SIP construction with Roxul on interior service cavity wall
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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