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Is 1″ enough?

sylvanrocks | Posted in Energy Efficiency and Durability on

We are constructing a home in far western SD (Climate Zone 6b)….There is a lot of bedrock here so we have opted to build a post frame (pole barn) style structure. From the outside in, our wall ‘sandwich’ will be
1-pole barn style tin wall covering
3- 1″ Tongue and Groove XPS foam
4-Half inch OSB attached to the girts
5- 7″ of cellulose filling the curtain walls
6- OSB on the shop walls and Drywall in the house.

The house is 27′ by 50′ and the shop is attached via a shared 27′ wall and is 36 by 48′ . for about 3000 sq foot under roof. The Black Hills of SD is Arizona dry (average of 17″ of precip per year with much of that being snow) and at times Canada cold but lots of sunshine. We have been through many recipes for a suitable wall, and have decided on this one for many reasons, but I’m worried about condensation on the OSB. My contractor/friend….said that if it were his, this is what he would do and assume that any moisture that may work its way into the wall will be able to dry “inward” without causing mold and moisture problems when things warm up. There are only 2 of us living in the house and we don’t have tons of plants or humidity producing habits. We plan to have an HRV.

Is the 1″ of foam enough? Or is it to big of a gamble to take? Not taking into consideration the post, girts and studs the will will be R31 with only R5 of that on the outside of the OSB. We really want an efficient home but marvel at the number of ways to ‘skin the cat’ and the difficulty in getting solid local knowledge on projects that are outside of ‘normal’ from area builders. Thank you.

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  1. GBA Editor
    Martin Holladay | | #1

    There is a GBA article that explains how you can calculate the minimum thickness of rigid foam installed on the exterior side of a wall. Here is the link: Calculating the Minimum Thickness of Rigid Foam Sheathing.

    In your climate zone, the minimum R-value for the exterior foam layer would be R-11.25 if you had 2x6 walls with 5.5 inches of insulation. But your walls will have 7 inches of cellulose -- so your rigid foam layer has to be proportionally thicker (27% thicker) to keep your OSB wall sheathing out of the danger range. Boosting R-11.25 by 27%, we come up with a minimum R-value for your foam layer of R-14.3.

    If you use XPS rigid foam for your house, the XPS has to be at least 3 inches thick to keep your OSB out of the danger zone.

  2. kloopster | | #2

    "Is 1" enough?"

  3. kloopster | | #3

    That's what she said.

  4. GBA Editor
    Martin Holladay | | #4

    I have no idea what point you are trying to make with your two comments.

  5. sylvanrocks | | #5

    Thank you for your thoughts...and all the info you have on this great site. Yes, I read the link you posted and I believe the first time I did the calc I did it wrong (probably late one night) and now with a week of snow and cold I revisited...and am now trying to figure out how to goto a thicker insulation. If I did not or could not goto 3" of foam...would I be better off with none, and air sealing on the inside? Seems like there are tons of remodel retrofits when people replace siding, add foam, and blow in cellulose....ending up with a situation that should by all accounts be problematic. How do they get away with that? I guess at this point I'm looking for other options beside the obvious, as I'm not sure how to build that in at this point.

  6. GBA Editor
    Martin Holladay | | #6

    The rule with exterior wall foam in cold climates is: Go thick or don't go there at all.

    If you can't manage to install 3 inches of rigid foam, it's better to have no foam at all.

    Q. "Seems like there are tons of remodel retrofits when people replace siding, add foam, and blow in cellulose....ending up with a situation that should by all accounts be problematic. How do they get away with that? "

    A. Stay tuned! Over the next few decades, contractors will be called in to repair many of these walls... and we'll have a lot more data on failure rates.

  7. sylvanrocks | | #7

    OK. So, if I leave off the foam and keep the house wrap and OSB, am I in a dry to the outside situation? Since going down the seal out...dry in bunny trail, I've not explored how to construct other options.

  8. GBA Editor
    Martin Holladay | | #8

    The best wall assemblies include a ventilated air space between the back of your siding and your sheathing (or housewrap).

    You chosen cladding ("pole barn style tin wall covering") is probably steel, not tin. If it is corrugated, with lots of air channels, you are all set.

    If it is mostly flat, so it sits tight to the sheathing, you might want to include furring strips to get better ventilation between your cladding and your sheathing.

  9. sylvanrocks | | #9

    So, taping and sealing the OSB sheets or tyvek?

  10. sylvanrocks | | #10

    can I put the foam layer inside the OSB?

  11. GBA Editor
    Martin Holladay | | #11

    Q. "No taping and sealing the OSB sheets or Tyvek?"

    A. I didn't say that. The seams between the sheets of OSB wall sheathing are taped to reduce air leakage; this action has nothing to do with drying by diffusion.

    All buildings need an air barrier; it's always a good idea to try to reduce air leakage. For more information, see Questions and Answers About Air Barriers.

    Tyvek seams should be taped because taping is part of the manufacturer's instructions; building codes require all materials to be installed in compliance with manufacturer's instructions. The main purpose of the Tyvek is to act as a water-resistive barrier (WRB). Tyvek is vapor-permeable, so it won't restrict diffusion drying.

  12. GBA Editor
    Martin Holladay | | #12

    Q. "Can I put the foam layer inside the OSB?"

    A. Possibly, as long as (a) your post frame includes diagonal bracing to provide racking resistance, and (b) the rigid foam is at least 3 inches thick.

    You still need to keep the inner surface of the rigid foam above the dew point in winter, to prevent condensation -- so you still need to make the foam at least 3 inches thick.

  13. sylvanrocks | | #13

    I can see the logic and safety in having 3" of foam, but what I cant seem to find an answer to is: If there is an air barrier/weather taped OSB covered in tyvec that is going to limit vapor flow either scenario, Why is condensation on the OSB so much more dangerous when it has less then optimal foam but is warmer and therefor less susceptible to condensation during more months of the year, then when it has no foam. I would assume that without any foam it might dry slightly better but I'd also assume that it would get it would be below the dew point for so many more days. I'm fascinated by it all.

  14. GBA Editor
    Martin Holladay | | #14

    Tyvek is vapor-permeable; it is designed to allow outward drying.

    OSB is somewhat permeable -- significantly more permeable than most types of rigid foam (other than thin EPS).

    So the basic answer to your question is that rigid foam inhibits outward drying more than Tyvek or OSB. While OSB that is not protected by a layer of exterior foam gets damp every February and March, it dries out every April.

    For more information on this issue, see How Risky Is Cold OSB Wall Sheathing?

  15. wjrobinson | | #15

    Daryl, you can put any thickness of foam on the inside of your framing behind your interior sheathing. So put an inch there and tape the seams.caulk your framing leaks like at sill plates etc.

  16. sylvanrocks | | #16

    AJ Builder
    So, you are saying to put the foam on the inside, and use that as the air seal. I've considered that, but doesn't that defeat the purpose of using the foam reduce thermal bridging? It continues to impress me how when you try to do one thing reduce thermal bridging it bites you in the butt with something like moisture. Its all way more connected and complicated then I ever gave it credit for. The original plan was to use spray foam, but after reading a bunch...I realized how hard it was going to be to get a good spray foam job going onto cold steel in the winter. I decided it was not worth the risk. And the balancing act continues as I try to look at what is being done around the country, and choose the system with lesser problems for the site we are building in. I often find myself wishing I cared more about granite counter tops and less about being efficient.

  17. GBA Editor
    Martin Holladay | | #17

    Q. "You are saying to put the foam on the inside, and use that as the air seal. I've considered that, but doesn't that defeat the purpose of using the foam reduce thermal bridging?"

    A. No. If you install a continuous layer of rigid foam on the interior side of your wall framing, the rigid foam will interrupt thermal bridging through the framing.

  18. srenia | | #18

    Done the one inch foam on the interior side. Tape and mastic, caulk and let the extra long drywall screws with drywall fasten the interior foam. One inch gives you about 80% efficiency Uvalue with the rest of the insulation in the cavity doing the rest. No need to pile on the foam when you don't need to in your situation.

  19. Expert Member
    Dana Dorsett | | #19

    In a zone 6B climate on wood sheathed wall assemblies you need at least 35% of the total R to be on the exterior of the wood sheathing to be able to protect the sheathing from interior moisture drives. That is a minimum, not an optimal.

    With air-tight OSB on the interior you can get away with a bit less, due to the comparatively low vapor retardency of OSB vs. painted drywall, but it's also closing in on being a moisture trap- better to keep it at 35% anyway.

    At 7" you're looking at about R25 for the cellulose layer, which means you'd want a minimum of R13.5 on the exterior of the sheathing. That could either be 3" of Type-II EPS, or 1.5" of polyiso directly on the sheathing, with 1" of EPS on the exterior of that (to keep the polyiso warm enough to perform near it's rated-R), or 3" of high-density rigid rock wool. Rock wool would be the better solution, since that would give you many orders of magnitude drying capacity toward the interior.

    If you go with less than 35% you may be able to get away with using a smart vapor retarder such as Certainteed MemBrain on the interior side, under the gypsum (or interior OSB). Those types of vapor retarders severely restrict the rate of moisture accumulation in winter without reducing the drying rates during the shoulder seasons.

    Putting 1" XPS between the gypsum & studs also works. It's important that it be fully air tight however- tape the seams of the XPS with housewrap tape, then goop over the tape with duct mastic as a more permanent air seal, and stagger the seams of the gypsum with those of the XPS. Better than 1" XPS would be 1" foil-faced polyiso, which can be easily air-sealed with 2" FSK tape, and offers modestly better thermal bridging. The biggest PITA with an interior-side foam solution is getting all of the electrical & plumbing penetrations permanently well-sealed.

    You could also put just 2-3" wide strips of foam on the interior side stud edges held in place with 2x3 or 2x4 furring through-screwed to the studs, and install MORE than 7" of cellulose, with MemBrain on the interior, which is probably cheaper. You get about the same amount of thermal break with about 15-20% of the foam, without inserting a highly vapor retardent layer to slow the drying rate. You may have to hunt down a source for it, but MemBrain is a heluva lot cheaper per square foot than 1" foam, and comes in really wide long rolls which reduces the time & risk of air sealing.

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