GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Reverse Flash & Batt

bugbug | Posted in Energy Efficiency and Durability on


I’m having a new home built in a very cold climate(Edmonton, AB, CAN) and it will have an unvented cathedral ceiling.

In order to save money I was thinking of going the Flash&Batt route but after reading about dew points and cold climates I’m worried there might be a condensation risk between the foam and the batt.

I was wondering if its possible to reverse the method and put the vapour/air barrier(the ccSPF) on top of the permeable layer? It seems to make more sense in a cold climate to do it this way.

I doubt one can put the ccSPF over fiber or cellulose but what if I had open cell SPF spayed first into the cavity followed by a layer of closed cell?

Ive googled around and can only find examples of applying closed cell first and then open but I would have to think that the opposite order would be better in a cold climate.

Is my idea crazy or even possible?


GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. GBA Editor
    Martin Holladay | | #1

    I don't advise mixing open-cell foam and closed-cell foam in the same cavity, for a variety of reasons -- including some scary stories about lingering odors that may be related to unexpected interactions between different types of foam.

    If you want to use a mixture of closed-cell spray polyurethane foam and vapor-permeable insulation (like fiberglass batts), you can -- but you have to follow recommendations designed to keep the interior surface of the cured foam above the dew point in winter.

    Those recommendations can be found here: How to Build an Insulated Cathedral Ceiling.

    The closed-cell spray foam needs to be on the exterior side of the fiberglass batts. In your climate, the closed-cell spray foam needs to have a minimum R-value of R-35. That means that you need a minimum of 5 1/2 inches of spray foam. Once that foam has been installed, you can go ahead and add fiberglass batts on the interior side of the foam.

  2. bugbug | | #2

    Thank you for the quick reply!

    In your climate, the closed-cell spray foam needs to have a minimum R-value of R-35."

    Why is that? The average outdoor relative humidity here in January is 70%. If I take a particularly cold day (-25C) and a thermostat setting of 21C and plug those values into an indoor relative humidity calculator ( I get an indoor relative humidity of 2%(most people run humidifiers here in the winter but I don't). If I plug that into a dew point calculator ( I get a dew point of -30C.
    So how would it be possible for the interior side of 2" of cc spray foam to be -30C when the outdoor temperature is -25C and the indoor temperature is +21C? It doesn't seem possible.
    The 2% relative humidity doesn't take into account human activity and I have no idea how to calculate for that but even if it was 5X higher at 10%. the dew point would be -12C during a -25C day in January. How likely is it that the interior side of 2" of foam would be -12C when its -25C outside and +21C inside?

    Thanks again!

  3. GBA Editor
    Martin Holladay | | #3

    There are two errors in you calculation method:

    1. Your interior relative humidity in winter is likely to be significantly higher than 2%.

    2. The fiberglass batts isolate the interior face of the cured spray foam from the interior temperatures, making the foam colder than you think.

    Here is an article explaining the usual method of performing a dew point calculation: Are Dew-Point Calculations Really Necessary?

    Fortunately, you don't have to do your own dew-point calculations to design an unvented cathedral ceiling. The dew-point calculations have already been performed for you, and have been integrated into the IRC and IECC codes. Those code requirements are provided here: How to Build an Insulated Cathedral Ceiling.

    Remember, even if you were able, through extraordinary juggling of exhaust fans and the use of a dehumidifer, to keep your interior RH at 2%, the family that you sell the house to in 10 years could easily run the house at 35% interior RH. It's important that your cathedral ceiling doesn't rot -- either when you are living in the house, or when the next family moves in. You don't want to create a roof assembly that is so fragile that it only works in a narrow range of interior RH.

  4. bugbug | | #4

    Hi Martin

    Thanks again.

    I wouldn't say I made two errors. I acknowledged that the indoor relative humidity wouldn't be 2% and used a value of 10%. I also realize that the batts would cool the air and my question how much? Isn't that the real problem and not the thickness of the spray foam? If the spray foam is 2" or 20" wont the interior surface of the foam be relatively close to the indoor air temperature without batts?(It wont be 33 degrees cooler). So isn't the absolute thickness of the spray foam not the thing to be concerned about but rather the ratio of foam to batt?

    I'm not sure the RH could "easily run at 35%" in this climate when its -25C outside but assuming that a new owner 10 years from now is pumping moisture into the house on very cold days I'll use that number.

    I looked at the website you linked to and the chart for dew point is pretty much the same as the calculator I linked to. If I use 35% for the RH and 21C for the indoor temperature, the dew point is 4C.

    If its -25C outside and 21C inside, 4C falls at about 60/40. So using R values, in theory 60% of the R value would have to be in foam and 40% in fiberglass . In practice I would think it would have to be -25C for a long time for this to be true. Additionally I'm not convinced that the the R value in fiberglass performs as the same value in foam but Ill ignore these two points and continue with a worst case scenario.

    If I use a target R value of 40 for the roof and a spray foam R value of 6.5, 60% of 40 is 20. 20 divided 6.5 is 3.7 inches of foam needed

    If I use R50, 60% of 50 is 25 and 25 divided by 6.5 is 4.6 inches of foam needed.

    Using a RH of 35% on a very cold day, I'm having trouble understanding why I need R35 in foam?


  5. GBA Editor
    Martin Holladay | | #5

    The code calculations assume that the framing bays are filled with fluffy insulation -- the usual practice.

    Your hypothetical roof assembly is only R-40. That's a little low for your climate -- most builders who visit the GBA web site are aiming for R-60 -- but it's your house. (According to the 2009 International Residential Code, the minimum R-value for roof insulation in Zone 6 -- an area that includes Vermont, New Hampshire, and Wyoming -- is R-49.)

    You calculate that 60% of R-40 = R-20. My calculation comes up with 60% of R-40 = R-24. If you assume that your closed-cell spray foam is R-6.5 per inch, you would need about 4 inches of foam (so our calculations agree there). You propose making up the rest of your R-value with fiberglass, so you end up with R-40 minus R-24, or R-16 of fiberglass in your roof.

    Here in the U.S., no one installs R-16 fiberglass in a roof assembly. You could do it, but it would be unusual, and you'd have to install metal rods to keep the fiberglass batts tight to the spray foam.

    It is far more typical to fill the 2x10 or 2x12 rafter bays with fiberglass batts with a rating of R-38. That's what the code assumes. That makes it necessary to install thicker spray foam. Of course, you end up with a much better insulated roof assembly than your proposed R-40 assembly.

    If you want to install R-16 fiberglass batts in your ceiling, and if your local building inspector approves, I suppose you can figure out a way to do so. But I don't recommend it. I strongly suggest that you aim a little higher -- and make your wall assembly a little more robust while you are at it by raising the R-value of the foam layer.

  6. Expert Member
    Dana Dorsett | | #6

    In an air-tight house containing living breathing bathing mammals and a few plants or a fish-tank in there you'd have to ventilate like hell to get the interior RH down to (an unhealthful) 10%.

    If you only sleep there,always eat out (or eat out of chow-bags/cans, and never cook), never bathe and don't believe in houseplants or pets you might make it that low even with low ventilation rates though. :-)

    If you've never lived in tight house you probably haven't experienced this, but my (not super-tight) house never drops below 35% RH @ 21C, even when it's -25C outside (which it DOES hit occasionally here, despite a -20C design temp). But we have 3 humans who bathe daily, one hamster who doesn't, and a 6' ficus tree adding moisture to the air. :-) I'm sure most R2000 standard houses need active ventilation to keep the humidity down to 35% in winter, and they're tighter than mine. If you're using spray foam for insulation & air sealing, decent windows & doors and good air sealing practices throughout, you'll be tighter than my cellulose insulated plank-sheathed antique with wood-sash circa 1923 double-hungs too.)

    It is a reasonable assumption to use 30-35% RH @ 20-21C, since that is at the low end of what health professionals consider the healthy range (even though ASHRAE sez 25% RH is still comfortable.) That puts your dew point at between 2-5C. Use 4C if you like- it's good enough.

    Looking at datasets, the mean binned hourly January temps for Edmonton is about -10C, so working your ratios from there, not an arbitrary -25C or the -37C 99% heating design temp. With 10C outdoors, 21C interior temp, and a 4C dew point you'll manage to keep it mold free, yet healthy for humans with about 45% of the R as foam outside the condensing surface. A 50/50 split would have margin.!dashboard;q=edmonton%20alberta%20canada

    (^^scroll out for graph to cover the whole winter, use the cursors to estimate the historical mean temps)

    Using an arbitrary -25C as your outdoor temp is as ridiculous as using the 99% outside design temp (-37C), since the volume of moisture in the fiber portion of the R is bounded, and when it's -25C or lower outside you'd have frost, not condensation on the condensing surface, with little to no damage potential (and zero mold potential) or loss of R. Using the January mean temp for the location is usually even more stringent than IRC prescriptive methods.

    IIRC code min for roofs & attics is now R50 for most of Ontario- is that not the case in Alberta too? If yes, you can get there with huge margin, very low risk with 4" of rigid polyiso or 5" of Type-II EPS above the roof deck, a flash-inch of cc polyurethane on the interior of the roof deck as air seal and non-wicking condensing surface, and R15-23 Roxul or fiberglass in the rafter bays. And with the thermal break over the rafters, even an R40 total center-cavity stackup would perform comparably to or better than an R50 open-blow cellulose over 2x12s.

    Note- even though the iso is rated R6/inch or more @ 75F (R24 nominal) and the EPS is only rated R4.2/inch (R21 nominal), at the -10C & lower outdoor temps of interest the average temp through the exterior foam is -5C or less in this stackup. Using the proper derating the different foams for a -5C average temp, the 4" iso only performs at ~R5.6/inch (~R22.4 @ 4") and the 5" of Type-II EPS (1.5lb nominal density) performs at R4.7/inch (R23.5 @ 5"), characteristics of the slopes of the curves of their performance over center-foam temp.

    Folks like Joe Lstiburek seem comfortable using the 10-12 week winter mean-temp for the quick & dirty dew point calcs, which would be about -7C for Edmonton, not the -10C January mean, but I'm personally leery of taking it close to the edge, even if a WUFI simulation vets it as marginally-OK. I prefer having a bit more margin. YMMV.

  7. bugbug | | #7


    Sorry about my math. The original post I wrote used a 50/50 split but I decided to go more extreme and use 60/40.(personally I think its probably closer 40/60) I forgot to edit how I got to the result. 60% of R50 is 30 and 30 divided by 6.5 is 4.6 inches(same as above). So my result is correct, I just forgot to go back and edit how I got to 4.6 inches.
    I regret using R40 and R50 as an example because what I really want to get at is the ratio regardless of the target R value. I have no problems using R60 as an example because it shouldn't change the ratio of foam/fluffy.

    Thank you very much for joining the conversation

    Using an arbitrary -25C as your outdoor temp is as ridiculous as using the 99% outside design temp (-37C)

    I agree with this 100%. This is why I said: "I would think it would have to be -25C for a long time for this to be true. " I was being extreme to try and understand the justification for using a high ratio spray foam. I would have used the mean but I figured someone would say I was not being safe.

    You're right I have no idea what its like to live in an airtight house. My current house is 100 years old and about as airtight as a leaky tent. I have no problems using 35% and used it in my last calculation.

    So you would agree that regardless of R-values, the split is around 45/65 foam/fluff? That is really what I'm trying to get a handle on.

    I don't know what the code requirement is for Alberta since they don't publish that data online but according to this NRCan document the December 2012 EngeryStar for new home standard for 5000-6000 heating days is 59.2 for ceilings below attics but 28.5 for cathedral and flat roofs.(see table 2 on page 30) so technically its 28.5...but before peoples heads explode I'm not aiming for 28.5 and Im not wishing to get into recommended R values above lets just pretend I'm aiming for R100 ;-)

    ...very low risk with 4" of rigid polyiso or 5" of Type-II EPS above the roof deck, a flash-inch of cc polyurethane on the interior of the roof deck...

    I'm glad you brought this up because I had this idea originally but I think I read in one of Martin's articles that having foam on both sides of the sheathing is a bad idea.

    Martin did I interpret that correctly? Is it a bad idea to have rigid foam on the exterior of the sheathing and spray foam on the interior?


  8. GBA Editor
    Martin Holladay | | #8

    Q. "Is it a bad idea to have rigid foam on the exterior of the sheathing and spray foam on the interior?"

    A. In general, yes, it is a bad idea. However, Dana is suggesting that you limit the interior spray foam to just 1 inch -- and a 1-inch layer of spray foam can dry (very slowly) to the interior. The permeance of 1 inch of closed-cell spray polyurethane foam is 1.9 to 2.5 perm. That's not much, but over the course of a year, there will be some drying to the interior.

  9. Expert Member
    Dana Dorsett | | #9

    From the specs I've been reading most 2lb foam is more liken 1- .2 perms @ 1" but it varies. Some of the BASF goods are more like 0.8 perms @ 1", whereas Icynene MD-R-200 is about 3 perms @ 1". If you double the thickness the permeance drops by roughly half- it's not perfectly linear, but close. (IIRC Straube assumed 1.2 perms @ 1" foam for his WUFI simulations in the BSC RR-1001 report: < see page 5 under "Material Properties".)

    For comparative purposes, standard latex paint runs 2-5 perms, "vapor barrier latex" runs ~0.5 perms. Anything in the 1 range or less is pretty much a vapor barrier for building purposes in climates other than arctic extremes.

    The ubiquitous 6-mil polyethylene sheeting is under 0.1 perms, and would take more than a year to pass the amount of moisture that an inch of 2lb foam passes in a month- a "dries never" scenario, making it a very sharp double-edged sword. In US climates it's probably responsible for creating more moisture problems than it ever solves, and if it can be designed-out, it should be. (Usually can.)

    In Edmonton's climate the summertime "drying period" is pretty substantial in length (and temperature), and at 1perm you will not have created a moisture trap. At ~1-2 perms between the interior and the roof deck, with a 45/65 ratio for foam/fiber, the roof deck won't be cold enough long enough to accumulate excessive moisture, but any moisture that does find it's way is has ample time over the course of the seasons to dry toward the interior.

  10. bugbug | | #10

    Thanks Dana & Martin

    Would having grooves in the rigid foam eliminate worries of a vapour sandwich?

    Could I put spacers or battens between the sheathing and the rafters and then if I used spray foam it could expand and fill the space between the rafter and sheathing creating a thermal break thus eliminating the need for rigid foam on the exterior for that purpose?(I'm not a builder so I have no idea if this is even possible or worthwhile but in my head it seems like an easy solution)

    Going back to the title of this thread, could I do a reverse flash&batt by putting plywood (or even drywall) at the bottom of the rafter and then flashing down from the top with an inch of ccSPF and then filling the rest of the cavity with fiber or cellulose?(wouldn't ceilings below attics be insulated this way?) I'm guessing this would eliminate any dew-point concerns because the barrier is on the inside. Is it ok to dry to the outside instead of the inside in my climate? I would think this solution would be cheaper because I could use less spray foam(1 or 2 inches) and just insulate the rest with the fluffy stuff. At the same time I'm guessing the installation might be more expensive as I imagine some guy hovering 10m in the air spraying down the foam. I'm also guessing the sequencing would be a factor too. The drywall(or interior plywood), the insulation and the roofing would all have to be done the same day and in that order. Again I'm not a builder so I could be describing something that is utter nonsense.

    Ah to be rich and just fill the whole thing with spray foam :)

  11. GBA Editor
    Martin Holladay | | #11

    There are lots of ways to build a roof assembly, but I think you are trying too hard to come up with something unconventional.

    If you don't want to invest in the thick foam required for an unvented cathedral ceiling, you can always install a vented cathedral ceiling. Then you won't need any foam at all.

    Your ventilation channels can be installed under the roof sheathing (using 1"x1" or 1 1/2" x 1 1/2" sticks in the upper corners of the rafter bays, followed by cardboard, thin plywood, or some other rigid material as a baffle). Or you can install ventilation channels above the roof sheathing (using 2x4s perpendicular to the ridge on top of each rafter, followed by a second layer of OSB or plywood).

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |