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Wall and roof assemblies – mineral wool or foam?

Devin Ratliff | Posted in Energy Efficiency and Durability on

This site is such a great source of information – I’ve been planning a weekend house (straw bale, zone 6) for the last several years, and this site along with Dr. Joe’s BSC archive have provided a great education on all aspects of sustainable residential design. So that’s a thank you to everyone who frequents and posts here.

But this isn’t only a shout-out — I do have a few questions and am hoping for some feedback. I’m under construction and have finally worked out my wall and roof assemblies to my satisfaction — I’m pretty sure that I’m on the right track, and I’m almost certain that I’m overthinking this, but anyway:

• Zone 6
• 1500sf (primarily weekend use, but always planning for the future)
• Slab on grade, post and beam, with a straw bale wrap
• 1-1/2 stories, with insulated cathedral ceiling (no can lights, no mechanical)

With straw bale, I get about R30 for the walls and with the insulated cathedral ceiling, I have to work awfully hard to get to the code minimum of R49. So I can’t achieve the 10-20-40-60 rational ideal. But I think that I can get close to a proportional 15-33-50. Here’s how.

Roof (from the inside out):
• ½” drywall
• 1×4 furring strips
• 2” foil faced polyiso with taped seams and sealed edges (vapor and air barrier)
• 2x12s with 8 ¼” fiberglass/mineral wool batts or cellulose
• 1” XPS or unfaced polyiso site-built ventilation baffle with taped seams and sealed edges (2nd air barrier)
• 2” ventilation airspace
• Roof decking

I spent some time worrying about the foam sandwich that I created here, but I think the difference in permeance from in to out (.05 to 1 perms) is adequate, and following Dr. Joe’s advice here https://www.greenbuildingadvisor.com/blogs/dept/building-science/lstiburek-s-rules-venting-roofs, I decided to focus on the air-tightness of the ceiling and the quality of the ventilation space, and not worry about the vapor diffusion.

Anyone disagree or have concerns with the above?

My one concern with this approach is that I’d love to get the foam outside of the house (fire and offgassing, as well as GHG and embodied energy concerns). I’m wondering if I could replace the foam with mineral wool board. It’s a bit lesser performing, but could I use 3” Roxul comfortboard IS under the rafters, held in place with the same 1×4 strapping? Anyone tried that?

Walls. A vast majority are straw bale with earth/lime plaster on both sides, but there are a few outliers:

Stem wall (24″ high) supporting the straw bale (the site is on a slope and the SOG steps). From the inside out:
• Wood wainscot (3/4” t&g)
• 2x2s on the horizontal providing a service cavity for wiring
• 1/2″ OSB with taped seams and sealed edges (air barrier, class II vapor retarder, and shear bracing for the wall)
• 2×6 studs with fiberglass/mineral wool batts or cellulose
• Vapor permeable WRB/air barrier (#15 felt or housewrap)
• 2” polyiso or 2 ½” XPS or 3” mineral wool board (Roxul comfortboard IS). If foam, need to maintain R11.25. If mineral wool, need to maintain class II vapor retarder at the interior face
• 1” Airspace
• 8″ Stone facing anchored with wall ties

I actually asked a question on this a couple of years ago, and I’ve improved on my original detail based on the recommendations. But my question now is about the foam/roxul. I like the roxul for the reasons mentioned above and the vapor permeance. That allows the stem wall to act much like my straw bale wall. But I wonder about the stone – am I likely to get inward vapor drive in summer? Is it better to go with the lower perm foam, isolate the stone, and let the wall ‘breathe’ to the inside?

Other walls. These are basically the same as the above without the service chase and without the stone — from the inside out:
• GWB with standard latex paint (class III vapor retarder)
• 2×6 studs with fiberglass/mineral wool batts or cellulose
• 1/2″ OSB structural sheathing
• Vapor permeable WRB/air barrier (#15 felt or housewrap)
• 2” polyiso or 2 ½” XPS or 3” mineral wool board (Roxul comfortboard IS). If foam, need to maintain R11.25. If mineral wool, need to maintain class II vapor retarder at the interior face
• 1×4 furring strips
• Paper faced lath & plaster

I haven’t seen much on mineral wool insulated sheathing as an underlalyment for plaster. Anyone have any experiences?

Appreciate your thoughts,

Devin Ratliff AIA

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Devin,
    Q. "Anyone disagree or have concerns with the above [foam sandwich]?"

    A. No.

    Q. "I’m wondering if I could replace the [ceiling] foam with mineral wool board."

    A. I've never heard of anyone trying that. You might find that mineral wool is too squishy for this assembly, seeing as how it would only be supported on the top side by the rafter edges, 16" or 24" on center.

    Q. "My question now is about the foam/Roxul. I like the Roxul for the reasons mentioned above and the vapor permeance. That allows the stem wall to act much like my straw bale wall. But I wonder about the stone - am I likely to get inward vapor drive in summer?"

    A. Indeed, rigid foam is preferred behind stone veneer because rigid foam resists inward solar vapor drive. If your house has air conditioning, your OSB could accumulate moisture over the summer, which isn't ideal.

    Q. "I haven't seen much on mineral wool insulated sheathing as an underlalyment for plaster. Anyone have any experiences?"

    A. I invite any GBA readers with experience with this approach to comment.

  2. Devin Ratliff | | #2

    Thanks for your responses, Martin.

    It seems that I'm reaching a bit on the mineral wool - each application seems to be suggesting foam as the preferred (or at least time tested) solution. Not that there's anything wrong with breaking new ground, but it's good to know that it is new ground, if that's where I am.

    One thing that you point out -- and I hadn't really focused on -- is the need for structural sheathing behind the mineral wool for support. I omitted that from both the rafter system and the stem wall system, despite the roxul suggested details. Neither application in foam requires/suggests struct sheathing, and that may be the thing that pushes the roxul into the harder and more expensive category.

    Going back to my first concern (offgassing and fire hazard from the foam insulation in my vaulted ceiling): The code addresses the latter by requiring encapsulation of the foam by a thermal/fire barrier (my 1/2" gwb). That's based on limiting the foam to a 250 degree rise in 15 minutes. So, is that safe? The code says yes, but does anyone know of any studies that demonstrate the effectiveness of the gwb? Residential fires (at least from the anecdotal evidence in the news) seem to be pretty fast, and 15 minutes doesn't seem unreasonable -- at that point, smoke and fire are much greater hazards -- but I'd appreciate any insights.

    As for general offgassing, I note that polyiso is more susceptible to this - does that push me towards EPS/XPS here? I'm inclined to think that the foil facing and gwb encapsulation go a long way towards making that concern moot - so the environmental preference for polyiso may hold. Thoughts?

    As for the stem wall, my preference would be for a class II or III vapor retarder (an unfaced foam) rather than a foil faced true vapor barrier. That would alleviate any concern about inward vapor drive, but allows some amount of drying to the exterior in the winter. Any real preference for XPS, unfaced PIC or EPS in this application? I would probably throw out EPS because of durability concerns. If I buy recycled, the ultimate choice will likely be based on cost/availability, but if one is preferred over the other, would be good to know.

    Thanks again.

  3. GBA Editor
    Martin Holladay | | #3

    Devin,
    Drywall is a perfectly acceptable thermal barrier or ignition barrier. If you worry enough to want better fire protection than is provided by 1/2-inch drywall, I suggest that you install a sprinkler system.

    I have never seen any data showing a link between rigid foam outgasing and negative health effects for human occupants.

    Most green builders consider EPS to be more environmentally friendly than XPS. For more information, see Avoiding the Global Warming Impact of Insulation.

  4. Devin Ratliff | | #4

    1. Fair enough.

    2. Nor have I, but plastics and foams always seem to be on the forefront of discovering the next negative health consequence, so it's worth asking what others may know more about. I'm perfectly willing to acknowledge that there are potential negative consequences with everything, so in this case, encapsulate and move on.

    3. My concern with EPS is durability. I've honestly never seen anything other than XPS in a cavity wall applications -- if EPS is appropriate here, then I'd consider it.

  5. Devin Ratliff | | #5

    I'd love to find an alternative to XPS, but I don't have much experience with either polyiso or EPS.

    Reading up on polyiso, I realize that the facing is part of the manufacture and so unfaced board doesn't exist (invalidating my previous comments). I'm not keen on a foil vapor barrier as my outside surface in a wall or roof assembly - seems ok for my interior ceiling application, but not as my exterior surface. There seem to be relatively few products that are faced with anything other than foil -- what I find is either paper faced or glass fiber faced. I'd be worried about paper faced in an exterior application, but I suppose glass fiber faced would be ok. Thoughts? Experiences?

    And regarding EPS: can I build ventilation baffles with it? I like the greater permeability if it's strong enough.

  6. GBA Editor
    Martin Holladay | | #6

    Devin,
    Paper-faced and fiberglass-faced polyiso products have been used by commercial roofers for decades. There is no reason to worry about the durability of these products.

    Yes, you can build ventilation baffles out of EPS. Visit a lumberyard and pick up pieces of different thicknesses. Choose the thickness that meets your needs for stiffness. (Stiffness requirements vary, and depend on what type of insulation you will be installing.)

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