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Insulating vented cathedral ceiling with Roxul comfort batts and foil-faced polyiso, HRV and Mini-split recommendations, etc.

edkrause | Posted in General Questions on

More than one question, I’m afraid, but here goes:

I’m building a 1400 sq. foot house using materials and techniques to make it an energy efficient (relatively) passive solar in the woods of western Wisconsin, zone 6:

~Triple pane windows with various coatings and gases

~R12 3″ Roxul comfort board for outside continuous blanket

~R23 Roxul Comfort Batts in stud cavities. air sealed carefully.

~ I know from reading this forum off and on over the years that a cathedral ceiling is difficult to do well.. I’m doing it anyway, for reasons of ease of build with one occasional semi-skilled helper and aesthetics. The vault is one long run on a simple shed roof on the second story, which is for only 35% of the buildings footprint. My plan is for R38 of Roxul in rafter cavities (12″ TJI rafters) plus 2″ rigid foam on the bottom side for thermal break and an additional R10. The roof has continuous 1.5″ site-built venting, bottom to top.

~ Standing seam metal roof. 3/12 pitch. large overhangs.

Questions:
1. I’ve been reading more about smart vapor retarders. My roof/ceiling plan was to tape the foam and run purloins for sheetrock and T+G aspen ceiling. This system will have a barrier, not a retarder, correct? A problem?

2. I’m planning on a smart vapor retarder for walls. I’ve been reading about the Intellio options and the MemBrain. I’m leaning towards the later. Recommendations, for price and performance?

3. HRV for this space? I’ll be doing it myself and would like to keep ducting as simple as possible. Recommendations?

4. And lastly (I hope). I put in-floor heat in the concrete ground floor — acid-etched, slab on grade — which runs on an off-peak electric micro-boiler. I’m planning on a ductless mini-split for the 2-4 weeks in summer we need the house closed and cooled, which will be mounted in the stairwell between ground and 2nd floor. It may also be used for back-up heat for the small woodstove I’ll eventually install. Mini-split model and size recommendations for this design?

Many thanks in advance for any observations/recommendations.

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Replies

  1. brp_nh | | #1

    Is this all just a plan or have you actually started building...and if so, where in the process are you?

  2. edkrause | | #2

    Brian, It is in process: Outside is finished. I've installed 80% of interior insulation. Thanks, Ed

  3. GBA Editor
    Martin Holladay | | #3

    Edward,
    Q. "I've been reading more about smart vapor retarders. My roof/ceiling plan was to tape the foam and run purlins for sheetrock and T+G aspen ceiling. This system will have a barrier, not a retarder, correct?"

    A. The ceiling assembly you have described includes an interior vapor retarder that may be a vapor barrier (the rigid foam). You do not need a smart retarder in this location.

    For everything to work, you also need an interior air barrier. The interior air barrier can be the rigid foam, the drywall, or both. Make sure that you address the penetrations -- you want an airtight assembly if possible.

    Q. "I'm planning on a smart vapor retarder for walls. I've been reading about the Intellio options and the MemBrain. I'm leaning towards the latter."

    A. Either one will work. I think that MemBrain is usually cheaper, but you'll need to do your own research on prices.

    Q. "HRV for this space? I'll be doing it myself and would like to keep ducting as simple as possible. Recommendations?"

    A. I recommend that you read as many articles as possible. Start with these three:

    Designing a Good Ventilation System

    “Installing a Heat-Recovery Ventilator”

    Commissioning Our Heat-Recovery Ventilator

    I also urge you to get some help to commission the system after you have installed it -- you need to verify air flow rates.

    Q. "I put in-floor heat in the concrete ground floor -- acid-etched, slab on grade -- which runs on an off-peak electric micro-boiler. I'm planning on a ductless minisplit for the 2-4 weeks in summer we need the house closed and cooled, which will be mounted in the stairwell between ground and 2nd floor."

    A. I think you will find that your upstairs is hot. If you are installing a minisplit for cooling, it really needs to be installed on the upper floor.

    Q. "It may also be used for back-up heat for the small woodstove I'll eventually install."

    A. In that case you need two ductless minisplits: one upstairs for cooling, and one downstairs for heating.

    -- Martin Holladay

  4. Expert Member
    Dana Dorsett | | #4

    A 2x6/R23 wall doesn't meet IRC 2015 code minimum for zone 6. The IRC spells out R20 cavity fill + R5 continuous insulation or R13 + R10 continuous insulation. See

    TABLE N1102.1.2 (R402.1.2) INSULATION AND FENESTRATION REQUIREMENTS BY COMPONENTa

    https://up.codes/viewer/int_residential_code_2015/chapter/11#11

    Cavity-R cannot be swapped for continuous R on a 1-1 swap either, due to the thermal bridging effects of the framing, but it can be compliant by U-factor using other R-values other than the code minimums.

    R5 of insulation outside the structural sheathing is not sufficient dew point control at the sheathing layer for R23 cavity fill. It needs at least R11.25 if going with 2x6 framing. But but a 2x4/R15 wall with R10 on the exterior would beat code slightly for thermal performance and would have plenty of dew point margin, enough that you can safely skip the MemBrain or Intello, with only standard latex paint for the interior side vapor retarder. There are probably several square miles of 2" reclaimed rigid polyiso foam in reclaimers and building salvage warehouses in the US, priced at less than 1/3 that of virgin-stock foam. It's fully labeled R-value would be anywhere from R11 to R13 so it would meet code min on a R15 2x4 wall, but from a dew point control in a zone 6 climate in that stackup derate it to it's mid-winter ~R10. You only need R7.5 on a 2x4 wall to skip the vapor retarder on an R13 wall, and the R10 has plenty of margin on R15.

  5. edkrause | | #5

    Thanks for the links and recommendations, Martin. I'll do some more reading about HRV's. It seems the Zehnder is an efficient and quiet machine, and I like their flex duct system, though I'm not sure we can afford the Mercedes. I was hoping one of the North American machines had risen to the top among builders of smaller homes in Northern climes... something quiet and efficient. Easy ducting would be great too. After a while all the options become mind-boggling. Won't someone do my homework for me?!

    Likewise with the mini-split(s). Those you who have specified mini-splits in regions with many sub-zero days and some hot humid summer days (we are in the woods and mostly in shade), which are the most versatile, efficient and trouble free?

    Dana, the walls are R23 in cavities with R12 Roxul Comfort Board on the exterior, rather than foam, which I've tried to use sparingly: under the slab, on frost walls and one layer of 2" foam in the cathedral roof assembly. The building is sided with cedar shingles over a 3/4" drainage/drying plane.

    Thanks very much,
    Ed

  6. GBA Editor
    Martin Holladay | | #6

    Edward,
    You may want to read the comments posted on this recent Q&A thread. Although the discussion focused mostly on ERVs, it also touched on HRVs: ERV selection.

    -- Martin Holladay

  7. Expert Member
    Dana Dorsett | | #7

    With high vapor permeance R12 on the exterior of R23 cavity fill, combined with a 3/4" air gap to maximize drying capacity toward the exterior an additional smart vapor retarder for the wall assembly isn't likely to buy you very much, even if the R-ratio is just a (tiny bit) marginal relative to the IRC prescriptives.

    If the wall sheathing were asphalted fiberboard instead of CDX or OSB there wouldn't be any question of wintertime moisture accumulation or damage in the sheathing.

  8. STEPHEN SHEEHY | | #8

    Edward: Re: minisplits, our Fujitsu RLS3H units work very well. They have only been in use for two years, but no issues at all so far.

  9. edkrause | | #9

    Thanks much, Martin, Dana and Stephen. -Ed

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