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XPS Instead of Drywall

Gregsko | Posted in Green Building Techniques on

Hi, I’m remodeling an attic in a brick house.  The outside is insulated with graphite EPS (4″ thick).  I’d like to add an inch of XPS boards on the inside for an added insulation and air tightness.  My understanding is that it would hold the air inside the house, but also stop water vapor from escaping.  My biggest concern is mold.  The vapor and stale air would then be ventilated through heat recuperator with a limited heat loss.

I do not intend to put up drywall.  I don’t see the point.  Vinyl netting would be glued (cement based adhesive) to the outside of the XPS, creating a uniformed wall structure, then the glue layer would be finished with polymer coating.  I could add some latex paint for the bathroom or a waterproof wallpaper.

Are there any holes in my reasoning?

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Replies

  1. insaneirish | | #1

    > The outside is insulated with graphite EPS (4″ thick).

    The outside of the roof? Or do you mean behind the brick? What climate zone are you in? 4" of GPS could be quite a lot for your climate zone and give you plenty of flexibility to use 'fluffy' insulation underneath the roof deck, but we need that info.

    What is the construction of the roof (i.e. rafter depth)?

    > Are there any holes in my reasoning?

    Yes. At least in the US, the situation you've described requires a thermal barrier over foam insulation. Two common options for that are 1/2" GWB (AKA drywall) or 23/32" WSP (AKA 3/4" plywood or OSB).

  2. creativedestruction | | #2

    Your solution sounds risky and unnecessary. With a continuous R20 above the roof deck I question the need for additional foam on the interior, but you haven't stated your climate zone or the rest of the assembly. Is there an air barrier beneath that existing foam on the structural sheathing? That would 'control' water vapor more safely than any attempt to "stop vapor". Waterproof wallpaper would likely be zero perm so that assembly can only dry upward (through shingles?). Not usually a good idea, but more information is needed.

    Most foam insulations require a code-approved thermal barrier separating them from occupied space. A polymer coating isn't prescribed in the code. There are few exceptions like Thermaxx and maybe Dow XPS -- double check the product data of what you plan to use. Drywall fits the bill, is cheaper and can be detailed as a suitable interior air barrier.

    Edit: Patrick beat me to it. Leaving this response to echo our similar concerns.

  3. Expert Member
    Peter Engle | | #3

    If you are going for a stucco-like look with the mesh and polymer coating, you could just go with traditional cement stucco plaster. It does meet the fire code requirements. Then again, it's way more expensive than gypsum which is why gypsum took over the market. Gypsum drywall is the cheapest and fastest sheet covering that meets fire, structure and other requirements.

  4. charlie_sullivan | | #4

    If you are in North America, an additional issue is that XPS is made with a high-global-warming impact blowing agent. GPS is a better alternative, if you need a foam board, but that might not be the right solution, depending on your answers to the other questions.

  5. Gregsko | | #5

    So, the climate would be a zone 5. Outside walls have graphite Expanded Polystyrene glued to brick walls and then covered with a thin-coat silicone render. There is no structural sheathing, since it's a brick wall.

    I'm attaching photos of the roof, so you can better see the progression, and gauge the rafter's depth. The roof and rafters are covered with open-cell spray polyurethane foam (pic). Then the former attic is divided into 6 feet (floor to ridge) section, where the ventilation system will be located, and the living section below. That dividing floor is made of ceiling joists filled with insulation (fiberglass or mineral wool) and covered with a vapor barrier (pic).

    Steve, by thermal barrier, you mean fire proofing walls, right? Couldn't I just use mesh and coating, as suggested by Peter, instead? It seems that XPS boards (which burn but don't sustain fire) covered with mesh (cement based) and the right coating (fire proof and water resistant) would be a better thermal barrier than say plywood.

    Jason, is there coating you'd suggest over polymer i.e. cement? Roof is covered with aluminum plates and ceiling has vapor barriers. The idea is to have the living space airtight and have the air being vented through a mechanical ventilation with heat recovery.

    Charlie, I already have the XPS boards.

    In an airtight space, gypsum drywall seems like an unnecessary layer of water-soaking material cladded in paper, which is an organic material that mold can feast on.

    1. creativedestruction | | #6

      So is there rigid foam above the roof deck or just outside the brick walls? I'm seeing interior polyethylene I believe (yellow membrane over the open cell)? And from the sound of it there is no air gap ventilation channels either above or below the "aluminum plates". This would seem to be a code violation as you either need to have an air gap at each rafter bay for ventilation, rigid foam above or closed cell foam in the rafters for zone 5.

      This roof has limited drying capacity. Paperless gypsum is an option that won't support mold growth but it's not going to reduce the risk of moisture accumulation within the open cell foam, in absence of rigid foam above the roof sheathing.

  6. Gregsko | | #7

    Aluminum panels is what the roof is made of, and there must be some kind of air gap between them and the roof deck (a professional roofing company did the roof).

    If by rigid foam you mean EPS Styrofoam, then it's on the outside of brick walls only. Then underneath the roof deck there is an open-cell polyurethane spray foam with no membrane (photo). Then the ceiling joists have a mineral wool insulation with a yellow membrane as a vapor barrier (photo). Then suspended ceiling made of XPS (extruded polystyrene) boards( + mesh & plaster) would be attached to the ceiling joists and roof rafters (only sections located below joists).

    My concern is the rigid foam XPS that I plan on cladding the interior walls and ceilings with. When it is covered with mesh and cement-based adhesive, and then an interior wall plaster, would it be fireproof enough? What would be the best plaster for bedroom vs bathroom? Would mechanical ventilation be sufficient to let the vapor escape?

    1. creativedestruction | | #8

      These articles may help:
      https://www.greenbuildingadvisor.com/article/thermal-barriers-and-ignition-barriers-for-spray-foam#
      https://www.greenbuildingadvisor.com/article/how-to-build-an-insulated-cathedral-ceiling
      https://www.greenbuildingadvisor.com/article/all-about-attic-venting

      Also you said:
      "Then the former attic is divided into 6 feet (floor to ridge) section, where the ventilation system will be located, and the living section below. That dividing floor is made of ceiling joists filled with insulation (fiberglass or mineral wool) and covered with a vapor barrier (pic)."

      It's not a good idea to put a mechanical ventilation system in an unconditioned attic space. The system would lose significant heat-recovery efficiency and may just fail. I know of no manufacturers that would warranty that.

    2. Expert Member
      MALCOLM TAYLOR | | #11

      Gregsko,

      In the absence of some evidence, it isn't wise to assume the roof is vented above the deck. Most metal roofs aren't, whether installed by processionals or not. That's important because open-cell foam on the underside can allow moisture to migrate up to the sheathing, and with no drying path it can cause it to rot. You will have to be very careful with the humidity levels in the attic for it to stay safe - if that's even possible with the XPS below.

      Drywall is more susceptible than plaster to moisture damage and mold. But the conditions that would led to those symptoms in the drywall mean your walls and roof are in big trouble. The drywall would just be a bell-weather that there were moisture problems elsewhere.

  7. Expert Member
    BILL WICHERS | | #9

    I think by installing the XPS on the interior, you're creating a moisture trap since the wall has no drying ability in either direction at that point. You're also going to need to put a thermal barrier on the interior side, which is normally done with drywall -- although there are other ways to do it. I'd expect stucco to be a lot more work, and also more expensive, than a layer of drywall would be.

    You'd be better off finishing the interior wall with drywall to allow for some drying. I'd save the XPS for something else. If you really want continuous rigid foam on the interior, you need something more vapor open, such as a thinner (1" or less) layer of EPS or kraft faced polyiso. You'd still need the thermal barrier with either of those materials though.

    Bill

  8. Gregsko | | #10

    Jason, I agree with you about ventilation system in an unconditioned attic space. In my case there is spray foam above the ventilation system sealing it off from the roof itself; therefore, it's a conditioned space.

    Bill, drywall is the customary material, because it's inexpensive and easy to work with, but is it the best? Gypsum soaks in moisture and paper is an organic food source for mold. My house is made of brick and has an aluminum roof. It doesn't use gas or firewood and I'm not a smoker. Fire while possible is highly unlikely. Still, I'm considering the possibility, so with a layer of cement based mesh and i.e. cement based plaster, the wall surface should also be fireproof. As for moisture, I want to move it out with the air through the ventilation system, not through the walls of the house.

    The question is, what plaster or plaster/paint combo would give me the best thermal barrier and best mold resistance?

    1. Expert Member
      BILL WICHERS | | #12

      You can get gypsum board that is not paper faced. Densglass is one example. Another possible material to consider would be MgO board, but that's not very available in this market, and I'm not sure if it would meet code requirements (not because the material isn't physically capable, but rather because I'm not sure it's been tested for code compliance in the North American market).

      Plaster has similar moisture issues to gypsum, so I don't think there is any advantage to wet plaster compared to drywall in that regard.

      Bill

  9. Gregsko | | #13

    Yes, gypsum plaster and gypsum board that's not paper faced should be quite similar.

    I was thinking of using some sort of waterproof plaster that's based on cement-calcium or cement-polymer. Since XPS board would make the wall straight and rigid and plaster would give it the thermal & waterproof qualities, I just don't see the point of drywall.

    1. Expert Member
      BILL WICHERS | | #15

      XPS won't make the wall very rigid, it's not a super strong material.

      I really think you're making things harder for yourself trying to avoid using drywall. The big reasons drywall is so commonly used is because it's a cheap material and it installs quickly.

      Bill

  10. creativedestruction | | #14

    Why not paperless drywall? No mold food, just fiberglass facings.

    Skip the XPS. You don't want more insulaton at the plane of the ceiling as it makes the attic space half-in-half-out if the conditioned space unless it has dedicated supply and return from the heating and cooling system, not just the ventilator.

  11. Gregsko | | #16

    I'd be open to fiberglass faced drywall on the ceiling in the areas outside of the bathroom.

    The attic is already half-in-half-out due to spray foam above it and mineral wool below.

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