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Community and Q&A

Dimple mat over SIP roof for ventilation space vs. strapping and plywood?

Nicolas_Bertrand | Posted in GBA Pro Help on

Building my SIP home in April 2020, and still not 100% sure of what to do on the roof.  Since SIP homes are always recommended to have a vent space above a vapor permeable underlayment on the roof, I have several options.

Option 1:  Permeable underlayment on the SIP, then lay down 1″ thick strapping vertically, then cover with 1/2″ plywood and then ice & water membrane with shingles.

Option 2:  Permeable underlayment on SIP, then cover roof with a dimple mat, like Delta MS or similar, which would leave a small space for any possible trapped moisture to escape out through a ridge vent.  Then ice & water over the dimple mat and then shingle.

There is a product called Sharkskin ventilated mat 
https://sharkskinroof.com/sharkskin-products/sharkskin-vent-roof-mat/

It seems like a similar idea, but I am wondering if the dimple mat would create enough ventilation space?  The ventilation is 99% to allow any possible moisture that gets through the SIP roof panels to have a place to escape.  The Delta MS I used on my foundation is very strong and you can walk on it without any type of deformation.  I just wonder about doing something nobody has really done before (as far as I know).

Thoughts/comments are appreciated!

Nick

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Replies

  1. Zdesign | | #1

    You want to have as few fasteners going into the SIPs as possible. If you are nailing shingles directly into the SIPs they are most likely going to fail. You're better off going with the tried and true 15# felt, 2x4s 24" OC laid flat, 5/8" OSB and then the rest of your assembly.

  2. ohioandy | | #2

    Joe Lstiburek wrote the definitive work on SIP construction. Anyone even considering the use of SIPs needs to buy this book! Builder's Guide to SIPs for All Climates, published in 2008 by SIPA, the industry association. That said, he doesn't address the use of a dimple mat on a SIP roof, so you're out on a ledge with that idea. He does allege that shingles can be installed directly on roofing underlayment over SIP assemblies, but there are drawbacks, and "shingles work best when installed on 'over-clad' SIP assemblies." Wood shingles, he says, "should be installed over a vented drainage space such as drainage mats (i.e. 'Cedar Breather') or batten and counter-batten wood assemblies." Of course, wood shingles have the advantage of being vapor permeable, so a thin drainage mat works.

    If you're using asphalt shingles, which are totally impermeable, I'd strongly recommend going with the most robust approach you can afford. Hopefully that would be option #1 (although you don't need "ice and water" membrane--just regular underlayment.)

  3. Expert Member
    MALCOLM TAYLOR | | #3

    Nick,

    Your second option seems to be missing a layer. I've never seen shingles that could be laid directly over a dimple mat without sheathing above. It would be worth checking with whatever manufacturer you are considering using to see if it's warrantied. Either way the mat (and very likely the top sheathing layer of the SIPS) is going to be penetrated by an awful lot of fasteners. Considering the consequences of SIPs failures, I'd go with the more conservative option of strapping.

  4. maine_tyler | | #4

    Is the fastener penetration issue one of exterior bulk water entering the panel and having poor drying potential? I don't see how a small roofing fastener would create interior driven moisture issues...

    I always thought that one of the 'advantages' of a SIP vs layering exterior foam was that the nail base was already in place, but it's sounding like its best not to even use it as such? I know that's not the only 'advantage,' but still... really makes me wonder why ever use SIPS, unless covering a timber-frame maybe.

    1. Expert Member
      AKOS TOTH | | #6

      The only real advantage of SIP is quick construction. You can get better performing assemblies for less money using "standard" build techniques.

      SIP roofs are especially easy to mess up. Don't take any chances and follow the guidelines from Zdesign.

      Most important of all, make sure your warm side air barrier on the SIP is PERFECT. Any air leaks at the seams means SIP rot down the road.

  5. Nicolas_Bertrand | | #5

    People install shingles right to SIPs all the time, I don't think the fasteners are the issue. The dimple mat is like a cedar breather mat, but it is rigid. Shingle warranties are basically useless (I own a construction/roofing company), so I'm not concerned about voiding a warranty.

    1. Expert Member
      MALCOLM TAYLOR | | #7

      Nicholas,

      Leaving aside any problems of air-leakage that can damage the panels, there is a more fundamental worry. When a roof leaks on a conventional roof, you almost invariably have to replace some sheathing. As a roofer you know this is this reflected in re-roofing contracts that assume a percentage will turn out to be damaged. With SIPS that sheathing is structural, and damage to it compromises the strength of the panels. You can't just piece in the parts that are damaged.

      I agree that shingle warranties are useless, but it's worth remembering that installing products outside the manufacturer's instructions is a code violation.

      1. Nicolas_Bertrand | | #8

        This is why I suggested full ice & water over the dimple mat, to help prevent a leak. The underlayment directly on the SIP would be there just in case anything got through. I even considered using Grace Vycor ENV-S as underlayment since it is a vapor permeable WRB that self-adheres (no fasteners needed) and also seals around fasteners like nails going through it.

        I'm not saying it's the right way, or best, but I am always trying to think of better solutions for SIP roofs because despite them being a great product, they have a lot of inconvenient drawbacks.

        1. Expert Member
          AKOS TOTH | | #9

          Most dimple mat is a vapour barrier, I really don't think it will create enough air flow underneath to allow for drying.

          The $/sqft of the stack up you are proposing is probably similar to the felt+strap+CDX+underlayment. It is harder to build and more risky, not sure what it buys you.

          1. Nicolas_Bertrand | | #10

            I think it would actually be quicker/easier. You roll out the dimple mat, attach with cap nails and then cover with ice & water and shingle. Versus strapping, then cutting all the plywood, then ice & water and shingles. Labor cost is less, materials cost is probably similar. Just thinking out loud here, trying to simplify the process. My house roof will be around 40 squares, so it's a decent size.

            The sharkskin product has been used on SIP roofs before and I think it has a little more air-space under it. Sometimes I overthink things, but sometimes that's how new ideas arise.

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