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Community and Q&A

Interior impenetrable membrane for SIP roof and floor panels

rockies63 | Posted in Green Building Techniques on

I’m in the detailing stage of my cabin and need some advice on air/moisture sealing of the SIP panels. The panels (12 1/4″ from Insulspan) willform shed roofs and the main floor only (the walls are 2×6’s with Roxul Comfortbatt insulation, interior Membrane barrier with 1/2″ drywall, exterior 1/2″ plywood sheathing, Stoguard, 2″ Roxul Comfortboard, 1 x 4 furring and then LP Smartside cladding). Inside the cabin there will be an 8″ deep false floor built within the exterior walls to run plumbing and electrical.

First the floor: The SIP floor panels will rest on beams on top of a space frame foundation system resulting in the underside of the panels being 3′ off the ground. I was thinking of using 30 lb asphalt paper on the underside of the floor panels and then using 5/8″ plywood between the beams (treated with Flamestop II fire protection coating). To provide a vent channel behind the plywood I was going to use Coravent venting strips around the plywood edges with FireUS Rainscreen barriers as a secondary safety measure.

For the cabin’s interior I’ve read that there should be a “fully adhered impenetrable membrane” applied to the SIP floor panels. I’m guessing that this “membrane” should be different than the “Membrane” product used on an interior stud wall but what to use?

Second, the SIP roof. I plan on using 12 1/4″ SIP panels that will have 2×12 wood splines at the edges. The connection points between panels will rest on top of timber rafters. I will place sealing tape on top of the rafters so that all the SIP seams will be foamed and taped. The exterior of the seams will also be taped. On the exterior of the SIP roof will be 30 lb asphalt paper, 2 x 4 furring strips running vertically up the roof and then another layer of 5/8″ plywood. On top of this “cold roof” will be Versashield fire protection membrane and then a 7/8″ Galvalume metal roof. The vent gap between the SIP roof and the cold roof will be filled with a Coravent product and the FireUs strip behind it. Any exposed fascia or soffit material will be wood covered in the Flamestop II.

Should there be a “fully adhered impenetrable membrane” sprayed or rolled onto the interior SIP roof surface? If so, which product?

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  1. user-6184358 | | #1

    Why not run the panels perpendicular to the rafters. The SIPS splines need to be nailed from below also. If the panels rest on Timber rafters why not go with the OSB splines and run the long splines perpendicular to the rafters?
    Look at the California Wildland fire code for how to build a fire resistant structure or the San Diego County BLDG Dept website
    . I took a quick look at the Cora-vent site and didn't see any mention of their stuff having a CLASS A rating. Look at Brandguard they have a Class A rated ridge vent.

  2. GBA Editor
    Martin Holladay | | #2

    I disagree with Tim's suggestion. Roof SIPs can only be oriented in one direction for structural reasons; you can't just change the orientation by 90 degrees without consulting an engineer.

    You should contact the SIP manufacturer to find out what the manufacturer means by a "fully adhered impenetrable membrane." Ask for a brand name.

    Most SIP manufacturers recommend the use of a high-quality tape at interior seams, not a membrane.

    For your roof assembly, I suggest that you switch from 30-pound asphalt felt to 15-pound asphalt felt. (The 15-pound asphalt felt is more vapor-permeable; it will help the SIP dry to the exterior if necessary.) Or you could use one of the (expensive) vapor-permeable synthetic roofing underlayments.

  3. GBA Editor
    Martin Holladay | | #3

    I just downloaded the Insulspan installation instructions from the Web, and I did a word search on the word "impenetrable," in hopes of finding your mysterious phrase, "fully adhered impenetrable membrane."

    The word "impenetrable" does not appear in the Insulspan instructions.

  4. user-6184358 | | #4

    Martin, If the Panels rest on top of timber rafters how will the bottom of the panel splines get nailed to the OSB? It is similar to the way metal buildings have frames and purlins. How do you build with SIPS with out involvement of an engineer? They are not a prescriptive item in the building code, thus engineering is required, as I understand.

  5. rockies63 | | #5

    Reply 1.
    Hello, thanks for everyone's reply.

    Tim, the SIP roof will have a 12" overhang at the east, west and north eaves but a 3' overhang on the south side (supported by timber brackets). Inside the main room there are interior ceiling beams running north/south. The 4' wide panels will be supported by these rafter beams inside and outside and the 2x wooden edge splines in the panels will also help to strengthen and stiffen the overhang.

    Martin: The phrase "impenetrable fully adhered membrane" was mentioned on a drawing in an article written by Joseph Lstiburek about building SIP houses in extreme cold environments. I have modified this drawing a bit because my walls are 2x6 stud (not SIP panels) and I don't need the original interior "secondary stud wall" in order to run mechanicals (this modified drawing also doesn't show my Roxul Comfortboard layer). It does show, however, the location of the membrane he recommends on the inside of the roof panels and the inside of the floor panels (below the false floor).

    I have also read an answer in another thread about sealing SIP panels written by Albert Rooks in which he suggests using SIGA Wigluv 100 on the exterior SIP seams followed by a layer of SIGA Majcoat on the exterior roof deck. He also suggests using SIGA nailing tape on the bottom of the cold roof battens and coarse thread screws to attach them to the SIP panels.

    For the interior roof surface Mr. Rooks suggests placing peel and stick tape across the tops of the rafter beams , placing the panels and then peeling off the paper layer to on each side of the beam and sealing the joint that way. Then he suggests using SIGA Majpell for the interior SIP panel surface. I'm guessing that this method could be used for the interior of the SIP floor panels too.

    Another question: should there be any air sealing around the edges of the false floor or should I leave it open to allow any moisture vapor to get out from under the false floor?

  6. rockies63 | | #6

    Reply 2.

    This is a detail I've done showing the basic wall/floor components I'm considering. I'm currently working on the window and door details. The openings will be "picture-framed" by 2x2 material so that the Roxul Comfortboard and siding have something to butt up against and the windows will be fiberglass framed units from Milgard..

    I did read a great article on the Building Science website by Joseph Lstiburek (BSI-085 Windows Can Be A Pain) about how to install outie windows in a wall clad with exterior mineral wool insulation panels but it doesn't show any metal window flashing (see Figure 8 in the article).

    As to using the Coravent venting strips (or possibly Brandguard) at the tops and bottoms of the exterior wall rainscreen channel and also under the SIP floor panels (shown on the drawing in red) along with nailing blocks and vertical furring strips I'm hoping that the siding will have enough solid material behind it so that it is properly secured to the wall.

    Am I forgetting anything?

  7. rockies63 | | #7

    I guess I should mention that I'm in climate zone 6B (Canadian Rockies). Is 2" of Roxul Comfortboard enough for the exterior if there is 6" of Roxul Comfortbatt inside the wall cavity?

  8. Expert Member

    What is the thinking behind providing a vented cavity under the floor SIP?

  9. GBA Editor
    Martin Holladay | | #9

    Thanks for providing additional information. Joe Lstiburek is intimately familiar with SIP failures, since he is one of the experts who investigated the famous cluster of SIP failures in Juneau, Alaska. All of these failures were due to air leakage. The colder the climate, the more essential it is to prevent air leaks.

    Most experts recommend that interior SIP seams be sealed with durable tape. These days -- after the Juneau failures -- most SIP manufacturers sell SIP tape. They're learned the error of their ways.

    If you are worried that SIP tape will telegraph through your finish flooring, there is no reason you can't install a membrane over your floor -- for example, a taped European air barrier membrane -- or a flooring underlayment (plywood subfloor) installed with attention to airtightness.

    When it comes to the SIP roof, once again, you need to follow the manufacturer's recommendations for installing canned spray foam at the seams, and you also need to tape the interior seams. That's what InsulSpan recommends. You've already figured out that you'll have to drape the tape over your beams before the SIPs are installed. Nobody likes this job -- it's fussy and prone to screw-ups -- which is one reason why SIP roof assemblies have a bad reputation in cold climates.

    If you are able to successfully install the interior tape, you don't need an additional fully adhered membrane, in my opinion.

  10. rockies63 | | #10

    Hmm, for some reason I'm not getting email notifications anymore whenever someone replies to my threads.

    Malcolm, the vented cavity under the floor SIP is my way of dealing with:

    1. Finishing the underside of the SIP panel with a protective weather barrier (and in case of wild fires).

    2. If I installed a plywood subfloor on top of the SIP panel (as shown on the Insulspan floor detailing drawings) then wouldn't I be decreasing the chances of any moisture within the panel seams from drying to the inside? If I then add an air or moisture barrier on top of the plywood subfloor then the drying potential is further lessened so I figured that the floor SIP should at least be more vapor permeable to the underside of the panel.

    That said, after looking at my design I was still concerned about wild fire getting under the cabin and getting into the air gap at the edge of the exterior plywood sheathing so I changed the asphalt paper to Versashield fire protection membrane and then added 2" of Roxul Comfortboard below that (keeping the Cor-a-vent and the plywood sheathing coated with Flamestop II.

    Thanks also to Malcolm for all your help and research.

  11. GBA Editor
    Martin Holladay | | #11

    Q. "If I installed a plywood subfloor on top of the SIP panel (as shown on the Insulspan floor detailing drawings) then wouldn't I be decreasing the chances of any moisture within the panel seams from drying to the inside?"

    A. All of the SIP rot horror stories I heard about were due to interior moisture condensing on cold materials near a seam, either on the roof or high on a wall. There is no reason to think that SIP seams need the ability to dry to the interior.

    Moreover, the stack effect carries interior moisture to roof seams in winter, while pulling exterior air through floor seams. Exterior air in winter is dry air. You get rot at the exfiltration locations, not at the infiltration locations. The stack effect protects your floor while it puts your roof at risk.

    All experts emphasize the need for the interior of SIP seams to be airtight. There is no need for inward drying. Remember, the foam is basically a vapor barrier.

    Adding a subfloor to your floor SIPs is not risky in any way.

  12. rockies63 | | #12

    Hi Martin, thanks for your reply.

    I looked at the wall to floor details again and the solution I'm leaning towards is this:

    The interior floor SIP seams will be foamed and taped as per the manufacturers instructions. On top of that will be a 5/8" plywood subfloor. I will NOT be adding an additional " fully adhered impermeable membrane" to the interior.

    Below the SIP panel (on the exterior) I will coat the underside of the panel with SIGA MajCoat (as suggested by Albert Rooks in his comments about how to finish a SIP roof panel) and then attach 2" of Roxul Comfortboard to the underside. The Roxul will become the "fire protection" layer placed right up against the bottom of the SIP panel. Over that will be and air space with the Cor-a-vent strips and then the 5/8" plywood protective panel treated with Flamestop II.

    Hopefully this should satisfy ALL my levels of paranoia lol.

  13. rockies63 | | #13

    I called Insulspan (did you know that manufacturers have information about their products and how to install them? lol) and the technician said my assembly details looked fine. For the sake of not having to source a ton of products I will be changing the SIGA Majcoat on the underside of the SIP panels to Stoguard Gold Coat (the same as the rest of the exterior).

    He did say that the floor seams didn't need to be taped (since there was a 5/8" plywood subfloor going down on top of the panels) but that perhaps I could add a layer of 6ml poly between the floor panel and the subfloor. I should probably still tape the exterior floor seams that fall over a foundation beam though.

    He also said that rather than laying seam tape across the tops of the ceiling beams and then peeling off the paper backing and sticking it to the interior OSB skin that I should use 6 ml poly across the ceiling beams and tape that instead, He said that the Stoguard Gold Coat didn't need to applied across the interior field of the panel but that I could apply it to the exterior of the SIP roof panels.


  14. Expert Member
    MALCOLM TAYLOR | | #14

    If you are worried about the seams on the floor SIPs, I'd tape them over using 6 mil poly. You want to secure the subfloor with adhesive and screws. Poly precludes this.

  15. rockies63 | | #15

    Malcolm, I plan on foaming and taping the floor seams anyway. The technician said that poly on the floor wasn't necessary since I was going to be putting down a plywood subfloor. I will probably tape any seams I see exposed on the underside of the floor panels too and then apply the Stoguard Gold Coat to the panel underside and also about 2 inches down the surface of the foundation beams (just to seal the joint between the beams and the panels).

  16. Expert Member
    MALCOLM TAYLOR | | #16

    i don't know much about SIPs, but I imagine most of the same factors that make conventional floors much less problematic than walls or roofs from an air-sealing and moisture perspective apply in your situation. With the measures you are suggesting it should be pretty bullet-proof.

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