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Community and Q&A

SIP construction with Roxul on interior service cavity wall

Rob Andrushko | Posted in Green Building Techniques on

I am new to the forum, but have been here a while reading allot of very informative posts. This seems like a great community to find like minded experts who are passionate about building environmentally friendly homes.

I am in Climate Zone 6 (On Georgian Bay, in Ontario Canada). I am currently designing a house that I will be building myself and want to achieve the Passive House standard. I am looking to utilize SIPs for the building envelope and also want to incorporate a service cavity. I am strongly considering utilizing ROXUL insulation on the interior service cavity walls to increase the insulation value and minimize the already small amount of thermal bridges that will be present at top and bottom plates, corners and wall intersection.
I am also open to using smaller SIPS and a thicker service cavity, possibly with dense packed cellulose if it makes more sense.

My overall wall design would be as follows from outside to inside:

James Hardie or similar siding,
3/4″ strapping over moisture barrier,
Insulspan or Thermapan 8″ structural walls (I am also considering utilizing 10″ Pressure Treated SIPs for the foundation on concrete footings (Available from Thermapan in my area) all floors will be suspended with Top Plate joist hangers,
Air barrier taped/ sealed on inside surface of SIPs,
4″ space filled with ROXUL,
2×4″ service cavity filled with another layer of ROXUL,

The roof will be constructed of 12″ SIPs with the 2X4 service cavity directly installed onto the bottom of the SIPS.

This will give me an effective wall R value of 66 and a roof R value of 72 (I could still increase the R value by installing 2X6 studs on the ceiling if required with further energy modelling)

I do not intend to put a vapour barrier behind the drywall and will be relying on the Vapour barrier on the inside surface of the SIPs where it is protected from penetration etc.

I plan on utilizing SIPs due to the cost (similar to frame construction in my jurisdiction) and speed of construction of the building envelope. This allows a rapidly erected structure to undergo more detailed air sealing and insulating once assembled, on the inside out of the weather in the bad months also.

My question: Could the insulation in the service cavity suffer from moisture issues without the vapour barrier directly behind the drywall? Is there any other recommended detail utilizing SIPs and a service cavity that is recommended? I appreciate all educated opinions.

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  1. Steve Knapp CZ 3A Georgia | | #1

    Rob. Are you considering Thermapan SIPs made with XPS?

  2. Rob Andrushko | | #2

    Steve. Thermapan has EPS SIPS.

  3. Steve Knapp CZ 3A Georgia | | #3

    Rob. I was trying to figure out how they were getting r-38 out of a 8.25 inch thick SIP.

  4. Rob Andrushko | | #4

    Steve, you are correct to suspect their rating. It is much higher than it should be for a EPS panel at that thickness. The Insulspan product lists a rating of R30 for the same thickness.

  5. GBA Editor
    Martin Holladay | | #5

    Q. "Could the insulation in the service cavity suffer from moisture issues without the vapour barrier directly behind the drywall?"

    A. No. Condensation (or moisture accumulation) happens on cold surfaces, and the surfaces in your service cavity should be warm.

    Q. "Is there any other recommended detail utilizing SIPs and a service cavity that is recommended?"

    A. As you evidently realize, the most important detail with SIPs is a bulletproof interior air barrier. In addition to sealing the SIP seams with canned spray foam, it's essential to install SIP tape on the interior seams.

    In a cold climate like yours, you should be planning to include a "cold roof" detail -- ventilation channels between the exterior OSB facing of the roof SIPs and the roofing. These ventilation channels will greatly reduce the risk of OSB rot at the seams (rot which has led to expensive repairs of SIP roofs in cold climates). For more information, see Air sealing SIP seams.


  6. D Dorsett | | #6

    With a SIP the OSB cladding is the vapor barrier. As long as the R-value of the Roxul is less than 2x that of the SIP in the wall you will not have a condensation/adsorption issue. For the roof, the R-value of the Roxul must be no more than the SIP-R.

    From your description it doesn't appear you are anywhere near the danger zone.

    Do NOT put sheet polyetheylene anywhere in the stack up!

    Regarding the R-value of the SIP, when the mean temp through the foam is 40F (about 5C, which is roughly the condition when it's 20C indoors, -10C outdoors, perhaps your average January condition), the performance runs about R4.5/inch. With an 8.5" SIP you'll have about 1" of OSB total (about R1), and 7.5" of EPS, about R34, totaling about R35, not R38. And that would be a clear-wall performance not whole-wall, not accounting for the thermal bridging of the necessary top/bottom plates and splines, window & door structural framing, etc

    Only they were using graphite-loaded would have a clear-wall performance of about R38, with whole-wall performance marginally lower.

  7. Rob Andrushko | | #7

    Thank you Martin Holladay and D. Dorsett for the advice. You both reinforce what I was already understanding.

    I am going to be utilizing a steel profiled roofing system. The SIPS roof will receive a roofing underlayment applied directly to the SIPS and then vertical and horizontal strapping to give airflow below the steel roofing. I will ensure that my ridge and eaves details allows air flow. My detail is similar to the above, but substitutes horizontal strapping in place of the 2nd layer of roof sheathing. The South face will also be covered with Solar PV (Ontario's Microfit program is a great investment that pays $0.32/Kwh).

    Do you have any concerns with Strapping in lieu of a full layer of sheathing on the roof deck?

    There are no roof windows and it is a very simple gable roof with the ridge running East and West with 2 shed roof dormers facing South and North at one end. I have done a few cold roof details similar to the above detail but always thought that the sheathing underside of the upper layer was at risk for condensation once the sun hits the roof deck on a warm day after a cool night.

  8. GBA Editor
    Martin Holladay | | #8

    Q. "Do you have any concerns with strapping in lieu of a full layer of sheathing on the roof deck?"

    A. No, as long as you are using a type of steel roofing that can be installed on strapping. I've installed a lot of through-fastened steel roofing on 24-inch-on-center purlins, and the technique works well.

    A few manufacturers of steel roofing require a full roof deck. When in doubt, call the manufacturer.

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