold surfaces... 

That's a good point about warm weather condensation. In trying to think this through, do you think the cooler indoor air is somehow making it up to the interior side of the roof deck (top-side of rigid foam), cooling that space and materials therein, and cooling that space enough to create condensation? Or, perhaps, a thermal bridging issue?

Hi Brennan--I'm assuming you had a chance to read the cut & cobble article that Martin Holladay wrote, and that I commented on with pictures (Comment #7):

Cut-and-Cobble Insulation: Does it ever make sense to cut rigid foam into strips and insert the strips between your studs or rafters?

I believe the problem is moisture accumulation in the sheathing due to air barrier imperfections, followed by solar drive pushing moisture out of the sheathing, and accumulation up at the ridge of the roof due to hygric buoyancy, ping pong water, or whatever mechanism you want to ascribe:

BSI-016: Ping Pong Water and The Chemical Engineer

The solution I would suggest is the same one I offered to my friends in that comment--install a diffusion vent (as you're familiar with) to release the moisture where it is accumulating, at the ridge.  With the incredibly high vapor pressure (crazy hot + ~100% RH), that will create an incredibly strong outward gradient for drying through the membrane.

For readers who haven't seen this diffusion vent detail, see column below.

BSI-088: Venting Vapor

Also, images of unvented roofs with diffusion vents in a test installation are in this PPT file.

13th Annual North American Passive House Conference
September 21, 2018
Kohta Ueno
Good luck, and feel free to reach out if you have any questions! 

Sorry this is happening, Brennan. Sounds like you're set on an unvented assembly (i.e., no air gap, soffit and ridge vents)? If it's the issue Ed is describing, maybe 1" polyiso continuous across the rafters could keep them from cooling from the inside? I feel like with unvented, you have to get all the details just right. FWIW, in the GBA article that Paul referenced, Holladay recommends explicitly against using cut and cobble with unvented cathedral roof assemblies because of moisture issues. 

Hi, Brennan,

Good to hear from you.  Wish the circumstances were better.  I think you'll have an easy solution.

We need more data.  Perhaps you could get temperatures of top of asphalt and bottom of roof sheathing boards for very early morning (4am, 5am, 6am, etc.).  I think Paul may have suggested the answer.  

In the mountains with a clear night sky, your black roofing is an almost permit heat emitter to the night sky.  It might get quite cold.  And the South-facing roofing might not warm up so fast with the sun rising in the Northeast near the solstice.  So, perhaps the underside of the roof deck gets cold enough to condense moisture in the air.  From your photos, I see there is a bit of a gap between top of polyiso and bottom of roof decking.  So there is air from which moisture could condense.

And the ridge beam and any blocking by it might be the slower than the bottom of the roof decking boards to warm up as their thickness absorbs the heat and keeps the bottom surface cold a bit longer than the bottom of the roof decking boards.  And if the steel connectors get cold, they will warm the slowest.

It seems odd that this would not be warm enough by 2pm to quit condensing, but regardless, you do not want it to ever condense.  

If you are going to use this insulation system, I suggest you must eliminate any air between top of polyiso and bottom of boards which are the roof sheathing.  And insure no air gets into that void as it will be extraordinarily cold much of the day as the roof deck gets well below dew point regularly.

I did a cobble wall insulation of a 2x6" stud wall which I could not insulate on the exterior of the plywood.  No problems that i know of, but I made sure the plywood sheathing was absolutely airtight.  Then I cut the XPS to fine tolerances.  Your polyiso tolerances look good.  I used spay foam, but then I did two more things because I never trust spray foam to be a long-term air barrier.  First, I took 3M sheathing tape and taped smooth surface of XPS across stud face to smooth surface of XPS in adjacent stud cavities.  And tapes top and bottom of study cavity well.  Then applies a 0.5" layer of foil-faced polyiso over the entire assembly with taping of all seams and nail holes.  I believe that if you can ensure no air enters the rafter cavities, you are safe; however, it is a risky assembly.  Today, I would also consider coating the entire surface of the roof sheathing and rafters with with a painted-on or sprayed-on long-term-durable air barrier.  However, I'm also not sure I would risk this assembly.  If I did, I would delay installing GWB for a year or two and then open a few areas to ensure no sign of moisture or decay.  Only then would I consider sealing with GWB.  Good luck.  Best, Tad Everhart

You talk about interior panels that you removed.  Is the surface exposed to the interior airtight?  If not, on a hot day you could get convection-driven air leakage from the interior through the panel joints.  Depending on your local climate, you could still have 50% RH inside even when air conditioning, so the metal gets cool from the A/C and has an endless supply of moist air passing over it.

Your moisture could be coming from either inside or outside.

I ran into something similar recently. I suspect your situation is similar but I can't say for certain from pictures and post on internet. So verify for yourself.

Joe Lstiburek has written articles on this and several others have made presentation (Lew Harriman sent me some of his data). During "cold" conditions (night sky radiation heat transfer can make for surprisingly low temperatures in the insulated roof), the wood structure is absorbing moisture due to very high RH. During the day, the high temperature are driving that same moisture out of the structure. So the dew point temperatures in that closed/insulated area are not likely to be what anyone would expect. The dew point can be well above the outdoor dew point. This problem gets worse the tighter the cavity is sealed in dry climates. In wet climates, it gets worse the more the cavity is vented. Yes - it can seem backwards.

If my memory is correct, Lstiburek also published a fix for this problem. My memory says he mentioned "ping pong water" in the title of the article. Try googling "Lstiburek ping pong" and see if you find an article about venting with a 'vapor throttle'. That amounts to a ridge vent with spun polyolefin (Tyvek) cover. If that article is not exactly what you are looking for I suspect you can find it from there.

I hope that helps.

Tad and Kent - bingo!

Nighttime extreme cooling of the structure from the roof down. Interior moisture migrates up during the morning until it reaches a critical RH and condenses on the metal enough to drip. Time delay because it takes a while for the humidity to build up in the cavity.

If this were my home, I'd cover all the metal with spray foam to eliminate the ability of moisture to reach it. Given the thermal mass of the large wood beam, I'd foam that over as well.

Further, if it were my home, I would have put two layers of foil-faced polyiso with taped, staggered-seams, across the rafters as well, before adding the interior ceiling material. That would block virtually all moisture from getting into the cavities from the living space.

I'd also carefully track interior humidity levels at the ceiling level with a remote-read temperature/humidity gauge. Those are dirt cheap these days and would encourage you to air-condition/dehumidify more as needed to keep the humidity levels low enough.

The thin application of expanding foam along the perimeter has a much lower R-value than the foam board. There are also small gaps between the foam board and the roof deck as others have pointed out.

When I install with foam board and expanding foam, I always seal the back side of the foam board as well as the inside. Cutting the foam board a little smaller (1/2" smaller than the cavity at least) helps get the foam to fill those side gaps. Sealing the sides is obviously important, but each piece needs to be sealed periodically across the cavity to prevent any type of convection happening in that tiny space.

I wish there were a good article on how convection can happen in these small spaces and how it can impact the performance.


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