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Is this no-foam, no-stud, no-OSB, no-ply, Passivhaus envelope healthy?

Gergman | Posted in PassivHaus on

We’re building a small passivhaus in central Vermont. (We’re shooting for PHIUS+ certification.) We’re eschewing the old paradigm of penetrating the outside envelope with HVAC, wiring, plumbing, sockets, switches, etc. (Anything on inside of the the outside wall be just that, on the wall — i.e., surface-mounted. We don’t like the idea of a service cavity, for several green, labor, material, & space reasons. We will evolutionarily adapt our aesthetic sensibilities.) It will be almost identical to our current house, except we will be using cellulose now where we used foam before. Both have rubber roofs of < 1 in 12. So, for our timberframe, we’re investigating a curtain wall (suspended 2” outside of the frame, actually) with vertical 16” TJI’s on 3’ ctrs., dense-packed w/ 3.5# cellulose, and wrapped with a high variable-perm internal air-tight membrane (Intello+) & slightly lower perm outside membrane (Solitex Mento 1000). (Pls see attached drawings.) The roof would match, except for one important “detail:” there’s no roof vent at the top of the assy. This is no architect and no designer on this low-budget project; none of us have worked on a passivhaus or an envelope assy. like this before. A couple of experts have weighed in on the side of “It’s fine. Cellulose is an extremely robust moisture-handler and just loves to move moisture.” But we’re open to other views. [I apologize for the length & complexity of this post. Thanks for your patience & any help offered.] Walls: – 24’x24′ timberframe (TF: post & beam), full basement, two floors, all joists & rafters on 3′ ctrs. running N./S.; – 2X3 spacers flat on the outside of the exterior TF surfaces (keeps the TF “inside” the envelope; allows windows & doors to be “independent” of the TF); – Horiz. & vert. 4×6’s frame the windows & door penetrations & serve as chair rails; they share the outside plane w/ the 2×3’s & come back into the TF space (4″ w/in TF, 2″ outside to reach the spacer plane); – 3/4″ T&VG cedar inside wall sheathing applied from the outside, nailed into the 2×3 spacers & the 4×6’s; – DB+ smart membrane applied over the outside of the cedar interior wall; – Vertical 16″ TJI’s on 3′ ctrs. all around the house fastened through the cedar sheathing & the 2×3 spacers & into the TF; – Solitex Mento 1000 on the outside of the TJI’s; – Vert. battens on the TJI’s; – Horiz. battens on the vert. battens with a Cor-A-Vent top & bottom; – Dense pack cellulose in the TJI cavity; – Vert. T&G cedar sheathing. Roof: – 2X8 T&G pine decking (ceiling) on top of the (3′ o.c.) rafters; – Intello+ smart membrane; – 16″ TJI’s running N./S. on top of the rafters; (as they run past the bldg. they are attached to the N. & S. wall TJI’s & then run by those to provide N. & S. eaves); – DP membrane; – 2×6’s on 12″ ctrs. running E./W. across the TJI’s (as they run past the bldg. they are attached to E. & W. wall TJI’s & then run by those to provide E. & W. eaves); – Dense pack cellulose the TJI cavity; – DP membrane on the 2×6’s; – 45deg. angled layer of rough-sawn 1×10 pine; – Dense pack cellulose the 2×6 cavity; [- Do we need an air vent here?] – 1/2″ fiberboard; – 60mil fully adhered EPDM. Air & Vapor Sealing: – The interior space is sealed with a continuous layer of Intello+, whose overlaps are sealed with a bead of 100% silicone caulk & then taped with the mfr’s. tape; – The exterior wall is sealed with a continuous layer of Solitex Mento 1000, taped & sealed as above; then sealed & taped to the DP webbing that covers the roof 2×6’s.

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  1. GBA Editor
    Martin Holladay | | #1

    You're putting a lot of faith in those European membranes. They might work for 100 years, and they might not. I don't think that any of us knows how long these membranes (or the tape used on the seams) will work. I would feel better with a plywood or gypsum wallboard air barrier. But that's just my opinion.

    Your text seems to say that the 2x6 cavity on the roof will be filled with cellulose. The way I read the drawing, however, the 2x6 cavity on the roof seems to be empty. If you fill it with cellulose, you have a code violation. If you are using cellulose to insulate your cathedral ceiling, you must include a ventilation channel above the insulation. More information here: How to Build an Insulated Cathedral Ceiling.

  2. Gergman | | #2

    Redirect to Martin:

    Thanks for weighing in on this, Martin. I’m very willing to learn from the vast experience of the bldg. code. However, I’m also aware of a few of its outright mistakes, its occasional slowness to learn from some of its over-generalizations, & its sometimes refusal to evolve along with bldg. science. (PassivHaus is already running into a few of these challenges.) We’re not covered by any bldg. codes where I live - we heat & cook w/ wood; use a composting toilet, etc.

    With your response in mind, I’ve taken another look at my “perfect” sketches and found them wanting. I’ve corrected the labeling of the exterior membrane; its attachment to the DP webbing above the 2x6’s; more clearly shown the cellulose; I’ve corrected the text in my posting - I hope.

    I have read that cathedral ceiling piece carefully. I’ve also read all that I could find of Dr. Joe’s papers on those related issues. Those presentations are often a few years old & do not address an assembly very much like the one I present. I agree that the long term viability of these new “smart” membranes will tested by assys. such as this one.

    So, we’re willing to take a bit of a risk here, but I’d like to know if the real risk is greater than we’re willing to tolerate. And we’re wondering if we could know a little more about the exact nature of the risks involved - perhaps we can ameliorate some w/o throwing out the whole idea.

  3. GBA Editor
    Martin Holladay | | #4

    Your roofing is EPDM. There will be no drying of this roof assembly to the exterior.

    You can't use fiberboard for roof sheathing. (And there is no point using a breathable sheathing for your roof, since you have chosen vapor-impermeable roofing). Your choices are boards, plywood, or OSB.

    There is a good reason that unvented roof assemblies don't use air-permeable and vapor-permeable insulation like cellulose. These insulation materials make the roof sheathing cold without stopping the flow of air or water vapor. As a result, the sheathing accumulates moisture over the winter.

    The solution is to include a ventilation gap above the insulation layer.

    Remember, you can get rotten roof sheathing even if you live somewhere without building codes.

  4. Gergman | | #5

    Reply to John Brooks.

    Yes, I have. To put my last line first & spare you the long presentation:

    We're mainly asking about the "missing" roof vent plane,

    but... We're attempting to avoid foam products as much as possible - even EPS. I understand where the condensation plane will be. I also understand how difficult it is to really SEAL the inner envelope, & make sure it stays sealed - or, semi-permeable, & make sure it stays that way. With essentially no bldg. training or experience I've spent the past few weeks studying these issues online & with available pros. (I'm obviously continuing that process here.) My surmise is that we're hard upon a major technology change in bldg. science. The structures we used to build were long-lasting & robust partly because we invested so much energy into continually flushing the moisture, CO2, CO, VOC's, & stale bacon odors through their walls & ceilings. Now that we've begun to attempt to stem that energy investment by controlling the bulk air & vapor movements, we've had to learn how & where, & then finally why, to draw our front lines of engagement. Bldg. science helps us understand why certain approaches fail, & gives us a hint as to where & how to proceed in our investigations.

    But now we've developed products that can control the physical properties of even residential construction to the point that we can "do" just about anything we want to do. (Think osmotic membranes, variably-performing plastics, expanding foams, CERV, etc.) The question is, exactly what works in which conditions? Responding to the variabilities Mother Nature throws at us, even at a single geographic location, can be most vexing. In our project, we've committed to trying to assist these advances in some small way - as long as we can tolerate the risk. (Like investing the stock mkt. - so I hear....)

    We're hoping to hear specifics from you experts as to where we need to pay attn. &/or perhaps change our plans.

  5. Gergman | | #6

    The guys at Efficiency Vt., agree that it'll take up moisture during the winter - but at a very slow rate & small amt. - but will happily give it back since the smart membrane is designed to do encourage/permit just that.

    The fiberboard (which we have on this house) is meant to protect the EPDM from injury: from above (we clods walking around on it to clean the chimneys & clear the snow off the solar panels); & from below (irregularities & changes in the lower sheathing layer. We're hoping to use local kiln-dried (cell-broken) 1x10 pine boards beneath.

    We've been told that cellulose "loves" to move moisture; & that as long as it can get back out more easily than it can get in, we're good. The fact that it might reach the condensation/freezing layer doesn't matter - we're told. (If I were completely comfortable with & convinced by all this, I wouldn't be *here*, though.)

    We're prepared to make the ventilation gap - if it's absolutely necessary. I'm not having any trouble finding folks who agree with you. I guess I'm asking you just how sure you are that it can't work otherwise. Again, I look to follow bldg. codes, as long as they're appropriate.

  6. user-1119462 | | #7

    Electrical code in Canada requires that there be no exposed wires or points lower than two metres (say 6 feet) at least in commercial spaces, and this may well be more stringent in domestic housing (I haven't asked). While you are not code restricted, it may be sensible to talk to someone appropriately licenced in your area so that you at least follow "good trade practice" and don't inadvertently do anything technically risky.

    Regards, Tony.

  7. Gergman | | #8


    Thanks for the notice. That sounds sensible, too. We do have that covered in our existing house. Spacing the interior wall 2" outside of frame gives us lots of "chases" for that sort of thing; & we've not been averse to surface-mtd. channels, power strips (stylish, of course), etc., either.

    We're actually looking for ideas & maybe new products that might help us make the outside-the-wall movement more attractive to folks.

  8. GBA Editor
    Martin Holladay | | #9

    Thanks for explaining the fiberboard on the roof; I had missed the fact that there were 1x10 boards under the fiberboard. My mistake.

    I still advise you to include a ventilation channel, however. In addition to the first reason I listed, here's another: Vermont gets a lot of snow, making cold roofs a good idea if you want to avoid ice dams.

  9. Gergman | | #10

    Actually, we like ice dams. Our current roof is 1-in-13, the cottage roof will be .5-in-12. We keep the snow up there for insulation & to protect Barbara's plants & bushes. Our roofs shed water only to the north, & only off its corners. (We're also looking at the possibility of making it a green roof.]

    Are you convinced that water will enter from the conditioned space & then not be able to exit? That it will stay too long? That it will remain in concentrations that are too high? (This is my concern.) Do you know of an assy. that is very similar to this one - but perhaps w/ a ventilation channel?

    I wonder (& so did one of the folks at Eff. Vt.) that such a large area of ventilation with such a relatively small "slit" of ventilation will, given certain not-uncommon weather conditions around here, become vapor-saturated. Then I (alone now) wondered if a sunny day might selectively drive too much of that moisture into the envelope while not enough escapes through the side vents. I suppose the driven moisture can still exit & then be expelled a little later, tho. (This also raises questions about screening those vents & the retarding effects of any screening.) I believe that, given its level-ness, & the fact that gravity-assisted convection cannot assist in the expulsion of this moisture, that maintaining a layer of high relative humidity air against the cellulose membrane isn't a good idea.

    [Boy, I thought *I* was up early this morning....]

  10. GBA Editor
    Martin Holladay | | #11

    OK, so you are designing a low-slope roof -- the topic of my blog this week: Insulating Low-Slope Residential Roofs.

    My article has several stories about wet roof sheathing. You may well be correct that the choice of cellulose will prevent your roof assembly from suffering the wet-sheathing problems that other roofs have suffered. But if I were building the roof, I would say that your bet isn't worth the risk.

  11. Gergman | | #12

    Fascinating piece. I can't remember seeing anything that addresses the peculiarities of our situation so closely. Implied in there is your oft-mentioned caution against allowing moisture to reach "dead end" surfaces such as fiberglass & polyethylene. (That's why we've kept the transporting mechanism (cellulose) against those surfaces & the others (fiberboard & pine) that in are contact with them.)

    We, too, are in touch with Bill (who's a great guy, and once a neighbor working at CVCAC - but he DOES sell the stuff, after all.... ;-)); and Bruce's cautions are instructive. But, again, we don't want to depend upon vapor-sealing the ceiling - forever. Just as the old envelopes were allowed to "breath," we're thinking of reverting to that paradigm, but under more controlled conditions. I.e., not allowing for the possibility of a pinhole expulsion of moisture into a large volume & then expecting to somehow extricate the escaped moisture back through that same pinhole. If we can accomplish a vapor-robust assy. that doesn't overly depend upon owner knowledge, upkeep, & protection we'd feel safer.

    Bill Rose's response, part 2 and Dana Dorsett's fourth paragraph are especially interesting to me.

    However, before this horse dies at our hands, it seems that we've come to an understanding. I think I understand your cautions & doubts. I'm tilting away from the idea now, as a bet that isn't worth the risk.

    Finally, this is such a great resource! During all the time I've been reading your work - on this site & others - I wondered if I might eventually be able to have a give-'n-take with you on some issue that was important to me (free, even!). It's all pretty amazing. The "regulars," who present original research and papers, the references, the commentators from the field, the wealth of experience, the considered responses. It's a wonderful service that is so appropriate for the promotion of the "green philosophy." Thanks so much.

  12. Expert Member
    MALCOLM TAYLOR | | #13

    Greg, Most electrical codes allow wiring to be surface mounted as long as it is run in conduits or uses BX (armoured) wire. You often see this in commercial applications on concrete or block walls and in residential loft conversions. When the project's aesthetic leans towards the modern or industrial, the exposed electrical and mechanical services often add to it .

  13. Gergman | | #14

    Yeah, thanks. My neighbor across the road is from New Yahk City & he did that industrial look with brushed aluminum tubing & hanging lights in his timberframe. My wife likes it a lot, & it looks pretty easy to do. I can imagine it in the kitchen, bath, & dining rooms a little more easily than in the (more rustic & cozy) bedroom & living rooms, tho. We'll see.

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