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Can I use R-15 rock wool batts in a REMOTE wall with R-20 exterior foam?

Gary Dick | Posted in Energy Efficiency and Durability on

I have read many articles on this site (thank you) and am currently using some of what I have learned in my house construction in Ridgway CO. For reasons I need not go into, the house has two wall systems – the north and west walls are built from ICFs. The south and east walls will be 2×4 studs, plywood sheathing, Tyvek and two layers of taped exterior foam with an R value of 20.

I have read that the rule of thumb is insulation of 1/3 of that exterior R value can go in between the stud bays while still allowing enough interior heat to get to the sheathing and prevent condensation (the sheathing will obviously only dry to the inside of the house).
I have read the advantages of Roxul mineral wool batts and like the idea of using them (compressed fit, no sagging or convection currents in the walls, water and mold resistant etc). The problem is they come in R-15 and not R-10 (R-10 would be my 1/3 insulation ratio). I read somewhere that in some climates you can get away with a higher ratio but I don’t know how to go about figuring out whether I can or not. Ridgway, CO is in climate zone 6 with an annual mean of 8,000 heating degree days and max of 9,000. Average minimum temps in December and January are around 5ºF although we have seen temps as low as minus 22ºF. We are in a fairly arid climate with an average rainfall of around 17”
So my question – given these are south and east facing walls which will see some sun and given the climate information, can anyone advise me whether the Roxul batts are a good choice – or should I punt and go to a standard R-13 fiberglass batt to avoid condensation on the sheathing?

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  1. user-869687 | | #1

    There's no reason for concern about R-15 batts in your situation. The 2/3 - 1/3 concept isn't a rule exactly. The most important way to reduce risk in a wall assembly is to have good flashing details at openings, and good air sealing.

  2. srenia | | #2

    Been playing with that same idea in zone 5. When you realize R10 continuous insulation is 90% effecient when air sealed. 5% on another R10 fiberglass. About 1% additional for rockwool. This is based off of Uvalues. The conclusion is 1% worth it? The other benefits are real. Rockwool will slow a house fire and will last longer. Those are important factors as well. For Inslation value alone its not worth the extra cost. For durability and safety it is worth it.

    Wet cellouse instalation could be a better solution. Cheap, fire hinderer and able to deal with most dew point issues. CO is fairly dry climate so its less of an issue for where you live.

  3. Gary Dick | | #3

    Thanks for the quick responses! The reason I did not understand why you could increase the ratio in some climates is, it seems to me the arid climate does not make this less of an issue because the amount of moisture at the sheathing will depend on the interior humidity rather than outside. If anything there will be more vapor drive from the inside to the outside because it's an arid climate. I will take care of that (I hope) with good sealing. However, I don't want the dew point to move from the exterior foam to the sheathing because I have too much r-value (per the rule of thumb) on the inside of the wall assembly. Having R15 Roxul on the inside and R20 foam on the outside puts me nearer 50/50 than the 1/3 rule. I am not sure whether I should be this concerned or if I am overthinking the issue and should just buy the Roxul batts.

  4. GBA Editor
    Martin Holladay | | #4

    You're fine.

    In Climate Zone 6, a 2x4 wall with exterior rigid foam only needs rigid foam with a minimum R-value of R-7.5. Since you are planning to install R-20 rigid foam -- far more than the minimum -- your wall assembly will be very safe.

    For more information on this issue, see Calculating the Minimum Thickness of Rigid Foam Sheathing.

  5. exeric | | #5

    I concur with S.E. about cellulose. Cellulose is an excellent product and far superior to fiberglass batts. I have no experience with rock wool so I can't say how it compares. It's often enjoyable to invest a little personal hands on work in addition to hiring out contractors. Blown in cellulose through netting is a good alternative to wet cellulose because its a job any homeowner can do in the construction stage and does not require any demolition. It would give you the feeling of contributing in the construction of the house and would save you a ton of money over any other option.

    Here is a video if your interested: I did my own house using the guidelines from this video and it came out as a great success. Bill Hulstrunk knows what he is talking about in this video and it works well. I rented a bog standard blowing machine from a local lumber yard and used a roller to attain an estimated 3.5 lbs/cubic foot. That should easily be sufficient for a 2/4 wall. The moisture buffering ability of cellulose should easily handle any sheathing wetting problems you might be worried about and would allow you to up the insulation values in the 2/4 cavity from the limits you are now anticipating.

    I should add that the embodied energy in producing cellulose is probably an order of magnitude less than fiberglass insulation and I'm sure is less than rock wool, though I don't know how much. That's important in the total scheme of things.

  6. Gary Dick | | #6

    Thanks Martin - I read that a while ago and it got lost somewhere in all the other reading I have been doing. The manual for the REMOTE wall system has a table which I looked at again. It concludes that I can have up to 57% of my insulation in the stud cavity (climate zone 6 - their example Juneau).Which is over half. That would imply that if I have R20 outside, I should be able to go up to R20 inside and still fall within the guidelines. So based on the responses here and my re-reading of the manual, I am going to go with the Roxul.

    Thanks to everyone that answered ........ Gary

  7. Expert Member
    ARMANDO COBO | | #7

    I'll propose a different approach. Condensation is based in temperature and humidity changes and differences (Delta Ts). The higher the humidity inside the home at a constant temperature, the more outsulation you need to use. Performing a dew point analysis on a 2x4 wall based on climate conditions in Montrose, with three month average temperatures at 30°F and 70°F interior temperature, IF you have interior RH= 30-35%, you would need R7.5 outsulation. If the RH= 40%, you'll need R10, and if the RH=45%, you'll need R13. Lots of showers, cooking and people increases the interior humidity in a home. If the interior temperature goes up, you need more outsulation too. So the point is that you need to make sure you control the interior temperature and humidity in the winter time, based on the outsulation you installed.
    Also, its worth noting that the outdoor RH% for the coldest three months in Montrose is 63,66 & 58, which is pretty high, so humidity in your area tends to move from the outside to the inside in the winter time. This means that you need to pay close attention to your moisture management and sealing details. This is no different than homes I've designed in NM & CO in CZ4-6.

  8. GBA Editor
    Martin Holladay | | #8

    I don't understand your warnings at all. You warn Gary that if his indoor RH is higher than normal, his walls might benefit from R-10 or even R-13 exterior foam.

    But he's planning on installing R-20 exterior foam. So everybody needs to relax.

  9. Expert Member
    ARMANDO COBO | | #9

    My point (obviously missed) is that if Gary wants to save some money on outsulation, he can; hence, "a different approach". Ultimately, if Gary's R20 outsulation is free or he really wants to use it regardless of costs, then more power to him. Most of my clients want the best protection for the best value. I guess I always think about diminishing returns when designing and building houses.

  10. Expert Member
    ARMANDO COBO | | #10

    Installing 3" or 4" of outsulation is great, but it also raises the cost of outsulation and cladding installation, not to mention less choices for cladding.

  11. Gary Dick | | #11

    Well - that boat (budget considerations vs insulation) has already left the dock as the house is under construction - we are framing now and so I am committed to going with more outsulation - two layers of exterior foam well taped and sealed (I read the article on different tapes and will be looking at the Siga Wigluv). So the concern I had - now resolved thanks to everyone here, was whether I might get the balance wrong by over insulating the interior and risking a dew point on the sheathing / studs. I plan to install an ERV as well to help mitigate humidity build up and provide adequate fresh air ventilation.

    Many thanks ....... Gary

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