Timber frame walls and air sealing
I’m planning to build a timber frame house and use SIPs or zip-r for the exterior walls, insulation, and air sealing. This seems a convenient and efficient system being used by many for other walls. I’m in Southern California USA, in a temperate climate, so I don’t require very thick SIPs, which makes the thinner zip-r panels look appealing.
My question is about the interior side of these walls…do I need to apply a uniform and complete layer of drywall throughout the house in order to have an air barrier on the interior side of the wall? I will definitely be air sealing on the exterior side of the panels. I’d prefer not to use drywall everywhere for a few reasons: aesthetics, avoid a needless layer of materials that are not great for the environment, labor costs/time, and I’d like to have easier access to wall internals by having easy to access paneling. I think ease of repair/improvements is highly underrated feature of well built homes, and drywall isn’t terrible but it’s not the best either.
I suppose a couple related questions that should be pondered are:
1. Is the interior side of the sips fire-rated? Would I need to air seal this side as well?
2. Is zip-r an alternative worth considering for my situation, and how would I meet fire-code with that product?
I’m still trying to dig up research on these topics myself, thought I’d drop this here to see if anyone had good links and/or information to share. Thanks all!
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Replies
Is it a fully diagonally braced post and beam, or a more modern faux timber frame?
If it is diagonally braced, the walls are not load bearing and you can buy non structural panels with drywall instead of osb on the inside.
what would be your finish wall instead of drywall?
Hi Joseph.
In your situation, one excellent exterior air barrier should be enough (although some SIP manufacturers may require redundant layers of spray foam in the seams and tape on the interior of the joints to address past failures caused by leaky seams).
I have the same question as Keith regarding your intended interior finish. Whether or not you need interior drywall to meet fire rating is a question for your local building official. When Matt Risinger built a house using Joe Lstiburek's "perfect wall" assembly, his building official was happy to let him go without drywall and if I remember correctly, saw the open cavities as a benefit to fire fighters. I believe the article mentions that (you can read it here: Perfect Wall House). But that wall had no cavity, the back of the sheathing was the interior finish; framing was exposed.
Hi Keith & Brian, thanks for the responses! If it weren't obvious, I'm very early on in the design process on this project, haven't made every decision yet, but I'm trying to focus on the envelope first.
Keith - I haven't yet spoken to an engineer, but I'm in earthquake country and I am assuming for now that I will need diagonal braces throughout the structure. Code here often requires "sheer wall assemblies" on many/most walls, but I am unsure if the outer panel of SIPs (or zip R) will meet those requirements. I will figure that out soon.
As for interior wall finishes I was planning on having it vary throughout the house. Some areas will indeed have drywall. In others I will build ~6 foot tall wainscoting with a couple feet of drywall above. Some spaces will probably have shiplap as well, whether it is real shiplap or faux depends on whether I need that to be an air barrier or not). My goal/hope is to avoid having a layer of drywall applied over 100% of the interior even though I'm hoping to cover more than 50% of it with wood paneling.
I have seen/read about the perfect wall from Joe Lstiburek and Matt Risinger...in Matt's execution of that system, he has basically laid out the layers the opposite order of the way zip-r or SIPs would work (in order from interior to exterior he has: studs, 1x6 boards, osb, water & vapor barrier, insulation, cladding - He has additional layers for a rainscreen between the insulation and cladding, but I won't be doing that). I have often read the line "a vapor barrier should be placed on the warm side of your insulation" ... how would someone use SIPs or zip-R in that case, in a warm climate?