Exterior Roxul batt insulation in semi-conditioned space?
I’m building a 10×14 timber frame shed in upstate NY climate zone 5. It’s under 144 sq.ft. so doesn’t require building permits or inspection. I have pine 2x T&G roofing and 1x wall cladding from a local mill. The building will be used occasionally as a workshop, including during winter. For this reason I’d like to insulate it. I’m grumpy about foam for various reasons including rampant carpenter ants around my property and so would like to use rock wool.
I’ve read the articles on PERSIST insulation, air barrier in cathedral ceilings, etc., and would like to use roxul. However, in a variation I’d like to use the comfortbatt vs comfortboard product because the board product is roughly 5x the cost per foot for equivalent R value – too expensive for an outbuilding. My idea is to use the comfortbatt on the outside of the T&G cladding. I’d attach 2×4 on edge vertically to the cladding to provide bays for the batts. Then house wrap then 1x furring strips to create a rain screen gap, then the exterior pine siding. No structural sheathing is necessary due to it being a timber frame.
So the wall (from interior): 1x pine T&G, air barrier, roxul comfortbatt in vertical 2×4 bays, house wrap, furring strips, exterior pine siding. Roof: 2x pine T&G, air barrier (grace ice/water?), roxul in 2×6 bays, plywood, felt, furring strips for air gap, metal roof.
My questions are:
1. Is it advisable to use comfortbatt in this manner? Can I skip the exterior sheathing?
2. What can i use as an air barrier that is relatively inexpensive? I’ve read about using sheetrock but not sure if it works in this unconditioned scenario due to mold or other issues.
3. Do I need plywood on the roof above the batts or just something like felt if I’m doing an air gap?
4. Is it overkill to be doing the grace ice and water or other air barriers for an occasional use outbuilding?
Thank you.
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Replies
Since the sheathing isn 't structural, using asphalted fiberboard for the exterior sheathing would be sufficient to use class-III vapor retarders on the interior, even in zone 5. Seal the seams with fiber-reinforced roofing patch to make it an air barrier. (Give the goop several days of drying time before adding layers to the exterior.) While not technically a weather resistant barrier, it's fairly weather tolerant, but a housewrap drain-plane is still a good idea. At 3/4" the fiberboard adds ~R1.5 to the whole-wall stackup over what it would be for half-inch plywood, and it's cheaper than half-inch plywood.
The interior side air barrier can be a layer of housewrap. Tape any rips or dings that you pick up with housewrap tape as hou go.
On the roof, the air gap has to be on the under side of the plywood. A layer of housewrap over the fully-filled rafter works as an iar barrier for the batts. Sister on some 2x2s over the rafters on which to mount the plywood. Grace Ice & Water is fine, above the structural roof deck, but so is #30 felt. It isn't being used as an air barrier when you have a vent channel running below the roof deck.
Thanks Dana. So it appears as if you are saying I do need an exterior side air barrier (ex. fiberboard) under the exterior siding on the walls to create both interior and exterior side air barriers.
The roof has house wrap as an air barrier over the rafters filled with batts, 2x2 over that, then plywood, underlayment, then roofing.
I don't understand all the mechanics of why the walls seem to need interior and exterior side air barriers while the roof just needs an exterior side one, but perhaps I'm reading/interpreting incorrectly. Wouldn't be the first time!
Michael,
My advice is different. I think that your plan for the walls is fine. The better you detail your housewrap (for example, by taping the seams), the better it will perform as an air barrier. After all, this is a shed / occasional workshop.
For the roof, your plan would also work -- but you might consider using taped OSB as an air barrier over the existing tongue-and-groove sheathing, or perhaps one of the European membranes sold by 475 High-Performance Building Products -- but only if these options are less expensive than Ice & Water Shield.
Needless to say, you aren't planning to install much insulation in your roof -- sounds like about R-21, which isn't much. But I assume that you know that.
If you are installing metal roofing, you don't need any roof sheathing above your insulation bays if you don't want to install any. You could top the insulated bays with housewrap, and then install 2x4 purlins above the housewrap. Metal roofing can be fastened directly to the purlins.
Thanks Martin. Upon further reading, I'm wondering about a few additional details:
1. I believe there is a chance for staining where the 2x6s are nailed to the roof, but only if the air barrier is compromised(?)
2. It seems doing a vented roof assembly is insurance in case the air barrier fails, but the most important thing is the air barrier. However I've also read that a vented roof assembly could possibly contribute to issues by pulling more air through the assembly. I'll go with venting in any case.
3. In the case of no sheathing over the insulation due to a metal roof, with purlins over the house wrap for a vent gap, I assume I install the insulation and house wrap all the way to the ridge, not stopping short as you would if the vent was under the sheathing in the insulation bays?
Thanks for your assistance.
Michael,
Q. "I assume I install the insulation and housewrap all the way to the ridge, not stopping short as you would if the vent was under the sheathing in the insulation bays?"
A. That's correct.