Designing wiring with SIPs and keeping building tight
I am in the process of designing a Timber Frame to be enclosed by SIPs. I have a solid design for my plumbing fully enclosed within an internal service cavity and I am now turning my attention to the electrical. I always thought the electrical would be a bit easier to wrangle but I am stuck… My goal is to avoid hacking up the SIPs because I want to keep the building envelope as tight as possible to keep my heat load down. It also seems counterintuitive to me to spend all the money on SIPs and then cut them up. From what I understand these are the basic design approaches:
1. Spec and use pre-designed vertical and horizontal chases in the panel.
2. Attach a vertical furring strip on the back of the panels and use horizontal strapping on the inside face of the OSB to create a cavity behind the sheet rock (which will slide in behind the posts). This approach would require the SIP be mounted 2” off the surface of the posts. Not sure I like this idea from a Seismic and Wind loading design perspective.
3. Drill vertical chases (post installation) down from all outlet locations down into the basement and run Romex horizontally below. I have a 2nd floor so this won’t really work for the upstairs. This idea seems particularly messy.
4. Run a wire chase at floor level under a baseboard and stop the drywall course before the floor to ensure the Romex is as far back as possible. I like this idea but I am worried that I won’t have the distance (1 1/2”) by code below the outer surface. Also, since this is a baseboard, there wouldn’t be any issues of a picture frame nail striking a conductor. I also like that this solution provides access to the wiring perhaps for inspection, maintenance and for the lack of damage to the SIP. Unfortunately, this doesn’t solve the issue of running wire to a wall switch.
As an aside I will have some internal walls on the 2nd floor so that should help a bit, but I want to make sure that my planning is sound and my engagement with the electrician is successful. From what I have read, the electricians don’t have as much experience with them and the panel companies are huge advocates of how easy it will be. Seems like a disconnect. I am thinking one possible solution is to go with the pre-installed chases, but have all the fish tape/string in place when the electrician shows up. It also seems confusing to me how the NEC doesn’t really make any accommodations for SIPs when it comes to securing Romex near boxes. The chases are internal and made of foam. I certainly understand how the code constraints can make an electrician nervous. That being said, I am trying to figure out how to best approach the design. I am thinking perhaps a call to the electrical inspector would be a good place to start. SIPs aren’t exactly new anymore so I hope I can get some clarity soon. I apologize for the long-winded post, but I wanted to fully describe the design dilemma I am working through.
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Replies
John,
Your questions should be directed first to the SIP manufacturer. Different manufacturers offer different options for electrical chases.
That said, you have put your finger on one of several hurdles that accompany the use of SIPs. Every construction method comes with pluses and minuses.
Another hurdle you face is draping the SIP tape that is required to seal the interior of each SIP seam over your timber frame beams and purlins before the SIPs are installed. Air sealing details get tricky fast when you have a timber frame. You might want to read these two articles:
How to Protect Structural Insulated Panels from Decay
A SIP Roof Repair in Wisconsin
John,
In your situation with a "true" timber frame home, the manufacturer "cut into foam" chases should be pretty straightforward to utilize. This was not the case with our house where we utilized the SIPs structurally and ended up with a lot of LVLs etc at critical locations. Worse than that we converted to an ICF foundation as our last major architectural modification, which ended up making the wiring transition from ground floor to first floor particularly tricky. (Insert shooting self in foot sound effect.)
I can see each of your alternative scenarios working too. For instance holding the SIP and sheetrock back from the timbers could be turned into a very cool feature, multiple shadow lines at play. However you are correct that this might make a structural engineer crazy as he figured out the connections between offset panels and timbers.
I thought about doing electric at the baseboard level in our home. That is very clean, but sets in motion cosmetics that will likely alter the overall look and feel of your home's trim.
Not using the SIP manufacturer "cut into foam" chases at all, yep there are times I wish I did that. Those chases have been a bit of an ant superhighway in our house, forcing me to utilize more perimeter pesticide than I would like.
Re the lack of a means to secure nonmetallic cable proximate to the electric boxes in a SIP panel; my initial thought is that the cut-in foam chases are much like a conduit from box to box, so little chance for any subsequent tugging, pulling, or other mechanical damage to those wires. Your inspector has the last word on that though.
And yes Martin is right, think a lot about those air sealing details. They are essential, but they burn up a lot of time and money.
BTW, some more pix and discussion about our SIP home (including about air sealing) in this non SIP titled thread. https://www.greenbuildingadvisor.com/community/forum/green-products-and-materials/57161/geothermaldhwradiant-heat
Is "hot balling" part of the SIP orthodoxy, or an Al Cobb invention? As a great mess-free alternative to the drilling you describe in #3, use a super-heated 1-inch ball bearing to melt vertical chases. You'll need a second 4" hole just above the bottom plate of the SIP, directly underneath the hole for receptacle or switch. After heating the ball bearing with a propane torch, it's inserted into the top hole and dropped at the panel's midpoint. In about 2 seconds, the ball rolls out the bottom hole, after melting a perfect 1.5" chase.
Thanks for raising those issues Martin and Andrew. I just spent a bit of time familiarizing myself with the failures in Juneau and the fellow from Wisconsin who also had issues. I am hopeful I can manage the installation if I go the SIP route. Tight seams, interior air sealing and of course, the best practice assembly of tar paper/furring strip/next layer sheathing/roofing material to ensure that the outer OSB could dry out.
Any suggestions on how one might be able to manage the seam taping of the panels with a common rafter system? The roof will be 32' long so I am hoping for basically four long seams. Again, it's important to have to a plan to ensure something is buildable. I just saw that the Siga tape has a primer step to really get it to stick properly (looks like a great product). Perhaps a gasket on the surface of the rafter might work?
Also, thanks for that tip Andy. Perhaps that might allow me to do the vertical chases on site (and as needed).
I'm hoping my site (southeastern Idaho) isn't so wet that the risk of using SIPs might not be worth it. I'm also hopeful that the detailing work is something I can do myself (seems tedious but not overly technical).
Your best option for determining how to wire your SIPS is to consult with the panel manufacturer, timber frame supplier, and your electrician now before you finalize your plans. Engaging your electrician early will ensure details such as vertical chases aren't missed when the panels are installed and your electrician is comfortable with your installation approach.
I am just finishing-up building my SIPS home and similar to you I had planned horizontal and vertical chases for all of my wiring. Unfortunately, when it came time to install the rough wiring, some of the vertical chases weren't cut, horizontal chases weren't large enough to accommodate all the required circuits in the kitchen, and other areas weren't accessible due to structural elements.
We ended up using a number of different methods, including strapping the kitchen wall, placing some outlets in the floor instead of on the wall, and minimizing lighting fixtures on the ceiling to get the house wired appropriately. However, quite a few of the last-minute solutions could have been avoided if I had engaged the electrician sooner.