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How important when insulating with foam board is it to be for the joints be overlapped, or pieces be foamed in place?

user-6612786 | Posted in GBA Pro Help on

My project is a retrofit of foam over the sheathing of a geodesic dome. Every triangle has a gentle angle, which will interrupt the five inches of XPS foam that’s being added. The sheathing below the foam will be primed with an acrylic sealer and taped. Over the insulation will be a water barrier, and strapping, 2 3/8 inch vent space, sheathing – with Densdeck Prime (sealed and taped) and a single ply adhered roofing of TPO. My feeling is that the foam insulation in this case is not the primary air barrier, and that I could use mineral wool slivers to fill the gaps, as this adds some fire resistance laterally between foam panels and a vapor conduit for any tiny amounts of condensation, that may occur. Which leads to the question of what perm rating should the weather barrier (underlayment, wrap, or slip sheet ) be over the foam? Should I be thinking also of the fire rating of this foam board cover? Even using two types of wrapping, like Tyvek, with a perforated aluminum reflective cover? The strapping is to be notched at the corners, and the intermediate to allow water draining in the event of moisture getting past the outer roofing material. Retrofit will start from the top, working down, weathering and leaving exposure to a bare minimum.
The weather zone is a 5 in central WA, high desert, with cold winters, and hot summers, generally low humidity. Interior existing poly air barrier to be removed during retrofit. The TPO roofing material is highly reflective, and there will be a sprinkler system on top of roof, as this is an area with annual wild fires. There is existing fiberglass insulation in the triangular strut cavities, and the retrofit will bring the insulation value from R21 to a R46. While the existing home was under insulated to the default code, it was allowed per the alternative energy code, by computer analysis, due to the buildings high volume to skin ratio. Comments, answers, recommendations, etc, highly appreciated. Thanks!

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  1. GBA Editor
    Martin Holladay | | #1

    You say that the sheathing below the rigid foam will be taped. Does that mean that it will be detailed as an air barrier, addressing all seams and penetrations? I hope so, because that is the most important air barrier in this assembly.

    I don't think that the mineral wool you propose installing (at the rigid foam seams) will provide any benefit -- there is no real need to provide "a vapor conduit for any tiny amounts of condensation that may occur." I would be inclined to use canned spray foam at this location.

    You write that you will install a "water barrier" between the rigid foam insulation and the ventilation air space. What is a water barrier? Will this layer be airtight?

  2. user-6612786 | | #2

    Your first point is yes, the sheathing below will be coated on all sides with an acrylic air barrier, called DRAW-TITE, primer - sealer, Class 1 barrier, with a perm of less than 1, and edges filled with quality caulking, and taped over with roofing seam tape, and saturated with the same DRAW-TITE, creating an uninterrupted barrier on the foam board side.
    The other reason to use the mineral wool gap filler, was because of speed, cost and fire resistance. I figured it would be faster to precut slices of mineral wool than use canned foam, but maybe with the specialty nozzles, the can would win. Admittedly,when I have used foam it often clogs if stopped, and a pain to clean the nozzle out, also those cans are pricey. Which is greener, cans, or mineral wool is hard to say, as everything has hidden ecological / health costs.
    The, "Water Barrier", is a the barrier over the foam and under the strapping and ventilation area. If it was in a wall it would be the house wrap under the rain screen, and in an assembly with shingles, it would be underlayment, under a flat roof it may be called an ignition barrier. Basically there are a plethora of different options from non-breathing coatings, and rain barriers, house wraps, and roofing felts. I have considered the options, and am looking for something that is easy to cut, lap and tape or caulked in a weatherized shingle pattern of large triangles. Translucent would be nice but not necessary. Ideas? This would keep leaks out of the foam board and beyond, and act as a stabilizer to help keep the foam as stable from unwanted cupping or dimensional changes as possible..

  3. GBA Editor
    Martin Holladay | | #3

    The barrier you are describing sounds like roofing underlayment to me. It's OK to have two layers of roofing underlayment in this type of assembly.

    The ideal roofing underlayment to use directly above the rigid foam would be a vapor-permeable synthetic roofing underlayment that can be detailed as an air barrier -- for example, Cosella-Dörken Delta-Maxx Titan, Cosella-Dörken Vent S, Cosella-Dörken Delta-Foxx, VaproShield SlopeShield, or Solitex Mento.

    Since you'll have a ventilation space above the roofing underlayment, you want the underlayment to be airtight. So that rules out asphalt felt. If there is ever moisture under the membrane -- which is possible if you use mineral wool at the seams of your rigid foam -- then it makes sense for this roofing underlayment to be vapor-permeable.

  4. user-6612786 | | #4

    This is the guidance I was hoping to get from someone. Since there is doubt about the mineral wool as a gap filler, I will use foam, and shop around for a product or nozzle better than what I've used in the past. As for the roofing underlayment, I will choose one based on slope, or even two products, as there is an almost flat area at the top, and getting vertical at bottom. The other criteria, would be fire resistance, as neither the TPO or the foam board offer much in this category, and any small pull towards fire resistance is important. I did not consider the acrylic coating (DRAW-TITE) to be a roofing underlayment but an air barrier; perhaps the whole assembly of DRAW-TITE , foam board, and roofing underlayment should be considered one barrier, with the tightest perm resistance facing the warm side of the assembly. Thanks for your help. Will learn to use the Greenbuilding personal project page, so I can share with you or your readers on this project

  5. user-6612786 | | #5

    As I move forward on the project, I see two important issues that should be answered, and I mention here because I want others who proceed with a outside insulation roof to think about. First is about the sheathing on the warm side of the assembly. I have read of this sheathing being taped, but I think that is weak, when examined as to its importance. I am using an acrylic sealer, on all sides and edges, and caulking to the frame to make as complete a air and vapor barrier as possible. Also taping and sealing the panels together on the outside. There may be better products or procedures, and I did consider a barrier like Ice Shield over this sheathing, but that doesn't change the porosity of OSB or plywood, like hopefully this solution does.
    The second issue is the potential drying out of the framing bays, where the fiberglass insulation resides. Its been stated there shouldn't be a plastic vapor against the dry wall, and there shouldn't be non breathable paints or vinyl on the interior of the dry wall. A question in my mind is; How porous is 5/8's dry wall, should I be doing something to help the wall breath, like drilling small holes in the wall and filling with plaster, or even making larger holes and decorating with a porous wall covering? Any thoughts on this, or science?
    Right now, I use a large dehumidifier in the upper loft of the dome as insurance. It would probably be good for the occupants of the home to have higher humidity. With a house that has framing and insulation that dries to the inside, what is a good rate of humidity in the home?

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