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Best Material to go above T&G under SIP roof panels

mike@thecabin | Posted in General Questions on

This is a follow up question from this discussion

I have shingles and I need to know the proper material to go under the sip above the t&G and the best tape for sealing this material. There was no definitive answer given for the best product in this discussion. Also while it was covered for metal roofs, I need to know what is the best material to to go under the shingles and above the sip.

My home is a log cabin with real wood rafters and the t&g installed over the rafters. I’m replacing 4″ sip’s with thicker ones. I’m located in Alabama which is obviously very humid.

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  1. GBA Editor
    Martin Holladay | | #1

    Above the SIPs, you install roofing underlayment (asphalt felt). Don't use synthetic felt, since synthetic felt is only approved for use over vented roof assemblies.

    Under the SIPs, you install SIP tape at all of the SIP seams, and then you install your finish ceiling material (T&G boards).

    And if you have any further questions, I strongly advise you to contact your SIP supplier.

  2. albertrooks | | #2


    Here is the outcome of that thread after being "vetted" by GBA experts:

    Note that all of these folks are primarily cold climate guys and are working from a cold climate perspective.

    As Martin suggested, whatever you do, work within the technical specs and your SIP vendors installation recommendation to maintain your warranty.

    That said, I really do like the SIGA products better than the R Guard tape that he suggested. Not because we sell it, but because I know how well I can get it to stick and stay stuck to OSB while maintaing permeability. I've learned that if you want a "bullet proof bond" on OSB, use the SIGA dockskin primer. I usually have other products from other companies that I'll recommend in addition to our products to keep some balance and perspective in the Q7A forum, but in this case these SIGA tapes are really far and away the best in this application. Second up would be 3M 8067 + a 3M or our primer.

    The SIGA Majcoat in the article is really for a vented application. It's not a good idea to put under shingles since there are so many fasteners. I like Martin's suggestion of felt if you are going to put a thousand holes in your SIP roof. It's durable and permeable.

    If you want to avoid the fastener penetrations, treat it like a metal roof with a vented gap and lay the shingles on the cold sheathing layer. You'll really protect your SIP panel.

    I'm curious if you've been able to see the condition of your existing SIP panels under the roofing. How long they've been there and how they look.

  3. mike@thecabin | | #3

    Guys T&g already down this is existing house. What goes on top T&g before sip goes down. I will let you guys know the condition of osb once I replace it, but I've go to figure what I need and purchase it before I can start. I will check with sip manufacture but wanted to know what everyone thinks I need between sip and T&g Please advise. Hot and humid climate.

  4. albertrooks | | #4

    it sounds like you can lay down some kind of interior air sealing membrane over the T&G. The focus is to replace the joint sealing that you can't reach because the T&G is in the way. The goal is to stop any air infiltration into the SIP joints. To do this, you'll need to be able to seal the membrane to/at the edges of the SIP so that you've got a continuos seal to the SIP around all edges.

    Your membrane can match the OSB perm rating, so a good interior air sealing membrane will work fine. If your in a rush, look around your area. Perhaps there is something like "Membrain" in stock. If not, and you need help finding something, call us and we can get some Siga Majpell out to you: 855-FOR-SIGA.

    It's a probably a "duh" and not the case, but if all your SIPS are joined on the ground and can be tapped before you set them, you wouldn't need a membrane over the T&G. I was able to do that on a SIP roof on a project...

    DON'T use poly. It's will tear, and if it doesn't then it won't let the OSB dry as it needs to.

  5. mike@thecabin | | #5

    So if I get a membrane or underlayment and seal all the edges and overlaps over the T&g will I be good? It just seems like sealing the T&g on top before the sips go down would be my best bet. Then I could attempt to seal the joints on the sips and tape the top of the sip joints(connections). After this do I need a high perm material above my sips under my shingles?

    Sealing the bottom joints of the sips seems next to impossible.

  6. albertrooks | | #6


    In my opinion you've got a good approach for the underside. Connect that membrane to the SIP with no breaks or gaps and you should be good.

    On the top side, Sealing the joints is really important. Some experienced builders only rely on the injected foam as the joints come together. Adding a permeable tape to the joint creates a much more durable seam.

    For the roofing: Since you are directly applying shingles to the SIP, a good permeable felt should work. However... When you take a part your existing roof... If you find that the OSB face of the SIP does not look good, like it was failing in place, then you can consider protecting the new SIP with a gapped cold sheathing approach: SIP+Felt or Majcoat+batten+Plywood+felt+shingles.

    I'm only putting this out there, since the SIPS OSB layer does better when it's a little farther out of "harms way": Less fastener penetrations & good venting to change with the exterior ambient conditions.

  7. mike@thecabin | | #7

    Ok, just out of curosity what membranes will work. I've seen people on these boards talk about certainteed product, ice and water, weather guard etc. I have time to order the product so what are some options on the type of product I need to go over the T&G.

  8. albertrooks | | #8


    Here are 3 that I typically recommend:

    1, You can track where to buy in your local market. For tape to go with it, look in your market for 3m 8067.

    2, The good folks at 475 are fellow Passivhaus enthusiasts like me and have Pro Klima Intello: They also have a less expensive DB+ membrane which maybe all you need in this application. Your welcome to give them a call and ask.

    3, We have the Siga Maijpell that we like for the application. We ship it all over the country along with SIGA tape to apply it with. Your welcome to call us also. We'd be happy to help.

    The membrane is actually the easy part, getting tape stuck to OSB requires good tape. For that I like SIGA Rissan 60 + Dockskin primer.

    I suggest that you don't use a impermeable peel and stick exterior WRN like Ice & Water and the like. By keeping some level of permeability, moisture can dry from the OSB layer as the levels rise and fall.

  9. mike@thecabin | | #9

    Thanks for all your help I will be ordering some of the products. I will also let the board know what I find.

  10. mike@thecabin | | #10

    One last consideration I just thought of the contractor will need to walk on what ever membrane I choose. Do you know of any that you can walk on top of a slope?

  11. albertrooks | | #11


    I've not handled the Membrain product enough to know about walking on it. Both #2, and #3 will take a lot of punishment, not tear easily and aren't slippery.

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