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Insulation for condensation control changes from IRC 2009 to 2012

Tom Frisch | Posted in Building Code Questions on

I’m building a small house in Portland Oregon, climate zone 4C marine (according to 2009 IRC). I have 2×12 rafters with blown cellulose, then a layer of taped plywood sheathing for air barrier, then 3″ of XPS followed by another layer of ply and the roofing on top of that. Since the ceilings are vaulted, it’s an unvented attic situation, so I’m worried about moisture. I’ve got R36 in the cellulose and R15 in the XPS.

I was reading in the 2009 IRC about the prescriptive rules for condensation control in unvented attics and in table 806.4 it says in order to avoid my first layer of plywood getting wet, I need R20 of exterior insulation as part of the required R38 minimum ceiling insulation.

So while I’m way over the minimum IRC2009 insulation requirements, my ratio of exterior foam to interior cellulose is way off- 52% of the stack should be exterior according to IRC2009.

Then I looked at the 2012 IRC, they have upped the minimum ceiling insulation to R49 but kept the exterior portion in an unvented attic to R20, meaning the ratio is now 41%.

Does this mean that the ratio was wrong in 2009, or that ratio isn’t the important thing, but rather then total amount of insulation above the first layer of sheathing?

I would think that the important thing is to keep the difference between outside air temp and the temp of the sheathing below a certain amount. (no idea what that amount is). I would also think that the more insulation you have below, the less of a problem it would be, since the heat from the house wouldn’t be warming the sheathing as much in the first place. So I’m questioning the idea that the ratio of above/below insulation is the important metric.

I’m a diy builder on my first project, so could someone more experienced and educated weigh in on this seeming inconsistency?

BTW, my city building plans inspectors (notoriously nitpicky) had no problem with my plans for R36 below and R15 above- either they missed it, or it’s not an issue for them. They use IRC2009 currently.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Tom,
    Q. "Does this mean that the ratio was wrong in 2009, or that ratio isn't the important thing, but rather then total amount of insulation above the first layer of sheathing?"

    A. First of all, good catch. You correctly noted that the 2012 code increased the minimum R-value for ceilings in Climate Zone 4 from R-38 to R-49 without adjusting the requirements shown in Table R 806.4 or R806.5. I would call this a code inconsistency or a lapse by the writers of the code, because (as you guessed) the ratio matters.

    (For curious GBA readers, the minimum prescriptive requirements for ceiling insulation can be found in Table N1102.1 of the 2009 IRC and in Table R402.1.1 of the 2012 IECC.)

    The original ratios were developed with a level of safety, because they were developed for worst-case levels of interior relative humidity. If you want to do your own calculations, the calculation method is explained in this article: Are Dew-Point Calculations Really Necessary?

    If your house is already built, my advice to you is to keep an eye on your indoor relative humidity -- don't let it get too high -- but otherwise, not to worry. If you haven't started building your house yet, you may want to include an interior layer of MemBrain or a similar smart vapor retarder.

    I welcome any input from other GBA readers.

  2. Tom Frisch | | #2

    House is being built now, framing 2nd story starts today. I can easily add a layer of membrain or similar. Following your helpful advice, I ran the simplified dewpoint calcuation, and got the following results:

    Portland has a mean winter temp of 42F (I know, not shoveling snow is great, however rain all winter kinda sucks). That makes my delta-T 28F at 70F indoor temp. If I have R36 inside and R25 outside, that's 60% interior insulation ratio, so the sheathing temp would be 53F. That is well over the 40F dewpoint. Even if I stick with my original plan of 3"XPS on the roof, at R15, the sheathing temp would still be a somewhat balmy 50F.

    So if I'm reading the psychrometric chart correctly, as long as the sheathing is kept below 50% RH, it won't be taking on water from condensation.

    This leads me to my followup question- since it's very cold and humid here in Oregon while it's raining all winter, does that factor in to indoor RH levels?

  3. Charlie Sullivan | | #3

    If it's 42 outside and 95% humidity, that's a dew point of 40.7 F. That means that no amount of ventilation is going to get the interior humidity down to below 40.7 F, because there are also moisture sources inside like showers and cooking and breathing. With good ventilation you should be able to keep the dew point inside below 50. Do you have an active ventilation system planned?

    Another option is to run a dehumidifier. That is an energy hog in the summer, but in the winter when you are trying to heat anyway, it's actually better than a regular electric heater, by about a factor of 2 for one that just meets the energy star rating, because you are recovering the latent heat of vaporization of the water and turning that into heat. That isn't interesting in most climates, because when it's cold there's rarely enough moisture around to remove, but it might work at least some of the time in your climate.

  4. Expert Member
    Dana Dorsett | | #4

    The short answer is yes, in 2009 the prescriptive levels for exterior R were far too conservative for your climate.

    In zone 4C the wintertime indoor dew points tend to be much higher than 40F, as Charlie alludes to. But 45F is still pretty easy to hit with ventilation. The mid-winter outdoor dew point averages in Portland OR are in the mid-30sF, not the foggy-dew 40.7F he mentions. While outdoor dew points north of 40F are common, the mean high dew point temps in January are only about 38F. It's the averages, not the peaks that count.

    Pull up a dew-point graph for your location and study it:

    https://weatherspark.com/#!dashboard;a=USA/OR/Portland

    As long at the mean temp at your sheathing over the 10-12 coldest weeks of winter is reliably above the 45F presumptive indoor dew point you'll be fine. Even if it takes on some moisture in the winter, by the time it's warm enough out for mold & rot to really get going it will have long since dried toward the interior.

  5. Tom Frisch | | #5

    I'll have an HRV for ventillation, wasn't planning on dehumidifier if I can avoid it. Heat will be a ductless minisplit and possibly some electric resistance heaters for the coldest days.

    Seems like I should be right on the border of safe with the 60% ratio of insulation making for 53F sheathing temp, and a variable vapor retarder added behind the drywall should help ease my worried mind. Also a good air seal and solid dense pack should help minimize moisture laden air moving through the wall.

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